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

X-linked adrenoleukodystrophy (X-ALD) involves the central or peripheral nervous system and the adrenal cortex. The nervous system and adrenal glands are involved independently; thus, an affected male may be diagnosed with cerebral adrenoleukodystrophy (CALD), adrenomyeloneuropathy (AMN), and/or primary adrenocortical insufficiency. CALD is characterized by progressive behavioral, cognitive, and neurologic deficits; onset of symptoms ranges from childhood (typically ages 4 to 8 years) to adolescence (ages 11 to 21 years) and adulthood. AMN is characterized by leg weakness, spasticity, clumsy gait, pain, and bladder and bowel dysfunction; onset is typically in the 20s and 30s. Onset of primary adrenocortical insufficiency ranges from age two years to adulthood (most commonly by age 7.5 years). Heterozygous females are not at increased risk to develop CALD, but are at increased risk to develop AMN and primary adrenocortical insufficiency with increasing age.

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

EZH2-related overgrowth is a variable overgrowth syndrome characterized by tall stature, macrocephaly, variable intellect (ranging from normal intellect to severe intellectual disability), characteristic facial appearance, and a range of associated clinical features including advanced bone age, poor coordination, soft, doughy skin, camptodactyly of the fingers and/or toes, umbilical hernia, abnormal tone, and hoarse, low cry in infancy. Brain MRI has identified abnormalities in a few individuals with EZH2-related overgrowth. Neuroblastoma occurs at a slightly increased frequency in individuals with a heterozygous EZH2 pathogenic variant, but data are insufficient to determine absolute risk. There is currently no evidence that additional malignancies (including hematologic malignancies) occur with increased frequency, though a few have been reported.

Purine nucleoside phosphorylase deficiency is a rare autosomal recessive immunodeficiency disorder characterized mainly by decreased T-cell function. Some patients also have neurologic impairment (review by Aust et al., 1992).

Succinic semialdehyde dehydrogenase (SSADH) deficiency is characterized by a relatively non-progressive encephalopathy typically presenting with hypotonia and delayed acquisition of motor and language developmental milestones in the first two years of life. Common clinical features include an almost universal intellectual disability and adaptive function deficits, as well as epilepsy, autism spectrum disorder, movement disorders (such as ataxia, dystonia, and exertional dyskinesia), sleep disturbances, attention problems, anxiety, and obsessive-compulsive behaviors. Notably, seizures, autism spectrum disorder features, and behavioral problems tend to worsen around the time of late childhood or early adolescence. Affected individuals do not usually have episodic decompensation following metabolic stressors, as is typical of other organic acidemias and metabolic encephalopathies, although some have been diagnosed after having unanticipated difficulty recovering from otherwise ordinary childhood illnesses. Clinical presentation with acute onset of generalized hypotonia and choreiform movement following upper-respiratory tract infection has been observed.

The diagnosis of severe breath-holding spells (BHS) in childhood is based on a distinctive and stereotyped sequence of clinical events beginning with a provocation resulting in crying or emotional upset that leads to a noiseless state of expiration accompanied by color change and ultimately loss of consciousness and postural tone (Lombroso and Lerman, 1967; DiMario, 1992). Two clinical types are recognized based on the child's coloration (cyanotic or pallid) during these events. Most children experience the cyanotic type, although some experience mixed types. BHS is an involuntary, nonvolitional, reflexic, nonepileptic paroxysmal phenomenon of childhood. The episodes occur during full expiration despite its misnomer. Autonomic dysregulation has been hypothesized as an underlying mechanism that results in loss of consciousness (Hunt, 1990; DiMario and Burleson, 1993; Dimario et al., 1998).

