Chediak-Higashi syndrome (CHS) is characterized by partial oculocutaneous albinism, immunodeficiency, and a mild bleeding tendency. Approximately 85% of affected individuals develop the accelerated phase, or hemophagocytic lymphohistiocytosis, a life-threatening, hyperinflammatory condition. All affected individuals including adolescents and adults with atypical CHS and children with classic CHS who have successfully undergone allogenic hematopoietic stem cell transplantation (HSCT) develop neurologic findings during early adulthood.

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

Normal-pressure hydrocephalus-1 (HYDNP1) is an autosomal dominant neurologic disorder characterized by the clinical triad of slowly progressive gait instability, urinary incontinence, and cognitive decline associated with ventricular enlargement on brain imaging with normal pressure of the cerebrospinal fluid (CSF). The onset of symptoms is usually in late adulthood; the symptoms are responsive to shunting. The disorder has been associated with recurrent respiratory infections and possible infertility issues, but the latter has not been confirmed (summary by Takahashi et al., 2011 and Morimoto et al., 2019).

Arylsulfatase A deficiency (also known as metachromatic leukodystrophy or MLD) is characterized by three clinical subtypes: late-infantile MLD, juvenile MLD, and adult MLD. Age of onset within a family is usually similar. The disease course may be from several years in the late-infantile-onset form to decades in the juvenile- and adult-onset forms. Late-infantile MLD. Onset is before age 30 months. Typical presenting findings include weakness, hypotonia, clumsiness, frequent falls, toe walking, and dysarthria. As the disease progresses, language, cognitive, and gross and fine motor skills regress. Later signs include spasticity, pain, seizures, and compromised vision and hearing. In the final stages, children have tonic spasms, decerebrate posturing, and general unawareness of their surroundings. Juvenile MLD. Onset is between age 30 months and 16 years. Initial manifestations include decline in school performance and emergence of behavioral problems, followed by gait disturbances. Progression is similar to but slower than in the late-infantile form. Adult MLD. Onset occurs after age 16 years, sometimes not until the fourth or fifth decade. Initial signs can include problems in school or job performance, personality changes, emotional lability, or psychosis; in others, neurologic symptoms (weakness and loss of coordination progressing to spasticity and incontinence) or seizures initially predominate. Peripheral neuropathy is common. Disease course is variable – with periods of stability interspersed with periods of decline – and may extend over two to three decades. The final stage is similar to earlier-onset forms.

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.

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.

'Behr syndrome' is a clinical term that refers to the constellation of early-onset optic atrophy accompanied by neurologic features, including ataxia, pyramidal signs, spasticity, and mental retardation (Behr, 1909; Thomas et al., 1984). Patients with mutations in genes other than OPA1 can present with clinical features reminiscent of Behr syndrome. Mutations in one of these genes, OPA3 (606580), result in type III 3-methylglutaconic aciduria (MGCA3; 258501). Lerman-Sagie (1995) noted that the abnormal urinary pattern in MGCA3 may not be picked up by routine organic acid analysis, suggesting that early reports of Behr syndrome with normal metabolic features may actually have been 3-methylglutaconic aciduria type III.

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.

Kniest dysplasia is characterized by skeletal and craniofacial anomalies. Skeletal anomalies include disproportionate dwarfism, a short trunk and small pelvis, kyphoscoliosis, short limbs, and prominent joints and premature osteoarthritis that restrict movement. Craniofacial manifestations include midface hypoplasia, cleft palate, early-onset myopia, retinal detachment, and hearing loss. The phenotype is severe in some patients and mild in others. There are distinct radiographic changes including coronal clefts of vertebrae and dumbbell-shaped femora. The chondrooseous morphology is pathognomonic with perilacunar 'foaminess' and sparse, aggregated collagen fibrils resulting in an interterritorial matrix with a 'Swiss-cheese' appearance (summary by Wilkin et al., 1999).

GLB1-related disorders comprise two phenotypically distinct lysosomal storage disorders: GM1 gangliosidosis and mucopolysaccharidosis type IVB (MPS IVB). The phenotype of GM1 gangliosidosis constitutes a spectrum ranging from severe (infantile) to intermediate (late-infantile and juvenile) to mild (chronic/adult). Type I (infantile) GM1 gangliosidosis begins before age 12 months. Prenatal manifestations may include nonimmune hydrops fetalis, intrauterine growth restriction, and placental vacuolization; congenital dermal melanocytosis (Mongolian spots) may be observed. Macular cherry-red spot is detected on eye exam. Progressive central nervous system dysfunction leads to spasticity and rapid regression; blindness, deafness, decerebrate rigidity, seizures, feeding difficulties, and oral secretions are observed. Life expectancy is two to three years. Type II can be subdivided into the late-infantile (onset age 1-3 years) and juvenile (onset age 3-10 years) phenotypes. Central nervous system dysfunction manifests as progressive cognitive, motor, and speech decline as measured by psychometric testing. There may be mild corneal clouding, hepatosplenomegaly, and/or cardiomyopathy; the typical course is characterized by progressive neurologic decline, progressive skeletal disease in some individuals (including kyphosis and avascular necrosis of the femoral heads), and progressive feeding difficulties leading to aspiration risk. Type III begins in late childhood to the third decade with generalized dystonia leading to unsteady gait and speech disturbance followed by extrapyramidal signs including akinetic-rigid parkinsonism. Cardiomyopathy develops in some and skeletal involvement occurs in most. Intellectual impairment is common late in the disease with prognosis directly related to the degree of neurologic impairment. MPS IVB is characterized by skeletal dysplasia with specific findings of axial and appendicular dysostosis multiplex, short stature (below 15th centile in adults), kyphoscoliosis, coxa/genu valga, joint laxity, platyspondyly, and odontoid hypoplasia. First signs and symptoms may be apparent at birth. Bony involvement is progressive, with more than 84% of adults requiring ambulation aids; life span does not appear to be limited. Corneal clouding is detected in some individuals and cardiac valvular disease may develop.

Oculopharyngeal muscular dystrophy (OPMD) is characterized by ptosis and dysphagia due to selective involvement of the muscles of the eyelids and pharynx, respectively. For the vast majority of individuals with typical OPMD, the mean age of onset of ptosis is usually 48 years and of dysphagia 50 years; in 5%-10% of individuals with severe OPMD, onset of ptosis and dysphagia occur before age 45 years and is associated with lower limb girdle weakness starting around age 60 years. Swallowing difficulties, which determine prognosis, increase the risk for potentially life-threatening aspiration pneumonia and poor nutrition. Other manifestations as the disease progresses can include limitation of upward gaze, tongue atrophy and weakness, chewing difficulties, wet voice, facial muscle weakness, axial muscle weakness, and proximal limb girdle weakness predominantly in lower limbs. Some individuals with severe involvement will eventually need a wheelchair. Neuropsychological tests have shown altered scores in executive functions in some.

NKX2-1-related disorders range from benign hereditary chorea (BHC) to choreoathetosis, congenital hypothyroidism, and neonatal respiratory distress (also known as brain-lung-thyroid syndrome). Childhood-onset chorea, the hallmark of NKX2-1-related disorders, may or may not be associated with respiratory distress syndrome or congenital hypothyroidism. Chorea generally begins in early infancy or about age one year (most commonly) or in late childhood or adolescence, and progresses into the second decade after which it remains static or (rarely) remits. Pulmonary disease, the second most common manifestation, can include respiratory distress syndrome in neonates, interstitial lung disease in young children, and pulmonary fibrosis in older persons. The risk for pulmonary carcinoma is increased in young adults with an NKX2-1-related disorder. Thyroid dysfunction, the result of dysembryogenesis, can present as congenital hypothyroidism or compensated hypothyroidism. The risk for thyroid cancer is unknown and may not be increased. In one review, 50% of affected individuals had the full brain-lung-thyroid syndrome, 30% had involvement of brain and thyroid only, and 13% had isolated chorea only.

GJB1 disorders are typically characterized by peripheral motor and sensory neuropathy with or without fixed CNS abnormalities and/or acute, self-limited episodes of transient neurologic dysfunction (especially weakness and dysarthria). Peripheral neuropathy typically manifests in affected males between ages five and 25 years. Although both men and women are affected, manifestations tend to be less severe in women, some of whom may remain asymptomatic. Less commonly, initial manifestations in some affected individuals are stroke-like episodes (acute fulminant episodes of reversible CNS dysfunction).

