The dystrophinopathies cover a spectrum of X-linked muscle disease ranging from mild to severe that includes Duchenne muscular dystrophy, Becker muscular dystrophy, and DMD-associated dilated cardiomyopathy (DCM). The mild end of the spectrum includes the phenotypes of asymptomatic increase in serum concentration of creatine phosphokinase (CK) and muscle cramps with myoglobinuria. The severe end of the spectrum includes progressive muscle diseases that are classified as Duchenne/Becker muscular dystrophy when skeletal muscle is primarily affected and as DMD-associated DCM when the heart is primarily affected. Duchenne muscular dystrophy (DMD) usually presents in early childhood with delayed motor milestones including delays in walking independently and standing up from a supine position. Proximal weakness causes a waddling gait and difficulty climbing stairs, running, jumping, and standing up from a squatting position. DMD is rapidly progressive, with affected children being wheelchair dependent by age 12 years. Cardiomyopathy occurs in almost all individuals with DMD after age 18 years. Few survive beyond the third decade, with respiratory complications and progressive cardiomyopathy being common causes of death. Becker muscular dystrophy (BMD) is characterized by later-onset skeletal muscle weakness. With improved diagnostic techniques, it has been recognized that the mild end of the spectrum includes men with onset of symptoms after age 30 years who remain ambulatory even into their 60s. Despite the milder skeletal muscle involvement, heart failure from DCM is a common cause of morbidity and the most common cause of death in BMD. Mean age of death is in the mid-40s. DMD-associated DCM is characterized by left ventricular dilation and congestive heart failure. Females heterozygous for a DMD pathogenic variant are at increased risk for DCM.

Fucosidosis is an autosomal recessive lysosomal storage disease caused by defective alpha-L-fucosidase with accumulation of fucose in the tissues. Clinical features include angiokeratoma, progressive psychomotor retardation, neurologic signs, coarse facial features, and dysostosis multiplex. Fucosidosis has been classified into 2 major types. Type 1 is characterized by rapid psychomotor regression and severe neurologic deterioration beginning at about 6 months of age, elevated sweat sodium chloride, and death within the first decade of life. Type 2 is characterized by milder psychomotor retardation and neurologic signs, the development of angiokeratoma corporis diffusum, normal sweat salinity, and longer survival (Kousseff et al., 1976).

The clinical manifestations of glycogen storage disease type IV (GSD IV) discussed in this entry span a continuum of different subtypes with variable ages of onset, severity, and clinical features. Clinical findings vary extensively both within and between families. The fatal perinatal neuromuscular subtype presents in utero with fetal akinesia deformation sequence, including decreased fetal movements, polyhydramnios, and fetal hydrops. Death usually occurs in the neonatal period. The congenital neuromuscular subtype presents in the newborn period with profound hypotonia, respiratory distress, and dilated cardiomyopathy. Death usually occurs in early infancy. Infants with the classic (progressive) hepatic subtype may appear normal at birth, but rapidly develop failure to thrive; hepatomegaly, liver dysfunction, and progressive liver cirrhosis; hypotonia; and cardiomyopathy. Without liver transplantation, death from liver failure usually occurs by age five years. Children with the non-progressive hepatic subtype tend to present with hepatomegaly, liver dysfunction, myopathy, and hypotonia; however, they are likely to survive without progression of the liver disease and may not show cardiac, skeletal muscle, or neurologic involvement. The childhood neuromuscular subtype is rare and the course is variable, ranging from onset in the second decade with a mild disease course to a more severe, progressive course resulting in death in the third decade.

FBN1-related Marfan syndrome (Marfan syndrome), a systemic disorder of connective tissue with a high degree of clinical variability, comprises a broad phenotypic continuum ranging from mild (features of Marfan syndrome in one or a few systems) to severe and rapidly progressive neonatal multiorgan disease. Cardinal manifestations involve the ocular, skeletal, and cardiovascular systems. Ocular findings include myopia (>50% of affected individuals); ectopia lentis (seen in approximately 60% of affected individuals); and an increased risk for retinal detachment, glaucoma, and early cataracts. Skeletal system manifestations include bone overgrowth and joint laxity; disproportionately long extremities for the size of the trunk (dolichostenomelia); overgrowth of the ribs that can push the sternum in (pectus excavatum) or out (pectus carinatum); and scoliosis that ranges from mild to severe and progressive. The major morbidity and early mortality in Marfan syndrome relate to the cardiovascular system and include dilatation of the aorta at the level of the sinuses of Valsalva (predisposing to aortic tear and rupture), mitral valve prolapse with or without regurgitation, tricuspid valve prolapse, and enlargement of the proximal pulmonary artery. Severe and prolonged regurgitation of the mitral and/or aortic valve can predispose to left ventricular dysfunction and occasionally heart failure. With proper management, the life expectancy of someone with Marfan syndrome approximates that of the general population.

Marinesco-Sjögren syndrome (MSS) is characterized by cerebellar ataxia with cerebellar atrophy, dysarthria, nystagmus, early-onset (not necessarily congenital) cataracts, myopathy, muscle weakness, and hypotonia. Additional features may include psychomotor delay, hypergonadotropic hypogonadism, short stature, and various skeletal abnormalities. Children with MSS usually present with muscular hypotonia in early infancy; distal and proximal muscular weakness is noticed during the first decade of life. Later, cerebellar findings of truncal ataxia, dysdiadochokinesia, nystagmus, and dysarthria become apparent. Motor function worsens progressively for some years, then stabilizes at an unpredictable age and degree of severity. Cataracts can develop rapidly and typically require lens extraction in the first decade of life. Although many adults have severe disabilities, life span in MSS appears to be near normal.

Mucopolysaccharidosis type II (MPS II; also known as Hunter syndrome) is an X-linked multisystem disorder characterized by glycosaminoglycan (GAG) accumulation. The vast majority of affected individuals are male; on rare occasion heterozygous females manifest findings. Age of onset, disease severity, and rate of progression vary significantly among affected males. In those with early progressive disease, CNS involvement (manifest primarily by progressive cognitive deterioration), progressive airway disease, and cardiac disease usually result in death in the first or second decade of life. In those with slowly progressive disease, the CNS is not (or is minimally) affected, although the effect of GAG accumulation on other organ systems may be early progressive to the same degree as in those who have progressive cognitive decline. Survival into the early adult years with normal intelligence is common in the slowly progressing form of the disease. Additional findings in both forms of MPS II include: short stature; macrocephaly with or without communicating hydrocephalus; macroglossia; hoarse voice; conductive and sensorineural hearing loss; hepatosplenomegaly; dysostosis multiplex; spinal stenosis; and carpal tunnel syndrome.

Mucopolysaccharidosis type VI (MPS6) is an autosomal recessive lysosomal storage disorder resulting from a deficiency of arylsulfatase B. Clinical features and severity are variable, but usually include short stature, hepatosplenomegaly, dysostosis multiplex, stiff joints, corneal clouding, cardiac abnormalities, and facial dysmorphism. Intelligence is usually normal (Azevedo et al., 2004).

Sjogren-Larsson syndrome (SLS) is an autosomal recessive, early childhood-onset disorder characterized by ichthyosis, impaired intellectual development, spastic paraparesis, macular dystrophy, and leukoencephalopathy. It is caused by deficiency of fatty aldehyde dehydrogenase (summary by Lossos et al., 2006).

Dystrophic epidermolysis bullosa (DEB) is a genetic skin disorder affecting skin and nails that usually presents at birth. DEB is divided into two major types depending on inheritance pattern: recessive dystrophic epidermolysis bullosa (RDEB) and dominant dystrophic epidermolysis bullosa (DDEB). Each type is further divided into multiple clinical subtypes. Absence of a known family history of DEB does not preclude the diagnosis. Clinical findings in severe generalized RDEB include skin fragility manifest by blistering with minimal trauma that heals with milia and scarring. Blistering and erosions affecting the whole body may be present in the neonatal period. Oral involvement may lead to mouth blistering, fusion of the tongue to the floor of the mouth, and progressive diminution of the size of the oral cavity. Esophageal erosions can lead to webs and strictures that can cause severe dysphagia. Consequently, malnutrition and vitamin and mineral deficiency may lead to growth restriction in young children. Corneal erosions can lead to scarring and loss of vision. Blistering of the hands and feet followed by scarring fuses the digits into "mitten" hands and feet, with contractures and pseudosyndactyly. The lifetime risk of aggressive squamous cell carcinoma is higher than 90%. In contrast, the blistering in the less severe forms of RDEB may be localized to hands, feet, knees, and elbows with or without involvement of flexural areas and the trunk, and without the mutilating scarring seen in severe generalized RDEB. In DDEB, blistering is often mild and limited to hands, feet, knees, and elbows, but nonetheless heals with scarring. Dystrophic nails, especially toenails, are common and may be the only manifestation of DDEB.

Mucopolysaccharidosis type VII (MPS7) is an autosomal recessive lysosomal storage disease characterized by the inability to degrade glucuronic acid-containing glycosaminoglycans. The phenotype is highly variable, ranging from severe lethal hydrops fetalis to mild forms with survival into adulthood. Most patients with the intermediate phenotype show hepatomegaly, skeletal anomalies, coarse facies, and variable degrees of mental impairment (Shipley et al., 1993). MPS VII was the first autosomal mucopolysaccharidosis for which chromosomal assignment was achieved.

Mucopolysaccharidosis type I (MPS I) is a progressive multisystem disorder with features ranging over a continuum of severity. While affected individuals have traditionally been classified as having one of three MPS I syndromes (Hurler syndrome, Hurler-Scheie syndrome, or Scheie syndrome), no easily measurable biochemical differences have been identified and the clinical findings overlap. Affected individuals are best described as having either a phenotype consistent with either severe (Hurler syndrome) or attenuated MPS I, a distinction that influences therapeutic options. Severe MPS I. Infants appear normal at birth. Typical early manifestations are nonspecific (e.g., umbilical or inguinal hernia, frequent upper respiratory tract infections before age 1 year). Coarsening of the facial features may not become apparent until after age one year. Gibbus deformity of the lower spine is common and often noted within the first year. Progressive skeletal dysplasia (dysostosis multiplex) involving all bones is universal, as is progressive arthropathy involving most joints. By age three years, linear growth decreases. Intellectual disability is progressive and profound but may not be readily apparent in the first year of life. Progressive cardiorespiratory involvement, hearing loss, and corneal clouding are common. Without treatment, death (typically from cardiorespiratory failure) usually occurs within the first ten years of life. Attenuated MPS I. Clinical onset is usually between ages three and ten years. The severity and rate of disease progression range from serious life-threatening complications leading to death in the second to third decade, to a normal life span complicated by significant disability from progressive joint manifestations and cardiorespiratory disease. While some individuals have no neurologic involvement and psychomotor development may be normal in early childhood, learning disabilities and psychiatric manifestations can be present later in life. Hearing loss, cardiac valvular disease, respiratory involvement, and corneal clouding are common.

Williams syndrome (WS) is characterized by cardiovascular disease (elastin arteriopathy, peripheral pulmonary stenosis, supravalvar aortic stenosis, hypertension), distinctive facies, connective tissue abnormalities, intellectual disability (usually mild), a specific cognitive profile, unique personality characteristics, growth abnormalities, and endocrine abnormalities (hypercalcemia, hypercalciuria, hypothyroidism, and early puberty). Feeding difficulties often lead to poor weight gain in infancy. Hypotonia and hyperextensible joints can result in delayed attainment of motor milestones.

A very rare genetic disorder with characteristics of the following congenital malformations: hydrocephalus (due to Dandy-Walker anomaly), cleft palate and severe joint contractures. Less than 20 cases have been reported in the literature. The fingers are thin with absent knuckles and reduced creases over the joints and patients show an inability to make a full fist. Additional findings may include deformed ears, ptosis, an inability to open the mouth fully, heart defects, and clubfoot. There are currently no human genes associated with this disease.

An autosomal recessive subtype of cerebrooculofacioskeletal syndrome caused by mutation(s) in the ERCC6 gene, encoding DNA excision repair protein ERCC-6.

Acromesomelic dysplasia-2A (AMD2A), or Grebe chondrodysplasia, is an autosomal recessive disorder characterized by severe abnormality of the limbs and limb joints. The severity of limb shortening progresses in a proximal-distal gradient, with the hands and feet being most affected. The fingers and toes lack articulation and appear as skin appendages. In contrast, axial skeletal structures and the craniofacial skeleton are not affected. Heterozygous individuals are of average stature and have mild skeletal abnormalities (summary by Thomas et al., 1997). Because Grebe syndrome exhibits increasing severity in a proximal-distal gradient, it is classified as a form of acromesomelic dysplasia (Costa et al., 1998). For discussion of the genetic heterogeneity of acromesomelic dysplasia, see AMD1 (602875).

Multiple pterygium syndromes comprise a group of multiple congenital anomaly disorders characterized by webbing (pterygia) of the neck, elbows, and/or knees and joint contractures (arthrogryposis) (Morgan et al., 2006). The multiple pterygium syndromes are phenotypically and genetically heterogeneous but are traditionally divided into prenatally lethal (253290) and nonlethal (Escobar) types.

The NSDHL-related disorders include: CHILD (congenital hemidysplasia with ichthyosiform nevus and limb defects) syndrome, an X-linked condition that is usually male lethal during gestation and thus predominantly affects females; and CK syndrome, an X-linked disorder that affects males. CHILD syndrome is characterized by unilateral distribution of ichthyosiform (yellow scaly) skin lesions and ipsilateral limb defects that range from shortening of the metacarpals and phalanges to absence of the entire limb. Intellect is usually normal. The ichthyosiform skin lesions are usually present at birth or in the first weeks of life; new lesions can develop in later life. Nail changes are also common. The heart, lung, and kidneys can also be involved. CK syndrome (named for the initials of the original proband) is characterized by mild to severe cognitive impairment and behavior problems (aggression, attention deficit hyperactivity disorder, and irritability). All affected males reported have developed seizures in infancy and have cerebral cortical malformations and microcephaly. All have distinctive facial features, a thin habitus, and relatively long, thin fingers and toes. Some have scoliosis and kyphosis. Strabismus is common. Optic atrophy is also reported.

The autosomal dominant TRPV4 disorders (previously considered to be clinically distinct phenotypes before their molecular basis was discovered) are now grouped into neuromuscular disorders and skeletal dysplasias; however, the overlap within each group is considerable. Affected individuals typically have either neuromuscular or skeletal manifestations alone, and in only rare instances an overlap syndrome has been reported. The three autosomal dominant neuromuscular disorders (mildest to most severe) are: Charcot-Marie-Tooth disease type 2C. Scapuloperoneal spinal muscular atrophy. Congenital distal spinal muscular atrophy. The autosomal dominant neuromuscular disorders are characterized by a congenital-onset, static, or later-onset progressive peripheral neuropathy with variable combinations of laryngeal dysfunction (i.e., vocal fold paresis), respiratory dysfunction, and joint contractures. The six autosomal dominant skeletal dysplasias (mildest to most severe) are: Familial digital arthropathy-brachydactyly. Autosomal dominant brachyolmia. Spondylometaphyseal dysplasia, Kozlowski type. Spondyloepiphyseal dysplasia, Maroteaux type. Parastremmatic dysplasia. Metatropic dysplasia. The skeletal dysplasia is characterized by brachydactyly (in all 6); the five that are more severe have short stature that varies from mild to severe with progressive spinal deformity and involvement of the long bones and pelvis. In the mildest of the autosomal dominant TRPV4 disorders life span is normal; in the most severe it is shortened. Bilateral progressive sensorineural hearing loss (SNHL) can occur with both autosomal dominant neuromuscular disorders and skeletal dysplasias.

Pallister-Killian syndrome (PKS) is a dysmorphic condition involving most organ systems, but is also characterized by a tissue-limited mosaicism; most fibroblasts have 47 chromosomes with an extra small metacentric chromosome, whereas the karyotype of lymphocytes is normal. The extra metacentric chromosome is an isochromosome for part of the short arm of chromosome 12: i(12)(p10) (Peltomaki et al., 1987; Warburton et al., 1987).

Buschke-Ollendorff syndrome (BOS) is an autosomal dominant connective tissue disorder manifest by multiple subcutaneous nevi or nodules. They may be either elastin-rich (elastoma) or collagen-rich (dermatofibrosis lenticularis disseminata) on histologic examination. The lesions are usually nontender and firm. Affected individuals also have osteopoikilosis (OPK), literally meaning 'spotted bones,' which are osteosclerotic foci that occur in the epiphyses and metaphyses of long bones, wrist, foot, ankle, pelvis, and scapula. Some individuals have both skin and bone manifestations, whereas others may lack skin or bone manifestations. Some individuals may also have melorheostosis (155950), which is characterized by 'flowing' hyperostosis of the cortex of tubular bones. Most reported cases of BOS and OPK are benign, and the bone lesions are found incidentally, although some patients may have joint pain (reviews by Hellemans et al., 2004 and Zhang et al., 2009).

Xeroderma pigmentosum (XP) is characterized by: Acute sun sensitivity (severe sunburn with blistering, persistent erythema on minimal sun exposure) with marked freckle-like pigmentation of the face before age two years; Sunlight-induced ocular involvement (photophobia, severe keratitis, atrophy of the skin of the lids, ocular surface neoplasms); Greatly increased risk of sunlight-induced cutaneous neoplasms (basal cell carcinoma, squamous cell carcinoma, melanoma) within the first decade of life. Approximately 25% of affected individuals have neurologic manifestations (acquired microcephaly, diminished or absent deep tendon stretch reflexes, progressive sensorineural hearing loss, progressive cognitive impairment, and ataxia). The most common causes of death are skin cancer, neurologic degeneration, and internal cancer. The median age at death in persons with XP with neurodegeneration (29 years) was found to be younger than that in persons with XP without neurodegeneration (37 years).

Most infants with short-chain acyl-CoA dehydrogenase deficiency (SCADD) identified through newborn screening programs have remained well, and asymptomatic relatives who meet diagnostic criteria are reported. Thus, SCADD is now viewed as a biochemical phenotype rather than a disease. A broad range of clinical findings was originally reported in those with confirmed SCADD, including severe dysmorphic facial features, feeding difficulties / failure to thrive, metabolic acidosis, ketotic hypoglycemia, lethargy, developmental delay, seizures, hypotonia, dystonia, and myopathy. However, individuals with no symptoms were also reported. In a large series of affected individuals detected on metabolic evaluation for developmental delay, 20% had failure to thrive, feeding difficulties, and hypotonia; 22% had seizures; and 30% had hypotonia without seizures. In contrast, the majority of infants with SCADD have been detected by expanded newborn screening, and the great majority of these infants remain asymptomatic. As with other fatty acid oxidation deficiencies, characteristic biochemical findings of SCADD may be absent except during times of physiologic stress such as fasting and illness. A diagnosis of SCADD based on clinical findings should not preclude additional testing to look for other causes.

