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.

Dejerine-Sottas neuropathy is a demyelinating peripheral neuropathy with onset in infancy. It can show autosomal dominant or recessive inheritance. Affected individuals have delayed motor development due to severe distal motor and sensory impairment, resulting in difficulties in gait. Some patients have generalized hypotonia in infancy. Other features may include pes cavus, scoliosis, and sensory ataxia. Nerve conduction velocities are severely decreased (sometimes less than 10 m/s), and sural nerve biopsy shows severe loss of myelinated fibers (summary by Baets et al., 2011).

FMR1 disorders include fragile X syndrome (FXS), fragile X-associated tremor/ataxia syndrome (FXTAS), and fragile X-associated primary ovarian insufficiency (FXPOI). Fragile X syndrome occurs in individuals with an FMR1 full mutation or other loss-of-function variant and is nearly always characterized in affected males by developmental delay and intellectual disability along with a variety of behavioral issues. Autism spectrum disorder is present in 50%-70% of individuals with FXS. Affected males may have characteristic craniofacial features (which become more obvious with age) and medical problems including hypotonia, gastroesophageal reflux, strabismus, seizures, sleep disorders, joint laxity, pes planus, scoliosis, and recurrent otitis media. Adults may have mitral valve prolapse or aortic root dilatation. The physical and behavioral features seen in males with FXS have been reported in females heterozygous for the FMR1 full mutation, but with lower frequency and milder involvement. FXTAS occurs in individuals who have an FMR1 premutation and is characterized by late-onset, progressive cerebellar ataxia and intention tremor followed by cognitive impairment. Psychiatric disorders are common. Age of onset is typically between 60 and 65 years and is more common among males who are hemizygous for the premutation (40%) than among females who are heterozygous for the premutation (16%-20%). FXPOI, defined as hypergonadotropic hypogonadism before age 40 years, has been observed in 20% of women who carry a premutation allele compared to 1% in the general population.

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

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

The X-linked otopalatodigital (X-OPD) spectrum disorders, characterized primarily by skeletal dysplasia, include the following: Otopalatodigital syndrome type 1 (OPD1). Otopalatodigital syndrome type 2 (OPD2). Frontometaphyseal dysplasia type 1 (FMD1). Melnick-Needles syndrome (MNS). Terminal osseous dysplasia with pigmentary skin defects (TODPD). In OPD1, most manifestations are present at birth; females can present with severity similar to affected males, although some have only mild manifestations. In OPD2, females are less severely affected than related affected males. Most males with OPD2 die during the first year of life, usually from thoracic hypoplasia resulting in pulmonary insufficiency. Males who live beyond the first year of life are usually developmentally delayed and require respiratory support and assistance with feeding. In FMD1, females are less severely affected than related affected males. Males do not experience a progressive skeletal dysplasia but may have joint contractures and hand and foot malformations. Progressive scoliosis is observed in both affected males and females. In MNS, wide phenotypic variability is observed; some individuals are diagnosed in adulthood, while others require respiratory support and have reduced longevity. MNS in males results in perinatal lethality in all recorded cases. TODPD, seen only in females, is characterized by a skeletal dysplasia that is most prominent in the digits, pigmentary defects of the skin, and recurrent digital fibromata.

Nail-patella syndrome (NPS) (previously referred to as Fong's disease), encompasses the classic clinical tetrad of changes in the nails, knees, and elbows, and the presence of iliac horns. Nail changes are the most constant feature of NPS. Nails may be absent, hypoplastic, or dystrophic; ridged longitudinally or horizontally; pitted; discolored; separated into two halves by a longitudinal cleft or ridge of skin; and thin or (less often) thickened. The patellae may be small, irregularly shaped, or absent. Elbow abnormalities may include limitation of extension, pronation, and supination; cubitus valgus; and antecubital pterygia. Iliac horns are bilateral, conical, bony processes that project posteriorly and laterally from the central part of the iliac bones of the pelvis. Renal involvement, first manifest as proteinuria with or without hematuria, occurs in 30%-50% of affected individuals; end-stage renal disease occurs up to 15% of affected individuals. Primary open-angle glaucoma and ocular hypertension occur at increased frequency and at a younger age than in the general population.

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.

Sotos syndrome is characterized by a distinctive facial appearance (broad and prominent forehead with a dolichocephalic head shape, sparse frontotemporal hair, downslanting palpebral fissures, malar flushing, long and narrow face, long chin); learning disability (early developmental delay, mild-to-severe intellectual impairment); and overgrowth (height and/or head circumference =2 SD above the mean). These three clinical features are considered the cardinal features of Sotos syndrome. Major features of Sotos syndrome include behavioral problems (most notably autistic spectrum disorder), advanced bone age, cardiac anomalies, cranial MRI/CT abnormalities, joint hyperlaxity with or without pes planus, maternal preeclampsia, neonatal complications, renal anomalies, scoliosis, and seizures.

Aarskog-Scott syndrome is a genetic disorder that affects the development of many parts of the body, most commonly the head and face, the hands and feet, and the genitals and urinary system (genitourinary tract). This condition mainly affects males, although females may have mild features of the syndrome.\n\nPeople with Aarskog-Scott syndrome often have distinctive facial features, such as widely spaced eyes (hypertelorism), a small nose, a long area between the nose and mouth (philtrum), and a widow's peak hairline. They frequently have mild to moderate short stature during childhood, but their growth usually catches up with that of their peers during puberty. Hand abnormalities are common in this syndrome and include short fingers (brachydactyly), curved pinky fingers (fifth finger clinodactyly), webbing of the skin between some fingers (cutaneous syndactyly), and a single crease across the palm. Affected individuals can also have wide, flat feet with broad, rounded toes. Other abnormalities in people with Aarskog-Scott syndrome include heart defects and a split in the upper lip (cleft lip) with or without an opening in the roof of the mouth (cleft palate).\n\nMost males with Aarskog-Scott syndrome have a shawl scrotum, in which the scrotum surrounds the penis instead of hanging below. Less often, they have undescended testes (cryptorchidism) or a soft out-pouching around the belly-button (umbilical hernia) or in the lower abdomen (inguinal hernia).\n\nThe intellectual development of people with Aarskog-Scott syndrome varies widely. Most individuals with Aarskog-Scott syndrome have normal intelligence; however, some may have mild learning and behavior problems, and in rare cases, severe intellectual disability has been reported.

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.

Mietens syndrome is a very rare syndrome consisting of corneal opacity, nystagmus, strabismus, flexion contracture of the elbows with dislocation of the head of the radius and abnormally short ulnae and radii.

Coffin-Lowry syndrome (CLS) is usually characterized by severe-to-profound intellectual disability in males; less severely impaired individuals have been reported. Neuropsychiatric concerns can include behavioral problems, loss of strength, progressive spasticity or paraplegia, sleep apnea, or stroke. Stimulus-induced drop attacks (SIDAs) in which unexpected tactile or auditory stimuli or excitement triggers a brief collapse but no loss of consciousness are present in approximately 20% of affected individuals. Typically SIDAs begin between mid-childhood and the teens. Characteristic facial features may be more apparent with age. Upper-extremity differences may be subtle and include short, soft, fleshy hands with tapered fingers as well as fleshy forearms. Progressive kyphoscoliosis is one of the most difficult aspects of long-term care. Affected females tend to have intellectual disability in the mild-to-moderate range and may also have the typical facial, hand, and skeletal findings noted in males.

Dyggve-Melchior-Clausen disease (DMC) is an autosomal recessive disorder characterized by progressive spondyloepimetaphyseal dysplasia and impaired intellectual development. Short-trunk dwarfism and microcephaly are present, and specific radiologic appearances most likely reflect abnormalities of the growth plates, including platyspondyly with notched end plates, metaphyseal irregularities, laterally displaced capital femoral epiphyses, and small iliac wings with lacy iliac crests (summary by El Ghouzzi et al., 2003).

Classic Ehlers-Danlos syndrome (cEDS) is a connective tissue disorder characterized by skin hyperextensibility, atrophic scarring, and generalized joint hypermobility (GJH). The skin is soft and doughy to the touch, and hyperextensible, extending easily and snapping back after release (unlike lax, redundant skin, as in cutis laxa). The skin is fragile, as manifested by splitting of the dermis following relatively minor trauma, especially over pressure points (knees, elbows) and areas prone to trauma (shins, forehead, chin). Wound healing is poor, and stretching of scars after apparently successful primary wound healing is characteristic. Complications of joint hypermobility, such as dislocations of the shoulder, patella, digits, hip, radius, and clavicle, usually resolve spontaneously or are easily managed by the affected individual. Other features include hypotonia with delayed motor development, fatigue and muscle cramps, and easy bruising. Mitral valve prolapse can occur infrequently, but tends to be of little clinical consequence. Aortic root dilatation has been reported, appears to be more common in young individuals, and rarely progresses.

PLOD1-related kyphoscoliotic Ehlers-Danlos syndrome (kEDS) is an autosomal recessive generalized connective tissue disorder characterized by hypotonia, early-onset kyphoscoliosis, and generalized joint hypermobility in association with skin fragility and ocular abnormality. Intelligence is normal. Life span may be normal, but affected individuals are at risk for rupture of medium-sized arteries. Adults with severe kyphoscoliosis are at risk for complications from restrictive lung disease, recurrent pneumonia, and cardiac failure.

Occipital horn syndrome (OHS) is a rare connective tissue disorder characterized by hyperelastic and bruisable skin, hernias, bladder diverticula, hyperextensible joints, varicosities, and multiple skeletal abnormalities. The disorder is sometimes accompanied by mild neurologic impairment, and bony abnormalities of the occiput are a common feature, giving rise to the name (summary by Das et al., 1995).

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.

Alström syndrome is characterized by cone-rod dystrophy, obesity, progressive bilateral sensorineural hearing impairment, acute infantile-onset cardiomyopathy and/or adolescent- or adult-onset restrictive cardiomyopathy, insulin resistance / type 2 diabetes mellitus (T2DM), nonalcoholic fatty liver disease (NAFLD), and chronic progressive kidney disease. Cone-rod dystrophy presents as progressive visual impairment, photophobia, and nystagmus usually starting between birth and age 15 months. Many individuals lose all perception of light by the end of the second decade, but a minority retain the ability to read large print into the third decade. Children usually have normal birth weight but develop truncal obesity during their first year. Sensorineural hearing loss presents in the first decade in as many as 70% of individuals and may progress to the severe or moderately severe range (40-70 db) by the end of the first to second decade. Insulin resistance is typically accompanied by the skin changes of acanthosis nigricans, and proceeds to T2DM in the majority by the third decade. Nearly all demonstrate hypertriglyceridemia. Other findings can include endocrine abnormalities (hypothyroidism, hypogonadotropic hypogonadism in males, and hyperandrogenism in females), urologic dysfunction / detrusor instability, progressive decrease in renal function, and hepatic disease (ranging from elevated transaminases to steatohepatitis/NAFLD). Approximately 20% of affected individuals have delay in early developmental milestones, most commonly in gross and fine motor skills. About 30% have a learning disability. Cognitive impairment (IQ <70) is very rare. Wide clinical variability is observed among affected individuals, even within the same family.

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.

Trichorhinophalangeal syndrome (TRPS) comprises TRPS I (caused by a heterozygous pathogenic variant in TRPS1) and TRPS II (caused by contiguous gene deletion of TRPS1, RAD21, and EXT1). Both types of TRPS are characterized by distinctive facial features; ectodermal features (fine, sparse, depigmented, and slow growing hair; dystrophic nails; and small breasts); and skeletal findings (short stature; short feet; brachydactyly with ulnar or radial deviation of the fingers; and early, marked hip dysplasia). TRPS II is characterized by multiple osteochondromas (typically first observed clinically on the scapulae and around the elbows and knees between ages 1 month and 6 years) and an increased risk of mild-to-moderate intellectual disability.

Monosomy 18q is a partial deletion of the long arm of chromosome 18 characterized by highly variable phenotype, most commonly including hypotonia, developmental delay, short stature, growth hormone deficiency, hearing loss and external ear anomalies, intellectual disability, palatal defects, dysmorphic facial features, skeletal anomalies (foot deformities, tapering fingers, scoliosis) and mood disorders.

The Strudwick type of spondyloepimetaphyseal dysplasia (SEMD) is characterized by disproportionate short stature, pectus carinatum, and scoliosis, as well as dappled metaphyses (summary by Tiller et al., 1995).