Homocystinuria caused by cystathionine ß-synthase (CBS) deficiency is characterized by involvement of the eye (ectopia lentis and/or severe myopia), skeletal system (excessive height, long limbs, scolioisis, and pectus excavatum), vascular system (thromboembolism), and CNS (developmental delay/intellectual disability). All four ? or only one ? of the systems can be involved; expressivity is variable for all of the clinical signs. It is not unusual for a previously asymptomatic individual to present in adult years with only a thromboembolic event that is often cerebrovascular. Two phenotypic variants are recognized, B6-responsive homocystinuria and B6-non-responsive homocystinuria. B6-responsive homocystinuria is usually milder than the non-responsive variant. Thromboembolism is the major cause of early death and morbidity. IQ in individuals with untreated homocystinuria ranges widely, from 10 to 138. In B6-responsive individuals the mean IQ is 79 versus 57 for those who are B6-non-responsive. Other features that may occur include: seizures, psychiatric problems, extrapyramidal signs (e.g., dystonia), hypopigmentation of the skin and hair, malar flush, livedo reticularis, and pancreatitis.

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.

Males with deafness-dystonia-optic neuronopathy (DDON) syndrome have prelingual or postlingual sensorineural hearing impairment in early childhood, slowly progressive dystonia or ataxia in the teens, slowly progressive decreased visual acuity from optic atrophy beginning at approximately age 20 years, and dementia beginning at approximately age 40 years. Psychiatric symptoms such as personality change and paranoia may appear in childhood and progress. The hearing impairment appears to be consistent in age of onset and progression, whereas the neurologic, visual, and neuropsychiatric signs vary in degree of severity and rate of progression. Females may have mild hearing impairment and focal dystonia.

Kumar-type X-linked syndromic intellectual developmental disorder (MRXSK) is an X-linked recessive disorder that shows phenotypic variability and multisystem involvement apparent from birth or early infancy. Most affected individuals are male, although 1 severely affected girl with a de novo THOC2 mutation has been reported. At the severe end of the spectrum, affected individuals have hypotonia, neonatal difficulties, failure to thrive with poor overall growth, feeding difficulties, respiratory insufficiency, visual impairment, profoundly impaired intellectual development with poor or absent speech, and motor abnormalities, such as inability to walk and hyperkinetic movements. Less severely affected individuals have mildly to moderately impaired intellectual development and speech delay. Additional features include behavioral abnormalities, hearing or visual defects, seizures, dysmorphic facial features, and brain imaging abnormalities (Kumar et al., 2015; Kumar et al., 2018; Kumar et al., 2020).

CLCN4-related neurodevelopmental disorder (CLCN4-NDD), an X-linked disorder, is characterized in the 36 males reported to date by developmental delay or intellectual disability, behavioral/mental health issues (e.g., autism spectrum disorder, anxiety, hyperactivity, and bipolar disorder), epilepsy, and gastrointestinal dysfunction. The five heterozygous females with a de novo CLCN4 variant reported to date had findings very similar to those of affected males. Twenty-two of 25 heterozygous females identified in family studies following identification of an affected male were unaffected or had only mild specific learning difficulties and/or mental health concerns, whereas three were more severely affected.

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

Autosomal dominant sleep-related hypermotor (hyperkinetic) epilepsy (ADSHE) is a seizure disorder characterized by clusters of nocturnal motor seizures that are often stereotyped and brief (<2 minutes). They vary from simple arousals from sleep to dramatic, often hyperkinetic events with tonic or dystonic features. Affected individuals may experience an aura. Retained awareness during seizures is common. A minority of individuals experience daytime seizures. Age of onset ranges from infancy to adulthood. About 80% of individuals develop ADSHE in the first two decades of life; mean age of onset is ten years. Clinical neurologic examination is normal and intellect is usually preserved, but reduced intellect, psychiatric comorbidities, or cognitive deficits may occur. Within a family, the manifestations of the disorder may vary considerably. ADSHE is lifelong but not progressive. As an individual reaches middle age, seizures may become milder and less frequent.

X-linked intellectual disability-retinitis pigmentosa syndrome is characterized by moderate intellectual deficit and severe, early-onset retinitis pigmentosa. It has been described in five males spanning three generations of one family. Some patients also had microcephaly. It is transmitted as an X-linked recessive trait.

