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).

Glycogen storage disease type III (GSD III) is characterized by variable liver, cardiac muscle, and skeletal muscle involvement. GSD IIIa is the most common subtype, present in about 85% of affected individuals; it manifests with liver and muscle involvement. GSD IIIb, with liver involvement only, comprises about 15% of all affected individuals. In infancy and early childhood, liver involvement presents as hepatomegaly and failure to thrive, with fasting ketotic hypoglycemia, hyperlipidemia, and elevated hepatic transaminases. In adolescence and adulthood, liver disease becomes less prominent. Most individuals develop cardiac involvement with cardiac hypertrophy and/or cardiomyopathy. Skeletal myopathy manifesting as weakness may be evident in childhood and slowly progresses, typically becoming prominent in the third to fourth decade. The overall prognosis is favorable but cannot be predicted on an individual basis. Long-term complications such as muscular and cardiac symptoms as well as liver fibrosis/cirrhosis and hepatocellular carcinoma may have a severe impact on prognosis and quality of life. To date, it is unknown if long-term complications can be alleviated and/or avoided by dietary interventions.

Spinocerebellar ataxia type 3 (SCA3), also known as Machado-Joseph disease (MJD), is characterized by progressive cerebellar ataxia and variable findings including pyramidal signs, a dystonic-rigid extrapyramidal syndrome, significant peripheral amyotrophy and generalized areflexia, progressive external ophthalmoplegia, action-induced facial and lingual fasciculations, and bulging eyes. Neurologic findings tend to evolve as the disorder progresses.

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.

Spinal muscular atrophy (SMA) is characterized by muscle weakness and atrophy resulting from progressive degeneration and irreversible loss of the anterior horn cells in the spinal cord (i.e., lower motor neurons) and the brain stem nuclei. The onset of weakness ranges from before birth to adulthood. The weakness is symmetric, proximal > distal, and progressive. Before the genetic basis of SMA was understood, it was classified into clinical subtypes based on maximum motor function achieved; however, it is now apparent that the phenotype of SMN1-associated SMA spans a continuum without clear delineation of subtypes. With supportive care only, poor weight gain with growth failure, restrictive lung disease, scoliosis, and joint contractures are common complications; however, newly available targeted treatment options are changing the natural history of this disease.

Roussy-Levy syndrome is an autosomal dominant disorder characterized by early onset of prominent ataxia followed by late onset of mild motor involvement. Symptoms progress very slowly, and affected individuals may remain ambulatory throughout life (Auer-Grumbach et al., 1998; Plante-Bordeneuve et al., 1999).

Welander distal myopathy (WDM) is an autosomal dominant disorder characterized by adult onset of distal muscle weakness predominantly affecting the distal long extensors of the hands, with slow progression to involve all small hand muscles and the lower legs. Skeletal muscle biopsy shows myopathic changes and prominent rimmed vacuoles. Rare homozygous patients showed earlier onset, faster progression, and proximal muscle involvement. This disorder is common in Sweden and Finland (summary by Hackman et al., 2013).

For a general phenotypic description and a discussion of genetic heterogeneity of Charcot-Marie-Tooth disease type 1, see CMT1B (118200). CMT1A is the most common form of CMT. The average age of onset of clinical symptoms is 12.2 +/- 7.3 years. Slow nerve conduction velocity (NCV) less than 38 m/s is highly diagnostic and is a 100% penetrant phenotype independent of age (Lupski et al., 1991, 1992).

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).

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

Troyer syndrome is characterized by progressive spastic paraparesis, dysarthria, pseudobulbar palsy, distal amyotrophy, short stature, and subtle skeletal abnormalities. Most affected children exhibit delays in walking and speech and difficulty in managing oral secretions, followed by increased lower-limb spasticity and slow deterioration in both gait and speech. Mild cerebellar signs are common. The most severely affected individuals have choreoathetosis. Emotional lability / difficulty in controlling emotions and affective disorders, such as inappropriate euphoria and/or crying, are frequently described. Life expectancy is normal.

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.

Spinocerebellar ataxia type 1 (SCA1) is characterized by progressive cerebellar ataxia, dysarthria, and eventual deterioration of bulbar functions. Early in the disease, affected individuals may have gait disturbance, slurred speech, difficulty with balance, brisk deep tendon reflexes, hypermetric saccades, nystagmus, and mild dysphagia. Later signs include slowing of saccadic velocity, development of up-gaze palsy, dysmetria, dysdiadochokinesia, and hypotonia. In advanced stages, muscle atrophy, decreased deep tendon reflexes, loss of proprioception, cognitive impairment (e.g., frontal executive dysfunction, impaired verbal memory), chorea, dystonia, and bulbar dysfunction are seen. Onset is typically in the third or fourth decade, although childhood onset and late-adult onset have been reported. Those with onset after age 60 years may manifest a pure cerebellar phenotype. Interval from onset to death varies from ten to 30 years; individuals with juvenile onset show more rapid progression and more severe disease. Anticipation is observed. An axonal sensory neuropathy detected by electrophysiologic testing is common; brain imaging typically shows cerebellar and brain stem atrophy.

Spinocerebellar ataxia type 2 (SCA2) is characterized by progressive cerebellar ataxia, including nystagmus, slow saccadic eye movements, and in some individuals, ophthalmoparesis or parkinsonism. Pyramidal findings are present; deep tendon reflexes are brisk early on and absent later in the course. Age of onset is typically in the fourth decade with a ten- to 15-year disease duration.

Primrose syndrome is characterized by macrocephaly, hypotonia, developmental delay, intellectual disability with expressive speech delay, behavioral issues, a recognizable facial phenotype, radiographic features, and altered glucose metabolism. Additional features seen in adults: sparse body hair, distal muscle wasting, and contractures. Characteristic craniofacial features include brachycephaly, high anterior hairline, deeply set eyes, ptosis, downslanted palpebral fissures, high palate with torus palatinus, broad jaw, and large ears with small or absent lobes. Radiographic features include calcification of the external ear cartilage, multiple Wormian bones, platybasia, bathrocephaly, slender bones with exaggerated metaphyseal flaring, mild epiphyseal dysplasia, and spondylar dysplasia. Additional features include hearing impairment, ocular anomalies, cryptorchidism, and nonspecific findings on brain MRI.

Wieacker-Wolff syndrome (WRWF) is a severe X-linked recessive neurodevelopmental disorder affecting the central and peripheral nervous systems. It is characterized by onset of muscle weakness in utero (fetal akinesia), which results in arthrogryposis multiplex congenita (AMC) apparent at birth. Affected boys are born with severe contractures, show delayed motor development, facial and bulbar weakness, characteristic dysmorphic facial features, and skeletal abnormalities, such as hip dislocation, scoliosis, and foot deformities. Additional features include global developmental delay with poor or absent speech and impaired intellectual development, feeding difficulties and poor growth, hypotonia, hypogenitalism, and spasticity. Carrier females may be unaffected or have mild features of the disorder (summary by Hirata et al., 2013 and Frints et al., 2019).

The phenotypic spectrum of GARS1-associated axonal neuropathy ranges from GARS1 infantile-onset SMA (GARS1-iSMA) to GARS1 adolescent- or early adult-onset hereditary motor/sensory neuropathy (GARS1-HMSN). GARS1-iSMA. Age of onset ranges from the neonatal period to the toddler years. Initial manifestations are typically respiratory distress, poor feeding, and muscle weakness (distal greater than proximal). Weakness is slowly progressive, ultimately requiring mechanical ventilation and feeding via gastrostomy tube. GARS1-HMSN. Age of onset is most commonly during the second decade (range eight to 36 years). Initial manifestations are typically muscle weakness in the hands sometimes with sensory deficits. Lower limb involvement (seen in ~50% of individuals) ranges from weakness and atrophy of the extensor digitorum brevis and weakness of toe dorsiflexors to classic peroneal muscular atrophy with foot drop and a high steppage gait.

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).

