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

MFN2 hereditary motor and sensory neuropathy (MFN2-HMSN) is a classic axonal peripheral sensorimotor neuropathy, inherited in either an autosomal dominant (AD) manner (~90%) or an autosomal recessive (AR) manner (~10%). MFN2-HMSN is characterized by more severe involvement of the lower extremities than the upper extremities, distal upper-extremity involvement as the neuropathy progresses, more prominent motor deficits than sensory deficits, and normal (>42 m/s) or only slightly decreased nerve conduction velocities (NCVs). Postural tremor is common. Median onset is age 12 years in the AD form and age eight years in the AR form. The prevalence of optic atrophy is approximately 7% in the AD form and approximately 20% in the AR form.

Retinopathy-sensory neuropathy syndrome (RETSNS) is an autosomal recessive disorder characterized by progressive visual impairment due to retinopathy (usually retinitis pigmentosa) and progressive sensory neuropathy resulting in distal sensory loss of various modalities (vibration, proprioception, pain). Affected individuals have noncerebellar gait ataxia, presumably due to degeneration of dorsal root ganglia in the posterior column of the spinal cord. The phenotypic manifestations and severity of the disorder are highly variable, and the age at onset can range from infancy to young adulthood. Individuals can present with either visual problems or sensory impairment with gait ataxia, but most patients eventually develop both. More severely affected individuals have congenital insensitivity to pain presenting in infancy, resulting in chronic ulceration and osteomyelitis. Autonomic abnormalities may also be apparent, consistent with hereditary sensory and autonomic neuropathy (HSAN; see, e.g., 162400). Developmental delay or impaired intellectual development is sometimes observed (Higgins et al., 1997; Grudzinska Pechhacker et al., 2020; Calame et al., 2025).

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).

POLG-related disorders comprise a continuum of overlapping phenotypes that were clinically defined before the molecular basis was known. POLG-related disorders can therefore be considered an overlapping spectrum of disease presenting from early childhood to late adulthood. The age of onset broadly correlates with the clinical phenotype. In individuals with early-onset disease (prior to age 12 years), liver involvement, feeding difficulties, seizures, hypotonia, and muscle weakness are the most common clinical features. This group has the worst prognosis. In the juvenile/adult-onset form (age 12-40 years), disease is typically characterized by peripheral neuropathy, ataxia, seizures, stroke-like episodes, and, in individuals with longer survival, progressive external ophthalmoplegia (PEO). This group generally has a better prognosis than the early-onset group. Late-onset disease (after age 40 years) is characterized by ptosis and PEO, with additional features such as peripheral neuropathy, ataxia, and muscle weakness. This group overall has the best prognosis.

X-linked spinocerebellar ataxia-6 with or without sideroblastic anemia (SCAX6) is an X-linked recessive disorder characterized by delayed motor development apparent in infancy with delayed walking (often by several years) due to ataxia and poor coordination. Additional features may include dysmetria, dysarthria, spasticity of the lower limbs, hyperreflexia, dysdiadochokinesis, strabismus, and nystagmus. The disorder is slowly progressive, and patients often lose ambulation. Brain imaging usually shows cerebellar atrophy. Most affected individuals have mild hypochromic, microcytic sideroblastic anemia, which may be asymptomatic. Laboratory studies show increased free erythrocyte protoporphyrin (FEP) and ringed sideroblasts on bone marrow biopsy. Female carriers do not have neurologic abnormalities, but may have subtle findings on peripheral blood smear (Pagon et al., 1985; D'Hooghe et al., 2012). For a discussion of genetic heterogeneity of X-linked spinocerebellar ataxia (SCAX), see SCAX1 (302500).

Spinocerebellar ataxia type 17 (SCA17) is characterized by ataxia, dementia, and involuntary movements, including chorea and dystonia. Psychiatric symptoms, pyramidal signs, and rigidity are common. The age of onset ranges from three to 55 years. Individuals with full-penetrance alleles develop neurologic and/or psychiatric symptoms by age 50 years. Ataxia and psychiatric abnormalities are frequently the initial findings, followed by involuntary movement, parkinsonism, dementia, and pyramidal signs. Brain MRI shows variable atrophy of the cerebrum, brain stem, and cerebellum. The clinical features correlate with the length of the polyglutamine expansion but are not absolutely predictive of the clinical course.