Congenital myopathy-1A (CMYP1A) with susceptibility to malignant hyperthermia is an autosomal dominant disorder of skeletal muscle characterized by muscle weakness primarily affecting the proximal muscles of the lower limbs beginning in infancy or early childhood, although later onset of symptoms has been reported. There is significant phenotypic variability, even within families, and the wide clinical diversity most likely depends on the severity of the RYR1 mutation. The disorder is static or slowly progressive; affected individuals typically show delayed motor development and usually achieve independent walking, although many have difficulty running or climbing stairs. Additional features often include mild facial weakness, joint laxity, shoulder girdle weakness, and skeletal manifestations, such as dislocation of the hips, foot deformities, scoliosis, and Achilles tendon contractures. Some patients present with orthopedic deformities. Serum creatine kinase is usually not elevated. Respiratory involvement is rare and there is no central nervous system or cardiac involvement. Patients with dominant mutations in the RYR1 gene are at risk for malignant hyperthermia and both disorders may segregate in the same family. Historically, patients with congenital myopathy due to RYR1 mutations were diagnosed based on the finding of pathologic central cores (central core disease; CCD) on muscle biopsy, which represent areas that lack oxidative enzymes and mitochondrial activity in type 1 muscle fibers. However, additional pathologic findings may also be observed, including cores and rods, central nuclei, fiber type disproportion, multiminicores, and uniform type 1 fibers. These histopathologic features are not always specific to RYR1 myopathy and often change over time (Quinlivan et al., 2003; Jungbluth et al., 2007; Klein et al., 2012; Ogasawara and Nishino, 2021). Some patients with RYR1 mutations have pathologic findings on muscle biopsy, but are clinically asymptomatic (Shuaib et al., 1987; Quane et al., 1993). Rare patients with a more severe phenotype have been found to carry a heterozygous mutation in the RYR1 gene inherited from an unaffected parent. However, in these cases, there is a possibility of recessive inheritance (CMYP1B; 255320) with either a missed second RYR1 mutation in trans or a genomic rearrangement on the other allele that is undetectable on routine genomic sequencing, since the RYR1 gene is very large and genetic analysis may be difficult (Klein et al., 2012). Genetic Heterogeneity of Congenital Myopathy See also CMYP1B (255320), caused by mutation in the RYR1 gene (180901) on chromosome 19q13; CMYP2A (161800), CMYP2B (620265), and CMYP2C (620278), caused by mutation in the ACTA1 gene (102610) on chromosome 1q42; CMYP3 (602771), caused by mutation in the SELENON gene (606210) on chromosome 1p36; CMYP4A (255310) and CMYP4B (609284), caused by mutation in the TPM3 gene (191030) on chromosome 1q21; CMYP5 (611705), caused by mutation in the TTN gene (188840) on chromosome 2q31; CMYP6 (605637), caused by mutation in the MYH2 gene (160740) on chromosome 17p13; CMYP7A (608358) and CMYP7B (255160), caused by mutation in the MYH7 gene (160760) on chromosome 14q11; CMYP8 (618654), caused by mutation in the ACTN2 gene (102573) on chromosome 1q43; CMYP9A (618822) and CMYP9B (618823), caused by mutation in the FXR1 gene (600819) on chromosome 3q28; CMYP10A (614399) and CMYP10B (620249), caused by mutation in the MEGF10 gene (612453) on chromosome 5q23; CMYP11 (619967), caused by mutation in the HACD1 gene (610467) on chromosome 10p12; CMYP12 (612540), caused by mutation in the CNTN1 gene (600016) on chromosome 12q12; CMYP13 (255995), caused by mutation in the STAC3 gene (615521) on chromosome 12q13; CMYP14 (618414), caused by mutation in the MYL1 gene (160780) on chromosome 2q34; CMYP15 (620161), caused by mutation in the TNNC2 gene (191039) on chromosome 20q13; CMYP16 (618524), caused by mutation in the MYBPC1 gene (160794) on chromosome 12q23; CMYP17 (618975), caused by mutation in the MYOD1 gene (159970) on chromosome 11p15; CMYP18 (620246), caused by mutation in the CACNA1S gene (114208) on chromosome 1q32; CMYP19 (618578), caused by mutation in the PAX7 gene (167410) on chromosome 1p36; CMYP20 (620310), caused by mutation in the RYR3 gene (180903) on chromosome 15q13; CMYP21 (620326), caused by mutation in the DNAJB4 gene (611327) on chromosome 1p31; CMYP22A (620351) and CMYP22B (620369), both caused by mutation in the SCN4A gene (603967) on chromosome 17q23; CMYP23 (609285), caused by mutation in the TPM2 gene (190990) on chromosome 9p13; and CMYP24 (617336), caused by mutation in the MYPN gene (608517) on chromosome 10q21.

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.

Smith-Magenis syndrome (SMS) is characterized by distinctive physical features (particularly coarse facial features that progress with age), developmental delay, cognitive impairment, behavioral abnormalities, sleep disturbance, and childhood-onset abdominal obesity. Infants have feeding difficulties, failure to thrive, hypotonia, hyporeflexia, prolonged napping or need to be awakened for feeds, and generalized lethargy. The majority of individuals function in the mild-to-moderate range of intellectual disability. The behavioral phenotype, including significant sleep disturbance, stereotypies, and maladaptive and self-injurious behaviors, is generally not recognized until age 18 months or older and continues to change until adulthood. Sensory issues are frequently noted; these may include avoidant behavior, as well as repetitive seeking of textures, sounds, and experiences. Toileting difficulties are common. Significant anxiety is common as are problems with executive functioning, including inattention, distractibility, hyperactivity, and impulsivity. Maladaptive behaviors include frequent outbursts / temper tantrums, attention-seeking behaviors, opposition, aggression, and self-injurious behaviors including self-hitting, self-biting, skin picking, inserting foreign objects into body orifices (polyembolokoilamania), and yanking fingernails and/or toenails (onychotillomania). Among the stereotypic behaviors described, the spasmodic upper-body squeeze or "self-hug" seems to be highly associated with SMS. An underlying developmental asynchrony, specifically emotional maturity delayed beyond intellectual functioning, may also contribute to maladaptive behaviors in people with SMS.

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.

The cardinal findings of Neuhauser syndrome, also known as MMR syndrome, are impaired intellectual development or developmental delay, megalocornea, hypotonia, prominent forehead, micrognathia, prominent nasal bridge, and thin upper lip or carp-like mouth (Naritomi et al., 1997). Reviews Gutierrez-Amavizca et al. (2013) reviewed published reports and tabulated the clinical features of 35 patients with Neuhauser syndrome. Primary megalocornea and psychomotor delay were present in all patients. Characteristics observed in more than half of patients included hypotonia, growth retardation, abnormal electroencephalography (EEG) and/or seizures, micro- or macrocephaly, brain malformations such as cerebral atrophy and hypoplastic corpus callosum, craniofacial dysmorphisms, cardiac anomalies, osteoarticular abnormalities, and refractive errors. Additional features found at low frequency included primary hypothyroidism, recurrent infections, feeding difficulties, cerebral hypomyelination, dyslipidemia, sensorineural deafness, laryngomalacia, large fleshy and cup-shaped ears, obesity, and cryptorchidism. The authors stated that the classification suggested by Verloes et al. (1993) did not seem to be applicable, and proposed that the diagnosis of Neuhauser syndrome should be made in the presence of intellectual disability and megalocornea in the absence of elevated intraocular pressure, with at least 1 minor feature from among those observed in more than half of patients.

Intellectual developmental disorder-29 (XLID29) is a nonspecific form of XLID. It 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 lissencephaly (LISX2; 300215) to Proud syndrome (300004) to infantile spasms without brain malformations (DEE1; 308350) to Partington syndrome (309510) (Kato et al., 2004; Wallerstein et al., 2008).

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.

SALL4-related disorders include Duane-radial ray syndrome (DRRS, Okihiro syndrome), acro-renal-ocular syndrome (AROS), and SALL4-related Holt-Oram syndrome (HOS) – three phenotypes previously thought to be distinct entities. DRRS is characterized by uni- or bilateral Duane anomaly and radial ray malformation that can include thenar hypoplasia and/or hypoplasia or aplasia of the thumbs, hypoplasia or aplasia of the radii, shortening and radial deviation of the forearms, triphalangeal thumbs, and duplication of the thumb (preaxial polydactyly). AROS is characterized by radial ray malformations, renal abnormalities (mild malrotation, ectopia, horseshoe kidney, renal hypoplasia, vesicoureteral reflux, bladder diverticula), ocular coloboma, and Duane anomaly. Rarely, pathogenic variants in SALL4 may cause clinically typical HOS (i.e., radial ray malformations and cardiac malformations without additional features).

Traboulsi syndrome is characterized by dislocated crystalline lenses and anterior segment abnormalities in association with a distinctive facies involving flat cheeks and a beaked nose. Some affected individuals develop highly unusual nontraumatic conjunctival cysts (filtering blebs), presumably caused by abnormal thinning of the sclera (Patel et al., 2014).

Cayman cerebellar ataxia (ATCAY) is an autosomal recessive neurologic disorder characterized by hypotonia from birth, variable psychomotor retardation, and cerebellar dysfunction, including nystagmus, intention tremor, dysarthria, ataxic gait, and truncal ataxia. Although the disorder was initially believed to be restricted to an isolated region of Grand Cayman Island (summary by Nystuen et al., 1996; Bomar et al., 2003), one Pakistani family with the disorder and an ATCAY mutation has been reported, thus expanding the ethnic distribution (Manzoor et al., 2018).

A severe form of axonal Charcot-Marie-Tooth disease, a peripheral sensorimotor neuropathy. Onset in the second or third decade has manifestations of ulceration and infection of the feet. Symmetric and distal weakness develops mostly in the legs together with a severe symmetric distal sensory loss. Tendon reflexes are only reduced at ankles and foot deformities including pes cavus or planus and hammer toes, appear in childhood.

Satoyoshi syndrome is a rare disorder characterized by progressive, painful, intermittent muscle spasms, diarrhea or unusual malabsorption, endocrinopathy with amenorrhea, and secondary skeletal abnormalities. The disorder is also called komuragaeri disease by the Japanese; in Japanese 'komura' means calf and 'gaeri' means 'turnover' or spasm. All cases have apparently been sporadic, even when occurring in large families (Ehlayel and Lacassie, 1995).

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

Autosomal dominant multiple epiphyseal dysplasia (MED) presents in early childhood, usually with pain in the hips and/or knees after exercise. Affected children complain of fatigue with long-distance walking. Waddling gait may be present. Adult height is either in the lower range of normal or mildly shortened. The limbs are relatively short in comparison to the trunk. Pain and joint deformity progress, resulting in early-onset osteoarthritis, particularly of the large weight-bearing joints.

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

Wilson-Turner syndrome (WTS) is an X-linked recessive neurologic disorder characterized by intellectual disability, dysmorphic facial features, hypogonadism, short stature, and truncal obesity. Females are unaffected (Wilson et al., 1991).

L1 syndrome involves a phenotypic spectrum ranging from severe to mild and includes three clinical phenotypes: X-linked hydrocephalus with stenosis of the aqueduct of Sylvius (HSAS). MASA (mental retardation [intellectual disability], aphasia [delayed speech], spastic paraplegia [shuffling gait], adducted thumbs) syndrome including X-linked complicated hereditary spastic paraplegia type 1. X-linked complicated corpus callosum agenesis. Males with HSAS are born with severe hydrocephalus, adducted thumbs, and spasticity; intellectual disability is severe. In less severely affected males, hydrocephalus may be subclinically present and documented only because of developmental delay; intellectual disability ranges from mild (IQ: 50-70) to moderate (IQ: 30-50). It is important to note that all phenotypes can be observed in affected individuals within the same family.

Ischiocoxopodopatellar syndrome (ICPPS) is a rare autosomal dominant disorder characterized by a/hypoplasia of the patellas and various anomalies of the pelvis and feet. Pelvic anomalies include bilateral absent or delayed ossification of the ischiopubic junction and infraacetabular axe cut notches. Other major signs are a wide gap between the first and second toes, short fourth and fifth rays of the feet, and pes planus (summary by Bongers et al., 2001). Pediatric-onset pulmonary arterial hypertension may be seen in association with ICPPS (Kerstjens-Frederikse et al., 2013 and Levy et al., 2016).

Ichthyosis-cheek-eyebrow syndrome is characterized by ichthyosis, prominent full cheeks and sparse lateral eyebrows. It has been described in several individuals from four generations of one family. Transmission is autosomal dominant.

A rare genetic autosomal recessive cerebellar ataxia disease with characteristics of nonprogressive cerebellar ataxia, with onset in infancy, manifesting with delayed motor and speech development, gait ataxia, dysmetria, hypotonia, increased deep tendon reflexes and dysarthria. Additional variable manifestations include moderate nystagmus on lateral gaze, mild spasticity, intention tremor, short stature and pes planus. Brain imaging reveals cerebellar vermis atrophy.

Craniolenticulosutural dysplasia is characterized by facial dysmorphism, late-closing fontanels, cataract, and skeletal defects (summary by Boyadjiev et al., 2011).

