[Polymyositis followed by myasthenia gravis]

A 63-year-old woman was admitted to the hospital because of swelling of both forearms and muscle weakness in the limbs. Laboratory examination revealed abnormally high levels of serum creatine kinase, glutamic-oxaloacetic transaminase, glutamic-pyruvic transaminase, and lactate dehydrogenase. Polymyositis was diagnosed, and prednisolone was administered. Her condition had been well controlled on prednisolone until several months before admission, when bilateral blepharoptosis and diplopia developed. An edrophonium test was positive: muscle weakness was transiently alleviated. High titers of anti-acetylcholine receptor antibodies were found. A chest CT scan showed a mass in the anterior part of the mediastinum. We diagnosed myasthenia gravis associated with thymoma. After an extended thymectomy, the patient's condition improved. When muscle weakness recurs after remission of polymyositis, myasthenia gravis should be considered.