Barth syndrome is characterized in affected males by cardiomyopathy, neutropenia, skeletal myopathy, prepubertal growth delay, and distinctive facial gestalt (most evident in infancy); not all features may be present in a given affected male. Cardiomyopathy, which is almost always present before age five years, is typically dilated cardiomyopathy with or without endocardial fibroelastosis or left ventricular noncompaction; hypertrophic cardiomyopathy can also occur. Heart failure is a significant cause of morbidity and mortality; risk of arrhythmia and sudden death is increased. Neutropenia is most often associated with mouth ulcers, pneumonia, and sepsis. The nonprogressive myopathy predominantly affects the proximal muscles, and results in early motor delays. Prepubertal growth delay is followed by a postpubertal growth spurt with remarkable "catch-up" growth. Heterozygous females who have a normal karyotype are asymptomatic and have normal biochemical studies.

Cockayne syndrome (referred to as CS in this GeneReview) spans a continuous phenotypic spectrum that includes: CS type I, the "classic" or "moderate" form; CS type II, a more severe form with symptoms present at birth; this form overlaps with cerebrooculofacioskeletal (COFS) syndrome; CS type III, a milder and later-onset form; COFS syndrome, a fetal form of CS. CS type I is characterized by normal prenatal growth with the onset of growth and developmental abnormalities in the first two years. By the time the disease has become fully manifest, height, weight, and head circumference are far below the fifth percentile. Progressive impairment of vision, hearing, and central and peripheral nervous system function leads to severe disability; death typically occurs in the first or second decade. CS type II is characterized by growth failure at birth, with little or no postnatal neurologic development. Congenital cataracts or other structural anomalies of the eye may be present. Affected children have early postnatal contractures of the spine (kyphosis, scoliosis) and joints. Death usually occurs by age five years. CS type III is a phenotype in which major clinical features associated with CS only become apparent after age two years; growth and/or cognition exceeds the expectations for CS type I. COFS syndrome is characterized by very severe prenatal developmental anomalies (arthrogryposis and microphthalmia).

X-linked intellectual developmental disorder-12 (XLID12) is characterized by borderline to severe intellectual disability with variable neurologic features, short stature, and elevated body mass index (BMI) (Kumar et al., 2015).

Charcot-Marie-Tooth disease type 4D (CMT4D) is an autosomal recessive disorder of the peripheral nervous system characterized by early-onset distal muscle weakness and atrophy, foot deformities, and sensory loss affecting all modalities. Affected individuals develop deafness by the third decade of life (summary by Okamoto et al., 2014). For a phenotypic description and a discussion of genetic heterogeneity of autosomal recessive Charcot-Marie-Tooth disease, see CMT4A (214400).

CHMP2B frontotemporal dementia (CHMP2B-FTD) has been described in a single family from Denmark, in one individual with familial FTD from Belgium, and in one individual with FTD and no family history. It typically starts between ages 46 and 65 years with subtle personality changes and slowly progressive behavioral changes, dysexecutive syndrome, dyscalculia, and language disturbances. Disinhibition or loss of initiative is the most common presenting symptom. The disease progresses over a few years into profound dementia with extrapyramidal symptoms and mutism. Several individuals have developed an asymmetric akinetic rigid syndrome with arm and gait dystonia and pyramidal signs that may be related to treatment with neuroleptic drugs. Symptoms and disease course are highly variable. Disease duration may be as short as three years or longer than 20 years.

Progressive external ophthalmoplegia is characterized by multiple mitochondrial DNA deletions in skeletal muscle. The most common clinical features include adult onset of weakness of the external eye muscles and exercise intolerance. Patients with C10ORF2-linked adPEO may have other clinical features including proximal muscle weakness, ataxia, peripheral neuropathy, cardiomyopathy, cataracts, depression, and endocrine abnormalities (summary by Fratter et al., 2010). For a general phenotypic description and a discussion of genetic heterogeneity of autosomal dominant progressive external ophthalmoplegia, see PEOA1 (157640). PEO caused by mutations in the POLG gene (174763) are associated with more complicated phenotypes than those forms caused by mutations in the SLC25A4 (103220) or C10ORF2 genes (Lamantea et al., 2002).

Nemaline myopathy-6 is an autosomal dominant skeletal muscle disorder characterized by childhood onset of slowly progressive proximal muscle weakness, exercise intolerance, and slow movements with stiff muscles. Patients are unable to run or correct themselves from falling over. Histopathologic changes seen on skeletal muscle biopsy include nemaline rods, cores devoid of oxidative enzyme activity, and predominance of hypertrophic type 1 fibers. There is no cardiac or respiratory involvement (summary by Sambuughin et al., 2010).

Autosomal dominant sensory ataxia-1 (SNAX1) is a peripheral neuropathy resulting from the degeneration of dorsal root ganglia that affects both central and peripheral neurites of sensory neurons. Affected individuals show adult onset of slowly progressive clumsiness, gait ataxia, walking difficulties, and distal sensory loss which may be associated with abnormal sensory nerve conduction values. Some patients have vestibular ocular dysfunction. Muscle weakness and atrophy are not observed, and brain imaging is normal (summary by Cortese et al., 2020).

HTRA1 disorder is a phenotypic spectrum in which some individuals have few to no symptoms and others manifest with the more severe CARASIL (cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy) phenotype. Those who have a heterozygous HTRA1 pathogenic variant may have mild neurologic findings (sometimes identified only on neuroimaging) or mild-to-moderate neurologic signs and symptoms of CARASIL. In this chapter, the term "classic CARASIL" refers to the more severe phenotype associated with biallelic pathogenic variants, and "HTRA1 cerebral small vessel disease" (HTRA1-CSVD) refers to the milder phenotype associated with a heterozygous HTRA1 pathogenic variant. Classic CARASIL is characterized by early-onset changes in the deep white matter of the brain observed on MRI, and associated neurologic findings. The most frequent initial symptom is gait disturbance from spasticity beginning between ages 20 and 40 years. Forty-four percent of affected individuals have stroke-like episodes before age 40 years. Mood changes (apathy and irritability), pseudobulbar palsy, and cognitive dysfunction begin between ages 20 and 50 years. The disease progresses slowly following the onset of neurologic symptoms. Scalp alopecia and acute mid- to lower-back pain (lumbago) before age 30 years are characteristic. The most frequent initial symptom in individuals with HTRA1-CSVD is slowly progressive gait disturbance after age 40 years, which may be followed by the development of mood changes and cognitive dysfunction. A majority of affected individuals have a stroke-like episode after age 40 years. Spondylosis and alopecia are seen in a minority of individuals with HTRA1-CSVD.

X-linked Charcot-Marie-Tooth neuropathy type 5 (CMTX5), part of the spectrum of PRPS1-related disorders, is characterized by peripheral neuropathy, early-onset (prelingual) bilateral profound sensorineural hearing loss, and optic neuropathy. The onset of peripheral neuropathy is between ages five and 12 years. The lower extremities are affected earlier and more severely than upper extremities. Initial manifestations often include foot drop or gait disturbance. Onset of visual impairment is between ages seven and 20 years. Intellect and life span are normal. Carrier females do not have findings of CMTX5.

Alzheimer's disease can be classified as early-onset or late-onset. The signs and symptoms of the early-onset form appear between a person's thirties and mid-sixties, while the late-onset form appears during or after a person's mid-sixties. The early-onset form of Alzheimer's disease is much less common than the late-onset form, accounting for less than 10 percent of all cases of Alzheimer's disease.\n\nIndividuals with Alzheimer's disease usually survive 8 to 10 years after the appearance of symptoms, but the course of the disease can range from 1 to 25 years. Survival is usually shorter in individuals diagnosed after age 80 than in those diagnosed at a younger age. In Alzheimer's disease, death usually results from pneumonia, malnutrition, or general body wasting (inanition).\n\nAs the disorder progresses, some people with Alzheimer's disease experience personality and behavioral changes and have trouble interacting in a socially appropriate manner. Other common symptoms include agitation, restlessness, withdrawal, and loss of language skills. People with Alzheimer's disease usually require total care during the advanced stages of the disease.\n\nMemory loss is the most common sign of Alzheimer's disease. Forgetfulness may be subtle at first, but the loss of memory worsens over time until it interferes with most aspects of daily living. Even in familiar settings, a person with Alzheimer's disease may get lost or become confused. Routine tasks such as preparing meals, doing laundry, and performing other household chores can be challenging. Additionally, it may become difficult to recognize people and name objects. Affected people increasingly require help with dressing, eating, and personal care.\n\nAlzheimer's disease is a degenerative disease of the brain that causes dementia, which is a gradual loss of memory, judgment, and ability to function. This disorder usually appears in people older than age 65, but less common forms of the disease appear earlier in adulthood.

