RESUMEN
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.