We report biochemical, immunological, and morphological findings in a patient with fatal Kearns-Sayre syndrome. Histochemical and biochemical findings from muscle biopsy specimens obtained 7 years apart documented the disease's evolution from a mild mitochondrial disorder affecting a small proportion of muscle fibers to a severe disorder affecting a large proportion of muscle fibers. Cytochrome c oxidase activity in muscle declined profoundly as the disease progressed, although the level of enzyme protein was normal, as shown by immunochemical techniques. Other organs were severely affected by the disease. Examination of postmortem tissue showed spongiosis in the frontal cortex, diffuse loss of Purkinje cells in the cerebellum, liver steatosis, and heart fibrosis with mitochondrial abnormalities. Cytochrome c oxidase activity was only slightly reduced in these organs.
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