Walsh & Hoyt: Background

The mitochondrial encephalomyopathies are a genetically and biochemically diverse set of disorders that are defined by structural abnormalities of mitochondria on muscle biopsy. The histologic hallmark of these disorders is the abnormal accumulation of increased numbers of enlarged mitochondria beneath the sarcolemma of affected muscle fibers. Because of their irregular appearance and their strikingly dark-red color when stained with the modified Gomori trichrome stain, these abnormal muscle fibers are called ragged-red fibers (RRF). In patients with ophthalmoplegia, RRFs are usually present in skeletal muscles. They are also found in the orbicularis oculi muscle and in the extraocular muscles of affected patients. Affected muscle fibers are irregularly distributed and often arranged in small groups.