Charcot-Marie-Tooth disease, type 4B1 (CMT4B1) is a severe early-onset demyelinating CMT peripheral sensorimotor polyneuropathy. It was initially described in an Italian family and around 10 additional families have been described so far. Onset occurs during early childhood with distal and proximal muscular weakness starting in the lower extremities, sensory loss and cranial nerve involvement. Foot deformities are frequent and diaphragmatic and facial involvement has been reported. CMT4B1 is caused by mutations in the gene encoding myotubularin-related protein 2 (MTMR2; 11q22), involved in polyphosphoinositide signaling. Transmitted in an autosomal recessive manner.

A severe form of axonal Charcot-Marie-Tooth disease, a peripheral sensorimotor neuropathy. Onset in the second or third decade has manifestations of ulceration and infection of the feet. Symmetric and distal weakness develops mostly in the legs together with a severe symmetric distal sensory loss. Tendon reflexes are only reduced at ankles and foot deformities including pes cavus or planus and hammer toes, appear in childhood.

Autosomal dominant distal hereditary motor neuronopathy-7 (HMND7) is a neurologic disorder characterized by onset in the second decade of progressive distal muscle wasting and weakness affecting the upper and lower limbs and resulting in walking difficulties and hand grip. There is significant muscle atrophy of the hands and lower limbs. The disorder is associated with vocal cord paresis due to involvement of the tenth cranial nerve (summary by Barwick et al., 2012). For a general phenotypic description and a discussion of genetic heterogeneity of autosomal dominant distal HMN, see HMND1 (182960).

Spastic paraplegia, optic atrophy, and neuropathy (SPOAN) is an autosomal recessive neurodegenerative disorder characterized by early-onset progressive spastic paraplegia resulting in loss of independent ambulation in the teenage years. Additional features include optic atrophy, later onset of sensorimotor peripheral neuropathy, and progressive joint contractures; cognition remains intact (summary by Melo et al., 2015).

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.

Age-related macular degeneration-3 (ARMD3) is characterized by numerous small round yellow lesions visible at the temporal edge of the macula. Larger, less distinct yellow areas near the center of the macula are also observed, which represent areas of pigment epithelial detachment (Stone et al., 2004). For a phenotypic description and a discussion of genetic heterogeneity of age-related macular degeneration, see 603075.

A form of axonal Charcot-Marie-Tooth disease, a peripheral sensorimotor neuropathy. In the single family reported to date, CMT2L onset is between 15 and 33 years. Patients present with a symmetric distal weakness of legs and occasionally of the hands, absent or reduced tendon reflexes, distal legs sensory loss and frequently a pes cavus. Progression is slow.

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 VAPB gene on chromosome 20q13.

The autosomal dominant TRPV4 disorders (previously considered to be clinically distinct phenotypes before their molecular basis was discovered) are now grouped into neuromuscular disorders and skeletal dysplasias; however, the overlap within each group is considerable. Affected individuals typically have either neuromuscular or skeletal manifestations alone, and in only rare instances an overlap syndrome has been reported. The three autosomal dominant neuromuscular disorders (mildest to most severe) are: Charcot-Marie-Tooth disease type 2C. Scapuloperoneal spinal muscular atrophy. Congenital distal spinal muscular atrophy. The autosomal dominant neuromuscular disorders are characterized by a congenital-onset, static, or later-onset progressive peripheral neuropathy with variable combinations of laryngeal dysfunction (i.e., vocal fold paresis), respiratory dysfunction, and joint contractures. The six autosomal dominant skeletal dysplasias (mildest to most severe) are: Familial digital arthropathy-brachydactyly. Autosomal dominant brachyolmia. Spondylometaphyseal dysplasia, Kozlowski type. Spondyloepiphyseal dysplasia, Maroteaux type. Parastremmatic dysplasia. Metatropic dysplasia. The skeletal dysplasia is characterized by brachydactyly (in all 6); the five that are more severe have short stature that varies from mild to severe with progressive spinal deformity and involvement of the long bones and pelvis. In the mildest of the autosomal dominant TRPV4 disorders life span is normal; in the most severe it is shortened. Bilateral progressive sensorineural hearing loss (SNHL) can occur with both autosomal dominant neuromuscular disorders and skeletal dysplasias.

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.

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 rare hereditary motor and sensory neuropathy characterized by intermediate motor median nerve conduction velocities (usually between 25 and 60 m/s). It presents with moderately severe, slowly progressive usual clinical features of Charcot-Marie-Tooth disease (muscle weakness and atrophy of the distal extremities, distal sensory loss, reduced or absent deep tendon reflexes, feet deformities, extensor digitorum brevis atrophy). Findings in nerve biopsies include age-dependent axonal degeneration, reduced number of large myelinated fibers, segmental remyelination, and no onion bulbs.

A severe early-onset form of axonal Charcot-Marie-Tooth peripheral sensorimotor polyneuropathy. Onset occurs in the neonatal period or early infancy with a clinical picture including hypotonia, scoliosis, a hoarse voice, vocal cord paralysis and respiratory insufficiency. However, nerve conduction velocities and pathological findings from sural nerve biopsies are indicative of a predominantly axonal neuropathy with some demyelinating features. Caused by mutations in the GDAP1 gene (8q13.3), encoding a protein required for mitochondrial fission.

A rare hereditary motor and sensory neuropathy with characteristics of intermediate motor median nerve conduction velocities (usually between 25 and 45 m/s) and signs of both axonal degeneration and demyelination without onion bulbs in nerve biopsies. It presents with usual Charcot-Marie-Tooth disease clinical features of variable severity (progressive muscle weakness and atrophy of the distal extremities, distal sensory loss, reduced or absent deep tendon reflexes, and feet deformities). Other findings in some of the families include debilitating neuropathic pain and mild postural/kinetic upper limb tremor.

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

A form of Charcot-Marie-Tooth disease type 1, with a variable clinical presentation that can range from severe impairment with onset in childhood to mild impairment appearing during adulthood. The disease has characteristics of progressive peripheral motor and sensory neuropathy with distal paresis in the lower limbs that varies from mild weakness to complete paralysis of the distal muscle groups, absent tendon reflexes and reduced nerve conduction. Caused by mutations in the NEFL gene (8p21.2).

An axonal peripheral sensorimotor polyneuropathy associated with pyramidal involvement. So far, it has been described in 13 members of a large Tunisian family. Onset occurred during the first decade of life with progressive distal atrophy involving both the upper and lower limbs, associated with a mild pyramidal syndrome (brisk patellar and upper limb reflexes, absent ankle reflexes and unattainable plantar reflexes). Transmitted in an autosomal recessive manner and the disease-causing locus has been mapped to 8q13-21.1.

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.

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

The spectrum of DCTN1-related neurodegeneration includes Perry syndrome, distal hereditary motor neuronopathy type 7B (dHMN7B), frontotemporal dementia (FTD), motor neuron disease / amyotrophic lateral sclerosis (ALS), and progressive supranuclear palsy. Some individuals present with overlapping phenotypes (e.g., FTD-ALS, Perry syndrome-dHMN7B). Perry syndrome (the most common of the phenotypes associated with DCTN1) is characterized by parkinsonism, neuropsychiatric symptoms, hypoventilation, and weight loss. The mean age of onset in those with Perry syndrome is 49 years (range: 35-70 years), and the mean disease duration is five years (range: 2-14 years). In most affected persons, the reported cause/circumstance of death relates to sudden death/hypoventilation or suicide.

