Glycogen storage disease type III (GSD III) is characterized by variable liver, cardiac muscle, and skeletal muscle involvement. GSD IIIa is the most common subtype, present in about 85% of affected individuals; it manifests with liver and muscle involvement. GSD IIIb, with liver involvement only, comprises about 15% of all affected individuals. In infancy and early childhood, liver involvement presents as hepatomegaly and failure to thrive, with fasting ketotic hypoglycemia, hyperlipidemia, and elevated hepatic transaminases. In adolescence and adulthood, liver disease becomes less prominent. Most individuals develop cardiac involvement with cardiac hypertrophy and/or cardiomyopathy. Skeletal myopathy manifesting as weakness may be evident in childhood and slowly progresses, typically becoming prominent in the third to fourth decade. The overall prognosis is favorable but cannot be predicted on an individual basis. Long-term complications such as muscular and cardiac symptoms as well as liver fibrosis/cirrhosis and hepatocellular carcinoma may have a severe impact on prognosis and quality of life. To date, it is unknown if long-term complications can be alleviated and/or avoided by dietary interventions.

Hallermann-Streiff syndrome is characterized by a typical skull shape (brachycephaly with frontal bossing), hypotrichosis, microphthalmia, cataracts, beaked nose, micrognathia, skin atrophy, dental anomalies, and proportionate short stature (Hallermann, 1948; Streiff, 1950; Francois, 1958). Mental retardation is present in a minority of cases (Gorlin et al., 1990).

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

Baller-Gerold syndrome (BGS) can be suspected at birth in an infant with craniosynostosis and upper limb abnormality. The coronal suture is most commonly affected; the metopic, lambdoid, and sagittal sutures may also be involved alone or in combination. Upper limb abnormality can include a combination of thumb hypo- or aplasia and radial hypo- or aplasia and may be asymmetric. Malformation or absence of carpal or metacarpal bones has also been described. Skin lesions may appear anytime within the first few years after birth, typically beginning with erythema of the face and extremities and evolving into poikiloderma. Slow growth is apparent in infancy with eventual height and length typically at 4 SD below the mean.

Vascular Ehlers-Danlos syndrome (vEDS) is characterized by arterial, intestinal, and/or uterine fragility; thin, translucent skin; easy bruising; characteristic facial appearance (thin vermilion of the lips, micrognathia, narrow nose, prominent eyes); and an aged appearance to the extremities, particularly the hands. Vascular dissection or rupture, gastrointestinal perforation, or organ rupture are the presenting signs in most adults with vEDS. Arterial rupture may be preceded by aneurysm, arteriovenous fistulae, or dissection but also may occur spontaneously. The majority (60%) of individuals with vEDS who are diagnosed before age 18 years are identified because of a positive family history. Neonates may present with clubfoot, hip dislocation, limb deficiency, and/or amniotic bands. Approximately half of children tested for vEDS in the absence of a positive family history present with a major complication at an average age of 11 years. Four minor diagnostic features – distal joint hypermobility, easy bruising, thin skin, and clubfeet – are most often present in those children ascertained without a major complication.

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

A rare chromosomal anomaly with characteristics of psychomotor developmental delay, facial dysmorphism (trigonocephaly, midface hypoplasia, upslanting palpebral fissures, dysplastic small ears, flat nasal bridge with anteverted nostrils and long philtrum, micrognathia, choanal atresia, short neck), single umbilical artery, omphalocele, inguinal or umbilical hernia, genital abnormalities (hypospadia, cryptorchidism), muscular hypotonia and scoliosis.

Filippi syndrome is characterized by short stature, microcephaly, syndactyly, intellectual disability, and facial dysmorphism consisting of bulging forehead, broad and prominent nasal bridge, and diminished alar flare. Common features include cryptorchidism, speech impairment, and clinodactyly of the fifth finger, Some patients exhibit visual disturbances, polydactyly, seizures, and/or ectodermal abnormalities, such as nail hypoplasia, long eyelashes, hirsutism, and microdontia (summary by Hussain et al., 2014).

