Diagnostic yield of muscle biopsy in patients with clinical evidence of mitochondrial cytopathy

We retrospectively reviewed 118 muscle biopsy, specimens from 113 patients with clinical and/or biochemical evidence of mitochondrial cytopathy. Light microscopic evaluation revealed histologic abnormalities in 65 specimens. The most common histologic findings included angular atrophic esterase-posi tive muscle fibers, type II muscle atrophy, regenerating muscle fibers, and scattered cytochrome-oxidase deficient fibers. Ragged red fibers were note d in 3 specimens on a Gomori trichrome stain. Electron microscopic evaluati on was performed in 113 muscle specimens, and in 34, no abnormalities were identified. Increased numbers of mitochondria, particularly in the subsarco lemmal region, were identified in 54 specimens. Increased mitochondrial siz e was seen in 8 specimens and paracrystalline mitochondrial inclusions in 3 . Other ultrastructural findings included focally increased glycogen deposi tion, focal Z-band streaming, and focally increased lipid accumulation. For 39 cases, concomitant skin biopsy specimens were available; abnormalities were identified by, electron microscopy, in 12. The majority of biopsy spec imens demonstrated some light or electron microscopic, abnormality. Specifi c histologic findings suggestive of mitochondrial abnormalities (partial cy tochrome oxidase deficiency, ragged red,fibers) were noted in a minority of cases. Ultrastructural evidence of mitochondrial abnormalities was noted i n the majority of cases.