Untreated ataxia with vitamin E deficiency (AVED) generally manifests between ages five and 15 years. The first manifestations 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. Although age of onset and disease course are more uniform within a given family, disease manifestations and their severity can vary even among sibs. When lifelong high-dose vitamin E supplementation is initiated in presymptomatic individuals, manifestations of AVED do not develop.

MPV17-related mitochondrial DNA (mtDNA) maintenance defect presents in the vast majority of affected individuals as an early-onset encephalohepatopathic (hepatocerebral) disease that is typically associated with mtDNA depletion, particularly in the liver. A later-onset neuromyopathic disease characterized by myopathy and neuropathy, and associated with multiple mtDNA deletions in muscle, has also rarely been described. MPV17-related mtDNA maintenance defect, encephalohepatopathic form is characterized by: Hepatic manifestations (liver dysfunction that typically progresses to liver failure, cholestasis, hepatomegaly, and steatosis); Neurologic involvement (developmental delay, hypotonia, microcephaly, and motor and sensory peripheral neuropathy); Gastrointestinal manifestations (gastrointestinal dysmotility, feeding difficulties, and failure to thrive); and Metabolic derangements (lactic acidosis and hypoglycemia). Less frequent manifestations include renal tubulopathy, nephrocalcinosis, and hypoparathyroidism. Progressive liver disease often leads to death in infancy or early childhood. Hepatocellular carcinoma has been reported.

Spinocerebellar ataxia-29 (SCA29) is an autosomal dominant neurologic disorder characterized by onset in infancy of delayed motor development and mild cognitive delay. Affected individuals develop a very slowly progressive or nonprogressive gait and limb ataxia associated with cerebellar atrophy on brain imaging. Additional variable features include nystagmus, dysarthria, and tremor (summary by Huang et al., 2012). For a general discussion of autosomal dominant spinocerebellar ataxia, see SCA1 (164400).

Hereditary transthyretin amyloidosis (ATTRv amyloidosis) is characterized by a slowly progressive peripheral sensorimotor and/or autonomic neuropathy. Amyloidosis can involve the heart, central nervous system (CNS), eyes, and kidneys. 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 paresthesia and hypesthesia 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 restrictive 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. Ocular involvement includes vitreous opacity, glaucoma, dry eye, and ocular amyloid angiopathy. Mild-to-severe kidney disease can develop.

The phenotypic spectrum associated with biallelic RFC1 AAGGG repeat expansion encompasses a range including (1) typical cerebellar ataxia, neuropathy, vestibular areflexia syndrome (CANVAS); (2) cerebellar, sensory, and vestibular impairment; (3) more limited phenotypes involving predominantly or exclusively one of the systems involved in balance control; (4) autonomic dysfunction; and (5) cough. Onset begins after age 35 years. In a retrospective study of 100 affected individuals after ten years of disease duration, two thirds had clinical features of CANVAS; 16 had a complex sensory ataxia with cerebellar or vestibular involvement; and 15 had a sensory neuropathy as the only clinically detectable manifestation.

Perrault syndrome is characterized by sensorineural hearing loss (SNHL) in males and females and ovarian dysfunction in females. SNHL is bilateral and ranges from profound with prelingual (congenital) onset to moderate with early-childhood onset. When onset is in early childhood, hearing loss can be progressive. Ovarian dysfunction ranges from gonadal dysgenesis (absent or streak gonads) manifesting as primary amenorrhea to primary ovarian insufficiency (POI) defined as cessation of menses before age 40 years. Fertility in affected males is reported as normal (although the number of reported males is limited). Neurologic features described in some individuals with Perrault syndrome include learning difficulties and developmental delay, cerebellar ataxia, and motor and sensory peripheral neuropathy.

POLG-related disorders comprise a continuum of overlapping phenotypes that were clinically defined before the molecular basis was known. POLG-related disorders can therefore be considered an overlapping spectrum of disease presenting from early childhood to late adulthood. The age of onset broadly correlates with the clinical phenotype. In individuals with early-onset disease (prior to age 12 years), liver involvement, feeding difficulties, seizures, hypotonia, and muscle weakness are the most common clinical features. This group has the worst prognosis. In the juvenile/adult-onset form (age 12-40 years), disease is typically characterized by peripheral neuropathy, ataxia, seizures, stroke-like episodes, and, in individuals with longer survival, progressive external ophthalmoplegia (PEO). This group generally has a better prognosis than the early-onset group. Late-onset disease (after age 40 years) is characterized by ptosis and PEO, with additional features such as peripheral neuropathy, ataxia, and muscle weakness. This group overall has the best prognosis.