Gomez-Lopez-Hernandez syndrome (GLHS), also known as cerebellotrigeminal dermal dysplasia, is a rare neurocutaneous syndrome classically characterized by the triad of rhombencephalosynapsis, trigeminal anesthesia, often giving rise to corneal opacities, and bilateral parietal or parietooccipital alopecia. However, trigeminal anesthesia is an inconsistent finding (summary by Sukhudyan et al., 2010).

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.

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.

This syndrome is characterized by microcephaly, severe intellectual deficit, phalangeal anomalies (cutaneous syndactyly of the fingers, toe brachyclinodactyly and nail hypoplasia) and neurological manifestations (epilepsy, spastic/dystonic paraplegia and brisk reflexes).

Transaldolase deficiency (TALDOD) is a rare inborn error of pentose metabolism. Typical features include intrauterine growth restriction, triangular face, loose wrinkly skin at birth, and development of progressive liver failure (summary by Lee-Barber et al., 2019).

Barber-Say syndrome (BBRSAY) is a rare congenital condition characterized by severe hypertrichosis, especially of the back, skin abnormalities such as hyperlaxity and redundancy, and facial dysmorphism, including macrostomia, eyelid deformities, ocular telecanthus, abnormal and low-set ears, bulbous nasal tip with hypoplastic alae nasi, and low frontal hairline (summary by Roche et al., 2010).

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

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.

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.

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.

Hypoparathyroidism-retardation-dysmorphism syndrome (HRDS) is an autosomal recessive multisystem disorder characterized by intrauterine and postnatal growth retardation, infantile-onset hypoparathyroidism that can result in severe hypocalcemic seizures, dysmorphic facial features, and developmental delay (summary by Padidela et al., 2009 and Ratbi et al., 2015).

Yunis-Varon syndrome (YVS) is a severe autosomal recessive disorder characterized by skeletal defects, including cleidocranial dysplasia and digital anomalies, and severe neurologic involvement with neuronal loss. Enlarged cytoplasmic vacuoles are found in neurons, muscle, and cartilage. The disorder is usually lethal in infancy (summary by Campeau et al., 2013).

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

Shohat-type spondyloepimetaphyseal dysplasia (SEMDSH) is a chondrodysplasia characterized by vertebral, epiphyseal, and metaphyseal abnormalities, including scoliosis with vertebral compression fractures, flattened vertebral bodies, and hypomineralization of long bones. Affected individuals may exhibit a small trunk, short neck, small limbs, joint laxity, bowlegs, and/or abdominal distention with hepatosplenomegaly (summary by Egunsola et al., 2017).

Pierpont syndrome (PRPTS) is a multiple congenital anomaly syndrome associated with learning disability. Key features include distinctive facial characteristics, especially when smiling, plantar fat pads, and other limb anomalies (summary by Burkitt Wright et al., 2011).

Penttinen syndrome (PENTT) is characterized by a prematurely aged appearance involving lipoatrophy and epidermal and dermal atrophy, as well as hypertrophic lesions that resemble scars, thin hair, proptosis, underdeveloped cheekbones, and marked acroosteolysis (Johnston et al., 2015).

A very rare inherited malformation syndrome with characteristics of omphalocele, scoliosis, mild dysmorphic features (downslanted palpebral fissures, s-shaped eyelids and thin upper lip), laryngeal and pharyngeal hypoplasia and learning disabilities.

Syndrome with characteristics of severe psychomotor retardation, failure to thrive and intolerance to wheat and dairy products. So far, only two cases have been described. The disease is caused by mutations in the COG8 gene, which encodes a subunit of the COG complex. This complex is involved vesicle transport in the Golgi apparatus.

The 1q21.1 recurrent microdeletion itself does not appear to lead to a clinically recognizable syndrome as some persons with the deletion have no obvious clinical findings and others have variable findings that most commonly include microcephaly (50%), mild intellectual disability (30%), mildly dysmorphic facial features, and eye abnormalities (26%). Other findings can include cardiac defects, genitourinary anomalies, skeletal malformations, and seizures (~15%). Psychiatric and behavioral abnormalities can include autism spectrum disorders, attention deficit hyperactivity disorder, autistic features, and sleep disturbances.

