Fucosidosis is an autosomal recessive lysosomal storage disease caused by defective alpha-L-fucosidase with accumulation of fucose in the tissues. Clinical features include angiokeratoma, progressive psychomotor retardation, neurologic signs, coarse facial features, and dysostosis multiplex. Fucosidosis has been classified into 2 major types. Type 1 is characterized by rapid psychomotor regression and severe neurologic deterioration beginning at about 6 months of age, elevated sweat sodium chloride, and death within the first decade of life. Type 2 is characterized by milder psychomotor retardation and neurologic signs, the development of angiokeratoma corporis diffusum, normal sweat salinity, and longer survival (Kousseff et al., 1976).

Wilson disease is a disorder of copper metabolism that can present with hepatic, neurologic, or psychiatric disturbances, or a combination of these, in individuals ranging from age three years to older than 50 years; symptoms vary among and within families. Liver disease includes recurrent jaundice, simple acute self-limited hepatitis-like illness, autoimmune-type hepatitis, fulminant hepatic failure, or chronic liver disease. Neurologic presentations include movement disorders (tremors, poor coordination, loss of fine-motor control, chorea, choreoathetosis) or rigid dystonia (mask-like facies, rigidity, gait disturbance, pseudobulbar involvement). Psychiatric disturbance includes depression, neurotic behaviors, disorganization of personality, and, occasionally, intellectual deterioration. Kayser-Fleischer rings, frequently present, result from copper deposition in Descemet's membrane of the cornea and reflect a high degree of copper storage in the body.

X-linked adrenoleukodystrophy (X-ALD) affects the nervous system white matter and the adrenal cortex. Three main phenotypes are seen in affected males: The childhood cerebral form manifests most commonly between ages four and eight years. It initially resembles attention-deficit disorder or hyperactivity; progressive impairment of cognition, behavior, vision, hearing, and motor function follow the initial symptoms and often lead to total disability within six months to two years. Most individuals have impaired adrenocortical function at the time that neurologic disturbances are first noted. Adrenomyeloneuropathy (AMN) manifests most commonly in an individual in his twenties or middle age as progressive stiffness and weakness of the legs, sphincter disturbances, sexual dysfunction, and often, impaired adrenocortical function; all symptoms are progressive over decades. "Addison disease only" presents with primary adrenocortical insufficiency between age two years and adulthood and most commonly by age 7.5 years, without evidence of neurologic abnormality; however, some degree of neurologic disability (most commonly AMN) usually develops by middle age. More than 20% of female carriers develop mild-to-moderate spastic paraparesis in middle age or later. Adrenal function is usually normal.

The adult form of metachromatic leukodystrophy affects approximately 15 to 20 percent of individuals with the disorder. In this form, the first symptoms appear during the teenage years or later. Often behavioral problems such as alcohol use disorder, drug abuse, or difficulties at school or work are the first symptoms to appear. The affected individual may experience psychiatric symptoms such as delusions or hallucinations. People with the adult form of metachromatic leukodystrophy may survive for 20 to 30 years after diagnosis. During this time there may be some periods of relative stability and other periods of more rapid decline.\n\nIn 20 to 30 percent of individuals with metachromatic leukodystrophy, onset occurs between the age of 4 and adolescence. In this juvenile form, the first signs of the disorder may be behavioral problems and increasing difficulty with schoolwork. Progression of the disorder is slower than in the late infantile form, and affected individuals may survive for about 20 years after diagnosis.\n\nThe most common form of metachromatic leukodystrophy, affecting about 50 to 60 percent of all individuals with this disorder, is called the late infantile form. This form of the disorder usually appears in the second year of life. Affected children lose any speech they have developed, become weak, and develop problems with walking (gait disturbance). As the disorder worsens, muscle tone generally first decreases, and then increases to the point of rigidity. Individuals with the late infantile form of metachromatic leukodystrophy typically do not survive past childhood.\n\nMetachromatic leukodystrophy gets its name from the way cells with an accumulation of sulfatides appear when viewed under a microscope. The sulfatides form granules that are described as metachromatic, which means they pick up color differently than surrounding cellular material when stained for examination.\n\nIn people with metachromatic leukodystrophy, white matter damage causes progressive deterioration of intellectual functions and motor skills, such as the ability to walk. Affected individuals also develop loss of sensation in the extremities (peripheral neuropathy), incontinence, seizures, paralysis, an inability to speak, blindness, and hearing loss. Eventually they lose awareness of their surroundings and become unresponsive. While neurological problems are the primary feature of metachromatic leukodystrophy, effects of sulfatide accumulation on other organs and tissues have been reported, most often involving the gallbladder.\n\nMetachromatic leukodystrophy is an inherited disorder characterized by the accumulation of fats called sulfatides in cells. This accumulation especially affects cells in the nervous system that produce myelin, the substance that insulates and protects nerves. Nerve cells covered by myelin make up a tissue called white matter. Sulfatide accumulation in myelin-producing cells causes progressive destruction of white matter (leukodystrophy) throughout the nervous system, including in the brain and spinal cord (the central nervous system) and the nerves connecting the brain and spinal cord to muscles and sensory cells that detect sensations such as touch, pain, heat, and sound (the peripheral nervous system).

