Autoimmune Targets of Heart and Skeletal Muscles in Myasthenia Gravis

Objective: To investigate the clinical, histological, and immunological features of patients with myasthenia gravis (MG) who also developed myocarditis and/or myositis. Design: Observational and retrospective case series. Setting: Keio University, Hanamaki General Hospital, Kanazawa University, Nagasaki University, and Juntendo University. Patients: A cohort of 8 patients with MG with clinically defined inflammatory myopathies. Interventions: Clinical and histological features were described. Serological analyses included MG-related antistriational autoantibodies (those to titin, ryanodine receptor, muscular voltage-gated potassium channel Kv1.4) and myositis-specific autoantibodies. Results: Of 924 patients with MG, 8 (0.9%) had inflammatory myopathies. The mean (SD) onset age of MG was 55.3(10.3) years. All patients showed severe symptoms with bulbar involvement; 5 patients had myasthenic crisis and 4 had invasive thymoma. Myocarditis was found in 3 patients and myositis in 6. Myocarditis, developing 13 to 211 months after the MG onset, was characterized by heart failure and arrhythmias. Myositis, developing before or at the same time as MG, affected limb and paraspinal muscles. Histological findings of skeletal muscles showed CD8(+) lymphocyte infiltration. Seven patients had 1 of these antistriational autoantibodies but not myositis-specific autoantibodies. Immunomodulatory therapy was required for all patients and was effective for both MG and inflammatory myopathies, although 1 patient died. Conclusions: Heart and skeletal muscles are autoimmune targets in some patients with MG. This autoimmunity has a broad clinical spectrum with antistriational autoantibodies.