Mitochondrial myopathy with reversible cytochrome C oxidase deficiency

Mitochondrial myopathy with reversible cytochrome C oxidase (COX) deficiency, also known as benign infantile mitochondrial myopathy or reversible infantile respiratory chain deficiency, is a rare mitochondrial disorder that primarily affects skeletal muscle in early infancy. It is caused by homozygous mutations in the MT-TE gene (encoding mitochondrial tRNA for glutamic acid) on mitochondrial DNA, or in nuclear genes such as TRMU (encoding tRNA 5-methylaminomethyl-2-thiouridylate methyltransferase), which is involved in mitochondrial tRNA modification. The condition is characterized by severe muscular hypotonia, feeding difficulties, respiratory insufficiency, and lactic acidosis presenting in the neonatal or early infantile period. Muscle biopsy typically reveals markedly reduced or absent cytochrome C oxidase (complex IV) activity in skeletal muscle fibers. The distinguishing and remarkable feature of this condition is its reversibility. Unlike most mitochondrial myopathies, affected infants who survive the critical early period — often requiring intensive supportive care including mechanical ventilation and nutritional support — show spontaneous and progressive improvement in muscle strength and respiratory function, typically by one to two years of age. COX activity in muscle gradually normalizes over time. The prognosis is generally favorable if patients receive adequate supportive care through the acute phase, though some residual mild weakness may persist. The exact mechanism underlying the spontaneous recovery is not fully understood but is thought to relate to developmental changes in mitochondrial tRNA modification or heteroplasmy shifts in the case of MT-TE mutations. There is no specific curative treatment for this condition. Management is primarily supportive and includes respiratory support, nutritional optimization, and physical therapy during the period of severe weakness. Some clinicians have used supplementation with cofactors such as coenzyme Q10 and riboflavin, though evidence for their efficacy in this specific condition is limited. Genetic counseling is important, as the inheritance pattern depends on the underlying genetic cause — mitochondrial for MT-TE mutations and autosomal recessive for TRMU mutations.

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