RESUMEN
We report a 57-year-old male with myasthenia gravis complicated with dermatomyositis and rheumatoid arthritis without evidence of thymoma. He showed prominent muscle wasting and weakness in the four extremities and trunk in addition to swallowing disturbance. He showed intolerance to exercise on a bicycle ergometer, and muscle biopsy specimens demonstrated ragged-red fibers. An anti-acetylcholine receptor (AChR) antibody was detected in his serum but no anti-mitochondrial M2 component antibody was found. In contrast, results of immunohistochemical study indicated that his serum sample reacted to muscle mitochondria as well as AChR. These results indicate the presence of an unidentified anti-mitochondrial antibody that may be involved in the development of mitochondrial dysfunction in skeletal muscle of the present patient.