A form of axonal Charcot-Marie-Tooth disease a peripheral sensorimotor neuropathy. Onset is in the first to sixth decade with a gait anomaly and a leg weakness that reaches the arms secondarily. Tendon reflexes are reduced or absent and after years all patients have a pes cavus. Other signs may be present including hearing loss and postural tremor.

The Hedera type of X-linked syndromic intellectual developmental disorder (MRXSH) is characterized by global developmental delay apparent from infancy and progressive neurologic decline with abnormal movements, spasticity, and seizures. Brain imaging shows volume loss of cortical white and gray matter, thin corpus callosum, and myelination defects, consistent with a neurodegenerative process. Only males are affected (summary by Hirose et al., 2019).

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

Kufor-Rakeb syndrome is a rare autosomal recessive form of juvenile-onset atypical Parkinson disease (PARK9) associated with supranuclear gaze palsy, spasticity, and dementia. Some patients have neuroradiologic evidence of iron deposition in the basal ganglia, indicating that the pathogenesis of PARK9 can be considered among the syndromes of neurodegeneration with brain iron accumulation (NBIA; see 234200) (summary by Bruggemann et al., 2010). For a phenotypic description and a discussion of genetic heterogeneity of Parkinson disease (PD), see 168600. Biallelic mutation in the ATP13A2 gene also causes autosomal recessive spastic paraplegia-78 (SPG78; 617225), an adult-onset neurodegenerative disorder with overlapping features. Patients with SPG78 have later onset and prominent spasticity, but rarely parkinsonism. Loss of ATP13A2 function results in a multidimensional spectrum of neurologic features reflecting various regions of the brain and nervous system, including cortical, pyramidal, extrapyramidal, brainstem, cerebellar, and peripheral (summary by Estrada-Cuzcano et al., 2017).

Ataxia with vitamin E deficiency (AVED) generally manifests in late childhood or early teens between ages five and 15 years. The first symptoms include progressive ataxia, clumsiness of the hands, loss of proprioception, and areflexia. Other features often observed are dysdiadochokinesia, dysarthria, positive Romberg sign, head titubation, decreased visual acuity, and positive Babinski sign. The phenotype and disease severity vary widely among families with different pathogenic variants; age of onset and disease course are more uniform within a given family, but symptoms and disease severity can vary even among sibs.

Most individuals with classic GBE1 adult polyglucosan body disease (GBE1-APBD) present after age 40 years with unexplained progressive neurogenic bladder, gait difficulties (i.e., spasticity and weakness) from mixed upper and lower motor neuron involvement, sensory loss predominantly in the distal lower extremities, autonomic dysfunction (associated with orthostatic hypotension and constipation), and mild cognitive difficulties (often executive dysfunction). Some affected individuals without classic GBE1-APBD have atypical phenotypes including Alzheimer disease-like dementia and axonal neuropathy, stroke-like episodes, and diaphragmatic failure; others may have a history of infantile liver disease.

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).

Nemaline myopathy-2 (NEM2) is an autosomal recessive skeletal muscle disorder with a wide range of severity. The most common clinical presentation is early-onset (in infancy or childhood) muscle weakness predominantly affecting proximal limb muscles. Muscle biopsy shows accumulation of Z-disc and thin filament proteins into aggregates named 'nemaline bodies' or 'nemaline rods,' usually accompanied by disorganization of the muscle Z discs. The clinical and histologic spectrum of entities caused by variants in the NEB gene is a continuum, ranging in severity. The distribution of weakness can vary from generalized muscle weakness, more pronounced in proximal limb muscles, to distal-only involvement, although neck flexor weakness appears to be rather consistent. Histologic patterns range from a severe usually nondystrophic disturbance of the myofibrillar pattern to an almost normal pattern, with or without nemaline bodies, sometimes combined with cores (summary by Lehtokari et al., 2014). Genetic Heterogeneity of Nemaline Myopathy See also NEM1 (255310), caused by mutation in the tropomyosin-3 gene (TPM3; 191030) on chromosome 1q22; NEM3 (161800), caused by mutation in the alpha-actin-1 gene (ACTA1; 102610) on chromosome 1q42; NEM4 (609285), caused by mutation in the beta-tropomyosin gene (TPM2; 190990) on chromosome 9p13; NEM5A (605355), also known as Amish nemaline myopathy, NEM5B (620386), and NEM5C (620389), all caused by mutation in the troponin T1 gene (TNNT1; 191041) on chromosome 19q13; NEM6 (609273), caused by mutation in the KBTBD13 gene (613727) on chromosome 15q22; NEM7 (610687), caused by mutation in the cofilin-2 gene (CFL2; 601443) on chromosome 14q13; NEM8 (615348), caused by mutation in the KLHL40 gene (615340), on chromosome 3p22; NEM9 (615731), caused by mutation in the KLHL41 gene (607701) on chromosome 2q31; NEM10 (616165), caused by mutation in the LMOD3 gene (616112) on chromosome 3p14; and NEM11 (617336), caused by mutation in the MYPN gene (608517) on chromosome 10q21. Several of the genes encode components of skeletal muscle sarcomeric thin filaments (Sanoudou and Beggs, 2001). Mutations in the NEB gene are the most common cause of nemaline myopathy (Lehtokari et al., 2006).

The Lafora type of progressive myoclonic epilepsy is an autosomal recessive disorder characterized by insidious onset of progressive neurodegeneration between 8 and 18 years of age. Initial features can include headache, difficulties in school work, myoclonic jerks, generalized seizures, and often visual hallucination. The myoclonus, seizures, and hallucinations gradually worsen and become intractable. This is accompanied by progressive cognitive decline, resulting in dementia. About 10 years after onset, affected individuals are in near-continuous myoclonus with absence seizures, frequent generalized seizures, and profound dementia or a vegetative state. Histologic studies of multiple tissues, including brain, muscle, liver, and heart show intracellular Lafora bodies, which are dense accumulations of malformed and insoluble glycogen molecules, termed polyglucosans (review by Ramachandran et al., 2009). There is a slower progression of disease and later age at death in Lafora disease-2 than in Lafora disease-1 (MELF1, EPM2A; 254780); see Genotype/Phenotype Correlations. Myoclonic epilepsy of Lafora-1 is caused by mutation in the EPM2A gene (608072), which encodes laforin, on chromosome 6q24. For a discussion of genetic heterogeneity of progressive myoclonic epilepsy, see EPM1A (254800).

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.

GNE myopathy is a slowly progressive muscle disease that typically presents between age 20 and 40 years with bilateral foot drop caused by anterior tibialis weakness. Lower-extremity muscle involvement progresses from the anterior to the posterior compartment of the lower leg, followed by hamstrings, then hip girdle muscles, with relative sparing of the quadriceps. A wheelchair may be needed about ten to 20 years after the onset of manifestations. The upper extremities, which may be affected within five to ten years of disease onset, do not necessarily follow a distal-to-proximal progression. In advanced stages, neck and core muscles can become affected.

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.

Mast syndrome (MASTS) is an autosomal recessive complicated form of hereditary spastic paraplegia in which progressive spastic paraparesis is associated in more advanced cases with cognitive decline, dementia, and other neurologic abnormalities. Symptom onset usually occurs in adulthood, and the disorder is progressive with variable severity. Brain imaging shows thinning of the corpus callosum. The disorder occurs with high frequency in the Old Order Amish (summary by Simpson et al., 2003). For a discussion of genetic heterogeneity of autosomal recessive spastic paraplegia, see SPG5A (270800).

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.

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).

7q11.23 duplication syndrome is characterized by delayed motor, speech, and social skills in early childhood; neurologic abnormalities (hypotonia, adventitious movements, and abnormal gait and station); speech sound disorders including motor speech disorders (childhood apraxia of speech and/or dysarthria) and phonologic disorders; behavior problems including anxiety disorders (especially social anxiety disorder [social phobia]), selective mutism, attention-deficit/hyperactivity disorder, oppositional disorders, physical aggression, and autism spectrum disorder; and intellectual disability in some individuals. Distinctive facial features are common. Cardiovascular disease includes dilatation of the ascending aorta. Approximately 30% of individuals have one or more congenital anomalies.