Familial and <i>de novo</i> recurrent Xp11.22-p11.23 microduplication has been recently identified in males and females.

Autosomal recessive form of oculodentodigital dysplasia.

Growth retardation, developmental delay, and facial dysmorphism (GDFD) is an autosomal recessive multiple congenital anomaly syndrome characterized by severe psychomotor retardation, poor overall growth, and dysmorphic facial features. Additional features may include cardiac malformations and deafness (summary by Daoud et al., 2016).

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

Congenital disorders of glycosylation (CDGs) are a genetically heterogeneous group of autosomal recessive disorders caused by enzymatic defects in the synthesis and processing of asparagine (N)-linked glycans or oligosaccharides on glycoproteins. These glycoconjugates play critical roles in metabolism, cell recognition and adhesion, cell migration, protease resistance, host defense, and antigenicity, among others. CDGs are divided into 2 main groups: type I CDGs (see, e.g., CDG1A, 212065) comprise defects in the assembly of the dolichol lipid-linked oligosaccharide (LLO) chain and its transfer to the nascent protein, whereas type II CDGs refer to defects in the trimming and processing of the protein-bound glycans either late in the endoplasmic reticulum or the Golgi compartments. The biochemical changes of CDGs are most readily observed in serum transferrin (TF; 190000), and the diagnosis is usually made by isoelectric focusing of this glycoprotein (reviews by Marquardt and Denecke, 2003; Grunewald et al., 2002). Genetic Heterogeneity of Congenital Disorder of Glycosylation Type II Multiple forms of CDG type II have been identified; see CDG2B (606056) through CDG2Z (620201), and CDG2AA (620454) to CDG2BB (620546).

BBS2 is an autosomal recessive ciliopathy characterized by retinal degeneration, polydactyly, renal disease, hypogonadism, obesity, dysmorphic features, and variable degrees of cognitive impairment (Innes et al., 2010). Mutation in the BBS2 gene is the third most frequent cause of BBS, accounting for approximately 8% of cases (Zaghloul and Katsanis, 2009). For a general phenotypic description and a discussion of genetic heterogeneity of Bardet-Biedl syndrome, see BBS1 (209900).

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

Ogden syndrome (OGDNS) is an X-linked neurodevelopmental disorder characterized by postnatal growth failure, severely delayed psychomotor development, variable dysmorphic features, and hypotonia. Many patients also have cardiac malformations or arrhythmias (summary by Popp et al., 2015).

The Nascimento type of X-linked syndromic intellectual developmental disorder (MRXSN) is characterized by dysmorphic features, including large head, synophrys, prominent supraorbital ridges, almond-shaped and deep-set eyes, large ears, wide mouth, myxedematous appearance, hirsutism, abnormal hair whorls, micropenis, and onychodystrophy. Female carriers have normal cognition, but may show subtle facial features (summary by Budny et al., 2010).

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

Chromosome Xq27.3-q28 duplication syndrome is an X-linked recessive neurodevelopmental disorder characterized by mild mental retardation, mild facial dysmorphism, short stature, and primary testicular failure manifest as high-pitched voice, sparse body hair, abdominal obesity, and small testes. Female carriers may have short stature and premature ovarian failure (summary by Rio et al., 2010).