The Cabezas type of X-linked syndromic intellectual developmental disorder is characterized primarily by short stature, hypogonadism, and abnormal gait, with other more variable features such as speech delay, prominent lower lip, and tremor (Cabezas et al., 2000).

Siderius-type syndromic intellectual developmental disorder (MRXSSD) is an X-linked disorder in which affected males have mildly impaired intellectual development, mild dysmorphic features, and bilateral or unilateral cleft lip/palate (summary by Koivisto et al., 2007).

The FLNB disorders include a spectrum of phenotypes ranging from mild to severe. At the mild end are spondylocarpotarsal synostosis (SCT) syndrome and Larsen syndrome; at the severe end are the phenotypic continuum of atelosteogenesis types I (AOI) and III (AOIII) and Piepkorn osteochondrodysplasia (POCD). SCT syndrome is characterized by postnatal disproportionate short stature, scoliosis and lordosis, clubfeet, hearing loss, dental enamel hypoplasia, carpal and tarsal synostosis, and vertebral fusions. Larsen syndrome is characterized by congenital dislocations of the hip, knee, and elbow; clubfeet (equinovarus or equinovalgus foot deformities); scoliosis and cervical kyphosis, which can be associated with a cervical myelopathy; short, broad, spatulate distal phalanges; distinctive craniofacies (prominent forehead, depressed nasal bridge, malar flattening, and widely spaced eyes); vertebral anomalies; and supernumerary carpal and tarsal bone ossification centers. Individuals with SCT syndrome and Larsen syndrome can have midline cleft palate and hearing loss. AOI and AOIII are characterized by severe short-limbed dwarfism; dislocated hips, knees, and elbows; and clubfeet. AOI is lethal in the perinatal period. In individuals with AOIII, survival beyond the neonatal period is possible with intensive and invasive respiratory support. Piepkorn osteochondrodysplasia (POCD) is a perinatal-lethal micromelic dwarfism characterized by flipper-like limbs (polysyndactyly with complete syndactyly of all fingers and toes, hypoplastic or absent first digits, and duplicated intermediate and distal phalanges), macrobrachycephaly, prominant forehead, hypertelorism, and exophthalmos. Occasional features include cleft palate, omphalocele, and cardiac and genitourinary anomalies. The radiographic features at mid-gestation are characteristic.

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.

Multicentric osteolysis nodulosis and arthropathy (MONA) is a skeletal dysplasia characterized by progressive osteolysis (particularly of the carpal and tarsal bones), osteoporosis, subcutaneous nodules on the palms and soles, and progressive arthropathy (joint contractures, pain, swelling, and stiffness). Other manifestations include coarse facies, pigmented skin lesions, cardiac defects, and corneal opacities. Onset is usually between ages six months and six years (range: birth to 11 years).

GAN-related neurodegeneration comprises a phenotypic continuum ranging from severe (sometimes called classic giant axonal neuropathy) to milder pure early-onset peripheral motor and sensory neuropathies. The classic giant axonal neuropathy phenotype typically manifests as an infantile-onset neurodegenerative disorder, starting as a severe peripheral motor and sensory neuropathy and evolving into central nervous system impairment (intellectual disability, seizures, cerebellar signs, and pyramidal tract signs). Most affected individuals become wheelchair dependent in the second decade of life and eventually bedridden with severe polyneuropathy, ataxia, and dementia. Death usually occurs in the third decade. At the milder end of the spectrum are predominantly motor and sensory neuropathies (with little to no CNS involvement) that overlap with the axonal form of Charcot-Marie-Tooth neuropathies.

Ehlers-Danlos syndrome is a group of disorders that affect connective tissues supporting the skin, bones, blood vessels, and many other organs and tissues. Defects in connective tissues cause the signs and symptoms of these conditions, which range from mildly loose joints to life-threatening complications.\n\nThe various forms of Ehlers-Danlos syndrome have been classified in several different ways. Originally, 11 forms of Ehlers-Danlos syndrome were named using Roman numerals to indicate the types (type I, type II, and so on). In 1997, researchers proposed a simpler classification (the Villefranche nomenclature) that reduced the number of types to six and gave them descriptive names based on their major features. In 2017, the classification was updated to include rare forms of Ehlers-Danlos syndrome that were identified more recently. The 2017 classification describes 13 types of Ehlers-Danlos syndrome.\n\nAn unusually large range of joint movement (hypermobility) occurs in most forms of Ehlers-Danlos syndrome, and it is a hallmark feature of the hypermobile type. Infants and children with hypermobility often have weak muscle tone (hypotonia), which can delay the development of motor skills such as sitting, standing, and walking. The loose joints are unstable and prone to dislocation and chronic pain. In the arthrochalasia type of Ehlers-Danlos syndrome, infants have hypermobility and dislocations of both hips at birth.\n\nMany people with the Ehlers-Danlos syndromes have soft, velvety skin that is highly stretchy (elastic) and fragile. Affected individuals tend to bruise easily, and some types of the condition also cause abnormal scarring. People with the classical form of Ehlers-Danlos syndrome experience wounds that split open with little bleeding and leave scars that widen over time to create characteristic "cigarette paper" scars. The dermatosparaxis type of the disorder is characterized by loose skin that sags and wrinkles, and extra (redundant) folds of skin may be present.\n\nBleeding problems are common in the vascular type of Ehlers-Danlos syndrome and are caused by unpredictable tearing (rupture) of blood vessels and organs. These complications can lead to easy bruising, internal bleeding, a hole in the wall of the intestine (intestinal perforation), or stroke. During pregnancy, women with vascular Ehlers-Danlos syndrome may experience rupture of the uterus. Additional forms of Ehlers-Danlos syndrome that involve rupture of the blood vessels include the kyphoscoliotic, classical, and classical-like types.\n\nOther types of Ehlers-Danlos syndrome have additional signs and symptoms. The cardiac-valvular type causes severe problems with the valves that control the movement of blood through the heart. People with the kyphoscoliotic type experience severe curvature of the spine that worsens over time and can interfere with breathing by restricting lung expansion. A type of Ehlers-Danlos syndrome called brittle cornea syndrome is characterized by thinness of the clear covering of the eye (the cornea) and other eye abnormalities. The spondylodysplastic type features short stature and skeletal abnormalities such as abnormally curved (bowed) limbs. Abnormalities of muscles, including hypotonia and permanently bent joints (contractures), are among the characteristic signs of the musculocontractural and myopathic forms of Ehlers-Danlos syndrome. The periodontal type causes abnormalities of the teeth and gums.

Nemaline myopathy-7 is an autosomal recessive congenital myopathy characterized by very early onset of hypotonia and delayed motor development. Affected individuals have difficulty walking and running due to proximal muscle weakness. The disorder is slowly progressive, and patients may lose independent ambulation. Muscle biopsy shows nemaline rods and may later show minicores, abnormal protein aggregates, and dystrophic changes (summary by Ockeloen et al., 2012). For a discussion of genetic heterogeneity of nemaline myopathy, see 161800.

Wiedemann-Steiner syndrome (WSS) is characterized by developmental delay, intellectual disability, and characteristic facial features, with or without additional congenital anomalies. The facial features include thick eyebrows with lateral flare, vertically narrow and downslanted palpebral fissures, widely spaced eyes, long eyelashes, wide nasal bridge, broad nasal tip, thin vermilion of the upper lip, and thick scalp hair. About 60% of affected individuals have hypertrichosis cubiti ("hairy elbows"), which was once thought to be pathognomic for the syndrome, with a majority having hypertrichosis of other body parts. Other clinical features include feeding difficulties, prenatal and postnatal growth restriction, epilepsy, ophthalmologic anomalies, congenital heart defects, hand anomalies (such as brachydactyly and clinodactyly), hypotonia, vertebral anomalies (especially fusion anomalies of the cervical spine), renal and uterine anomalies, immune dysfunction, brain malformations, and dental anomalies.

Mucolipidosis III gamma (ML III?) is a slowly progressive inborn error of metabolism mainly affecting skeletal, joint, and connective tissues. Clinical onset is in early childhood; the progressive course results in severe functional impairment and significant morbidity from chronic pain. Cardiorespiratory complications (restrictive lung disease from thoracic involvement, and thickening and insufficiency of the mitral and aortic valves) are rarely clinically significant. A few (probably <10%) affected individuals display mild cognitive impairment.

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.

Haim-Munk syndrome is an autosomal recessive disorder characterized by palmoplantar keratoderma, severe periodonitis, arachnodactyly, acroosteolysis, atrophic changes of the nails, and a radiographic deformity of the fingers (summary by Hart et al., 2000).

Temtamy syndrome is a mental retardation/multiple congenital anomaly syndrome characterized by variable craniofacial dysmorphism, ocular coloboma, seizures, and brain abnormalities, including abnormalities of the corpus callosum and thalamus (summary by Akizu et al., 2013).

Campomelic dysplasia (CD) is a skeletal dysplasia characterized by distinctive facies, Pierre Robin sequence with cleft palate, shortening and bowing of long bones, and clubfeet. Other findings include laryngotracheomalacia with respiratory compromise and ambiguous genitalia or normal female external genitalia in most individuals with a 46,XY karyotype. Many affected infants die in the neonatal period; additional findings identified in long-term survivors include short stature, cervical spine instability with cord compression, progressive scoliosis, and hearing impairment.

The histiocytosis-lymphadenopathy plus syndrome comprises features of 4 histiocytic disorders previously thought to be distinct: Faisalabad histiocytosis (FHC), sinus histiocytosis with massive lymphadenopathy (SHML), H syndrome, and pigmented hypertrichosis with insulin-dependent diabetes mellitus syndrome (PHID). FHC was described as an autosomal recessive disease involving joint deformities, sensorineural hearing loss, and subsequent development of generalized lymphadenopathy and swellings in the eyelids that contain histiocytes (summary by Morgan et al., 2010). SHML, or familial Rosai-Dorfman disease, was described as a rare cause of lymph node enlargement in children, consisting of chronic massive enlargement of cervical lymph nodes frequently accompanied by fever, leukocytosis, elevated erythrocyte sedimentation rate, and polyclonal hypergammaglobulinemia. Extranodal sites were involved in approximately 25% of patients, including salivary glands, orbit, eyelid, spleen, and testes. The involvement of retropharyngeal lymphoid tissue sometimes caused snoring and sleep apnea (summary by Kismet et al., 2005). H syndrome was characterized by cutaneous hyperpigmentation and hypertrichosis, hepatosplenomegaly, heart anomalies, and hypogonadism; hearing loss was also found in about half of patients, and many had short stature. PHID was characterized by predominantly antibody-negative insulin-dependent diabetes mellitus associated with pigmented hypertrichosis and variable occurrence of other features of H syndrome, with hepatosplenomegaly occurring in about half of patients (Cliffe et al., 2009). Bolze et al. (2012) noted that mutations in the SLC29A3 gene (612373) had been implicated in H syndrome, PHID, FHC, and SHML, and that some patients presented a combination of features from 2 or more of these syndromes, leading to the suggestion that these phenotypes should be grouped together as 'SLC29A3 disorder.' Bolze et al. (2012) suggested that the histologic features of the lesions seemed to be the most uniform phenotype in these patients. In addition, the immunophenotype of infiltrating cells in H syndrome patients was shown to be the same as that seen in patients with the familial form of Rosai-Dorfman disease, further supporting the relationship between these disorders (Avitan-Hersh et al., 2011; Colmenero et al., 2012).

Pitt-Hopkins syndrome (PTHS) is characterized by significant developmental delays with moderate-to-severe intellectual disability and behavioral differences, characteristic facial features, and episodic hyperventilation and/or breath-holding while awake. Speech is significantly delayed and most individuals are nonverbal with receptive language often stronger than expressive language. Other common findings are autism spectrum disorder symptoms, sleep disturbance, stereotypic hand movements, seizures, constipation, and severe myopia.

Any non-syndromic X-linked intellectual disability in which the cause of the disease is a mutation in the BRWD3 gene.

Loeys-Dietz syndrome (LDS) is characterized by vascular findings (cerebral, thoracic, and abdominal arterial aneurysms and/or dissections), skeletal manifestations (pectus excavatum or pectus carinatum, scoliosis, joint laxity, arachnodactyly, talipes equinovarus, cervical spine malformation and/or instability), craniofacial features (widely spaced eyes, strabismus, bifid uvula / cleft palate, and craniosynostosis that can involve any sutures), and cutaneous findings (velvety and translucent skin, easy bruising, and dystrophic scars). Individuals with LDS are predisposed to widespread and aggressive arterial aneurysms and pregnancy-related complications including uterine rupture and death. Individuals with LDS can show a strong predisposition for allergic/inflammatory disease including asthma, eczema, and reactions to food or environmental allergens. There is also an increased incidence of gastrointestinal inflammation including eosinophilic esophagitis and gastritis or inflammatory bowel disease. Wide variation in the distribution and severity of clinical features can be seen in individuals with LDS, even among affected individuals within a family who have the same pathogenic variant.

