Chronic primary adrenal insufficiency (CPAI) is a chronic disorder of the adrenal cortex resulting in the inadequate production of glucocorticoid and mineralocorticoid hormones.

Cri-du-chat syndrome was first described by Lejeune et al. (1963) as a hereditary congenital syndrome associated with deletion of part of the short arm of chromosome 5. The deletions can vary in size from extremely small and involving only band 5p15.2 to the entire short arm. Although the majority of deletions arise as new mutations, approximately 12% result from unbalanced segregation of translocations or recombination involving a pericentric inversion in one of the parents.

Familial dysautonomia, which affects the development and survival of sensory, sympathetic, and parasympathetic neurons, is a debilitating disorder present from birth. Neuronal degeneration progresses throughout life. Affected individuals have gastrointestinal dysfunction, autonomic crises (i.e., hypertensive vomiting attacks), recurrent pneumonia, altered pain sensitivity, altered temperature perception, and blood pressure instability. Hypotonia contributes to delay in acquisition of motor milestones. Optic neuropathy results in progressive vision loss. Older individuals often have a broad-based and ataxic gait that deteriorates over time. Developmental delay / intellectual disability occur in about 21% of individuals. Life expectancy is decreased.

Pantothenate kinase-associated neurodegeneration (PKAN) is a type of neurodegeneration with brain iron accumulation (NBIA). The phenotypic spectrum of PKAN includes classic PKAN and atypical PKAN. Classic PKAN is characterized by early-childhood onset of progressive dystonia, dysarthria, rigidity, and choreoathetosis. Pigmentary retinal degeneration is common. Atypical PKAN is characterized by later onset (age >10 years), prominent speech defects, psychiatric disturbances, and more gradual progression of disease.

Pierre Robin sequence is a craniofacial anomaly comprising mandibular hypoplasia, cleft secondary palate, and glossoptosis leading to life-threatening obstructive apnea and feeding difficulties during the neonatal period (summary by Tan et al., 2013).

Prader-Willi syndrome (PWS) is characterized by severe hypotonia and feeding difficulties in early infancy, followed in later infancy or early childhood by excessive eating and gradual development of morbid obesity (unless eating is externally controlled). Motor milestones and language development are delayed. All individuals have some degree of cognitive impairment. A distinctive behavioral phenotype (with temper tantrums, stubbornness, manipulative behavior, and obsessive-compulsive characteristics) is common. Hypogonadism is present in both males and females and manifests as genital hypoplasia, incomplete pubertal development, and, in most, infertility. Short stature is common (if not treated with growth hormone); characteristic facial features, strabismus, and scoliosis are often present.

Angelman syndrome (AS) is characterized by severe developmental delay or intellectual disability, severe speech impairment, gait ataxia and/or tremulousness of the limbs, and unique behavior with an apparent happy demeanor that includes frequent laughing, smiling, and excitability. Microcephaly and seizures are also common. Developmental delays are first noted at around age six months; however, the unique clinical features of AS do not become manifest until after age one year.

Dubowitz syndrome (DS) is a rare multiple congenital syndrome characterized primarly by growth retardation, microcephaly, distinctive facial dysmorphism, cutaneous eczema, a mild to severe intellectual deficit and genital abnormalities.

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.

If untreated, young children with profound biotinidase deficiency usually exhibit neurologic abnormalities including seizures, hypotonia, ataxia, developmental delay, vision problems, hearing loss, and cutaneous abnormalities (e.g., alopecia, skin rash, candidiasis). Older children and adolescents with profound biotinidase deficiency often exhibit motor limb weakness, spastic paresis, and decreased visual acuity. Once vision problems, hearing loss, and developmental delay occur, they are usually irreversible, even with biotin therapy. Individuals with partial biotinidase deficiency may have hypotonia, skin rash, and hair loss, particularly during times of stress.

The most basic description of Moebius syndrome is a congenital facial palsy with impairment of ocular abduction. The facial nerve (cranial nerve VII) and abducens nerve (CN VI) are most frequently involved, but other cranial nerves may be involved as well. Other variable features include orofacial dysmorphism and limb malformations. Mental retardation has been reported in a subset of patients. Most cases of Moebius syndrome are sporadic, but familial occurrence has been reported (Verzijl et al., 2003). The definition of and diagnostic criteria for Moebius syndrome have been controversial and problematic. The syndrome has most frequently been confused with hereditary congenital facial paresis (HCFP; see 601471), which is restricted to involvement of the facial nerve and no other abnormalities. Verzijl et al. (2003) and Verzijl et al. (2005) concluded that HCFP and Moebius syndrome are distinct disorders, and that Moebius syndrome is a complex developmental disorder of the brainstem. Moebius syndrome was defined at the Moebius Syndrome Foundation Research Conference in 2007 as congenital, nonprogressive facial weakness with limited abduction of one or both eyes. Additional features can include hearing loss and other cranial nerve dysfunction, as well as motor, orofacial, musculoskeletal, neurodevelopmental, and social problems (summary by Webb et al., 2012). Kumar (1990) provided a review of Moebius syndrome, which was critiqued by Lipson et al. (1990). Briegel (2006) provided a review of Moebius sequence with special emphasis on neuropsychiatric findings.

Cohen syndrome is characterized by failure to thrive in infancy and childhood; truncal obesity in the teen years; early-onset hypotonia and developmental delays; microcephaly developing during the first year of life; moderate to profound psychomotor retardation; progressive retinochoroidal dystrophy and high myopia; neutropenia in many with recurrent infections and aphthous ulcers in some; a cheerful disposition; joint hypermobility; and characteristic facial features.

Freeman-Sheldon syndrome (FSS), or DA2A, is phenotypically similar to DA1. In addition to contractures of the hands and feet, FSS is characterized by oropharyngeal abnormalities, scoliosis, and a distinctive face that includes a very small oral orifice (often only a few millimeters in diameter at birth), puckered lips, and an H-shaped dimple of the chin; hence, FSS has been called 'whistling face syndrome.' The limb phenotypes of DA1 and FSS may be so similar that they can only be distinguished by the differences in facial morphology (summary by Bamshad et al., 2009). For a general phenotypic description and a discussion of genetic heterogeneity of distal arthrogryposis, see DA1 (108120).

The phenotype of acid sphingomyelinase deficiency (ASMD) occurs along a continuum. Individuals with the severe early-onset form, infantile neurovisceral ASMD, were historically diagnosed with Niemann-Pick disease type A (NPD-A). The later-onset, chronic visceral form of ASMD is also referred to as Niemann-Pick disease type B (NPD-B). A phenotype with intermediate severity is also known as chronic neurovisceral ASMD (NPD-A/B). The most common presenting symptom in NPD-A is hepatosplenomegaly, usually detectable by age three months; over time the liver and spleen become massive in size. Psychomotor development progresses no further than the 12-month level, after which neurologic deterioration is relentless. Failure to thrive typically becomes evident by the second year of life. A classic cherry-red spot of the macula of the retina, which may not be present in the first few months, is eventually present in all affected children. Interstitial lung disease caused by storage of sphingomyelin in pulmonary macrophages results in frequent respiratory infections and often respiratory failure. Most children succumb before the third year of life. NPD-B generally presents later than NPD-A, and the manifestations are less severe. NPD-B is characterized by progressive hepatosplenomegaly, gradual deterioration in liver and pulmonary function, osteopenia, and atherogenic lipid profile. No central nervous system (CNS) manifestations occur. Individuals with NPD-A/B have symptoms that are intermediate between NPD-A and NPD-B. The presentation in individuals with NPD-A/B varies greatly, although all are characterized by the presence of some CNS manifestations. Survival to adulthood can occur in individuals with NPD-B and NPD-A/B.

CMO type I deficiency is an autosomal recessive disorder caused by a defect in the penultimate biochemical step of aldosterone biosynthesis, the 18-hydroxylation of corticosterone (B) to 18-hydroxycorticosterone (18-OHB). This enzymatic defect results in decreased aldosterone and salt-wasting. In CMO I deficiency, aldosterone is undetectable, whereas its immediate precursor, 18-OHB, is low or normal. These patients have an increased ratio of corticosterone to 18-OHB (Portrat-Doyen et al., 1998). The CYP11B2 gene product also catalyzes the final step in aldosterone biosynthesis: the 18-oxidation of 18-OHB to aldosterone. A defect in that enzymatic step results in CMO type II deficiency (610600), an allelic disorder with an overlapping phenotype but distinct biochemical features. In CMO II deficiency, aldosterone can be low or normal, but at the expense of increased secretion of 18-OHB. These patients have a low ratio of corticosterone to 18-OHB (Portrat-Doyen et al., 1998).

ATP6V0A2-related cutis laxa is characterized by generalized cutis laxa, findings associated with generalized connective tissue disorder, developmental delays, and a variety of neurologic findings including abnormality on brain MRI. At birth, hypotonia, overfolded skin, and distinctive facial features are present and enlarged fontanelles are often observed. During childhood, the characteristic facial features and thick or coarse hair may become quite pronounced. The skin findings decrease with age, although easy bruising and Ehlers-Danlos-like scars have been described in some. In most (not all) affected individuals, cortical and cerebellar malformations are observed on brain MRI. Nearly all affected individuals have developmental delays, seizures, and neurologic regression.

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis is a progressive renal disorder characterized by excessive urinary Ca(2+) and Mg(2+) excretion. There is progressive loss of kidney function, and in about 50% of cases, the need for renal replacement therapy arises as early as the second decade of life (summary by Muller et al., 2006). Amelogenesis imperfecta may also be present in some patients (Bardet et al., 2016). A similar disorder with renal magnesium wasting, renal failure, and nephrocalcinosis (HOMG5; 248190) is caused by mutations in another tight-junction gene, CLDN19 (610036), and is distinguished by the association of severe ocular involvement. For a discussion of phenotypic and genetic heterogeneity of familial hypomagnesemia, see HOMG1 (602014).

Deficiency of argininosuccinate lyase (ASL), the enzyme that cleaves argininosuccinic acid to produce arginine and fumarate in the fourth step of the urea cycle, may present as a severe neonatal-onset form or a late-onset form: The severe neonatal-onset form is characterized by hyperammonemia within the first few days after birth that can manifest as increasing lethargy, somnolence, refusal to feed, vomiting, tachypnea, and respiratory alkalosis. Absence of treatment leads to worsening lethargy, seizures, coma, and even death. In contrast, the manifestations of late-onset form range from episodic hyperammonemia triggered by acute infection or stress to cognitive impairment, behavioral abnormalities, and/or learning disabilities in the absence of any documented episodes of hyperammonemia. Manifestations of ASL deficiency that appear to be unrelated to the severity or duration of hyperammonemic episodes: Neurocognitive deficiencies (attention-deficit/hyperactivity disorder, developmental delay, seizures, and learning disability). Liver disease (hepatitis, cirrhosis). Trichorrhexis nodosa (coarse brittle hair that breaks easily). Systemic hypertension.

The spectrum of propionic acidemia (PA) ranges from neonatal-onset to late-onset disease. Neonatal-onset PA, the most common form, is characterized by a healthy newborn with poor feeding and decreased arousal in the first few days of life, followed by progressive encephalopathy of unexplained origin. Without prompt diagnosis and management, this is followed by progressive encephalopathy manifesting as lethargy, seizures, or coma that can result in death. It is frequently accompanied by metabolic acidosis with anion gap, lactic acidosis, ketonuria, hypoglycemia, hyperammonemia, and cytopenias. Individuals with late-onset PA may remain asymptomatic and suffer a metabolic crisis under catabolic stress (e.g., illness, surgery, fasting) or may experience a more insidious onset with the development of multiorgan complications including vomiting, protein intolerance, failure to thrive, hypotonia, developmental delays or regression, movement disorders, or cardiomyopathy. Isolated cardiomyopathy can be observed on rare occasion in the absence of clinical metabolic decompensation or neurocognitive deficits. Manifestations of neonatal and late-onset PA over time can include growth impairment, intellectual disability, seizures, basal ganglia lesions, pancreatitis, and cardiomyopathy. Other rarely reported complications include optic atrophy, hearing loss, premature ovarian insufficiency, and chronic renal failure.

Holocarboxylase synthetase deficiency, a biotin-responsive multiple carboxylase deficiency (MCD), is characterized by metabolic acidosis, lethargy, hypotonia, convulsions, and dermatitis. Most patients present in the newborn or early infantile period, but some become symptomatic in the later infantile period (summary by Suzuki et al., 2005). Also see biotinidase deficiency (253260), another form of MCD with a later onset. Care must be taken to differentiate the inherited multiple carboxylase deficiencies from acquired biotin deficiencies, such as those that develop after excessive dietary intake of avidin, an egg-white glycoprotein that binds specifically and essentially irreversibly to biotin (Sweetman et al., 1981) or prolonged parenteral alimentation without supplemental biotin (Mock et al., 1981).

3-Methylcrotonylglycinuria is an autosomal recessive disorder of leucine catabolism. The clinical phenotype is highly variable, ranging from neonatal onset with severe neurologic involvement to asymptomatic adults. There is a characteristic organic aciduria with massive excretion of 3-hydroxyisovaleric acid and 3-methylcrotonylglycine, usually in combination with a severe secondary carnitine deficiency. MCC activity in extracts of cultured fibroblasts of patients is usually less than 2% of control (summary by Baumgartner et al., 2001). Also see 3-methylcrotonylglycinuria II (MCC2D; 210210), caused by mutation in the beta subunit of 3-methylcrotonyl-CoA carboxylase (MCCC2; 609014).

