Microthymoma in elderly-onset myasthenia gravis detected preoperatively.

A 77-year-old woman with a 3-month history of muscle weakness was diagnosed with elderly-onset generalized myasthenia gravis (Myasthenia Gravis Foundation of America classification IIa) based on a high serum acetylcholine receptor antibody level (25.4 nmol·L-1) and neurological findings. Computed tomography detected a small nodule (diameter 15 mm) in the anterior mediastinum, which was suspected to be a thymoma. An extended thymectomy was performed. The pathological examination revealed a 6-mm-diameter thymoma, termed a microthymoma, accompanied with a unilocular thymic cyst without capsule formation (type B2 according to the World Health Organization classification). Some fat tissue was also found within the tumor.