Okinawa-type hereditary motor and sensory neuropathy (HMSNO) is an autosomal dominant neurodegenerative disorder characterized by young adult onset of proximal or distal muscle weakness and atrophy, muscle cramps, and fasciculations, with later onset of distal sensory impairment. The disorder is slowly progressive and clinically resembles amyotrophic lateral sclerosis (ALS; see 105400) (summary by Ishiura et al., 2012).

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).

Parkinson disease is the second most common neurogenic disorder after Alzheimer disease (AD; 104300), affecting approximately 1% of the population over age 50. Clinical manifestations include resting tremor, muscular rigidity, bradykinesia, and postural instability. Additional features are characteristic postural abnormalities, dysautonomia, dystonic cramps, and dementia (Polymeropoulos et al., 1996). For a general phenotypic description and a discussion of genetic heterogeneity of Parkinson disease, see 168600.

Parkin type of early-onset Parkinson disease (PARK-Parkin) is characterized by the cardinal signs of Parkinson disease (PD): bradykinesia, resting tremor, and rigidity. The median age at onset is 31 years (range: 3-81 years). The disease is slowly progressive: disease duration of more than 50 years has been reported. Clinical findings vary; hyperreflexia is common. Lower-limb dystonia may be a presenting sign and cognitive decline appears to be no more frequent than in the general population. Dyskinesia as a result of treatment with levodopa frequently occurs.

Charcot-Marie-Tooth disease type 4J is an autosomal recessive progressive neurologic disorder with a highly variable phenotype and onset ranging from early childhood to adulthood. Most patients have both proximal and distal asymmetric muscle weakness of the upper and lower limbs. There is significant motor dysfunction, followed by variably progressive sensory loss, which may be mild. Nerve conduction studies and nerve biopsies indicate demyelination as well as axonal loss (summary by Nicholson et al., 2011). For a phenotypic description and a discussion of genetic heterogeneity of autosomal recessive demyelinating Charcot-Marie-Tooth disease, see CMT4A (214400).

Hereditary spastic ataxia comprises a heterogeneous group of progressive neurodegenerative disorders characterized by lower-limb spasticity and generalized ataxia with dysarthria, impaired ocular movements, and gait disturbance. Spastic ataxia-1 (SPAX1) is an autosomal dominant form of the disorder with onset between the ages of 10 and 20 years. Other clinical features are supranuclear gaze palsy, hyperreflexia, hypertonicity, dystonia, pes cavus, mild ptosis, and decreased vibration sense in the lower limbs. Symptom severity is variable, but neither life span nor cognition is affected (summary by Meijer et al., 2002 and Bourassa et al., 2012). Genetic Heterogeneity of Spastic Ataxia See also SPAX2 (611302), caused by mutation in the KIF1C gene (603060) on chromosome 17p13; SPAX3 (611390), caused by rearrangements of the MARS2 gene (609728) on chromosome 2q33; SPAX4 (613672), caused by mutation in the MTPAP gene (613669) on chromosome 10p11; SPAX5 (614487), caused by mutation in the AFG3L2 gene (604581) on chromosome 18p11; SPAX6 (270550), caused by mutation in the SACS gene (604490) on chromosome 13q12; SPAX7 (108650); SPAX8 (617560), caused by mutation in the NKX6-2 gene (605955) on chromosome 8q21; SPAX9 (618438), caused by mutation in the CHP1 gene (606988) on chromosome 15q15; and SPAX10 (620666), caused by mutation in the COQ4 gene (612898) on chromosome 9q34.

Dystonia 16 is one of many forms of dystonia, which is a group of conditions characterized by involuntary movements, twisting (torsion) and tensing of various muscles, and unusual positioning of affected body parts. Dystonia 16 can appear at any age from infancy through adulthood, although it most often begins in childhood.\n\nThe signs and symptoms of dystonia 16 vary among people with the condition. In many affected individuals, the disorder first affects muscles in one or both arms or legs. Tensing (contraction) of the muscles often sets the affected limb in an abnormal position, which may be painful and can lead to difficulty performing tasks, such as walking. In others, muscles in the neck are affected first, causing the head to be pulled backward and positioned with the chin in the air (retrocollis).\n\nIn dystonia 16, muscles of the jaw, lips, and tongue are also commonly affected (oromandibular dystonia), causing difficulty opening and closing the mouth and problems with swallowing and speech. Speech can also be affected by involuntary tensing of the muscles that control the vocal cords (laryngeal dystonia), resulting in a quiet, breathy voice or an inability to speak clearly. Dystonia 16 gradually gets worse, eventually involving muscles in most parts of the body.\n\nSome people with dystonia 16 develop a pattern of movement abnormalities known as parkinsonism. These abnormalities include unusually slow movement (bradykinesia), muscle rigidity, tremors, and an inability to hold the body upright and balanced (postural instability). In dystonia 16, parkinsonism is relatively mild if it develops at all.\n\nThe signs and symptoms of dystonia 16 usually do not get better when treated with drugs that are typically used for movement disorders.

PNPLA6 disorders span a phenotypic continuum characterized by variable combinations of cerebellar ataxia; upper motor neuron involvement manifesting as spasticity and/or brisk reflexes; chorioretinal dystrophy associated with variable degrees of reduced visual function; and hypogonadotropic hypogonadism (delayed puberty and lack of secondary sex characteristics). The hypogonadotropic hypogonadism occurs either in isolation or as part of anterior hypopituitarism (growth hormone, thyroid hormone, or gonadotropin deficiencies). Common but less frequent features are peripheral neuropathy (usually of axonal type manifesting as reduced distal reflexes, diminished vibratory sensation, and/or distal muscle wasting); hair anomalies (long eyelashes, bushy eyebrows, or scalp alopecia); short stature; and impaired cognitive functioning (learning disabilities in children; deficits in attention, visuospatial abilities, and recall in adults). Some of these features can occur in distinct clusters on the phenotypic continuum: Boucher-Neuhäuser syndrome (cerebellar ataxia, chorioretinal dystrophy, and hypogonadotropic hypogonadism); Gordon Holmes syndrome (cerebellar ataxia, hypogonadotropic hypogonadism, and – to a variable degree – brisk reflexes); Oliver-McFarlane syndrome (trichomegaly, chorioretinal dystrophy, short stature, intellectual disability, and hypopituitarism); Laurence-Moon syndrome; and spastic paraplegia type 39 (SPG39) (upper motor neuron involvement, peripheral neuropathy, and sometimes reduced cognitive functioning and/or cerebellar ataxia).

Spastic paraplegia-43 (SPG43) is an autosomal recessive neurodegenerative disorder characterized by childhood onset of progressive spasticity affecting the lower and upper limbs (summary by Meilleur et al., 2010). For a general phenotypic description and a discussion of genetic heterogeneity of autosomal recessive spastic paraplegia, see 270800.

Spastic paraplegia-18B (SPG18B) is a severe autosomal recessive neurologic disorder characterized by onset in early childhood of progressive spastic paraplegia resulting in motor disability. Most affected individuals have severe psychomotor retardation. Some may develop significant joint contractures (summary by Alazami et al., 2011 and Yildirim et al., 2011).

Emery-Dreifuss muscular dystrophy is characterized classically by the triad of weakness of the shoulder and pelvic girdle muscles, contractures of the elbows, neck, and Achilles tendon, and cardiac involvement, most commonly arrhythmias (summary by Jimenez-Escrig et al., 2012). For a discussion of genetic heterogeneity of EDMD, see 310300.

Spondylo-megaepiphyseal-metaphyseal dysplasia is a rare autosomal recessive skeletal dysplasia characterized by disproportionate short stature with a short and stiff neck and trunk; relatively long limbs that may show flexion contractures of the distal joints; delayed and impaired ossification of the vertebral bodies and the presence of large epiphyseal ossification centers and wide growth plates in the long tubular bones; and numerous pseudoepiphyses of the short tubular bones in hands and feet (summary by Hellemans et al., 2009).

A neurodegenerative disease with characteristics of progressive muscular paralysis reflecting degeneration of motor neurons in the primary motor cortex, corticospinal tracts, brainstem and spinal cord. Caused by heterozygous mutation in the FUS gene on chromosome 16p11.