Hereditary folate malabsorption (HFM) is characterized by folate deficiency due to impaired intestinal folate absorption and impaired folate transport into the central nervous system. Findings include poor feeding, failure to thrive, and anemia. There can be leukopenia and thrombocytopenia, diarrhea and/or oral mucositis, hypoimmunoglobulinemia, and other immunologic dysfunction resulting in infections, most often Pneumocystis jirovecii pneumonia. Neurologic manifestations include developmental delays, cognitive and motor disorders, behavioral disorders, and seizures.

D-bifunctional protein deficiency is a disorder of peroxisomal fatty acid beta-oxidation. See also peroxisomal acyl-CoA oxidase deficiency (264470), caused by mutation in the ACOX1 gene (609751) on chromosome 17q25. The clinical manifestations of these 2 deficiencies are similar to those of disorders of peroxisomal assembly, including X-linked adrenoleukodystrophy (ALD; 300100), Zellweger cerebrohepatorenal syndrome (see 214100) and neonatal adrenoleukodystrophy (NALD; see 601539) (Watkins et al., 1995). DBP deficiency has been classified into 3 subtypes depending upon the deficient enzyme activity. Type I is a deficiency of both 2-enoyl-CoA hydratase and 3-hydroxyacyl-CoA dehydrogenase; type II is a deficiency of hydratase activity alone; and type III is a deficiency of dehydrogenase activity alone. Virtually all patients with types I, II, and III have a severe phenotype characterized by infantile-onset of hypotonia, seizures, and abnormal facial features, and most die before age 2 years. McMillan et al. (2012) proposed a type IV deficiency on the basis of less severe features; these patients have a phenotype reminiscent of Perrault syndrome (PRLTS1; 233400). Pierce et al. (2010) noted that Perrault syndrome and DBP deficiency overlap clinically and suggested that DBP deficiency may be underdiagnosed.

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.

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.

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

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.

Kabuki syndrome (KS) is characterized by typical facial features (long palpebral fissures with eversion of the lateral third of the lower eyelid; arched and broad eyebrows; short columella with depressed nasal tip; large, prominent, or cupped ears), minor skeletal anomalies, persistence of fetal fingertip pads, mild-to-moderate intellectual disability, and postnatal growth deficiency. Other findings may include: congenital heart defects, genitourinary anomalies, cleft lip and/or palate, gastrointestinal anomalies including anal atresia, ptosis and strabismus, and widely spaced teeth and hypodontia. Functional differences can include: increased susceptibility to infections and autoimmune disorders, seizures, endocrinologic abnormalities (including isolated premature thelarche in females), feeding problems, and hearing loss.

Peters plus syndrome is characterized by anterior chamber eye anomalies, short limbs with broad distal extremities, characteristic facial features, cleft lip/palate, and variable developmental delay / intellectual disability. The most common anterior chamber defect is Peters' anomaly, consisting of central corneal clouding, thinning of the posterior cornea, and iridocorneal adhesions. Cataracts and glaucoma are common. Developmental delay is observed in about 80% of children; intellectual disability can range from mild to severe.

A rare multiple congenital malformation syndrome with characteristics of blepharophimosis, ptosis, dental hypoplasia, hearing impairment and intellectual disability. Abnormal ears, microcephaly, and growth retardation have been reported occasionally. Male patients may show cryptorchidism and scrotal hypoplasia. Most reported cases are sporadic, except the original cases of Ohdo who described two affected sisters and a first cousin, suggesting autosomal recessive inheritance. Autosomal dominant, X-linked- and mitochondrial inheritance have also been suggested.

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.

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).

Cardiofaciocutaneous (CFC) syndrome is characterized by cardiac abnormalities (pulmonic stenosis and other valve dysplasias, septal defects, hypertrophic cardiomyopathy, rhythm disturbances), distinctive craniofacial appearance, and cutaneous abnormalities (including xerosis, hyperkeratosis, ichthyosis, keratosis pilaris, ulerythema ophryogenes, eczema, pigmented moles, hemangiomas, and palmoplantar hyperkeratosis). The hair is typically sparse, curly, fine or thick, woolly or brittle; eyelashes and eyebrows may be absent or sparse. Nails may be dystrophic or fast growing. Some form of neurologic and/or cognitive delay (ranging from mild to severe) is seen in all affected individuals. Neoplasia, mostly acute lymphoblastic leukemia, has been reported in some individuals.

3-hydroxyacyl-CoA dehydrogenase deficiency is an inherited condition that prevents the body from converting certain fats to energy, particularly during prolonged periods without food (fasting).\n\nInitial signs and symptoms of this disorder typically occur during infancy or early childhood and can include poor appetite, vomiting, diarrhea, and lack of energy (lethargy). Affected individuals can also have muscle weakness (hypotonia), liver problems, low blood glucose (hypoglycemia), and abnormally high levels of insulin (hyperinsulinism). Insulin controls the amount of glucose that moves from the blood into cells for conversion to energy. Individuals with 3-hydroxyacyl-CoA dehydrogenase deficiency are also at risk for complications such as seizures, life-threatening heart and breathing problems, coma, and sudden death. This condition may explain some cases of sudden infant death syndrome (SIDS), which is defined as unexplained death in babies younger than 1 year.\n\nProblems related to 3-hydroxyacyl-CoA dehydrogenase deficiency can be triggered by periods of fasting or by illnesses such as viral infections. This disorder is sometimes mistaken for Reye syndrome, a severe disorder that may develop in children while they appear to be recovering from viral infections such as chicken pox or flu. Most cases of Reye syndrome are associated with the use of aspirin during these viral infections.

Aromatic L-amino acid decarboxylase deficiency (AADCD) is an autosomal recessive inborn error in neurotransmitter metabolism that leads to combined serotonin and catecholamine deficiency (Abeling et al., 2000). The disorder is clinically characterized by vegetative symptoms, oculogyric crises, dystonia, and severe neurologic dysfunction, usually beginning in infancy or childhood (summary by Brun et al., 2010).

Shprintzen-Goldberg syndrome (SGS) is characterized by: delayed motor and cognitive milestones and mild-to-moderate intellectual disability; craniosynostosis of the coronal, sagittal, or lambdoid sutures; distinctive craniofacial features; and musculoskeletal findings including olichostenomelia, arachnodactyly, camptodactyly, pectus excavatum or carinatum, scoliosis, joint hypermobility or contractures, pes planus, foot malposition, and C1-C2 spine malformation. Cardiovascular anomalies may include mitral valve prolapse, secundum atrial septal defect, and aortic root dilatation. Minimal subcutaneous fat, abdominal wall defects, and myopia are also characteristic findings.

Hereditary nephrogenic diabetes insipidus (NDI) is characterized by inability to concentrate the urine, which results in polyuria (excessive urine production) and polydipsia (excessive thirst). Affected untreated infants usually have poor feeding and failure to thrive, and rapid onset of severe dehydration with illness, hot environment, or the withholding of water. Short stature and secondary dilatation of the ureters and bladder from the high urine volume is common in untreated individuals.

Hereditary nephrogenic diabetes insipidus (NDI) is characterized by inability to concentrate the urine, which results in polyuria (excessive urine production) and polydipsia (excessive thirst). Affected untreated infants usually have poor feeding and failure to thrive, and rapid onset of severe dehydration with illness, hot environment, or the withholding of water. Short stature and secondary dilatation of the ureters and bladder from the high urine volume is common in untreated individuals.

Neonatal severe hyperparathyroidism usually manifests in the first 6 months of life with severe hypercalcemia, bone demineralization, and failure to thrive. Early diagnosis is critical because untreated NSHPT can be a devastating neurodevelopmental disorder, which in some cases is lethal without parathyroidectomy. Some infants have milder hyperparathyroidism and a substantially milder clinical presentation and natural history (summary by Egbuna and Brown, 2008).

Carnitine palmitoyltransferase II (CPT II) deficiency is a disorder of long-chain fatty-acid oxidation. The three clinical presentations are lethal neonatal form, severe infantile hepatocardiomuscular form, and myopathic form (which is usually mild and can manifest from infancy to adulthood). While the former two are severe multisystemic diseases characterized by liver failure with hypoketotic hypoglycemia, cardiomyopathy, seizures, and early death, the latter is characterized by exercise-induced muscle pain and weakness, sometimes associated with myoglobinuria. The myopathic form of CPT II deficiency is the most common disorder of lipid metabolism affecting skeletal muscle and the most frequent cause of hereditary myoglobinuria. Males are more likely to be affected than females.

Nemaline myopathy is divided into six types. In order of decreasing severity, the types are: severe congenital, Amish, intermediate congenital, typical congenital, childhood-onset, and adult-onset. The types are distinguished by the age when symptoms first appear and the severity of symptoms; however, there is overlap among the various types. The severe congenital type is the most life-threatening. Most individuals with this type do not survive past early childhood due to respiratory failure. The Amish type solely affects the Old Order Amish population of Pennsylvania and is typically fatal in early childhood. The most common type of nemaline myopathy is the typical congenital type, which is characterized by muscle weakness and feeding problems beginning in infancy. Most of these individuals do not have severe breathing problems and can walk unassisted. People with the childhood-onset type usually develop muscle weakness in adolescence. The adult-onset type is the mildest of all the various types. People with this type usually develop muscle weakness between ages 20 and 50.\n\nNemaline myopathy is a disorder that primarily affects skeletal muscles, which are muscles that the body uses for movement. People with nemaline myopathy have muscle weakness (myopathy) throughout the body, but it is typically most severe in the muscles of the face; neck; trunk; and other muscles close to the center of the body (proximal muscles), such as those of the upper arms and legs. This weakness can worsen over time. Affected individuals may have feeding and swallowing difficulties, foot deformities, abnormal curvature of the spine (scoliosis), and joint deformities (contractures). Most people with nemaline myopathy are able to walk, although some affected children may begin walking later than usual. As the condition progresses, some people may require wheelchair assistance. In severe cases, the muscles used for breathing are affected and life-threatening breathing difficulties can occur.

Salt and pepper developmental regression syndrome, also known as Amish infantile epilepsy syndrome, is an autosomal recessive neurocutaneous disorder characterized by infantile onset of refractory and recurrent seizures associated with profoundly delayed psychomotor development and/or developmental regression as well as abnormal movements and visual loss (summary by Fragaki et al., 2013). Affected individuals develop hypo- or hyperpigmented skin macules on the trunk, face, and extremities in early childhood (summary by Boccuto et al., 2014). Not all patients have overt seizures (Lee et al., 2016).

Sudden infant death with dysgenesis of the testes syndrome (SIDDT) is characterized by sudden cardiac or respiratory arrest, disordered testicular development, and neurologic dysfunction, and is uniformly fatal before 1 year of age (Slater et al., 2020).

TXNL4A-related craniofacial disorders comprise a range of phenotypes that includes: isolated choanal atresia; choanal atresia with minor anomalies; and Burn-McKeown syndrome (BMKS), which is characterized by typical craniofacial features (bilateral choanal atresia/stenosis, short palpebral fissures, coloboma of the lower eyelids, prominent nasal bridge with widely spaced eyes, short philtrum, thin vermilion of the upper lip, and prominent ears). Hearing loss is common and cardiac defects and short stature have been reported. Intellectual disability is rare.

Isolated complex I deficiency is a rare inborn error of metabolism due to mutations in nuclear or mitochondrial genes encoding subunits or assembly factors of the human mitochondrial complex I (NADH: ubiquinone oxidoreductase) and is characterized by a wide range of manifestations including marked and often fatal lactic acidosis, cardiomyopathy, leukoencephalopathy, pure myopathy and hepatopathy with tubulopathy. Among the numerous clinical phenotypes observed are Leigh syndrome, Leber hereditary optic neuropathy and MELAS syndrome (see these terms).

The constitutional deletion of chromosome 1p36 results in a syndrome with multiple congenital anomalies and mental retardation (Shapira et al., 1997). Monosomy 1p36 is the most common terminal deletion syndrome in humans, occurring in 1 in 5,000 births (Shaffer and Lupski, 2000; Heilstedt et al., 2003). See also neurodevelopmental disorder with or without anomalies of the brain, eye, or heart (NEDBEH; 616975), which shows overlapping features and is caused by heterozygous mutation in the RERE gene (605226) on proximal chromosome 1p36. See also Radio-Tartaglia syndrome (RATARS; 619312), caused by mutation in the SPEN gene (613484) on chromosome 1p36, which shows overlapping features.

