Abetalipoproteinemia typically presents in infancy with failure to thrive, diarrhea, vomiting, and malabsorption of fat. Hematologic manifestations may include acanthocytosis (irregularly spiculated erythrocytes), anemia, reticulocytosis, and hemolysis with resultant hyperbilirubinemia. Malabsorption of fat-soluble vitamins (A, D, E, and K) can result in an increased international normalized ratio (INR). Untreated individuals may develop atypical pigmentation of the retina that may present with progressive loss of night vision and/or color vision in adulthood. Neuromuscular findings in untreated individuals including progressive loss of deep tendon reflexes, vibratory sense, and proprioception; muscle weakness; dysarthria; and ataxia typically manifest in the first or second decades of life.

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

Krabbe disease comprises a spectrum ranging from infantile-onset disease (i.e., onset of extreme irritability, spasticity, and developmental delay before age 12 months) to later-onset disease (i.e., onset of manifestations after age 12 months and as late as the seventh decade). Although historically 85%-90% of symptomatic individuals with Krabbe disease diagnosed by enzyme activity alone have infantile-onset Krabbe disease and 10%-15% have later-onset Krabbe disease, the experience with newborn screening (NBS) suggests that the proportion of individuals with possible later-onset Krabbe disease is higher than previously thought. Infantile-onset Krabbe disease is characterized by normal development in the first few months followed by rapid severe neurologic deterioration; the average age of death is 24 months (range 8 months to 9 years). Later-onset Krabbe disease is much more variable in its presentation and disease course.

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.

Tangier disease is characterized by severe deficiency or absence of high-density lipoprotein (HDL) in the circulation resulting in tissue accumulation of cholesteryl esters throughout the body, particularly in the reticuloendothelial system. The major clinical signs of Tangier disease include hyperplastic yellow-orange tonsils, hepatosplenomegaly, and peripheral neuropathy, which may be either relapsing-remitting or chronic progressive in nature. Rarer complications may include corneal opacities that typically do not affect vision, premature atherosclerotic coronary artery disease occurring in the sixth and seventh decades of life (not usually before age 40 years), and mild hematologic manifestations, such as mild thrombocytopenia, reticulocytosis, stomatocytosis, or hemolytic anemia. The clinical expression of Tangier disease is variable, with some affected individuals only showing biochemical perturbations.

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\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 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\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).

Initial symptoms of multiple sulfatase deficiency (MSD) can develop from infancy through early childhood, and presentation is widely variable. Some individuals display the multisystemic features characteristic of mucopolysaccharidosis disorders (e.g., developmental regression, organomegaly, skeletal deformities) while other individuals present primarily with neurologic regression (associated with leukodystrophy). Based on age of onset, rate of progression, and disease severity, several different clinical subtypes of MSD have been described: Neonatal MSD is the most severe with presentation in the prenatal period or at birth with rapid progression and death occurring within the first two years of life. Infantile MSD is the most common variant and may be characterized as attenuated (slower clinical course with cognitive disability and neurodegeneration identified in the 2nd year of life) or severe (loss of the majority of developmental milestones by age 5 years). Juvenile MSD is the rarest subtype with later onset of symptoms and subacute clinical presentation. Many of the features found in MSD are progressive, including neurologic deterioration, heart disease, hearing loss, and airway compromise.

Methionine adenosyltransferase deficiency is an inborn error of metabolism resulting in isolated hypermethioninemia. Most patients have no clinical abnormalities, although some with the autosomal recessive form have have neurologic abnormalities (Mudd et al., 2003; Kim et al., 2016).