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.

Ehlers-Danlos syndrome spondylodysplastic type 3 (EDSSPD3) is characterized by short stature, hyperelastic skin and hypermobile joints, protuberant eyes with bluish sclerae, finely wrinkled palms, and characteristic radiologic features (Giunta et al., 2008). For a discussion of genetic heterogeneity of the spondylodysplastic type of Ehlers-Danlos syndrome, see 130070.

X-linked intellectual disability-craniofacioskeletal syndrome is a rare, hereditary, syndromic intellectual disability characterized by craniofacial and skeletal abnormalities in association with mild intellectual disability in females and early postnatal lethality in males. In addition to mild cognitive impairment, females present with microcephaly, short stature, skeletal features and extra temporal lobe gyrus. In males, intrauterine growth impairment, cardiac and urogenital anomalies have been reported.

Three M syndrome is characterized by severe pre- and postnatal growth deficiency (final height 5-6 SD below the mean; i.e., 120-130 cm), characteristic facies, and normal intelligence. Additional features of three M syndrome include short broad neck, prominent trapezii, deformed sternum, short thorax, square shoulders, winged scapulae, hyperlordosis, short fifth fingers, prominent heels, and loose joints. Males with three M syndrome have hypogonadism and occasionally hypospadias.

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

3q29 microduplication syndrome (also known as 3q29 duplication syndrome) is a condition that results from the copying (duplication) of a small piece of chromosome 3 in each cell. The duplication occurs on the long (q) arm of the chromosome at a position designated q29.\n\nThe features associated with 3q29 microduplication syndrome vary widely. Some individuals with this chromosomal change have very mild or no related signs and symptoms, and the duplication is discovered because they undergo genetic testing only after a family member is diagnosed. Other people with a 3q29 microduplication have delayed development (particularly speech delay) and intellectual disability or learning difficulties. Although most affected individuals have no major birth defects, eye abnormalities, heart defects, and an unusually small head (microcephaly) can occur. 3q29 microduplication syndrome may increase the likelihood of being overweight or having obesity, although it is hard to determine whether these weight issues are caused by the duplication.

A very rare inherited connective tissue disorder with characteristics of macrocephaly, sparse scalp hair, soft redundant and hyperextensible skin, joint hypermobility, and scoliosis. Patients have progressive facial coarsening with downslanted palpebral fissures, upper eyelid fullness/infraorbital folds, thick/everted vermillion, gingival overgrowth and abnormal position of the teeth. Rare manifestations such as abnormal high-pitched voice, bronchiectasis, hypergonadotropic hypergonadism and brachydactyly have also been reported. Caused by homozygous mutation in the RIN2 gene on chromosome 20p11.

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).

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 forms of OI are autosomal dominant with mutations in one of the 2 genes that code for type I collagen alpha chains, COL1A1 (120150) and COL1A2 (120160). Glorieux et al. (2000) described a novel autosomal dominant form of OI, which they designated OI type V (OI5), in 7 patients. The disorder was similar to OI type IV but had distinctive clinical, histologic, and molecular characteristics. OI type V is characterized by calcification of the forearm interosseous membrane, radial head dislocation, a subphyseal metaphyseal radiodense line, and hyperplastic callus formation (summary by Cho et al., 2012). OI type V has a variable phenotype. For example, in patients with the more common c.-14C-T variant (614757.0001), distinctive radiographic findings (calcification of the forearm interosseous membrane, radial head dislocation, a subphyseal metaphyseal radiodense line, and hyperplastic callus formation) are often seen, whereas these findings are not seen in patients with the less common S40L variant (614757.0002).

Congenital hypertryptophanemia, which is accompanied by hyperserotonemia, does not appear to have significant clinical consequences (Ferreira et al., 2017).

17q23.1q23.2 microdeletion syndrome is a recently described syndrome characterized by developmental delay, microcephaly, short stature, heart defects and limb abnormalities.

The congenital variant of Rett syndrome is a severe neurodevelopmental disorder with features of classic Rett syndrome (RTT; 312750), but earlier onset in the first months of life. Classic Rett syndrome shows later onset and is caused by mutation in the MECP2 gene (300005).

The cardinal features of chromosome 6q11-q14 interstitial deletions include hypotonia, short stature, skeletal/limb anomalies, umbilical hernia, and urinary tract anomalies, as well as characteristic facial features including upslanting palpebral fissures, low-set and/or dysplastic ears, and high-arched palate (summary by Wang et al., 2009).

Loeys-Dietz syndrome (LDS) is characterized by vascular findings (cerebral, thoracic, and abdominal arterial aneurysms and/or dissections), skeletal manifestations (pectus excavatum or pectus carinatum, scoliosis, joint laxity, arachnodactyly, talipes equinovarus, cervical spine malformation and/or instability), craniofacial features (widely spaced eyes, strabismus, bifid uvula / cleft palate, and craniosynostosis that can involve any sutures), and cutaneous findings (velvety and translucent skin, easy bruising, and dystrophic scars). Individuals with LDS are predisposed to widespread and aggressive arterial aneurysms and pregnancy-related complications including uterine rupture and death. Individuals with LDS can show a strong predisposition for allergic/inflammatory disease including asthma, eczema, and reactions to food or environmental allergens. There is also an increased incidence of gastrointestinal inflammation including eosinophilic esophagitis and gastritis or inflammatory bowel disease. Wide variation in the distribution and severity of clinical features can be seen in individuals with LDS, even among affected individuals within a family who have the same pathogenic variant.

Seckel syndrome is an autosomal recessive disorder characterized by proportionate short stature, severe microcephaly, mental retardation, and a typical 'bird-head' facial appearance (summary by Kalay et al., 2011). For a general phenotypic description and a discussion of genetic heterogeneity of Seckel syndrome, see 210600.

GRIN2B-related neurodevelopmental disorder is characterized by mild to profound developmental delay / intellectual disability (DD/ID) in all affected individuals. Muscle tone abnormalities (spasticity and/or hypotonia, occasionally associated with feeding difficulties), as well as epilepsy and autism spectrum disorder (ASD) / behavioral issues, are common. Other infantile- or childhood-onset findings include microcephaly; dystonic, dyskinetic, or choreiform movement disorder; and/or cortical visual impairment. Brain MRI reveals a malformation of cortical development in a minority of affected individuals. To date, fewer than 100 individuals with GRIN2B-related neurodevelopmental disorder have been reported.

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

Raymond-type X-linked syndromic intellectual developmental disorder (MRXSR) is characterized by mildly to severely impaired intellectual development with speech and language difficulties associated with variable additional features, including marfanoid habitus, epilepsy, facial dysmorphism, hypotonia, and behavioral problems (summary by Baker et al., 2015 and Schirwani et al., 2018).

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).

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

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

Stickler syndrome is a connective tissue disorder that can include ocular findings of myopia, cataract, and retinal detachment; hearing loss that is both conductive and sensorineural; midfacial underdevelopment and cleft palate (either alone or as part of the Robin sequence); and mild spondyloepiphyseal dysplasia and/or precocious arthritis. Variable phenotypic expression of Stickler syndrome occurs both within and among families; interfamilial variability is in part explained by locus and allelic heterogeneity.

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.

FKBP14 kyphoscoliotic Ehlers-Danlos syndrome (FKBP14-kEDS) is characterized by congenital muscle hypotonia and weakness (typically improving during childhood), progressive scoliosis, joint hypermobility, hyperelastic skin, gross motor developmental delay, myopathy, and hearing impairment. Most affected children achieve independent walking between ages two and four years. A decline of motor function in adulthood may be seen, but affected individuals are likely to be able to participate in activities of daily living in adulthood and maintain independent walking. Occasional features underlying systemic connective tissue involvement include aortic rupture and arterial dissection, subdural hygroma, insufficiency of cardiac valves, bluish sclerae, bladder diverticula, inguinal or umbilical herniae, and premature rupture of membranes during pregnancy. Rarer findings may include bifid uvula with submucous or frank cleft palate, speech/language delay without true cognitive impairment, and rectal prolapse.

Isolated gonadotropin-releasing hormone (GnRH) deficiency (IGD) is characterized by inappropriately low serum concentrations of the gonadotropins LH (luteinizing hormone) and FSH (follicle-stimulating hormone) in the presence of low circulating concentrations of sex steroids. IGD is associated with a normal sense of smell (normosmic IGD) in approximately 40% of affected individuals and an impaired sense of smell (Kallmann syndrome) in approximately 60%. IGD can first become apparent in infancy, adolescence, or adulthood. Infant boys with congenital IGD often have micropenis and cryptorchidism. Adolescents and adults with IGD have clinical evidence of hypogonadism and incomplete sexual maturation on physical examination. Adult males with IGD tend to have prepubertal testicular volume (i.e., <4 mL), absence of secondary sexual features (e.g., facial and axillary hair growth, deepening of the voice), decreased muscle mass, diminished libido, erectile dysfunction, and infertility. Adult females have little or no breast development and primary amenorrhea. Although skeletal maturation is delayed, the rate of linear growth is usually normal except for the absence of a distinct pubertal growth spurt.

Loeys-Dietz syndrome (LDS) is characterized by vascular findings (cerebral, thoracic, and abdominal arterial aneurysms and/or dissections), skeletal manifestations (pectus excavatum or pectus carinatum, scoliosis, joint laxity, arachnodactyly, talipes equinovarus, cervical spine malformation and/or instability), craniofacial features (widely spaced eyes, strabismus, bifid uvula / cleft palate, and craniosynostosis that can involve any sutures), and cutaneous findings (velvety and translucent skin, easy bruising, and dystrophic scars). Individuals with LDS are predisposed to widespread and aggressive arterial aneurysms and pregnancy-related complications including uterine rupture and death. Individuals with LDS can show a strong predisposition for allergic/inflammatory disease including asthma, eczema, and reactions to food or environmental allergens. There is also an increased incidence of gastrointestinal inflammation including eosinophilic esophagitis and gastritis or inflammatory bowel disease. Wide variation in the distribution and severity of clinical features can be seen in individuals with LDS, even among affected individuals within a family who have the same pathogenic variant.

PACS1 neurodevelopmental disorder (PACS1-NDD) is characterized by mild-to-severe neurodevelopmental delays. Language skills are more severely affected than motor skills. Hypotonia is reported in about a third of individuals and is noted to improve over time. Approximately 60% of individuals are ambulatory. Feeding difficulty is common, with 25% requiring gastrostomy tube to maintain appropriate caloric intake. Other common features include constipation, seizures, behavioral issues, congenital heart anomalies, short stature, and microcephaly. Common facial features include hypertelorism, downslanting palpebral fissures, bulbous nasal tip, low-set and simple ears, smooth philtrum, wide mouth with downturned corners, thin upper vermilion, and wide-spaced teeth. To date approximately 35 individuals with PACS1-NDD have been reported.

CHD8-related neurodevelopmental disorder with overgrowth (CHD8-NDD) is characterized by generalized overgrowth, developmental delay / intellectual disability (DD/ID), autism spectrum disorder (ASD), neuropsychiatric issues, neurologic problems, sleep disturbance, and gastrointestinal issues The most common findings are the development of macrocephaly (most often during infancy) and tall stature (most typically during puberty), which is often accompanied by ASD and/or DD/ID. Most, if not all, affected individuals have some degree of DD, most commonly speech and motor delays. When present, ID is most often in the mild-to-moderate range. Sleep disturbance is characterized by difficulty with both initiation (delayed sleep onset) and maintenance (frequent night awakenings) of sleep. The most common gastrointestinal issue is constipation with or without periods of diarrhea. Less common features are hypotonia (about 30% of affected individuals), seizures (10%-15%), dystonia (rare), and Chiari I malformation (rare).

CHCHD10-related disorders are characterized by a spectrum of adult-onset neurologic phenotypes that can include: Mitochondrial myopathy (may also be early onset): weakness, amyotrophy, exercise intolerance. Amyotrophic lateral sclerosis (ALS): progressive degeneration of upper motor neurons and lower motor neurons. Frontotemporal dementia (FTD): slowly progressive behavioral changes, language disturbances, cognitive decline, extrapyramidal signs. Late-onset spinal motor neuronopathy (SMA, Jokela type): weakness, cramps, and/or fasciculations; areflexia. Axonal Charcot-Marie-Tooth neuropathy: slowly progressive lower-leg muscle weakness and atrophy, small hand muscle weakness, loss of tendon reflexes, sensory abnormalities. Cerebellar ataxia: gait ataxia, kinetic ataxia (progressive loss of coordination of lower- and upper-limb movements), dysarthria/dysphagia, nystagmus, cerebellar oculomotor disorder. Because of the recent discovery of CHCHD10-related disorders and the limited number of affected individuals reported to date, the natural history of these disorders (except for SMAJ caused by the p.Gly66Val pathogenic variant) is largely unknown.