Pontocerebellar hypoplasia (PCH) refers to a group of severe neurodegenerative disorders affecting growth and function of the brainstem and cerebellum, resulting in little or no development. Different types were classified based on the clinical picture and the spectrum of pathologic changes. PCH type 1 is characterized by central and peripheral motor dysfunction associated with anterior horn cell degeneration resembling infantile spinal muscular atrophy (SMA; see SMA1, 253300); death usually occurs early. Genetic Heterogeneity of Pontocerebellar Hypoplasia Also see PCH1B (614678), caused by mutation in the EXOSC3 gene (606489); PCH1C (616081), caused by mutation in the EXOSC8 gene (606019); PCH1D (618065), caused by mutation in the EXOSC9 gene (606180); PCH1E (619303), caused by mutation in the SLC25A46 gene (610826); PCH1F (619304), caused by mutation in the EXOSC1 gene (606493); PCH2A (277470), caused by mutation in the TSEN54 gene (608755); PCH2B (612389), caused by mutation in the TSEN2 gene (608753); PCH2C (612390), caused by mutation in the TSEN34 gene (608754); PCH2D (613811), caused by mutation in the SEPSECS gene (613009); PCH3 (608027), caused by mutation in the PCLO gene (604918); PCH4 (225753), caused by mutation in the TSEN54 gene; PCH5 (610204), caused by mutation in the TSEN54 gene; PCH6 (611523), caused by mutation in the RARS2 gene (611524); PCH7 (614969), caused by mutation in the TOE1 gene (613931); PCH8 (614961), caused by mutation in the CHMP1A gene (164010); PCH9 (615809), caused by mutation in the AMPD2 gene (102771); PCH10 (615803), caused by mutation in the CLP1 gene (608757); PCH11 (617695), caused by mutation in the TBC1D23 gene (617687); PCH12 (618266), caused by mutation in the COASY gene (609855); PCH13 (618606), caused by mutation in the VPS51 gene (615738); PCH14 (619301), caused by mutation in the PPIL1 gene (601301); PCH15 (619302), caused by mutation in the CDC40 gene (605585); PCH16 (619527), caused by mutation in the MINPP1 gene (605391); and PCH17 (619909), caused by mutation in the PRDM13 gene (616741) on chromosome 6q16.

CASK disorders include a spectrum of phenotypes in both females and males. Two main types of clinical presentation are seen: Microcephaly with pontine and cerebellar hypoplasia (MICPCH), generally associated with pathogenic loss-of-function variants in CASK. X-linked intellectual disability (XLID) with or without nystagmus, generally associated with hypomorphic CASK pathogenic variants. MICPCH is typically seen in females with moderate-to-severe intellectual disability, progressive microcephaly with or without ophthalmologic anomalies, and sensorineural hearing loss. Most are able to sit independently; 20%-25% attain the ability to walk; language is nearly absent in most. Neurologic features may include axial hypotonia, hypertonia/spasticity of the extremities, and dystonia or other movement disorders. Nearly 40% have seizures by age ten years. Behaviors may include sleep disturbances, hand stereotypies, and self biting. MICPCH in males may occur with or without severe epileptic encephalopathy in addition to severe-to-profound developmental delay. When seizures are present they occur early and may be intractable. In individuals and families with milder (i.e., hypomorphic) pathogenic variants, the clinical phenotype is usually that of XLID with or without nystagmus and additional clinical features. Males have mild-to-severe intellectual disability, with or without nystagmus and other ocular features. Females typically have normal intelligence with some displaying mild-to-severe intellectual disability with or without ocular features.

The creatine deficiency disorders (CDDs), inborn errors of creatine metabolism and transport, comprise three disorders: the creatine biosynthesis disorders guanidinoacetate methyltransferase (GAMT) deficiency and L-arginine:glycine amidinotransferase (AGAT) deficiency; and creatine transporter (CRTR) deficiency. Developmental delay and cognitive dysfunction or intellectual disability and speech-language disorder are common to all three CDDs. Onset of clinical manifestations of GAMT deficiency (reported in ~130 individuals) is between ages three months and two years; in addition to developmental delays, the majority of individuals have epilepsy and develop a behavior disorder (e.g., hyperactivity, autism, or self-injurious behavior), and about 30% have movement disorder. AGAT deficiency has been reported in 16 individuals; none have had epilepsy or movement disorders. Clinical findings of CRTR deficiency in affected males (reported in ~130 individuals) in addition to developmental delays include epilepsy (variable seizure types and may be intractable) and behavior disorders (e.g., attention deficit and/or hyperactivity, autistic features, impulsivity, social anxiety), hypotonia, and (less commonly) a movement disorder. Poor weight gain with constipation and prolonged QTc on EKG have been reported. While mild-to-moderate intellectual disability is commonly observed up to age four years, the majority of adult males with CRTR deficiency have been reported to have severe intellectual disability. Females heterozygous for CRTR deficiency are typically either asymptomatic or have mild intellectual disability, although a more severe phenotype resembling the male phenotype has been reported.

X-linked lissencephaly-2 (LISX2) is a developmental disorder characterized by structural brain anomalies, early-onset intractable seizures, severe psychomotor retardation, and ambiguous genitalia. Males are severely affected and often die within the first days or months of life, whereas females may be unaffected or have a milder phenotype (Bonneau et al., 2002). LISX2 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 to Proud syndrome (300004) to infantile spasms without brain malformations (DEE1; 308350) to syndromic (309510) and nonsyndromic (300419) mental retardation (Kato et al., 2004; Wallerstein et al., 2008). For a general phenotypic description and a discussion of genetic heterogeneity of lissencephaly, see LIS1 (607432).

A rare, genetic disorder of amino acid absorption and transport, characterized by generalized hypotonia at birth, neonatal/infantile failure to thrive (followed by hyperphagia and rapid weight gain in late childhood), cystinuria type 1, nephrolithiasis, growth retardation due to growth hormone deficiency, and minor facial dysmorphism. Dysmorphic features mainly include dolichocephaly and ptosis. Nephrolithiasis occurs at variable ages.

TSEN54 pontocerebellar hypoplasia (TSEN54-PCH) comprises three PCH phenotypes (PCH2, 4, and 5) that share characteristic neuroradiologic and neurologic findings. The three PCH phenotypes (which differ mainly in life expectancy) were considered to be distinct entities before their molecular basis was known. PCH2. Children usually succumb before age ten years (those with PCH4 and 5 usually succumb as neonates). Children with PCH2 have generalized clonus, uncoordinated sucking and swallowing, impaired cognitive development, lack of voluntary motor development, cortical blindness, and an increased risk for rhabdomyolysis during severe infections. Epilepsy is present in approximately 50%. PCH4. Neonates often have seizures, multiple joint contractures ("arthrogryposis"), generalized clonus, and central respiratory impairment. PCH5 resembles PCH4 and has been described in one family.

Disorders of intracellular cobalamin metabolism have a variable phenotype and age of onset that are influenced by the severity and location within the pathway of the defect. The prototype and best understood phenotype is cblC; it is also the most common of these disorders. The age of initial presentation of cblC spans a wide range: In utero with fetal presentation of nonimmune hydrops, cardiomyopathy, and intrauterine growth restriction. Newborns, who can have microcephaly, poor feeding, and encephalopathy. Infants, who can have poor feeding and slow growth, neurologic abnormality, and, rarely, hemolytic uremic syndrome (HUS). Toddlers, who can have poor growth, progressive microcephaly, cytopenias (including megaloblastic anemia), global developmental delay, encephalopathy, and neurologic signs such as hypotonia and seizures. Adolescents and adults, who can have neuropsychiatric symptoms, progressive cognitive decline, thromboembolic complications, and/or subacute combined degeneration of the spinal cord.

Cold-induced sweating syndrome (CISS) and its infantile presentation, Crisponi syndrome(CS) is characterized by dysmorphic features (distinctive facies, lower facial weakness, flexion deformity at the elbows, camptodactyly with fisted hands, misshapen feet, and overriding toes); intermittent contracture of facial and oropharyngeal muscles when crying or being handled with puckering of lips and drooling of foamy saliva often associated with laryngospasm and respiratory distress; excessive startling and opisthotonus-like posturing with unexpected tactile or auditory stimuli; poor suck reflex and severely impaired swallowing; and a scaly erythematous rash. During the first decade of life, children with CISS/CS develop profuse sweating of the face, arms, and chest with ambient temperatures below 18º to 22º C, and with other stimuli including nervousness or ingestion of sweets. Affected individuals sweat very little in hot environments and may feel overheated. Progressive thoracolumbar kyphoscoliosis occurs, requiring intervention in the second decade.

PEHO is a severe autosomal recessive neurodevelopmental disorder characterized by extreme cerebellar atrophy due to almost total loss of granule neurons. Affected individuals present in early infancy with hypotonia, profoundly delayed psychomotor development, optic atrophy, progressive atrophy of the cerebellum and brainstem, and dysmyelination. Most patients also develop infantile seizures that are often associated with hypsarrhythmia on EEG, and many have peripheral edema (summary by Anttonen et al., 2017).

Mosaic variegated aneuploidy (MVA) syndrome is a rare disorder in which some cells in the body have an abnormal number of chromosomes instead of the usual 46 chromosomes, a situation known as aneuploidy. Most commonly, cells have an extra chromosome, which is called trisomy, or are missing a chromosome, which is known as monosomy. In MVA syndrome, some cells are aneuploid and others have the normal number of chromosomes, which is a phenomenon known as mosaicism. Typically, at least one-quarter of cells in affected individuals have an abnormal number of chromosomes. Because the additional or missing chromosomes vary among the abnormal cells, the aneuploidy is described as variegated.\n\nIn MVA syndrome, growth before birth is slow (intrauterine growth restriction). After birth, affected individuals continue to grow at a slow rate and are shorter than average. In addition, they typically have an unusually small head size (microcephaly). Another common feature of MVA syndrome is an increased risk of developing cancer in childhood. Cancers that occur most frequently in affected individuals include a cancer of muscle tissue called rhabdomyosarcoma, a form of kidney cancer known as Wilms tumor, and a cancer of the blood-forming tissue known as leukemia.\n\nLess commonly, people with MVA syndrome have eye abnormalities or distinctive facial features, such as a broad nasal bridge and low-set ears. Some affected individuals have brain abnormalities, the most common of which is called Dandy-Walker malformation. Intellectual disability, seizures, and other health problems can also occur in people with MVA syndrome.\n\nThere are at least three types of MVA syndrome, each with a different genetic cause. Type 1 is the most common and displays the classic signs and symptoms described above. Type 2 appears to have slightly different signs and symptoms than type 1, although the small number of affected individuals makes it difficult to define its characteristic features. Individuals with MVA syndrome type 2 grow slowly before and after birth; however, their head size is typically normal. Some people with MVA syndrome type 2 have unusually short arms. Individuals with MVA syndrome type 2 do not seem to have an increased risk of cancer. Another form of MVA syndrome is characterized by a high risk of developing Wilms tumor. Individuals with this form may also have other signs and symptoms typical of MVA syndrome type 1.

Congenital myopathy-1B (CMYO1B) is an autosomal recessive disorder of skeletal muscle characterized by severe hypotonia and generalized muscle weakness apparent soon after birth or in early childhood with delayed motor development, generalized muscle weakness and atrophy, and difficulty walking or running. Affected individuals show proximal muscle weakness with axial and shoulder girdle involvement, external ophthalmoplegia, and bulbar weakness, often resulting in feeding difficulties and respiratory insufficiency. Orthopedic complications such as joint laxity, distal contractures, hip dislocation, cleft palate, and scoliosis are commonly observed. Serum creatine kinase is normal. The phenotype is variable in severity (Jungbluth et al., 2005; Bharucha-Goebel et al., 2013). Some patients show symptoms in utero, including reduced fetal movements, polyhydramnios, and intrauterine growth restriction. The most severely affected patients present in utero with fetal akinesia, arthrogryposis, and lung hypoplasia resulting in fetal or perinatal death (McKie et al., 2014). Skeletal muscle biopsy of patients with recessive RYR1 mutations can show variable features, including multiminicores (Ferreiro and Fardeau, 2002), central cores (Jungbluth et al., 2002), congenital fiber-type disproportion (CFTD) (Monnier et al., 2009), and centronuclear myopathy (Wilmshurst et al., 2010). For a discussion of genetic heterogeneity of congenital myopathy, see CMYO1A (117000).

Beare-Stevenson cutis gyrata syndrome (BSTVS) is an autosomal dominant condition characterized by the furrowed skin disorder of cutis gyrata, acanthosis nigricans, craniosynostosis, craniofacial dysmorphism, digital anomalies, umbilical and anogenital abnormalities, and early death (summary by Przylepa et al., 1996).

Cornelia de Lange syndrome (CdLS) encompasses a spectrum of findings from mild to severe. Severe (classic) CdLS is characterized by distinctive facial features, growth restriction (prenatal onset; <5th centile throughout life), hypertrichosis, and upper-limb reduction defects that range from subtle phalangeal abnormalities to oligodactyly (missing digits). Craniofacial features include synophrys, highly arched and/or thick eyebrows, long eyelashes, short nasal bridge with anteverted nares, small widely spaced teeth, and microcephaly. Individuals with a milder phenotype have less severe growth, cognitive, and limb involvement, but often have facial features consistent with CdLS. Across the CdLS spectrum IQ ranges from below 30 to 102 (mean: 53). Many individuals demonstrate autistic and self-destructive tendencies. Other frequent findings include cardiac septal defects, gastrointestinal dysfunction, hearing loss, myopia, and cryptorchidism or hypoplastic genitalia.

A form of congenital disorders of N-linked glycosylation characterized by generalized hypotonia, craniofacial dysmorphism (prominent occiput, short palpebral fissures, long eyelashes, broad nose, high arched palate, retrognathia), hypoplastic genitalia, seizures, feeding difficulties, hypoventilation, severe hypogammaglobulinemia with generalized edema and increased resistance to particular viral infections (particularly to enveloped viruses). The disease is caused by loss-of-function mutations in the gene MOGS (2p13.1).