Phelan-McDermid syndrome-SHANK3 related (PMS-SHANK3 related) is characterized by neonatal hypotonia, absent to severely delayed speech, developmental delay, and minor dysmorphic facial features. Most affected individuals have moderate-to-profound intellectual disability. Other features include relatively large fleshy hands, dysplastic toenails, and decreased perspiration that results in a tendency to overheat. Normal stature and normal head size distinguish PMS-SHANK3 related from other autosomal chromosome disorders. Neurobehavioral characteristics include mouthing or chewing non-food items, decreased perception of pain, and autism spectrum disorder or autistic-like affect and behavior. Some individuals experience regression / loss of skills, epilepsy, ataxic/abnormal gait, and sleep disturbance (difficulty falling asleep and staying asleep, hypersomnia, and parasomnias). Less commonly, affected individuals may have strabismus, vision problems (hyperopia or myopia), cardiac anomalies, renal anomalies, and lymphedema. Those who have PMS-SHANK3 related due to a ring chromosome 22 also have a high risk of developing features of NF2-related schwannomatosis (NF2).

Methylenetetrahydrofolate reductase deficiency is a common inborn error of folate metabolism. The phenotypic spectrum ranges from severe neurologic deterioration and early death to asymptomatic adults. In the classic form, both thermostable and thermolabile enzyme variants have been identified (Rosenblatt et al., 1992).

Retinal vasculopathy with cerebral leukoencephalopathy and systemic manifestations (RVCL-S) is a small-vessel disease that affects highly vascularized tissues including the retina, brain, liver, and kidneys. Age of onset is often between 35 and 50 years. The most common presenting finding is decreased visual acuity and/or visual field defects. Neurologic manifestations may include hemiparesis, facial weakness, aphasia, and hemianopsia. Migraines and seizures are less frequently described. Renal manifestations may include mild-to-moderate increase in serum creatinine and mild proteinuria; progression to end-stage renal disease (ESRD) is uncommon. Hepatic manifestations frequently include mildly elevated levels of alkaline phosphatase and gamma-glutamyltransferase (GGT). Less common findings include psychiatric disorders, hypertension, mild-to-moderate anemia, and Raynaud phenomenon.

Neuronal intranuclear inclusion disease (NIID) is an autosomal dominant, slowly progressive neurodegenerative disorder characterized by a wide range of clinical manifestations, including pyramidal and extrapyramidal symptoms, cerebellar ataxia, cognitive decline and dementia, peripheral neuropathy, and autonomic dysfunction. The age at onset varies, but most individuals present as adults between about 30 and 70 years of age. Pathologic investigation shows eosinophilic intranuclear inclusions in almost all cell types, including neurons, skin cells, fibroblasts, and skeletal muscle. Brain imaging shows a characteristic leukoencephalopathy with high intensity signals in the corticomedullary junction on diffusion-weighted imaging (DWI), as well as white matter abnormalities in subcortical and brainstem regions. Skin biopsy combined with brain imaging is useful for diagnosis (summary by Sone et al., 2016). The phenotype in some cases is suggestive of Parkinson disease (see 168600) and/or Alzheimer disease (see 104300), consistent with an evolving phenotypic spectrum of adult-onset NIID (summary by Tian et al., 2019).

Autosomal dominant sleep-related hypermotor (hyperkinetic) epilepsy (ADSHE) is a seizure disorder characterized by clusters of nocturnal motor seizures that are often stereotyped and brief (<2 minutes). They vary from simple arousals from sleep to dramatic, often hyperkinetic events with tonic or dystonic features. Affected individuals may experience an aura. Retained awareness during seizures is common. A minority of individuals experience daytime seizures. Age of onset ranges from infancy to adulthood. About 80% of individuals develop ADSHE in the first two decades of life; mean age of onset is ten years. Clinical neurologic examination is normal and intellect is usually preserved, but reduced intellect, psychiatric comorbidities, or cognitive deficits may occur. Within a family, the manifestations of the disorder may vary considerably. ADSHE is lifelong but not progressive. As an individual reaches middle age, seizures may become milder and less frequent.

