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.

X-linked infantile spinal muscular atrophy (XL-SMA) is characterized by congenital hypotonia, areflexia, and evidence of degeneration and loss of anterior horn cells (i.e., lower motor neurons) in the spinal cord and brain stem. Often congenital contractures and/or fractures are present. Intellect is normal. Life span is significantly shortened because of progressive ventilatory insufficiency resulting from chest muscle involvement.

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.

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).

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).

An extremely rare subtype of autosomal recessive intermediate Charcot-Marie-Tooth (CMT) disease characterized by a CMT neuropathy associated with developmental delay, self-abusive behavior, dysmorphic features and vestibular Schwannoma. Motor nerve conduction velocities demonstrate features of both demyelinating and axonal pathology.

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.

Autosomal dominant limb-girdle muscular dystrophy is characterized by proximal and/or distal muscle weakness and atrophy. The age at onset is variable and can range from the first to the sixth decade, although later onset is less common. Most patients present with proximal muscle weakness that progresses to distal involvement, but some can present with distal impairment. The severity is variable: patients with a more severe phenotype can lose ambulation after several decades and have facial weakness with bulbar and respiratory involvement. Muscle biopsy shows dystrophic changes with protein aggregates, myofibrillar degeneration, and rimmed vacuoles (summary by Ruggieri et al., 2015). Genetic Heterogeneity of Autosomal Dominant Limb-Girdle Muscular Dystrophy Other forms of autosomal dominant LGMD include LGMDD2 (608423), previously LGMD1F, caused by mutation in the TNPO3 gene (610032) on chromosome 7q32; LGMDD3 (609115), previously LGMD1G, caused by mutation in the HNRNPDL gene (607137) on chromosome 4q21; and LGMDD4 (618129), previously LGMD1I, caused by mutation in the CAPN3 gene (114240) on chromosome 15q15. For a discussion of autosomal recessive LGMD, see 253600.

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).

Congenital myasthenic syndrome-25 (CMS25) is an autosomal recessive neuromuscular disorder characterized by hypotonia and generalized muscle weakness apparent from birth. Affected individuals have feeding difficulties and delayed motor development, usually never achieving independent ambulation. Additional variable features include eye movement abnormalities, joint contractures, and rigid spine. Pyridostigmine treatment may be partially effective (summary by Shen et al., 2017). For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

Autosomal dominant distal hereditary motor neuronopathy-13 (HMND13) is a neurologic disorder characterized by distal muscle weakness and atrophy affecting both the upper and lower limbs, resulting in difficulty walking and poor fine hand motor skills. Some patients show spasticity and hyperreflexia, mainly of the lower limbs: these features overlap with those observed in Silver syndrome, an allelic disorder. In addition, some patients with BSCL2 mutations show features of Charcot-Marie-Tooth type 2 (CMT2) with distal sensory impairment. HMND13, Silver syndrome (SPG17), and features of axonal sensorimotor peripheral neuropathy (CMT2) thus represent a phenotypic spectrum associated with heterozygous mutations in the BSCL2 gene. Individuals with the same mutation may manifest features consistent with any of those disorders; variability is even observed within the same family (summary by Van de Warrenburg et al., 2006; Luigetti et al., 2010; Choi et al., 2013). For a general phenotypic description and a discussion of genetic heterogeneity of distal HMN, see HMND1 (182960).

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

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).

Autosomal dominant distal hereditary motor neuronopathy-10 (HMND10) is a neurologic disorder of the peripheral nerves characterized clinically by length-dependent motor neuropathy primarily affecting the lower limbs. Affected individuals have onset of distal muscle weakness and atrophy in early childhood that results in walking difficulties and gait abnormalities. Some have pyramidal signs, including hyperreflexia, suggesting the involvement of upper motor neurons. Electrophysiologic studies are consistent with a neurogenic process. More variable features may include mild intellectual disability, minor gyration defects on brain imaging, foot deformities, and connective tissue defects (1 family) (Capuano et al., 2016; Iacomino et al., 2020). For a discussion of genetic heterogeneity of autosomal dominant distal HMN, see HMND1 (182960).

Autosomal dominant distal hereditary motor neuronopathy-11 (HMND11) is a peripheral axonal motor neuropathy characterized by juvenile or young-adult onset of distal limb muscle weakness and atrophy mainly affecting the lower limbs, resulting in gait instability and walking difficulties. Foot deformities may also be present. The disorder is usually slowly progressive, and patients remain ambulatory until late adulthood. Some affected individuals may have distal upper limb and hand involvement or mild distal sensory abnormalities, but motor symptoms dominate the clinical picture. Electrophysiologic studies are consistent with a length-dependent axonal motor or sensorimotor neuropathy. Seizures are not present and brain imaging is normal (Beijer et al., 2019). One reported affected individual had a marfanoid habitus and mild speech delay with learning disabilities, suggesting possible expansion of the phenotypic spectrum (Ylikallio et al., 2020). For a discussion of genetic heterogeneity of autosomal dominant distal HMN, see HMND1 (182960).