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

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

PAX6-related aniridia occurs either as an isolated ocular abnormality or as part of the Wilms tumor-aniridia-genital anomalies-retardation (WAGR) syndrome. Aniridia is a pan ocular disorder affecting the cornea, iris, intraocular pressure (resulting in glaucoma), lens (cataract and lens subluxation), fovea (foveal hypoplasia), and optic nerve (optic nerve coloboma and hypoplasia). Individuals with aniridia characteristically show nystagmus and impaired visual acuity (usually 20/100 - 20/200); however, milder forms of aniridia with subtle iris architecture changes, good vision, and normal foveal structure do occur. Other ocular involvement may include strabismus and occasionally microphthalmia. Although the severity of aniridia can vary between and within families, little variability is usually observed in the two eyes of an affected individual. WAGR syndrome. The risk for Wilms tumor is 42.5%-77%; of those who develop Wilms tumor, 90% do so by age four years and 98% by age seven years. Genital anomalies in males can include cryptorchidism and hypospadias (sometimes resulting in ambiguous genitalia), urethral strictures, ureteric abnormalities, and gonadoblastoma. While females typically have normal external genitalia, they may have uterine abnormalities and streak ovaries. Intellectual disability (defined as IQ <74) is observed in 70%; behavioral abnormalities include attention-deficit/hyperactivity disorder (ADHD), autism spectrum disorder, anxiety, depression, and obsessive-compulsive disorder. Other individuals with WAGR syndrome can have normal intellect without behavioral problems.

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

King-Denborough syndrome (KDS) is an autosomal dominant disorder characterized by the triad of congenital myopathy, dysmorphic features, and susceptibility to malignant hyperthermia (summary by Dowling et al., 2011).

Hereditary myopathy with lactic acidosis (HML) is an autosomal recessive muscular disorder characterized by childhood onset of exercise intolerance with muscle tenderness, cramping, dyspnea, and palpitations. Biochemical features include lactic acidosis and, rarely, rhabdomyolysis. It is a chronic disorder with remission and exacerbation of the muscle phenotype (summary by Sanaker et al., 2010).

Congenital fibrosis of the extraocular muscles (CFEOM) encompasses several different inherited strabismus syndromes characterized by congenital restrictive ophthalmoplegia affecting extraocular muscles innervated by the oculomotor and/or trochlear nerves. Classic CFEOM is characterized by bilateral blepharoptosis and ophthalmoplegia with the eyes fixed in an infraducted position about 20 to 30 degrees below the horizontal midline. Involvement of the horizontal extraocular muscles is variable. If all affected members of a family have the classic phenotype with bilateral involvement, the disorder is referred to as 'CFEOM1' (Engle et al., 1997; Heidary et al., 2008). CFEOM2 (602078), an autosomal recessive disorder caused by mutation in the ARIX gene (PHOX2A; 602753) on chromosome 11q13, is characterized by bilateral ptosis with eyes fixed in an exotropic position. The CFEOM3 phenotype shows more variable clinical features: affected individuals may have unilateral eye involvement, may be able raise their eyes above midline, or may not have blepharoptosis. CFEOM3 is diagnosed in a family if even 1 member does not have classic findings of the disorder. CFEOM3 is a genetically heterogeneous disorder; CFEOM3A with or without extraocular involvement (600638) is caused by mutation in the TUBB3 gene (602661) on chromosome 16q24; CFEOM3B is caused by mutation in the KIF21A gene (608283) on chromosome 12q12; and CFEOM3C (609384) maps to chromosome 13q. CFEOM4 (609428), also known as Tukel syndrome, maps to chromosome 21q. CFEOM5 (616219) is caused by mutation in the COL25A1 gene (610004) on chromosome 4q25. See also NOMENCLATURE.

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.

A rare hereditary disorder with the combination of congenital bilateral recurrent laryngeal nerve paralysis and congenital bilateral ptosis. There have been no further descriptions in the literature since 1983.