For a phenotypic description and a discussion of genetic heterogeneity of autosomal dominant Charcot-Marie-Tooth disease type 1, see CMT1B (118200).

A psychotic syndrome caused by damage to the brain by lack of thiamine (vitamin B1). Signs and symptoms include anterograde and retrograde amnesia, confabulation, apathy, ataxia, and coma.

PMM2-CDG, the most common of a group of disorders of abnormal glycosylation of N-linked oligosaccharides, is divided into three clinical stages: infantile multisystem, late-infantile and childhood ataxia–intellectual disability, and adult stable disability. The clinical manifestations and course are highly variable, ranging from infants who die in the first year of life to mildly affected adults. Clinical findings tend to be similar in sibs. In the infantile multisystem presentation, infants show axial hypotonia, hyporeflexia, esotropia, and developmental delay. Feeding problems, vomiting, faltering growth, and developmental delay are frequently seen. Subcutaneous fat may be excessive over the buttocks and suprapubic region. Two distinct clinical courses are observed: (1) a nonfatal neurologic course with faltering growth, strabismus, developmental delay, cerebellar hypoplasia, and hepatopathy in infancy followed by neuropathy and retinitis pigmentosa in the first or second decade; and (2) a more severe neurologic-multivisceral course with approximately 20% mortality in the first year of life. The late-infantile and childhood ataxia–intellectual disability stage, which begins between ages three and ten years, is characterized by hypotonia, ataxia, severely delayed language and motor development, inability to walk, and IQ of 40 to 70; other findings include seizures, stroke-like episodes or transient unilateral loss of function, coagulopathy, retinitis pigmentosa, joint contractures, and skeletal deformities. In the adult stable disability stage, intellectual ability is stable; peripheral neuropathy is variable, progressive retinitis pigmentosa and myopia are seen, thoracic and spinal deformities with osteoporosis worsen, and premature aging is observed; females may lack secondary sexual development and males may exhibit decreased testicular volume. Hypogonadotropic hypogonadism and coagulopathy may occur. The risk for deep venous thrombosis is increased.

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).

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).

TBC1D24-related disorders comprise a continuum of features that were originally described as distinct, recognized phenotypes: DOORS syndrome (deafness, onychodystrophy, osteodystrophy, mental retardation, and seizures). Profound sensorineural hearing loss, onychodystrophy, osteodystrophy, intellectual disability / developmental delay, and seizures. Familial infantile myoclonic epilepsy (FIME). Early-onset myoclonic seizures, focal epilepsy, dysarthria, and mild-to-moderate intellectual disability. Progressive myoclonus epilepsy (PME). Action myoclonus, tonic-clonic seizures, progressive neurologic decline, and ataxia. Early-infantile epileptic encephalopathy 16 (EIEE16). Epileptiform EEG abnormalities which themselves are believed to contribute to progressive disturbance in cerebral function. Autosomal recessive nonsyndromic hearing loss, DFNB86. Profound prelingual deafness. Autosomal dominant nonsyndromic hearing loss, DFNA65. Slowly progressive deafness with onset in the third decade, initially affecting the high frequencies.