Pontocerebellar hypoplasia (PCH) refers to a group of severe neurodegenerative disorders affecting growth and function of the brainstem and cerebellum, resulting in little or no development. Different types were classified based on the clinical picture and the spectrum of pathologic changes. PCH type 1 is characterized by central and peripheral motor dysfunction associated with anterior horn cell degeneration resembling infantile spinal muscular atrophy (SMA; see SMA1, 253300); death usually occurs early. Genetic Heterogeneity of Pontocerebellar Hypoplasia Also see PCH1B (614678), caused by mutation in the EXOSC3 gene (606489); PCH1C (616081), caused by mutation in the EXOSC8 gene (606019); PCH1D (618065), caused by mutation in the EXOSC9 gene (606180); PCH1E (619303), caused by mutation in the SLC25A46 gene (610826); PCH1F (619304), caused by mutation in the EXOSC1 gene (606493); PCH2A (277470), caused by mutation in the TSEN54 gene (608755); PCH2B (612389), caused by mutation in the TSEN2 gene (608753); PCH2C (612390), caused by mutation in the TSEN34 gene (608754); PCH2D (613811), caused by mutation in the SEPSECS gene (613009); PCH3 (608027), caused by mutation in the PCLO gene (604918); PCH4 (225753), caused by mutation in the TSEN54 gene; PCH5 (610204), caused by mutation in the TSEN54 gene; PCH6 (611523), caused by mutation in the RARS2 gene (611524); PCH7 (614969), caused by mutation in the TOE1 gene (613931); PCH8 (614961), caused by mutation in the CHMP1A gene (164010); PCH9 (615809), caused by mutation in the AMPD2 gene (102771); PCH10 (615803), caused by mutation in the CLP1 gene (608757); PCH11 (617695), caused by mutation in the TBC1D23 gene (617687); PCH12 (618266), caused by mutation in the COASY gene (609855); PCH13 (618606), caused by mutation in the VPS51 gene (615738); PCH14 (619301), caused by mutation in the PPIL1 gene (601301); PCH15 (619302), caused by mutation in the CDC40 gene (605585); PCH16 (619527), caused by mutation in the MINPP1 gene (605391); and PCH17 (619909), caused by mutation in the PRDM13 gene (616741) on chromosome 6q16.

A rare genetic peripheral sensorimotor neuropathy with an X-linked recessive inheritance pattern and the childhood to adolescent-onset of progressive, distal muscle weakness and atrophy (beginning in the lower extremities and then affecting the upper extremities), as well as distal, pan sensory loss in the upper and lower extremities, pes cavus, and absent or reduced distal tendon reflexes. Pain and paraesthesia are frequently the initial sensory symptoms. Spastic paraparesis (manifested by clasp-knife sign, hyperactive deep-tendon reflexes, and Babinski sign) has also been reported.

A rare genetic peripheral sensorimotor neuropathy with an X-linked recessive inheritance pattern and the infantile to childhood-onset of progressive, distal muscle weakness and atrophy (more prominent in the lower extremities than in the upper extremities), pes cavus, and absent tendon reflexes. Sensory impairment and intellectual disability has been reported in some individuals.

Phosphorylase kinase (PhK) deficiency causing glycogen storage disease type IX (GSD IX) results from deficiency of the enzyme phosphorylase b kinase, which has a major regulatory role in the breakdown of glycogen. The two types of PhK deficiency are liver PhK deficiency (characterized by early childhood onset of hepatomegaly and growth restriction, and often, but not always, fasting ketosis and hypoglycemia) and muscle PhK deficiency, which is considerably rarer (characterized by any of the following: exercise intolerance, myalgia, muscle cramps, myoglobinuria, and progressive muscle weakness). While symptoms and biochemical abnormalities of liver PhK deficiency were thought to improve with age, it is becoming evident that affected individuals need to be monitored for long-term complications such as liver fibrosis and cirrhosis.

A rare distal hereditary motor neuropathy with characteristics of slowly progressive atrophy and weakness of distal muscles of hands and feet with normal deep tendon reflexes or absent ankle reflexes and minimal or no sensory loss, sometimes mild proximal weakness in the legs and feet and hand deformities in males.

VPS13D movement disorder is a hyperkinetic movement disorder (dystonia, chorea, and/or ataxia) of variable age of onset that can be associated with developmental delay. Onset ranges from birth to adulthood. Individuals can present in childhood with motor delays and gait instability. Cognitive impairment ranging from mild intellectual disability to developmental delay has been reported, and several individuals have normal cognitive function. Individuals have also presented as young adults with gait difficulties caused by spastic ataxia or ataxia. In addition to gait ataxia, affected individuals had limb ataxia, dysarthria, and eye movement abnormalities (macro-saccadic oscillations, nystagmus, and saccadic pursuit). Additional features reported in some individuals include peripheral neuropathy and/or seizures. The disorder progresses to spastic ataxia or generalized dystonia, which can lead to loss of independent ambulation.

Autosomal recessive distal hereditary motor neuronopathy-3 (HMNR3), also known as distal spinal muscular atrophy (DSMA) and distal hereditary motor neuronopathy (dHMN or HMN), is characterized by distal muscle weakness and wasting without significant sensory involvement. For a discussion of genetic heterogeneity of autosomal recessive HMN, see HMNR1 (604320). Harding (1993) classified autosomal recessive distal hereditary motor neuronopathy as dHMN IV (HMN4) and dHMN III (HMN3). Both have juvenile onset and differ only by less severe involvement in HMN3. However, Viollet et al. (2004) reported an extended Lebanese kindred in which both HMN III and HMN IV occurred, suggesting that the same gene was involved in both phenotypes (see Irobi et al., 2006).

Charcot-Marie-Tooth disease is a clinically and genetically heterogeneous disorder of the peripheral nervous system, characterized by progressive weakness and atrophy, initially of the peroneal muscles and later of the distal muscles of the arms. Classification CMT neuropathy is subdivided into CMT1 (see 118200) and CMT2 (see 118210) types on the basis of electrophysiologic and neuropathologic criteria. CMT1, or hereditary motor and sensory neuropathy type I (HMSN I), is a demyelinating neuropathy, whereas CMT2, or HMSN II, is an axonal neuropathy. Most patients with CMT are classified as having CMT1 or CMT2 by use of a cut-off value of 38 m/s for the motor median nerve conduction velocity (NCV). However, in some families with CMT, patients have motor median NCVs ranging from 25 to 45 m/s. Families of this type were reported by Salisachs (1974) and Davis et al. (1978). Davis et al. (1978) proposed that this form be designated 'intermediate' CMT. Claeys et al. (2009) stated that some CMT families may have an even broader range of NCV than 25 to 45 m/s, with the lowest levels around 25 and the highest levels within the normal range (50+ m/s). They also suggested that the term 'intermediate' should not be used to describe a single NCV value, but rather the CMT subtype at the level of the family (e.g., in families with a range or combinations of NCV values). Berciano et al. (2017) provided a detailed review of the different forms of intermediate CMT, noting that diagnoses may be controversial because of variable classification issues. The authors presented an algorithm for the interpretation of electrophysiologic studies in CMT, and suggested that nerve conduction studies should be conducted on the upper arm (axilla to elbow). They noted that distal axonal degeneration can result in secondary myelination defects, which may cause significantly decreased motor NCV and CMAP values that may be misinterpreted. Genetic Heterogeneity of Autosomal Dominant Intermediate CMT In addition to CMTDIB, which is caused by mutation in the DNM2 gene, other forms of dominant intermediate CMT include CMTDIA (620378), mapped to chromosome 10q24-q25; CMTDIC (608323), caused by mutation in the YARS gene (603623) on chromosome 1p35; CMTDID (607791), caused by mutation in the MPZ gene (159440) on chromosome 1q22; CMTDIE with focal segmental glomerulosclerosis (CMTDIE; 614455), caused by mutation in the INF2 gene (610982) on chromosome 14q32; CMTDIF (615185), caused by mutation in the GNB4 gene (610863) on chromosome 3q26; and CMTDIG (617882), caused by mutation in the NEFL gene (162280) on chromosome 8p21.

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).

Infantile-onset spinocerebellar ataxia (IOSCA) is a severe, progressive neurodegenerative disorder characterized by normal development until age one year, followed by onset of ataxia, muscle hypotonia, loss of deep-tendon reflexes, and athetosis. Ophthalmoplegia and sensorineural deafness develop by age seven years. By adolescence, affected individuals are profoundly deaf and no longer ambulatory; sensory axonal neuropathy, optic atrophy, autonomic nervous system dysfunction, and hypergonadotropic hypogonadism in females become evident. Epilepsy can develop into a serious and often fatal encephalopathy: myoclonic jerks or focal clonic seizures that progress to epilepsia partialis continua followed by status epilepticus with loss of consciousness.