PMM2-CDG, the most common of a group of disorders of abnormal glycosylation of N-linked oligosaccharides, is divided into three clinical stages: infantile multisystem, late-infantile and childhood ataxia–intellectual disability, and adult stable disability. The clinical manifestations and course are highly variable, ranging from infants who die in the first year of life to mildly affected adults. Clinical findings tend to be similar in sibs. In the infantile multisystem presentation, infants show axial hypotonia, hyporeflexia, esotropia, and developmental delay. Feeding problems, vomiting, faltering growth, and developmental delay are frequently seen. Subcutaneous fat may be excessive over the buttocks and suprapubic region. Two distinct clinical courses are observed: (1) a nonfatal neurologic course with faltering growth, strabismus, developmental delay, cerebellar hypoplasia, and hepatopathy in infancy followed by neuropathy and retinitis pigmentosa in the first or second decade; and (2) a more severe neurologic-multivisceral course with approximately 20% mortality in the first year of life. The late-infantile and childhood ataxia–intellectual disability stage, which begins between ages three and ten years, is characterized by hypotonia, ataxia, severely delayed language and motor development, inability to walk, and IQ of 40 to 70; other findings include seizures, stroke-like episodes or transient unilateral loss of function, coagulopathy, retinitis pigmentosa, joint contractures, and skeletal deformities. In the adult stable disability stage, intellectual ability is stable; peripheral neuropathy is variable, progressive retinitis pigmentosa and myopia are seen, thoracic and spinal deformities with osteoporosis worsen, and premature aging is observed; females may lack secondary sexual development and males may exhibit decreased testicular volume. Hypogonadotropic hypogonadism and coagulopathy may occur. The risk for deep venous thrombosis is increased.

Troyer syndrome is characterized by progressive spastic paraparesis, dysarthria, pseudobulbar palsy, distal amyotrophy, short stature, and subtle skeletal abnormalities. Most affected children exhibit delays in walking and speech and difficulty in managing oral secretions, followed by increased lower-limb spasticity and slow deterioration in both gait and speech. Mild cerebellar signs are common. The most severely affected individuals have choreoathetosis. Emotional lability / difficulty in controlling emotions and affective disorders, such as inappropriate euphoria and/or crying, are frequently described. Life expectancy is normal.

Wiedemann-Rautenstrauch syndrome (WDRTS) is a rare autosomal recessive neonatal progeroid disorder characterized by intrauterine growth retardation, failure to thrive, short stature, a progeroid appearance, hypotonia, and variable mental impairment (summary by Toriello, 1990). Average survival in WDRTS is 7 months, although survival into the third decade of life has been reported (Akawi et al., 2013).

EEC syndrome is a genetic developmental disorder characterized by ectrodactyly, ectodermal dysplasia, and orofacial clefts (cleft lip/palate).

A subtype of autosomal recessive limb-girdle muscular dystrophy characterized by a childhood onset of progressive shoulder and pelvic girdle muscle weakness and atrophy frequently associated with calf hypertrophy, diaphragmatic weakness, and/or variable cardiac abnormalities. Mild to moderate elevated serum creatine kinase levels and positive Gowers sign are reported.

Fukuyama congenital muscular dystrophy (FCMD) is characterized by hypotonia, symmetric generalized muscle weakness, and CNS migration disturbances that result in changes consistent with cobblestone lissencephaly with cerebral and cerebellar cortical dysplasia. Mild, typical, and severe phenotypes are recognized. Onset typically occurs in early infancy with poor suck, weak cry, and floppiness. Affected individuals have contractures of the hips, knees, and interphalangeal joints. Later features include myopathic facial appearance, pseudohypertrophy of the calves and forearms, motor and speech delays, intellectual disability, seizures, ophthalmologic abnormalities including visual impairment and retinal dysplasia, and progressive cardiac involvement after age ten years. Swallowing disturbance occurs in individuals with severe FCMD and in individuals older than age ten years, leading to recurrent aspiration pneumonia and death.

Collagen VI-related dystrophies (COL6-RDs) represent a continuum of overlapping clinical phenotypes with Bethlem muscular dystrophy at the milder end, Ullrich congenital muscular dystrophy (UCMD) at the more severe end, and a phenotype in between UCMD and Bethlem muscular dystrophy, referred to as intermediate COL6-RD. Bethlem muscular dystrophy is characterized by a combination of proximal muscle weakness and joint contractures. Hypotonia and delayed motor milestones occur in early childhood; mild hypotonia and weakness may be present congenitally. By adulthood, there is evidence of proximal weakness and contractures of the elbows, Achilles tendons, and long finger flexors. The progression of weakness is slow, and more than two thirds of affected individuals older than age 50 years remain independently ambulatory indoors, while relying on supportive means for mobility outdoors. Respiratory involvement is not a consistent feature. UCMD is characterized by congenital weakness, hypotonia, proximal joint contractures, and striking hyperlaxity of distal joints. Decreased fetal movements are frequently reported. Some affected children acquire the ability to walk independently; however, progression of the disease results in a loss of ambulation by age ten to eleven years. Early and severe respiratory insufficiency occurs in all individuals, resulting in the need for nocturnal noninvasive ventilation (NIV) in the form of bilevel positive airway pressure (BiPAP) by age 11 years. Intermediate COL6-RD is characterized by independent ambulation past age 11 years and respiratory insufficiency that is later in onset than in UCMD and results in the need for NIV in the form of BiPAP by the late teens to early 20s. In contrast to individuals with Bethlem muscular dystrophy, those with intermediate COL6-RD typically do not achieve the ability to run, jump, or climb stairs without use of a railing.

Rigid spine muscular dystrophy (RSMD) is a form of congenital muscular dystrophy. Disorders in this group cause muscle weakness and wasting (atrophy) beginning very early in life. In particular, RSMD involves weakness of the muscles of the torso and neck (axial muscles). Other characteristic features include spine stiffness and serious breathing problems.\n\nIn RSMD, muscle weakness is often apparent at birth or within the first few months of life. Affected infants can have poor head control and weak muscle tone (hypotonia), which may delay the development of motor skills such as crawling or walking. Over time, muscles surrounding the spine atrophy, and the joints of the spine develop deformities called contractures that restrict movement. The neck and back become stiff and rigid, and affected children have limited ability to move their heads up and down or side to side. Affected children eventually develop an abnormal curvature of the spine (scoliosis). In some people with RSMD, muscles in the inner thighs also atrophy, although it does not impair the ability to walk.\n\nA characteristic feature of RSMD is breathing difficulty (respiratory insufficiency) due to restricted movement of the torso and weakness of the diaphragm, which is the muscle that separates the abdomen from the chest cavity. The breathing problems, which tend to occur only at night, can be life-threatening. Many affected individuals require a machine to help them breathe (mechanical ventilation) during sleep.\n\nThe combination of features characteristic of RSMD, particularly axial muscle weakness, spine rigidity, and respiratory insufficiency, is sometimes referred to as rigid spine syndrome. While these features occur on their own in RSMD, they can also occur along with additional signs and symptoms in other muscle disorders. The features of rigid spine syndrome typically appear at a younger age in people with RSMD than in those with other muscle disorders.

X-linked myotubular myopathy (X-MTM), also known as myotubular myopathy (MTM), is characterized by muscle weakness that ranges from severe to mild. Approximately 80% of affected males present with severe (classic) X-MTM characterized by polyhydramnios, decreased fetal movement, and neonatal weakness, hypotonia, and respiratory failure. Motor milestones are significantly delayed and most individuals fail to achieve independent ambulation. Weakness is profound and often involves facial and extraocular muscles. Respiratory failure is nearly uniform, with most individuals requiring 24-hour ventilatory assistance. It is estimated that at least 25% of boys with severe X-MTM die in the first year of life, and those who survive rarely live into adulthood. Males with mild or moderate X-MTM (~20%) achieve motor milestones more quickly than males with the severe form; many ambulate independently, and may live into adulthood. Most require gastrostomy tubes and/or ventilator support. In all subtypes of X-MTM, the muscle disease is not obviously progressive. Female carriers of X-MTM are generally asymptomatic, although manifesting heterozygotes are increasingly being identified. In affected females, symptoms range from severe, generalized weakness presenting in childhood, with infantile onset similar to affected male patients, to mild (often asymmetric) weakness manifesting in adulthood. Affected adult females may experience progressive respiratory decline and ultimately require ventilatory support.

Any centronuclear myopathy in which the cause of the disease is a mutation in the BIN1 gene.

Jacobsen syndrome (JBS) is a contiguous gene deletion syndrome with major clinical features of growth retardation, psychomotor retardation, trigonocephaly, divergent intermittent strabismus, epicanthus, telecanthus, broad nasal bridge, short nose with anteverted nostrils, carp-shaped upper lip, retrognathia, low-set dysmorphic ears, bilateral camptodactyly, hammertoes, and isoimmune thrombocytopenia (Fryns et al., 1986, Epstein, 1986).

Allan-Herndon-Dudley syndrome (AHDS), an X-linked disorder, is characterized in males by neurologic findings (hypotonia and feeding difficulties in infancy, developmental delay / intellectual disability ranging from mild to profound) and later-onset pyramidal signs, extrapyramidal findings (dystonia, choreoathetosis, paroxysmal movement disorder, hypokinesia, masked facies), and seizures, often with drug resistance. Additional findings can include dysthyroidism (manifest as poor weight gain, reduced muscle mass, and variable cold intolerance, sweating, elevated heart rate, and irritability) and pathognomonic thyroid test results. Most heterozygous females are not clinically affected but may have minor thyroid test abnormalities.

A form of ectodermal dysplasia syndrome characterized by a short stature of prenatal onset, alopecia, ichthyosis, photophobia, ectrodactyly, seizures, scoliosis, multiple contractures, fusions of various bones (particularly elbows, carpals, metacarpals, and spine), intellectual disability, and facial dysmorphism (microdolichocephaly, madarosis, large ears and long nose). ACD syndrome overlaps with ichthyosis follicularis-alopecia-photophobia syndrome.

Hereditary motor and sensory neuropathy with agenesis of the corpus callosum (HMSN/ACC), a neurodevelopmental and neurodegenerative disorder, is characterized by severe progressive sensorimotor neuropathy with resulting hypotonia, areflexia, and amyotrophy, and by variable degrees of dysgenesis of the corpus callosum. Mild-to-severe intellectual disability and "psychotic episodes" during adolescence are observed. Sensory modalities are moderately to severely affected beginning in infancy. The average age of onset of walking is 3.8 years; the average age of loss of walking is 13.8 years; the average age of death is 33 years.

MED12-related disorders include the phenotypes of FG syndrome type 1 (FGS1), Lujan syndrome (LS), X-linked Ohdo syndrome (XLOS), Hardikar syndrome (HS), and nonspecific intellectual disability (NSID). FGS1 and LS share the clinical findings of cognitive impairment, hypotonia, and abnormalities of the corpus callosum. FGS1 is further characterized by absolute or relative macrocephaly, tall forehead, downslanted palpebral fissures, small and simple ears, constipation and/or anal anomalies, broad thumbs and halluces, and characteristic behavior. LS is further characterized by large head, tall thin body habitus, long thin face, prominent nasal bridge, high narrow palate, and short philtrum. Carrier females in families with FGS1 and LS are typically unaffected. XLOS is characterized by intellectual disability, blepharophimosis, and facial coarsening. HS has been described in females with cleft lip and/or cleft palate, biliary and liver anomalies, intestinal malrotation, pigmentary retinopathy, and coarctation of the aorta. Developmental and cognitive concerns have not been reported in females with HS. Pathogenic variants in MED12 have been reported in an increasing number of males and females with NSID, with affected individuals often having clinical features identified in other MED12-related disorders.

Primrose syndrome is characterized by macrocephaly, hypotonia, developmental delay, intellectual disability with expressive speech delay, behavioral issues, a recognizable facial phenotype, radiographic features, and altered glucose metabolism. Additional features seen in adults: sparse body hair, distal muscle wasting, and contractures. Characteristic craniofacial features include brachycephaly, high anterior hairline, deeply set eyes, ptosis, downslanted palpebral fissures, high palate with torus palatinus, broad jaw, and large ears with small or absent lobes. Radiographic features include calcification of the external ear cartilage, multiple Wormian bones, platybasia, bathrocephaly, slender bones with exaggerated metaphyseal flaring, mild epiphyseal dysplasia, and spondylar dysplasia. Additional features include hearing impairment, ocular anomalies, cryptorchidism, and nonspecific findings on brain MRI.

The spectrum of MECP2-related phenotypes in females ranges from classic Rett syndrome to variant Rett syndrome with a broader clinical phenotype (either milder or more severe than classic Rett syndrome) to mild learning disabilities; the spectrum in males ranges from severe neonatal encephalopathy to pyramidal signs, parkinsonism, and macroorchidism (PPM-X) syndrome to severe syndromic/nonsyndromic intellectual disability. Females: Classic Rett syndrome, a progressive neurodevelopmental disorder primarily affecting girls, is characterized by apparently normal psychomotor development during the first six to 18 months of life, followed by a short period of developmental stagnation, then rapid regression in language and motor skills, followed by long-term stability. During the phase of rapid regression, repetitive, stereotypic hand movements replace purposeful hand use. Additional findings include fits of screaming and inconsolable crying, autistic features, panic-like attacks, bruxism, episodic apnea and/or hyperpnea, gait ataxia and apraxia, tremors, seizures, and acquired microcephaly. Males: Severe neonatal-onset encephalopathy, the most common phenotype in affected males, is characterized by a relentless clinical course that follows a metabolic-degenerative type of pattern, abnormal tone, involuntary movements, severe seizures, and breathing abnormalities. Death often occurs before age two years.

Bohring-Opitz syndrome (BOS) is characterized by distinctive facial features and posture, growth failure, variable but usually severe intellectual disability, and variable anomalies. The facial features may include microcephaly or trigonocephaly / prominent (but not fused) metopic ridge, hypotonic facies with full cheeks, synophrys, glabellar and eyelid nevus flammeus (simplex), prominent globes, widely set eyes, palate anomalies, and micrognathia. The BOS posture, which is most striking in early childhood and often becomes less apparent with age, is characterized by flexion at the elbows with ulnar deviation and flexion of the wrists and metacarpophalangeal joints. Feeding difficulties in early childhood, including cyclic vomiting, have a significant impact on overall health; feeding tends to improve with age. Seizures are common and typically responsive to standard epileptic medications. Minor cardiac anomalies and transient bradycardia and apnea may be present. Affected individuals may experience recurrent infections, which also tend to improve with age. Isolated case reports suggest that individuals with BOS are at greater risk for Wilms tumor than the general population, but large-scale epidemiologic studies have not been conducted.

Partington syndrome (PRTS) is an X-linked developmental disorder characterized by impaired intellectual development and variable movement disturbances. Partington syndrome is part of a phenotypic spectrum of disorders caused by mutation in the ARX gene comprising a nearly continuous series of developmental disorders ranging from hydranencephaly and lissencephaly (LISX2; 300215) to Proud syndrome (300004) to infantile spasms without brain malformations (see 308350) to nonsyndromic intellectual disability (300419). Although males with ARX mutations are often more severely affected, female mutation carriers may also be affected (Kato et al., 2004; Wallerstein et al., 2008).

X-linked Dandy-Walker malformation with intellectual disability, basal ganglia disease and seizures (XDIBS), or Pettigrew syndrome is a central nervous system malformation characterized by severe intellectual deficit, early hypotonia with progression to spasticity and contractures, choreoathetosis, seizures, dysmorphic face (long face with prominent forehead), and brain imaging abnormalities such as Dandy-Walker malformation, and iron deposition. (From Mondo:0010574)

Merosin-deficient congenital muscular dystrophy is an autosomal recessive form of muscular dystrophy characterized by muscle weakness apparent at birth or in the first 6 months of life. Patients show hypotonia, poor suck and cry, and delayed motor development; most never achieve independent ambulation. Most patients also have periventricular white matter abnormalities on brain imaging, but mental retardation and/or seizures occur only rarely (summary by Xiong et al., 2015).

Congenital disorders of glycosylation (CDGs) are a genetically heterogeneous group of autosomal recessive disorders caused by enzymatic defects in the synthesis and processing of asparagine (N)-linked glycans or oligosaccharides on glycoproteins. Type I CDGs comprise defects in the assembly of the dolichol lipid-linked oligosaccharide (LLO) chain and its transfer to the nascent protein. These disorders can be identified by a characteristic abnormal isoelectric focusing profile of plasma transferrin (Leroy, 2006). CDG1D is a type I CDG that generally presents with severe neurologic involvement associated with dysmorphism and visual impairment. Liver involvement is sometimes present (summary by Marques-da-Silva et al., 2017). For a discussion of the classification of CDGs, see CDG1A (212065).

Brody disease (BROD) is an autosomal recessive skeletal muscle disorder characterized by exercise-induced muscle stiffness and cramps primarily affecting the arms, legs, and eyelids, although more generalized muscle involvement may also occur. Symptom onset is most often in the first decade, but many patients present and are diagnosed later in life. Skeletal muscle biopsy typically shows variation in fiber size, increased internal nuclei, and atrophy of type II muscle fibers. Rare patients have been reported to develop malignant hyperthermia after administration of anesthesia, suggesting that patients with the disorder should be tested. The disorder results from defective relaxation of fast-twitch (type II) skeletal muscle fibers due to defects in calcium homeostasis and reuptake in the muscle fiber (summary by Odermatt et al., 2000 and Molenaar et al., 2020).

Spastic paraplegia, optic atrophy, and neuropathy (SPOAN) is an autosomal recessive neurodegenerative disorder characterized by early-onset progressive spastic paraplegia resulting in loss of independent ambulation in the teenage years. Additional features include optic atrophy, later onset of sensorimotor peripheral neuropathy, and progressive joint contractures; cognition remains intact (summary by Melo et al., 2015).

Limb-girdle muscular dystrophies resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239) represent the mildest end of the phenotypic spectrum of muscular dystrophies collectively known as dystroglycanopathies. The limb-girdle phenotype is characterized by onset of muscular weakness apparent after ambulation is achieved; mental retardation and mild brain anomalies are variable (Balci et al., 2005; review by Godfrey et al., 2007). The most severe end of the phenotypic spectrum of dystroglycanopathies is represented by congenital muscular dystrophy-dystroglycanopathy with brain and eye anomalies (type A; see MDDGA1, 236670), previously designated Walker-Warburg syndrome (WWS) or muscle-eye-brain disease (MEB), and the intermediate range of the spectrum is represented by congenital muscular dystrophy-dystroglycanopathy with or without mental retardation (type B; see MDDGB1, 613155). Genetic Heterogeneity of Limb-Girdle Muscular Dystrophy-Dystroglycanopathy (Type C) Limb-girdle muscular dystrophy due to defective glycosylation of DAG1 is genetically heterogeneous. See also MDDGC2 (613158), caused by mutation in the POMT2 gene (607439); MDDGC3 (613157), caused by mutation in the POMGNT1 gene (606822); MDDGC4 (611588), caused by mutation in the FKTN gene (607440); MDDGC5 (607155), caused by mutation in the FKRP gene (606596); MDDGC7 (616052), caused by mutation in the ISPD gene (CRPPA; 614631); MDDGC8 (618135), caused by mutation in the POMGNT2 gene (614828); MDDGC9 (613818) caused by mutation in the DAG1 gene (128239); MDDGC12 (616094), caused by mutation in the POMK gene (615247); MDDGC14 (615352) caused by mutation in the GMPPB gene (615320); and MDDGC15 (612937), caused by mutation in the DPM3 gene (605951).