Steel syndrome is characterized by characteristic facies, dislocated hips and radial heads, carpal coalition (fusion of carpal bones), short stature, scoliosis, and cervical spine anomalies. The dislocated hips are resistant to surgical intervention (summary by Flynn et al., 2010).

A subtype of Charcot-Marie-Tooth type 4 with characteristics of childhood onset of slowly progressing, demyelinating sensorimotor neuropathy, focally folded myelin sheaths in nerve biopsy, reduced nerve conduction velocities and the typical Charcot-Marie-Tooth phenotype (i.e. distal muscle weakness and atrophy, and sensory loss). There is evidence this disease is caused by homozygous or compound heterozygous mutation in the SBF1 gene on chromosome 22q.

Smith-McCort dysplasia is a rare autosomal recessive osteochondrodysplasia characterized by short trunk dwarfism with a barrel-shaped chest, rhizomelic limb shortening, and specific radiologic features including marked platyspondyly with double-humped end-plates, kyphoscoliosis, metaphyseal irregularities, laterally displaced capital femoral epiphyses, and small pelvis with a lace-like appearance of iliac crests. These clinical and radiologic features are also common to Dyggve-Melchior-Clausen syndrome (DMC; 223800), which is distinguished from SMC by the additional feature of mental retardation (summary by Dupuis et al., 2013). For a discussion of genetic heterogeneity of Smith-McCort dysplasia, see SMC1 (607326).

Any proximal symphalangism in which the cause of the disease is a mutation in the GDF5 gene.

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

Loeys-Dietz syndrome (LDS) is characterized by vascular findings (cerebral, thoracic, and abdominal arterial aneurysms and/or dissections), skeletal manifestations (pectus excavatum or pectus carinatum, scoliosis, joint laxity, arachnodactyly, talipes equinovarus, cervical spine malformation and/or instability), craniofacial features (widely spaced eyes, strabismus, bifid uvula / cleft palate, and craniosynostosis that can involve any sutures), and cutaneous findings (velvety and translucent skin, easy bruising, and dystrophic scars). Individuals with LDS are predisposed to widespread and aggressive arterial aneurysms and pregnancy-related complications including uterine rupture and death. Individuals with LDS can show a strong predisposition for allergic/inflammatory disease including asthma, eczema, and reactions to food or environmental allergens. There is also an increased incidence of gastrointestinal inflammation including eosinophilic esophagitis and gastritis or inflammatory bowel disease. Wide variation in the distribution and severity of clinical features can be seen in individuals with LDS, even among affected individuals within a family who have the same pathogenic variant.

Desbuquois dysplasia (DBQD) is an autosomal recessive chondrodysplasia belonging to the multiple dislocation group and characterized by severe prenatal and postnatal growth retardation (stature less than -5 SD), joint laxity, short extremities, and progressive scoliosis. The main radiologic features are short long bones with metaphyseal splay, a 'Swedish key' appearance of the proximal femur (exaggerated trochanter), and advanced carpal and tarsal bone age with a delta phalanx (summary by Huber et al., 2009). Desbuquois dysplasia is clinically and radiographically heterogeneous, and had been classified into 2 types based on the presence (type 1) or absence (type 2) of characteristic hand anomalies, including an extra ossification center distal to the second metacarpal, delta phalanx, bifid distal thumb phalanx, and dislocation of the interphalangeal joints (Faivre et al., 2004). However, patients with and without these additional hand anomalies have been reported to have mutations in the same gene (see, e.g., CANT1); thus, these features are not distinctive criteria to predict the molecular basis of DBQD (Furuichi et al., 2011). In addition, Kim et al. (2010) described another milder variant of DBQD with almost normal outwardly appearing hands, but significant radiographic changes, including short metacarpals, elongated phalanges, and remarkably advanced carpal bone age. However, there is no accessory ossification center distal to the second metacarpal, and patients do not have thumb anomalies. Similar changes occur in the feet. These patients also tend to develop precocious osteoarthritis of the hand and spine with age. This phenotype is sometimes referred to as the 'Kim variant' of DBQD (Furuichi et al., 2011). Genetic Heterogeneity of Desbuquois Dysplasia DBQD2 (615777) is caused by mutation in the XYLT1 gene (608124) on chromosome 16p12. Two unrelated patients with immunodeficiency-23 (IMD23; 615816), due to mutation in the PGM3 gene (172100), were reported to have skeletal features reminiscent of DBQD.

Desbuquois dysplasia, which belongs to the multiple dislocation group of disorders, is characterized by dislocations of large joints, severe pre- and postnatal growth retardation, joint laxity, and flat face with prominent eyes. Radiologic features include short long bones with an exaggerated trochanter that gives a 'monkey wrench' appearance to the proximal femur, and advanced carpal and tarsal ossification (summary by Bui et al., 2014). For a discussion of genetic heterogeneity of Desbuquois dysplasia, see DBQD1 (251450).

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.

Colobomatous microphthalmia-rhizomelic dysplasia syndrome is a rare, genetic developmental defect during embryogenesis characterized by a range of developmental eye anomalies (including anophthalmia, microphthalmia, colobomas, microcornea, corectopia, cataract) and symmetric limb rhizomelia with short stature and contractures of large joints. Intellectual disability with autistic features, macrocephaly, dysmorphic features, urogenital anomalies (hypospadia, cryptorchidism), cutaneous syndactyly and precocious puberty may also be present.

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).

Presynaptic congenital myasthenic syndrome-7A with distal motor neuropathy (CMS7A) is an autosomal dominant neuromuscular disorder characterized by onset of foot deformities, delayed motor development, and slowly progressive distal muscle weakness resulting in gait difficulties in early childhood. Other features may include hyporeflexia, muscle atrophy, and upper limb involvement. Electrophysiologic studies show low compound muscle action potentials (CMAPs), consistent with a distal hereditary motor neuropathy (dHMN), as well as a decremental response to repetitive stimulation, indicating presynaptic defects at the neuromuscular junction (NMJ), consistent with myasthenic syndrome (summary by Fionda et al., 2021). The complex phenotype of patients with dominant SYT2 mutations likely results from impairment of 2 fundamental functions of SYT2: (1) disturbance of calcium-dependent synchronous presynaptic neurotransmitter release, resulting in a myasthenic disorder, and (2) disruption of exocytosis and endocytosis, causing a degenerative process affecting peripheral motor nerve terminals and resulting in a motor neuropathy (Maselli et al., 2021). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462). For a discussion of genetic heterogeneity of dHMN, see 182960.

Any autosomal recessive non-syndromic intellectual disability in which the cause of the disease is a mutation in the NDST1 gene.

Ruijs et al. (2003) reported a Moroccan boy with a chromosomal breakage who died of hepatocellular carcinoma at age 17 years. The boy was noted to have growth retardation at age 3 years; at age 7 he was found to have thoracic kyphosis, frontal bossing, and a delayed bone age of approximately 3 years. He underwent surgery for severe bilateral posterior subcapsular cataracts at age 14. Examination at age 15 showed short stature and low weight, with premature graying of scalp hair, small frontotemporal diameter, small deep-set eyes, bulbous nose with high nasal bridge, small upper lip, and micrognathia. In addition, he had thoracic kyphoscoliosis, sloping shoulders, mild pectus excavatum, moderate bilateral contractures of both elbows, bilateral clinodactyly, and pes planus. At age 17, he developed abdominal pain, and ultrasonography revealed a liver mass; biopsy confirmed hepatocellular carcinoma. Because of the advanced stage, no treatment was possible, and he died 2 months later. Although his parents were not known to be consanguineous, they originated from the same small Moroccan village. Lessel et al. (2014) studied 2 brothers from a nonconsanguineous Australian family of European ancestry who exhibited low body weight, micrognathia, triangular face, muscular atrophy, lipodystrophy, bilateral simian creases, delayed bone age, and mild joint restrictions in the fingers and elbows. In addition, both brothers developed early-onset hepatocellular carcinoma, at ages 16 and 14 years, respectively. The older brother died at age 18 from complications of acute fulminant hepatic failure. Analysis of patient tumor biopsies showed strong focal accumulations of cancer biomarkers as well as a high proliferative index compared to healthy liver or to cells from idiopathic hepatocellular carcinoma.

Cerebellofaciodental syndrome is an autosomal recessive neurodevelopmental disorder characterized by delayed development, intellectual disability, abnormal facial and dental findings, and cerebellar hypoplasia (summary by Borck et al., 2015).

Congenital myasthenic syndrome-14 is an autosomal recessive neuromuscular disorder characterized by onset of limb-girdle muscle weakness in early childhood. The disorder is slowly progressive, and some patients may become wheelchair-bound. There is no respiratory or cardiac involvement. Treatment with anticholinesterase medication may be beneficial (summary by Cossins et al., 2013). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

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

Lamb-Shaffer syndrome is a neurodevelopmental disorder characterized by global developmental delay, intellectual disability, poor expressive speech, and mild dysmorphic facial features. Additional variable skeletal abnormalities may also be present (summary by Nesbitt et al., 2015).

Congenital symmetric circumferential skin creases is characterized by the folding of excess skin, which leads to ringed creases, primarily of the limbs. Affected individuals also exhibit intellectual disability, cleft palate, and dysmorphic features (summary by Isrie et al., 2015). For a discussion of genetic heterogeneity of congenital symmetric circumferential skin creases, see CSCSC1 (156610).

Seizures, scoliosis, and macrocephaly/microcephaly syndrome (SSMS) is an autosomal recessive neurodevelopmental disorder characterized by global developmental delay apparent from early infancy, impaired intellectual development, behavioral problems, poor or absent speech, seizures, dysmorphic facial features with macro- or microcephaly, and skeletal abnormalities, including scoliosis and delayed bone age. Other features may include hypotonia, gastrointestinal problems, and exostoses (summary by Gentile et al., 2019).

Yuan-Harel-Lupski syndrome is a complex neurodevelopmental disorder characterized by global developmental delay and early-onset peripheral neuropathy. The disorder comprises features of both demyelinating Charcot-Marie-Tooth disease type 1A (CMT1A; 118220), which results from duplication of the PMP22 gene on 17p12, and Potocki-Lupski syndrome (PTLS; 610883), which results from duplication of a slightly proximal region on 17p11.2 that includes the RAI1 gene. These 2 loci are about 2.5 Mb apart. The resultant YUHAL phenotype may be more severe in comparison to the individual contributions of each gene, with particularly early onset of peripheral neuropathy and features of both central and peripheral nervous system involvement (summary by Yuan et al., 2015).

Heimler syndrome, which represents the mildest end of the peroxisomal biogenesis disorder spectrum (see PBD1A, 214100), is a rare autosomal recessive disorder characterized by sensorineural hearing loss, enamel hypoplasia of the secondary dentition, and nail abnormalities (Ratbi et al., 2015). For a discussion of genetic heterogeneity of Heimler syndrome, see HMLR1 (234580).

Arboleda-Tham syndrome (ARTHS) is an autosomal dominant disorder with the core features of impaired intellectual development, speech delay, microcephaly, cardiac anomalies, and gastrointestinal complications (summary by Kennedy et al., 2019).

Spondylo-ocular syndrome is a very rare association of spinal and ocular manifestations that is characterized by dense cataracts, and retinal detachment along with generalized osteoporosis and platyspondyly. Mild craniofacial dysphormism has been reported including short neck, large head and prominent eyebrows.

X-linked syndromic intellectual developmental disorder-34 (MRXS34) is an X-linked recessive neurodevelopmental disorder characterized by delayed psychomotor development, intellectual disability with poor speech, dysmorphic facial features, and mild structural brain abnormalities, including thickening of the corpus callosum (summary by Mircsof et al., 2015).

Tonne-Kalscheuer syndrome (TOKAS) is an X-linked recessive multiple congenital anomaly disorder with 2 main presentations. Most patients exhibit global developmental delay apparent from early infancy, impaired intellectual development, speech delay, behavioral abnormalities, and abnormal gait. Affected individuals also have dysmorphic facial features that evolve with age, anomalies of the hands, feet, and nails, and urogenital abnormalities with hypogenitalism. A subset of more severely affected males develop congenital diaphragmatic hernia in utero, which may result in perinatal or premature death. Carrier females may have very mild skeletal or hormonal abnormalities (summary by Frints et al., 2019). Also see Fryns syndrome (229850), an autosomal recessive disorder with overlapping features.