Spastic paraplegia 15 (SPG15), typically an early-onset complex hereditary spastic paraplegia, is characterized by progressive spasticity that begins in the lower extremities and is associated with several manifestations resulting from central and peripheral nervous system dysfunction. While onset of spasticity is typically in mid- to late childhood or adolescence (i.e., between ages 5 and 18 years), other manifestations, such as developmental delay or learning disability, may be present earlier, often preceding motor involvement. Individuals with adult onset have also been reported.

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is clinically characterized by a progressive cerebellar ataxia, peripheral neuropathy, and spasticity. Disease onset of classic ARSACS is often in early childhood, leading to delayed walking because of gait unsteadiness in very young toddlers, while an increasing number of individuals with disease onset in teenage or early-adult years are now being described. Typically the ataxia is followed by lower-limb spasticity and later by peripheral neuropathy – although pronounced peripheral neuropathy has been observed as a first sign of ARSACS. Oculomotor disturbances, dysarthria, and upper-limb ataxia develop with slower progression than the other findings. Brain imaging demonstrates atrophy of the superior vermis and the cerebellar hemisphere with additional findings on MRI, such as linear hypointensities in the pons and hyperintense rims around the thalami. Many affected individuals (though not all) have yellow streaks of hypermyelinated fibers radiating from the edges of the optic disc noted on ophthalmologic exam, and thickened retinal fibers can be demonstrated by optical coherence tomography. Mild intellectual disability, hearing loss, and urinary urgency and incontinence have been reported in some individuals.

GAN-related neurodegeneration comprises a phenotypic continuum ranging from severe (sometimes called classic giant axonal neuropathy) to milder pure early-onset peripheral motor and sensory neuropathies. The classic giant axonal neuropathy phenotype typically manifests as an infantile-onset neurodegenerative disorder, starting as a severe peripheral motor and sensory neuropathy and evolving into central nervous system impairment (intellectual disability, seizures, cerebellar signs, and pyramidal tract signs). Most affected individuals become wheelchair dependent in the second decade of life and eventually bedridden with severe polyneuropathy, ataxia, and dementia. Death usually occurs in the third decade. At the milder end of the spectrum are predominantly motor and sensory neuropathies (with little to no CNS involvement) that overlap with the axonal form of Charcot-Marie-Tooth neuropathies.

This syndrome is characterized by the association of an axonal sensory and autonomic neuropathy with spastic paraplegia.

Congenital myasthenic syndromes (CMS) are a group of inherited disorders affecting the neuromuscular junction (NMJ). Patients present clinically with onset of variable muscle weakness between infancy and adulthood. These disorders have been classified according to the location of the defect: presynaptic, synaptic, and postsynaptic. CMS10 is an autosomal recessive CMS resulting from a postsynaptic defect affecting endplate maintenance of the NMJ. Patients present with limb-girdle weakness in the first decade. Treatment with ephedrine or salbutamol may be beneficial; cholinesterase inhibitors should be avoided (summary by Engel et al., 2015). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

Spastic paraplegia-31 (SPG31) is an autosomal dominant neurologic disorder characterized primarily by spasticity of the lower limbs, resulting in gait abnormalities and muscle weakness. There is a bimodal age at onset with a peak in the second and fourth decades of life. Most affected individuals have a 'pure' form of the disorder with gait difficulties and hyperreflexia of the lower limbs. However, there is phenotypic heterogeneity, and some patients have a 'complex' form of the disorder with additional signs and symptoms, such as peripheral neuropathy, cerebellar ataxia, postural tremor, or urinary symptoms. The disorder is slowly progressive, and there is intrafamilial variability and incomplete penetrance (summary by Goizet et al., 2011 and Toft et al., 2019). For a general phenotypic description and a discussion of genetic heterogeneity of autosomal dominant spastic paraplegia, see SPG3A (182600).

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).

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.

Autosomal recessive distal hereditary motor neuronopathy-2 (HMNR2) is a neuromuscular disorder characterized by onset of distal muscle weakness and wasting affecting the lower and upper limbs in the first decade; there is no sensory involvement (summary by Li et al., 2015). For a general phenotypic description and a discussion of genetic heterogeneity of autosomal recessive HMN, see HMNR1 (604320).

Spinal muscular atrophy is characterized by degeneration of the anterior horn cells in the spinal cord, leading to symmetric muscle weakness and wasting. See also autosomal recessive adult-onset proximal spinal muscular atrophy (SMA4; 271150), caused by defect in the SMN1 gene (600354), and autosomal dominant childhood-onset proximal SMA (158600).

Charcot-Marie-Tooth disease type 2B2 (CMT2B2) is an autosomal recessive sensorineural axonal peripheral neuropathy manifest as distal muscle weakness and atrophy and distal sensory impairment. The disorder predominantly affects the lower limbs, resulting in gait impairment, although upper limb and hand involvement also occurs. The age at onset and severity is variable: most have onset in the third decade, although earlier onset has been reported. The disorder is slowly progressive, and some patients may lose independent ambulation later in life. More variable features may include ataxia, dysarthria, cerebellar atrophy, and eye movement abnormalities (summary by Leal et al., 2018). For a phenotypic description and a discussion of genetic heterogeneity of axonal CMT type 2, see CMT2A1 (118210).

Charcot-Marie-Tooth disease constitutes a clinically and genetically heterogeneous group of hereditary motor and sensory neuropathies. On the basis of electrophysiologic criteria, CMT is divided into 2 major types: type 1, the demyelinating form, characterized by a motor median nerve conduction velocity less than 38 m/s (see CMT1B; 118200); and type 2, the axonal form, with a normal or slightly reduced nerve conduction velocity. For a phenotypic description and a discussion of genetic heterogeneity of axonal CMT type 2, see CMT2A1 (118210).

Autosomal recessive Charcot-Marie-Tooth disease type 4B2 (CMT4B2) is a demyelinating hereditary motor and sensory neuropathy characterized by abnormal folding of myelin sheaths. CMT4B1 (601382) is a clinically similar disorder caused by mutation in the MTMR2 gene (603557) on 11q22. For a phenotypic description and a discussion of genetic heterogeneity of autosomal recessive demyelinating CMT, see CMT4A (214400).

Autosomal recessive distal hereditary motor neuronopathy-1 (HMNR1) is characterized by distal and proximal muscle weakness and diaphragmatic palsy that leads to respiratory distress. Without intervention, most infants with the severe form of the disease die before 2 years of age. Affected individuals present in infancy with inspiratory stridor, weak cry, recurrent bronchopneumonia, and swallowing difficulties. The disorder is caused by distal and progressive motor neuronopathy resulting in muscle weakness (summary by Perego et al., 2020). Genetic Heterogeneity of Autosomal Recessive Distal Hereditary Motor Neuronopathy See also HMNR2 (605726), caused by mutation in the SIGMAR1 gene (601978); HMNR3 (607088) (encompassing Harding HMN types III and IV), which maps to chromosome 11q13; HMNR4 (611067), caused by mutation in the PLEKHG5 gene (611101); HMNR5 (614881), caused by mutation in the DNAJB2 gene (604139); HMNR6 (620011), caused by mutation in the REEP1 gene (609139); HMNR7 (619216), caused by mutation in the VWA1 gene (611901); HMNR8 (618912), caused by mutation in the SORD gene (182500); HMNR9 (620402), caused by mutation in the COQ7 gene (601683); and HMRN10 (620542), caused by mutation in the VRK1 gene (602168).

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).

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.

Ataxia with oculomotor apraxia type 1 (AOA1) is characterized by childhood onset of slowly progressive cerebellar ataxia, followed by oculomotor apraxia and a severe primary motor peripheral axonal motor neuropathy. The first manifestation is progressive gait imbalance (mean age of onset: 4.3 years; range: 2-10 years), followed by dysarthria, then upper-limb dysmetria with mild intention tremor. Oculomotor apraxia, usually noticed a few years after the onset of ataxia, progresses to external ophthalmoplegia. All affected individuals have generalized areflexia followed by a peripheral neuropathy and quadriplegia with loss of ambulation about seven to ten years after onset. Hands and feet are short and atrophic. Chorea and upper-limb dystonia are common. Intellect remains normal in some individuals; in others, different degrees of cognitive impairment have been observed.