Molybdenum cofactor deficiency (MoCD) represents a spectrum, with some individuals experiencing significant signs and symptoms in the neonatal period and early infancy (termed early-onset or severe MoCD) and others developing signs and symptoms in childhood or adulthood (termed late-onset or mild MoCD). Individuals with early-onset MoCD typically present in the first days of life with severe encephalopathy, including refractory seizures, opisthotonos, axial and appendicular hypotonia, feeding difficulties, and apnea. Head imaging may demonstrate loss of gray and white matter differentiation, gyral swelling, sulci injury (typically assessed by evaluating the depth of focal lesional injury within the sulci), diffusely elevated T2-weighted signal, and panlobar diffusion restriction throughout the forebrain and midbrain with relative sparring of the brain stem. Prognosis for early-onset MoCD is poor, with about 75% succumbing in infancy to secondary complications of their neurologic disability (i.e., pneumonia). Late-onset MoCD is typically characterized by milder symptoms, such as acute neurologic decompensation in the setting of infection. Episodes vary in nature but commonly consist of altered mental status, dystonia, choreoathetosis, ataxia, nystagmus, and fluctuating hypotonia and hypertonia. These features may improve after resolution of the inciting infection or progress in a gradual or stochastic manner over the lifetime. Brain imaging may be normal or may demonstrate T2-weighted hyperintense or cystic lesions in the globus pallidus, thinning of the corpus callosum, and cerebellar atrophy.

For this GeneReview, the term "isolated methylmalonic acidemia" refers to a group of inborn errors of metabolism associated with elevated methylmalonic acid (MMA) concentration in the blood and urine that result from the failure to isomerize (convert) methylmalonyl-coenzyme A (CoA) into succinyl-CoA during propionyl-CoA metabolism in the mitochondrial matrix, without hyperhomocysteinemia or homocystinuria, hypomethioninemia, or variations in other metabolites, such as malonic acid. Isolated MMA is caused by complete or partial deficiency of the enzyme methylmalonyl-CoA mutase (mut0 enzymatic subtype or mut– enzymatic subtype, respectively), a defect in the transport or synthesis of its cofactor, 5-deoxy-adenosyl-cobalamin (cblA, cblB, or cblD-MMA), or deficiency of the enzyme methylmalonyl-CoA epimerase. Prior to the advent of newborn screening, common phenotypes included: Infantile/non-B12-responsive form (mut0 enzymatic subtype, cblB), the most common phenotype, associated with infantile-onset lethargy, tachypnea, hypothermia, vomiting, and dehydration on initiation of protein-containing feeds. Without appropriate treatment, the infantile/non-B12-responsive phenotype could rapidly progress to coma due to hyperammonemic encephalopathy. Partially deficient or B12-responsive phenotypes (mut– enzymatic subtype, cblA, cblB [rare], cblD-MMA), in which symptoms occur in the first few months or years of life and are characterized by feeding problems, failure to thrive, hypotonia, and developmental delay marked by episodes of metabolic decompensation. Methylmalonyl-CoA epimerase deficiency, in which findings range from complete absence of symptoms to severe metabolic acidosis. Affected individuals can also develop ataxia, dysarthria, hypotonia, mild spastic paraparesis, and seizures. In those individuals diagnosed by newborn screening and treated from an early age, there appears to be decreased early mortality, less severe symptoms at diagnosis, favorable short-term neurodevelopmental outcome, and lower incidence of movement disorders and irreversible cerebral damage. However, secondary complications may still occur and can include intellectual disability, tubulointerstitial nephritis with progressive impairment of renal function, "metabolic stroke" (bilateral lacunar infarction of the basal ganglia during acute metabolic decompensation), pancreatitis, growth failure, functional immune impairment, bone marrow failure, optic nerve atrophy, arrhythmias and/or cardiomyopathy (dilated or hypertrophic), liver steatosis/fibrosis/cancer, and renal cancer.

For this GeneReview, the term "isolated methylmalonic acidemia" refers to a group of inborn errors of metabolism associated with elevated methylmalonic acid (MMA) concentration in the blood and urine that result from the failure to isomerize (convert) methylmalonyl-coenzyme A (CoA) into succinyl-CoA during propionyl-CoA metabolism in the mitochondrial matrix, without hyperhomocysteinemia or homocystinuria, hypomethioninemia, or variations in other metabolites, such as malonic acid. Isolated MMA is caused by complete or partial deficiency of the enzyme methylmalonyl-CoA mutase (mut0 enzymatic subtype or mut– enzymatic subtype, respectively), a defect in the transport or synthesis of its cofactor, 5-deoxy-adenosyl-cobalamin (cblA, cblB, or cblD-MMA), or deficiency of the enzyme methylmalonyl-CoA epimerase. Prior to the advent of newborn screening, common phenotypes included: Infantile/non-B12-responsive form (mut0 enzymatic subtype, cblB), the most common phenotype, associated with infantile-onset lethargy, tachypnea, hypothermia, vomiting, and dehydration on initiation of protein-containing feeds. Without appropriate treatment, the infantile/non-B12-responsive phenotype could rapidly progress to coma due to hyperammonemic encephalopathy. Partially deficient or B12-responsive phenotypes (mut– enzymatic subtype, cblA, cblB [rare], cblD-MMA), in which symptoms occur in the first few months or years of life and are characterized by feeding problems, failure to thrive, hypotonia, and developmental delay marked by episodes of metabolic decompensation. Methylmalonyl-CoA epimerase deficiency, in which findings range from complete absence of symptoms to severe metabolic acidosis. Affected individuals can also develop ataxia, dysarthria, hypotonia, mild spastic paraparesis, and seizures. In those individuals diagnosed by newborn screening and treated from an early age, there appears to be decreased early mortality, less severe symptoms at diagnosis, favorable short-term neurodevelopmental outcome, and lower incidence of movement disorders and irreversible cerebral damage. However, secondary complications may still occur and can include intellectual disability, tubulointerstitial nephritis with progressive impairment of renal function, "metabolic stroke" (bilateral lacunar infarction of the basal ganglia during acute metabolic decompensation), pancreatitis, growth failure, functional immune impairment, bone marrow failure, optic nerve atrophy, arrhythmias and/or cardiomyopathy (dilated or hypertrophic), liver steatosis/fibrosis/cancer, and renal cancer.

A maple syrup urine disease caused by mutations in BCKDHA.

Kaufman oculocerebrofacial syndrome (KOS) is characterized by developmental delay, severe intellectual disability, and distinctive craniofacial features. Most affected children have prenatal-onset microcephaly, hypotonia, and growth deficiency. Feeding issues, ocular abnormalities, hearing impairment, and respiratory tract abnormalities are common. Ocular abnormalities can include structural abnormalities (microcornea or microphthalmia, coloboma, optic nerve hypoplasia), refractive errors (myopia ± astigmatism, hyperopia), strabismus, and entropion. Both conductive and sensorineural hearing loss have been reported as well as mixed conductive-sensorineural hearing loss of variable severity. Breathing problems can lead to prolonged hospitalization after birth in more than half of individuals. Less common findings include ectodermal abnormalities, cardiac manifestations, urogenital abnormalities, seizures, and skeletal abnormalities.

A very rare syndrome consisting of microcephaly with facial dysmorphism, spondylometaphyseal dysplasia and severe intellectual deficit. Eight cases have been reported in the literature in two unrelated families. Dysmorphic features include hypertelorism, depressed nasal bridge, and large nose with a large nasal tip, anteverted nostrils and wide mouth with thick lips. Affected patients do not achieve language ability. The condition is probably hereditary, and transmitted as an autosomal recessive trait.

Blepharophimosis-intellectual disability syndrome, Verloes type is a rare, genetic multiple congenital anomalies/dysmorphic syndrome characterized by congenital microcephaly, severe epilepsy with hypsarrhythmia, adducted thumbs, abnormal genitalia, and normal thyroid function. Hypotonia, moderate to severe psychomotor delay, and characteristic facial dysmorphism (including round face with prominent cheeks, blepharophimosis, large, bulbous nose with wide alae nasi, posteriorly rotated ears with dysplastic conchae, narrow mouth, cleft palate, and mild micrognathia) are additional characteristic features.

Congenital myasthenic syndromes (CMS) are a group of inherited disorders affecting the neuromuscular junction. Patients present clinically with onset of variable muscle weakness between infancy and adulthood. These disorders have been classified according to the location of the defect: presynaptic, synaptic, and postsynaptic. Endplate acetylcholinesterase deficiency is an autosomal recessive congenital myasthenic syndrome characterized by a defect within the synapse at the neuromuscular junction (NMJ). Mutations in COLQ result in a deficiency of acetylcholinesterase (AChE), which causes prolonged synaptic currents and action potentials due to extended residence of acetylcholine in the synaptic space. Treatment with ephedrine may be beneficial; AChE inhibitors and amifampridine should be avoided (summary by Engel et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

Chromosome 16p13.3deletion syndrome is a chromosome abnormality that can affect many parts of the body. People with this condition are missing a small piece (deletion) of chromosome 16 at a location designated p13.3. Although once thought to be a severe form of Rubinstein-Taybi syndrome, it is now emerging as a unique syndrome. Signs and symptoms may include failure to thrive, hypotonia (reduced muscle tone), short stature, microcephaly (unusually small head), characteristic facial features, mild to moderate intellectual disability, organ anomalies (i.e. heart and/or kidney problems), and vulnerability to infections. Chromosome testing of both parents can provide information about whether the deletion was inherited. In most cases, parents do not have any chromosome abnormalities. However, sometimes one parent has a balanced translocation where a piece of a chromosome has broken off and attached to another one with no gain or loss of genetic material. The balanced translocation normally does not cause signs or symptoms, but it increases the risk for having a child with a chromosome abnormality like a deletion. Treatment is based on the signs and symptoms present in each person.To learn more about chromosome abnormalities in general, view our GARD fact sheet on Chromosome Disorders.

Mandibulofacial dysostosis with microcephaly (MFDM) is characterized by malar and mandibular hypoplasia, microcephaly (congenital or postnatal onset), intellectual disability (mild, moderate, or severe), malformations of the external ear, and hearing loss that is typically conductive. Associated craniofacial malformations may include cleft palate, choanal atresia, zygomatic arch cleft (identified on cranial CT scan), and facial asymmetry. Other relatively common findings (present in 25%-35% of individuals) can include cardiac anomalies, thumb anomalies, esophageal atresia/tracheoesophageal fistula, short stature, spine anomalies, and epilepsy.

Untreated pyridox(am)ine 5'-phosphate oxidase (PNPO) deficiency, characterized by a range of seizure types, is "classic" (i.e., seizure onset in the neonatal period) in about 90% of affected individuals and "late onset" (seizure onset after the neonatal period) in about 10%. In classic PNPO deficiency, seizures (including status epilepticus) often begin on the first day of life and typically before age two weeks. In both classic and late-onset untreated PNPO deficiency, seizure semiology varies from myoclonic to clonic or tonic seizures, and seizures are typically resistant to common anti-seizure medications. Independent of age of onset, seizures respond to life-long treatment with a B6 vitamer: pyridoxal 5'-phosphate (PLP) in about 60% of affected individuals and pyridoxine (PN) in about 40%. About 60% of individuals with PNPO deficiency have developmental impairment, affecting speech, cognition, and behavior; some individuals have neurologic impairment such as muscular hypotonia or dystonia. Severe neurodevelopmental impairment is more likely to occur in individuals with PNPO deficiency who experienced diagnostic delay and prolonged periods of uncontrolled seizures.

Combined oxidative phosphorylation deficiency type 3 is an extremely rare clinically heterogenous disorder described in about 5 patients to date. Clinical signs included hypotonia, lactic acidosis, and hepatic insufficiency, with progressive encephalomyopathy or hypertrophic cardiomyopathy.

A rare mitochondrial disorder due to a defect in mitochondrial protein synthesis characterized by severe intrauterine growth retardation, neonatal limb edema and redundant skin on the neck (hydrops), developmental brain defects (corpus callosum agenesis, ventriculomegaly), brachydactyly, dysmorphic facial features with low set ears, severe intractable neonatal lactic acidosis with lethargy, hypotonia, absent spontaneous movements and fatal outcome. Markedly decreased activity of complex I, II + III and IV in muscle and liver have been determined.

Koolen-de Vries syndrome (KdVS) is characterized by developmental delay / intellectual disability, neonatal/childhood hypotonia, dysmorphisms, congenital malformations, and behavioral features. Psychomotor developmental delay is noted in all individuals from an early age. The majority of individuals with KdVS function in the mild-to-moderate range of intellectual disability. Other findings include speech and language delay (100%), epilepsy (~33%), congenital heart defects (25%-50%), renal and urologic anomalies (25%-50%), and cryptorchidism (71% of males). Behavior in most is described as friendly, amiable, and cooperative.

