Polymyositis with atypical pathological features associated with thymic carcinoma

A 66-year-old man was admitted for progressive proximal weakness and myalgia in his shoulder girdles without skin lesions. A muscle biopsy showed infiltration of inflammatory cells, degeneration of muscle fibers, and perifascicular atrophy. Remarkably, MHC-I antigen was expressed in the muscle membrane and most of the inflammatory cells were CD8-positive, suggesting that class-I antigen-dependent cytotoxic T-cells played a crucial role in the muscle pathology, which supported a diagnosis of polymyositis rather than dermatomyositis. Magnetic resonance imaging of his chest revealed two mediastinal tumors, which were extirpated and diagnosed as thymic carcinoma. The muscle weakness was completely recovered after the operation and subsequent administration of oral prednisolone. Postoperative clinical improvement and decline of serum creatinine kinase suggested a paraneoplastic nature of the polymyositis in this patient.