Charcot-Marie-Tooth disease is a sensorineural peripheral polyneuropathy. Affecting approximately 1 in 2,500 individuals, Charcot-Marie-Tooth disease is the most common inherited disorder of the peripheral nervous system (Skre, 1974). Autosomal dominant, autosomal recessive, and X-linked forms have been recognized. Classification On the basis of electrophysiologic properties and histopathology, CMT has been divided into primary peripheral demyelinating (type 1, or HMSNI) and primary peripheral axonal (type 2, or HMSNII) neuropathies. The demyelinating neuropathies classified as CMT type 1 are characterized by severely reduced motor NCVs (less than 38 m/s) and segmental demyelination and remyelination with onion bulb formations on nerve biopsy. The axonal neuropathies classified as CMT type 2 are characterized by normal or mildly reduced NCVs and chronic axonal degeneration and regeneration on nerve biopsy (see CMT2A1; 118210). Distal hereditary motor neuropathy (dHMN) (see 158590), or spinal CMT, is characterized by exclusive motor involvement and sparing of sensory nerves (Pareyson, 1999). McAlpine (1989) proposed that the forms of CMT with very slow nerve conduction be given the gene symbol CMT1A (118220) and CMT1B, CMT1A being the gene on chromosome 17 and CMT1B being the gene on chromosome 1. CMT2 was the proposed symbol for the autosomal locus responsible for the moderately slow nerve conduction form of the disease (axonal). For a phenotypic description and discussion of genetic heterogeneity of the various subtypes of CMT, see CMTX1 (302800), CMT2A1 (118210), CMT3 (DSS; 145900), CMT4A (214400), and CMTDIB (606482). Genetic Heterogeneity of Autosomal Dominant Demyelinating CMT1 Autosomal dominant demyelinating CMT1 is a genetically heterogeneous disorder and can be caused by mutations in different genes; see CMT1A (118220), CMT1C (601098), CMT1D (607678), CMT1E (607734), CMT1F (607734), CMT1G (618279), CMT1H (619764), CMT1I (619742), and CMT1J (620111). See also 608236 for a related phenotype characterized by isolated slowed nerve conduction velocities (NCVs).

PCWH syndrome is a complex neurocristopathy that includes features of 4 distinct syndromes: peripheral demyelinating neuropathy (see 118200), central dysmyelination, Waardenburg syndrome, and Hirschsprung disease (see 142623) (Inoue et al., 2004). Inoue et al. (2004) proposed the acronym PCWH for this disorder.

Posterior column ataxia with retinitis pigmentosa (AXPC1) is an autosomal recessive neurologic disorder characterized by childhood-onset retinitis pigmentosa and later onset of gait ataxia due to sensory loss (summary by Ishiura et al., 2011).

GDAP1-related hereditary motor and sensory neuropathy (GDAP1-HMSN) is a peripheral neuropathy (also known as a subtype of Charcot-Marie-Tooth disease) that typically affects the lower extremities earlier and more severely than the upper extremities. As the neuropathy progresses, the distal upper extremities also become severely affected. Proximal muscles can also become weak. Age at onset ranges from infancy to early childhood. In most cases, disease progression causes disabilities within the first or second decade of life. At the end of the second decade, most individuals are wheelchair bound. Disease progression varies considerably even within the same family. The neuropathy can be either of the demyelinating type with reduced nerve conduction velocities or the axonal type with normal nerve conduction velocities. Vocal cord paresis is common. Intelligence is normal. Life expectancy is usually normal, but on occasion may be reduced because of secondary complications.

A hereditary demyelinating motor and sensory neuropathy with characteristics of slowed nerve conduction velocities in the absence of clinically apparent neurological deficits, gait abnormalities or muscular atrophy, associated with a germline mutation in the ARGHEF10 gene.

For a phenotypic description and a discussion of genetic heterogeneity of axonal CMT, see CMT2A1 (118210).

Charcot-Marie-Tooth disease, dominant intermediate-A (CMTDIA) is an autosomal dominant peripheral neuropathy characterized by onset of symptoms in the first or second decades of life. Affected individuals have difficulty walking with muscle cramps of the lower limbs; the motor symptoms may be worsened by cold. The disorder is slowly progressive, eventually involving all 4 limbs, but patients remain ambulatory. After age 40, patients develop more severe features, including distal muscle weakness and atrophy, pes cavus, areflexia, and distal sensory loss. Electrophysiologic studies yield nerve conduction velocities with 'intermediate' values between demyelinating and axonal neuropathy (see below). One such family has been reported (Rossi et al., 1985).