FRA12A is a folate-sensitive chromosomal fragile site prone to breakage. No consistent phenotype has been observed with FRA12A, and it can be inherited without phenotypic effect (Berg et al., 2000). However, impaired intellectual development with or without other anomalies has been described in patients with over 40% of cells expressing FRA12A (Winnepenninckx et al., 2007).

The 1q21.1 recurrent deletion itself does not lead to a clinically recognizable syndrome, as some persons with the deletion have no obvious clinical findings. Others have variable findings that most commonly include mildly dysmorphic but nonspecific facial features (>75%), mild intellectual disability or learning disabilities (25%), microcephaly (43%), and eye abnormalities (26%). Other findings can include cardiac defects, genitourinary anomalies, skeletal malformations, joint laxity, and seizures (~23%). Psychiatric and behavioral abnormalities can include autism spectrum disorder, attention-deficit/hyperactivity disorder, and sleep disturbances. Sensorineural hearing loss and recurrent infections /otitis media are rare.

Individuals with the 15q13.3 recurrent deletion may have a wide range of clinical manifestations. The deletion itself may not lead to a clinically recognizable syndrome and a subset of persons with the recurrent deletion have no obvious clinical findings, implying that penetrance for the deletion is incomplete. A little over half of individuals diagnosed with this recurrent deletion have intellectual disability or developmental delay, mainly in the areas of speech acquisition and cognitive function. In the majority of individuals, cognitive impairment is mild. Other features reported in diagnosed individuals include epilepsy (in ~30%), mild hypotonia, and neuropsychiatric disorders (including autism spectrum disorder, attention-deficit/hyperactivity disorder, mood disorder, schizophrenia, and aggressive or self-injurious behavior). Congenital malformations are uncommon.

A rare chromosomal anomaly syndrome, resulting from the partial deletion of the long arm of chromosome 22, outside the DiGeorge critical region. The phenotype is characterized by prematurity, pre- and post-natal growth retardation, developmental delay (particularly speech), mild intellectual disability, variable cardiac defects, and minor skeletal anomalies (such as clinodactyly). Dysmorphic features present in half of the individuals include microcephaly, arched eyebrows, deep set eyes, narrow upslanting palpebral fissures, ear abnormalities (low-set ears, tags and pits), hypoplastic alae nasi, smooth philtrum, down-turned mouth, thin upper lip, retro/micrognatia and pointed chin. For certain very distal deletions including the <i>SMARCB1</i> gene, there is a risk of developing malignant rhabdoid tumours. Most deletions are <i>de novo </i>.

16p12.2 recurrent deletion is characterized by variable clinical findings that do not constitute a recognizable syndrome. Of note, the significant bias in ascertainment of individuals undergoing clinical chromosomal microarray analysis (i.e., children with intellectual disability and developmental delay; individuals with schizophrenia) makes it difficult to accurately associate specific phenotypes with the 16p12.2 recurrent deletion. Findings commonly observed in children (probands) with this deletion include: developmental delay, cognitive impairment (ranging from mild to profound), growth impairment (including short stature), cardiac malformations, epilepsy, and psychiatric and/or behavioral issues. Other findings can include: hearing loss, dental abnormalities, renal and genital anomalies (the latter in males), and cleft palate ± cleft lip.

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.

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).

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.

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

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.

Autosomal dominant sleep-related hypermotor (hyperkinetic) epilepsy (ADSHE) is a seizure disorder characterized by clusters of nocturnal motor seizures that are often stereotyped and brief (<2 minutes). They vary from simple arousals from sleep to dramatic, often hyperkinetic events with tonic or dystonic features. Affected individuals may experience an aura. Retained awareness during seizures is common. A minority of individuals experience daytime seizures. Age of onset ranges from infancy to adulthood. About 80% of individuals develop ADSHE in the first two decades of life; mean age of onset is ten years. Clinical neurologic examination is normal and intellect is usually preserved, but reduced intellect, psychiatric comorbidities, or cognitive deficits may occur. Within a family, the manifestations of the disorder may vary considerably. ADSHE is lifelong but not progressive. As an individual reaches middle age, seizures may become milder and less frequent.