A juvenile amyotrophic lateral sclerosis that is slowly progressive with concomitantly progressive dementia.

ALS2-related disorder involves retrograde degeneration of the upper motor neurons of the pyramidal tracts and comprises a clinical continuum of the following three phenotypes: Infantile ascending hereditary spastic paraplegia (IAHSP), characterized by onset of spasticity with increased reflexes and sustained clonus of the lower limbs within the first two years of life, progressive weakness and spasticity of the upper limbs by age seven to eight years, and wheelchair dependence in the second decade with progression toward severe spastic tetraparesis and a pseudobulbar syndrome caused by progressive cranial nerve involvement. Juvenile primary lateral sclerosis (JPLS), characterized by upper motor neuron findings of pseudobulbar palsy and spastic quadriplegia without dementia or cerebellar, extrapyramidal, or sensory signs. Juvenile amyotrophic lateral sclerosis (JALS or ALS2), characterized by onset between ages three and 20 years. All affected individuals show a spastic pseudobulbar syndrome (spasticity of speech and swallowing) together with spastic paraplegia. Some individuals are bedridden by age 12 to 50 years.

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.

Giant axonal neuropathy-2 is an autosomal dominant peripheral axonal neuropathy characterized by onset of distal sensory impairment and lower extremity muscle weakness and atrophy after the second decade. Foot deformities may be present in childhood. More severely affected individuals may develop cardiomyopathy. Sural nerve biopsy shows giant axonal swelling with neurofilament accumulation (summary by Klein et al., 2014).

Distal myopathy-3 (MPD3) is an autosomal dominant skeletal muscle disorder characterized by adult onset of slowly progressive distal muscular weakness and atrophy affecting the upper and lower limbs, leading to difficulties using the hands and walking difficulties. Proximal muscle involvement may occur later in the disease, but patients typically remain ambulatory. Muscle biopsy shows myopathic changes with rimmed vacuoles (Hackman et al., 2021).

Juvenile amyotrophic lateral sclerosis-4 (ALS4) is an autosomal dominant disorder characterized by distal muscle weakness and atrophy, normal sensation, and pyramidal signs, with onset of symptoms before the age of 25 years, a slow rate of progression, and a normal life span (summary by Chen et al., 2004). For a phenotypic description and a discussion of genetic heterogeneity of amyotrophic lateral sclerosis, see ALS1 (105400).

Autosomal recessive juvenile amyotrophic lateral sclerosis-5 (ALS5) is a neurodegenerative disorder characterized by onset of upper and lower motor neuron signs before age 25. Affected individuals have progressive spasticity of limb and facial muscles associated with distal amyotrophy. The disorder is slowly progressive, with cases of prolonged survival of more than 3 decades (summary by Orlacchio et al., 2010). For a phenotypic description and a discussion of genetic heterogeneity of amyotrophic lateral sclerosis (ALS), see ALS1 (105400).

SH3TC2-related hereditary motor and sensory neuropathy (SH3TC2-HMSN) is a demyelinating neuropathy characterized by severe spine deformities (scoliosis or kyphoscoliosis) and foot deformities (pes cavus, pes planus, or pes valgus) that typically present in the first decade of life or early adolescence. Other findings can include cranial nerve involvement (most commonly tongue involvement, facial weakness/paralysis, hearing impairment, dysarthria) and respiratory problems.

Distal hereditary motor neuronopathy (dHMN or HMN) is a heterogeneous group of neuromuscular disorders caused by anterior horn cell degeneration and characterized by progressive distal motor weakness and muscular atrophy of the peripheral nervous system without sensory impairment. Distal HMN is also referred to as spinal Charcot-Marie-Tooth disease (spinal CMT). Distal HMN is often referred to as a 'neuronopathy' instead of a 'neuropathy' based on the hypothesis that the primary pathologic process resides in the neuron cell body and not in the axons (Irobi et al., 2006). Historically, Harding (1993) proposed a clinical classification of distal HMN into 7 phenotypic subtypes according to age at onset, mode of inheritance, and presence of additional features; see NOMENCLATURE. Genetic Heterogeneity of Autosomal Dominant Distal Hereditary Motor Neuronopathy Genetically distinct forms of autosomal dominant distal hereditary motor neuropathy include HMND1; HMND2 (158590), caused by mutation in the HSPB8 gene (608014); HMND3 (608634), caused by mutation in the HSPB1 gene (602195); HMND4 (613376), caused by mutation in the HSPB3 gene (604624); HMND5 (600794), caused by mutation in the GARS gene (600287); HMND6 (615575), caused by mutation in the FBXO38 gene (608533); HMND7 (158580), caused by mutation in the SLC5A7 gene (608761); HMND8 (600175), caused by mutation in the TRPV4 gene (605427); HMND9 (617721), caused by mutation in the WARS gene (191050); HMND10 (620080), caused by mutation in the EMILIN1 gene (130660); HMND11 (620528), caused by mutation in the SPTAN1 gene (182810); HMND12 (614751), caused by mutation in the REEP1 gene (609139); HMND13 (619112), caused by mutation in the BSCL2 gene (606158); and HMND14 (607641), caused by mutation in the DCTN1 gene (601143). See also X-linked HMN (HMNX; 300489), caused by mutation in the ATP7A gene (300011) on chromosome Xq21. Additional disorders with overlapping features include autosomal dominant ALS4 (602433), caused by mutation in the SETX gene (608465); and CMS7A (616040), caused by mutation in the SYT2 gene (600104).

A complex form of hereditary spastic paraplegia characterized by spastic paraplegia, demyelinating peripheral sensorimotor neuropathy, poikiloderma (manifesting with loss of eyebrows and eyelashes in childhood in addition to delicate, smooth, and wasted skin) and distal amyotrophy (presenting after puberty). There have been no further descriptions in the literature since 1992.

Autosomal recessive spastic ataxia-2 (SPAX2) is a neurologic disorder characterized by onset in the first 2 decades of cerebellar ataxia, dysarthria, and variable spasticity of the lower limbs. Cognition is not affected (summary by Dor et al., 2014). For a discussion of genetic heterogeneity of spastic ataxia, see SPAX1 (108600).

A rare genetic neuromuscular disease characterized by proximal muscle weakness with an early involvement of foot and hand muscles following normal motor development in early childhood, a rapidly progressive disease course leading to generalized areflexic tetraplegia with contractures, severe scoliosis, hyperlordosis, and progressive respiratory insufficiency leading to assisted ventilation. Cranial nerve functions are normal and tongue wasting and fasciculations are absent. Milder phenotype with a moderate generalized weakness and slower disease progress was reported. There is evidence the disease is caused by homozygous mutation in the gene encoding pleckstrin homology domain-containing protein, family G member 5 (PLEKHG5) on chromosome 1p36.

Fiskerstrand type peripheral neuropathy is a slowly-progressive Refsum-like disorder associating signs of peripheral neuropathy with late-onset hearing loss, cataract and pigmentary retinopathy that become evident during the third decade of life.

A complex hereditary spastic paraplegia with characteristics of mild to severe lower limbs spasticity, hyperreflexia, extensor plantar responses, pes cavus and significant wasting and weakness of the small hand muscles. Impaired vibration sensation, temporal lobe epilepsy and cognitive dysfunction were also reported.

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).

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 angiogenin gene (ANG) on chromosome 14q11.

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.

The spectrum of BSCL2-related neurologic disorders includes Silver syndrome and variants of Charcot-Marie-Tooth neuropathy type 2, distal hereditary motor neuropathy (dHMN) type V, and spastic paraplegia 17. Features of these disorders include onset of symptoms ranging from the first to the seventh decade, slow disease progression, upper motor neuron involvement (gait disturbance with pyramidal signs ranging from mild to severe spasticity with hyperreflexia in the lower limbs and variable extensor plantar responses), lower motor neuron involvement (amyotrophy of the peroneal muscles and small muscles of the hand), and pes cavus and other foot deformities. Disease severity is variable among and within families.