Syndromic optic atrophy, also known as DOA+ syndrome, is a neurologic disorder characterized most commonly by an insidious onset of visual loss and sensorineural hearing loss in childhood with variable presentation of other clinical manifestations including progressive external ophthalmoplegia (PEO), muscle cramps, hyperreflexia, and ataxia. There appears to be a wide range of intermediate phenotypes (Yu-Wai-Man et al., 2010).

Brittle cornea syndrome (BCS) is characterized by blue sclerae, corneal rupture after minor trauma, keratoconus or keratoglobus, hyperelasticity of the skin, and hypermobility of the joints (Al-Hussain et al., 2004). It is classified as a form of Ehlers-Danlos syndrome (Malfait et al., 2017). For a discussion of genetic heterogeneity of brittle cornea syndrome, see BCS1 (229200).

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).

Mitochondrial membrane protein-associated neurodegeneration (MPAN) is characterized initially by gait changes followed by progressive spastic paresis, progressive dystonia (which may be limited to the hands and feet or more generalized), neuropsychiatric abnormalities (emotional lability, depression, anxiety, impulsivity, compulsions, hallucinations, perseveration, inattention, and hyperactivity), and cognitive decline. Additional early findings can include dysphagia, dysarthria, optic atrophy, axonal neuropathy, parkinsonism, and bowel/bladder incontinence. Survival is usually well into adulthood. End-stage disease is characterized by severe dementia, spasticity, dystonia, and parkinsonism.

Sclerosteosis is a severe sclerosing bone dysplasia characterized by progressive skeletal overgrowth. Syndactyly is a variable manifestation. The disorder is rare and the majority of affected individuals have been reported in the Afrikaner population of South Africa (summary by Brunkow et al., 2001). For a discussion of genetic heterogeneity of sclerosteosis, see SOST1 (269500).

Thiamine metabolism dysfunction syndrome-5 (THMD5) is an autosomal recessive metabolic disorder due to an inborn error of thiamine metabolism. The phenotype is highly variable, but in general, affected individuals have onset in early childhood of acute encephalopathic episodes associated with increased serum and CSF lactate. These episodes result in progressive neurologic dysfunction manifest as gait disturbances, ataxia, dystonia, and spasticity, which in some cases may result in loss of ability to walk. Cognitive function is usually preserved, although mildly delayed development has been reported. These episodes are usually associated with infection and metabolic decompensation. Some patients may have recovery of some neurologic deficits (Mayr et al., 2011). For a discussion of genetic heterogeneity of disorders due to thiamine metabolism dysfunction, see THMD1 (249270).

Leukoencephalopathy, brain calcifications, and cysts (LCC), also known as Labrune syndrome, is characterized by a constellation of features restricted to the central nervous system, including leukoencephalopathy, brain calcifications, and cysts, resulting in spasticity, dystonia, seizures, and cognitive decline (summary by Labrune et al., 1996). See also cerebroretinal microangiopathy with calcifications and cysts (CRMCC; 612199), an autosomal recessive disorder caused by mutation in the CTC1 gene (613129) that shows phenotypic similarities to Labrune syndrome. CRMCC includes the neurologic findings of intracranial calcifications, leukodystrophy, and brain cysts, but also includes retinal vascular abnormalities and other systemic manifestations, such as osteopenia with poor bone healing, a high risk of gastrointestinal bleeding, hair, skin, and nail changes, and anemia and thrombocytopenia. Although Coats plus syndrome and Labrune syndrome were initially thought to be manifestations of the same disorder, namely CRMCC, molecular evidence has excluded mutations in the CTC1 gene in patients with Labrune syndrome, suggesting that the 2 disorders are not allelic (Anderson et al., 2012; Polvi et al., 2012).

Complex cortical dysplasia with other brain malformations-13 (CDCBM13) is an autosomal dominant neurodevelopmental disorder characterized by global developmental delay with impaired intellectual development. Brain imaging shows variable neuronal migration defects resulting in cortical malformations, including pachygyria. More variable features include early-onset seizures and dysmorphic features. Some patients may also show signs of peripheral neuropathy, such as abnormal gait, hyporeflexia, and foot deformities (summary by Willemsen et al., 2012 and Poirier et al., 2013). For a discussion of genetic heterogeneity of CDCBM, see CDCBM1 (614039).

A rare subtype of CMT1 characterized by a variable clinical presentation. Onset within the first two years of life with a delay in walking is not uncommon; however, onset may occur later. CMT1E is caused by point mutations in the <i>PMP22</i> (17p12) gene. The disease severity depends on the particular <i>PMP22</i> mutation, with some cases being very mild and even resembling hereditary neuropathy with liability to pressure palsies, while others having an earlier onset with a more severe phenotype (reminiscent of Dejerine-Sottas syndrome) than that seen in CMT1A, caused by gene duplication. These severe cases may also report deafness and much slower motor nerve conduction velocities compared to CMT1A patients.

A rare genetic isolated dystonia with characteristics of adult-onset non-progressive focal cervical dystonia typically manifesting with torticollis and occasionally accompanied by mild head tremor and essential-type limb tremor.

SPG53 is an autosomal recessive neurologic disorder characterized by onset in infancy of delayed motor development progressing to upper and lower limb spasticity with impaired walking. Affected individuals also show mild to moderate cognitive impairment (summary by Zivony-Elboum et al., 2012).

Peroxisome biogenesis disorder-4B (PBD4B) includes the overlapping phenotypes of neonatal adrenoleukodystrophy (NALD) and infantile Refsum disease (IRD), which represent milder manifestations of the Zellweger syndrome spectrum (ZSS) of peroxisome biogenesis disorders (PBDs). The clinical course of patients with the NALD and IRD presentation is variable and may include developmental delay, hypotonia, liver dysfunction, sensorineural hearing loss, retinal dystrophy, and visual impairment. Children with the NALD presentation may reach their teens, and those with the IRD presentation may reach adulthood (summary by Waterham and Ebberink, 2012). For a complete phenotypic description and a discussion of genetic heterogeneity of PBD(NALD/IRD), see 601539. Individuals with mutations in the PEX6 gene have cells of complementation group 4 (CG4, equivalent to CG6 and CGC). For information on the history of PBD complementation groups, see 214100.

PEOA6 is characterized by muscle weakness, mainly affecting the lower limbs, external ophthalmoplegia, exercise intolerance, and mitochondrial DNA (mtDNA) deletions on muscle biopsy. Symptoms may appear in childhood or adulthood and show slow progression (summary by Ronchi et al., 2013). For a general phenotypic description and a discussion of genetic heterogeneity of autosomal dominant progressive external ophthalmoplegia, see PEOA1 (157640).

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.

Parkinson disease-20 is an autosomal recessive neurodegenerative disorder characterized by young adult-onset of parkinsonism. Additional features may include seizures, cognitive decline, abnormal eye movements, and dystonia (summary by Krebs et al., 2013 and Quadri et al., 2013). For a phenotypic description and a discussion of genetic heterogeneity of Parkinson disease, see PD (168600).

Spastic paraplegia-64 (SPG64) is a neurologic disorder characterized by childhood onset of progressive spastic paraplegia with impaired intellectual development, gait impairment, dysarthria, and white matter abnormalities on brain imaging. Some individuals show neurocognitive regression (Calame et al., 2022). For a discussion of genetic heterogeneity of autosomal recessive SPG, see SPG5A (270800).

An extremely rare and complex form of hereditary spastic paraplegia with characteristics of onset in infancy of spastic paraplegia (presenting with delayed walking and a scissors gait) associated with short stature and normal cognition. Periventricular deep white matter changes in the corpus callosum are noted on brain imaging. SPG63 is caused by a homozygous mutation in the AMPD2 gene (1p13.3) encoding AMP deaminase 2.

Vulto-van Silfout-de Vries syndrome (VSVS) is an intellectual developmental disorder characterized by delayed psychomotor development, poor expressive speech, and behavioral abnormalities, including autistic features and poor eye contact. Most patients have additional nonspecific features, including hypotonia and gait abnormalities, seizures, which may be refractory, high pain threshold, and sleep disturbances (summary by Nabais Sa et al., 2019).