Inclusion body myopathy associated with Paget disease of bone (PDB) and/or frontotemporal dementia (IBMPFD) is characterized by adult-onset proximal and distal muscle weakness (clinically resembling a limb-girdle muscular dystrophy syndrome), early-onset PDB, and premature frontotemporal dementia (FTD). Muscle weakness progresses to involve other limb and respiratory muscles. PDB involves focal areas of increased bone turnover that typically lead to spine and/or hip pain and localized enlargement and deformity of the long bones; pathologic fractures occur on occasion. Early stages of FTD are characterized by dysnomia, dyscalculia, comprehension deficits, and paraphasic errors, with minimal impairment of episodic memory; later stages are characterized by inability to speak, auditory comprehension deficits for even one-step commands, alexia, and agraphia. Mean age at diagnosis for muscle disease and PDB is 42 years; for FTD, 56 years. Dilated cardiomyopathy, amyotrophic lateral sclerosis, and Parkinson disease are now known to be part of the spectrum of findings associated with IBMPFD.

Takenouchi-Kosaki syndrome is a highly heterogeneous autosomal dominant complex congenital developmental disorder affecting multiple organ systems. The core phenotype includes delayed psychomotor development with variable intellectual disability, dysmorphic facial features, and cardiac, genitourinary, and hematologic or lymphatic defects, including thrombocytopenia and lymphedema. Additional features may include abnormalities on brain imaging, skeletal anomalies, and recurrent infections. Some patients have a milder disease course reminiscent of Noonan syndrome (see, e.g., NS1, 163950) (summary by Martinelli et al., 2018).

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.

Band heterotopia (BH) is a neuronal migration disorder in which aberrantly located neurons, in the form of a band in the brain white matter, are present below a cortex that appears relatively normal by magnetic resonance imaging (MRI). Clinically, patients show severe developmental delay with intellectual disability, seizures, hypotonia, and hydrocephalus (Kielar et al., 2014, Shaheen et al., 2017).

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

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.

X-linked intellectual developmental disorder-105 (XLID105) is characterized by different combinations of impaired intellectual development, developmental delay, autism spectrum disorder, ADHD, and anxiety. Some patients have ophthalmologic abnormalities (summary by Koch et al., 2024).

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

WFS1 spectrum disorder (WFS1-SD) comprises classic WFS1 spectrum disorder and nonclassic WFS1 spectrum disorder. Classic WFS1-SD, a progressive neurodegenerative disorder, is characterized by onset of diabetes mellitus and optic atrophy before age 16 years. Additional complications may include one or more of the following: variable hearing impairment / deafness, diabetes insipidus, neurologic abnormalities, neurogenic bladder, and psychiatric abnormalities. Nonclassic WFS1-SD is less common than classic WFS1-SD. Phenotypes that appear to be milder than classic WFS1-SD include: optic atrophy and hearing impairment; neonatal diabetes, profound congenital deafness, and cataracts; isolated diabetes mellitus; isolated congenital cataracts; and isolated congenital, slowly progressive, and low-frequency (<2000 Hz) sensorineural hearing loss.

CADASIL (cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy) is characterized by mid-adult onset of recurrent ischemic stroke, cognitive decline progressing to dementia, a history of migraine with aura, mood disturbance, apathy, and diffuse white matter lesions and subcortical infarcts on neuroimaging.

ALKKUCS is an autosomal recessive severe neurodevelopmental disorder characterized by arthrogryposis, brain abnormalities associated with cerebral parenchymal underdevelopment, and global developmental delay. Most affected individuals die in utero or soon after birth. Additional abnormalities may include hypotonia, dysmorphic facial features, and involvement of other organ systems, such as cardiac or renal. The few patients who survive have variable intellectual disability and may have seizures (summary by Gueneau et al., 2018).