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.

Peripheral neuropathy-myopathy-hoarseness-hearing loss syndrome is a rare, syndromic genetic deafness characterized by a combination of muscle weakness, chronic neuropathic and myopathic features, hoarseness and sensorineural hearing loss. A wide range of disease onset and severity has been reported even within the same family.

A rare genetic axonal hereditary motor and sensory neuropathy disorder with characteristics of adulthood-onset of slowly progressive, occasionally asymmetrical, distal muscle weakness and atrophy (predominantly in the lower limbs), pan-modal sensory loss, muscle cramping in extremities and/or trunk, pes cavus and absent or reduced deep tendon reflexes. Gait anomalies and variable autonomic disturbances, such as erectile dysfunction and urinary urgency, may be associated. The disease can be caused by homozygous or heterozygous mutation in the LRSAM1 gene on chromosome 9q33.

Spastic ataxia-5 (SPAX5) is an autosomal recessive neurodegenerative disorder characterized by early-onset spasticity resulting in significantly impaired ambulation, cerebellar ataxia, oculomotor apraxia, dystonia, and myoclonic epilepsy (summary by Pierson et al., 2011). For a discussion of genetic heterogeneity of spastic ataxia, see SPAX1 (108600).

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.

Charcot-Marie-Tooth disease type 4F 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. Some patients with PEX10 mutations have a milder disorder characterized by childhood-onset cerebellar ataxia and neuropathy without mental retardation (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 PEX10 gene have cells of complementation group 7 (CG7, equivalent to CGB). For information on the history of PBD complementation groups, see 214100.

AOA3 is an autosomal recessive progressive neurologic disorder with onset in the second decade of life (Al Tassan et al., 2012). For a discussion of genetic heterogeneity of ataxia-oculomotor apraxia, see AOA1 (208920).

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).

CMTRIC is an autosomal recessive peripheral neuropathy characterized by distal sensory impairment predominantly affecting the lower limbs and resulting in walking difficulties due to muscle weakness and atrophy. The upper limbs may also be affected. Electrophysiologic studies and sural nerve biopsy show mixed features of demyelinating and axonal neuropathy. The age at onset and the severity of the disease are variable (summary by Azzedine et al., 2013). For a discussion of genetic heterogeneity of autosomal recessive intermediate CMT, see CMTRIA (608340).

A form of axonal Charcot-Marie-Tooth disease a peripheral sensorimotor neuropathy. A late onset with severe sensory loss associated with distal weakness mainly of the legs and absent or reduced deep tendon reflexes.

Ataxia-telangiectasia-like disorder-1 is an autosomal recessive disorder characterized clinically by progressive cerebellar degeneration resulting in ataxia and oculomotor apraxia. Laboratory studies of patient cells showed increased susceptibility to radiation, consistent with a defect in DNA repair. The disorder shares some phenotypic features of ataxia-telangiectasia (AT; 208900), but telangiectases and immune deficiency are not present in ATLD1 (summary by Hernandez et al., 1993 and Stewart et al., 1999). Genetic Heterogeneity of Ataxia-Telangiectasia-Like Disorder See also ATLD2 (615919), caused by mutation in the PCNA gene (176740) on chromosome 20p12.

Presynaptic congenital myasthenic syndrome-7A with distal motor neuropathy (CMS7A) is an autosomal dominant neuromuscular disorder characterized by onset of foot deformities, delayed motor development, and slowly progressive distal muscle weakness resulting in gait difficulties in early childhood. Other features may include hyporeflexia, muscle atrophy, and upper limb involvement. Electrophysiologic studies show low compound muscle action potentials (CMAPs), consistent with a distal hereditary motor neuropathy (dHMN), as well as a decremental response to repetitive stimulation, indicating presynaptic defects at the neuromuscular junction (NMJ), consistent with myasthenic syndrome (summary by Fionda et al., 2021). The complex phenotype of patients with dominant SYT2 mutations likely results from impairment of 2 fundamental functions of SYT2: (1) disturbance of calcium-dependent synchronous presynaptic neurotransmitter release, resulting in a myasthenic disorder, and (2) disruption of exocytosis and endocytosis, causing a degenerative process affecting peripheral motor nerve terminals and resulting in a motor neuropathy (Maselli et al., 2021). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462). For a discussion of genetic heterogeneity of dHMN, see 182960.

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).

Hereditary motor and sensory neuropathy type VIB is an autosomal recessive complex progressive neurologic disorder characterized mainly by early-onset optic atrophy resulting in progressive visual loss and peripheral axonal sensorimotor neuropathy with highly variable age at onset and severity. Affected individuals may also have cerebellar or pontocerebellar atrophy on brain imaging, and they may show abnormal movements such as ataxia, dysmetria, and myoclonus (summary by Abrams et al., 2015). For a general phenotypic description and a discussion of genetic heterogeneity of HMSN6, see HMSN6A (601152).

Lethal congenital contracture syndrome-8 (LCCS8), an axoglial form of arthrogryposis multiplex congenita, is characterized by congenital distal joint contractures, reduced fetal movements, and severe motor paralysis leading to death early in the neonatal period (Laquerriere et al., 2014). For a general phenotypic description and a discussion of genetic heterogeneity of lethal congenital contracture syndrome, see LCCS1 (253310).

Progressive encephalopathy with amyotrophy and optic atrophy (PEAMO) is a severe autosomal recessive neurodegenerative disorder characterized by delayed development with hypotonia apparent in infancy and subsequent motor regression. Most affected individuals are unable to or lose the ability to sit and show distal amyotrophy and weakness of all 4 limbs. The patients are cognitively impaired and unable to speak or have severe dysarthria. Additional features include optic atrophy, thin corpus callosum, and cerebellar atrophy (Sferra et al., 2016).

Harel-Yoon syndrome is a syndromic neurodevelopmental disorder characterized by delayed psychomotor development, intellectual disability, truncal hypotonia, spasticity, and peripheral neuropathy. Other more variable features such as optic atrophy may also occur. Laboratory studies in some patients show evidence of mitochondrial dysfunction (summary by Harel et al., 2016).

Spinocerebellar ataxia-43 is an autosomal dominant, slowly progressive neurologic disorder characterized by adult-onset gait and limb ataxia and often associated with peripheral neuropathy mainly affecting the motor system, although some patients may have distal sensory impairment (summary by Depondt et al., 2016). For a general discussion of autosomal dominant spinocerebellar ataxia, see SCA1 (164400).

SPTBN4 disorder is typically characterized by severe-to-profound developmental delay and/or intellectual disability, although two individuals in one family had a milder phenotype, including one individual with normal cognitive development. Speech and language skills are often severely limited. Affected individuals rarely achieve head control. Most are unable to sit, stand, or walk. Affected individuals typically have congenital hypotonia that may transition to hypertonia. Axonal motor neuropathy leads to hyporeflexia/areflexia and weakness, which can result in respiratory difficulties requiring ventilatory support. Most affected individuals require tube feeding for nutrition. Half of affected individuals develop seizures. Cortical visual impairment and auditory neuropathy have also been reported.

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).

Mitochondrial myopathy and ataxia (MMYAT) is an autosomal recessive mtDNA depletion disorder characterized by cerebellar ataxia, congenital muscle involvement with histologic findings ranging from myopathic to dystrophic, and pigmentary retinopathy (summary by Donkervoort et al., 2019).

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.

Dysferlinopathy includes a spectrum of muscle disease characterized by two major phenotypes: Miyoshi muscular dystrophy (MMD) and limb-girdle muscular dystrophy type 2B (LGMD2B); and two minor phenotypes: asymptomatic hyperCKemia and distal myopathy with anterior tibial onset (DMAT). MMD (median age of onset 19 years) is characterized by muscle weakness and atrophy, most marked in the distal parts of the legs, especially the gastrocnemius and soleus muscles. Over a period of years, the weakness and atrophy spread to the thighs and gluteal muscles. The forearms may become mildly atrophic with decrease in grip strength; the small muscles of the hands are spared. LGMD2B is characterized by early weakness and atrophy of the pelvic and shoulder girdle muscles in adolescence or young adulthood, with slow progression. Other phenotypes in this spectrum are scapuloperoneal syndrome and congenital muscular dystrophy. Asymptomatic hyperCKemia is characterized by marked elevation of serum CK concentration only. DMAT is characterized by early and predominant distal muscle weakness, particularly of the muscles of the anterior compartment of the legs.