Congenital hypothyroidism can also occur as part of syndromes that affect other organs and tissues in the body. These forms of the condition are described as syndromic. Some common forms of syndromic hypothyroidism include Pendred syndrome, Bamforth-Lazarus syndrome, and brain-lung-thyroid syndrome.\n\nSigns and symptoms of congenital hypothyroidism result from the shortage of thyroid hormones. Affected babies may show no features of the condition, although some babies with congenital hypothyroidism are less active and sleep more than normal. They may have difficulty feeding and experience constipation. If untreated, congenital hypothyroidism can lead to intellectual disability and slow growth. In the United States and many other countries, all hospitals test newborns for congenital hypothyroidism. If treatment begins in the first two weeks after birth, infants usually develop normally.\n\nCongenital hypothyroidism occurs when the thyroid gland fails to develop or function properly. In 80 to 85 percent of cases, the thyroid gland is absent, severely reduced in size (hypoplastic), or abnormally located. These cases are classified as thyroid dysgenesis. In the remainder of cases, a normal-sized or enlarged thyroid gland (goiter) is present, but production of thyroid hormones is decreased or absent. Most of these cases occur when one of several steps in the hormone synthesis process is impaired; these cases are classified as thyroid dyshormonogenesis. Less commonly, reduction or absence of thyroid hormone production is caused by impaired stimulation of the production process (which is normally done by a structure at the base of the brain called the pituitary gland), even though the process itself is unimpaired. These cases are classified as central (or pituitary) hypothyroidism.\n\nCongenital hypothyroidism is a partial or complete loss of function of the thyroid gland (hypothyroidism) that affects infants from birth (congenital). The thyroid gland is a butterfly-shaped tissue in the lower neck. It makes iodine-containing hormones that play an important role in regulating growth, brain development, and the rate of chemical reactions in the body (metabolism). People with congenital hypothyroidism have lower-than-normal levels of these important hormones.

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.

KCNK9 imprinting syndrome is characterized by congenital central hypotonia (manifest as decreased movement, lethargy, and weak cry), severe feeding difficulties (resulting from facial weakness and poor suck), delayed development/intellectual disability, and dysmorphic manifestations. Poor feeding can cause failure to thrive during infancy unless managed appropriately. Significant dysphagia of solid foods typically persists until puberty. Intellectual disability can be severe. To date 19 individuals with a molecularly confirmed diagnosis have been reported.

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

FLNA deficiency is associated with a phenotypic spectrum that includes FLNA-related periventricular nodular heterotopia (Huttenlocher syndrome), congenital heart disease (patent ductus arteriosus, atrial and ventricular septal defects), valvular dystrophy, dilation and rupture of the thoracic aortic, pulmonary disease (pulmonary hypertension, alveolar hypoplasia, emphysema, asthma, chronic bronchitis), gastrointestinal dysmotility and obstruction, joint hypermobility, and macrothrombocytopenia.

Hypoglossia with situs inversus is a very rare congenital condition that likely represents a developmental field defect. Only sporadic cases have been reported (Faqeih et al., 2008). Hypoglossia is part of a group of malformation syndromes collectively termed 'oromandibular limb hypogenesis syndromes,' that usually include limb defects. Hall (1971) provided a classification system (see 103300). See also agnathia with holoprosencephaly (202650), which shows hypoglossia and situs inversus in addition to severe neurodevelopmental defects.

Classic Joubert syndrome (JS) is characterized by three primary findings: A distinctive cerebellar and brain stem malformation called the molar tooth sign (MTS). Hypotonia. Developmental delays. Often these findings are accompanied by episodic tachypnea or apnea and/or atypical eye movements. In general, the breathing abnormalities improve with age, truncal ataxia develops over time, and acquisition of gross motor milestones is delayed. Cognitive abilities are variable, ranging from severe intellectual disability to normal. Additional findings can include retinal dystrophy, renal disease, ocular colobomas, occipital encephalocele, hepatic fibrosis, polydactyly, oral hamartomas, and endocrine abnormalities. Both intra- and interfamilial variation are seen.

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.

SUCLA2-related mitochondrial DNA (mtDNA) depletion syndrome, encephalomyopathic form with methylmalonic aciduria is characterized by onset of the following features in infancy or childhood (median age of onset 2 months; range of onset birth to 6 years): psychomotor retardation, hypotonia, dystonia, muscular atrophy, sensorineural hearing impairment, postnatal growth retardation, and feeding difficulties. Other less frequent features include distinctive facial features, contractures, kyphoscoliosis, gastroesophageal reflux, ptosis, choreoathetosis, ophthalmoplegia, and epilepsy (infantile spasms or generalized convulsions). The median survival is 20 years; approximately 30% of affected individuals succumb during childhood. Affected individuals may have hyperintensities in the basal ganglia, cerebral atrophy, and leukoencephalopathy on head MRI. Elevation of methylmalonic acid (MMA) in the urine and plasma is found in a vast majority of affected individuals, although at levels that are far below those typically seen in individuals with classic methylmalonic aciduria.

Hereditary sensory and autonomic neuropathy type II (HSAN2) is characterized by progressively reduced sensation to pain, temperature, and touch. Onset can be at birth and is often before puberty. The sensory deficit is predominantly distal with the lower limbs more severely affected than the upper limbs. Over time sensory function becomes severely reduced. Unnoticed injuries and neuropathic skin promote ulcerations and infections that result in spontaneous amputation of digits or the need for surgical amputation. Osteomyelitis is common. Painless fractures can complicate the disease. Autonomic disturbances are variable and can include hyperhidrosis, tonic pupils, and urinary incontinence in those with more advanced disease.

An extremely rare form of carbohydrate deficient glycoprotein syndrome with, in the few cases reported to date, variable signs including microcephaly, growth retardation, psychomotor retardation and facial dysmorphism.

Potocki-Lupski syndrome (PTLS) is characterized by cognitive, behavioral, and medical manifestations. Cognitively, most individuals present with developmental delay, later meeting criteria for moderate intellectual disability. Behaviorally, issues with attention, hyperactivity, withdrawal, and anxiety may be seen. Some individuals meet criteria for autism spectrum disorder. Medically, hypotonia, oropharyngeal dysphagia leading to failure to thrive, congenital heart disease, hypoglycemia associated with growth hormone deficiency, and mildly dysmorphic facial features are observed. Medical manifestations typically lead to identification of PTLS in infancy; however, those with only behavioral and cognitive manifestations may be identified in later childhood.

The 16p11.2 recurrent deletion phenotype is characterized by motor speech disorder, language disorder, motor coordination difficulties, psychiatric conditions, and autistic features. While most, if not all, individuals with the 16p11.2 recurrent deletion experience some degree of developmental delay, the severity varies significantly. Most affected individuals do not have intellectual disability (defined as an IQ of <70), but many have below average cognition and learning disabilities in both verbal and nonverbal domains. Obesity is a feature of this disorder and generally emerges in childhood; BMI in individuals with the 16p11.2 recurrent deletion is significantly higher than in the general population by age five years. Seizures are observed in approximately 25% of individuals with the recurrent deletion. Vertebral anomalies, hearing impairment, macrocephaly, and cardiovascular malformation have each been observed in some individuals. Clinical follow-up data from adults suggests that the greatest medical challenges are obesity and related comorbidities that can be exacerbated by medications used to treat behavioral and psychiatric problems.

Syndromic multisystem autoimmune disease due to Itch deficiency is a rare, genetic, systemic autoimmune disease characterized by failure to thrive, global developmental delay, distinctive craniofacial dysmorphism (relative macrocephaly, dolichocephaly, frontal bossing, orbital proptosis, flattened midface with a prominent occiput, low, posteriorly rotated ears, micrognatia), hepato- and/or splenomegaly, and multisystemic autoimmune disease involving the lungs, liver, gut and/or thyroid gland.

Noonan syndrome (NS) is characterized by characteristic facies, short stature, congenital heart defect, and developmental delay of variable degree. Other findings can include broad or webbed neck, unusual chest shape with superior pectus carinatum and inferior pectus excavatum, cryptorchidism, varied coagulation defects, lymphatic dysplasias, and ocular abnormalities. Although birth length is usually normal, final adult height approaches the lower limit of normal. Congenital heart disease occurs in 50%-80% of individuals. Pulmonary valve stenosis, often with dysplasia, is the most common heart defect and is found in 20%-50% of individuals. Hypertrophic cardiomyopathy, found in 20%-30% of individuals, may be present at birth or develop in infancy or childhood. Other structural defects include atrial and ventricular septal defects, branch pulmonary artery stenosis, and tetralogy of Fallot. Up to one fourth of affected individuals have mild intellectual disability, and language impairments in general are more common in NS than in the general population.

Myotonic dystrophy type 1 (DM1) is a multisystem disorder that affects skeletal and smooth muscle as well as the eye, heart, endocrine system, and central nervous system. The clinical findings, which span a continuum from mild to severe, have been categorized into three somewhat overlapping phenotypes: mild, classic, and congenital. Mild DM1 is characterized by cataract and mild myotonia (sustained muscle contraction); life span is normal. Classic DM1 is characterized by muscle weakness and wasting, myotonia, cataract, and often cardiac conduction abnormalities; adults may become physically disabled and may have a shortened life span. Congenital DM1 is characterized by hypotonia and severe generalized weakness at birth, often with respiratory insufficiency and early death; intellectual disability is common.

Kabuki syndrome (KS) is characterized by typical facial features (long palpebral fissures with eversion of the lateral third of the lower eyelid; arched and broad eyebrows; short columella with depressed nasal tip; large, prominent, or cupped ears), minor skeletal anomalies, persistence of fetal fingertip pads, mild-to-moderate intellectual disability, and postnatal growth deficiency. Other findings may include: congenital heart defects, genitourinary anomalies, cleft lip and/or palate, gastrointestinal anomalies including anal atresia, ptosis and strabismus, and widely spaced teeth and hypodontia. Functional differences can include: increased susceptibility to infections and autoimmune disorders, seizures, endocrinologic abnormalities (including isolated premature thelarche in females), feeding problems, and hearing loss.

Acute infantile liver failure resulting from TRMU mutation is a transient disorder of hepatic function. In addition to elevated liver enzymes, jaundice, vomiting, coagulopathy, and hyperbilirubinemia, the presence of increased serum lactate is consistent with a defect in mitochondrial respiratory function. With supportive care, patients who survive the initial acute episode can recover and show normal development (Zeharia et al., 2009). See also transient infantile mitochondrial myopathy (MMIT; 500009), which is a similar disorder. A more severe, permanent disorder with some overlapping features is associated with mitochondrial DNA depletion (251880). See ILFS1 (615438) for information on syndromic infantile liver failure.

Chromosome 20q11-q12 deletion syndrome is characterized by global developmental delay, poor overall growth, sometimes with severe feeding difficulties, facial dysmorphism, and distal skeletal anomalies. Some patients may have hearing impairment, retinopathy, or cardiac defects. It is a multisystemic disorder with variable features (summary by Loddo et al., 2018).

Coffin-Siris syndrome (CSS) is classically characterized by aplasia or hypoplasia of the distal phalanx or nail of the fifth and additional digits, developmental or cognitive delay of varying degree, distinctive facial features, hypotonia, hirsutism/hypertrichosis, and sparse scalp hair. Congenital anomalies can include malformations of the cardiac, gastrointestinal, genitourinary, and/or central nervous systems. Other findings commonly include feeding difficulties, slow growth, ophthalmologic abnormalities, and hearing impairment.

Autosomal recessive mitochondrial complex III deficiency is a severe multisystem disorder with onset at birth of lactic acidosis, hypotonia, hypoglycemia, failure to thrive, encephalopathy, and delayed psychomotor development. Visceral involvement, including hepatopathy and renal tubulopathy, may also occur. Many patients die in early childhood, but some may show longer survival (de Lonlay et al., 2001; De Meirleir et al., 2003). Genetic Heterogeneity of Mitochondrial Complex III Deficiency Mitochondrial complex III deficiency can be caused by mutation in several different nuclear-encoded genes. See MC3DN2 (615157), caused by mutation in the TTC19 gene (613814) on chromosome 17p12; MC3DN3 (615158), caused by mutation in the UQCRB gene (191330) on chromosome 8q; MC3DN4 (615159), caused by mutation in the UQCRQ gene (612080) on chromosome 5q31; MC3DN5 (615160), caused by mutation in the UQCRC2 gene (191329) on chromosome 16p12; MC3DN6 (615453), caused by mutation in the CYC1 gene (123980) on chromosome 8q24; MC3DN7 (615824), caused by mutation in the UQCC2 gene (614461) on chromosome 6p21; MC3DN8 (615838), caused by mutation in the LYRM7 gene (615831) on chromosome 5q23; MC3DN9 (616111), caused by mutation in the UQCC3 gene (616097) on chromosome 11q12; and MC3DN10 (618775), caused by mutation in the UQCRFS1 gene (191327) on chromosome 19q12. See also MTYCB (516020) for a discussion of a milder phenotype associated with isolated mitochondrial complex III deficiency and mutations in a mitochondrial-encoded gene.

Cornelia de Lange syndrome (CdLS) encompasses a spectrum of findings from mild to severe. Severe (classic) CdLS is characterized by distinctive facial features, growth restriction (prenatal onset; <5th centile throughout life), hypertrichosis, and upper-limb reduction defects that range from subtle phalangeal abnormalities to oligodactyly (missing digits). Craniofacial features include synophrys, highly arched and/or thick eyebrows, long eyelashes, short nasal bridge with anteverted nares, small widely spaced teeth, and microcephaly. Individuals with a milder phenotype have less severe growth, cognitive, and limb involvement, but often have facial features consistent with CdLS. Across the CdLS spectrum IQ ranges from below 30 to 102 (mean: 53). Many individuals demonstrate autistic and self-destructive tendencies. Other frequent findings include cardiac septal defects, gastrointestinal dysfunction, hearing loss, myopia, and cryptorchidism or hypoplastic genitalia.