Congenital fibrosis of extraocular muscles-2 (CFEOM2) is an autosomal recessive disorder in which affected individuals are born with bilateral ptosis and restrictive ophthalmoplegia with the globes fixed in extreme abduction (exotropia) (Wang et al., 1998, Nakano et al., 2001). For a general phenotypic description and a discussion of genetic heterogeneity of various forms of CFEOM, see CFEOM1 (135700).

Robinow-Sorauf syndrome is a condition with features similar to those of Saethre-Chotzen syndrome, including craniosynostosis and broad or duplicated great toes. It was once considered a separate disorder, but was found to result from mutations in the same gene and is now thought to be a variant of Saethre-Chotzen syndrome.\n\nThe signs and symptoms of Saethre-Chotzen syndrome vary widely, even among affected individuals in the same family. This condition can cause mild changes in the hands and feet, such as partial fusion of the skin between the second and third fingers on each hand and a broad or duplicated first (big) toe. Delayed development and learning difficulties have been reported, although most people with this condition are of normal intelligence. Less common signs and symptoms of Saethre-Chotzen syndrome include short stature, abnormalities of the bones of the spine (the vertebra), hearing loss, and heart defects.\n\nMost people with Saethre-Chotzen syndrome have prematurely fused skull bones along the coronal suture, the growth line that goes over the head from ear to ear. Other parts of the skull may be malformed as well. These changes can result in an abnormally shaped head, a high forehead, a low frontal hairline, droopy eyelids (ptosis), widely spaced eyes, and a broad nasal bridge. One side of the face may appear noticeably different from the other (facial asymmetry). Most people with Saethre-Chotzen syndrome also have small, rounded ears.\n\nSaethre-Chotzen syndrome is a genetic condition characterized by the premature fusion of certain skull bones (craniosynostosis). This early fusion prevents the skull from growing normally and affects the shape of the head and face.

Mungan syndrome (MGS) is characterized by chronic intestinal pseudoobstruction (CIPO), megaduodenum, long-segment Barrett esophagus, and cardiac abnormalities of variable severity (summary by Bonora et al., 2015).

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.

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

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

Congenital myasthenic syndrome is a disorder characterized by variable degrees of muscle fatigability caused by impaired transmission of electrical signals at the neuromuscular junction (NMJ) (summary by Arnold et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

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

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.

POLG-related disorders comprise a continuum of overlapping phenotypes that were clinically defined long before their molecular basis was known. Most affected individuals have some, but not all, of the features of a given phenotype; nonetheless, the following nomenclature can assist the clinician in diagnosis and management. Onset of the POLG-related disorders ranges from infancy to late adulthood. Alpers-Huttenlocher syndrome (AHS), one of the most severe phenotypes, is characterized by childhood-onset progressive and ultimately severe encephalopathy with intractable epilepsy and hepatic failure. Childhood myocerebrohepatopathy spectrum (MCHS) presents between the first few months of life and about age three years with developmental delay or dementia, lactic acidosis, and a myopathy with failure to thrive. Other findings can include liver failure, renal tubular acidosis, pancreatitis, cyclic vomiting, and hearing loss. Myoclonic epilepsy myopathy sensory ataxia (MEMSA) now describes the spectrum of disorders with epilepsy, myopathy, and ataxia without ophthalmoplegia. MEMSA now includes the disorders previously described as spinocerebellar ataxia with epilepsy (SCAE). The ataxia neuropathy spectrum (ANS) includes the phenotypes previously referred to as mitochondrial recessive ataxia syndrome (MIRAS) and sensory ataxia neuropathy dysarthria and ophthalmoplegia (SANDO). About 90% of persons in the ANS have ataxia and neuropathy as core features. Approximately two thirds develop seizures and almost one half develop ophthalmoplegia; clinical myopathy is rare. Autosomal recessive progressive external ophthalmoplegia (arPEO) is characterized by progressive weakness of the extraocular eye muscles resulting in ptosis and ophthalmoparesis (or paresis of the extraocular muscles) without associated systemic involvement; however, caution is advised because many individuals with apparently isolated arPEO at the onset develop other manifestations of POLG-related disorders over years or decades. Of note, in the ANS spectrum the neuropathy commonly precedes the onset of PEO by years to decades. Autosomal dominant progressive external ophthalmoplegia (adPEO) typically includes a generalized myopathy and often variable degrees of sensorineural hearing loss, axonal neuropathy, ataxia, depression, parkinsonism, hypogonadism, and cataracts (in what has been called "chronic progressive external ophthalmoplegia plus," or "CPEO+").