Molybdenum cofactor deficiency (MoCD) represents a spectrum, with some individuals experiencing significant signs and symptoms in the neonatal period and early infancy (termed early-onset or severe MoCD) and others developing signs and symptoms in childhood or adulthood (termed late-onset or mild MoCD). Individuals with early-onset MoCD typically present in the first days of life with severe encephalopathy, including refractory seizures, opisthotonos, axial and appendicular hypotonia, feeding difficulties, and apnea. Head imaging may demonstrate loss of gray and white matter differentiation, gyral swelling, sulci injury (typically assessed by evaluating the depth of focal lesional injury within the sulci), diffusely elevated T2-weighted signal, and panlobar diffusion restriction throughout the forebrain and midbrain with relative sparring of the brain stem. Prognosis for early-onset MoCD is poor, with about 75% succumbing in infancy to secondary complications of their neurologic disability (i.e., pneumonia). Late-onset MoCD is typically characterized by milder symptoms, such as acute neurologic decompensation in the setting of infection. Episodes vary in nature but commonly consist of altered mental status, dystonia, choreoathetosis, ataxia, nystagmus, and fluctuating hypotonia and hypertonia. These features may improve after resolution of the inciting infection or progress in a gradual or stochastic manner over the lifetime. Brain imaging may be normal or may demonstrate T2-weighted hyperintense or cystic lesions in the globus pallidus, thinning of the corpus callosum, and cerebellar atrophy.

Molybdenum cofactor deficiency (MoCD) represents a spectrum, with some individuals experiencing significant signs and symptoms in the neonatal period and early infancy (termed early-onset or severe MoCD) and others developing signs and symptoms in childhood or adulthood (termed late-onset or mild MoCD). Individuals with early-onset MoCD typically present in the first days of life with severe encephalopathy, including refractory seizures, opisthotonos, axial and appendicular hypotonia, feeding difficulties, and apnea. Head imaging may demonstrate loss of gray and white matter differentiation, gyral swelling, sulci injury (typically assessed by evaluating the depth of focal lesional injury within the sulci), diffusely elevated T2-weighted signal, and panlobar diffusion restriction throughout the forebrain and midbrain with relative sparring of the brain stem. Prognosis for early-onset MoCD is poor, with about 75% succumbing in infancy to secondary complications of their neurologic disability (i.e., pneumonia). Late-onset MoCD is typically characterized by milder symptoms, such as acute neurologic decompensation in the setting of infection. Episodes vary in nature but commonly consist of altered mental status, dystonia, choreoathetosis, ataxia, nystagmus, and fluctuating hypotonia and hypertonia. These features may improve after resolution of the inciting infection or progress in a gradual or stochastic manner over the lifetime. Brain imaging may be normal or may demonstrate T2-weighted hyperintense or cystic lesions in the globus pallidus, thinning of the corpus callosum, and cerebellar atrophy.

Mitochondrial complex IV deficiency nuclear type 5 (MC4DN5) is an autosomal recessive severe metabolic multisystemic disorder with onset in infancy. Features include delayed psychomotor development, impaired intellectual development with speech delay, mild dysmorphic facial features, hypotonia, ataxia, and seizures. There is increased serum lactate and episodic hypoglycemia. Some patients may have cardiomyopathy, abnormal breathing, or liver abnormalities, reflecting systemic involvement. Brain imaging shows lesions in the brainstem and basal ganglia, consistent with a diagnosis of Leigh syndrome (see 256000). Affected individuals tend to have episodic metabolic and/or neurologic crises in early childhood, which often lead to early death (summary by Debray et al., 2011). For a discussion of genetic heterogeneity of mitochondrial complex IV (cytochrome c oxidase) deficiency, see 220110.