Mitochondrial complex III deficiency, nuclear type 8, is an autosomal recessive disorder characterized by progressive neurodegeneration with onset in childhood. Affected individuals may have normal or delayed early development, and often have episodic acute neurologic decompensation and regression associated with febrile illnesses. The developmental regression results in variable intellectual disability and motor deficits, such as hypotonia, axial hypertonia, and spasticity; some patients may lose the ability to walk independently. Laboratory studies show increased serum lactate and isolated deficiency of mitochondrial complex III in skeletal muscle and fibroblasts. Brain imaging shows a characteristic pattern of multifocal small cystic lesions in the periventricular and deep cerebral white matter (summary by Dallabona et al., 2016). For a discussion of genetic heterogeneity of mitochondrial complex III deficiency, see MC3DN1 (124000).

Charcot-Marie-Tooth disease type 2S is a relatively pure form of autosomal recessive axonal neuropathy characterized by onset in the first decade of slowly progressive distal muscle weakness and atrophy affecting the lower and upper limbs. Patients have decreased reflexes and variable distal sensory impairment (summary by Cottenie et al., 2014). For a phenotypic description and a discussion of genetic heterogeneity of axonal CMT, see CMT2A1 (118210).

Tenorio syndrome (TNORS) is characterized by overgrowth, macrocephaly, and impaired intellectual development. Some patients may have mild hydrocephaly, hypoglycemia, and inflammatory diseases resembling Sjogren syndrome (270150) (summary by Tenorio et al., 2014).

Charcot-Marie-Tooth disease type 2U (CMT2U) is an autosomal dominant neurologic disorder characterized by late-adult onset of distal sensory impairment resulting in distal muscle weakness and atrophy affecting the upper and lower limbs. The disorder is slowly progressive (summary by Gonzalez et al., 2013). For a phenotypic description and a discussion of genetic heterogeneity of axonal CMT, see CMT2A1 (118210).

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

Childhood-onset spasticity with hyperglycinemia is an autosomal recessive disorder characterized by onset of slowly progressive spasticity that results in impaired gait in the first decade of life. Imaging of the central nervous system shows leukodystrophy and/or lesions in the upper spinal cord. More variable features include visual defects and mild learning disabilities. Serum glycine is increased, but CSF glycine is only mildly increased or normal; serum lactate is normal. The disorder represents a form of 'variant' nonketotic hyperglycinemia and is distinct from classic nonketotic hyperglycinemia (NKH, or GCE; 605899), which is characterized by significantly increased CSF glycine. Several forms of 'variant' NKH, including SPAHGC, appear to result from defects of mitochondrial lipoate biosynthesis (summary by Baker et al., 2014).

HTRA1 disorder is a phenotypic spectrum in which some individuals have few to no symptoms and others manifest with the more severe CARASIL (cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy) phenotype. Those who have a heterozygous HTRA1 pathogenic variant may have mild neurologic findings (sometimes identified only on neuroimaging) or mild-to-moderate neurologic signs and symptoms of CARASIL. In this chapter, the term "classic CARASIL" refers to the more severe phenotype associated with biallelic pathogenic variants, and "HTRA1 cerebral small vessel disease" (HTRA1-CSVD) refers to the milder phenotype associated with a heterozygous HTRA1 pathogenic variant. Classic CARASIL is characterized by early-onset changes in the deep white matter of the brain observed on MRI, and associated neurologic findings. The most frequent initial symptom is gait disturbance from spasticity beginning between ages 20 and 40 years. Forty-four percent of affected individuals have stroke-like episodes before age 40 years. Mood changes (apathy and irritability), pseudobulbar palsy, and cognitive dysfunction begin between ages 20 and 50 years. The disease progresses slowly following the onset of neurologic symptoms. Scalp alopecia and acute mid- to lower-back pain (lumbago) before age 30 years are characteristic. The most frequent initial symptom in individuals with HTRA1-CSVD is slowly progressive gait disturbance after age 40 years, which may be followed by the development of mood changes and cognitive dysfunction. A majority of affected individuals have a stroke-like episode after age 40 years. Spondylosis and alopecia are seen in a minority of individuals with HTRA1-CSVD.

Any Parkinson disease in which the cause of the disease is a mutation in the CHCHD2 gene.

In patients with SSMED, short stature and microcephaly are apparent at birth, and there is progressive postnatal growth failure. Endocrine dysfunction, including hypergonadotropic hypogonadism, multinodular goiter, and diabetes mellitus, is present in affected adults. Progressive ataxia has been reported in some patients, with onset ranging from the second to fifth decade of life. In addition, a few patients have developed tumors, suggesting that there may be a predisposition to tumorigenesis. In contrast to syndromes involving defects in other components of the nonhomologous end-joining (NHEJ) complex (see, e.g., 606593), no clinically overt immunodeficiency has been observed in SSMED, although laboratory analysis has revealed lymphopenia or borderline leukopenia in some patients (Murray et al., 2015; Bee et al., 2015; de Bruin et al., 2015; Guo et al., 2015).

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).

X-linked syndromic intellectual developmental disorder-33 (MRXS33) is an X-linked recessive neurodevelopmental disorder characterized by delayed psychomotor development, intellectual disability, and characteristic facial features (summary by O'Rawe et al., 2015).

KMT2B-related dystonia (DYT-KMT2B) is a complex childhood-onset (mean age 7 years) movement disorder described to date in 39 individuals. It is characterized by a progressive disease course evolving commonly from lower-limb focal dystonia into generalized dystonia with prominent cervical, cranial, and laryngeal involvement. Communication difficulties, secondary to articulation difficulties and low speech volume, are common. Bulbar dysfunction leads to impaired swallowing. Intellectual disability (ID) / developmental delay (DD) are commonly reported. Additional findings can include eye movement abnormalities, skin changes, psychiatric comorbidities (attention-deficit/hyperactivity disorder, anxiety, depression, and obsessive-compulsive disorder), myoclonus, seizures, spasticity, and sensorineural hearing loss. Many affected individuals follow a similar disease course, though milder and atypical findings have been described.

MECR-related neurologic disorder is characterized by a progressive childhood-onset movement disorder and optic atrophy; intellect is often – but not always – preserved. The movement disorder typically presents between ages one and 6.5 years and is mainly dystonia that can be accompanied by chorea and/or ataxia. Over time some affected individuals require assistive devices for mobility. Speech fluency and intelligibility are progressively impaired due to dysarthria. Optic atrophy typically develops between ages four and 12 years and manifests as reduced visual acuity, which can include functional blindness (also known as legal blindness) in adulthood. Because only 13 affected individuals are known to the authors, and because nearly half of them were diagnosed retrospectively as adults, the natural history of disease progression and other aspects of the phenotype have not yet been completely defined.

Developmental and epileptic encephalopathy-47 (DEE47) is a neurologic disorder characterized by onset of intractable seizures in the first days or weeks of life. EEG shows background slowing and multifocal epileptic spikes, and may show hypsarrhythmia. Most patients have developmental regression after seizure onset and show persistent intellectual disability and neurologic impairment, although the severity is variable. Treatment with phenytoin, a voltage-gated sodium channel blocker, may be beneficial (summary by Guella et al., 2016). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Childhood-onset neurodegeneration with ataxia, dystonia, and gaze palsy (NADGP) is an autosomal recessive progressive disorder characterized by onset of gait ataxia, cognitive decline, and gaze palsy in the first or second decades. Additional features include dysarthria, dystonia, and athetoid movements. Some patients may become wheelchair-bound as young adults (summary by Haack et al., 2016).

SLC39A14 deficiency is characterized by evidence between ages six months and three years of delay or loss of motor developmental milestones (e.g., delayed walking, gait disturbance). Early in the disease course, children show axial hypotonia followed by dystonia, spasticity, dysarthria, bulbar dysfunction, and signs of parkinsonism including bradykinesia, hypomimia, and tremor. By the end of the first decade they develop severe, generalized, pharmaco-resistant dystonia, limb contractures, and scoliosis, and lose independent ambulation. Cognitive impairment appears to be less prominent than motor disability. Some affected children have succumbed in their first decade due to secondary complications such as respiratory infections.