The peroxisomal biogenesis disorder (PBD) Zellweger syndrome (ZS) is an autosomal recessive multiple congenital anomaly syndrome resulting from disordered peroxisome biogenesis. Affected children present in the newborn period with profound hypotonia, seizures, and inability to feed. Characteristic craniofacial anomalies, eye abnormalities, neuronal migration defects, hepatomegaly, and chondrodysplasia punctata are present. Children with this condition do not show any significant development and usually die in the first year of life (summary by Steinberg et al., 2006). For a complete phenotypic description and a discussion of genetic heterogeneity of Zellweger syndrome, see 214100. Individuals with PBDs of complementation group 3 (CG3) have mutations in the PEX12 gene. For information on the history of PBD complementation groups, see 214100.

The peroxisomal biogenesis disorder (PBD) Zellweger syndrome (ZS) is an autosomal recessive multiple congenital anomaly syndrome. Affected children present in the newborn period with profound hypotonia, seizures, and inability to feed. Characteristic craniofacial anomalies, eye abnormalities, neuronal migration defects, hepatomegaly, and chondrodysplasia punctata are present. Children with this condition do not show any significant development and usually die in the first year of life (summary by Steinberg et al., 2006). For a complete phenotypic description and a discussion of genetic heterogeneity of Zellweger syndrome, see 214100. Individuals with PBDs of complementation group 4 (CG4, equivalent to CG6 and CGC) have mutations in the PEX6 gene. For information on the history of PBD complementation groups, see 214100.

The peroxisomal biogenesis disorder (PBD) Zellweger syndrome (ZS) is an autosomal recessive multiple congenital anomaly syndrome. Affected children present in the newborn period with profound hypotonia, seizures, and inability to feed. Characteristic craniofacial anomalies, eye abnormalities, neuronal migration defects, hepatomegaly, and chondrodysplasia punctata are present. Children with this condition do not show any significant development and usually die in the first year of life (summary by Steinberg et al., 2006). For a complete phenotypic description and a discussion of genetic heterogeneity of Zellweger syndrome, see 214100. Individuals with PBDs of complementation group 5 (CG5, equivalent to CG10 and CGF) have mutations in the PEX2 gene. For information on the history of PBD complementation groups, see 214100.

Zellweger syndrome (ZS) is an autosomal recessive multiple congenital anomaly syndrome resulting from disordered peroxisome biogenesis. Affected children present in the newborn period with profound hypotonia, seizures, and inability to feed. Characteristic craniofacial anomalies, eye abnormalities, neuronal migration defects, hepatomegaly, and chondrodysplasia punctata are present. Children with this condition do not show any significant development and usually die in the first year of life (summary by Steinberg et al., 2006). For a complete phenotypic description and a discussion of genetic heterogeneity of Zellweger syndrome, see 214100. Individuals with PBDs of complementation group 7 (CG7, equivalent to CGB) have mutations in the PEX10 gene. For information on the history of PBD complementation groups, see 214100.

Zellweger syndrome (ZS) is an autosomal recessive multiple congenital anomaly syndrome resulting from disordered peroxisome biogenesis. Affected children present in the newborn period with profound hypotonia, seizures, and inability to feed. Characteristic craniofacial anomalies, eye abnormalities, neuronal migration defects, hepatomegaly, and chondrodysplasia punctata are present. Children with this condition do not show any significant development and usually die in the first year of life (summary by Steinberg et al., 2006). For a complete phenotypic description and a discussion of genetic heterogeneity of Zellweger syndrome, see 214100. Individuals with PBDs of complementation group 9 (CG9, equivalent to CGD) have mutations in the PEX16 gene. For information on the history of PBD complementation groups, see 214100.

Congenital myopathy-2A (CMYO2A) is an autosomal dominant disorder of the skeletal muscle characterized by infantile- or childhood-onset myopathy with delayed motor milestones and nonprogressive muscle weakness. Of the patients with congenital myopathy caused by mutation in the ACTA1 gene, about 90% carry heterozygous mutations that are usually de novo and cause the severe infantile phenotype (CMYO2C; 620278). Some patients with de novo mutations have a more typical and milder disease course with delayed motor development and proximal muscle weakness, but are able to achieve independent ambulation. Less frequently, autosomal dominant transmission of the disorder within a family may occur when the ACTA1 mutation produces a phenotype compatible with adult life. Of note, intrafamilial variability has also been reported: a severely affected proband may be identified and then mildly affected or even asymptomatic relatives are found to carry the same mutation. The severity of the disease most likely depends on the detrimental effect of the mutation, although there are probably additional modifying factors (Ryan et al., 2001; Laing et al., 2009; Sanoudou and Beggs, 2001; Agrawal et al., 2004; Nowak et al., 2013; Sewry et al., 2019; Laitila and Wallgren-Pettersson, 2021). The most common histologic finding on muscle biopsy in patients with ACTA1 mutations is the presence of 'nemaline rods,' which represent abnormal thread- or rod-like structures ('nema' is Greek for 'thread'). However, skeletal muscle biopsy from patients with mutations in the ACTA1 gene can show a range of pathologic phenotypes. These include classic rods, intranuclear rods, clumped filaments, cores, or fiber-type disproportion, all of which are nonspecific pathologic findings and not pathognomonic of a specific congenital myopathy. Most patients have clinically severe disease, regardless of the histopathologic phenotype (Nowak et al., 2007; Sewry et al., 2019). ACTA1 mutations are the second most common cause of congenital myopathies classified histologically as 'nemaline myopathy' after mutations in the NEB gene (161650). ACTA1 mutations are overrepresented in the severe phenotype with early death (Laing et al., 2009). For a discussion of genetic heterogeneity of congenital myopathy, see CMYO1A (117000). For a discussion of genetic heterogeneity of nemaline myopathy, see NEM2 (256030).

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.

Intellectual developmental disorder with language impairment and with or without autistic features (IDDLA) is a neurodevelopmental disorder characterized by global developmental delay with moderate to severe speech delay that particularly affects expressive speech. Most patients have articulation defects, but frank verbal dyspraxia is not observed. Common dysmorphic features include broad forehead, downslanting palpebral fissures, short nose with broad tip, relative macrocephaly, frontal hair upsweep, and prominent digit pads. Gross motor skills are also delayed. Some patients have autistic features and/or behavioral problems. All reported cases have occurred de novo (review by Le Fevre et al., 2013).

Autoinflammation with infantile enterocolitis is an autosomal dominant disorder characterized by onset of recurrent flares of autoinflammation in early infancy. Affected individuals tend to have poor overall growth and gastrointestinal symptoms in infancy associated with laboratory evidence of activated inflammation. This initial presentation is followed by recurrent febrile episodes with splenomegaly and sometimes hematologic disturbances, arthralgias, or myalgias. The disorder results from overactivation of an arm of the immune response system (Romberg et al., 2014; Canna et al., 2014).

Any mitochondrial DNA depletion syndrome in which the cause of the disease is a mutation in the OPA1 gene.

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; acetylcholinesterase inhibitors and amifampridine should be avoided (summary by Engel et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

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 a genetically heterogeneous autosomal recessive disorder with characteristic brain and eye malformations, profound mental retardation, congenital muscular dystrophy, and early death. The phenotype commonly includes cobblestone (type II) lissencephaly, cerebellar malformations, and retinal malformations. More variable features include macrocephaly or microcephaly, hypoplasia of midline brain structures, ventricular dilatation, microphthalmia, cleft lip/palate, and congenital contractures (Dobyns et al., 1989). Those with a more severe phenotype characterized as Walker-Warburg syndrome often die within the first year of life, whereas those characterized as having muscle-eye-brain disease may rarely acquire the ability to walk and to speak a few words. These are part of a group of disorders resulting from defective glycosylation of DAG1 (128239), collectively known as 'dystroglycanopathies' (Godfrey et al., 2007). Genetic Heterogeneity of Congenital Muscular Dystrophy-Dystroglycanopathy with Brain and Eye Anomalies (Type A) Muscular dystrophy-dystroglycanopathy with brain and eye anomalies (type A) is genetically heterogeneous and can be caused by mutation in other genes involved in DAG1 glycosylation: see MDDGA2 (613150), caused by mutation in the POMT2 gene (607439); MDDGA3 (253280), caused by mutation in the POMGNT1 gene (606822); MDDGA4 (253800), caused by mutation in the FKTN gene (607440); MDDGA5 (613153), caused by mutation in the FKRP gene (606596); MDDGA6 (613154), caused by mutation in the LARGE gene (603590); MDDGA7 (614643), caused by mutation in the ISPD gene (CRPPA; 614631); MDDGA8 (614830) caused by mutation in the GTDC2 gene (POMGNT2; 614828); MDDGA9 (616538), caused by mutation in the DAG1 gene (128239); MDDGA10 (615041), caused by mutation in the TMEM5 gene (RXYLT1; 605862); MDDGA11 (615181), caused by mutation in the B3GALNT2 gene (610194); MDDGA12 (615249), caused by mutation in the SGK196 gene (POMK; 615247); MDDGA13 (615287), caused by mutation in the B3GNT1 gene (B4GAT1; 605517); and MDDGA14 (615350), caused by mutation in the GMPPB gene (615320).

Shashi-Pena syndrome is a neurodevelopmental syndrome characterized by delayed psychomotor development, variable intellectual disability, hypotonia, facial dysmorphism, and some unusual features, including enlarged head circumference, glabellar nevus flammeus, and deep palmar creases. Some patients may also have atrial septal defect, episodic hypoglycemia, changes in bone mineral density, and/or seizures (summary by Shashi et al., 2016).

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).

Frontometaphyseal dysplasia (FMD) is a progressive sclerosing skeletal dysplasia characterized by supraorbital hyperostosis, undermodeling of the small bones, and small and large joint contractures, as well as extraskeletal developmental abnormalities, primarily of the cardiorespiratory system and genitourinary tract. Patients with FMD2 appear to have a propensity for keloid formation (summary by Wade et al., 2016). For a discussion of genetic heterogeneity of frontometaphyseal dysplasia, see FMD1 (305620).

The marfanoid-progeroid-lipodystrophy syndrome (MFLS) is characterized by congenital lipodystrophy, premature birth with an accelerated linear growth disproportionate to weight gain, and progeroid appearance with distinct facial features, including proptosis, downslanting palpebral fissures, and retrognathia. Other characteristic features include arachnodactyly, digital hyperextensibility, myopia, dural ectasia, and normal psychomotor development (Takenouchi et al., 2013). Takenouchi et al. (2013) noted phenotypic overlap with Marfan syndrome (154700) and Shprintzen-Goldberg craniosynostosis syndrome (182212).

Witteveen-Kolk syndrome (WITKOS) is an autosomal dominant disorder with characteristic distinctive facial features, microcephaly, short stature, and mildly impaired intellectual development with delayed cognitive and motor development and subtle anomalies on MRI-brain imaging (summary by Balasubramanian et al., 2021).

Distal chromosome 19q13.11 deletion syndrome is an autosomal dominant neurodevelopmental disorder characterized by poor overall growth, slender habitus, microcephaly, delayed development, intellectual disability with poor or absent speech, and feeding difficulties. Additional features include dysmorphic facies, signs of ectodermal dysplasia, hand and foot anomalies, and genitourinary anomalies, particularly in males (summary by Chowdhury et al., 2014).

An inherited condition caused by autosomal dominant mutation(s) in the PPP1CB gene, encoding serine/threonine-protein phosphatase PP1-beta catalytic subunit. The condition is characterized by facial features similar to those seen in Noonan syndrome but may also include short stature, cognitive deficits, relative macrocephaly, small posterior fossa resulting in Chiari I malformation, hypernasal voice, cardiac defects, and ectodermal abnormalities, which typically presents as slow-growing, sparse, and/or unruly hair.

A rare genetic multiple congenital anomalies/dysmorphic syndrome characterized by global developmental delay and moderate to severe intellectual disability, as well as variable other manifestations, such as macro- or microcephaly, epilepsy, hypotonia, behavioral problems, stereotypic movements, and facial dysmorphism (including arched eyebrows, long palpebral fissures, prominent nasal bridge, upturned nose, dysplastic ears, and broad mouth), among others. Brain imaging may show cerebellar anomalies, hypoplastic corpus callosum, enlarged ventricles, polymicrogyria, or white matter abnormalities.

Noonan syndrome (NS) is characterized by characteristic facies, short stature, congenital heart defect, and developmental delay of variable degree. Other findings can include broad or webbed neck, unusual chest shape with superior pectus carinatum and inferior pectus excavatum, cryptorchidism, varied coagulation defects, lymphatic dysplasias, and ocular abnormalities. Although birth length is usually normal, final adult height approaches the lower limit of normal. Congenital heart disease occurs in 50%-80% of individuals. Pulmonary valve stenosis, often with dysplasia, is the most common heart defect and is found in 20%-50% of individuals. Hypertrophic cardiomyopathy, found in 20%-30% of individuals, may be present at birth or develop in infancy or childhood. Other structural defects include atrial and ventricular septal defects, branch pulmonary artery stenosis, and tetralogy of Fallot. Up to one fourth of affected individuals have mild intellectual disability, and language impairments in general are more common in NS than in the general population.

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.

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.