Multiple congenital anomalies-hypotonia-seizures syndrome is an autosomal recessive disorder characterized by neonatal hypotonia, lack of psychomotor development, seizures, dysmorphic features, and variable congenital anomalies involving the cardiac, urinary, and gastrointestinal systems. Most affected individuals die before 3 years of age (summary by Maydan et al., 2011). The disorder is caused by a defect in glycosylphosphatidylinositol biosynthesis; see GPIBD1 (610293). Genetic Heterogeneity of Multiple Congenital Anomalies-Hypotonia-Seizures Syndrome MCAHS2 (300868) is caused by mutation in the PIGA gene (311770) on chromosome Xp22, MCAHS3 (615398) is caused by mutation in the PIGT gene (610272) on chromosome 20q13, and MCAHS4 (618548) is caused by mutation in the PIGQ gene (605754) on chromosome 16p13. Knaus et al. (2018) provided a review of the main clinical features of the different types of MCAHS, noting that patients with mutations in the PIGN, PIGA, and PIGT genes have distinct patterns of facial anomalies that can be detected by computer-assisted comparison. Some individuals with MCAHS may have variable increases in alkaline phosphatase (AP) as well as variable decreases in GPI-linked proteins that can be detected by flow cytometry. However, there was no clear correlation between AP levels or GPI-linked protein abnormalities and degree of neurologic involvement, mutation class, or gene involved. Knaus et al. (2018) concluded that a distinction between MCAHS and HPMRS1 (239300), which is also caused by mutation in genes involved in GPI biosynthesis, may be artificial and even inaccurate, and that all these disorders should be considered and classified together under the more encompassing term of 'GPI biosynthesis defects' (GPIBD).

Cranioectodermal dysplasia (CED) is a ciliopathy with skeletal involvement (narrow thorax, shortened proximal limbs, syndactyly, polydactyly, brachydactyly), ectodermal features (widely spaced hypoplastic teeth, hypodontia, sparse hair, skin laxity, abnormal nails), joint laxity, growth deficiency, and characteristic facial features (frontal bossing, low-set simple ears, high forehead, telecanthus, epicanthal folds, full cheeks, everted lower lip). Most affected children develop nephronophthisis that often leads to end-stage kidney disease in infancy or childhood, a major cause of morbidity and mortality. Hepatic fibrosis and retinal dystrophy are also observed. Dolichocephaly, often secondary to sagittal craniosynostosis, is a primary manifestation that distinguishes CED from most other ciliopathies. Brain malformations and developmental delay may also occur.

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

Among the Yakuts, an Asian population isolate that is located in the northeastern part of Siberia, Maksimova et al. (2010) ascertained an autosomal recessive short stature syndrome involving postnatal growth failure, small hands and feet, loss of visual acuity with abnormalities of color vision, abnormal nuclear shape in neutrophil granulocytes (Pelger-Huet anomaly; see 169400), and normal intelligence.

Metaphyseal dysplasia and maxillary hypoplasia with or without brachydactyly (MDMHB) is an autosomal dominant bone dysplasia characterized by metaphyseal flaring of long bones, enlargement of the medial halves of the clavicles, maxillary hypoplasia, variable brachydactyly, and dystrophic teeth (summary by Moffatt et al., 2013).

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.

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.

Cerebellar dysfunction with variable cognitive and behavioral abnormalities (CECBA) is an autosomal dominant neurologic disorder with significant phenotypic heterogeneity, even within families. The disorder is most often diagnosed through genetic analysis with retrospective clinical phenotyping. Symptom onset is usually in early childhood, although later onset, even in adulthood, has been reported. Most affected individuals show global developmental delay from early childhood, particularly of motor and language skills. Many have mild intellectual disability; behavioral and psychiatric abnormalities such as autism and obsessive-compulsive disorder are also often observed. The movement disorder is prominent and may include cerebellar signs such as ataxia, tremor, dysmetria, poor coordination, and dysarthria. Other abnormal movements including spasticity, myoclonus, and dystonia have been reported, thus widening the phenotypic spectrum. Brain imaging is usually normal, but may show cerebellar atrophy or nonspecific white matter lesions. Variable dysmorphic facial features may also be present (summary by Thevenon et al., 2012; Jacobs et al., 2021; Wijnen et al., 2020).

Osteogenesis imperfecta (OI) is a connective tissue disorder characterized by bone fragility and low bone mass. Due to considerable phenotypic variability, Sillence et al. (1979) developed a classification of OI subtypes based on clinical features and disease severity: OI type I, with blue sclerae (166200); perinatal lethal OI type II, also known as congenital OI (166210); OI type III, a progressively deforming form with normal sclerae (259420); and OI type IV, with normal sclerae (166220). Most cases of OI are autosomal dominant with mutations in 1 of the 2 genes that code for type I collagen alpha chains, COL1A1 (120150) and COL1A2 (120160). Martinez-Glez et al. (2012) described osteogenesis imperfecta type XIII, an autosomal recessive form of the disorder characterized by normal teeth, faint blue sclerae, severe growth deficiency, borderline osteoporosis, and an average of 10 to 15 fractures a year affecting both upper and lower limbs and with severe bone deformity.