Any autosomal dominant non-syndromic intellectual disability in which the cause of the disease is a mutation in the COL4A3BP gene.

Autosomal recessive progressive external ophthalmoplegia with mitochondrial DNA deletions-2 (PEOB2) is a mitochondrial disorder characterized by adult onset of progressive external ophthalmoplegia, exercise intolerance, muscle weakness, and signs and symptoms of spinocerebellar ataxia, such as impaired gait and dysarthria. Some patients may have respiratory insufficiency. Laboratory studies are consistent with a defect in mtDNA replication (summary by Reyes et al., 2015). For a discussion of genetic heterogeneity of autosomal recessive PEO, see PEOB1 (258450).

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). For a discussion of genetic heterogeneity of classic-like Ehlers-Danlos syndrome, see 606408. For a discussion of the classification of EDS, see 130000.

Oculopharyngodistal myopathy-1 (OPDM1) is an autosomal dominant disorder characterized by adult-onset ptosis, external ophthalmoplegia, facial muscle weakness, distal limb muscle weakness and atrophy, and pharyngeal involvement, resulting in dysphagia and dysarthria. Skeletal muscle biopsy shows myopathic changes with rimmed vacuoles. There are variable manifestations of the disorder regarding muscle involvement and severity (summary by Ishiura et al., 2019). Genetic Heterogeneity of Oculopharyngodistal Myopathy See also OPDM2 (618940), caused by trinucleotide repeat expansion in the GIPC1 gene (605072) on chromosome 19p13; OPDM3 (619473), caused by trinucleotide repeat expansion in the NOTCH2NLC gene (618025) on chromosome 1q21; and OPDM4 (619790), caused by trinucleotide repeat expansion in the RILPL1 gene (614092) on chromosome 12q24. Oculopharyngeal muscular dystrophy (OPMD; 164300) is a similar disorder with overlapping features. It is caused by a similar heterozygous trinucleotide repeat expansion in the PABPN1 gene (602279) (summary by Durmus et al., 2011).

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

Neurodevelopmental disorder with or without autistic features and/or structural brain abnormalities (NEDASB) is an early-onset neurologic disorder characterized by global developmental delay, poor or absent speech and language development, and behavioral abnormalities reminiscent of autism spectrum disorder (ASD; 209850) or Angelman syndrome (AS; 105830). Additional features may include poor overall growth with small head circumference, axial hypotonia, spasticity, and seizures. Some patients have abnormal findings on brain imaging, including cerebral atrophy, cerebellar atrophy, and/or thin corpus callosum (summary by Mattioli et al., 2020).

Retinal dystrophy and leukodystrophy (RDLKD) is a peroxisomal enzyme deficiency caused by impaired very long chain fatty acid (VLCFA) metabolism. Patients exhibit ataxia and spastic paraparesis as well as developmental delay, and may show facial dysmorphism (Ferdinandusse et al., 2017).

Neurodevelopmental disorder with hypotonia and brain abnormalities (NEDHYBA) is characterized by impaired development of motor skills, cognitive function, and speech acquisition beginning in infancy or early childhood. Some affected individuals may have feeding difficulties, seizures, behavioral abnormalities, and nonspecific dysmorphic facial features. Brain imaging shows variable abnormalities, including corpus callosum defects, cerebellar defects, and decreased white matter volume. There is significant phenotypic variability (summary by Duncan et al., 2021).