Hereditary motor and sensory neuropathy with agenesis of the corpus callosum (HMSN/ACC), a neurodevelopmental and neurodegenerative disorder, is characterized by severe progressive sensorimotor neuropathy with resulting hypotonia, areflexia, and amyotrophy, and by variable degrees of dysgenesis of the corpus callosum. Mild-to-severe intellectual disability and "psychotic episodes" during adolescence are observed. Sensory modalities are moderately to severely affected beginning in infancy. The average age of onset of walking is 3.8 years; the average age of loss of walking is 13.8 years; the average age of death is 33 years.

A rare genetic disease characterized by mild intellectual deficit, congenital cataract, progressive sensorineural hearing impairment, ataxia, peripheral neuropathy, and short stature. There have been no further descriptions in the literature since 1991.

Leber hereditary optic neuropathy (LHON) typically presents in young adults as bilateral, painless, subacute visual failure. The peak age of onset in LHON is in the second and third decades of life, with 90% of those who lose their vision doing so before age 50 years. Very rarely, individuals first manifest LHON in the seventh and eighth decades of life. Males are four to five times more likely to be affected than females, but neither sex nor mutational status significantly influences the timing and severity of the initial visual loss. Neurologic abnormalities such as postural tremor, peripheral neuropathy, nonspecific myopathy, and movement disorders have been reported to be more common in individuals with LHON than in the general population. Some individuals with LHON, usually women, may also develop a multiple sclerosis-like illness.

Ribose 5-phosphate isomerase deficiency (RPIAD) is an autosomal recessive inborn error of metabolism of the pentose phosphate pathway that presents with leukoencephalopathy and peripheral neuropathy (Huck et al., 2004).

The Finnish type of systemic amyloidosis is characterized clinically by a unique constellation of features including lattice corneal dystrophy, and cranial neuropathy, bulbar signs, and skin changes. Some patients may develop peripheral neuropathy and renal failure. The disorder is usually inherited in an autosomal dominant pattern; however, homozygotes with a more severe phenotype have also been reported (Meretoja, 1973). Finnish hereditary amyloidosis, also known as Meretoja syndrome or AGel amyloidosis, is one of the most common diseases in the Finnish disease heritage. Symptoms commonly appear by age 40, with the first finding usually corneal lattice dystrophy (CLD), diagnosed by an ophthalmologist. Impaired vision, polyneuropathy, facial nerve paresis, and cutis laxa follow. These symptoms may develop slowly and simultaneously, since amyloid accumulates systemically at a constant rate (summary by Nikoskinen et al., 2015). For a discussion of genetic heterogeneity of hereditary systemic amyloidosis, see AMYLD1 (105210).

Waldenstrom macroglobulinemia (WM) is a malignant B-cell neoplasm characterized by lymphoplasmacytic infiltration of the bone marrow and hypersecretion of monoclonal immunoglobulin M (IgM) protein (review by Vijay and Gertz, 2007). The importance of genetic factors is suggested by the observation of familial clustering of WM (McMaster, 2003). Whereas WM is rare, an asymptomatic elevation of monoclonal IgM protein, termed 'IgM monoclonal gammopathy of undetermined significance' (IgM MGUS) is more common. Patients with IgM MGUS can progress to develop WM, at the rate of 1.5% to 2% per year (Kyle et al., 2003). Genetic Heterogeneity of Waldenstrom Macroglobulinemia A locus for susceptibility to Waldenstrom macroglobulinemia (WM2; 610430) has been mapped to chromosome 4q.

Cerebral dysgenesis, neuropathy, ichthyosis, and keratoderma syndrome (CEDNIK) refers to a unique constellation of clinical manifestations including global developmental delay with hypotonia, roving eye movements or nystagmus, poor motor skills, and impaired intellectual development with speech delay. More variable features include microcephaly, feeding difficulties, seizures, ocular anomalies, hearing loss, and nonspecific dysmorphic facial features. Palmoplantar keratoderma and ichthyosis or neuropathy develop in some patients. Brain magnetic resonance imaging (MRI) shows varying degrees of cerebral dysgenesis, including absence of the corpus callosum and cortical dysplasia, as well as hypomyelination, white matter loss, and white matter signal anomalies suggestive of a leukodystrophy. Some patients may show developmental regression; many die in childhood (Fuchs-Telem et al., 2011; Mah-Som et al., 2021). With more patients being reported, several authors (Diggle et al., 2017; Llaci et al., 2019; Mah-Som et al., 2021) have observed that the dermatologic features and peripheral neuropathy show reduced penetrance and are more variable manifestations of this disorder, as they are not observed in all patients with biallelic SNAP29 mutations.

