Neuronal losses, gliosis, parenchymal necrosis, spongiosis, and calcifications are common. The distribution of these lesions, however, is fairly characteristic for a particular disease.

Mitochondrial myopathy is characterized by the presence of ragged-red fibers. These result from subsar-colemmal mitochondrial aggregates and are readily demonstrated in histologic sections stained with Gomori modified trichrome method. Histochemical examination shows absent cytochrome oxidase in the region of ragged-red fibers and increased succinic dehydrogenase activity. Electron microscopic study reveals paracrystal-line inclusions from precipitations of mitochondrial proteins.

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