Ehlers-Danlos syndrome is a group of disorders that affect connective tissues supporting the skin, bones, blood vessels, and many other organs and tissues. Defects in connective tissues cause the signs and symptoms of these conditions, which range from mildly loose joints to life-threatening complications.\n\nThe various forms of Ehlers-Danlos syndrome have been classified in several different ways. Originally, 11 forms of Ehlers-Danlos syndrome were named using Roman numerals to indicate the types (type I, type II, and so on). In 1997, researchers proposed a simpler classification (the Villefranche nomenclature) that reduced the number of types to six and gave them descriptive names based on their major features. In 2017, the classification was updated to include rare forms of Ehlers-Danlos syndrome that were identified more recently. The 2017 classification describes 13 types of Ehlers-Danlos syndrome.\n\nMany people with the Ehlers-Danlos syndromes have soft, velvety skin that is highly stretchy (elastic) and fragile. Affected individuals tend to bruise easily, and some types of the condition also cause abnormal scarring. People with the classical form of Ehlers-Danlos syndrome experience wounds that split open with little bleeding and leave scars that widen over time to create characteristic "cigarette paper" scars. The dermatosparaxis type of the disorder is characterized by loose skin that sags and wrinkles, and extra (redundant) folds of skin may be present.\n\nBleeding problems are common in the vascular type of Ehlers-Danlos syndrome and are caused by unpredictable tearing (rupture) of blood vessels and organs. These complications can lead to easy bruising, internal bleeding, a hole in the wall of the intestine (intestinal perforation), or stroke. During pregnancy, women with vascular Ehlers-Danlos syndrome may experience rupture of the uterus. Additional forms of Ehlers-Danlos syndrome that involve rupture of the blood vessels include the kyphoscoliotic, classical, and classical-like types.\n\nOther types of Ehlers-Danlos syndrome have additional signs and symptoms. The cardiac-valvular type causes severe problems with the valves that control the movement of blood through the heart. People with the kyphoscoliotic type experience severe curvature of the spine that worsens over time and can interfere with breathing by restricting lung expansion. A type of Ehlers-Danlos syndrome called brittle cornea syndrome is characterized by thinness of the clear covering of the eye (the cornea) and other eye abnormalities. The spondylodysplastic type features short stature and skeletal abnormalities such as abnormally curved (bowed) limbs. Abnormalities of muscles, including hypotonia and permanently bent joints (contractures), are among the characteristic signs of the musculocontractural and myopathic forms of Ehlers-Danlos syndrome. The periodontal type causes abnormalities of the teeth and gums.\n\nAn unusually large range of joint movement (hypermobility) occurs in most forms of Ehlers-Danlos syndrome, and it is a hallmark feature of the hypermobile type. Infants and children with hypermobility often have weak muscle tone (hypotonia), which can delay the development of motor skills such as sitting, standing, and walking. The loose joints are unstable and prone to dislocation and chronic pain. In the arthrochalasia type of Ehlers-Danlos syndrome, infants have hypermobility and dislocations of both hips at birth.

GDACCF is an intellectual disability syndrome apparent soon after birth with neonatal hypotonia, poor feeding, and respiratory insufficiency followed by delayed psychomotor development and intellectual disability with poor speech. Brain imaging shows aplasia or hypoplasia of the corpus callosum. Affected individuals have variable dysmorphic facial features, and some may have dysplastic, cystic kidneys or mild cardiac defects (summary by Stevens et al., 2016).

Myofibrillar myopathy-8 (MFM8) is an autosomal recessive myopathy characterized by slowly progressive proximal muscle weakness and atrophy affecting the upper and lower limbs, resulting in increased falls, gait problems, difficulty running or climbing stairs, and upper limb weakness or scapular winging. Some patients develop distal muscle weakness and atrophy. The phenotype may also be consistent with a clinical diagnosis of limb-girdle muscular dystrophy (LGMD). Age at symptom onset ranges from infancy to adulthood. Ambulation is generally preserved and cardiac involvement is rare, but respiratory compromise with decreased forced vital capacity often occurs. Muscle biopsy shows a mix of myopathic features, including myofibrillar inclusions and sarcomeric disorganization; some patients have been reported to have dystrophic changes on muscle biopsy (O'Grady et al., 2016; Daimaguler et al., 2021). There is significant phenotypic variation, even in patients with the same mutation, which must be taken into account when counseling affecting individuals (Woods et al., 2020). For a general phenotypic description and a discussion of genetic heterogeneity of myofibrillar myopathy, see MFM1 (601419).

GRIN2D-related developmental and epileptic encephalopathy (GRIN2D-related DEE) is characterized by mild-to-profound developmental delay or intellectual disability, epilepsy, abnormal muscle tone (hypotonia and spasticity), movement disorders (dystonia, dyskinesia, chorea), autism spectrum disorder, and cortical visual impairment. Additional findings can include sleep disorders and feeding difficulties. To date 22 individuals with GRIN2D-related DEE have been reported.

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).

Congenital myasthenic syndrome-20 is an autosomal recessive neuromuscular disorder characterized by severe hypotonia associated with episodic apnea soon after birth. Patients have muscle weakness resulting in delayed walking, ptosis, poor sucking and swallowing, and generalized limb fatigability and weakness. EMG studies usually show a decremental response to repetitive nerve stimulation, and some patients may show a good response to AChE inhibitors (summary by Bauche et al., 2016). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

Thauvin-Robinet-Faivre syndrome is an autosomal recessive disorder characterized by generalized overgrowth, mainly of height, and mildly delayed psychomotor development with mild or severe learning difficulties. More variable features may include congenital heart defects, kidney abnormalities, and skeletal defects. Patients may have an increased risk for Wilms tumor (summary by Akawi et al., 2016).

Macrocephaly, dysmorphic facies, and psychomotor retardation (MDFPMR) is an autosomal recessive neurodevelopmental disorder characterized by large head and somatic overgrowth apparent at birth followed by global developmental delay. Affected individuals have characteristic dysmorphic facial features and persistently large head, but increased birth weight normalizes with age. Additional neurologic features, including seizures, hypotonia, and gait ataxia, may also occur. Patients show severe intellectual impairment (summary by Ortega-Recalde et al., 2015).

HIVEP2-related intellectual disability is a neurological disorder characterized by moderate to severe developmental delay and intellectual disability and mild physical abnormalities (dysmorphic features). Early symptoms of the condition include weak muscle tone (hypotonia) and delayed development of motor skills, such as sitting, standing, and walking. After learning to walk, many affected individuals continue to have difficulty with this activity; their walking style (gait) is often unbalanced and wide-based. Speech is also delayed, and some people with this condition never learn to talk. Most people with HIVEP2-related intellectual disability also have unusual physical features, such as widely spaced eyes (hypertelorism), a broad nasal bridge, or fingers with tapered ends, although there is no characteristic pattern of such features among affected individuals. Many people with the condition exhibit neurodevelopmental disorders, such as hyperactivity, attention deficit disorder, aggression, anxiety, and autism spectrum disorder, which is a group of developmental disorders characterized by impaired communication and social interaction.\n\nOther features of HIVEP2-related intellectual disability include mild abnormalities in the structure of the brain and an abnormally small brain and head size (microcephaly). Less common health problems include seizures; recurrent ear infections; and eye disorders, such as eyes that do not look in the same direction (strabismus), "lazy eye" (amblyopia), and farsightedness (hyperopia). Some people with HIVEP2-related intellectual disability have gastrointestinal problems, which can include backflow of acidic stomach contents into the esophagus (gastroesophageal reflux) and constipation.

GNB1 encephalopathy (GNB1-E) is characterized by moderate-to-severe developmental delay / intellectual disability, structural brain abnormalities, and often infantile hypotonia and seizures. Other less common findings include dystonia, reduced vision, behavior issues, growth delay, gastrointestinal (GI) problems, genitourinary (GU) abnormalities in males, and cutaneous mastocytosis.

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).

A rare genetic multiple congenital anomalies/dysmorphic syndrome with characteristics of craniofacial dysmorphism (including an abnormal skull shape, hypertelorism, downslanting palpebral fissures, epicanthal folds, low-set ears, depressed nasal bridge, micrognathia), short stature, ectodermal anomalies (such as sparse eyebrows, eyelashes, and scalp hair, hypoplastic toenails), developmental delay, and intellectual disability. Additional features may include cerebral/cerebellar malformations and mild renal involvement.

Midface hypoplasia, hearing impairment, elliptocytosis, and nephrocalcinosis is an X-linked recessive disorder with onset of features in early childhood. Anemia is sometimes present. Some patients may show mild early motor or speech delay, but cognition is normal (summary by Andreoletti et al., 2017).

Meester-Loeys syndrome (MRLS) is an X-linked disorder characterized by early-onset aortic aneurysm and dissection. Other recurrent findings include hypertelorism, pectus deformity, joint hypermobility, contractures, and mild skeletal dysplasia (Meester et al., 2017).

Most individuals with HNRNPH2-related neurodevelopmental disorder (HNRNPH2-NDD) have symptoms early in life, before age 12 months. The major features of HNRNPH2-NDD are developmental delay / intellectual disability, motor and language delays, behavioral and psychiatric disorders, and growth and musculoskeletal abnormalities. Minor features include dysmorphic facies, gastrointestinal disturbances, epilepsy, and visual defects. Although HNRNPH2-NDD is an X-linked condition, there is not enough information on affected females versus affected males to make any generalizations about phenotypic differences between the two sexes.

Autosomal recessive cutis laxa type IIC (ARCL2C) is characterized by generalized skin wrinkling with sparse subcutaneous fat and dysmorphic progeroid facial features. Most patients also exhibit severe hypotonia as well as cardiovascular involvement (summary by Van Damme et al., 2017). For a general phenotypic description and a discussion of genetic heterogeneity of autosomal recessive cutis laxa, see ARCL1A (219100).

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).

Al Kaissi syndrome (ALKAS) is an autosomal recessive developmental disorder characterized by growth retardation, spine malformation, particularly of the cervical spine, dysmorphic facial features, and delayed psychomotor development with moderate to severe intellectual disability (summary by Windpassinger et al., 2017).

Pontocerebellar hypoplasia type 11 (PCH11) is an autosomal recessive neurodevelopmental disorder characterized by severely delayed psychomotor development with impaired intellectual development and poor speech, microcephaly, dysmorphic features, and pontocerebellar hypoplasia on brain imaging. Additional features are more variable (summary by Marin-Valencia et al., 2017). For a general phenotypic description and a discussion of genetic heterogeneity of PCH, see PCH1 (607596).

Neurodevelopmental disorder with microcephaly, ataxia, and seizures (NEDMAS) is an autosomal recessive disorder characterized by global developmental delay and early-onset seizures. More variable features may include deafness, cardiomyopathy, and severe febrile decompensations (summary by Ravel et al., 2021).

NEDDFL is a neurodevelopmental disorder characterized by delayed psychomotor development and impaired intellectual development, poor growth with small head size, dysmorphic facial features, and mild abnormalities of the hands and feet (summary by Stankiewicz et al., 2017).

NEDMIAL is a neurodevelopmental disorder characterized by delayed psychomotor development and hypotonia apparent from early infancy, resulting in feeding difficulties, ataxic gait or inability to walk, delayed or absent speech development, and impaired intellectual development, sometimes with behavioral abnormalities, such as hand-flapping. Additional common features may include sleep disorder, nonspecific dysmorphic facial features, and joint hyperlaxity (summary by Lessel et al., 2017 and Mannucci et al., 2021).

Seckel syndrome is a rare autosomal recessive disorder characterized by intrauterine growth retardation, dwarfism, microcephaly with mental retardation, and a characteristic 'bird-headed' facial appearance (Shanske et al., 1997). Genetic Heterogeneity of Seckel Syndrome Other forms of Seckel syndrome include SCKL2 (606744), caused by mutation in the RBBP8 gene (604124) on chromosome 18q11; SCKL4 (613676), caused by mutation in the CENPJ gene (609279) on chromosome 13q12; SCKL5 (613823), caused by mutation in the CEP152 gene (613529) on chromosome 15q21; SCKL6 (614728), caused by mutation in the CEP63 gene (614724) on chromosome 3q22; SCKL7 (614851), caused by mutation in the NIN gene (608684) on chromosome 14q22; SCKL8 (615807), caused by mutation in the DNA2 gene (601810) on chromosome 10q21; SCKL9 (616777), caused by mutation in the TRAIP gene (605958) on chromosome 3p21; SCKL10 (617253), caused by mutation in the NSMCE2 gene (617246) on chromosome 8q24; and SCKL11 (620767), caused by mutation in the CEP295 gene (617728) on chromosome 11q21. The report of a Seckel syndrome locus on chromosome 14q, designated SCKL3, by Kilinc et al. (2003) was found to be in error; see History section.