Combined oxidative phosphorylation deficiency-13 (COXPD13) is an autosomal recessive multisystem disorder resulting from mitochondrial dysfunction. Affected individuals develop severe neurologic impairment in the first months of life, including hypotonia, abnormal dystonic movements, hearing loss, poor feeding, global developmental delay, and abnormal eye movements. Brain imaging shows signal abnormalities in putamen, basal ganglia, caudate nuclei, or corpus callosum, as well as delayed myelination. Analysis of patient tissues shows multiple defects in enzymatic activities of the mitochondrial respiratory chain, although some tissues may show normal values since tissue expression of the mitochondrial defect and metabolic needs of specific tissues are variable (summary by Vedrenne et al., 2012). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

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.

The two forms of deoxyguanosine kinase (DGUOK) deficiency are a neonatal multisystem disorder and an isolated hepatic disorder that presents later in infancy or childhood. The majority of affected individuals have the multisystem illness with hepatic disease (jaundice, cholestasis, hepatomegaly, and elevated transaminases) and neurologic manifestations (hypotonia, nystagmus, and psychomotor retardation) evident within weeks of birth. Those with isolated liver disease may also have renal involvement and some later develop mild hypotonia. Progressive hepatic disease is the most common cause of death in both forms.

Turnpenny-Fry syndrome (TPFS) is characterized by developmental delay, impaired intellectual development, impaired growth, and recognizable facial features that include frontal bossing, sparse hair, malar hypoplasia, small palpebral fissures and oral stoma, and dysplastic 'satyr' ears. Other common findings include feeding problems, constipation, and a range of brain, cardiac, vascular, and skeletal malformations (Turnpenny et al., 2018).

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.

Developmental and epileptic encephalopathy-75 (DEE75) is an autosomal recessive neurodevelopmental and neurodegenerative disorder characterized by onset of severe refractory seizures in the first months of life. Patients often have global developmental delay before the onset of seizures, and thereafter achieve few milestones. EEG usually shows multifocal spikes and hypsarrhythmia, consistent with a clinical diagnosis of West syndrome. They have severely impaired intellectual development with inability to walk, absent speech, and hypotonia with axial hyperreflexia. Brain imaging shows progressive cerebral atrophy, frontal lobe atrophy, white matter abnormalities, and delayed myelination. Since the disorder is due to mitochondrial dysfunction, some patients may develop other organ involvement, including cardiomyopathy or liver and renal dysfunction. Death may occur in childhood (summary by Yin et al., 2018). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Developmental and epileptic encephalopathy-76 (DEE76) is an autosomal recessive neurodevelopmental disorder characterized by early-onset, usually refractory, seizures, severely delayed global development, hypotonia, peripheral spasticity, and abnormalities on brain imaging, mainly cerebral atrophy and delayed myelination. Some patients may have additional features, such as scoliosis or microcephaly. The disorder may result in death in childhood (summary by Bell et al., 2019). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Noonan syndrome (NS) is characterized by characteristic facies, short stature, congenital heart defect, and developmental delay of variable degree. Other findings can include broad or webbed neck, unusual chest shape with superior pectus carinatum and inferior pectus excavatum, cryptorchidism, varied coagulation defects, lymphatic dysplasias, and ocular abnormalities. Although birth length is usually normal, final adult height approaches the lower limit of normal. Congenital heart disease occurs in 50%-80% of individuals. Pulmonary valve stenosis, often with dysplasia, is the most common heart defect and is found in 20%-50% of individuals. Hypertrophic cardiomyopathy, found in 20%-30% of individuals, may be present at birth or develop in infancy or childhood. Other structural defects include atrial and ventricular septal defects, branch pulmonary artery stenosis, and tetralogy of Fallot. Up to one fourth of affected individuals have mild intellectual disability, and language impairments in general are more common in NS than in the general population.

Autosomal dominant severe congenital neutropenia-8 (SCN8) is a pleiotropic disorder with the consistent feature of decreased neutrophils associated with recurrent bacterial infections apparent from early infancy. Other hematologic parameters are usually normal, although some patients may have mild anemia. Bone marrow examination shows hypocellularity with arrested maturation of the granulocyte lineage at the level of promyelocytes or myeloblasts. Treatment with granulocyte colony-stimulating factor (GCSF; 138970) is usually ineffective or only partially effective, whereas hematopoietic bone marrow transplantation is effective. A subset of patients have additional features, including exocrine pancreatic insufficiency, which resembles Shwachman-Diamond syndrome (see SDS1, 260400), and/or neurologic deficits, including developmental delay, impaired intellectual development, speech delay, and/or autistic features (summary by Carapito et al., 2017 and Bellanne-Chantelot et al., 2018). For discussion of genetic heterogeneity of severe congenital neutropenia, see SCN1 (202700).

Multiple congenital anomalies-hypotonia-seizures syndrome-4 (MCAHS4) is an autosomal recessive neurologic disorder characterized by onset of refractory seizures in the first months of life. Patients have severe global developmental delay, and may have additional variable features, including dysmorphic or coarse facial features, visual defects, and mild skeletal or renal anomalies. At the cellular level, the disorder is caused by a defect in the synthesis of glycosylphosphatidylinositol (GPI), and thus affects the expression of GPI-anchored proteins at the cell surface (summary by Starr et al., 2019). For a discussion of genetic heterogeneity of MCAHS, see MCAHS1 (614080). For a discussion of genetic heterogeneity of DEE, see 308350. For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293).

Intellectual developmental disorder with nasal speech, dysmorphic facies, and variable skeletal anomalies (IDNADFS) is characterized by mildly impaired global development, speech delay with nasal speech, and dysmorphic facial features, including high forehead, midface hypoplasia, micrognathia or high-arched palate, hypo/hypertelorism, upslanting palpebral fissures, and thin upper lip. Some patients may have skeletal anomalies, such as brachydactyly, 2-3 toe syndactyly, and flat feet (summary by Alesi et al., 2019 and Uehara et al., 2019).

Noonan syndrome (NS) is characterized by characteristic facies, short stature, congenital heart defect, and developmental delay of variable degree. Other findings can include broad or webbed neck, unusual chest shape with superior pectus carinatum and inferior pectus excavatum, cryptorchidism, varied coagulation defects, lymphatic dysplasias, and ocular abnormalities. Although birth length is usually normal, final adult height approaches the lower limit of normal. Congenital heart disease occurs in 50%-80% of individuals. Pulmonary valve stenosis, often with dysplasia, is the most common heart defect and is found in 20%-50% of individuals. Hypertrophic cardiomyopathy, found in 20%-30% of individuals, may be present at birth or develop in infancy or childhood. Other structural defects include atrial and ventricular septal defects, branch pulmonary artery stenosis, and tetralogy of Fallot. Up to one fourth of affected individuals have mild intellectual disability, and language impairments in general are more common in NS than in the general population.

Diencephalic-mesencephalic junction dysplasia syndrome-2 (DMJDS2) is an autosomal recessive neurodevelopmental disorder characterized by global developmental delay and hypotonia apparent from infancy. Affected individuals develop severe progressive hyperkinetic movements, including spastic tetraplegia, dystonia, and bulbar dysphagia necessitating tube feeding. Patients are unable to walk and have severely impaired intellectual development with absent speech. Brain imaging shows a unique malformation reflecting abnormal embryonic development of the diencephalic-mesencephalic junction (DMJ), with agenesis of the basal ganglia and olfactory bulb, hypoplasia of the thalamus, and abnormal course of the corticospinal tracts (summary by De Mori et al., 2019). For a discussion of genetic heterogeneity of DMJDS, see DMJDS1 (251280).

Congenital myopathy-8 (CMYO8) is an autosomal dominant disorder of the skeletal muscle characterized by hypotonia and delayed motor development apparent from infancy or childhood, resulting in difficulties walking or loss of ambulation within the first few decades. Affected individuals show respiratory insufficiency, high-arched palate, and scoliosis; external ophthalmoplegia may also be present. Skeletal muscle biopsy shows cores and myofibrillar disorganization (Lornage et al., 2019). For a discussion of genetic heterogeneity of congenital myopathy, see CMYO1A (117000).

Neurodevelopmental disorder with dysmorphic facies and distal skeletal anomalies (NEDDFSA) is a global neurodevelopmental disorder with highly variable features. Patients often show poor feeding, poor overall growth, and hypotonia from early infancy, followed by mildly delayed motor development, poor language acquisition, and behavioral abnormalities. Intellectual development varies from severe with absent speech to mild with the ability to attend special schools. Common features include dysmorphic facial features with notable eye anomalies, joint hypermobility, and mild skeletal anomalies of the hands and feet (summary by Carapito et al., 2019).

Developmental and epileptic encephalopathy-82 (DEE82) is an autosomal recessive mitochondriopathy manifest as early-onset metabolic epileptic encephalopathy. Soon after birth, affected individuals exhibit hypotonia, feeding difficulties, and global developmental delay even before the onset of seizures in the first year of life. The severity is variable, but all patients have severely impaired intellectual development with absent speech and spastic tetraplegia. Other features include poor overall growth with microcephaly and recurrent infections. Brain imaging shows cerebral atrophy, thin corpus callosum, cerebellar hypoplasia, and white matter abnormalities. Laboratory studies show increased serum lactate and ammonia. Importantly, treatment with combined pyridoxine and serine can result in significant improvement in seizures as well as some mild developmental progress (summary by van Karnebeek et al., 2019). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Developmental and epileptic encephalopathy-83 (DEE83) is a severe autosomal recessive neurodevelopmental disorder characterized by onset of frequent seizures in the first days to months of life that are usually refractory to medical treatment and are associated with significant EEG abnormalities. Affected individuals have profoundly impaired development, with no motor or language skill acquisition, poor or absent visual tracking, and poor oromotor function necessitating tube feeding. Many patients die in the first years of life (summary by Perenthaler et al., 2020). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Intellectual developmental disorder with hypotonia and behavioral abnormalities (IDDHBA) is a neurodevelopmental disorder characterized by onset of hypotonia and variably impaired global developmental delay in infancy. Affected individuals tend to have learning disability, usually requiring special schooling, as well as behavioral abnormalities, such as autistic features and attention deficit-hyperactivity disorder (ADHD). Additional more variable features may include nonspecific dysmorphic facial features, congenital heart defects, visual or ocular movement anomalies, and poor feeding and/or gastroesophageal reflux (summary by Calpena et al., 2019).

Neurogenic arthrogryposis multiplex congenita-4 with agenesis of the corpus callosum (AMC4) is a severe neurologic disorder with onset in utero. Affected individuals show little or no fetal movements and are born with significant contractures affecting the upper and lower limbs, as well as dysmorphic facial features. Other abnormalities include globally impaired development, optic atrophy, agenesis of the corpus callosum, seizures, and peripheral neuropathy. Many patients die in early childhood (summary by Seidahmed et al., 2020).

Coffin-Siris syndrome-11 (CSS11) is a syndromic neurodevelopmental disorder characterized by global developmental delay and impaired intellectual development associated with hypotonia, feeding difficulties, and variable dysmorphic features. Most patients have distal anomalies, such as small hands and feet and hypoplastic fifth toenails (summary by Nixon et al., 2019). For a general phenotypic description and a discussion of genetic heterogeneity of Coffin-Siris syndrome, see CSS1 (135900).

Neurodevelopmental disorder with epilepsy, spasticity, and brain atrophy (NEDESBA) is an autosomal recessive disorder characterized by severely impaired global development apparent soon after birth. Affected individuals develop seizures in the first year of life and achieve almost no psychomotor progress, resulting in feeding difficulties and an inability to walk or speak. Other features include hypotonia, peripheral spasticity with contractures, cortical visual impairment, and dysmorphic features, including microcephaly. Death in childhood may occur (summary by Van Bergen et al., 2020). Van Bergen et al. (2020) noted that the molecular mechanism of this disorder can be classified into a group of similar phenotypes resulting from mutations in genes associated with transport protein particles, sometimes referred to as 'TRAPPopathies' (review by Sacher et al., 2019).

Beck-Fahrner syndrome (BEFAHRS) is a developmental disorder characterized by global developmental delay with variably impaired intellectual development. Affected individuals often have behavioral abnormalities, such as autistic features or attention deficit-hyperactivity disorder (ADHD), as well as learning disabilities. Most patients have hypotonia and dysmorphic facies. Some may have growth abnormalities, including overgrowth or poor growth, poor feeding, and rarely, seizures. Although both monoallelic and biallelic mutations have been reported, some heterozygous carriers in autosomal recessive families may have milder symptoms; thus, both groups are included in this entry (summary by Beck et al., 2020).

Neonatal lethal pontocerebellar hypoplasia, hypotonia, and respiratory insufficiency syndrome (PHRINL) is an autosomal recessive multisystem disorder with onset in utero and death in the neonatal period. Rare patients may survive a few months. Affected infants show respiratory insufficiency and almost no spontaneous movement at birth, usually requiring mechanical ventilation and admission to the neonatal intensive care unit. Additional features include corneal clouding, seizures, dysmorphic facies, contractures, and progressive pontocerebellar hypoplasia with simplified gyral pattern and white matter abnormalities. Some patients may have cardiac anomalies or cardiac hypertrophy. Laboratory studies show evidence consistent with mitochondrial defects and/or abnormal cholesterol or lipid metabolism. Depending on the type of mutation or deletion, some patients may have a less severe disorder (see GENOTYPE/PHENOTYPE CORRELATIONS) (summary by Desai et al., 2017).