CTDP1-related congenital cataracts, facial dysmorphism, and neuropathy (CTDP1-CCFDN) is characterized by abnormalities of the eye (bilateral congenital cataracts, microcornea, microphthalmia, micropupils), mildly dysmorphic facial features apparent in late childhood, and a hypo-/demyelinating, symmetric, distal peripheral neuropathy. The neuropathy is predominantly motor at the onset and results in delays in early motor development, progressing to severe disability by the third decade of life. Secondary foot deformities and scoliosis are common. Sensory neuropathy develops after age ten years. Most affected individuals have a mild nonprogressive intellectual deficit and cerebellar involvement including ataxia, nystagmus, intention tremor, and dysmetria. All have short stature and most have subnormal weight. Adults have hypogonadotropic hypogonadism. Parainfectious rhabdomyolysis (profound muscle weakness, myoglobinuria, and excessively elevated serum concentration of creatine kinase usually following a viral infection) is a potentially life-threatening complication. To date all affected individuals and carriers identified have been from the Romani population.

Charcot-Marie-Tooth disease type 4F (CMT4F) is an autosomal recessive demyelinating neuropathy characterized by distal sensory impairment and distal muscle weakness and atrophy affecting the lower more than the upper limbs. Nerve conduction velocities are decreased and sural nerve biopsy shows loss of myelinated fibers. The age at onset is variable and can range from childhood to adult years. When the onset is in infancy, the phenotype is characterized as Dejerine-Sottas syndrome (DSS; 145900). For a phenotypic description and a discussion of genetic heterogeneity of autosomal recessive demyelinating Charcot-Marie-Tooth disease, see CMT4A (214400).

The overlapping phenotypes of neonatal adrenoleukodystrophy (NALD) and infantile Refsum disease (IRD) represent the milder manifestations of the Zellweger syndrome spectrum (ZSS) of peroxisome biogenesis disorders. 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 PEX16 gene have cells of complementation group 9 (CG9, equivalent to CGD). For information on the history of PBD complementation groups, see 214100.

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.

Charcot-Marie-Tooth disease type 4K (CMT4K) is an autosomal recessive demyelinating peripheral neuropathy characterized by onset in the first decade of distal muscle weakness and atrophy associated with impaired distal sensation. Both upper and lower limbs are affected. Affected individuals may also have nystagmus and late-onset cerebellar ataxia. Laboratory studies show increased serum lactate and isolated mitochondrial complex IV deficiency (summary by Echaniz-Laguna et al., 2013). For a phenotypic description and a discussion of genetic heterogeneity of autosomal recessive demyelinating Charcot-Marie-Tooth disease, see CMT4A (214400).

Childhood-onset neurodegeneration with brain atrophy (CONDBA) is a severe progressive neurodegenerative disorder characterized by loss of motor and cognitive skills between ages 2 and 7 years. Affected individuals may have normal development or mild developmental delay, but all eventually lose all motor skills, resulting in inability to walk, absence of language, and profound intellectual disability. Brain imaging shows progressive cerebral and cerebellar atrophy (summary by Edvardson et al., 2017).

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.

Charcot-Marie-Tooth disease type 1G is an autosomal dominant progressive peripheral sensorimotor neuropathy characterized by distal muscle weakness and atrophy with onset in the first or second decade. Affected individuals have difficulty walking, distal sensory impairment with decreased or absent reflexes, and often have foot deformities. Median motor nerve conduction velocities (NCV) are decreased (less than 38 m/s) and sural nerve biopsy shows myelin defects and onion bulb formation (summary by Hong et al., 2016 and Motley et al., 2016). For a phenotypic description and a discussion of genetic heterogeneity of autosomal dominant Charcot-Marie-Tooth disease type 1, see CMT1B (118200).

CMTDIF is an autosomal dominant neurologic disorder characterized by onset around adolescence of slowly progressive distal muscle atrophy and weakness affecting the upper and lower limbs and resulting in steppage gait. There is distal sensory impairment with decreased reflexes. Nerve conduction velocities are variable, ranging from the demyelinating to the axonal range (summary by Soong et al., 2013). For a discussion of genetic heterogeneity of CMTDI, see 606482.

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).