Bruck syndrome-2 (BRKS2) is an autosomal recessive disorder characterized by osteoporosis, joint contractures at birth, fragile bones, and short stature (Van der Slot et al., 2003). For a discussion of genetic heterogeneity of Bruck syndrome, see Bruck syndrome-1 (BRKS1; 259450).

Congenital disorders of glycosylation (CDGs) are metabolic deficiencies in glycoprotein biosynthesis that usually cause severe mental and psychomotor retardation. Different forms of CDGs can be recognized by altered isoelectric focusing (IEF) patterns of serum transferrin. For a general discussion of CDGs, see CDG Ia (212065) and CDG Ib (602579).

Czech dysplasia is an autosomal dominant skeletal dysplasia characterized by early-onset, progressive pseudorheumatoid arthritis, platyspondyly, and short third and fourth toes (Marik et al., 2004; Kozlowski et al., 2004).

Permanent neonatal diabetes mellitus-pancreatic and cerebellar agenesis syndrome is characterized by neonatal diabetes mellitus associated with cerebellar and/or pancreatic agenesis.

CHST3-related skeletal dysplasia is characterized by short stature of prenatal onset, joint dislocations (knees, hips, radial heads), clubfeet, and limitation of range of motion that can involve all large joints. Kyphosis and occasionally scoliosis with slight shortening of the trunk develop in childhood. Minor heart valve dysplasia has been described in several persons. Intellect and vision are normal.

Mandibuloacral dysplasia with type B lipodystrophy (MADB) is a rare autosomal recessive disorder characterized by postnatal growth retardation, craniofacial anomalies such as mandibular hypoplasia, skeletal anomalies such as progressive osteolysis of the terminal phalanges and clavicles, and skin changes such as mottled hyperpigmentation and atrophy. The lipodystrophy is characterized by generalized loss of subcutaneous fat involving the face, trunk, and extremities. Some patients have a progeroid appearance. Metabolic complications associated with insulin resistance have been reported (Schrander-Stumpel et al., 1992; summary by Simha et al., 2003). For a general phenotypic description of lipodystrophy associated with mandibuloacral dysplasia, see MADA (248370).

PLP1 disorders of central nervous system myelin formation include a range of phenotypes from Pelizaeus-Merzbacher disease (PMD) to spastic paraplegia 2 (SPG2). PMD typically manifests in infancy or early childhood with nystagmus, hypotonia, and cognitive impairment; the findings progress to severe spasticity and ataxia. Life span is shortened. SPG2 manifests as spastic paraparesis with or without CNS involvement and usually normal life span. Intrafamilial variation of phenotypes can be observed, but the signs are usually fairly consistent within families. Heterozygous females may manifest mild-to-moderate signs of the disease.

X-linked lethal multiple pterygium syndrome is a rare, genetic, developmental defect during embryogenesis characterized by the typical lethal multiple pterygium syndrome presentation (comprising of multiple pterygia, severe arthrogryposis, cleft palate, cystic hygromata and/or fetal hydrops, skeletal abnormalities and fetal death in the 2nd or 3rd trimester) with an X-linked pattern of inheritance.

X-linked myopathy with excessive autophagy (XMEA) is an X-linked recessive skeletal muscle disorder characterized by childhood onset of progressive muscle weakness and atrophy primarily affecting the proximal muscles. While onset is usually in childhood, it can range from infancy to adulthood. Many patients lose ambulation and become wheelchair-bound. Other organ systems, including the heart, are clinically unaffected. Muscle biopsy shows intracytoplasmic autophagic vacuoles with sarcolemmal features and a multilayered basal membrane (summary by Ramachandran et al., 2013; Kurashige et al., 2013, and Ruggieri et al., 2015). Danon disease (300257), caused by mutation in the LAMP2 gene (309060) on chromosome Xq24, is a distinct disorder with similar pathologic features.

A form of axonal Charcot-Marie-Tooth disease a peripheral sensorimotor neuropathy. Onset is in the first to sixth decade with a gait anomaly and a leg weakness that reaches the arms secondarily. Tendon reflexes are reduced or absent and after years all patients have a pes cavus. Other signs may be present including hearing loss and postural tremor.

X-linked infantile spinal muscular atrophy (XL-SMA) is characterized by congenital hypotonia, areflexia, and evidence of degeneration and loss of anterior horn cells (i.e., lower motor neurons) in the spinal cord and brain stem. Often congenital contractures and/or fractures are present. Intellect is normal. Life span is significantly shortened because of progressive ventilatory insufficiency resulting from chest muscle involvement.

Claes-Jensen type of X-linked syndromic intellectual developmental disorder (MRXSCJ) is characterized by impaired intellectual development with substantial clinical heterogeneity in affected males. However, males are usually reported to have short stature, microcephaly, hyperreflexia, and aggressive behavior. In rare cases, female carriers exhibit mildly impaired intellectual development or learning difficulties (summary by Guerra et al., 2020).

X-linked spondyloepimetaphyseal dysplasia with hypomyelinating leukodystrophy (SEMDHL) is an X-linked recessive developmental disorder characterized by slowly progressive skeletal and neurologic abnormalities, including short stature, large and deformed joints, significant motor impairment, visual defects, and sometimes cognitive deficits. Affected individuals typically have normal early development in the first year or so of life, followed by development regression and the development of symptoms. Brain imaging shows white matter abnormalities consistent with hypomyelinating leukodystrophy (summary by Miyake et al., 2017).

Oculofaciocardiodental (OFCD) syndrome is a condition that affects the development of the eyes (oculo-), facial features (facio-), heart (cardio-) and teeth (dental). This condition occurs only in females.\n\nThe eye abnormalities associated with OFCD syndrome can affect one or both eyes. Many people with this condition are born with eyeballs that are abnormally small (microphthalmia). Other eye problems can include clouding of the lens (cataract) and a higher risk of glaucoma, an eye disease that increases the pressure in the eye. These abnormalities can lead to vision loss or blindness.\n\nPeople with OFCD syndrome often have a long, narrow face with distinctive facial features, including deep-set eyes and a broad nasal tip that is divided by a cleft. Some affected people have an opening in the roof of the mouth called a cleft palate.\n\nHeart defects are another common feature of OFCD syndrome. Babies with this condition may be born with a hole between two chambers of the heart (an atrial or ventricular septal defect) or a leak in one of the valves that controls blood flow through the heart (mitral valve prolapse).\n\nTeeth with very large roots (radiculomegaly) are characteristic of OFCD syndrome. Additional dental abnormalities can include delayed loss of primary (baby) teeth, missing or abnormally small teeth, misaligned teeth, and defective tooth enamel.

Amish lethal microcephaly is characterized by severe congenital microcephaly and highly elevated 2-ketoglutarate or lactic acidosis. The occipitofrontal circumference is typically more than two standard deviations (occasionally >6 SD) below the mean; anterior and posterior fontanels are closed at birth and facial features are distorted. The average life span of an affected infant is between five and six months among the Lancaster Amish, although an affected Amish-Mennonite child was reported to be living with severe developmental delay at age seven years.

Bartsocas-Papas syndrome-1 (BPS1) is an autosomal recessive disorder characterized by multiple popliteal pterygia, ankyloblepharon, filiform bands between the jaws, cleft lip and palate, and syndactyly. Early lethality is common, although survival into childhood and beyond has been reported (summary by Mitchell et al., 2012). Genetic Heterogeneity of Bartsocas-Papas Syndrome Bartsocas-Papas syndrome-2 (BPS2) is caused by mutation in the CHUK gene (600664). A less severe form of popliteal pterygium syndrome (PPS; 119500) is caused by mutation in the IRF6 gene (607199).

Gillessen-Kaesbach-Nishimura syndrome is an autosomal recessive multiple congenital anomaly disorder characterized by skeletal dysplasia, dysmorphic facial features, and variable visceral abnormalities, including polycystic kidneys, diaphragmatic hernia, lung hypoplasia, and congenital heart defects. It may be lethal in utero or early in life. The disorder is at the severe end of the phenotypic spectrum of congenital disorders of glycosylation (summary by Tham et al., 2016).

The neuronal ceroid lipofuscinoses (NCL; CLN) are a clinically and genetically heterogeneous group of neurodegenerative disorders characterized by the intracellular accumulation of autofluorescent lipopigment storage material in different patterns ultrastructurally. The lipopigment pattern seen most often in CLN1 is referred to as granular osmiophilic deposits (GROD). The patterns most often observed in CLN2 and CLN3 are 'curvilinear' and 'fingerprint' profiles, respectively. CLN4, CLN5, CLN6, CLN7, and CLN8 show mixed combinations of granular, curvilinear, fingerprint, and rectilinear profiles. The clinical course includes progressive dementia, seizures, and progressive visual failure (Mole et al., 2005). Zeman and Dyken (1969) referred to these conditions as the 'neuronal ceroid lipofuscinoses.' Goebel (1995) provided a comprehensive review of the NCLs and noted that they are possibly the most common group of neurodegenerative diseases in children. Mole et al. (2005) provided a detailed clinical and genetic review of the neuronal ceroid lipofuscinoses. Genetic Heterogeneity of Neuronal Ceroid Lipofuscinosis See also CLN2 (204500), caused by mutation in the TPP1 gene (607998) on chromosome 11p15; CLN3 (204200), caused by mutation in the CLN3 gene (607042) on 16p12; CLN4 (162350), caused by mutation in the DNAJC5 gene (611203) on 20q13; CLN5 (256731), caused by mutation in the CLN5 gene (608102) on 13q22; CLN6A (601780) and CLN6B (204300), both caused by mutation in the CLN6 gene (606725) on 15q21; CLN7 (610951), caused by mutation in the MFSD8 gene (611124) on 4q28; CLN8 (600143) and the Northern epilepsy variant of CLN8 (610003), both caused by mutation in the CLN8 gene (607837) on 8p23; CLN10 (610127), caused by mutation in the CTSD gene (116840) on 11p15; CLN11 (614706), caused by mutation in the GRN gene (138945) on 17q21; CLN13 (615362), caused by mutation in the CTSF gene (603539) on 11q13; and CLN14 (611726), caused by mutation in the KCTD7 gene (611725) on 7q11. CLN9 (609055) has not been molecularly characterized. A disorder that was formerly designated neuronal ceroid lipofuscinosis-12 (CLN12) is now considered to be a variable form of Kufor-Rakeb syndrome (KRS; 606693).

Nemaline myopathy-2 (NEM2) is an autosomal recessive skeletal muscle disorder with a wide range of severity. The most common clinical presentation is early-onset (in infancy or childhood) muscle weakness predominantly affecting proximal limb muscles. Muscle biopsy shows accumulation of Z-disc and thin filament proteins into aggregates named 'nemaline bodies' or 'nemaline rods,' usually accompanied by disorganization of the muscle Z discs. The clinical and histologic spectrum of entities caused by variants in the NEB gene is a continuum, ranging in severity. The distribution of weakness can vary from generalized muscle weakness, more pronounced in proximal limb muscles, to distal-only involvement, although neck flexor weakness appears to be rather consistent. Histologic patterns range from a severe usually nondystrophic disturbance of the myofibrillar pattern to an almost normal pattern, with or without nemaline bodies, sometimes combined with cores (summary by Lehtokari et al., 2014). Genetic Heterogeneity of Nemaline Myopathy See also NEM1 (255310), caused by mutation in the tropomyosin-3 gene (TPM3; 191030) on chromosome 1q22; NEM3 (161800), caused by mutation in the alpha-actin-1 gene (ACTA1; 102610) on chromosome 1q42; NEM4 (609285), caused by mutation in the beta-tropomyosin gene (TPM2; 190990) on chromosome 9p13; NEM5A (605355), also known as Amish nemaline myopathy, NEM5B (620386), and NEM5C (620389), all caused by mutation in the troponin T1 gene (TNNT1; 191041) on chromosome 19q13; NEM6 (609273), caused by mutation in the KBTBD13 gene (613727) on chromosome 15q22; NEM7 (610687), caused by mutation in the cofilin-2 gene (CFL2; 601443) on chromosome 14q13; NEM8 (615348), caused by mutation in the KLHL40 gene (615340), on chromosome 3p22; NEM9 (615731), caused by mutation in the KLHL41 gene (607701) on chromosome 2q31; NEM10 (616165), caused by mutation in the LMOD3 gene (616112) on chromosome 3p14; and NEM11 (617336), caused by mutation in the MYPN gene (608517) on chromosome 10q21. Several of the genes encode components of skeletal muscle sarcomeric thin filaments (Sanoudou and Beggs, 2001). Mutations in the NEB gene are the most common cause of nemaline myopathy (Lehtokari et al., 2006).

STAC3 disorder is characterized by congenital myopathy, musculoskeletal involvement of the trunk and extremities, feeding difficulties, and delayed motor milestones. Most affected individuals have weakness with myopathic facies, scoliosis, kyphosis or kyphoscoliosis, and contractures. Other common findings are ptosis, abnormalities of the palate (including cleft palate), and short stature. Risk for malignant hyperthermia susceptibility and restrictive lung disease are increased. Intellect is typically normal. Originally described in individuals from the Lumbee Native American tribe (an admixture of Cheraw Indian, English, and African American ancestry) in the state of North Carolina and reported as Native American myopathy, STAC3 disorder has now been identified in numerous other populations worldwide.

Lethal multiple pterygium syndrome has many of the same signs and symptoms as the Escobar type. In addition, affected fetuses may develop a buildup of excess fluid in the body (hydrops fetalis) or a fluid-filled sac typically found on the back of the neck (cystic hygroma). Individuals with this type have severe arthrogryposis. Lethal multiple pterygium syndrome is associated with abnormalities such as underdevelopment (hypoplasia) of the heart, lung, or brain; twisting of the intestines (intestinal malrotation); kidney abnormalities; an opening in the roof of the mouth (a cleft palate); and an unusually small head size (microcephaly). Affected individuals may also develop a hole in the muscle that separates the abdomen from the chest cavity (the diaphragm), a condition called a congenital diaphragmatic hernia. Lethal multiple pterygium syndrome is typically fatal in the second or third trimester of pregnancy.\n\nIn people with multiple pterygium syndrome, Escobar type, the webbing typically affects the skin of the neck, fingers, forearms, inner thighs, and backs of the knee. People with this type may also have arthrogryposis. A side-to-side curvature of the spine (scoliosis) is sometimes seen. Affected individuals may also have respiratory distress at birth due to underdeveloped lungs (lung hypoplasia). People with multiple pterygium syndrome, Escobar type usually have distinctive facial features including droopy eyelids (ptosis), outside corners of the eyes that point downward (downslanting palpebral fissures), skin folds covering the inner corner of the eyes (epicanthal folds), a small jaw, and low-set ears. Males with this condition can have undescended testes (cryptorchidism). This condition does not worsen after birth, and affected individuals typically do not have muscle weakness later in life.\n\nThe two forms of multiple pterygium syndrome are differentiated by the severity of their symptoms. Multiple pterygium syndrome, Escobar type (sometimes referred to as Escobar syndrome) is the milder of the two types. Lethal multiple pterygium syndrome is fatal before birth or very soon after birth.\n\nMultiple pterygium syndrome is a condition that is evident before birth with webbing of the skin (pterygium) at the joints and a lack of muscle movement (akinesia) before birth. Akinesia frequently results in muscle weakness and joint deformities called contractures that restrict the movement of joints (arthrogryposis). As a result, multiple pterygium syndrome can lead to further problems with movement such as arms and legs that cannot fully extend.

Say-Barber-Miller syndrome is characterised by the association of unusual facial features, microcephaly, developmental delay, and severe postnatal growth retardation.

Rhizomelic chondrodysplasia punctata (RCDP) is a peroxisomal disorder characterized by disproportionately short stature primarily affecting the proximal parts of the extremities, a typical facial appearance including a broad nasal bridge, epicanthus, high-arched palate, dysplastic external ears, and micrognathia, congenital contractures, characteristic ocular involvement, dwarfism, and severe mental retardation with spasticity. Biochemically, plasmalogen synthesis and phytanic acid alpha-oxidation are defective. Most patients die in the first decade of life. RCDP1 (215100) is the most frequent form of RCDP (summary by Wanders and Waterham, 2005). Whereas RCDP1 is a peroxisomal biogenesis disorder (PBD), RCDP2 is classified as a single peroxisome enzyme deficiency (Waterham and Ebberink, 2012). For a discussion of genetic heterogeneity of rhizomelic chondrodysplasia punctata, see 215100.

A rare Huntington disease-like syndrome with characteristics of childhood-onset progressive neurologic deterioration with pyramidal and extrapyramidal abnormalities, chorea, dystonia, ataxia, gait instability, spasticity, seizures, mutism, and (on brain MRI) progressive frontal cortical atrophy and bilateral caudate atrophy.

Rhizomelic chondrodysplasia punctata type 1 (RCDP1), a peroxisome biogenesis disorder (PBD) has a classic (severe) form and a nonclassic (mild) form. Classic (severe) RCDP1 is characterized by proximal shortening of the humerus (rhizomelia) and to a lesser degree the femur, punctate calcifications in cartilage with epiphyseal and metaphyseal abnormalities (chondrodysplasia punctata, or CDP), coronal clefts of the vertebral bodies, and cataracts that are usually present at birth or appear in the first few months of life. Birth weight, length, and head circumference are often at the lower range of normal; postnatal growth deficiency is profound. Intellectual disability is severe, and the majority of children develop seizures. Most affected children do not survive the first decade of life; a proportion die in the neonatal period. Nonclassic (mild) RCDP1 is characterized by congenital or childhood cataracts, CDP or infrequently, chondrodysplasia manifesting only as mild epiphyseal changes, variable rhizomelia, and milder intellectual disability and growth restriction than classic RCDP1.

Microcephalic osteodysplastic primordial dwarfism type I (MOPD1) is a severe autosomal recessive skeletal dysplasia characterized by dwarfism, microcephaly, and neurologic abnormalities, including mental retardation, brain malformations, and ocular/auditory sensory deficits. Patients often die in early childhood (summary by Pierce and Morse, 2012).

Arterial tortuosity syndrome (ATS) is characterized by widespread elongation and tortuosity of the aorta and mid-sized arteries as well as focal stenosis of segments of the pulmonary arteries and/or aorta combined with findings of a generalized connective tissue disorder, which may include soft or doughy hyperextensible skin, joint hypermobility, inguinal hernia, and diaphragmatic hernia. Skeletal findings include pectus excavatum or carinatum, arachnodactyly, scoliosis, knee/elbow contractures, and camptodactyly. The cardiovascular system is the major source of morbidity and mortality with increased risk at any age for aneurysm formation and dissection both at the aortic root and throughout the arterial tree, and for ischemic vascular events involving cerebrovascular circulation (resulting in non-hemorrhagic stroke) and the abdominal arteries (resulting in infarctions of abdominal organs).

Congenital glutamine deficiency (GLND) is a severe autosomal recessive disorder characterized by onset at birth of encephalopathy, lack of normal development, seizures, and hypotonia associated with variable brain abnormalities (summary by Haberle et al., 2011).