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

X-linked intellectual developmental disorder-98 (XLID98) is a neurodevelopmental disorder characterized by delayed psychomotor development, poor speech, behavioral abnormalities, poor overall growth, dysmorphic facial features, and often early-onset seizures. Some carrier females are unaffected, whereas other females with mutations are affected; males tend to be more severely affected than females. It is believed that the phenotypic variability and disease manifestations in female carriers results from skewed X-inactivation or cellular mosaicism (summary by de Lange et al., 2016).

A rare, genetic, neurodevelopmental disorder characterized by global developmental delay, borderline to severe intellectual disability, feeding difficulties, behavioral anomalies, vision anomalies and mild facial dysmorphism. Other associated features may include microcephaly, short stature, urogenital or palatal anomalies (e.g. cleft palate), minor cardiac defects, recurrent infections or hearing loss.

ADNP-related disorder is characterized by hypotonia, severe speech and motor delay, mild-to-severe intellectual disability, and characteristic facial features (prominent forehead, high anterior hairline, wide and depressed nasal bridge, and short nose with full, upturned nasal tip) based on a cohort of 78 individuals. Features of autism spectrum disorder are common (stereotypic behavior, impaired social interaction). Other common findings include additional behavioral problems, sleep disturbance, brain abnormalities, seizures, feeding issues, gastrointestinal problems, visual dysfunction (hypermetropia, strabismus, cortical visual impairment), musculoskeletal anomalies, endocrine issues including short stature and hormonal deficiencies, cardiac and urinary tract anomalies, and hearing loss.

Tatton-Brown-Rahman syndrome (TBRS) is an overgrowth / intellectual disability syndrome characterized by length/height and/or head circumference =2 SD above the mean for age and sex, obesity / increased weight, intellectual disability that ranges from mild to severe, joint hypermobility, hypotonia, behavioral/psychiatric issues, kyphoscoliosis, and seizures. Individuals with TBRS have subtle dysmorphic features, including a round face with coarse features, thick horizontal low-set eyebrows, narrow (as measured vertically) palpebral fissures, and prominent upper central incisors. The facial gestalt is most easily recognizable in the teenage years. TBRS may be associated with an increased risk of developing acute myeloid leukemia. There are less clear associations with aortic root dilatation and increased risk of other hematologic and solid tumors.

Any autosomal recessive non-syndromic intellectual disability in which the cause of the disease is a mutation in the METTL23 gene.

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

Hypomyelinating leukodystrophy-10 (HLD10) is an autosomal recessive neurologic disorder characterized by postnatal progressive microcephaly, severely delayed psychomotor development, and hypomyelination on brain imaging (summary by Nakayama et al., 2015). For a general phenotypic description and a discussion of genetic heterogeneity of HLD, see 312080.

White-Sutton syndrome is a neurodevelopmental disorder characterized by a wide spectrum of cognitive dysfunction, developmental delays (particularly in speech and language acquisition), hypotonia, autism spectrum disorder, and other behavioral problems. Additional features commonly reported include seizures, refractive errors and strabismus, hearing loss, sleep disturbance (particularly sleep apnea), feeding and gastrointestinal problems, mild genital abnormalities in males, and urinary tract involvement in both males and females.

A rare genetic, multiple congenital anomalies syndrome characterized by short stature, hand brachydactyly with hypoplastic distal phalanges, global development delay, intellectual disability, and more variably seizures, obesity, and craniofacial dysmorphism that includes microcephaly, high forehead, flat face, hypertelorism, deep set eyes, flat nasal bridge, averted nostrils, long philtrum, thin lip vermilion, and short neck.

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).