Nabais Sa-de Vries syndrome type 2 (NSDVS2) is characterized by global developmental delay apparent from birth and distinctive dysmorphic facial features. Most patients have additional anomalies, including congenital heart defects, sleep disturbances, hypotonia, and variable endocrine abnormalities, such as hypothyroidism (summary by Nabais Sa et al., 2020).

Diets-Jongmans syndrome (DIJOS) is an autosomal dominant disorder characterized by mild to moderately impaired intellectual development with a recognizable facial gestalt (summary by Diets et al., 2019).

Nizon-Isidor syndrome (NIZIDS) is a neurodevelopmental disorder characterized by global developmental delay, mildly delayed walking, poor speech and language, variably impaired intellectual development, and behavioral abnormalities, such as autistic features or attention deficit-hyperactivity disorder (ADHD). Some patients may have additional features, including nonspecific facial dysmorphism, gastrointestinal difficulties, distal hand anomalies, and thin corpus callosum on brain imaging (summary by Nizon et al., 2019).

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).

Silver-Russell Syndrome (SRS) is typically characterized by asymmetric gestational growth restriction resulting in affected individuals being born small for gestational age, with relative macrocephaly at birth (head circumference =1.5 SD above birth weight and/or length), prominent forehead usually with frontal bossing, and frequently body asymmetry. This is followed by postnatal growth failure, and in some cases progressive limb length discrepancy and feeding difficulties. Additional clinical features include triangular facies, fifth-finger clinodactyly, and micrognathia with narrow chin. Except for the limb length asymmetry, the growth failure is proportionate and head growth normal. The average adult height in untreated individuals is ~3.1±1.4 SD below the mean. The Netchine-Harbison Clinical Scoring System (NH-CSS) is a sensitive diagnostic scoring system. Clinical diagnosis can be established in an individual who meets at least four of the NH-CSS clinical criteria – prominent forehead/frontal bossing and relative macrocephaly at birth plus two additional findings – and in whom other disorders have been ruled out.

Silver-Russell Syndrome (SRS) is typically characterized by asymmetric gestational growth restriction resulting in affected individuals being born small for gestational age, with relative macrocephaly at birth (head circumference =1.5 SD above birth weight and/or length), prominent forehead usually with frontal bossing, and frequently body asymmetry. This is followed by postnatal growth failure, and in some cases progressive limb length discrepancy and feeding difficulties. Additional clinical features include triangular facies, fifth-finger clinodactyly, and micrognathia with narrow chin. Except for the limb length asymmetry, the growth failure is proportionate and head growth normal. The average adult height in untreated individuals is ~3.1±1.4 SD below the mean. The Netchine-Harbison Clinical Scoring System (NH-CSS) is a sensitive diagnostic scoring system. Clinical diagnosis can be established in an individual who meets at least four of the NH-CSS clinical criteria – prominent forehead/frontal bossing and relative macrocephaly at birth plus two additional findings – and in whom other disorders have been ruled out.

Treacher Collins syndrome (TCS) is characterized by bilateral and symmetric downslanting palpebral fissures, malar hypoplasia, micrognathia, and external ear abnormalities. Hypoplasia of the zygomatic bones and mandible can cause significant feeding and respiratory difficulties. About 40%-50% of individuals have conductive hearing loss attributed most commonly to malformation of the ossicles and hypoplasia of the middle ear cavities. Inner ear structures tend to be normal. Other, less common abnormalities include cleft palate and unilateral or bilateral choanal stenosis or atresia. Typically intellect is normal.

Li-Ghorbani-Weisz-Hubshman syndrome (LIGOWS) is a neurodevelopmental disorder characterized by global developmental delay, mild to moderately impaired intellectual development with language delay, and mild dysmorphic features. Affected individuals may have behavioral abnormalities and difficulties with numbers and understanding certain concepts, such as money. Some patients have seizures. Brain imaging often shows enlarged ventricles, thin corpus callosum, and gray matter nodular heterotopia, suggesting abnormal cortical brain development. More variable additional features may be present (summary by Li et al., 2020).

Rajab interstitial lung disease with brain calcifications-2 (RILDBC2) is an autosomal recessive disorder characterized by growth delay, interstitial lung disease, liver disease, and abnormal brain MRI findings, including brain calcifications and periventricular cysts (Krenke et al., 2019). For a discussion of genetic heterogeneity of RILDBC, see RILDBC1 (613658).

Myopathy, epilepsy, and progressive cerebral atrophy (MEPCA) is a severe autosomal recessive disorder with onset in utero or at birth. Affected individuals have hypotonia with respiratory or feeding difficulties apparent from birth and often associated with contractures of the large joints. There is little spontaneous movement: skeletal muscle biopsy and electrophysiologic studies are consistent with a myopathy or myasthenic disorder. Patients also develop refractory seizures with burst-suppression pattern or hypsarrhythmia on EEG. Brain imaging shows progressive cerebral atrophy and myelination defects. All patients reported to date died within the first year of life (summary by Schorling et al., 2017).

Mitochondrial complex IV deficiency nuclear type 3 (MC4DN3) is an autosomal recessive multisystem metabolic disorder with a highly variable phenotype. Some patients present with encephalomyopathic features in early infancy, whereas others may present later in infancy or the first years of life after normal early development. Affected individuals show hypotonia, failure to thrive, and developmental delay or regression with poor eye contact and loss of motor skills with ataxia. Additional features observed in some patients include proximal renal tubulopathy, macrocytic anemia, sensorineural hearing loss, nystagmus, and hypertrophic cardiomyopathy, consistent with systemic involvement. Brain imaging in most patients shows lesions consistent with Leigh syndrome (see 256000). Laboratory studies show increased serum lactate and decreased levels and activity of mitochondrial respiratory complex IV. Most patients die in infancy (summary by Valnot et al., 2000 and Antonicka et al., 2003). For a discussion of genetic heterogeneity of mitochondrial complex IV (cytochrome c oxidase) deficiency, see 220110.

Mitochondrial complex IV deficiency nuclear type 4 (MC4DN4) is an autosomal recessive multisystem metabolic disorder characterized by the onset of symptoms in infancy. Affected individuals show hypotonia, failure to thrive, and neurologic distress. Additional features include hepatomegaly, hepatic steatosis, increased serum lactate, and metabolic acidosis. Some patients may develop hypertrophic cardiomyopathy. Patient tissues show decreased levels and activity of mitochondrial respiratory complex IV. Death usually occurs in infancy (summary by Valnot et al., 2000 and Stiburek et al., 2009). For a discussion of genetic heterogeneity of mitochondrial complex IV (cytochrome c oxidase) deficiency, see 220110.

Bachmann-Bupp syndrome (BABS) is characterized by a distinctive type of alopecia, global developmental delay in the moderate to severe range, hypotonia, nonspecific dysmorphic features, behavioral abnormalities (autism spectrum disorder, attention-deficit/hyperactivity disorder) and feeding difficulties. Hair is typically present at birth but may be sparse and of an unexpected color with subsequent loss of hair in large clumps within the first few weeks of life. Rare findings may include seizures with onset in later childhood and conductive hearing loss.

Cardiofacioneurodevelopmental syndrome (CFNDS) is characterized by microcephaly, midline facial defects, developmental delay, and cerebellar hypoplasia. Variable cardiac defects may be present, including atrioventricular canal and ventricular septal defects. Heterotaxy has also been reported (Harel et al., 2020).

Kaya-Barakat-Masson syndrome (KABAMAS) is a severe autosomal recessive neurodevelopmental disorder characterized by profoundly impaired global development with variable motor abnormalities, such as axial hypotonia, peripheral spasticity, dystonia, and poor coordination, resulting in the inability to sit or walk. Affected individuals have impaired intellectual development with absent speech, poor eye contact, and feeding difficulties, resulting in poor overall growth, sometimes with microcephaly. Dysmorphic features are generally not present. Additional more variable features include early-onset seizures, ocular anomalies, foot deformities, and nonspecific brain imaging findings, such as thin corpus callosum and cerebral, cerebellar, or pontine atrophy. Some patients may die in infancy or early childhood (summary by AlMuhaizea et al., 2020 and Diaz et al., 2020).

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.

X-linked multiple congenital anomalies-neurodevelopmental syndrome (MCAND) is an X-linked recessive congenital multisystemic disorder characterized by poor growth, global developmental delay with impaired intellectual development, and variable abnormalities of the cardiac, skeletal, and genitourinary systems. Most affected individuals also have hypotonia and dysmorphic craniofacial features. Brain imaging typically shows enlarged ventricles and thin corpus callosum; some have microcephaly, whereas others have hydrocephalus. The severity of the disorder is highly variable, ranging from death in early infancy to survival into the second or third decade. Pathogenetically, the disorder results from disrupted gene expression and signaling during embryogenesis, thus affecting multiple systems (summary by Tripolszki et al., 2021 and Beck et al., 2021). Beck et al. (2021) referred to the disorder as LINKED syndrome (LINKage-specific deubiquitylation deficiency-induced Embryonic Defects).

Myofibrillar myopathy-11 (MFM11) is an autosomal recessive skeletal muscle disorder characterized by onset of slowly progressive proximal muscle weakness in the first decade of life. Some patients may present at birth with hypotonia and feeding difficulties, whereas others present later in mid-childhood. Although most patients show delayed walking at 2 to 3 years, all remain ambulatory into adulthood. More variable features may include decreased respiratory forced vital capacity, variable cardiac features, and calf hypertrophy. Skeletal muscle biopsy shows myopathic changes with variation in fiber size, type 1 fiber predominance, centralized nuclei, eccentrically placed core-like lesions, and distortion of the myofibrillary pattern with Z-line streaming and abnormal myofibrillar aggregates or inclusions (summary by Donkervoort et al., 2020). For a phenotypic description and a discussion of genetic heterogeneity of myofibrillar myopathy, see MFM1 (601419).

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.

Hypomyelinating leukodystrophy-21 (HLD21) is an autosomal recessive neurodegenerative disorder characterized by global developmental delay apparent from infancy with loss of motor, speech, and cognitive milestones in the first decades of life. Affected individuals show cerebellar and pyramidal signs, including nystagmus, ataxia, dystonia, and spasticity, resulting in the loss of ambulation. Other more variable features include feeding difficulties, poor overall growth with microcephaly, optic atrophy, and seizures. Brain imaging shows diffuse hypomyelination of the white matter and atrophy of the cerebellum and corpus callosum. The disorder is progressive and may lead to premature death (summary by Dorboz et al., 2018). For a general phenotypic description and a discussion of genetic heterogeneity of HLD, see 312080.

Mitochondrial complex IV deficiency nuclear type 22 (MC4DN22) is an autosomal recessive metabolic disorder characterized by neonatal hypertrophic cardiomyopathy, encephalopathy, and severe lactic acidosis with fatal outcome (Wintjes et al., 2021). For a discussion of genetic heterogeneity of mitochondrial complex IV (cytochrome c oxidase) deficiency, see 220110.

Faundes-Banka syndrome (FABAS) is an autosomal dominant disorder characterized by variable combinations of developmental delay and microcephaly, as well as micrognathia and other dysmorphic features (Faundes et al., 2021).

Luo-Schoch-Yamamoto syndrome (LUSYAM) is a neurodevelopmental disorder characterized by global developmental delay and impaired intellectual development apparent from infancy. Affected individuals have delayed walking, early-onset seizures, hypotonia, dysmorphic facial features, and white matter abnormalities on brain imaging (Luo et al., 2021).

DYRK1A syndrome is characterized by intellectual disability including impaired speech development, autism spectrum disorder including anxious and/or stereotypic behavior problems, and microcephaly. Affected individuals often have a clinically recognizable phenotype including a typical facial gestalt, feeding problems, seizures, hypertonia, gait disturbances, and foot anomalies. The majority of affected individuals function in the moderate-to-severe range of intellectual disability; however, individuals with mild intellectual disability have also been reported. Other medical concerns relate to febrile seizures in infancy; the development of epilepsy with seizures of the atonic, absence, and generalized myoclonic types; short stature; and gastrointestinal problems. Ophthalmologic, urogenital, cardiac, and/or dental anomalies have been reported.

Autosomal recessive pseudohypoaldosteronism type I, including PHA1B1, is characterized by renal salt wasting and high concentrations of sodium in sweat, stool, and saliva. The disorder involves multiple organ systems and is especially threatening in the neonatal period. Laboratory evaluation shows hyponatremia, hyperkalemia, and increased plasma renin activity with high serum aldosterone concentrations. Respiratory tract infections are common in affected children and may be mistaken for cystic fibrosis (CF; 219700). Aggressive salt replacement and control of hyperkalemia results in survival, and the disorder appears to become less severe with age (review by Scheinman et al., 1999). A milder, autosomal dominant form of type I pseudohypoaldosteronism (PHA1A; 177735) is caused by mutations in the mineralocorticoid receptor gene (MCR, NR3C2; 600983). Gitelman syndrome (263800), another example of primary renal tubular salt wasting, is due to mutation in the thiazide-sensitive sodium-chloride cotransporter (SLC12A3; 600968). Hanukoglu and Hanukoglu (2016) provided a detailed review of the ENaC gene family, including structure, function, tissue distribution, and associated inherited diseases.

Developmental and epileptic encephalopathy-107 (DEE107) is a severe autosomal recessive neurologic disorder characterized by onset of seizures in the first months of life and severe global developmental delay, profound intellectual disability, progressive microcephaly, and hypotonia (Conroy et al., 2016). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

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).