Any autosomal recessive nonsyndromic deafness in which the cause of the disease is a mutation in the RIPOR2 gene.

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

A rare autosomal dominant cerebellar ataxia with characteristics of slowly progressive late-onset cerebellar ataxia variably combined with sensory axonal neuropathy. Patients may present gait and limb ataxia, dysarthria, abnormal oculomotor function and distal sensory impairment. Cerebellar atrophy is typically mild or absent.

IRF2BPL-related disorder is characterized by mild-to-profound developmental delay (with regression in many individuals), intellectual disability, seizures (generalized tonic-clonic, myoclonic, absence, focal tonic-clonic, complex partial, infantile spasms, and/or atonic seizures), movement disorder (ataxia, dystonia, tremor, and parkinsonism), spasticity, and neurobehavioral/psychiatric manifestations (autism spectrum disorder, autistic features, anxiety, depression, and psychosis). Feeding issues, gastrointestinal dysmotility, and ophthalmologic manifestations are also reported. Brain MRI can show focal or diffuse cortical and/or subcortical atrophy, cerebellar atrophy (particularly of the vermis), brain stem atrophy, and corpus callosum abnormalities including thinning/atrophy or thickening. Onset is highly variable and can be in the first year of life through the sixth decade. In some individuals the course of the disorder is progressive or debilitating.

Hereditary motor and sensory neuropathy type VIC with optic atrophy (HMSN6C) is an autosomal recessive axonal sensorimotor peripheral neuropathy characterized by progressive distal muscle weakness and atrophy primarily affecting the lower limbs. Onset of neuropathy is in the first decade, manifest by difficulty walking and running and followed by similar involvement of the upper limbs and hands. The disorder is associated with distal sensory impairment, particularly of position and vibration sense, as well as areflexia; individuals usually have pes cavus, hammertoes, and atrophy of the intrinsic hand muscles. In addition, progressive optic atrophy and visual impairment occur during adulthood. Treatment with pyridoxal 5-prime phosphate supplementation (vitamin B6) may result in amelioration of symptoms and slow progression of the disease (summary by Chelban et al., 2019). For a general phenotypic description and a discussion of genetic heterogeneity of HMSN6, see HMSN6A (601152).

Autosomal dominant deafness-76 (DFNA76) is characterized by progressive or nonprogressive hearing loss with variable age at onset. Hearing loss is more severe at higher frequencies in most patients (Schrauwen et al., 2019; Morgan et al., 2019; Diaz-Horta et al., 2019).

Mitochondrial complex IV deficiency nuclear type 2 (MC4DN2) is an autosomal recessive multisystem metabolic disorder characterized by the onset of symptoms at birth or in the first weeks or months of life. Affected individuals have severe hypotonia, often associated with feeding difficulties and respiratory insufficiency necessitating tube feeding and mechanical ventilation. The vast majority of patients develop hypertrophic cardiomyopathy in the first days or weeks of life, which usually leads to death in infancy or early childhood. Patients also show neurologic abnormalities, including developmental delay, nystagmus, fasciculations, dystonia, EEG changes, and brain imaging abnormalities compatible with a diagnosis of Leigh syndrome (see 256000). There may also be evidence of systemic involvement with hepatomegaly and myopathy, although neurogenic muscle atrophy is more common and may resemble spinal muscular atrophy type I (SMA1; 253300). Serum lactate is increased, and laboratory studies show decreased mitochondrial complex IV protein and activity levels in various tissues, including heart and skeletal muscle. Most patients die in infancy of cardiorespiratory failure (summary by Papadopoulou et al., 1999). For a discussion of genetic heterogeneity of mitochondrial complex IV (cytochrome c oxidase) deficiency, see 220110.

Autosomal recessive deafness-116 (DFNB116) is characterized by slowly progressive moderate to profound sensorineural hearing loss (SNHL), with a steeply sloping audiogram in the high frequencies in younger patients (Sineni et al., 2019).

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).