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.

Waardenburg syndrome type 4 (WS4), also known as Waardenburg-Shah syndrome, is an auditory-pigmentary syndrome characterized by pigmentary abnormalities of the hair, skin, and eyes, congenital sensorineural hearing loss, and Hirschsprung disease (reviews by Read and Newton, 1997 and Pingault et al., 2010). WS type 4A is caused by mutation in the EDNRB gene (131244). Clinical Variability of Waardenburg Syndrome Types 1-4 Waardenburg syndrome has been classified into 4 main phenotypes. Type I Waardenburg syndrome (WS1; 193500) is characterized by pigmentary abnormalities of the hair, including a white forelock and premature graying; pigmentary changes of the iris, such as heterochromia iridis and brilliant blue eyes; congenital sensorineural hearing loss; and 'dystopia canthorum.' WS type II (WS2) is distinguished from type I by the absence of dystopia canthorum. WS type III (WS3; 148820) has dystopia canthorum and is distinguished by the presence of upper limb abnormalities. WS type 4 has the additional feature of Hirschsprung disease (reviews by Read and Newton, 1997 and Pingault et al., 2010). Genetic Heterogeneity of Waardenburg Syndrome Type 4 Waardenburg syndrome type 4 is genetically heterogeneous. WS4B (613265) is caused by mutation in the EDN3 gene (131242) on chromosome 20q13, and WS4C (613266) is caused by mutation in the SOX10 gene (602229) on chromosome 22q13.

Spinocerebellar ataxia-23 (SCA23) is an adult-onset autosomal dominant neurodegenerative disorder characterized by slowly progressive gait and limb ataxia, with variable additional features, including peripheral neuropathy and dysarthria (Bakalkin et al., 2010). For a general discussion of autosomal dominant spinocerebellar ataxia, see SCA1 (164400).

Ataxia with oculomotor apraxia type 2 (AOA2) is characterized by onset of ataxia between age three and 30 years after initial normal development, axonal sensorimotor neuropathy, oculomotor apraxia, cerebellar atrophy, and elevated serum concentration of alpha-fetoprotein (AFP).

Gamma-glutamylcysteine synthetase deficiency is 1 of 4 diseases involving enzymes in the gamma-glutamyl cycle (Meister, 1974). The other 3 disorders are glutathione synthetase deficiency (231900), 5-oxoprolinuria, which is a severe or generalized form of glutathione synthetase deficiency (266130), and gamma-glutamyl transpeptidase deficiency (231950). All except gamma-glutamyl transpeptidase deficiency are accompanied by hemolytic anemia (Larsson and Anderson, 2001). Some patients with CNSHA7 may develop progressive neurologic abnormalities, including ataxic gait, speech impairment, and spinocerebellar degeneration (Konrad et al., 1972). The hemolytic anemia caused by mutation in the GCLC gene is here designated congenital nonspherocytic hemolytic anemia-7 (CNSHA7).

Celiac disease is a systemic autoimmune disease that can be associated with gastrointestinal findings (diarrhea, malabsorption, abdominal pain and distension, bloating, vomiting, and weight loss) and/or highly variable non-gastrointestinal findings (dermatitis herpetiformis, chronic fatigue, joint pain/inflammation, iron deficiency anemia, migraines, depression, attention-deficit disorder, epilepsy, osteoporosis/osteopenia, infertility and/or recurrent fetal loss, vitamin deficiencies, short stature, failure to thrive, delayed puberty, dental enamel defects, and autoimmune disorders). Classic celiac disease, characterized by mild to severe gastrointestinal symptoms, is less common than non-classic celiac disease, characterized by absence of gastrointestinal symptoms.

