Focal cytochrome c oxidase deficiency in various neuromuscular diseases.

To determine whether focal cytochrome c oxidase (CCO) deficiency characterized by scattered fibers with absent CCO activity among normal fibers was a specific finding for mitochondrial myopathies, we studied 389 muscle biopsies from various neuromuscular diseases other than mitochondrial myopathies. Focal CCO deficiency was found in 14 biopsies: 5 of 26 patients with myotonic dystrophy, 3 of 19 with nemaline myopathy, 1 of 7 with distal myopathy with rimmed vacuole formation, 3 of 22 with limb-girdle muscular dystrophy, 1 of 9 with amyotrophic lateral sclerosis, one of 79 with Duchenne muscular dystrophy. Focal CCO deficiency is known to be a crucial finding for chronic progressive external ophthalmoplegia, but it can also be seen in a variety of other neuromuscular disorders, probably as a secondarily induced phenomenon.