Early-onset progressive encephalopathy with brain edema and/or leukoencephalopathy-2 (PEBEL2) is an autosomal recessive severe neurometabolic disorder characterized by rapidly progressive neurologic deterioration that is usually associated with a febrile illness. Affected infants tend to show normal early development followed by acute psychomotor regression with ataxia, hypotonia, and sometimes seizures, resulting in death in the first years of life. Brain imaging shows multiple abnormalities, including brain edema and signal abnormalities in the cortical and subcortical regions (summary by Van Bergen et al., 2019). For a discussion of genetic heterogeneity of PEBEL, see PEBEL1 (617186).

Susceptibility to idiopathic generalized epilepsy-15 (EIG15) is an autosomal dominant seizure disorder characterized by onset of variable types of seizures in the first decade. Absence seizures are the most common manifestation, but most patients also develop other types, including clonic or generalized tonic-clonic seizures. EEG tends to show 3-Hz spike-wave discharges, whereas brain imaging is normal. The majority of patients also have developmental delay associated with impaired intellectual development apparent from infancy or early childhood (summary by Rudolf et al., 2016). For a general phenotypic description and a discussion of genetic heterogeneity of idiopathic generalized epilepsy, see EIG (600669).

Basilicata-Akhtar syndrome (MRXSBA) is characterized by global developmental delay apparent from infancy, feeding difficulties, hypotonia, and poor or absent speech. Most patients are able to walk, although they may have an unsteady gait or spasticity. Additional findings include dysmorphic facial features and mild distal skeletal anomalies. Males and females are similarly affected (summary by Basilicata et al., 2018).

Neurodevelopmental disorder with hypotonia and autistic features with or without hyperkinetic movements (NEDHAHM) is characterized by axial hypotonia apparent from birth, global developmental delay with impaired intellectual development and poor or absent language acquisition, and behavioral abnormalities, including autistic features, poor social interaction, and hang-wringing. Most patients have childhood-onset seizures that are usually responsive to medication, and a subset of patients develop cortical visual impairment and involuntary hyperkinetic movements, including chorea and dystonia. Some of the features are reminiscent of Rett syndrome (RTT; 312750) (summary by Salpietro et al., 2019).

DDX3X-related neurodevelopmental disorder (DDX3X-NDD) typically occurs in females and very rarely in males. All affected individuals reported to date have developmental delay / intellectual disability ranging from mild to severe; about 50% of affected girls remain nonverbal after age five years. Hypotonia, a common finding, can be associated with feeding difficulty in infancy. Behavioral issues can include autism spectrum disorder, attention-deficit/hyperactivity disorder and hyperactivity, self-injurious behavior, poor impulse control, and aggression. Other findings can include seizures, movement disorders (dyskinesia, spasticity, abnormal gait), vision and hearing impairment, congenital heart defects, respiratory difficulties, joint laxity, and scoliosis. Neuroblastoma has been observed in three individuals.

Frontotemporal dementia and/or amyotrophic lateral sclerosis-8 (FTDALS8) is an autosomal dominant neurodegenerative disorder characterized by adult-onset dementia manifest as memory impairment, executive dysfunction, and behavioral or personality changes. Some patients may develop ALS or parkinsonism. Neuropathologic studies show frontotemporal lobar degeneration (FTLD) with tau (MAPT; 157140)- and TDP43 (605078)-immunoreactive inclusions (summary by Dobson-Stone et al., 2020). For a discussion of genetic heterogeneity of FTDALS, see FTDALS1 (105550).

Frontotemporal dementia and/or amyotrophic lateral sclerosis-5 (FTDALS5) is an autosomal dominant neurodegenerative disorder characterized by onset of ALS or FTD symptoms in adulthood. The disease is progressive, and some patients may develop both diseases, although ALS seems to be more prevalent than FTD. The disorder usually results in premature death (summary by Williams et al., 2016). For a discussion of genetic heterogeneity of FTDALS, see FTDALS1 (105550).