Developmental and epileptic encephalopathy-37 (DEE37) is an autosomal recessive epileptic-dyskinetic neurologic disorder characterized by the onset of intractable seizures or abnormal movements in the first months or years of life. Patients typically have normal or only mildly delayed development in early infancy, but then show developmental regression and stagnation after the onset of seizures, which can occur between about 6 months to 2 years of age. In addition to epileptic encephalopathy, affected individuals also manifest a hyperkinetic movement disorder with choreoathetosis, spasticity, and rigidity. There is severely impaired intellectual development and function, loss of verbal skills with absent speech, and impaired volitional movements (summary by Madeo et al., 2016). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Autosomal dominant striatal degeneration is a neurologic disorder characterized by variable movement abnormalities due to dysfunction in the striatal part of the basal ganglia (summary by Kuhlenbaumer et al., 2004). Genetic Heterogeneity of Autosomal Dominant Striatal Degeneration See also ADSD2 (616922), caused by mutation in the PDE10A gene (610652) on chromosome 6q27.

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.

MDCCAID is an autosomal recessive form of muscular dystrophy with onset of progressive muscle weakness in early childhood. Almost all patients also have early-onset cataracts, most have intellectual disability of varying severity, and some have seizures (summary by Wiessner et al., 2017 and Osborn et al., 2017).

EED-related overgrowth is characterized by fetal or early childhood overgrowth (tall stature, macrocephaly, large hands and feet, and advanced bone age) and intellectual disability that ranges from mild to severe. To date, EED-related overgrowth has been reported in eight individuals.

Neurodegeneration with brain iron accumulation refers to a group of neurodegenerative disorders characterized by progressive motor and cognitive dysfunction beginning in childhood or young adulthood. Patients show extrapyramidal motor signs, such as spasticity, dystonia, and parkinsonism. Brain imaging shows iron accumulation in the basal ganglia (summary by Dusi et al., 2014). For a general phenotypic description and a discussion of genetic heterogeneity of NBIA, see NBIA1 (234200).

Primary familial brain calcification (PFBC) is a neurodegenerative disorder with characteristic calcium deposits in the basal ganglia and other brain areas visualized on neuroimaging. Most affected individuals are in good health during childhood and young adulthood and typically present in the fourth to fifth decade with a gradually progressive movement disorder and neuropsychiatric symptoms. The movement disorder first manifests as clumsiness, fatigability, unsteady gait, slow or slurred speech, dysphagia, involuntary movements, or muscle cramping. Neuropsychiatric symptoms, often the first or most prominent manifestations, range from mild difficulty with concentration and memory to changes in personality and/or behavior, to psychosis and dementia. Seizures of various types occur frequently, some individuals experience chronic headache and vertigo; urinary urgency or incontinence may be present.

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.

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.

Laing distal myopathy is characterized by early-onset weakness (usually before age 5 years) that initially involves the dorsiflexors of the ankles and great toes and then the finger extensors, especially those of the third and fourth fingers. Weakness of the neck flexors is seen in most affected individuals and mild facial weakness is often present. After distal weakness has been present for more than ten years, mild proximal weakness may be observed. Life expectancy is normal.

CMTDIG is an autosomal dominant neurologic disorder with a highly variable phenotype. Most affected individuals have onset in the first or second decades of slowly progressive distal motor weakness and atrophy, resulting in gait instability and distal upper limb impairment, as well as distal sensory impairment. More severely affected individuals may have pes cavus and claw hands and become wheelchair-bound, whereas other affected individuals have later onset with a milder disease course. Electrophysiologic studies tend to show median motor nerve conduction velocities (NCV) in the 'intermediate' range, between 25 and 45 m/s (summary by Berciano et al., 2017). In a review of intermediate CMT, Berciano et al. (2017) noted that advanced axonal degeneration may induce secondary demyelinating changes resulting in decreased NCV and attenuated compound muscle action potential (CMAP) in median nerve conduction studies. They thus suggested that testing the upper arm, axilla to elbow, may provide more accurate assessment of NCV and CMAP and reveal an intermediate phenotype (review by Berciano et al., 2017). For a discussion of genetic heterogeneity of CMTDI, see 606482.

NDPLHS is an autosomal dominant disorder characterized by developmental stagnation or regression apparent in the first years of life and manifest as loss of purposeful hand movements, loss of language, and intellectual disability. Additional features may include stereotypic movements, dystonia, gait abnormalities, sleep disturbances, and small hands and feet. The phenotype is reminiscent of Rett syndrome (RTT; 312750) (summary by Yoo et al., 2017).

Developmental and epileptic encephalopathy (DEE93) is an autosomal dominant neurologic disorder characterized by delayed psychomotor development, early-onset refractory seizures, and impaired intellectual development. The severity of the phenotype is highly variable: some patients may be nonverbal and nonambulatory with spastic quadriparesis and poor eye contact, whereas others have moderate intellectual disability (summary by Fassio et al., 2018). For a discussion of genetic heterogeneity of DEE, see 308350.

Polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy (PLOSL) is characterized by fractures (resulting from radiologically demonstrable polycystic osseous lesions), frontal lobe syndrome, and progressive presenile dementia beginning in the fourth decade. The clinical course of PLOSL can be divided into four stages: 1. The latent stage is characterized by normal early development. 2. The osseous stage (3rd decade of life) is characterized by pain and tenderness, mostly in ankles and feet, usually following strain or injury. Fractures are typically diagnosed several years later, most commonly in the bones of the extremities. 3. In the early neurologic stage (4th decade of life), a change of personality begins to develop insidiously. Affected individuals show a frontal lobe syndrome (loss of judgment, euphoria, loss of social inhibitions, disturbance of concentration, and lack of insight, libido, and motor persistence) leading to serious social problems. 4. The late neurologic stage is characterized by progressive dementia and loss of mobility. Death usually occurs before age 50 years.

Autosomal dominant limb-girdle muscular dystrophy-4 (LGMDD4) is characterized by onset of proximal muscle weakness in young adulthood. Affected individuals often have gait difficulties; some may have upper limb involvement. Other features include variably increased serum creatine kinase, myalgia, and back pain. The severity and expressivity of the disorder is highly variable, even within families (summary by Vissing et al., 2016). For a discussion of genetic heterogeneity of autosomal dominant limb-girdle muscular dystrophy, see 603511.

Developmental and epileptic encephalopathy-67 (DEE67) is characterized by the onset of various types of seizures in the first months of life, although later onset may occur in milder cases. The seizures tend to be resistant to treatment. Affected individuals have global developmental delay with impaired motor and intellectual development, poor or absent speech, movement disorders, and stereotypic or autistic behavior (summary by Chatron et al., 2018). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

DEE95 is a severe autosomal recessive developmental disorder characterized by severely impaired global development, hypotonia, weakness, ataxia, coarse facial features, and intractable seizures. More variable features may include abnormalities of the hands and feet, inguinal hernia, and feeding difficulties. The disorder is part of a group of similar neurologic disorders resulting from biochemical defects in the glycosylphosphatidylinositol (GPI) biosynthetic pathway (summary by Nguyen et al., 2018). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350. For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293).

Polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy-2 (PLOSL2), or Nasu-Hakola disease, is a recessively inherited presenile frontal dementia with leukoencephalopathy and basal ganglia calcification. In most cases the disorder first manifests in early adulthood as pain and swelling in ankles and feet, followed by bone fractures. Neurologic symptoms manifest in the fourth decade of life as a frontal lobe syndrome with loss of judgment, euphoria, and disinhibition. Progressive decline in other cognitive domains begins to develop at about the same time. The disorder culminates in a profound dementia and death by age 50 years (summary by Klunemann et al., 2005). For a discussion of genetic heterogeneity of polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy, see 221770.

HMNR5 is an autosomal recessive neurologic disorder characterized by young adult onset of slowly progressive distal muscle weakness and atrophy resulting in gait impairment and loss of reflexes due to impaired function of motor nerves. Sensation and cognition are not impaired (summary by Blumen et al., 2012). For a discussion of genetic heterogeneity of autosomal recessive HMN, see HMNR1 (604320).

A rare hereditary cerebellar ataxia disorder with characteristics of late-onset spinocerebellar ataxia, manifesting with slowly progressive gait disturbances, dysarthria, limb and truncal ataxia and smooth-pursuit eye movement disturbance, associated with a history of psychomotor delay from childhood. Mild atrophy of the cerebellar vermis and hemispheres is observed on brain imaging. There is evidence the disease is caused by homozygous mutation in the SYT14 gene on chromosome 1q32.