Majeed syndrome (MJDS) is an autosomal recessive pediatric multisystem autoinflammatory disorder characterized by chronic recurrent multifocal osteomyelitis (CRMO) and congenital dyserythropoietic anemia; some patients may also develop neutrophilic dermatosis. Additional features may include fever, failure to thrive, and neutropenia. Laboratory studies show elevated inflammatory markers consistent with activation of the proinflammatory IL1 (147760) pathway (summary by Ferguson and El-Shanti, 2021). Genetic Heterogeneity of Chronic Recurrent Multifocal Osteomyelitis See also CRMO2 (612852), caused by mutation in the IL1RN gene (147679) on chromosome 2q14; and CRMO3 (259680), caused by mutation in the IL1R1 gene (147810) on chromosome 2q12.

About half of all people with trichothiodystrophy have a photosensitive form of the disorder, which causes them to be extremely sensitive to ultraviolet (UV) rays from sunlight. They develop a severe sunburn after spending just a few minutes in the sun. However, for reasons that are unclear, they do not develop other sun-related problems such as excessive freckling of the skin or an increased risk of skin cancer. Many people with trichothiodystrophy report that they do not sweat.\n\nIntellectual disability and delayed development are common in people with trichothiodystrophy, although most affected individuals are highly social with an outgoing and engaging personality. Some people with trichothiodystrophy have brain abnormalities that can be seen with imaging tests. A common neurological feature of this disorder is impaired myelin production (dysmyelination). Myelin is a fatty substance that insulates nerve cells and promotes the rapid transmission of nerve impulses.\n\nTrichothiodystrophy is also associated with recurrent infections, particularly respiratory infections, which can be life-threatening. People with trichothiodystrophy may have abnormal red blood cells, including red blood cells that are smaller than normal. They may also have elevated levels of a type of hemoglobin called A2, which is a protein found in red blood cells. Other features of trichothiodystrophy can include dry, scaly skin (ichthyosis); abnormalities of the fingernails and toenails; clouding of the lens in both eyes from birth (congenital cataracts); poor coordination; and skeletal abnormalities including degeneration of both hips at an early age.\n\nMothers of children with trichothiodystrophy may experience problems during pregnancy including pregnancy-induced high blood pressure (preeclampsia) and a related condition called HELLP syndrome that can damage the liver. Babies with trichothiodystrophy are at increased risk of premature birth, low birth weight, and slow growth. Most children with trichothiodystrophy have short stature compared to others their age. \n\nThe signs and symptoms of trichothiodystrophy vary widely. Mild cases may involve only the hair. More severe cases also cause delayed development, significant intellectual disability, and recurrent infections; severely affected individuals may survive only into infancy or early childhood.\n\nIn people with trichothiodystrophy, tests show that the hair is lacking sulfur-containing proteins that normally gives hair its strength. A cross section of a cut hair shows alternating light and dark banding that has been described as a "tiger tail."\n\nTrichothiodystrophy, commonly called TTD, is a rare inherited condition that affects many parts of the body. The hallmark of this condition is hair that is sparse and easily broken. 

The autosomal dominant TRPV4 disorders (previously considered to be clinically distinct phenotypes before their molecular basis was discovered) are now grouped into neuromuscular disorders and skeletal dysplasias; however, the overlap within each group is considerable. Affected individuals typically have either neuromuscular or skeletal manifestations alone, and in only rare instances an overlap syndrome has been reported. The three autosomal dominant neuromuscular disorders (mildest to most severe) are: Charcot-Marie-Tooth disease type 2C. Scapuloperoneal spinal muscular atrophy. Congenital distal spinal muscular atrophy. The autosomal dominant neuromuscular disorders are characterized by a congenital-onset, static, or later-onset progressive peripheral neuropathy with variable combinations of laryngeal dysfunction (i.e., vocal fold paresis), respiratory dysfunction, and joint contractures. The six autosomal dominant skeletal dysplasias (mildest to most severe) are: Familial digital arthropathy-brachydactyly. Autosomal dominant brachyolmia. Spondylometaphyseal dysplasia, Kozlowski type. Spondyloepiphyseal dysplasia, Maroteaux type. Parastremmatic dysplasia. Metatropic dysplasia. The skeletal dysplasia is characterized by brachydactyly (in all 6); the five that are more severe have short stature that varies from mild to severe with progressive spinal deformity and involvement of the long bones and pelvis. In the mildest of the autosomal dominant TRPV4 disorders life span is normal; in the most severe it is shortened. Bilateral progressive sensorineural hearing loss (SNHL) can occur with both autosomal dominant neuromuscular disorders and skeletal dysplasias.

Calpainopathy is characterized by symmetric and progressive weakness of proximal limb-girdle muscles. The age at onset of muscle weakness ranges from two to 40 years. The phenotype shows intra- and interfamilial variability ranging from severe to mild. Three autosomal recessive calpainopathy phenotypes have been identified based on the distribution of muscle weakness and age at onset: Pelvifemoral limb-girdle muscular dystrophy (LGMD) (Leyden-Möbius LGMD) phenotype, the most frequently observed calpainopathy phenotype, in which muscle weakness is first evident in the pelvic girdle and later in the shoulder girdle, with onset that may occur as early as before age 12 years or as late as after age 30 years. Scapulohumeral LGMD (Erb LGMD) phenotype, usually a milder phenotype with infrequent early onset, in which muscle weakness is first evident in the shoulder girdle and later in the pelvic girdle. HyperCKemia, usually observed in children or young individuals, in which individuals are asymptomatic and have high serum creatine kinase (CK) concentrations. The autosomal dominant form of calpainopathy shows a variability of clinical phenotype, ranging from almost asymptomatic to wheelchair dependence after age 60 years in few cases with a generally milder phenotype than the recessive form. Clinical findings of calpainopathy include the tendency to walk on tiptoe, difficulty in running, scapular winging, waddling gait, and slight hyperlordosis. Other findings include symmetric weakness of proximal more than distal muscles in the limbs, trunk, and periscapular area; laxity of the abdominal muscles; Achilles tendon shortening; scoliosis; and joint contractures. Affected individuals typically do not have cardiac involvement or intellectual disability.

MDDGC4 is an autosomal recessive muscular dystrophy with onset in infancy or early childhood. Cognition and brain structure are usually normal (Godfrey et al., 2006). It is part of a group of similar disorders resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239), collectively known as 'dystroglycanopathies' (Mercuri et al., 2009).

Leukoencephalopathy with brain stem and spinal cord involvement and lactate elevation (LBSL) is characterized by slowly progressive cerebellar ataxia and spasticity with dorsal column dysfunction (decreased position and vibration sense) in most individuals. The neurologic dysfunction involves the legs more than the arms. The tendon reflexes are retained. Deterioration of motor skills usually starts in childhood or adolescence, but occasionally not until adulthood. Dysarthria develops over time. Occasional findings include epilepsy; learning problems; cognitive decline; and reduced consciousness, neurologic deterioration, and fever following minor head trauma. Individuals with neonatal or early-infantile onset have a severe disease course often associated with early death. Those with childhood onset have slow progression with wheelchair dependence in the teens or twenties. Adult onset is associated with slow progression and mild impairment.

An extremely rare type of spondyloepiphyseal dysplasia described in about 5 patients to date with clinical signs including short stature, peculiar facies with blepharophimosis, upward slanted eyes, abundant eyebrows and eyelashes, coarse voice, and short hands and feet.

Any leukodystrophy in which the cause of the disease is a mutation in the HSPD1 gene.

Epidermolysis bullosa simplex 5C with pyloric atresia (EBS5C) is an autosomal recessive genodermatosis characterized by severe skin blistering at birth and congenital pyloric atresia. Death usually occurs in infancy. In reports of 2 consensus meetings for EB, Fine et al. (2000, 2008) considered EBSPA to be a 'basal' form of simplex EB because the electron microscopy shows that skin cleavage occurs in the lower basal level of the keratinocyte, just above the hemidesmosome. There is often decreased integration of keratin filaments with hemidesmosomes. See also forms of junctional EB with pyloric atresia, JEB5B (226730) and JEB6 (619817), caused by mutation in the ITGB4 (147557) and ITGA6 (147556) genes, respectively. For a discussion of genetic heterogeneity of the subtypes of EBS, see EBS1A (131760).

Turner-type X-linked syndromic intellectual developmental disorder (MRXST) is a neurodevelopmental disorder with a highly variable phenotype. Some affected families show X-linked recessive inheritance, with only males being affected and carrier females having no abnormal findings. In other affected families, males are severely affected, and female mutation carriers show milder cognitive abnormalities or dysmorphic features. In addition, there are female patients with de novo mutations who show the full phenotype, despite skewed X-chromosome inactivation. Affected individuals show global developmental delay from infancy, with variably impaired intellectual development and poor or absent speech, often with delayed walking. Dysmorphic features are common and can include macrocephaly, microcephaly, deep-set eyes, hypotelorism, small palpebral fissures, dysplastic, large, or low-set ears, long face, bitemporal narrowing, high-arched palate, thin upper lip, and scoliosis or mild distal skeletal anomalies, such as brachydactyly or tapered fingers. Males tend to have cryptorchidism. Other features, such as hypotonia, seizures, and delayed bone age, are more variable (summary by Moortgat et al., 2018).

Christianson syndrome (referred to as CS in this GeneReview), an X-linked disorder, is characterized in males by cognitive dysfunction, behavioral disorder, and neurologic findings (e.g., seizures, ataxia, postnatal microcephaly, and eye movement abnormalities). Males with CS typically present with developmental delay, later meeting criteria for severe intellectual disability (ID). Behaviorally, autism spectrum disorder and hyperactivity are common, and may resemble the behaviors observed in Angelman syndrome. Hypotonia and oropharyngeal dysphagia in infancy may result in failure to thrive. Seizures, typically beginning before age three years, can include infantile spasms and tonic, tonic-clonic, myoclonic, and atonic seizures. Subsequently, regression (e.g., loss of ambulation and ability to feed independently) may occur. Manifestations in heterozygous females range from asymptomatic to mild ID and/or behavioral issues.

Most characteristically, Aicardi-Goutières syndrome (AGS) manifests as an early-onset encephalopathy that usually, but not always, results in severe intellectual and physical disability. A subgroup of infants with AGS present at birth with abnormal neurologic findings, hepatosplenomegaly, elevated liver enzymes, and thrombocytopenia, a picture highly suggestive of congenital infection. Otherwise, most affected infants present at variable times after the first few weeks of life, frequently after a period of apparently normal development. Typically, they demonstrate the subacute onset of a severe encephalopathy characterized by extreme irritability, intermittent sterile pyrexias, loss of skills, and slowing of head growth. Over time, as many as 40% develop chilblain skin lesions on the fingers, toes, and ears. It is becoming apparent that atypical, sometimes milder, cases of AGS exist, and thus the true extent of the phenotype associated with pathogenic variants in the AGS-related genes is not yet known.

Spondylo-megaepiphyseal-metaphyseal dysplasia is a rare autosomal recessive skeletal dysplasia characterized by disproportionate short stature with a short and stiff neck and trunk; relatively long limbs that may show flexion contractures of the distal joints; delayed and impaired ossification of the vertebral bodies and the presence of large epiphyseal ossification centers and wide growth plates in the long tubular bones; and numerous pseudoepiphyses of the short tubular bones in hands and feet (summary by Hellemans et al., 2009).

Congenital generalized lipodystrophy type 4 (CGL4) combines the phenotype of classic Berardinelli-Seip lipodystrophy (608594) with muscular dystrophy and cardiac conduction anomalies (Hayashi et al., 2009). For a general description and a discussion of genetic heterogeneity of congenital generalized lipodystrophy, see CGL1 (608594).

Like all CDGs, which are caused by a shortage of precursor monosaccharide phosphate or deficiencies in the glycosyltransferases required for lipid-linked oligosaccharide precursor (LLO) synthesis, CDG Ij is caused by a defect in the formation of DPAGT1, the first dolichyl-linked intermediate of the protein N-glycosylation pathway. For a general discussion of CDGs, see CDG1A (212065).

Congenital disorders of glycosylation (CDGs) comprise a group of multisystem diseases with mostly severe psychomotor and mental retardation. Type I CDG comprises those disorders in which there are defects that affect biosynthesis of dolichol-linked oligosaccharides in the cytosol or the endoplasmic reticulum (ER), as well as defects involving the transfer of oligosaccharides onto nascent glycoproteins. Type II CDG comprises all defects of further trimming and elongation of N-linked oligosaccharides in the ER and Golgi (Schwarz et al., 2004). CDG1K is a type I CDG characterized by predominant neurologic involvement. Survival ranges from the second day of life to adulthood. The liver is affected in a minority of patients and shows hepatomegaly, edema, ascites, cholestatic jaundice, portal hypertension, and Budd-Chiari syndrome (summary by Marques-da-Silva et al., 2017). For a general discussion of CDGs, see CDG1A (212065).

Autosomal recessive limb-girdle muscular dystrophy-3 (LGMDR3) affects mainly the proximal muscles and results in difficulty walking. Most individuals have onset in childhood; the disorder is progressive. Other features may include scapular winging, calf pseudohypertrophy, and contractures. Cardiomyopathy has rarely been reported (summary by Babameto-Laku et al., 2011). For a discussion of genetic heterogeneity of autosomal recessive limb-girdle muscular dystrophy, see LGMDR1 (253600).

Cytochrome P450 oxidoreductase deficiency (PORD) is a disorder of steroidogenesis with a broad phenotypic spectrum including cortisol deficiency, altered sex steroid synthesis, disorders of sex development (DSD), and skeletal malformations of the Antley-Bixler syndrome (ABS) phenotype. Cortisol deficiency is usually partial, with some baseline cortisol production but failure to mount an adequate cortisol response in stress. Mild mineralocorticoid excess can be present and causes arterial hypertension, usually presenting in young adulthood. Manifestations of altered sex steroid synthesis include ambiguous genitalia/DSD in both males and females, large ovarian cysts in females, poor masculinization and delayed puberty in males, and maternal virilization during pregnancy with an affected fetus. Skeletal malformations can manifest as craniosynostosis, mid-face retrusion with proptosis and choanal stenosis or atresia, low-set dysplastic ears with stenotic external auditory canals, hydrocephalus, radiohumeral synostosis, neonatal fractures, congenital bowing of the long bones, joint contractures, arachnodactyly, and clubfeet; other anomalies observed include urinary tract anomalies (renal pelvic dilatation, vesicoureteral reflux). Cognitive impairment is of minor concern and likely associated with the severity of malformations; studies of developmental outcomes are lacking.

Congenital muscular dystrophy-dystroglycanopathy with brain and eye anomalies (type A), which includes both the more severe Walker-Warburg syndrome (WWS) and the slightly less severe muscle-eye-brain disease (MEB), is an autosomal recessive disorder with characteristic brain and eye malformations, profound mental retardation, congenital muscular dystrophy, and death usually in the first years of life. It represents the most severe end of a phenotypic spectrum of similar disorders resulting from defective glycosylation of DAG1 (128239), collectively known as 'dystroglycanopathies' (Godfrey et al., 2007). For a general phenotypic description and a discussion of genetic heterogeneity of muscular dystrophy-dystroglycanopathy type A, see MDDGA1 (236670).

MDDGB2 is an autosomal recessive congenital muscular dystrophy associated with impaired intellectual development and mild structural brain abnormalities (Yanagisawa et al., 2007). It is part of a group of similar disorders, collectively known as 'dystroglycanopathies,' resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239) (Godfrey et al., 2007). For a discussion of genetic heterogeneity of congenital muscular dystrophy-dystroglycanopathy type B, see MDDGB1 (613155).

Autosomal recessive limb-girdle muscular dystrophy-17 (LGMDR17) is characterized by early childhood onset of proximal muscle weakness and atrophy without skin involvement. One family has shown rapid progression of the disorder in adolescence (summary by Gundesli et al., 2010). For a discussion of genetic heterogeneity of autosomal recessive limb-girdle muscular dystrophy, see LGMDR1 (253600).

AP-4-associated hereditary spastic paraplegia (HSP), also known as AP-4 deficiency syndrome, is a group of neurodegenerative disorders characterized by a progressive, complex spastic paraplegia with onset typically in infancy or early childhood. Early-onset hypotonia evolves into progressive lower-extremity spasticity. The majority of children become nonambulatory and usually wheelchair bound. Over time spasticity progresses to involve the upper extremities, resulting in a spastic tetraplegia. Associated complications include dysphagia, contractures, foot deformities, dysregulation of bladder and bowel function, and a pseudobulbar affect. About 50% of affected individuals have seizures. Postnatal microcephaly (usually in the -2SD to -3SD range) is common. All have developmental delay. Speech development is significantly impaired and many affected individuals remain nonverbal. Intellectual disability in older children is usually moderate to severe.

Nestor-Guillermo progeria syndrome (NGPS) is an autosomal recessive disorder characterized by lipoatrophy, osteoporosis, and very severe osteolysis. Patients have no cardiovascular impairment, diabetes mellitus, or hypertriglyceridemia, but suffer profound skeletal abnormalities that affect their quality of life. Onset is after 2 years of age, and lifespan is relatively long (summary by Cabanillas et al., 2011).

Multiple congenital anomalies-hypotonia-seizures syndrome-2 (MCAHS2) is an X-linked recessive neurodevelopmental disorder characterized by dysmorphic features, neonatal hypotonia, early-onset myoclonic seizures, and variable congenital anomalies involving the central nervous, cardiac, and urinary systems. Some affected individuals die in infancy (summary by Johnston et al., 2012). The phenotype shows clinical variability with regard to severity and extraneurologic features. However, most patients present in infancy with early-onset epileptic encephalopathy associated with developmental arrest and subsequent severe neurologic disability; these features are consistent with a form of developmental and epileptic encephalopathy (DEE) (summary by Belet et al., 2014, Kato et al., 2014). The disorder is caused by a defect in glycosylphosphatidylinositol (GPI) biosynthesis. For a discussion of genetic heterogeneity of MCAHS, see MCAHS1 (614080). For a discussion of nomenclature and genetic heterogeneity of DEE, see 308350. For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293).

A distinct group of inborn defects of complex V (ATP synthase) is represented by the enzyme deficiency due to nuclear genome mutations characterized by a selective inhibition of ATP synthase biogenesis. Biochemically, the patients show a generalized decrease in the content of ATP synthase complex which is less than 30% of normal. Most cases present with neonatal-onset hypotonia, lactic acidosis, hyperammonemia, hypertrophic cardiomyopathy, and 3-methylglutaconic aciduria. Many patients die within a few months or years (summary by Mayr et al., 2010). Genetic Heterogeneity of Mitochondrial Complex V Deficiency Other nuclear types of mitochondrial complex V deficiency include MC5DN2 (614052), caused by mutation in the TMEM70 gene (612418) on chromosome 8q21; MC5DN3 (614053), caused by mutation in the ATP5E gene (ATP5F1E; 606153) on chromosome 20q13; MC5DN4A (620358) and MC5DN4B (615228), both caused by mutation in the ATP5A1 gene (ATP5F1A; 164360) on chromosome 18q; MC5DN5 (618120), caused by mutation in the ATP5D gene (ATP5F1D; 603150) on chromosome 19p13; MC5DN6 (618683), caused by mutation in the USMG5 gene (ATP5MD; 615204) on chromosome 10q24; and MC5DN7 (620359), caused by mutation in the ATP5PO gene (600828) on chromosome 21q22. Mutations in the mitochondrial-encoded MTATP6 (516060) and MTATP8 (516070) genes can also cause mitochondrial complex V deficiency (see, e.g., 500015).