Digital extensor muscle aplasia-polyneuropathy is a rare, hereditary motor and sensory neuropathy characterized by flexion deformities of the thumb and fingers, sensory deficit in the hand and polyneuropathic electrophysiologic findings in the limbs. Operation on the hands reveals extensor muscles and their tendons to be absent or hypoplastic. There have been no further descriptions in the literature since 1986.

Hypomyelination and congenital cataract (HCC) is usually characterized by bilateral congenital cataracts and normal psychomotor or only mildly delayed development in the first year of life, followed by slowly progressive neurologic impairment manifest as ataxia, spasticity (brisk tendon reflexes and bilateral extensor plantar responses), and mild-to-moderate cognitive impairment. Dysarthria and truncal hypotonia are observed. Cerebellar signs (truncal titubation and intention tremor) and peripheral neuropathy (muscle weakness and wasting of the legs) are present in the majority of affected individuals. Seizures can occur. Cataracts may be absent in some individuals.

Spinocerebellar ataxia type 10 (SCA10) is characterized by slowly progressive cerebellar ataxia that usually starts as poor balance and unsteady gait, followed by upper-limb ataxia, scanning dysarthria, and dysphagia. Abnormal tracking eye movements are common. Recurrent seizures after the onset of gait ataxia have been reported with variable frequencies among different families. Some individuals have cognitive dysfunction, behavioral disturbances, mood disorders, mild pyramidal signs, and peripheral neuropathy. Age of onset ranges from 12 to 48 years.

CD59-mediated hemolytic anemia with immune-mediated polyneuropathy is an autosomal recessive disorder characterized by infantile onset of a relapsing-remitting polyneuropathy, often exacerbated by infection, and manifest as hypotonia, limb muscle weakness, and hyporeflexia. Immunosuppressive treatment may result in some clinical improvement (summary by Nevo et al., 2013).

Adult Refsum disease (ARD is associated with elevated plasma phytanic acid levels, late childhood-onset (or later) retinitis pigmentosa, and variable combinations of anosmia, polyneuropathy, deafness, ataxia, and ichthyosis. Onset of symptoms ranges from age seven months to older than age 50 years. Cardiac arrhythmia and heart failure caused by cardiomyopathy are potentially severe health problems that develop later in life.

Hereditary transthyretin (ATTR) amyloidosis is characterized by a slowly progressive peripheral sensorimotor and/or autonomic neuropathy as well as non-neuropathic changes of cardiomyopathy, nephropathy, vitreous opacities, and CNS amyloidosis. The disease usually begins in the third to fifth decade in persons from endemic foci in Portugal and Japan; onset is later in persons from other areas. Typically, sensory neuropathy starts in the lower extremities with paresthesias and hypesthesias of the feet, followed within a few years by motor neuropathy. In some persons, particularly those with early-onset disease, autonomic neuropathy is the first manifestation of the condition; findings can include: orthostatic hypotension, constipation alternating with diarrhea, attacks of nausea and vomiting, delayed gastric emptying, sexual impotence, anhidrosis, and urinary retention or incontinence. Cardiac amyloidosis is mainly characterized by progressive cardiomyopathy. Individuals with leptomeningeal amyloidosis may have the following CNS findings: dementia, psychosis, visual impairment, headache, seizures, motor paresis, ataxia, myelopathy, hydrocephalus, or intracranial hemorrhage.

A rare mitochondrial disease due to a defect in mitochondrial protein synthesis with a variable phenotype that includes onset in infancy or early childhood of failure to thrive and psychomotor regression (after initial normal development), as well as ocular manifestations (such as ptosis, nystagmus, optic atrophy, ophthalmoplegia and reduced vision). Additional manifestations include bulbar paresis with facial weakness, hypotonia, difficulty chewing, dysphagia, mild dysarthria, ataxia, global muscle atrophy, and areflexia. It has a relatively slow disease progression with patients often living into the third decade of life.