Progressive myoclonic epilepsy-8 (EPM8) is a rare autosomal recessive form of progressive myoclonic epilepsy with phenotypic variability including ataxia and other movement disorders in addition to myoclonus (summary by Godeiro et al., 2018). For a discussion of genetic heterogeneity of progressive myoclonic epilepsy, see EPM1A (254800).

Autosomal recessive spinocerebellar ataxia-27 (SCAR27) is an adult-onset neurologic disorder characterized by gait difficulties and other cerebellar signs, such as eye movement abnormalities, dysarthria, and difficulty writing. The disorder is progressive, and some patients may lose independent ambulation. Additional features include spasticity of the lower limbs and cognitive impairment. Brain imaging shows cerebellar atrophy (summary by Eidhof et al., 2018).

Charcot-Marie-Tooth disease type 2EE (CMT2EE) is an autosomal recessive sensorimotor peripheral axonal neuropathy with onset in the first or second decades of life. The disorder primarily affects the lower limbs and is slowly progressive, sometimes resulting in loss of ambulation, with later onset of upper limb involvement. There is significant distal muscle weakness and atrophy, usually with foot or hand deformities. Skeletal muscle biopsy shows findings of disturbed mitochondrial maintenance. Cognition is unaffected, and chronic liver disease is absent (summary by Baumann et al., 2019). For a phenotypic description and a discussion of genetic heterogeneity of axonal CMT type 2, see CMT2A (118210).

Spastic paraplegia-80 (SPG80) is an autosomal dominant juvenile-onset neurologic disorder characterized by onset of progressive spasticity and hyperreflexia affecting mainly the lower limbs and resulting in difficulty walking or loss of independent ambulation, sometimes as early as the second decade. Some patients may have cerebellar signs and mild cognitive impairment, but most have a pure form of the disorder (summary by Farazi Fard et al., 2019). For a general phenotypic description and a discussion of genetic heterogeneity of autosomal dominant spastic paraplegia, see SPG3A (182600).

Autosomal dominant adult-onset distal myopathy-6 (MPD6) is a muscle disorder characterized by slowly progressive distal muscle weakness, primarily affecting the lower limbs and resulting in gait difficulties. Some patients develop involvement of proximal and upper limb muscles (summary by Savarese et al., 2019)

Congenital myopathy-19 (CMYP19) is an autosomal recessive skeletal muscle disorder characterized by infantile-onset of progressive muscle weakness and atrophy associated with scoliosis, variably impaired walking, and dysmorphic facial features (Feichtinger et al., 2019). For a discussion of genetic heterogeneity of congenital myopathy, see CMYP1A (117000).

CATIFA syndrome is characterized by global developmental delay and impaired intellectual development ranging from mild to severe, with most patients exhibiting attention-deficit hyperactivity disorder (ADHD). Patients show an elongated face with long philtrum and small ears. Ocular anomalies include congenital cataracts, strabismus, and amblyopia, which may be associated with reduced vision; other anomalies include cleft lip and/or palate and misaligned teeth with extensive caries (Unlu et al., 2020).

Neurodegeneration with ataxia and late-onset optic atrophy (NDAXOA) is an autosomal dominant disorder with somewhat variable manifestations. Most affected individuals present in mid-adulthood with slowly progressive cerebellar and gait ataxia, optic atrophy, and myopathy or myalgia. Some patients may have a childhood history of neurologic features, including limited extraocular movements. Additional features can include cardiomyopathy, psychiatric disturbances, and peripheral sensory impairment (summary by Taylor et al., 1996 and Courage et al., 2017).

CSF1R-related adult-onset leukoencephalopathy with axonal spheroids and pigmented glia (ALSP) is characterized by executive dysfunction, memory decline, personality changes, motor impairments, and seizures. A frontal lobe syndrome (e.g., loss of judgment, lack of social inhibitors, lack of insight, and motor persistence) usually appears early in the disease course. The mean age of onset is usually in the fourth decade. Affected individuals eventually become bedridden with spasticity and rigidity. The disease course ranges from two to 30 or more years (mean: 8 years).

Charcot-Marie-Tooth disease type 2W is an autosomal dominant neurologic disorder characterized by a peripheral neuropathy mainly affecting the lower limbs and resulting in gait difficulties and distal sensory impairment, although most patients also have upper limb involvement. The age at onset is highly variable, ranging from childhood to late adulthood (summary by Safka Brozkova et al., 2015). For a phenotypic description and a discussion of genetic heterogeneity of axonal CMT, see CMT2A1 (118210).

Individuals with TANGO2-related metabolic encephalopathy and arrhythmias can present in acute metabolic crisis (hypoglycemia, elevated lactate, mild hyperammonemia) or with developmental delay, regression, and/or seizures. The acute presentation varies from profound muscle weakness, ataxia, and/or disorientation to a comatose state. Individuals can present with intermittent acute episodes of rhabdomyolysis. The first episode of myoglobinuria has been known to occur as early as age five months. Acute renal tubular damage due to myoglobinuria can result in acute kidney injury and renal failure. During acute illness, transient electrocardiogram changes can be seen; the most common is QT prolongation. Life-threatening recurrent ventricular tachycardia or torsade de pointes occurs primarily during times of acute illness. Individuals who do not present in metabolic crises may present with gait incoordination, progressively unsteady gait, difficulty with speech, or clumsiness. Intellectual disability of variable severity is observed in almost all individuals. Seizures are observed outside the periods of crises in more than 75% of individuals. Hypothyroidism has been reported in more than one third of individuals.

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

Autosomal dominant spastic paraplegia-9A is a neurologic disorder characterized by onset of slowly progressive spasticity mainly affecting the lower limbs. The age at onset usually ranges from adolescence to adulthood, and patients have gait difficulties, motor neuropathy, and dysarthria. Additional variable features include cerebellar signs, cataract, pes cavus, and urinary urgency (summary by Coutelier et al., 2015). For a general phenotypic description and a discussion of genetic heterogeneity of autosomal dominant spastic paraplegia, see SPG3A (182600).

Autosomal recessive SPG9B is a neurologic disorder characterized by early-onset complex spastic paraplegia. Affected individuals had delayed psychomotor development, intellectual disability, and severe motor impairment. More variable features include dysmorphic facial features, tremor, and urinary incontinence (summary by Coutelier et al., 2015). For a discussion of genetic heterogeneity of autosomal recessive SPG, see SPG5A (270800).

Charcot-Marie-Tooth disease type 2X (CMT2X) is an autosomal recessive, slowly progressive, axonal peripheral sensorimotor neuropathy characterized by lower limb muscle weakness and atrophy associated with distal sensory impairment and gait difficulties. Some patients also have involvement of the upper limbs. Onset usually occurs in the first 2 decades of life, although later onset can also occur (summary by Montecchiani et al., 2016) For a phenotypic description and a discussion of genetic heterogeneity of axonal CMT, see CMT2A1 (118210).

Childhood-onset remitting leukodystrophy (CORLK) is a very rare autosomal dominant disorder characterized in some patients by onset of a metabolic crisis at the end of the first year of life that leads to widespread demyelination and leukodystrophy on brain imaging and a dramatic loss of developmental abilities. Affected children recover over the following several months, regaining normal development accompanied by remyelination. Not all patients have documented acute episodes of metabolic demyelination in infancy, but individuals with the FBP2 mutation show persistent white matter abnormalities on brain imaging that resemble the abnormalities observed in infants with the acute crisis. Other neurologic disturbances that may or may not be related to the FBP2 mutation have been observed, including psychiatric manifestations, seizures, and mild learning difficulties (Gizak et al., 2021).

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.

Hijazi-Reis syndrome (HIJRS) is an X-linked dominant disorder characterized by global developmental delay with hypotonia, motor delay, impaired intellectual development, and speech and language delay. Affected individuals also have dysmorphic facial features, gastrointestinal issues, and ocular anomalies. Rare patients have seizures (Hijazi et al., 2022).

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).

Any leukoencephalopathy with vanishing white matter in which the cause of the disease is a variation in the EIF2B1 gene.

Juvenile amyotrophic lateral sclerosis-27 (ALS27) is an autosomal dominant disorder characterized by early childhood-onset lower extremity spasticity manifesting as toe walking and gait abnormalities, followed by progressive lower motor neuron-mediated weakness without sensory signs or symptoms (Mohassel et al., 2021). For a discussion of genetic heterogeneity of amyotrophic lateral sclerosis, see ALS1 (105400).