AP-4-associated hereditary spastic paraplegia (HSP), also known as AP-4 deficiency syndrome, is a group of neurodegenerative disorders characterized by a progressive, complex spastic paraplegia with onset typically in infancy or early childhood. Early-onset hypotonia evolves into progressive lower-extremity spasticity. The majority of children become nonambulatory and usually wheelchair bound. Over time spasticity progresses to involve the upper extremities, resulting in a spastic tetraplegia. Associated complications include dysphagia, contractures, foot deformities, dysregulation of bladder and bowel function, and a pseudobulbar affect. About 50% of affected individuals have seizures. Postnatal microcephaly (usually in the -2SD to -3SD range) is common. All have developmental delay. Speech development is significantly impaired and many affected individuals remain nonverbal. Intellectual disability in older children is usually moderate to severe.

AP-4-associated hereditary spastic paraplegia (HSP), also known as AP-4 deficiency syndrome, is a group of neurodegenerative disorders characterized by a progressive, complex spastic paraplegia with onset typically in infancy or early childhood. Early-onset hypotonia evolves into progressive lower-extremity spasticity. The majority of children become nonambulatory and usually wheelchair bound. Over time spasticity progresses to involve the upper extremities, resulting in a spastic tetraplegia. Associated complications include dysphagia, contractures, foot deformities, dysregulation of bladder and bowel function, and a pseudobulbar affect. About 50% of affected individuals have seizures. Postnatal microcephaly (usually in the -2SD to -3SD range) is common. All have developmental delay. Speech development is significantly impaired and many affected individuals remain nonverbal. Intellectual disability in older children is usually moderate to severe.

Keppen-Lubinsky syndrome (KPLBS) is a rare disorder characterized by severely delayed psychomotor development, hypertonia, hyperreflexia, generalized lipodystrophy giving an aged appearance, and distinctive dysmorphic features, including microcephaly, prominent eyes, narrow nasal bridge, and open mouth (summary by Masotti et al., 2015).

Rafiq syndrome (RAFQS) is an autosomal recessive disorder characterized by variably impaired intellectual and motor development, a characteristic facial dysmorphism, truncal obesity, and hypotonia. The facial dysmorphism comprises prominent eyebrows with lateral thinning, downward-slanting palpebral fissures, bulbous tip of the nose, large ears, and a thin upper lip. Behavioral problems, including overeating, verbal and physical aggression, have been reported in some cases. Serum transferrin isoelectric focusing shows a type 2 pattern (summary by Balasubramanian et al., 2019).

RAB18 deficiency is the molecular deficit underlying both Warburg micro syndrome (characterized by eye, nervous system, and endocrine abnormalities) and Martsolf syndrome (characterized by similar – but milder – findings). To date Warburg micro syndrome comprises >96% of reported individuals with genetically defined RAB18 deficiency. The hallmark ophthalmologic findings are bilateral congenital cataracts, usually accompanied by microphthalmia, microcornea (diameter <10), and small atonic pupils. Poor vision despite early cataract surgery likely results from progressive optic atrophy and cortical visual impairment. Individuals with Warburg micro syndrome have severe to profound intellectual disability (ID); those with Martsolf syndrome have mild to moderate ID. Some individuals with RAB18 deficiency also have epilepsy. In Warburg micro syndrome, a progressive ascending spastic paraplegia typically begins with spastic diplegia and contractures during the first year, followed by upper-limb involvement leading to spastic quadriplegia after about age five years, often eventually causing breathing difficulties. In Martsolf syndrome infantile hypotonia is followed primarily by slowly progressive lower-limb spasticity. Hypogonadism – when present – manifests in both syndromes, in males as micropenis and/or cryptorchidism and in females as hypoplastic labia minora, clitoral hypoplasia, and small introitus.

RAB18 deficiency is the molecular deficit underlying both Warburg micro syndrome (characterized by eye, nervous system, and endocrine abnormalities) and Martsolf syndrome (characterized by similar – but milder – findings). To date Warburg micro syndrome comprises >96% of reported individuals with genetically defined RAB18 deficiency. The hallmark ophthalmologic findings are bilateral congenital cataracts, usually accompanied by microphthalmia, microcornea (diameter <10), and small atonic pupils. Poor vision despite early cataract surgery likely results from progressive optic atrophy and cortical visual impairment. Individuals with Warburg micro syndrome have severe to profound intellectual disability (ID); those with Martsolf syndrome have mild to moderate ID. Some individuals with RAB18 deficiency also have epilepsy. In Warburg micro syndrome, a progressive ascending spastic paraplegia typically begins with spastic diplegia and contractures during the first year, followed by upper-limb involvement leading to spastic quadriplegia after about age five years, often eventually causing breathing difficulties. In Martsolf syndrome infantile hypotonia is followed primarily by slowly progressive lower-limb spasticity. Hypogonadism – when present – manifests in both syndromes, in males as micropenis and/or cryptorchidism and in females as hypoplastic labia minora, clitoral hypoplasia, and small introitus.

De Barsy syndrome, also known as autosomal recessive cutis laxa type III (ARCL3), is a rare autosomal recessive disorder characterized by an aged appearance with distinctive facial features, sparse hair, ophthalmologic abnormalities, intrauterine growth retardation (IUGR), and cutis laxa (summary by Lin et al., 2011). For a phenotypic description and a discussion of genetic heterogeneity of de Barsy syndrome, see 219150. For a phenotypic description and a discussion of genetic heterogeneity of autosomal recessive cutis laxa, see 219200.

Fanconi anemia (FA) is characterized by physical abnormalities, bone marrow failure, and increased risk for malignancy. Physical abnormalities, present in approximately 75% of affected individuals, include one or more of the following: short stature, abnormal skin pigmentation, skeletal malformations of the upper and/or lower limbs, microcephaly, and ophthalmic and genitourinary tract anomalies. Progressive bone marrow failure with pancytopenia typically presents in the first decade, often initially with thrombocytopenia or leukopenia. The incidence of acute myeloid leukemia is 13% by age 50 years. Solid tumors – particularly of the head and neck, skin, and genitourinary tract – are more common in individuals with FA.

Hereditary sensory and autonomic neuropathy type VI (HSAN6) is a severe autosomal recessive disorder characterized by neonatal hypotonia, respiratory and feeding difficulties, lack of psychomotor development, and autonomic abnormalities including labile cardiovascular function, lack of corneal reflexes leading to corneal scarring, areflexia, and absent axonal flare response after intradermal histamine injection (summary by Edvardson et al., 2012). For a discussion of genetic heterogeneity of hereditary sensory and autonomic neuropathy, see HSAN1 (162400).

EXOSC3 pontocerebellar hypoplasia (EXOSC3-PCH) is characterized by abnormalities in the posterior fossa and degeneration of the anterior horn cells. At birth, skeletal muscle weakness manifests as hypotonia (sometimes with congenital joint contractures) and poor feeding. In persons with prolonged survival, spasticity, dystonia, and seizures become evident. Within the first year of life respiratory insufficiency and swallowing difficulties are common. Intellectual disability is severe. Life expectancy ranges from a few weeks to adolescence. To date, 82 individuals (from 58 families) with EXOSC3-PCH have been described.

A rare genetic neurodevelopmental disorder with primordial microcephaly, with characteristics of primary microcephaly, moderate to severe intellectual disability and global developmental delay. Variable brain malformations are common ranging from simplified gyration, to cortical malformations such as pachygyria, polymicrogyria, reduced sulcation and midline defects. Craniofacial dysmorphism (e.g. sloping forehead, high and broad nasal bridge) are related to the primary microcephaly. Short stature is frequently observed, and may be severe. Germline biallelic variants in RTTN (18q22.2) are responsible for the disease. The pattern of inheritance is autosomal recessive.

Lethal congenital contracture syndrome-4 (LCCS4) is a severe form of neuromuscular arthrogryposis characterized by contractures leading to various degrees of flexion or extension limitations evident at birth (Markus et al., 2012). For a general phenotypic description and discussion of genetic heterogeneity of LCCS, see LCCS1 (253310).

Congenital muscular dystrophy-dystroglycanopathy with brain and eye anomalies (type A) is an autosomal recessive disorder with congenital muscular dystrophy resulting in muscle weakness early in life and brain and eye anomalies. It is usually associated with delayed psychomotor development and shortened life expectancy. The phenotype includes the alternative clinical designations Walker-Warburg syndrome (WWS) and muscle-eye-brain disease (MEB). The disorder represents the most severe end of a phenotypic spectrum of similar disorders resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239), collectively known as dystroglycanopathies (summary by Stevens et al., 2013). For a general phenotypic description and a discussion of genetic heterogeneity of muscular dystrophy-dystroglycanopathy type A, see MDDGA1 (236670).

Nemaline myopathy-8 is a severe autosomal recessive muscle disorder characterized by fetal akinesia or hypokinesia, followed by contractures, fractures, respiratory failure, and swallowing difficulties apparent at birth. Most patients die in infancy. Skeletal muscle biopsy shows numerous small nemaline bodies, often with no normal myofibrils (summary by Ravenscroft et al., 2013). For a discussion of genetic heterogeneity of nemaline myopathy, see NEM3 (161800).

Spastic paraplegia-79B (SPG79B) is an autosomal recessive progressive neurologic disorder characterized by onset of spastic paraplegia and optic atrophy in the first decade of life. Additional features are variable, but may include peripheral neuropathy, cerebellar ataxia, and cognitive impairment (summary by Rydning et al., 2017). For a discussion of genetic heterogeneity of autosomal recessive spastic paraplegia, see SPG5A (270800).

RAB18 deficiency is the molecular deficit underlying both Warburg micro syndrome (characterized by eye, nervous system, and endocrine abnormalities) and Martsolf syndrome (characterized by similar – but milder – findings). To date Warburg micro syndrome comprises >96% of reported individuals with genetically defined RAB18 deficiency. The hallmark ophthalmologic findings are bilateral congenital cataracts, usually accompanied by microphthalmia, microcornea (diameter <10), and small atonic pupils. Poor vision despite early cataract surgery likely results from progressive optic atrophy and cortical visual impairment. Individuals with Warburg micro syndrome have severe to profound intellectual disability (ID); those with Martsolf syndrome have mild to moderate ID. Some individuals with RAB18 deficiency also have epilepsy. In Warburg micro syndrome, a progressive ascending spastic paraplegia typically begins with spastic diplegia and contractures during the first year, followed by upper-limb involvement leading to spastic quadriplegia after about age five years, often eventually causing breathing difficulties. In Martsolf syndrome infantile hypotonia is followed primarily by slowly progressive lower-limb spasticity. Hypogonadism – when present – manifests in both syndromes, in males as micropenis and/or cryptorchidism and in females as hypoplastic labia minora, clitoral hypoplasia, and small introitus.

A rare pure or complex form of hereditary spastic paraplegia with characteristics of onset in infancy of progressive lower limb spasticity, abnormal gait, increased deep tendon reflexes and extensor plantar responses that may be associated with intellectual disability. Additional signs such as contractures in the lower limbs, amyotrophy, clubfoot and optic atrophy, have also been reported. Caused by homozygous mutation in the NT5C2 gene on chromosome 10q24.

Tubular aggregates in muscle, first described by Engel (1964), are structures of variable appearance consisting of an outer tubule containing either one or more microtubule-like structures or amorphous material. They are a nonspecific pathologic finding that may occur in a variety of circumstances, including alcohol- and drug-induced myopathies, exercise-induced cramps or muscle weakness, and inherited myopathies. Tubular aggregates are derived from the sarcoplasmic reticulum (Salviati et al., 1985) and are believed to represent an adaptive mechanism aimed at regulating an increased intracellular level of calcium in order to prevent the muscle fibers from hypercontraction and necrosis (Martin et al., 1997; Muller et al., 2001). Genetic Heterogeneity of Tubular Aggregate Myopathy See also TAM2 (615883), caused by mutation in the ORAI1 gene (610277) on chromosome 12q24.

Pontocerebellar hypoplasia type 2E is an autosomal recessive neurodegenerative disorder characterized by profound mental retardation, progressive microcephaly, spasticity, and early-onset epilepsy (summary by Feinstein et al., 2014). For a general phenotypic description and a discussion of genetic heterogeneity of pontocerebellar hypoplasia type 2, see PCH2A (277470).

CAGSSS, which comprises cataracts, growth hormone deficiency, sensory neuropathy, sensorineural hearing loss, and skeletal dysplasia, is an autosomal recessive multisystemic disorder with a highly variable phenotypic spectrum. Not all of these features are always present, and almost all the features may present at different times and/or become more apparent with age. The skeletal features are consistent with spondyloepimetaphyseal dysplasia (SEMD) (summary by Vona et al., 2018). One family had a distinctive presentation with infantile-onset intractable seizures and cortical abnormalities reminiscent of Leigh syndrome (see 256000). The correlation between genotype and phenotype remains unclear, but since the IARS2 gene is involved in mitochondrial function, heterogeneous manifestations can be expected (Takezawa et al., 2018).

Nemaline myopathy-10 (NEM10) is an autosomal recessive severe congenital myopathy characterized by early-onset generalized muscle weakness and hypotonia with respiratory insufficiency and feeding difficulties. Many patients present antenatally with decreased fetal movements, and most die of respiratory failure in early infancy (summary by Yuen et al., 2014). Patients with a stable and much milder disease course have been described (Schatz et al., 2018). For a discussion of genetic heterogeneity of nemaline myopathy, see NEM3 (161800).

Autosomal recessive spinocerebellar ataxia-18 (SCAR18) is a neurologic disorder characterized by delayed psychomotor development, severely impaired gait due to cerebellar ataxia, ocular movement abnormalities, and intellectual disability. Brain imaging shows progressive cerebellar atrophy (summary by Hills et al., 2013).

Temple syndrome is a short stature disorder of imprinting. The cardinal features are low birth weight, hypotonia and motor delay, feeding problems early in life, early puberty, and significantly reduced final height. Facial features include a broad forehead and short nose with a wide nasal tip, and the majority of patients have small hands and feet. However, many of the clinical features are nonspecific, making diagnosis difficult. In addition, isodisomy may uncover recessive disorders, which may influence the phenotype in maternal uniparental disomy of chromosome 14 (UPD14mat) cases (summary by Ioannides et al., 2014).

Any spondyloepimetaphyseal dysplasia with joint laxity in which the cause of the disease is a mutation in the B3GALT6 gene.

Reducing-body myopathy (RBM) is a rare myopathy characterized pathologically by the presence of intracytoplasmic inclusion bodies strongly stained by menadione-linked alpha-glycerophosphate dehydrogenase (MAG) in the absence of substrate, alpha-glycerophosphate. The term 'reducing body' refers to the reducing activity of the inclusions to nitroblue tetrazolium (NBT) in the absence of substrate. This condition is also commonly associated with rimmed vacuoles and cytoplasmic bodies. The clinical features of RBM are variable; a severe form has onset in infancy or early childhood and results in severe disability or early death (RBMX1A; 300717), and a less severe form has onset in late childhood or adulthood (RBMX1B) (summary by Liewluck et al., 2007 and Shalaby et al., 2009).

Complex lethal osteochondrodysplasia of the Symoens-Barnes-Gistelinck type is characterized by severe skeletal osteopenia, microcephaly, multiple fractures, and congenital anomalies including ascites, pleural effusion, and intracranial ventriculomegaly (Symoens et al., 2015).

Spinal muscular atrophy with congenital bone fractures is an autosomal recessive severe neuromuscular disorder characterized by onset of severe hypotonia with fetal hypokinesia in utero. This results in congenital contractures, consistent with arthrogryposis multiplex congenita, and increased incidence of prenatal fracture of the long bones. Affected infants have difficulty breathing and feeding and often die in the first days or months of life (summary by Knierim et al., 2016). For a discussion of genetic heterogeneity of spinal muscular atrophy with congenital bone fractures, see SMABF1 (616866).

Spinal muscular atrophy with congenital bone fractures is an autosomal recessive severe neuromuscular disorder characterized by onset of severe hypotonia with fetal hypokinesia in utero. This results in congenital contractures, consistent with arthrogryposis multiplex congenita, and increased incidence of prenatal fracture of the long bones. Affected infants have difficulty breathing and feeding and often die in the first days or months of life (summary by Knierim et al., 2016). Genetic Heterogeneity of Spinal Muscular Atrophy With Congenital Bone Fractures See also SMABF2 (616867), caused by mutation in the ASCC1 gene (614215) on chromosome 10q22.

Primary coenzyme Q10 (CoQ10) deficiency is usually associated with multisystem involvement, including neurologic manifestations such as fatal neonatal encephalopathy with hypotonia; a late-onset slowly progressive multiple-system atrophy-like phenotype (neurodegeneration with autonomic failure and various combinations of parkinsonism and cerebellar ataxia, and pyramidal dysfunction); and dystonia, spasticity, seizures, and intellectual disability. Steroid-resistant nephrotic syndrome (SRNS), the hallmark renal manifestation, is often the initial manifestation either as isolated renal involvement that progresses to end-stage renal disease (ESRD), or associated with encephalopathy (seizures, stroke-like episodes, severe neurologic impairment) resulting in early death. Hypertrophic cardiomyopathy (HCM), retinopathy or optic atrophy, and sensorineural hearing loss can also be seen.

Hypomyelinating leukodystrophy-12 (HLD12) is an autosomal recessive neurologic disorder characterized by severely delayed or even lack of psychomotor development that becomes apparent in the first months of life. Patients are markedly disabled, with acquired microcephaly, lack of speech, and often lack of spontaneous movement due to hypotonia and spasticity. Brain imaging shows delayed myelination (summary by Edvardson et al., 2015). For a general phenotypic description and a discussion of genetic heterogeneity of HLD, see 312080. In a review of the pathogenesis of disorders with prominent dystonia or opisthotonic posturing as a feature, Monfrini et al. (2021) classified HLD12 as belonging to a group of neurologic disorders termed 'HOPS-associated neurologic disorders (HOPSANDs), which are caused by mutations in genes encoding various components of the autophagic/endolysosomal system, including VPS11.

Klippel-Feil syndrome-4 with nemaline myopathy and facial dysmorphism (KFS4) is an autosomal recessive disorder characterized mainly by severe hypotonia apparent from infancy. Klippel-Feil anomaly is primarily defined by fusion of the cervical spine, with associated low posterior hairline and limited neck mobility being observed in about half of patients (summary by Alazami et al., 2015). For a general description and a discussion of genetic heterogeneity of Klippel-Feil syndrome, see KFS1 (118100).