Mitochondrial neurogastrointestinal encephalopathy (MNGIE) disease is characterized by progressive gastrointestinal dysmotility (manifesting as early satiety, nausea, dysphagia, gastroesophageal reflux, postprandial emesis, episodic abdominal pain and/or distention, and diarrhea); cachexia; ptosis/ophthalmoplegia or ophthalmoparesis; leukoencephalopathy; and demyelinating peripheral neuropathy (manifesting as paresthesias (tingling, numbness, and pain) and symmetric and distal weakness more prominently affecting the lower extremities). The order in which manifestations appear is unpredictable. Onset is usually between the first and fifth decades; in about 60% of individuals, symptoms begin before age 20 years.

Congenital hypomyelinating neuropathy (CHN) is characterized clinically by onset of hypotonia at birth, areflexia, distal muscle weakness, and very slow nerve conduction velocities (often less than 10 m/s). Warner et al. (1997, 1998) noted that pathologic findings on sural nerve biopsies show hypomyelination of most or all fibers. Based on these findings, CHN is considered to be a result of congenital impairment in myelin formation. There has been some controversy and difficulty in differentiating congenital hypomyelination from Dejerine-Sottas syndrome (DSS; 145900) because there is considerable overlap in clinical presentation. Based on pathologic findings of sural nerve biopsies (the absence of active myelin breakdown and the paucity of the onion bulbs in CHN and the presence of demyelination/remyelination and an abundance of well-organized onion bulbs in DSS; see Balestrini et al., 1991), CHN is considered to result from a congenital impairment in myelin formation, whereas DSS is thought to be due to aberrant demyelination and subsequent remyelination of the peripheral nerve. There is also variation in the prognosis of patients diagnosed with CHN. In patients with CHN, Harati and Butler (1985) showed correlation of morbidity and mortality with the presence/absence of onion bulbs: patients with few onion bulbs died in early infancy, usually because of difficulty in swallowing and respiration after birth. Patients with atypical onion bulbs survived but were affected with severe motor and sensory impairment. These differences in outcome may represent genetic heterogeneity such that mutations in essential early myelin gene(s) cause a severe phenotype, whereas mutations in other, possibly later acting gene(s), such as MPZ, lead to a less severe outcome. Genetic Heterogeneity of Congenital Hypomyelinating Neuropathy See also CHN2 (618184), caused by mutation in the MPZ gene (159440) on chromosome 1q23; and CHN3 (618186), caused by mutation in the CNTNAP1 gene (602346) on chromosome 17q21.

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.

Hereditary motor and sensory neuropathies (HMSN) are a heterogeneous group of peripheral nervous system disorders affecting motor and sensory function. HMSN I, also known as Charcot-Marie-Tooth (CMT) disease, or peroneal muscular atrophy, type 1, is a demyelinating neuropathy (see CMT1B; 118200) and HMSN II, also known as CMT type 2, is an axonal neuropathy (see CMT2A1; 118210). See also HMSN III (145900) and HMSN IV (266500). For an autosomal recessive disorder with similarities to HMSN V, see 607731.

Congenital hypomyelinating neuropathy-2 is an autosomal dominant neurologic disorder characterized by early-onset hypotonia, severely delayed motor development, muscle weakness with areflexia, and severely decreased nerve conduction velocities (NCV) resulting from improper myelination of axons. The severity is variable: some patients may present at birth with contractures and respiratory insufficiency, whereas others may achieve walking (summary by Warner et al., 1996). CHN shows significant phenotypic overlap with Dejerine-Sottas syndrome (DSS; 145900), which is also a neuropathy with early onset. Some classify the disorders differently, noting that CHN is characterized by hypo- or amyelination resulting from a congenital defect in myelin formation, whereas DSS has features of continuous myelin breakdown, with demyelination and remyelination (summary by Smit et al., 2008). For a discussion of genetic heterogeneity of CHN, see CHN1 (605253).

Charcot-Marie-Tooth disease type 2DD is an autosomal dominant peripheral sensorimotor neuropathy mainly affecting the lower limbs. Affected individuals have gait impairment due to distal muscle weakness and atrophy. Some patients may also have involvement of the distal upper limbs, resulting in atrophy of the intrinsic hand muscles. The age at onset and severity of the disorder is highly variable, even within families, and those with earlier onset in late childhood or the teenage years tend to have a more severe disease course. Patients remain ambulatory even late in the disease, although some may require orthotic devices (summary by Lassuthova et al., 2018). For a phenotypic description and a discussion of genetic heterogeneity of axonal CMT type 2, see CMT2A (118210).

MDDGC8 is an autosomal recessive muscular dystrophy with onset in childhood. The phenotype is highly variable: some patients may have gait difficulties and impaired intellectual development, whereas others may be clinically asymptomatic. Common features include calf hypertrophy and increased serum creatine kinase, and muscle biopsy often shows dystrophic features (summary by Endo et al., 2015). The disorder is part of a group of similar disorders resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239), collectively known as 'dystroglycanopathies' (Godfrey et al., 2007). For a discussion of genetic heterogeneity of muscular dystrophy- dystroglycanopathy type C, see MDDGC1 (609308).

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).

Spinocerebellar ataxia with axonal neuropathy type 1 (SCAN1) is characterized by late-childhood-onset slowly progressive cerebellar ataxia and distal sensorimotor axonal neuropathy. Gaze nystagmus and dysarthria usually develop after the onset of ataxic gait. As the disease advances, pain and touch sensation in the hands and feet become impaired; vibration sense is lost in hands and lower thighs. Individuals with advanced disease develop a steppage gait and pes cavus and eventually become wheelchair dependent. Cognitive dysfunction – present in some – manifests as mild intellectual disability and poor executive function. To date only seven affected individuals have been described from three apparently unrelated consanguineous families (one from Saudi Arabia and two from Oman); therefore, it is likely that the full phenotypic spectrum of this disorder is not yet known.

Spinocerebellar ataxia with axonal neuropathy-3 (SCAN3) is an autosomal recessive neuromuscular disorder characterized by onset in the first decade of slowly progressive distal muscle weakness and atrophy and distal sensory impairment due to an axonal peripheral neuropathy. Affected individuals have gait disturbances and sometimes manual dexterity difficulties, as well as cerebellar ataxia associated with cerebellar atrophy on brain imaging. Additional features usually include dysarthria, hyporeflexia, and increased serum creatine kinase. Some patients may have impaired intellectual development (summary by Higuchi et al., 2018). For a discussion of genetic heterogeneity of SCAN, see SCAN1 (607250).

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).

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)

Oculopharyngodistal myopathy-1 (OPDM1) is an autosomal dominant disorder characterized by adult-onset ptosis, external ophthalmoplegia, facial muscle weakness, distal limb muscle weakness and atrophy, and pharyngeal involvement, resulting in dysphagia and dysarthria. Skeletal muscle biopsy shows myopathic changes with rimmed vacuoles. There are variable manifestations of the disorder regarding muscle involvement and severity (summary by Ishiura et al., 2019). Genetic Heterogeneity of Oculopharyngodistal Myopathy See also OPDM2 (618940), caused by trinucleotide repeat expansion in the GIPC1 gene (605072) on chromosome 19p13; OPDM3 (619473), caused by trinucleotide repeat expansion in the NOTCH2NLC gene (618025) on chromosome 1q21; and OPDM4 (619790), caused by trinucleotide repeat expansion in the RILPL1 gene (614092) on chromosome 12q24. Oculopharyngeal muscular dystrophy (OPMD; 164300) is a similar disorder with overlapping features. It is caused by a similar heterozygous trinucleotide repeat expansion in the PABPN1 gene (602279) (summary by Durmus et al., 2011).