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).

CDK13-related disorder, reported in 43 individuals to date, is characterized in all individuals by developmental delay / intellectual disability (DD/ID); nearly all individuals older than age one year display impaired verbal language skills (either absent or restricted speech). Other common findings are recognizable facial features in some individuals, behavioral problems (autism spectrum disorder or autistic traits/stereotypies, attention-deficit/hyperactivity disorder), feeding difficulties in infancy, structural cardiac defects, and seizures.

BTDD is an autosomal dominant disorder characterized by brachycephaly, trichomegaly, and developmental delay. Although it is caused by dysfunction of the ribosome, patients do not have anemia (summary by Paolini et al., 2017).

Intellectual developmental disorder with neuropsychiatric features is an autosomal recessive disorder characterized by moderate intellectual disability, relatively mild seizures, and neuropsychiatric abnormalities, such as anxiety, obsessive-compulsive behavior, and autistic features. Mild facial dysmorphic features may also be present (summary by Srour et al., 2017).

Congenital heart defects and skeletal malformations syndrome (CHDSKM) is characterized by atrial and ventricular septal defects, with aortic root dilation in adulthood. Skeletal defects are variable and include pectus excavatum, scoliosis, and finger contractures, and some patients exhibit joint laxity. Failure to thrive is observed during infancy and early childhood (Wang et al., 2017).

Chung-Jansen syndrome (CHUJANS) is characterized by global developmental delay apparent from infancy, impaired intellectual development or learning difficulties, behavioral abnormalities, dysmorphic features, and obesity. The severity of the phenotype and additional features are variable (summary by Jansen et al., 2018).

Characteristic features of the distal 3p- syndrome include low birth weight, microcephaly, trigonocephaly, hypotonia, psychomotor and growth retardation, ptosis, telecanthus, downslanting palpebral fissures, and micrognathia. Postaxial polydactyly, renal anomalies, cleft palate, congenital heart defects (especially atrioventricular septal defects), preauricular pits, sacral dimple, and gastrointestinal anomalies are variable features. Although intellectual deficits are almost invariably associated with cytogenetically visible 3p deletions, rare patients with a 3p26-p25 deletion and normal intelligence or only mild abnormalities have been described (summary by Shuib et al., 2009).

Neurodevelopmental disorder with cerebellar atrophy and with or without seizures (NEDCAS) is an autosomal recessive disorder characterized by intellectual disability associated with ataxia (summary by Engel et al., 2023).

Paganini-Miozzo syndrome (MRXSPM) is a neurodevelopmental disorder characterized by global developmental delay, impaired intellectual development, high myopia, and mild dysmorphic facial features (summary by Paganini et al., 2019)

Polymicrogyria with or without vascular-type Ehlers-Danlos syndrome is an autosomal recessive disorder with a highly variable phenotype. Although all patients have polymicrogyria and other variable structural brain anomalies on imaging, only some show developmental delay and/or seizures. Similarly, only some patients have connective tissue defects that particularly affect the vascular system and can result in early death (summary by Vandervore et al., 2017).

Myoectodermal gonadal dysgenesis syndrome (MEGD) is characterized by 46,XY complete or partial gonadal dysgenesis, or 46,XX gonadal dysgenesis, in association with extragonadal anomalies, including low birth weight, typical facies, rod and cone dystrophy, sensorineural hearing loss, omphalocele, anal atresia, renal agenesis, skeletal abnormalities, dry and scaly skin, severe myopathy, and neuromotor delay. Dysmorphic facial features along with muscular habitus are the hallmarks of the syndrome. Abnormal hair patterning with frontal upsweep and additional whorls, eyebrow abnormalities comprising broad, arched, and sparse or thick eyebrows, underdeveloped alae nasi, smooth philtrum, and low-set ears with overfolded helices facilitate a gestalt diagnosis. (Guran et al., 2019; Altunoglu et al., 2022).