Hereditary motor and sensory neuropathy type VIB is an autosomal recessive complex progressive neurologic disorder characterized mainly by early-onset optic atrophy resulting in progressive visual loss and peripheral axonal sensorimotor neuropathy with highly variable age at onset and severity. Affected individuals may also have cerebellar or pontocerebellar atrophy on brain imaging, and they may show abnormal movements such as ataxia, dysmetria, and myoclonus (summary by Abrams et al., 2015). For a general phenotypic description and a discussion of genetic heterogeneity of HMSN6, see HMSN6A (601152).

Lethal congenital contracture syndrome-9 (LCCS9) is an autosomal recessive disorder characterized by multiple flexion and extension contractures resulting from reduced or absent fetal movement (Ravenscroft et al., 2015). For a general phenotypic description and discussion of genetic heterogeneity of lethal congenital contracture syndrome, see LCCS1 (253310).

Bethlem myopathy-2 (BTHLM2) is characterized by congenital hypotonia and myopathy. Motor development is delayed, but muscle strength improves with age, and patients are able to achieve ambulation. Proximal joint contractures that improve over time, as well as joint hyperlaxity, are also present. Muscle biopsy shows mild variability in fiber diameter, without degeneration or regeneration (Zou et al., 2014; Hicks et al., 2014). For a discussion of genetic heterogeneity of Bethlem myopathy, see BTHLM1 (158810).

Ullrich congenital muscular dystrophy-2 (UCMD2) is a severe autosomal recessive disorder characterized by joint hypermobility, proximal contractures, and muscle weakness precluding ambulation (summary by Zou et al., 2014). For a discussion of genetic heterogeneity of Ullrich congenital muscular dystrophy, see UCMD1A (254090).

Congenital myasthenic syndrome associated with AChR deficiency is a disorder of the postsynaptic neuromuscular junction (NMJ) clinically characterized by early-onset muscle weakness with variable severity. Electrophysiologic studies show low amplitude of the miniature endplate potential (MEPP) and current (MEPC) resulting from deficiency of AChR at the endplate. Patients may show a favorable response to amifampridine (summary by Engel et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

Slow-channel congenital myasthenic syndrome (SCCMS) is a disorder of the postsynaptic neuromuscular junction (NMJ) characterized by early-onset progressive muscle weakness. The disorder results from kinetic abnormalities of the acetylcholine receptor channel, specifically from prolonged opening and activity of the channel, which causes prolonged synaptic currents resulting in a depolarization block. This is associated with calcium overload, which may contribute to subsequent degeneration of the endplate and postsynaptic membrane. Treatment with quinine, quinidine, or fluoxetine may be helpful; cholinesterase inhibitors and amifampridine should be avoided (summary by Engel et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

Lethal congenital contracture syndrome-8 (LCCS8), an axoglial form of arthrogryposis multiplex congenita, is characterized by congenital distal joint contractures, reduced fetal movements, and severe motor paralysis leading to death early in the neonatal period (Laquerriere et al., 2014). For a general phenotypic description and a discussion of genetic heterogeneity of lethal congenital contracture syndrome, see LCCS1 (253310).

Neurodevelopmental disorder with spastic paraplegia and microcephaly (NEDSPM) is an autosomal recessive neurologic syndrome characterized by delayed psychomotor development with delayed walking, moderately to severely impaired intellectual development, and poor or absent speech. More severely affected individuals show poor overall growth with progressive microcephaly, axial hypotonia, oromotor dysfunction with drooling, joint contractures, and spastic paraplegia resulting in walking difficulties. Some patients may develop seizures; nonspecific dysmorphic features have also been reported (summary by Hengel et al., 2018 and Ouyang et al., 2019).

Reducing-body myopathy (RBM) is a rare myopathy characterized pathologically by the presence of intracytoplasmic inclusion bodies strongly stained by menadione-linked alpha-glycerophosphate dehydrogenase (MAG) in the absence of substrate, alpha-glycerophosphate. The term 'reducing body' refers to the reducing activity of the inclusions to nitroblue tetrazolium (NBT) in the absence of substrate. This condition is also commonly associated with rimmed vacuoles and cytoplasmic bodies. The clinical features of RBM are variable; a severe form has onset in infancy or early childhood and results in severe disability or early death, and a less severe form has onset in late childhood or adulthood (RBMX1B; 300718) (summary by Liewluck et al., 2007 and Shalaby et al., 2009).

MPSPS is an autosomal recessive inborn error of metabolism resulting in a multisystem disorder with features of the mucopolysaccharidosis lysosomal storage diseases (see, e.g., 607016). Patients present in infancy or early childhood with respiratory difficulties, cardiac problems, anemia, dysostosis multiplex, renal involvement, coarse facies, and delayed psychomotor development. Most patients die of cardiorespiratory failure in the first years of life (summary by Kondo et al., 2017).

Lethal congenital contracture syndrome-11 (LCCS11) is an autosomal recessive disorder characterized by multiple flexion and extension joint contractures resulting from reduced or absent fetal movements (Maluenda et al., 2016). For a general phenotypic description and a discussion of genetic heterogeneity of lethal congenital contracture syndrome, see LCCS1 (253310).

Distal arthrogryposis with impaired proprioception and touch is an autosomal recessive neurologic disorder characterized by loss of certain mechanosensation modalities resulting in ataxia, difficulty walking, dysmetria, muscle weakness and atrophy, and progressive skeletal contractures. Patients have onset of symptoms in early childhood (summary by Chesler et al., 2016 and Delle Vedove et al., 2016).

ZTTK syndrome (ZTTKS) is a severe multisystem developmental disorder characterized by delayed psychomotor development and intellectual disability. Affected individuals have characteristic dysmorphic facial features, hypotonia, poor feeding, poor overall growth, and eye or visual abnormalities. Most patients also have musculoskeletal abnormalities, and some have congenital defects of the heart and urogenital system. Brain imaging usually shows developmental abnormalities such as gyral changes, cortical and/or cerebellar atrophy, and thin corpus callosum (summary by Kim et al., 2016).

Myofibrillar myopathy-7 (MFM7) is an autosomal recessive muscle disorder characterized by early childhood onset of slowly progressive muscle weakness that primarily affects the lower limbs and is associated with joint contractures (summary by Straussberg et al., 2016). For a general phenotypic description and a discussion of genetic heterogeneity of myofibrillar myopathy, see MFM1 (601419).

Developmental and epileptic encephalopathy-42 (DEE42) is a neurologic disorder characterized by the onset of various types of seizures in the first hours or days of life, although rare patients may have onset in the first weeks of life. The seizures tend to be refractory and associated with EEG abnormalities, including multifocal spikes and generalized spike-wave complexes. Affected infants show global developmental delay with severely impaired intellectual development. Other features may include axial hypotonia, peripheral hypertonia with hyperreflexia, tremor, ataxia, and abnormal eye movements (summary by the Epi4K Consortium, 2016). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Developmental and epileptic encephalopathy-41 (DEE41) is a neurologic disorder characterized by the onset of seizures in the first days or weeks of life. Affected infants show severely impaired psychomotor development with hypotonia, spasticity, lack of speech, poor visual fixation, feeding difficulties sometimes necessitating tube feeding, poor overall growth and microcephaly, and contractures. Brain imaging may show delayed myelination, thin corpus callosum, and cerebral atrophy (summary by the EPI4K Consortium, 2016). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

GLYT1 encephalopathy is characterized in neonates by severe hypotonia, respiratory failure requiring mechanical ventilation, and absent neonatal reflexes; encephalopathy, including impaired consciousness and unresponsiveness, may be present. Arthrogryposis or joint laxity can be observed. Generalized hypotonia develops later into axial hypotonia with limb hypertonicity and a startle-like response to vocal and visual stimuli which should not be confused with seizures. To date, three of the six affected children reported from three families died between ages two days and seven months; the oldest reported living child is severely globally impaired at age three years. Because of the limited number of affected individuals reported to date, the phenotype has not yet been completely described.

Developmental and epileptic encephalopathy-36 (DEE36) is an X-linked neurodevelopmental disorder characterized by the onset of seizures at a mean age of 6.5 months. Most patients present with infantile spasms associated with hypsarrhythmia on EEG, consistent with a clinical diagnosis of West syndrome. The seizures tend to be refractory to treatment, although some patients may respond to benzodiazepines or a ketogenic diet. Affected individuals have severely delayed psychomotor development with poor motor function, severe intellectual disability, poor or absent speech, and limited eye contact. More variable features include feeding difficulties sometimes requiring tube feeding, ocular defects including cortical visual impairment, dysmorphic facial features, and scoliosis or osteopenia. The vast majority of patients reported have been females, although rare affected males with a similar phenotype have been described. Most patients show normal N-glycosylation on transferrin isoelectric focusing, but some show abnormal N-glycosylation consistent with CDG type I (summary by Ng et al., 2020). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350. For a discussion of the classification of CDGs, see CDG1A (212065).

Anauxetic dysplasia is a spondyloepimetaphyseal dysplasia characterized by severe short stature of prenatal onset, very short adult height (less than 1 meter), hypodontia, midface hypoplasia, and mild intellectual disability. Vertebrae are ovoid with concave dorsal surfaces in the lumbar region and show delayed bone maturation. Femoral heads and necks are hypoplastic, as are the iliac bodies. Long bones show irregular mineralization of the metaphyses. The first and fifth metacarpals are short and wide with small, late-ossifying epiphyses and bullet-shaped middle phalanges (summary by Barraza-Garcia et al., 2017). For a discussion of genetic heterogeneity of anauxetic dysplasia, see ANXD1 (607095).

IDDFSDA is an autosomal recessive severe multisystem disorder characterized by poor overall growth, developmental delay, early-onset seizures, intellectual disability, and dysmorphic features. There is phenotypic variability. The most severely affected patients have a neurodevelopmental disorder with microcephaly, absent speech, and inability to walk, and they require feeding tubes. Some patients have congenital heart defects or nonspecific abnormalities on brain imaging. Less severely affected individuals have mild to moderate intellectual disability with normal speech and motor development (summary by Santiago-Sim et al., 2017).

AMC1 is an autosomal recessive severe neurologic disorder with onset in utero. Most affected individuals die in utero or are subject to pregnancy termination because of lack of fetal movements and prenatal evidence of contractures of virtually all joints. Those who survive have generalized contractures and hypotonia. The disorder is caused by a neurogenic defect and poor or absent myelin formation around peripheral nerves rather than by a muscular defect (summary by Xue et al., 2017). <Genetic Heterogeneity of Arthrogryposis Multiplex Congenita Also see AMC2 (208100), caused by mutation in the ERGIC1 gene (617946); AMC3 (618484), caused by mutation in the SYNE1 gene (608441); AMC4 (618776), caused by mutation in the SCYL2 gene (616365); AMC5 (618947), caused by mutation in the TOR1A gene (605204), AMC6 (619334), caused by mutation in the NEB gene (161650), and AMC7 (301127), caused by mutation in the THOC2 gene (300395).

EED-related overgrowth is characterized by fetal or early childhood overgrowth (tall stature, macrocephaly, large hands and feet, and advanced bone age) and intellectual disability that ranges from mild to severe. To date, EED-related overgrowth has been reported in eight individuals.

Developmental and epileptic encephalopathy-55 (DEE55) is an autosomal recessive neurologic disorder characterized by onset of refractory seizures in the first weeks or months of life. Affected individuals have an extremely poor outcome, with profoundly impaired intellectual development, absent speech, spastic quadriplegia, and dyskinetic movements. Most have cortical visual impairment and require a feeding tube. Brain imaging shows nonspecific abnormalities, including cerebral atrophy, thin corpus callosum, and abnormal signals in the white matter. Death in childhood may occur. Biochemically, the disorder is associated with impaired synthesis of GPI-anchored proteins (summary by Vetro et al., 2020). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350. For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293).

Autosomal recessive congenital ichthyosis (ARCI) encompasses several forms of nonsyndromic ichthyosis. Although most neonates with ARCI are collodion babies, the clinical presentation and severity of ARCI may vary significantly, ranging from harlequin ichthyosis, the most severe and often fatal form, to lamellar ichthyosis (LI) and (nonbullous) congenital ichthyosiform erythroderma (CIE). These phenotypes are now recognized to fall on a continuum; however, the phenotypic descriptions are clinically useful for clarification of prognosis and management. Infants with harlequin ichthyosis are usually born prematurely and are encased in thick, hard, armor-like plates of cornified skin that severely restrict movement. Life-threatening complications in the immediate postnatal period include respiratory distress, feeding problems, and systemic infection. Collodion babies are born with a taut, shiny, translucent or opaque membrane that encases the entire body and lasts for days to weeks. LI and CIE are seemingly distinct phenotypes: classic, severe LI with dark brown, plate-like scale with no erythroderma and CIE with finer whiter scale and underlying generalized redness of the skin. Affected individuals with severe involvement can have ectropion, eclabium, scarring alopecia involving the scalp and eyebrows, and palmar and plantar keratoderma. Besides these major forms of nonsyndromic ichthyosis, a few rare subtypes have been recognized, such as bathing suit ichthyosis, self-improving collodion ichthyosis, or ichthyosis-prematurity syndrome.

Rubinstein-Taybi syndrome (RSTS) is characterized by distinctive facial features, broad and often angulated thumbs and halluces, short stature, and moderate-to-severe intellectual disability. The characteristic craniofacial features are downslanted palpebral fissures, low-hanging columella, high palate, grimacing smile, and talon cusps. Prenatal growth is often normal, then height, weight, and head circumference percentiles rapidly drop in the first few months of life. Short stature is typical in adulthood. Obesity may develop in childhood or adolescence. Average IQ ranges between 35 and 50; however, developmental outcome varies considerably. Some individuals with EP300-RSTS have normal intellect. Additional features include ocular abnormalities, hearing loss, respiratory difficulties, congenital heart defects, renal abnormalities, cryptorchidism, feeding problems, recurrent infections, and severe constipation.

Centronuclear myopathy-1 (CNM1) is an autosomal dominant congenital myopathy characterized by slowly progressive muscular weakness and wasting. The disorder involves mainly limb girdle, trunk, and neck muscles but may also affect distal muscles. Weakness may be present during childhood or adolescence or may not become evident until the third decade of life, and some affected individuals become wheelchair-bound in their fifties. Ptosis and limitation of eye movements occur frequently. The most prominent histopathologic features include high frequency of centrally located nuclei in a large number of extrafusal muscle fibers (which is the basis of the name of the disorder), radial arrangement of sarcoplasmic strands around the central nuclei, and predominance and hypotrophy of type 1 fibers (summary by Bitoun et al., 2005). Genetic Heterogeneity of Centronuclear Myopathy Centronuclear myopathy is a genetically heterogeneous disorder. See also X-linked CNM (CNMX; 310400), caused by mutation in the MTM1 gene (300415) on chromosome Xq28; CNM2 (255200), caused by mutation in the BIN1 gene (601248) on chromosome 2q14; CNM4 (614807), caused by mutation in the CCDC78 gene (614666) on chromosome 16p13; CNM5 (615959), caused by mutation in the SPEG gene (615950) on chromosome 2q35; and CNM6 (617760), caused by mutation in the ZAK gene (609479) on chromosome 2q31. The mutation in the MYF6 gene that was reported to cause a form of CNM, formerly designated CNM3, has been reclassified as a variant of unknown significance; see 159991.0001. Some patients with mutation in the RYR1 gene (180901) have findings of centronuclear myopathy on skeletal muscle biopsy (see 255320).

The Meier-Gorlin syndrome is a rare disorder characterized by severe intrauterine and postnatal growth retardation, microcephaly, bilateral microtia, and aplasia or hypoplasia of the patellae (summary by Shalev and Hall, 2003). While almost all cases have primordial dwarfism with substantial prenatal and postnatal growth retardation, not all cases have microcephaly, and microtia and absent/hypoplastic patella are absent in some. Despite the presence of microcephaly, intellect is usually normal (Bicknell et al., 2011). Genetic Heterogeneity of Meier-Gorlin Syndrome Most forms of Meier-Gorlin syndrome are autosomal recessive disorders, including Meier-Gorlin syndrome-1; Meier-Gorlin syndrome-2 (613800), caused by mutation in the ORC4 gene (603056) on chromosome 2q23; Meier-Gorlin syndrome-3 (613803), caused by mutation in the ORC6 gene (607213) on chromosome 16q11; Meier-Gorlin syndrome-4 (613804), caused by mutation in the CDT1 gene (605525) on chromosome 16q24; Meier-Gorlin syndrome-5 (613805), caused by mutation in the CDC6 gene (602627) on chromosome 17q21; Meier-Gorlin syndrome-7 (617063), caused by mutation in the CDC45L gene (603465) on chromosome 22q11; and Meier-Gorlin syndrome-8 (617564), caused by mutation in the MCM5 gene (602696) on chromosome 22q12. An autosomal dominant form of the disorder, Meier-Gorlin syndrome-6 (616835), is caused by mutation in the GMNN gene (602842) on chromosome 6p22.

Ehlers-Danlos syndrome spondylodysplastic type 1 (EDSSPD1) is characterized by short stature, developmental anomalies of the forearm bones and elbow, and bowing of extremities, in addition to the classic stigmata of Ehlers-Danlos syndrome, including joint laxity, skin hyperextensibility, and poor wound healing. Significant developmental delay is not a consistent feature (Guo et al., 2013). Genetic Heterogeneity of Ehlers-Danlos Syndrome, Spondylodysplastic Type See EDSSPD2 (615349), caused by mutation in the B3GALT6 gene (615291), and EDSSPD3 (612350), caused by mutation in the SLC39A13 gene (608735).

Hypomyelinating leukodystrophy-17 (HLD17) is an autosomal recessive neurodevelopmental disorder characterized by poor, if any, development apparent from infancy. Affected individuals never learn to walk or speak, and have early-onset multifocal seizures, spasticity, poor overall growth, and microcephaly (up to -10 SD). Brain imaging shows multiple abnormalities, including cerebral and cerebellar atrophy, thin corpus callosum, abnormal signals in the basal ganglia, and features suggesting hypo- or demyelination. Some patients may die in childhood (summary by Shukla et al., 2018). For a general phenotypic description and a discussion of genetic heterogeneity of hypomyelinating leukodystrophy, see 312080.

Hyperekplexia-4 is an autosomal recessive severe neurologic disorder apparent at birth. Affected infants have extreme hypertonia and appear stiff and rigid. They have little if any development, poor or absent visual contact, and no spontaneous movement, consistent with an encephalopathy. Some patients have early-onset refractory seizures, and many have inguinal or umbilical hernia. Most patients die in the first months of life due to respiratory failure or other complications (summary by Piard et al., 2018). For a general description and a discussion of genetic heterogeneity of hyperekplexia, see HKPX1 (149400).

Lethal congenital contracture syndrome-5 (LCCS5) is an autosomal recessive disorder characterized by decreased fetal movements, joint contractures, hypotonia, skeletal abnormalities with thin bones, and brain and retinal hemorrhages (Koutsopoulos et al., 2013). For a general phenotypic description and a discussion of genetic heterogeneity of LCCS, see LCCS1 (253310).