Combined oxidative phosphorylation deficiency-6 (COXPD6) is an X-linked recessive severe encephalomyopathic disorder with onset in utero or in infancy. Affected patients have hypotonia and severely impaired psychomotor development associated with variably decreased enzymatic activity of mitochondrial respiratory complexes in skeletal muscle or fibroblasts. More variable features may include sensorimotor neuropathy, seizures, severe muscle weakness, abnormal signals in the basal ganglia, hypertrophic cardiomyopathy, deafness, swallowing difficulties, and respiratory insufficiency. Death in childhood may occur (summary by Berger et al., 2011). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

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).

Mitochondrial encephalo-cardio-myopathy due to <i>TMEM70</i> mutation is characterized by early neonatal onset of hypotonia, hypetrophic cardiomyopathy and apneic spells within hours after birth accompanied by lactic acidosis, hyperammonemia and 3-methylglutaconic aciduria.

Zellweger spectrum disorder (ZSD) is a phenotypic continuum ranging from severe to mild. While individual phenotypes (e.g., Zellweger syndrome [ZS], neonatal adrenoleukodystrophy [NALD], and infantile Refsum disease [IRD]) were described in the past before the biochemical and molecular bases of this spectrum were fully determined, the term "ZSD" is now used to refer to all individuals with a defect in one of the ZSD-PEX genes regardless of phenotype. Individuals with ZSD usually come to clinical attention in the newborn period or later in childhood. Affected newborns are hypotonic and feed poorly. They have distinctive facies, congenital malformations (neuronal migration defects associated with neonatal-onset seizures, renal cysts, and bony stippling [chondrodysplasia punctata] of the patella[e] and the long bones), and liver disease that can be severe. Infants with severe ZSD are significantly impaired and typically die during the first year of life, usually having made no developmental progress. Individuals with intermediate/milder ZSD do not have congenital malformations, but rather progressive peroxisome dysfunction variably manifest as sensory loss (secondary to retinal dystrophy and sensorineural hearing loss), neurologic involvement (ataxia, polyneuropathy, and leukodystrophy), liver dysfunction, adrenal insufficiency, and renal oxalate stones. While hypotonia and developmental delays are typical, intellect can be normal. Some have osteopenia; almost all have ameleogenesis imperfecta in the secondary teeth.

Autosomal recessive spinocerebellar ataxia-13 (SCAR13) is an autosomal recessive neurologic disorder characterized by delayed psychomotor development beginning in infancy. Affected individuals show mildly to profoundly impaired intellectual development with poor or absent speech as well as gait and stance ataxia and hyperreflexia. Most individuals also have eye movement abnormalities. Brain MRI shows cerebellar atrophy and ventriculomegaly (Guergueltcheva et al., 2012).

PBD14B is an autosomal recessive peroxisome biogenesis disorder characterized clinically by mild intellectual disability, congenital cataracts, progressive hearing loss, and polyneuropathy (Ebberink et al., 2012), all of which had been observed in patients with mild peroxisomal biogenesis disorders (e.g., Kelley et al., 1986; Poll-The et al., 1987). Additionally, recurrent migraine-like episodes following mental stress or physical exertion, not a common feature in peroxisome disorders, was reported. Thoms and Gartner (2012) classified the disorder described by Ebberink et al. (2012) in their patient as a mild 'Zellweger syndrome (214100) spectrum' (ZSS) disorder. See PBD1B (601539) for a phenotypic description and discussion of genetic heterogeneity of less severe phenotypes on the Zellweger syndrome spectrum. See PBD9B (614879) for another atypical peroxisome biogenesis disorder.

Branched-chain ketoacid dehydrogenase kinase deficiency (BCKDKD) is a neurodevelopmental disorder characterized by autism, impaired intellectual development, and microcephaly (Tangeraas et al., 2023).

Myofibrillar myopathy refers to a genetically heterogeneous group of muscular disorders characterized by a pathologic morphologic pattern of myofibrillar degradation and abnormal accumulation of proteins involved with the sarcomeric Z disc (summary by Foroud et al., 2005). For a general phenotypic description and a discussion of genetic heterogeneity of myofibrillar myopathy, see MFM1 (601419).

