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).

Monomelic amyotrophy, also known as Hirayama disease, is characterized by insidious onset of weakness and wasting of the muscles of the hand and forearm. It is usually unilateral, but can be bilateral. It occurs most commonly as a sporadic condition, is most common in young men, and follows a relatively benign course after a few years of progression (Nalini et al., 2004; Misra et al., 2005).

Autosomal recessive myopathy with rigid spine and distal joint contractures (MRRSDC) is characterized by onset of slowly progressive muscle weakness in the first or second decades of life. There is initial involvement of the proximal lower limbs, followed by distal upper and lower limb muscle weakness and atrophy. Other features include joint contractures, rigid spine, and restricted pulmonary function; some patients may have mild cardiac involvement (summary by Kayman-Kurekci et al., 2014).

Mitochondrial form of axonal Charcot-Marie-Tooth disease-1 (CMTMA1) is inherited only through the maternal line. The disorder is characterized by onset of distal muscle weakness and atrophy mainly affecting the lower limbs and resulting in difficulty walking in the second decade of life, although both earlier and later onset can occur. Upper limb involvement often develops with time, and affected individuals have weakness and atrophy of the intrinsic hand muscles. Other features may include distal sensory impairment, foot deformities, scoliosis, hypo- or hyperreflexia, spastic paraparesis, and neurogenic bladder. Electrophysiologic studies are compatible with an axonal sensorimotor peripheral neuropathy, and muscle and nerve biopsy show evidence of mitochondrial dysfunction with decreased activities of respiratory complexes, mtDNA deletions, and mitochondrial hyperplasia (summary by Fay et al., 2020).

Childhood-onset biotin-responsive peripheral motor neuropathy (COMNB) is an autosomal recessive disorder characterized predominantly by the onset of distal muscle weakness and atrophy late in the first decade of life. The disorder predominantly affects the upper limbs and hands, resulting in difficulties with fine motor skills. Some patients may have lower limb involvement, resulting in gait difficulties. Electrophysiologic studies and muscle biopsy are consistent with chronic denervation with axonal and demyelinating features. Rare patients may have additional neurologic signs, including spasticity, ataxia, and cerebellar signs. Sensation is intact, and patients have normal cognitive development. Treatment with biotin, pantothenic acid, and lipoic acid may result in clinical improvement (Holling et al., 2022).