Megaconial-type congenital muscular dystrophy (MDCMC) is an autosomal recessive disorder characterized by early-onset muscle wasting and impaired intellectual development. Some patients develop fatal cardiomyopathy. Muscle biopsy shows peculiar enlarged mitochondria that are prevalent toward the periphery of the fibers but are sparse in the center (summary by Mitsuhashi et al., 2011).

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).

Muscular dystrophy, congenital hearing loss, and ovarian insufficiency syndrome (MDHLO) is an autosomal recessive systemic disorder characterized by progressive muscle weakness, sensorineural hearing loss, and endocrine abnormalities, mainly primary amenorrhea due to ovarian insufficiency. Features of the disorder appear soon after birth, although endocrine anomalies are not noted until puberty. The severity of the phenotype is variable: some patients may lose ambulation and have significant respiratory insufficiency, whereas others retain the ability to walk (Foley et al., 2020).