SIMHA syndrome is characterized by short stature, impaired intellectual development, microcephaly, hypotonia, and ocular anomalies. Inter- and intrafamilial phenotypic variability has been observed (Kambouris et al., 2014; Zahra et al., 2020).

Yoon-Bellen neurodevelopmental syndrome (YOBELN) is an autosomal recessive disorder characterized mainly by global developmental delay with variably impaired intellectual development. The manifestations and severity of the phenotype are highly variable. Additional neurologic features may include hypotonia, spasticity, ataxia, hearing loss, visual problems, seizures, and nonspecific anomalies on brain imaging (summary by Yap et al., 2021).

Combined oxidative phosphorylation deficiency-55 (COXPD55) is characterized by global developmental delay, hypotonia, short stature, and impaired intellectual development with speech disabilities in childhood. Indolent progressive external ophthalmoplegia phenotype has been described in 1 patient (summary by Olahova et al., 2021). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

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).

Intellectual developmental disorder with autism and dysmorphic facies (IDDADF) is an autosomal recessive neurodevelopmental disorder characterized by moderate to severely impaired cognitive development associated with behavioral abnormalities, including autism spectrum disorder. Affected individuals have variable dysmorphic facial features (Al-Amri et al., 2022)

Digenic dyskeratosis congenita (DKCD) is characterized clinically by a combination of mucocutaneous features including abnormal skin pigmentation, nail dystrophy, thin hair, and oral leukoplakia. Some patients may have evidence of bone marrow failure, manifest as immune defects such as recurrent infections or hypogammaglobulinemia. Telomeres are shortened in patient cells. Individuals with DKCD may show severe adverse reactions to treatment with 5-FU (Tummala et al., 2022). For a discussion of genetic heterogeneity of dyskeratosis congenita, see DKCA1 (127550).

Classic congenital myopathy-22A (CMYO22A) is an autosomal recessive muscle disorder characterized by onset of muscle weakness in utero or soon after birth. Early features may include fetal hypokinesia, breech presentation, and polyhydramnios. Affected individuals are born with severe hypotonia and require respiratory and feeding assistance. Those who survive the neonatal period show a 'classic' phenotype of congenital myopathy with delayed motor development, difficulty walking, proximal muscle weakness of the upper and lower limbs, facial and neck muscle weakness, easy fatigability, and mild limb contractures or foot deformities. Some have persistent respiratory insufficiency; dysmorphic facial features may be present (Zaharieva et al., 2016). For a discussion of genetic heterogeneity of congenital myopathy, see CMYO1A (117000).

Jeffries-Lakhani neurodevelopmental syndrome (JELANS) is an autosomal recessive disorder characterized by hypotonia, early-onset seizures, and global developmental delay apparent from infancy. Affected individuals have motor delay, speech delay, and impaired intellectual development, and about half of patients are nonambulatory and/or nonverbal. Some patients have cardiac arrhythmia, but congenital cardiac septal defects are only rarely observed. Additional features may include feeding difficulties, recurrent infections, ocular defects, and nonspecific dysmorphic features. Premature death due to cardiac arrhythmia or epilepsy may occur (Jeffries et al., 2024).

Teebi hypertelorism syndrome-1 (TBHS1) is an autosomal dominant disorder characterized by hypertelorism with upslanting palpebral fissures, prominent forehead, broad and depressed nasal bridge with short nose, thick eyebrows, and widow's peak. Additional features include small broad hands with mild interdigital webbing and shawl scrotum. Umbilical malformations, cardiac defects, natal teeth, cleft lip/palate, congenital diaphragmatic hernia, and malformations of the central nervous system (ventriculomegaly, abnormal corpus callosum) have also been reported. Development is typically normal, although some patients with developmental delays have been reported (summary by Bhoj et al., 2015). Genetic Heterogeneity of Teebi Hypertelorism Syndrome Teebi hypertelorism syndrome-2 (TBHS2; 619736) is caused by mutation in the CDH11 gene (600023) on chromosome 16q21.