De Barsy syndrome, or autosomal recessive cutis laxa type III (ARCL3), is characterized by cutis laxa, a progeria-like appearance, and ophthalmologic abnormalities (summary by Kivuva et al., 2008). For a phenotypic description and a discussion of genetic heterogeneity of autosomal recessive cutis laxa, see 219100. Genetic Heterogeneity of de Barsy Syndrome Also see ARCL3B (614438), caused by mutation in the PYCR1 gene (179035) on chromosome 17q25.

Mitochondrial DNA depletion syndrome-18 (MTDPS18) is an autosomal recessive neuromuscular disorder characterized by early-onset progressive weakness and atrophy of the distal limb muscles, resulting in loss of ambulation as well as atrophy of the intrinsic hand muscles with clawed hands. Affected individuals may also develop scoliosis and have hypo- or hyperreflexia and decreased pulmonary vital capacity. Examination of skeletal muscle shows neurogenic atrophy and combined mitochondrial oxidative phosphorylation deficiency associated with mtDNA depletion. The clinical phenotype is reminiscent of spinal muscular atrophy (see SMA, 253300) and the metabolic profile is reminiscent of 2-aminoadipic 2-oxoadipic aciduria (AMOXAD; 204750), which is caused by mutation in the DHTKD1 gene (614984) (summary by Boczonadi et al., 2018). For a discussion of genetic heterogeneity of autosomal recessive mtDNA depletion syndromes, see MTDPS1 (603041).

The phenotypic spectrum of GARS1-associated axonal neuropathy ranges from GARS1 infantile-onset SMA (GARS1-iSMA) to GARS1 adolescent- or early adult-onset hereditary motor/sensory neuropathy (GARS1-HMSN). GARS1-iSMA. Age of onset ranges from the neonatal period to the toddler years. Initial manifestations are typically respiratory distress, poor feeding, and muscle weakness (distal greater than proximal). Weakness is slowly progressive, ultimately requiring mechanical ventilation and feeding via gastrostomy tube. GARS1-HMSN. Age of onset is most commonly during the second decade (range eight to 36 years). Initial manifestations are typically muscle weakness in the hands sometimes with sensory deficits. Lower limb involvement (seen in ~50% of individuals) ranges from weakness and atrophy of the extensor digitorum brevis and weakness of toe dorsiflexors to classic peroneal muscular atrophy with foot drop and a high steppage gait.

The phenotypic spectrum of GARS1-associated axonal neuropathy ranges from GARS1 infantile-onset SMA (GARS1-iSMA) to GARS1 adolescent- or early adult-onset hereditary motor/sensory neuropathy (GARS1-HMSN). GARS1-iSMA. Age of onset ranges from the neonatal period to the toddler years. Initial manifestations are typically respiratory distress, poor feeding, and muscle weakness (distal greater than proximal). Weakness is slowly progressive, ultimately requiring mechanical ventilation and feeding via gastrostomy tube. GARS1-HMSN. Age of onset is most commonly during the second decade (range eight to 36 years). Initial manifestations are typically muscle weakness in the hands sometimes with sensory deficits. Lower limb involvement (seen in ~50% of individuals) ranges from weakness and atrophy of the extensor digitorum brevis and weakness of toe dorsiflexors to classic peroneal muscular atrophy with foot drop and a high steppage gait.

Intellectual developmental disorder with speech delay and axonal peripheral neuropathy (IDDSAPN) is an autosomal recessive neurologic disorder characterized by mild global developmental delay with motor impairment and severe speech delay apparent in the first years of life. Affected individuals begin to walk independently between 3 and 4 years of age, but often have an unsteady or ataxic gait. Most patients have progressive distal muscle weakness and atrophy of the lower limbs, foot or hand deformities, and dysarthria, consistent with a peripheral neuropathy. There is mildly impaired intellectual development. Some patients may have behavioral anomalies, such as autistic features or attention deficit-hyperactivity disorder (ADHD), and some can attend special schools. The overall clinical features indicate involvement of both the central and peripheral nervous systems (summary by Martin et al., 2020 and Ahmed et al., 2021)

Charcot-Marie-Tooth disease type 2GG (CMT2GG) is an autosomal dominant axonal peripheral neuropathy characterized by slowly progressive distal muscle weakness and atrophy primarily affecting the lower limbs and causing difficulty walking. The onset is usually in adulthood, although rare patients may have mild symptoms from childhood. Some individuals may also have involvement of the hands. Although most patients have hypo- or areflexia at the ankles, distal sensory impairment is not always present, indicating a spectrum of disease encompassing both distal hereditary neuropathy and axonal CMT. Electrophysiologic studies are consistent with a axonal process (summary by Mendoza-Ferreira et al., 2020). For a discussion of genetic heterogeneity of axonal CMT, see CMT2A1 (118210).

Oculopharyngodistal myopathy-3 (OPDM3) is a neuromyodegenerative disease characterized by progressive muscle weakness with ocular, facial, pharyngeal, and distal limb involvement, resulting in dysarthria and gait difficulties. The onset of the disorder is usually in adulthood, although childhood onset has rarely been reported. Additional features include hyporeflexia, proximal muscle weakness, neck muscle weakness, dysarthria, dysphagia, and ptosis. Some patients may develop pigmentary retinopathy, peripheral neuropathy, or hearing loss. Cognition is usually not affected, but there may be deficits or psychiatric manifestations. Brain imaging tends to show a leukoencephalopathy, often with a characteristic linear signal along the corticomedullary junction on brain imaging. Skin and muscle biopsy show intranuclear inclusions and rimmed vacuoles. Many of the clinical features are reminiscent of NIID, suggesting that these disorders likely fall within a broad phenotypic spectrum of diseases with neuromyodegenerative features associated with abnormal repeat expansions in this gene (summary by Ogasawara et al., 2020 and Yu et al., 2021). For a discussion of genetic heterogeneity of OPDM, see OPDM1 (164310).

Charcot-Marie-Tooth disease type 2FF (CMT2FF) is an autosomal dominant progressive axonal sensorimotor peripheral neuropathy characterized by early-childhood onset of difficulties walking or running due to atrophy and weakness of the lower limbs. Most patients have foot and ankle deformities, requiring surgery or walking aids. Some patients lose independent ambulation. There is also prominent involvement of the upper limbs, with weakness and atrophy of the forearm, wrist, and intrinsic hand muscles. Proximal muscle function is preserved. Affected individuals have variable distal sensory impairment. Most patients have hyporeflexia, although brisk reflexes, suggesting upper motor involvement, have been described in 1 family. Sural nerve biopsy showed abnormal myelination (Rebelo et al., 2021). For a phenotypic description and a discussion of genetic heterogeneity of axonal CMT type 2, see CMT2A (118210).

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).

Distal myopathy-5 (MPD5) is an autosomal recessive, slowly progressive muscle disorder characterized by adolescent onset of distal muscle weakness and atrophy predominantly affecting the lower limbs. Other features include facial weakness and hyporeflexia. Patients remain ambulatory even after long disease duration (summary by Park et al., 2016).

Charcot-Marie-Tooth disease type 2Z (CMT2Z) is an autosomal dominant axonal peripheral neuropathy characterized by onset, usually in the first decade, of distal lower limb muscle weakness and sensory impairment. The disorder is progressive, and affected individuals tend to develop upper limb and proximal muscle involvement in an asymmetric pattern, resulting in severe disability late in adulthood. Rare occurrence of global developmental delay with impaired intellectual development or learning difficulties has been observed. In some instances, the same mutation may result in different phenotypic manifestations (CMT2Z or DIGFAN), which highlights the clinical spectrum associated with MORC2 mutations and may render the classification of patients into one or the other disorder challenging (summary by Sevilla et al., 2016, Ando et al., 2017, Guillen Sacoto et al., 2020). For a phenotypic description and a discussion of genetic heterogeneity of axonal CMT, see CMT2A1 (118210).