Schwartz-Jampel syndrome type 1 (SJS1) is a rare autosomal recessive disorder characterized by muscle stiffness (myotonia) and chondrodysplasia. Affected individuals usually present in childhood with permanent muscle stiffness or bone deformities. Common clinical features include mask-like facies (narrow palpebral fissures, blepharospasm, and pursed lips); permanent muscle stiffness with continuous skeletal muscle activity recorded on electromyography; dwarfism; pectus carinatum; kyphoscoliosis; bowing of long bones; and epiphyseal, metaphyseal, and hip dysplasia. The disorder is slowly progressive but does not appear to alter life span (summary by Stum et al., 2006).

VLDLR cerebellar hypoplasia (VLDLR-CH) is characterized by non-progressive congenital ataxia that is predominantly truncal and results in delayed ambulation, moderate-to-profound intellectual disability, dysarthria, strabismus, and seizures. Children either learn to walk very late (often after age 6 years) or never achieve independent ambulation. Brain MRI findings include hypoplasia of the inferior portion of the cerebellar vermis and hemispheres, simplified gyration of the cerebral hemispheres, and small brain stem – particularly the pons.

Arthrochalasia-type EDS is distinguished from other types of EDS by the frequency of congenital hip dislocation and extreme joint laxity with recurrent joint subluxations and minimal skin involvement (Byers et al., 1997; Giunta et al., 2008). Genetic Heterogeneity of Arthrochalasia-type Ehlers-Danlos Syndrome See EDSARTH2 (617821), caused by mutation in the COL1A2 gene (120160).

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.

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

Multiple synostoses syndrome-4 is characterized by fusion of carpal and tarsal bones, as well as conductive hearing loss (Terhal et al., 2018). For a general phenotypic description and a discussion of genetic heterogeneity of multiple synostoses syndrome, see SYNS1 (186500).

Ehlers-Danlos syndrome classic-like-2 (EDSCLL2) is characterized by severe joint and skin laxity, osteoporosis involving the hips and spine, osteoarthritis, soft redundant skin that can be acrogeria-like, delayed wound healing with abnormal atrophic scarring, and shoulder, hip, knee, and ankle dislocations. Variable features include gastrointestinal and genitourinary manifestations, such as bowel rupture, gut dysmotility, cryptorchidism, and hernias; vascular complications, such as mitral valve prolapse and aortic root dilation; and skeletal anomalies (Blackburn et al., 2018). See 606408 for another classic-like EDS syndrome. For a discussion of the classification of EDS, see 130000.

Congenital hypomyelinating neuropathy-2 is an autosomal dominant neurologic disorder characterized by early-onset hypotonia, severely delayed motor development, muscle weakness with areflexia, and severely decreased nerve conduction velocities (NCV) resulting from improper myelination of axons. The severity is variable: some patients may present at birth with contractures and respiratory insufficiency, whereas others may achieve walking (summary by Warner et al., 1996). CHN shows significant phenotypic overlap with Dejerine-Sottas syndrome (DSS; 145900), which is also a neuropathy with early onset. Some classify the disorders differently, noting that CHN is characterized by hypo- or amyelination resulting from a congenital defect in myelin formation, whereas DSS has features of continuous myelin breakdown, with demyelination and remyelination (summary by Smit et al., 2008). For a discussion of genetic heterogeneity of CHN, see CHN1 (605253).

SMALED2A is an autosomal dominant form of spinal muscular atrophy characterized by early childhood onset of muscle weakness and atrophy predominantly affecting the proximal and distal muscles of the lower extremity, although some patients may show upper extremity involvement. The disorder results in delayed walking, waddling gait, difficulty walking, and loss of distal reflexes. Some patients may have foot deformities or hyperlordosis, and some show mild upper motor signs, such as spasticity. Sensation, bulbar function, and cognitive function are preserved. The disorder shows very slow progression throughout life (summary by Oates et al., 2013). For discussion of genetic heterogeneity of lower extremity-predominant spinal muscular atrophy, see SMALED1 (158600).

MRD57 is an autosomal dominant neurodevelopmental disorder with a highly variable phenotype. Most affected individuals have delayed psychomotor development apparent in infancy or early childhood, language delay, and behavioral abnormalities. Additional features may include hypotonia, feeding problems, gastrointestinal issues, and dysmorphic facial features (summary by Reijnders et al., 2018).

CAFDADD is a multisystemic developmental disorder with variable cardiac and digital anomalies and facial dysmorphism. Some patients may have seizures and ocular/aural abnormalities (Tokita et al., 2018).

ASXL3-related disorder is characterized by developmental delay or intellectual disability, typically in the moderate to severe range, with speech and language delay and/or absent speech. Affected individuals may also display autistic features. There may be issues with feeding. While dysmorphic facial features have been described, they are typically nonspecific. Affected individuals may also have hypotonia that can transition to spasticity resulting in unusual posture with flexion contractions of the elbows, wrists, and fingers. Other findings may include poor postnatal growth, strabismus, seizures, sleep disturbance, and dental anomalies.

Alpha-thalassemia X-linked intellectual disability (ATR-X) syndrome is characterized by distinctive craniofacial features, genital anomalies, hypotonia, and mild-to-profound developmental delay / intellectual disability (DD/ID). Craniofacial abnormalities include small head circumference, telecanthus or widely spaced eyes, short triangular nose, tented upper lip, and thick or everted lower lip with coarsening of the facial features over time. While all affected individuals have a normal 46,XY karyotype, genital anomalies comprise a range from hypospadias and undescended testicles, to severe hypospadias and ambiguous genitalia, to normal-appearing female external genitalia. Alpha-thalassemia, observed in about 75% of affected individuals, is mild and typically does not require treatment. Osteosarcoma has been reported in a few males with germline pathogenic variants.

Mullegama-Klein-Martinez syndrome (MKMS) is an X-linked recessive disorder with features of microcephaly, microtia, hearing loss, developmental delay, dysmorphic features, congenital heart defect, and digit abnormalities. Females are generally affected more severely than males (Mullegama et al., 2019).

X-linked intellectual developmental disorder-108 (MRX108) is characterized by early hypotonia, global developmental delay, and moderately to severely impaired intellectual development. Brisk tendon reflexes, variable facial dysmorphism, and fifth finger clinodactyly may be present (Khayat et al., 2019).

Short stature, amelogenesis imperfecta, and skeletal dysplasia with scoliosis (SSASKS)is characterized by disproportionate short stature, defective tooth enamel formation, and skeletal dysplasia with severe scoliosis in some patients. Variable features include facial dysmorphism, moderate hearing impairment, and mildly impaired intellectual development (Ashikov et al., 2018).

Spondyloepimetaphyseal dysplasia with joint laxity-3 (SEMDJL3) is characterized by multiple joint dislocations at birth, severe joint laxity, scoliosis, gracile metacarpals and metatarsals, delayed bone age, and poorly ossified carpal and tarsal bones (Girisha et al., 2016). For a discussion of genetic heterogeneity of SEMD with joint laxity, see SEMDJL1 (271640).

Neurodevelopmental disorder with visual defects and brain anomalies (NEDVIBA) is characterized by global developmental delay with impaired intellectual development and speech delay, variable visual defects, including retinitis pigmentosa and optic atrophy, hypotonia or hypertonia, and variable structural brain abnormalities. Other nonspecific features may be found (summary by Okur et al., 2019).

DLG4-related synaptopathy is a condition that affects neurological development. This condition is characterized by delayed development and mild to moderate intellectual disabilities that typically becomes evident before age 2. Over time, many individuals with DLG4-related synaptopathy lose skills that they have learned, such as speech or motor skills. About 20 percent of people with this condition cannot speak. Affected individuals often have neurodevelopmental disorders, such as autism spectrum disorder or attention-deficit/hyperactivity disorder. About half of individuals with this condition have recurrent seizures (epilepsy) that typically begin in childhood. Brain changes can also occur. These include brain tissue loss (atrophy) and abnormalities of the tissue connecting the left and right halves of the brain (corpus callosum) or the  hippocampus, which is a region of the brain that is involved in learning and memory.\n\nIndividuals with DLG4-related synaptopathy can also have weak muscle tone (hyptonia), loose joints (joint laxity), or a spine that curves to the side (scoliosis). Movement problems, including impaired muscle coordination (ataxia), involuntary muscle coordination (dystonia), or rhythmic shaking (tremor) are common in people with this condition. Other problems can include migraine, sleep problems, or anxiety. Some people with DLG4-related synaptopathy have a distinctive body type that includes a long face, slim body, and long fingers.\n\nLess commonly, DLG4-related synaptopathy can affect a person's vision. Affected individuals can have eyes that do not point in the same direction (strabismus), farsightedness (hyperopia), or involuntary movements of the eyes (nystagmus). Some affected individuals have blindness because the area of the brain responsible for processing vision is impaired. \n\nDLG4-related synaptopathy can also cause gastrointestinal difficulties that make it difficult to eat. These can include a backflow of stomach acids into the esophagus (gastroesophageal reflux disease or GERD).\n\n

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).

CSGALNACT1 deficiency is characterized by mild skeletal dysplasia, joint hypermobility, and advanced bone age. Shortness of long bones is evident prenatally, and patients exhibit short stature and relative macrocephaly. Advanced carpotarsal bone age and monkey-wrench appearance of the femur observed in infancy may disappear with age (Mizumoto et al., 2020).

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).

The phenotypic spectrum of GARS1-associated axonal neuropathy ranges from GARS1 infantile-onset SMA (GARS1-iSMA) to GARS1 adolescent- or early adult-onset hereditary motor/sensory neuropathy (GARS1-HMSN). GARS1-iSMA. Age of onset ranges from the neonatal period to the toddler years. Initial manifestations are typically respiratory distress, poor feeding, and muscle weakness (distal greater than proximal). Weakness is slowly progressive, ultimately requiring mechanical ventilation and feeding via gastrostomy tube. GARS1-HMSN. Age of onset is most commonly during the second decade (range eight to 36 years). Initial manifestations are typically muscle weakness in the hands sometimes with sensory deficits. Lower limb involvement (seen in ~50% of individuals) ranges from weakness and atrophy of the extensor digitorum brevis and weakness of toe dorsiflexors to classic peroneal muscular atrophy with foot drop and a high steppage gait.

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.

Vertebral hypersegmentation and orofacial anomalies (VHO) is characterized by supernumerary cervical, thoracic, and/or lumbar vertebrae, in association with supernumerary ribs. Most patients also exhibit orofacial clefting and ear anomalies (Cox et al., 2019).

Osteogenesis imperfecta type XXI (OI21) is a progressively deforming disorder, characterized by multiple fractures that often occur after minor trauma. Fractures may be present at birth in some affected individuals. Patients exhibit disproportionate short stature and scoliosis, and are often wheelchair-bound by adulthood (van Dijk et al., 2020).

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

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.

Baralle-Macken syndrome (BARMACS) is an autosomal recessive disorder characterized by global developmental delay apparent from infancy, difficulty walking or inability to walk, and impaired intellectual development with poor or absent speech. Affected individuals develop early-onset cataracts; some may have microcephaly. Additional more variable features may include dysmorphic facial features, metabolic abnormalities, spasticity, and lymphopenia (summary by Macken et al., 2021).

KINSSHIP syndrome (KINS) is an autosomal dominant disorder characterized by a recognizable pattern of anomalies including developmental delay, impaired intellectual development, seizures, mesomelic dysplasia, dysmorphic facial features, horseshoe or hypoplastic kidney, and failure to thrive (summary by Voisin et al., 2021).

Radio-Tartaglia syndrome (RATARS) is a neurodevelopmental disorder characterized by global developmental delay with impaired intellectual development, speech delay, and variable behavioral abnormalities. Affected individuals show hypotonia, mild motor difficulties, and craniofacial dysmorphism. Brain imaging may show nonspecific defects; rare patients have seizures or pyramidal signs. A subset of individuals may have congenital heart defects, precocious puberty, and obesity in females. Some of the features are similar to those observed in patients with chromosome 1p36 deletion syndrome (607872) (summary by Radio et al., 2021).

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

Multifocal fibromuscular dysplasia (FMDMF) is characterized histologically by medial fibroplasia and angiographically by multiple arterial stenoses with intervening mural dilations. Arterial tortuosity, macroaneurysms, dissections, and rupture may occur (summary by Richer et al., 2020).

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).

X-linked syndromic intellectual developmental disorder with pigmentary mosaicism and coarse facies (MRXSPF) is characterized by a phenotypic triad of severe developmental delay, coarse facial dysmorphisms, and Blaschkoid pigmentary mosaicism. Additional clinical features may include epilepsy, orthopedic abnormalities, hypotonia, and growth abnormalities. The disorder affects both males and females (Villegas et al., 2019; Diaz et al., 2020).

