A case of thymoma-associated myasthenia gravis accompanied with myositis showing the clusters of histiocyte along the fascicles in perimysium.

Patients with myasthenia gravis (MG) often have other autoimmune disorders. However, the coexistence of MG and myositis is rare. Here, we report a case of a 77-year-old woman who developed mild fatigable muscle weakness and diplopia in 3 months. Serum creatine kinase was elevated to 1385 IU/L. Antibodies to acetylcholine receptor (AChR), titin and voltage-gated potassium channel 1.4 (Kv 1.4) were all positive while all tested myositis-specific autoantibodies were negative. Standard needle electromyography showed fibrillation potential and early recruitment of motor units. The repetitive nerve stimulations were consistent with a disorder of the neuromuscular junction. Muscle biopsy showed that the clusters of histiocyte were present along the fascicles in perimysium and some of them invaded into endomysium.