Autosomal dominant limb-girdle muscular dystrophy is characterized by proximal and/or distal muscle weakness and atrophy. The age at onset is variable and can range from the first to the sixth decade, although later onset is less common. Most patients present with proximal muscle weakness that progresses to distal involvement, but some can present with distal impairment. The severity is variable: patients with a more severe phenotype can lose ambulation after several decades and have facial weakness with bulbar and respiratory involvement. Muscle biopsy shows dystrophic changes with protein aggregates, myofibrillar degeneration, and rimmed vacuoles (summary by Ruggieri et al., 2015). Genetic Heterogeneity of Autosomal Dominant Limb-Girdle Muscular Dystrophy Other forms of autosomal dominant LGMD include LGMDD2 (608423), previously LGMD1F, caused by mutation in the TNPO3 gene (610032) on chromosome 7q32; LGMDD3 (609115), previously LGMD1G, caused by mutation in the HNRNPDL gene (607137) on chromosome 4q21; and LGMDD4 (618129), previously LGMD1I, caused by mutation in the CAPN3 gene (114240) on chromosome 15q15. For a discussion of autosomal recessive LGMD, see 253600.

MFN2 hereditary motor and sensory neuropathy (MFN2-HMSN) is a classic axonal peripheral sensorimotor neuropathy, inherited in either an autosomal dominant (AD) manner (~90%) or an autosomal recessive (AR) manner (~10%). MFN2-HMSN is characterized by more severe involvement of the lower extremities than the upper extremities, distal upper-extremity involvement as the neuropathy progresses, more prominent motor deficits than sensory deficits, and normal (>42 m/s) or only slightly decreased nerve conduction velocities (NCVs). Postural tremor is common. Median onset is age 12 years in the AD form and age eight years in the AR form. The prevalence of optic atrophy is approximately 7% in the AD form and approximately 20% in the AR form.

Proteasome-associated autoinflammatory syndrome-1 (PRAAS1) is an autosomal recessive disorder characterized by early childhood onset of annular erythematous plaques on the face and extremities with subsequent development of partial lipodystrophy and laboratory evidence of immune dysregulation. More variable features include recurrent fever, severe joint contractures, muscle weakness and atrophy, hepatosplenomegaly, basal ganglia calcifications, and microcytic anemia (summary by Agarwal et al., 2010; Kitamura et al., 2011; Arima et al., 2011). This disorder encompasses Nakajo-Nishimura syndrome (NKJO); joint contractures, muscular atrophy, microcytic anemia, and panniculitis-induced lipodystrophy (JMP syndrome); and chronic atypical neutrophilic dermatosis with lipodystrophy and elevated temperature syndrome (CANDLE). Among Japanese patients, this disorder is best described as Nakajo-Nishimura syndrome, since both Nakajo (1939) and Nishimura et al. (1950) contributed to the original phenotypic descriptions. Genetic Heterogeneity of Proteasome-Associated Autoinflammatory Syndrome See also PRAAS2 (618048), caused by mutation in the POMP gene (613386) on chromosome 13q12; PRAAS3 (617591), caused by mutation in the PSMB4 gene (602177) on chromosome 1q21; PRAAS4 (619183), caused by mutation in the PSMG2 gene (609702) on chromosome 18p11; PRAAS5 (619175), caused by mutation in the PSMB10 gene (176847) on chromosome 16q22; and PRAAS6 (620796), caused by mutation in the PSMB9 gene (177045) on chromosome 6p21.

Proteasome-associated autoinflammatory syndrome-3 (PRAAS3) is an autosomal recessive syndrome with onset in early infancy. Affected individuals present with nodular dermatitis, recurrent fever, myositis, panniculitis-induced lipodystrophy, lymphadenopathy, and dysregulation of the immune response, particularly associated with abnormal type I interferon-induced gene expression patterns. Additional features are highly variable, but may include joint contractures, hepatosplenomegaly, anemia, thrombocytopenia, recurrent infections, autoantibodies, and hypergammaglobulinemia. Some patients may have intracranial calcifications (summary by Brehm et al., 2015). For a discussion of genetic heterogeneity of PRAAS, see PRAAS1 (256040).

Pontocerebellar hypoplasia type 1D (PCH1D) is a severe autosomal recessive neurologic disorder characterized by severe hypotonia and a motor neuronopathy apparent at birth or in infancy. Patients have respiratory insufficiency, feeding difficulties, and severely delayed or minimal gross motor development. Other features may include eye movement abnormalities, poor overall growth, contractures. Brain imaging shows progressive cerebellar atrophy with relative sparing of the brainstem (summary by Burns et al., 2018). For a general phenotypic description and a discussion of genetic heterogeneity of PCH, see PCH1A (607596).

Congenital hypomyelinating neuropathy-3 is an autosomal recessive neurologic disorder characterized by onset of neurogenic muscle impairment in utero. Affected individuals present at birth with severe hypotonia, often causing respiratory insufficiency or failure and inability to swallow or feed properly. They have profoundly impaired psychomotor development and may die in infancy or early childhood. Those that survive are unable to sit or walk. Sural nerve biopsy shows hypomyelination of the nerve fibers, and brain imaging often shows impaired myelination and cerebral and cerebellar atrophy. Nerve conduction velocities are severely decreased (about 10 m/s) or absent due to improper myelination (summary by Vallat et al., 2016 and Low et al., 2018). For a discussion of genetic heterogeneity of CHN, see CHN1 (605253).

Developmental and epileptic encephalopathy-68 (DEE68) is an autosomal recessive neurologic disorder characterized by onset of twitching and/or myoclonic jerks in infancy. The disorder progresses to refractory generalized tonic-clonic seizures, often resulting in status epilepticus, loss of developmental milestones, and early death. Other features include delayed development, axial hypotonia, spasticity of the limbs, and clonus. Brain imaging may show cortical atrophy (summary by Barel et al., 2017). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

SMALED2B is a severe neuromuscular disorder with onset in utero. Affected individuals show decreased fetal movements and are usually born with congenital contractures consistent with arthrogryposis multiplex congenita (AMC). After birth, they have severe hypotonia and muscle atrophy as well as respiratory insufficiency due to muscle weakness. Some patients may have dysmorphic facial features and/or abnormalities on brain imaging. Many patients die in early childhood (summary by Storbeck et al., 2017) For discussion of genetic heterogeneity of lower extremity-predominant spinal muscular atrophy, see SMALED1 (158600).

Developmental and epileptic encephalopathy-70 (DEE70) is neurologic disorder characterized by the onset of epileptic spasms or seizures in the first months of life. EEG may show hypsarrhythmia, consistent with a clinical diagnosis of West syndrome. Affected individuals show severely delayed psychomotor development with impaired or absent walking and language skills; intellectual impairment ranges from moderate to severe (summary by Hamada et al., 2018). For a discussion of genetic heterogeneity of DEE, see 308350.

A rare genetic metabolic liver disease with characteristics of progressive neurodegeneration, cutaneous abnormalities including varying degrees of ichthyosis or seborrheic dermatitis, and systemic iron overload. Patients manifest with infantile-onset seizures, encephalopathy, abnormal eye movements, axial hypotonia with peripheral hypertonia, brisk reflexes, cortical blindness and deafness, myoclonus and hepato/splenomegaly, as well as oral manifestations including microdontia, widely spaced and pointed teeth with delayed eruption and gingival overgrowth.

The fetal akinesia deformation sequence (FADS) refers to a clinically and genetically heterogeneous constellation of features including fetal akinesia, intrauterine growth retardation, arthrogryposis, and developmental anomalies, including lung hypoplasia, cleft palate, and cryptorchidism (Vogt et al., 2009). It shows phenotypic overlap with the lethal form of multiple pterygium syndrome (see 253290). For a general phenotypic description and a discussion of genetic heterogeneity of FADS, see 208150.

Congenital myasthenic syndrome-25 (CMS25) is an autosomal recessive neuromuscular disorder characterized by hypotonia and generalized muscle weakness apparent from birth. Affected individuals have feeding difficulties and delayed motor development, usually never achieving independent ambulation. Additional variable features include eye movement abnormalities, joint contractures, and rigid spine. Pyridostigmine treatment may be partially effective (summary by Shen et al., 2017). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

Menke-Hennekam syndrome-1 (MKHK1) is a congenital disorder characterized by variable impairment of intellectual development and facial dysmorphisms. Feeding difficulties, autistic behavior, recurrent upper airway infections, hearing impairment, short stature, and microcephaly are also frequently seen. Although mutations in the same gene cause Rubinstein-Taybi syndrome-1 (RSTS1; 180849), patients with MKHK1 do not resemble the striking phenotype of RSTS1. Genetic Heterogeneity of Menke-Hennekam Syndrome Menke-Hennekam syndrome-2 (MKHK2; 618333) is caused by heterozygous mutation in exons 30 or 31 of the EP300 gene (602700). Mutation elsewhere in that gene results in RSTS2 (613684).

Microcephaly, growth deficiency, seizures, and brain malformations (MIGSB) is a severe autosomal recessive disorder characterized by intrauterine growth retardation, postnatal growth deficiency with severe microcephaly, and poor or absent psychomotor development. Additional features include optic atrophy, early-onset seizures, dysmorphic facial features, and brain malformations, such as partial agenesis of the corpus callosum and simplified gyration (summary by Shaheen et al., 2015).

Developmental and epileptic encephalopathy-73 (DEE73) is a neurologic disorder characterized by the onset of refractory seizures in the first months of life. Affected individuals meet almost no developmental milestones: they have hypotonia and are unable to walk, speak, or feed properly. They have poor overall growth with small head circumference and dysmorphic facial features. Additional manifestations include cortical visual impairment with roving eye movements and variable hearing loss (summary by Edvardson et al., 2019). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Combined oxidative phosphorylation deficiency-39 (COXPD39) is an autosomal recessive multisystem disorder resulting from a defect in mitochondrial energy metabolism. Affected individuals show global developmental delay, sometimes with regression after normal early development, axial hypotonia with limb spasticity or abnormal involuntary movements, and impaired intellectual development with poor speech. More variable features may include hypotonia, seizures, and features of Leigh syndrome (256000) on brain imaging. There are variable deficiencies of the mitochondrial respiratory chain enzyme complexes in patient tissues (summary by Glasgow et al., 2017). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Hypomyelinating leukodystrophy-18 (HLD18) is an autosomal recessive neurologic disorder characterized by onset of global developmental delay usually in early infancy. Affected individuals have very poor psychomotor development, including inability to sit or walk independently in the more severe cases, as well as poor or absent speech, dystonia, and spasticity. A subset of patients may develop seizures. Brain imaging shows hypomyelinating leukodystrophy affecting various brain regions; some patients may also have progressive atrophy of the corpus callosum, thalami, and cerebellum (summary by Pant et al., 2019). For a general phenotypic description and a discussion of genetic heterogeneity of hypomyelinating leukodystrophy, see 312080.

Congenital myopathy-14 (CMYO14) is an autosomal recessive skeletal muscle disorder characterized by onset of severe muscle weakness apparent at birth and sometimes in utero. Affected infants have difficulty breathing independently and usually require mechanical ventilation for variable lengths of time. Other features include delayed motor development with delayed walking, hypo- or areflexia, and high-arched palate. Skeletal muscle biopsy shows variation in fiber size with specific atrophy of the fast-twitch type II fibers. Cardiac muscle is not affected (summary by Ravenscroft et al., 2018). For a discussion of genetic heterogeneity of congenital myopathy, see CMYO1A (117000).

Contractures, pterygia, and spondylocarpotarsal fusion syndrome-1B (CPSFS1B) is characterized by contractures of proximal and distal joints, pterygia involving the neck, elbows, fingers, and/or knees, and variable vertebral, carpal, and tarsal fusions. Inter- and intrafamilial variability has been observed (Cameron-Christie et al., 2018). An autosomal dominant form of contractures, pterygia, and spondylocarpotarsal fusion syndrome (CPSFS1A; 178110) is caused by heterozygous mutation in the MYH3 gene.

Myogenic-type arthrogryposis multiplex congenita-3 (AMC3) is an autosomal recessive disorder characterized by decreased fetal movements, hypotonia, variable skeletal defects, including clubfoot and scoliosis, and delayed motor milestones with difficulty walking (summary by Baumann et al., 2017).

Congenital myopathy-16 (CMYO16) is an autosomal dominant muscle disorder characterized by onset of hypotonia and tremor in infancy. Patients have mildly delayed walking, unsteady gait, proximal muscle weakness, and a high-frequency tremor of the limbs. Some may develop secondary mild contractures or spinal deformities. Cognition is normal and the disease course tends to stabilize after adolescence (summary by Stavusis et al., 2019). For a discussion of genetic heterogeneity of congenital myopathy, see CMYO1A (117000).

Siddiqi syndrome (SIDDIS) is an autosomal recessive disorder characterized by global developmental delay, early-onset progressive sensorineural hearing impairment, regression of motor skills, dystonia, poor overall growth, and low body mass index (BMI). More variable features may include ichthyosis-like skin abnormalities or sensory neuropathy (summary by Zazo Seco et al., 2017).

Halperin-Birk syndrome (HLBKS) is an autosomal recessive neurodevelopmental disorder characterized by structural brain defects, spastic quadriplegia with multiple contractures, profound developmental delay, seizures, dysmorphism, cataract, and optic nerve atrophy. Death occurs in early childhood (Halperin et al., 2019).

Zimmermann-Laband syndrome-3 (ZLS3) is characterized by developmental delay, intellectual disability, coarse face, gingival hyperplasia, and nail hypoplasia/aplasia (Bauer et al., 2019). For a general phenotypic description and a discussion of genetic heterogeneity of Zimmermann-Laband syndrome, see ZLS1 (135500).

Female-restricted Wieacker-Wolff syndrome (WRWFFR) is an X-linked dominant syndromic form of neurogenic arthrogryposis multiplex congenita (AMC) with central and peripheral nervous system involvement. Affected individuals have decreased fetal movements causing the development of contractures in utero and resulting in AMC and diffuse contractures involving the large and small joints apparent at birth. There is global developmental delay with difficulty walking or inability to walk, hypotonia that often evolves to spasticity, and variably impaired intellectual development with poor or absent speech and language. Dysmorphic facial features, including hypotonic facies, ptosis, microretrognathia, and small mouth, are seen in most patients. Seizures are uncommon; some patients have evidence of a peripheral motor neuropathy with distal muscle weakness. The level of X inactivation in lymphocytes and fibroblasts is often skewed, but may not predict the severity of the phenotype. Most cases occur sporadically; rare X-linked dominant inheritance has been reported in families (summary by Frints et al., 2019).

NESCAV syndrome (NESCAVS) is a neurodegenerative disorder characterized by onset of features in infancy or early childhood. Affected individuals show global developmental delay with delayed walking or difficulty walking due to progressive spasticity mainly affecting the lower limbs and often leading to loss of independent ambulation. There is variably impaired intellectual development, speech delay, and learning disabilities and/or behavioral abnormalities. Additional features may include cortical visual impairment, often associated with optic atrophy, axonal peripheral neuropathy, seizures, dysautonomia, ataxia, and dystonia. Brain imaging often shows progressive cerebellar atrophy and thin corpus callosum. Some patients may show developmental regression, particularly of motor skills. The phenotype and presentation are highly variable (summary by Nemani et al., 2020).

Autosomal dominant chromosome 1p36.33 duplication syndrome is a severe multisystemic disorder characterized by neonatal respiratory insufficiency, hypotonia, and cardiomyopathy, resulting in death in the first weeks of life. Affected infants may also have seizures, contractures, and corneal opacities. Brain imaging shows variable anomalies, such as white matter changes, and laboratory studies suggest that the phenotype results from metabolic defects in mitochondrial and cholesterol homeostasis (summary by Gunning et al., 2020).

Permanent neonatal diabetes mellitus-2 (PNDM2) is characterized by onset of insulin-requiring hyperglycemia within the first months of life that requires insulin therapy throughout life. Some patients additionally have marked developmental delay, muscle weakness, and epilepsy (Gloyn et al., 2004). The triad of developmental delay, epilepsy, and neonatal diabetes is known as DEND (Shimomura et al., 2007). Proks et al. (2006) stated that heterozygous activating mutations in KCNJ11 are the most common cause of PNDM and account for 26 to 64% of cases, and that neurologic features are found in 20% of patients with KCNJ11 mutations. For a discussion of genetic heterogeneity of permanent neonatal diabetes mellitus, see PNDM1 (606176).

Microcephaly, developmental delay, and brittle hair syndrome (MDBH) is a multisystem disorder with clinical variability. Affected individuals show cognitive and motor disabilities, as well as some degree of fine, brittle hair with microscopic shaft abnormalities. Other shared features include failure to thrive in early childhood and short stature, with some patients exhibiting feeding difficulties and hepatic steatosis (Kuo et al., 2019).

Mandibuloacral dysplasia with type A lipodystrophy (MADA) is an autosomal recessive disorder characterized by growth retardation, craniofacial anomalies with mandibular hypoplasia, skeletal abnormalities with progressive osteolysis of the distal phalanges and clavicles, and pigmentary skin changes. The lipodystrophy is characterized by a marked acral loss of fatty tissue with normal or increased fatty tissue in the neck and trunk. Some patients may show progeroid features. Metabolic complications can arise due to insulin resistance and diabetes (Young et al., 1971; Simha and Garg, 2002; summary by Garavelli et al., 2009). See also MAD type B (MADB; 608612), which is caused by mutation in the ZMPSTE24 gene (606480).

Arthrogryposis multiplex congenita-5 (AMC5) is an autosomal recessive disorder characterized by severe joint contractures apparent at birth. Affected individuals usually have hypertonia and abnormal movements suggestive of dystonia, as well as feeding and/or breathing difficulties. More variable features may include poor overall growth, strabismus, dysmorphic facies, and global developmental delay with impaired speech (summary by Kariminejad et al., 2017).

Neurodevelopmental disorder with progressive spasticity and brain white matter abnormalities (NEDSWMA) is an autosomal recessive disorder characterized by impaired psychomotor development apparent in infancy. Affected individuals show poor overall growth, progressive microcephaly, and axial hypotonia, with later onset of spasticity. The disorder is progressive. Some patients show normal early development, but later have regression of motor, cognitive, and language skills. More variable features include seizures, joint contractures, ocular disturbances, episodic respiratory failure, and nonspecific dysmorphic facial features. The intellectual impairment is variable, ranging from poor visual contact with inability to walk or speak to milder intellectual disability with the ability to say some words. Brain imaging shows variable white matter abnormalities, including thin corpus callosum and poor myelination (summary by Husain et al., 2020).

Neurodevelopmental disorder with microcephaly, seizures, and brain atrophy (NEDMISB) is an autosomal recessive disorder characterized by severe global developmental delay, developmental regression with loss of milestones, severe microcephaly, and brain abnormalities, primarily cerebral atrophy and hypoplasia of the corpus callosum. Affected individuals develop seizures in the first year of life; eventually they are unable to sit, feed, or communicate, and may be unresponsive to stimuli. Other features include muscle weakness, spasticity with hyperreflexia, irritability, and contractures (Coulter et al., 2020).