Autosomal dominant Usmani-Riazzudin syndrome (USRISD) is a neurodevelopmental disorder characterized by global developmental delay with impaired intellectual development and speech delay, hypotonia, and behavioral abnormalities, most commonly aggressive behavior. More variable additional features may include seizures and distal limb anomalies (summary by Usmani et al., 2021).

VISS syndrome is a generalized connective tissue disorder characterized by early-onset thoracic aortic aneurysm and other connective tissue findings, such as aneurysm and tortuosity of other arteries, joint hypermobility, skin laxity, and hernias, as well as craniofacial dysmorphic features, structural cardiac defects, skeletal anomalies, and motor developmental delay (Van Gucht et al., 2021). Immune dysregulation has been observed in some patients (Ziegler et al., 2021).

Charcot-Marie-Tooth disease type 2FF (CMT2FF) is an autosomal dominant progressive axonal sensorimotor peripheral neuropathy characterized by early-childhood onset of difficulties walking or running due to atrophy and weakness of the lower limbs. Most patients have foot and ankle deformities, requiring surgery or walking aids. Some patients lose independent ambulation. There is also prominent involvement of the upper limbs, with weakness and atrophy of the forearm, wrist, and intrinsic hand muscles. Proximal muscle function is preserved. Affected individuals have variable distal sensory impairment. Most patients have hyporeflexia, although brisk reflexes, suggesting upper motor involvement, have been described in 1 family. Sural nerve biopsy showed abnormal myelination (Rebelo et al., 2021). For a phenotypic description and a discussion of genetic heterogeneity of axonal CMT type 2, see CMT2A (118210).

Congenital disorder of glycosylation type IIw (CDG2W) is an autosomal dominant metabolic disorder characterized by liver dysfunction, coagulation deficiencies, and profound abnormalities in N-glycosylation of serum specific proteins. All reported patients carry the same mutation (602671.0017) (summary by Ng et al., 2021). For an overview of congenital disorders of glycosylation, see CDG1A (212065) and CDG2A (212066).

Boudin-Mortier syndrome (BOMOS) is characterized by tall stature, arachnodactyly, disproportionately elongated great toes, and multiple extra epiphyses. Some patients also show joint hypermobility and dilation of the aortic root (Boudin et al., 2018). Mutation in the NPR2 gene (108961) results in a similar phenotype of increased stature and elongation of the digits, particularly of the great toes, with multiple extra epiphyses (epiphyseal chondrodysplasia, Miura type; 615923).

Developmental delay with or without intellectual impairment or behavioral abnormalities (DDIB) is an autosomal dominant disorder with a nonspecific phenotype of developmental delay. Additional features may include neonatal feeding problems, hypotonia, and dysmorphic facial features (Dulovic-Mahlow et al., 2019; van Woerden et al., 2021).

Spondylometaphyseal dysplasia Pagnamenta type (SMDP) is characterized by short stature and mild platyspondyly with no disproportion between the limbs. Mild metaphyseal changes are present (Pagnamenta et al., 2022).

Loeys-Dietz syndrome (LDS) is characterized by vascular findings (cerebral, thoracic, and abdominal arterial aneurysms and/or dissections), skeletal manifestations (pectus excavatum or pectus carinatum, scoliosis, joint laxity, arachnodactyly, talipes equinovarus, cervical spine malformation and/or instability), craniofacial features (widely spaced eyes, strabismus, bifid uvula / cleft palate, and craniosynostosis that can involve any sutures), and cutaneous findings (velvety and translucent skin, easy bruising, and dystrophic scars). Individuals with LDS are predisposed to widespread and aggressive arterial aneurysms and pregnancy-related complications including uterine rupture and death. Individuals with LDS can show a strong predisposition for allergic/inflammatory disease including asthma, eczema, and reactions to food or environmental allergens. There is also an increased incidence of gastrointestinal inflammation including eosinophilic esophagitis and gastritis or inflammatory bowel disease. Wide variation in the distribution and severity of clinical features can be seen in individuals with LDS, even among affected individuals within a family who have the same pathogenic variant.

Charcot-Marie-Tooth disease type 2Z (CMT2Z) is an autosomal dominant axonal peripheral neuropathy characterized by onset, usually in the first decade, of distal lower limb muscle weakness and sensory impairment. The disorder is progressive, and affected individuals tend to develop upper limb and proximal muscle involvement in an asymmetric pattern, resulting in severe disability late in adulthood. Rare occurrence of global developmental delay with impaired intellectual development or learning difficulties has been observed. In some instances, the same mutation may result in different phenotypic manifestations (CMT2Z or DIGFAN), which highlights the clinical spectrum associated with MORC2 mutations and may render the classification of patients into one or the other disorder challenging (summary by Sevilla et al., 2016, Ando et al., 2017, Guillen Sacoto et al., 2020). For a phenotypic description and a discussion of genetic heterogeneity of axonal CMT, see CMT2A1 (118210).

Autosomal recessive intellectual developmental disorder-73 (MRT73) is characterized by global developmental delay with hypotonia and mildly delayed walking, impaired intellectual development with poor or absent speech, and mildly dysmorphic features (summary by Morrison et al., 2021).

Bryant-Li-Bhoj neurodevelopmental syndrome-2 (BRYLIB2) is a highly variable phenotype characterized predominantly by moderate to severe global developmental delay with impaired intellectual development, poor or absent speech, and delayed motor milestones. Most patients have hypotonia, although some have peripheral hypertonia. Common features include variable dysmorphic facial features, oculomotor abnormalities, feeding problems, and nonspecific brain imaging abnormalities. Additional features may include hearing loss, seizures, short stature, and mild skeletal defects (summary by Bryant et al., 2020). For a discussion of genetic heterogeneity of Bryant-Li-Bhoj neurodevelopmental syndrome, see BRYLIB1 (619720).

Noonan syndrome-14 (NS14) is a recessive developmental disorder within the RASopathy clinical spectrum. Patients exhibit developmental delay, impaired intellectual development, and short stature, as well as distinctive dysmorphic features including bitemporal narrowing, hypertelorism, low-set posteriorly rotated ears, prominent nasal bridge, low posterior hairline with a short webbed neck, and pectus excavatum (Motta et al., 2021). For a general phenotypic description and discussion of genetic heterogeneity of Noonan syndrome, see NS1 (163950).

Tessadori-Bicknell-van Haaften neurodevelopmental syndrome-2 (TEBIVANED2) is characterized by poor overall growth, profound global developmental delay with absent speech, and characteristic dysmorphic facial features, including hypertelorism, abnormal nose, and wide mouth (Tessadori et al., 2020). For a discussion of genetic heterogeneity of Tessadori-Bicknell-van Haaften neurodevelopmental syndrome, see TEBIVANED1 (619758).

Craniofacial dysmorphism, skeletal anomalies, and impaired intellectual development syndrome-1 (CFSMR1) is characterized by cranial involvement with macrocrania at birth, brachycephaly, anomalies of middle fossa structures including hypoplasia of corpus callosum, enlargement of septum pellucidum, and dilated lateral ventricles, as well as cortical atrophy and hypodensity of the gray matter. Facial dysmorphisms include flat face, hypertelorism, epicanthal folds, synophrys, broad nasal bridge, cleft lip and cleft palate, and low-set posteriorly rotated ears. Patients also exhibit short neck and multiple costal and vertebral anomalies. The face is rather characteristic, and various authors have consistently reported affable/friendly personality, despite intellectual delay (summary by Alanay et al., 2014). Genetic Heterogeneity of Craniofacial Dysmorphism, Skeletal Anomalies, and Impaired Intellectual Development Syndrome CFSMR2 (616994) is caused by mutation in the RAB5IF gene (619960) on chromosome 20q11.

Chilton-Okur-Chung neurodevelopmental syndrome (CHOCNS) is characterized mainly by global developmental delay with variably impaired intellectual development and occasional speech delay. Most patients have behavioral abnormalities, including autism spectrum disorder, ADHD, and aggression. About half of patients have dysmorphic facial features, and about half have nonspecific brain abnormalities, including thin corpus callosum. Rare involvement of other organ systems may be present. At least 1 child with normal development at age 2.5 years has been reported (Chilton et al., 2020).

Stickler syndrome type VI (STL6) is characterized by early-onset progressive hearing loss and progressive myopia, with variable manifestation of facial dysmorphism and skeletal anomalies (Nixon et al., 2019; Rad et al., 2022). For a general phenotypic description and discussion of genetic heterogeneity of Stickler syndrome, see STL1 (108300).

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).

Neurodevelopmental disorder with short stature, prominent forehead, and feeding difficulties (NEDSFF) is an autosomal recessive disorder characterized by distinct craniofacial features, multisystem dysfunction, profound neurodevelopmental delays, and neonatal death (Shankar et al., 2022).

Diamond-Blackfan anemia-21 (DBA21) is an autosomal recessive bone marrow failure syndrome that includes selective erythroid hypoplasia, anemia with transient thrombocytopenia, short stature, facial dysmorphism, limb abnormalities, cardiac defects, and intellectual disability (O'Donohue et al., 2022). For a general phenotypic description and discussion of genetic heterogeneity of Diamond-Blackfan anemia, see DBA1 (105650).

Autosomal dominant distal hereditary motor neuronopathy-10 (HMND10) is a neurologic disorder of the peripheral nerves characterized clinically by length-dependent motor neuropathy primarily affecting the lower limbs. Affected individuals have onset of distal muscle weakness and atrophy in early childhood that results in walking difficulties and gait abnormalities. Some have pyramidal signs, including hyperreflexia, suggesting the involvement of upper motor neurons. Electrophysiologic studies are consistent with a neurogenic process. More variable features may include mild intellectual disability, minor gyration defects on brain imaging, foot deformities, and connective tissue defects (1 family) (Capuano et al., 2016; Iacomino et al., 2020). For a discussion of genetic heterogeneity of autosomal dominant distal HMN, see HMND1 (182960).

Developmental delay with variable intellectual disability and dysmorphic facies (DIDDF) is a clinically heterogeneous disorder characterized by neurologic deficits and characteristic dysmorphic facial features apparent from infancy or early childhood. Affected individuals usually show impaired intellectual development, speech delay, learning difficulties, and/or behavioral problems. Additional features may include hypotonia, hand or foot deformities, and palatal defects (Verberne et al., 2021; Verberne et al., 2022).

Cleidocranial dysplasia-2 (CLCD2) is characterized by clavicular anomalies, ranging from unilateral 'clavicula bipartita' to bilateral clavicular aplasia, and dental anomalies, including delayed or absent eruption of deciduous teeth and supernumerary teeth. Skull abnormalities such as delayed closure of fontanels have been reported; other skeletal features include delayed bone age, short distal phalanges, and pseudoepiphyses of the metacarpals and/or metatarsals. Phenotypic variability, including intrafamilial, has been observed (Beyltjens et al., 2023). For a general phenotypic description and a discussion of genetic heterogeneity of cleidocranial dysplasia, see CLCD1 (119600).

Atelis syndrome-2 (ATELS2) is an autosomal recessive disorder characterized by poor overall growth with microcephaly and short stature, dysmorphic facial features, and congenital cardiac defects. Additional more variable features may include hematologic abnormalities, variable ocular abnormalities, motor delay, and anxiety. Patient cells exhibit a unique chromosomal instability phenotype consisting of segmented and dicentric chromosomes with mosaic variegated hyperploidy (Grange et al., 2022). See also ATELS1 (620184), caused by mutation in the SLF2 gene (610348). For a discussion of genetic heterogeneity of MVA, see MVA1 (257300).

Tessadori-Bicknell-van Haaften neurodevelopmental syndrome-3 (TEBIVANED3) is characterized by global developmental delay with poor overall growth, impaired intellectual development, and speech difficulties. More variable features include hypotonia, microcephaly, and dysmorphic facies. The severity and manifestations of the disorder are highly variable (Tessadori et al., 2022). For a discussion of genetic heterogeneity of Tessadori-Bicknell-van Haaften neurodevelopmental disorder, see TEBIVANED1 (619758).

Neurodevelopmental disorder with hypotonia, dysmorphic facies, and skeletal anomalies, with or without seizures (NEDFSS), is characterized by these features and global developmental delay with delayed or absent walking, moderate to severely impaired intellectual development, and poor or absent speech acquisition. Affected individuals may also have behavioral abnormalities. About half of patients develop various types of seizures that are usually well-controlled with medication. Rare patients are noted to have heat intolerance or insensitivity to pain (Lines et al., 2022).

Autosomal recessive intellectual developmental disorder-79 (MRT79) is characterized by global developmental delay apparent from infancy. Affected individuals have mildly delayed walking with an ataxic gait and severely impaired intellectual development with poor or absent speech. Additional features may include postnatal microcephaly and dysmorphic features (Van Bergen et al., 2022).