Coffin-Siris syndrome-12 (CSS12) is a neurodevelopmental disorder characterized by global developmental delay with variably impaired intellectual development, speech and language delay, and behavioral abnormalities, such as autism or hyperactivity. Affected individuals may have hypotonia and poor feeding in infancy. There are variable dysmorphic facial features, although most patients do not have the classic hypoplastic fifth digit/nail abnormalities that are often observed in other forms of CSS (Barish et al., 2020). For a general phenotypic description and a discussion of genetic heterogeneity of Coffin-Siris syndrome, see CSS1 (135900).

Developmental delay with dysmorphic facies and dental anomalies (DEFDA) is characterized by generally mild global developmental delay with variably impaired intellectual development, walking by 2 to 3 years, and slow language acquisition. The severity of the disorder ranges from moderate cognitive deficits to mild learning difficulties or behavioral abnormalities. Most patients have dysmorphic facial features, often with abnormal dentition and nonspecific visual defects, such as myopia, astigmatism, and strabismus. Although rare, involvement of other systems, such as skeletal, cardiac, and gastrointestinal, may be present (summary by den Hoed et al., 2021).

The spectrum of CSF1R-related disorder ranges from early-onset disease (age <18 years) to late-onset disease (age =18 years). Early-onset disease is associated with hypotonia, delayed acquisition of developmental milestones, and non-neurologic manifestations (such as skeletal abnormalities); both early- and late-onset disease have similar neurodegenerative involvement. Most affected individuals eventually become bedridden with spasticity, rigidity, and loss of the ability to walk. They lose speech and voluntary movement and appear to be generally unaware of their surroundings. The last stage of disease progresses to a vegetative state with presence of primitive reflexes, such as visual and tactile grasp, mouth-opening reflex, and sucking reflex. Death most commonly results from pneumonia or other infections. About 500 individuals with CSF1R-related disorder have been reported to date.

Developmental delay, impaired speech, and behavioral abnormalities (DDISBA) is characterized by global developmental delay apparent from early childhood. Intellectual disability can range from mild to severe. Additional variable features may include dysmorphic facial features, seizures, hypotonia, motor abnormalities such as Tourette syndrome or dystonia, and hearing loss (summary by Cousin et al., 2021).

Developmental delay, hypotonia, musculoskeletal defects, and behavioral abnormalities (DEHMBA) is an early-onset neurodevelopmental disorder characterized by these features. Affected individuals also have nonspecific and variable dysmorphic facial features that do not constitute a recognizable gestalt. Although the disorder is caused by truncating mutations in the SRCAP gene as is FLHS, the DEHMBA phenotype is clinically distinguishable from FLHS by the lack of short stature, brachydactyly, and delayed bone age, as well as absence of a specific facial appearance. There are some overlapping features between the 2 disorders, mainly impaired intellectual development and speech delay (summary by Rots et al., 2021).

Developmental and epileptic encephalopathy-102 (DEE102) is an autosomal recessive neurodevelopmental disorder characterized by global developmental delay and severe to profoundly impaired intellectual development with inability to walk or speak. Most patients have onset of variable types of seizures within the first year of life, and the seizures tend to be refractory. Additional features include progressive microcephaly, visual impairment, axial hypotonia, peripheral hypertonia, and nonspecific brain imaging abnormalities (Marafi et al., 2022). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

WAC-related intellectual disability (ID) is typically characterized by variable degrees of developmental delay and/or intellectual disability. Behavioral abnormalities including anxiety, attention-deficit/hyperactivity disorder, and/or autism spectrum disorder are observed in the majority of older children and adults. Most affected infants have significant but nonspecific features at birth such as neonatal hypotonia and feeding problems. Some affected individuals come to medical attention with respiratory or vision problems. Facial features may be mildly dysmorphic, but are nonspecific. To date, 18 individuals have been identified with WAC-related ID.

X-linked intellectual disorder-112 (XLID112) is a neurodevelopmental disorder characterized by developmental delay, with speech delay more prominent than motor delay, autism or autism traits, and variable dysmorphic features. Affected females have been reported, which appears to be related to skewed X-inactivation (summary by Hiatt et al., 2023).