Neurodevelopmental disorder with microcephaly, arthrogryposis, and structural brain anomalies (NEDMABA) is an autosomal recessive disorder characterized by severe global developmental delay, usually with hypotonia and absence of spontaneous movements other than head control, impaired intellectual development with absent speech, distal contractures, progressive microcephaly, dysmorphic features, and distal skeletal abnormalities, such as rocker-bottom feet and clenched hands with camptodactyly. Brain imaging tends to show a simplified gyral pattern of the cerebral cortex, delayed myelination, thin corpus callosum, and hypoplasia of the brainstem and cerebellum. The disorder may be complicated by feeding and/or breathing difficulties, often resulting in death in infancy (summary by Magini 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).

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).

Distal arthrogryposis type 1C (DA1C) is characterized by multiple congenital contractures, scoliosis, and short stature. Contractures involving the proximal joints appear to be more common in MYLPF-associated DA than in other forms of DA, and segmental amyoplasia has been observed (Chong et al., 2020).

Rauch-Steindl syndrome (RAUST) is characterized by poor pre- and postnatal growth, sometimes with short stature and small head circumference, characteristic dysmorphic facial features, and variable developmental delay with delayed motor and speech acquisition and impaired intellectual function that can be mild. Other features may include hypotonia and behavioral abnormalities. The phenotype represents a mild form of Wolf-Hirschhorn syndrome (WHS; 194190), which is a contiguous gene deletion syndrome caused by heterozygous deletion of several genes on chromosome 4p16. The clinical features of RAUST are similar to but milder than those of WHS, with less severe dysmorphic facial features, less severe developmental disabilities in general, and absence of a seizure disorder. The phenotype and expressivity of RAUST is highly variable (summary by Rauch et al., 2001; Zanoni et al., 2021).

Stuve-Wiedemann syndrome is an autosomal recessive disorder characterized by bowing of the long bones and other skeletal anomalies, episodic hyperthermia, respiratory distress, and feeding difficulties usually resulting in early death (Dagoneau et al., 2004). See also 'classic' Schwartz-Jampel syndrome type 1 (SJS1; 255800), a phenotypically similar but genetically distinct disorder caused by mutation in the HSPG2 gene (142461) on chromosome 1p36. Genetic Heterogeneity of Stuve-Wiedemann Syndrome Stuve-Wiedemann syndrome-2 (STWS2; 619751) is caused by mutation in the IL6ST gene (600694) on chromosome 5q11.

Combined oxidative phosphorylation deficiency-54 (COXPD54) is an autosomal recessive disorder with pleiotropic multisystem presentations resulting from a disruption in mitochondrial transcription and translation. The phenotype is highly variable. Many patients have early-onset sensorineural hearing loss, sometimes in isolation, and sometimes associated with global developmental delay or primary ovarian failure. Other features may include peripheral hypertonia, seizures, muscle weakness, behavioral abnormalities, and leukoencephalopathy on brain imaging. Serum lactate may or may not be elevated (summary by Hochberg et al., 2021). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Neurocardiofaciodigital syndrome (NCFD) is characterized by severe developmental delay, variable brain anomalies, congenital heart defects, dysmorphic facial features, and a distinctive type of synpolydactyly with a supernumerary hypoplastic digit between the fourth and fifth digits of the hands and/or feet. Other features include eye abnormalities, hearing impairment, and electroencephalogram anomalies (summary by Horn et al., 2021).

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

RECON progeroid syndrome (RECON) is a chromosomal instability disorder characterized by postnatal growth retardation, progeroid facial appearance, hypoplastic nose, prominent premaxilla, skin photosensitivity and xeroderma, muscle wasting with reduced subcutaneous fat, and slender elongated thumbs (Abu-Libdeh et al., 2022).

Congenital disorder of glycosylation type IIbb (CDG2BB) is an autosomal recessive disorder characterized by global developmental delay, severely impaired intellectual development, microcephaly, epilepsy, facial dysmorphism, and variable neurologic findings (Duan et al., 2023).

Alfadhel syndrome (AFDL) is an autosomal recessive neurodevelopmental disorder with features of global developmental delay, hypotonia, and facial dysmorphism (Asiri et al., 2020, Bertoli-Avella et al., 2021).