A rare genetic disease with characteristics of childhood onset of multiple endocrine manifestations in combination with central and peripheral nervous system abnormalities. Reported signs and symptoms include postnatal growth retardation, moderate intellectual disability, hypogonadotropic hypogonadism, insulin-dependent diabetes mellitus, central hypothyroidism, demyelinating sensorimotor polyneuropathy, cerebellar and pyramidal signs. Progressive hearing loss and a hypoplastic pituitary gland have also been described. Brain imaging shows moderate white matter abnormalities.

Primary coenzyme Q10 (CoQ10) deficiency is usually associated with multisystem involvement, including neurologic manifestations such as fatal neonatal encephalopathy with hypotonia; a late-onset slowly progressive multiple-system atrophy-like phenotype (neurodegeneration with autonomic failure and various combinations of parkinsonism and cerebellar ataxia, and pyramidal dysfunction); and dystonia, spasticity, seizures, and intellectual disability. Steroid-resistant nephrotic syndrome (SRNS), the hallmark renal manifestation, is often the initial manifestation either as isolated renal involvement that progresses to end-stage renal disease (ESRD), or associated with encephalopathy (seizures, stroke-like episodes, severe neurologic impairment) resulting in early death. Hypertrophic cardiomyopathy (HCM), retinopathy or optic atrophy, and sensorineural hearing loss can also be seen.

Myofibrillar myopathy-4 (MFM4) is an autosomal dominant disorder characterized by adult-onset distal muscle weakness primarily affecting the lower limbs at onset. Affected individuals usually present with gait difficulties in their forties, followed by slow progression with eventual involvement of the hands and proximal muscles of the lower limbs. Rare patients may develop cardiomyopathy. Skeletal muscle biopsy shows myopathic changes with myofibrillar changes (Selcen and Engel, 2005; Griggs et al., 2007). For a phenotypic description and a discussion of genetic heterogeneity of myofibrillar myopathy, see MFM1 (601419).

The two forms of deoxyguanosine kinase (DGUOK) deficiency are a neonatal multisystem disorder and an isolated hepatic disorder that presents later in infancy or childhood. The majority of affected individuals have the multisystem illness with hepatic disease (jaundice, cholestasis, hepatomegaly, and elevated transaminases) and neurologic manifestations (hypotonia, nystagmus, and psychomotor retardation) evident within weeks of birth. Those with isolated liver disease may also have renal involvement and some later develop mild hypotonia. Progressive hepatic disease is the most common cause of death in both forms.

46,XY gonadal dysgenesis-motor and sensory neuropathy syndrome is a rare, genetic, developmental defect during embryogenesis disorder characterized by partial (unilateral testis, persistence of Müllerian duct structures) or complete (streak gonads only) gonadal dysgenesis, usually manifesting with primary amenorrhea in individuals with female phenotype but 46,XY karyotype, and sensorimotor dysmyelinating minifascicular polyneuropathy, which presents with numbness, weakness, exercise-induced muscle cramps, sensory disturbances and reduced/absent deep tendon reflexes. Germ cell tumors (seminoma, dysgerminoma, gonadoblastoma) may develop from the gonadal tissue.

Parkinsonism with polyneuropathy (PKNPY) is an autosomal dominant disorder characterized by asymmetrical tremor-dependent parkinsonism. The age of onset ranges from the late forties to mid-sixties, and patients have a good response to levodopa (summary by Lin et al., 2020).

Deafness, cataract, impaired intellectual development, and polyneuropathy (DCIDP) is characterized by early-onset of deafness, cataract, severe developmental delay, and severely impaired intellectual development. Patients later develop polyneuropathy of the lower extremities, associated with depigmentation of the hair in that area (Kroll-Hermi et al., 2020).

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

Individuals with NGLY1-related congenital disorder of deglycosylation (NGLY1-CDDG) typically display a clinical tetrad of developmental delay / intellectual disability in the mild to profound range, hypo- or alacrima, elevated liver transaminases that may spontaneously resolve in childhood, and a complex hyperkinetic movement disorder that can include choreiform, athetoid, dystonic, myoclonic, action tremor, and dysmetric movements. About half of affected individuals will develop clinical seizures. Other findings may include obstructive and/or central sleep apnea, oral motor defects that affect feeding ability, auditory neuropathy, constipation, scoliosis, and peripheral neuropathy.