Mitochondrial Myopathy due to Complex III Deficiency With Normal Reducible Cytochrome b Concentration

Defects of the respiratory chain of muscle mitochondria have been described in increasing number in recent years; such defects appear to comprise a large proportion of the biochemically defined "mitochondrial myopathies." 1,2 Abnormalities of complexes 1, III, and IV have been identified by studies of oxidative phosphorylation, spectra of reduced-minus-oxidized cytochromes, and enzyme assays. The clinical picture may be confined to skeletal muscle or may involve other tissues, especially the central nervous system ("mitochondrial encephalomyopathy"). Of the eight described patients with complex III deficiency, five had only muscle symptoms 1,3-5 and three suffered from encephalomyopathy. 6,7 Biochemically, five patients showed a lack of reducible cytochrome b in the spectra, 3,5-7 one had an incomplete defect of cytochrome b and lack of cytochrome c 1 , 4 and two had normal cytochrome spectra 1 .