Neurodevelopmental disorder with microcephaly, language delay, and gait abnormalities (NEDMILG) is an autosomal recessive disorder characterized by global developmental delay apparent in infancy. Affected individuals have delayed walking with variable gait abnormalities, impaired intellectual development with poor or absent speech and language, and progressive microcephaly. More variable features include hypotonia, early-onset seizures, and a peripheral demyelinating or axonal peripheral sensorimotor neuropathy. The disease follows a neurodegenerative course in many patients; clinical features suggest involvement of both the central and peripheral nervous systems (Manole et al., 2020).

Developmental and epileptic encephalopathy-89 (DEE89) is a severe autosomal recessive disorder characterized by profound global developmental delay with impaired intellectual development, absent speech, inability to sit or walk due to axial hypotonia and spastic quadriparesis, and onset of seizures in the first days or months of life. EEG shows suppression-burst pattern or hypsarrhythmia, consistent with DEE or a clinical diagnosis of West syndrome. More variable features include joint contractures with foot deformities, dysmorphic facial features with cleft palate, and omphalocele. Affected individuals have poor motor skills, poor eye contact, and lack of language development; some die in infancy or early childhood. Brain imaging may be normal or show nonspecific abnormalities (summary by Chatron et al., 2020).

Mandibuloacral dysplasia progeroid syndrome (MDPS) is an autosomal recessive severe laminopathy-like disorder characterized by growth retardation, bone resorption, arterial calcification, renal glomerulosclerosis, and hypertension (Elouej et al., 2020).

Congenital muscular dystrophies resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239) are characterized by early onset of muscle weakness, usually before ambulation is achieved; intellectual disability mild brain anomalies are variable (Balci et al., 2005; Godfrey et al., 2007). Congenital muscular dystrophy-dystroglycanopathies with or without impaired intellectual development (type B) represent the intermediate range of the spectrum of dystroglycanopathies. They are less severe than muscular dystrophy-dystroglycanopathy with brain and eye anomalies (type A; see MDDGA1, 236670), previously designated Walker-Warburg syndrome (WWS) or muscle-eye-brain disease (MEB), and more severe than limb-girdle muscular dystrophy-dystroglycanopathy (type C; see MDDGC1, 609308). Genetic Heterogeneity of Congenital Muscular Dystrophy-Dystroglycanopathy with or without Impaired Intellectual Development (Type B) Congenital muscular dystrophy with impaired intellectual development due to defective glycosylation of DAG1 is genetically heterogeneous. See also MDDGB2 (613156), caused by mutation in the POMT2 gene (607439); MDDGB3 (613151), caused by mutation in the POMGNT1 gene (606822); MDDGB4 (613152), caused by mutation in the FKTN gene (607440); MDDGB5 (616612), caused by mutation in the FKRP gene (606596); MDDGB6 (608840), caused by mutation in the LARGE gene (603590); MDDGB14 (615351), caused by mutation in the GMPPB gene (615320); and MDDGB15 (618992), caused by mutation in the DPM3 gene (605951).

Blepharophimosis-impaired intellectual development syndrome (BIS) is a congenital disorder characterized by a distinct facial appearance with blepharophimosis and global development delay. Affected individuals have delayed motor skills, sometimes with inability to walk, and impaired intellectual development with poor or absent speech; some patients show behavioral abnormalities. There are recognizable facial features, including epicanthal folds, sparse eyebrows, broad nasal bridge, short nose with downturned tip, and open mouth with thin upper lip. Other more variable features include distal skeletal anomalies, feeding difficulties with poor growth, respiratory infections, and hypotonia with peripheral spasticity (summary by Cappuccio et al., 2020).

Olmsted syndrome-1 (OLMS1) is a rare congenital disorder characterized by bilateral mutilating palmoplantar keratoderma (PPK) and periorificial keratotic plaques with severe pruritus of lesions. Diffuse alopecia, constriction of digits, and onychodystrophy have also been reported. Infections and squamous cell carcinomas can arise on the keratotic areas (summary by Lin et al., 2012). The digital constriction ('pseudoainhum') may progress to autoamputation of fingers and toes (Olmsted, 1927). Genetic Heterogeneity of Olmsted Syndrome Olmsted syndrome-2 (OLMS2; 619208) is caused by mutation in the PERP gene (609301) on chromosome 6q23. An X-linked form of Olmsted syndrome (OLMSX; 300918) is caused by mutation in the MBTPS2 gene (300294) on chromosome Xp22.

Proteasome-associated autoinflammatory syndrome-4 (PRAAS4) is an autosomal recessive immunologic disorder characterized by onset of panniculitis and erythematous skin lesions in early infancy. Additional features include hepatosplenomegaly, lymphadenopathy, fever, generalized lipodystrophy, myositis, and joint contractures, as well as delayed motor and speech development. Autoimmune features, such as hemolytic anemia, may also occur. Laboratory studies show elevation of acute phase reactants and abnormal activation of the type I interferon response. Treatment with the JAK (see 147795) inhibitor ruxolitinib may result in clinical improvement (summary by de Jesus et al., 2019). For a discussion of genetic heterogeneity of PRAAS, see PRAAS1 (256040).

Autosomal recessive distal hereditary motor neuronopathy-7 (HMNR7) is characterized by onset of lower leg weakness in the first decade. Affected individuals have difficulty climbing stairs and problems standing on the heels. Some patients have later onset well into the adult years. Most patients have foot deformities, and some may have leg muscle atrophy. The disorder is slowly progressive and often involves the upper limbs. Muscle biopsy and electrophysiologic studies are consistent with both a myopathic process and an axonal motor neuropathy. Sensory abnormalities are not typically present, and patients remain ambulatory. The phenotype shows phenotypic overlap with distal hereditary motor neuropathy, but can distinguished by the presence of myopathic features (summary by Deschauer et al., 2021 and Pagnamenta et al., 2021). For a discussion of genetic heterogeneity of autosomal recessive HMN, see HMNR1 (604320).

Mitochondrial complex II deficiency nuclear type 4 (MC2DN4) is a severe autosomal recessive disorder characterized by early-onset progressive neurodegeneration with leukoencephalopathy. Acute episodes of neurodegeneration are often triggered by catabolic stress such as infection or fasting.

Neurodevelopmental disorder with dysmorphic facies and cerebellar hypoplasia (NEDFACH) is an autosomal recessive disorder characterized by global developmental delay and intellectual disability. The phenotype is variable: more severely affected individuals have poor overall growth with microcephaly, delayed walking, spasticity, and poor or absent speech, whereas others may achieve more significant developmental milestones and even attend special schooling. Brain imaging shows abnormalities of the cerebellum, most commonly cerebellar hypoplasia, although other features, such as thin corpus callosum and delayed myelination, may also be present. Dysmorphic facial features include sloping forehead, upslanting palpebral fissures, and hypertelorism. Additional more variable manifestations may include cardiac ventricular septal defect, spasticity, cataracts, optic nerve hypoplasia, seizures, and joint contractures (summary by Van Bergen et al., 2020).

Growth restriction, hypoplastic kidneys, alopecia, and distinctive facies (GKAF) is characterized by microcephaly, congenital alopecia, distinctive craniofacial features, severe congenital sensorineural hearing loss, global developmental delay, hydrocephalus, hypoplastic kidneys with renal insufficiency, genital hypoplasia, and early mortality (Ito et al., 2018).

Hypomyelinating leukodystrophy-22 (HLD22) is a neurologic disorder characterized by global developmental delay with mildly impaired intellectual development and marked motor impairment with limited or no ability to walk and dysarthria. Affected individuals have limb spasticity with pyramidal signs, as well as nystagmus, hypermetropia, and astigmatism. Brain imaging shows hypomyelination and a delay in myelination, although serial imaging shows some progress in both the central and peripheral white matter regions (Riedhammer et al., 2021). For a general phenotypic description and a discussion of genetic heterogeneity of HLD, see 312080.

Neurodevelopmental disorder with hypotonia, facial dysmorphism, and brain abnormalities (NEDHFBA) is an autosomal recessive neurologic syndrome characterized by global developmental delay with severely impaired intellectual development, hypotonia and muscle weakness, often resulting in the inability to walk or sit, and characteristic coarse facial features. Additional features include feeding difficulties, respiratory distress, scoliosis, poor visual function, and rotary nystagmus. Brain imaging shows variable abnormalities, including enlarged ventricles, decreased white matter volume, white matter changes, thin corpus callosum, and cerebellar hypoplasia (summary by Loddo et al., 2020).

Otospondylomegaepiphyseal dysplasia (OSMED) is characterized by sensorineural hearing loss, enlarged epiphyses, disproportionate shortness of the limbs, abnormalities in vertebral bodies, and typical facial features (summary by Harel et al., 2005).

Short-rib thoracic dysplasia-21 (SRTD21) is characterized by rhizomelic limb shortening with bowing of long bones and metaphyseal abnormalities, narrow chest with short broad ribs, and trident pelvis. Other features include hypotonia and global developmental delay, with corpus callosum hypoplasia and cerebellar vermis abnormalities on brain imaging, which may show the 'molar tooth' sign (Hammarsjo et al., 2017). For a general phenotypic description and discussion of genetic heterogeneity of SRTD, see SRTD1 (208500). Mutation in the KIAA0753 gene also causes orofaciodigital syndrome (OFD15; 617127) and Joubert syndrome (JBTS28; 619476), phenotypes with features overlapping those of SRTD21.

Neurodevelopmental disorder with hypotonia and dysmorphic facies (NEDHYDF) is characterized by global developmental delay and hypotonia apparent from birth. Affected individuals have variably impaired intellectual development, often with speech delay and delayed walking. Seizures are generally not observed, although some patients may have single seizures or late-onset epilepsy. Most patients have prominent dysmorphic facial features. Additional features may include congenital cardiac defects (without arrhythmia), nonspecific renal anomalies, joint contractures or joint hyperextensibility, dry skin, and cryptorchidism. There is significant phenotypic variability in both the neurologic and extraneurologic manifestations (summary by Tan et al., 2022).

Hyaline fibromatosis syndrome (HFS) is characterized by hyaline deposits in the papillary dermis and other tissues. It can present at birth or in infancy with severe pain with movement, progressive joint contractures, and often with severe motor disability, thickened skin, and hyperpigmented macules/patches over bony prominences of the joints. Gingival hypertrophy, skin nodules, pearly papules of the face and neck, and perianal masses are common. Complications of protein-losing enteropathy and failure to thrive can be life threatening. Cognitive development is normal. Many children with the severe form (previously called infantile systemic hyalinosis) have a significant risk of morbidity or mortality in early childhood; some with a milder phenotype (previously called juvenile hyaline fibromatosis) survive into adulthood.

Schaaf-Yang syndrome (SYS) is a rare neurodevelopmental disorder that shares multiple clinical features with the genetically related Prader-Willi syndrome. It usually manifests at birth with muscular hypotonia in all and distal joint contractures in a majority of affected individuals. Gastrointestinal/feeding problems are particularly pronounced in infancy and childhood, but can transition to hyperphagia and obesity in adulthood. Respiratory distress is present in many individuals at birth, with approximately half requiring intubation and mechanical ventilation, and approximately 20% requiring tracheostomy. Skeletal manifestations such as joint contractures, scoliosis, and decreased bone mineral density are frequently observed. All affected individuals show developmental delay, resulting in intellectual disability of variable degree, from low-normal intelligence to severe intellectual disability. Other findings may include short stature, seizures, eye anomalies, and hypogonadism.

Carey-Fineman-Ziter syndrome-1 (CFZS1) is a multisystem congenital disorder characterized by hypotonia, Moebius sequence (bilateral congenital facial palsy with impairment of ocular abduction), Pierre Robin complex (micrognathia, glossoptosis, and high-arched or cleft palate), delayed motor milestones, and failure to thrive. More variable features include dysmorphic facial features, brain abnormalities, and intellectual disability. It has been postulated that many clinical features in CFZS1 may be secondary effects of muscle weakness during development or brainstem anomalies (summary by Pasetti et al., 2016). Di Gioia et al. (2017) determined that CFZS1 represents a slowly progressive congenital myopathy resulting from a defect in myoblast fusion. Genetic Heterogeneity of Carey-Fineman-Ziter Syndrome Carey-Fineman-Ziter syndrome-2 (CFZS2) is caused by mutation in the MYMX gene (619912) on chromosome 6p21.

A restrictive dermopathy that has material basis in homozygous or compound heterozygous mutation in the ZMPSTE24 gene on chromosome 1p34.

CLPB (caseinolytic peptidase B) deficiency is characterized by neurologic involvement and neutropenia, which can range from severe to mild. In severe CLPB deficiency, death usually occurs at a few months of age due to significant neonatal neurologic involvement (hyperekplexia or absence of voluntary movements, hypotonia or hypertonia, swallowing problems, respiratory insufficiency, and epilepsy) and severe neutropenia associated with life-threatening infections. Individuals with moderate CLPB deficiency present with neurologic abnormalities in infancy including hypotonia and feeding problems, and develop spasticity, a progressive movement disorder (ataxia, dystonia, and/or dyskinesia), epilepsy, and intellectual disability. Neutropenia is variable, but not life threatening. In those with mild CLPB deficiency there is no neurologic involvement, intellect is normal, neutropenia is mild and intermittent, and life expectancy is normal.

Bryant-Li-Bhoj neurodevelopmental syndrome-1 (BRYLIB1) is a highly variable phenotype characterized predominantly by moderate to severe global developmental delay with impaired intellectual development, poor or absent speech, and delayed motor milestones. Most patients have hypotonia, although some have peripheral hypertonia. Common features include abnormal head shape, variable dysmorphic facial features, oculomotor abnormalities, feeding problems, and nonspecific brain imaging abnormalities. Additional features may include hearing loss, seizures, short stature, and mild skeletal defects (summary by Bryant et al., 2020). Genetic Heterogeneity of Bryant-Li-Bhoj Neurodevelopmental Syndrome See also BRYLIB2 (619721), caused by heterozygous mutation in the H3F3B gene (601058).

Hypomyelinating leukodystrophy-24 (HLD24) is an autosomal dominant disorder characterized by global developmental delay and neurologic deterioration (Segawa et al., 2021). For a general phenotypic description and a discussion of genetic heterogeneity of HLD, see 312080.

Kagami-Ogata syndrome (KOS) is a rare imprinting disorder characterized prenatally by polyhydramnios, macrosomia, and placentomegaly. After birth, infants often have respiratory distress, feeding difficulties, and postnatal growth retardation. Thoracic abnormalities include small bell-shaped thorax, 'coat-hanger' ribs, narrow chest wall, and cardiac anomalies. Abdominal wall defects include omphalocele, diastasis recti, and inguinal hernias. Hepatoblastoma is present in some patients. Dysmorphic facial features include frontal bossing, depressed nasal bridge, hairy forehead, anteverted nares, micrognathia, and a short neck. Developmental findings include hypotonia, speech and/or motor delays, and normal to mildly impaired intellectual development (summary by Prasasya et al., 2020).

Mitochondrial complex II deficiency is an autosomal recessive multisystemic metabolic disorder with a highly variable phenotype. Some patients have multisystem involvement of the brain, heart, and muscle with onset in infancy, whereas others have only isolated cardiac or muscle involvement. Measurement of complex II activity in muscle is the most reliable means of diagnosis; however, there is no clear correlation between residual complex II activity and severity or clinical outcome. In some cases, treatment with riboflavin may have clinical benefit (summary by Jain-Ghai et al., 2013). Complex II, also known as succinate dehydrogenase, is part of the mitochondrial respiratory chain. Genetic Heterogeneity of Mitochondrial Complex II Deficiency See MC2DN2 (619166), caused by mutation in the SDHAF1 gene (612848) on chromosome 19q13; MC2DN3 (619167), caused by mutation in the SDHD gene (602690) on chromosome 11q23; and MC2DN4 (619224), caused by mutation in the SDHB gene (185470) on chromosome 1p36. Fullerton et al. (2020) reviewed the genetic basis of isolated mitochondrial complex II deficiency.

Neurodevelopmental disorder with spasticity, seizures, and brain abnormalities (NEDSSBA) is an autosomal recessive disorder characterized by global developmental delay apparent in infancy, axial hypotonia, peripheral spasticity, and early-onset seizures of various types and severity. Affected individuals have delayed walking or are unable to walk and show impaired intellectual development with poor or absent speech. Brain imaging may show developmental defects of the operculum, cerebellum, and corpus callosum. Death in early childhood may occur (Calame et al., 2021).

Neurodevelopmental disorder with hypotonia, language delay, and skeletal defects with or without seizures (NEDHLSS) is characterized by global developmental delay apparent from infancy. Affected individuals show severe hypotonia with delayed walking or inability to walk, poor or absent speech, and impaired intellectual development with behavioral abnormalities. Most patients have early-onset seizures, mild skeletal defects that are usually distal, and nonspecific dysmorphic features. More severely affected individuals have additional congenital abnormalities; however, cardiac involvement is rare (summary by Rodan et al., 2021).

Congenital myopathy-4B (CMYO4B) is an autosomal recessive disorder of the skeletal muscle characterized by the onset of muscle weakness in infancy or early childhood. The severity and pattern of muscle weakness varies, but most affected individuals show congenital contractures, delayed motor development, hypotonia, generalized muscle weakness, and weakness of the proximal limb muscles and neck muscles, resulting in difficulty walking or inability to walk. Affected individuals have respiratory insufficiency due to muscle weakness, which may be life-threatening. Other common features include myopathic facies, chest deformities, distal joint laxity, and scoliosis. Variable histologic findings on skeletal muscle biopsy are observed, including nemaline rods, type 1 fiber predomination, and centralized nuclei (Tan et al., 1999; Lehtokari et al., 2008). For a discussion of genetic heterogeneity of congenital myopathy, see CMYO1A (117000).

Developmental delay with hypotonia, myopathy, and brain abnormalities (DEDHMB) is an autosomal recessive disorder characterized by global developmental delay and muscle weakness apparent in infancy. Affected individuals show severe motor delay and may not achieve independent walking due to central hypotonia and skeletal muscle myopathy. Some have poor overall growth with microcephaly, subtle dysmorphic features, and delayed language acquisition. Brain imaging shows cerebral atrophy, thinning of the corpus callosum, and delayed myelination (Shamseldin et al., 2016; Kotecha et al., 2021).

Severe fetal congenital myopathy-22B (CMYO22B) is an autosomal recessive muscle disorder characterized by in utero onset of severe muscle weakness manifest as fetal akinesia. The pregnancies are often complicated by polyhydramnios, and affected individuals develop fetal hydrops with pulmonary hypoplasia, severe joint contractures, and generalized muscle hypoplasia. Those who are born have respiratory failure resulting in death. Dysmorphic facial features may be present. The features in these patients overlap with fetal akinesia deformation sequence (FADS; see 208150) and lethal congenital contractures syndrome (LCCS; see 253310) (Zaharieva et al., 2016). For a discussion of genetic heterogeneity of congenital myopathy, see CMYO1A (117000).

Autosomal dominant intellectual developmental disorder-73 (MRD73) is a highly variable neurodevelopmental disorder characterized by impaired intellectual development that ranges from mild to severe, speech delay, behavioral abnormalities, and nonspecific dysmorphic facial features (Janssen et al., 2022).