New clinical entity of myasthenia gravis with autoimmune targets of heart and skeletal muscles

Since previous case reports have shown inflammatory myopathies in a few patients with myasthenia gravis (MG), heart and skeletal muscles are speculated to be autoimmune targets in MG. We screened to investigate the clinical, histological and immunological features of MG patients, who also developed myocarditis and/or myositis at 5 Japanese hospitals. Of 924 MG patients, eight (0.9%) had inflammatory myopathies. The onset age of MG was 55.3 ± 10.3 years. All patients showed severe symptoms with bulbar involvement accompanied by myasthenic crisis in 5 and invasive thymoma in 4. Myocarditis was found in 3 patients and myositis in 6. Myocarditis, developing 13-211 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 one of these anti-striational autoantibodies (those to titin, ryanodine receptor, muscular voltage-gated potassium channel, Kv l.4). Immunomodulatory therapy was required in all patients and was effective for both MG and inflammatory myopathies, except for one autopsy case. Heart and skeletal muscles are autoimmune targets in some MG patients. This autoimmunity has a broad clinical spectrum with anti-striational autoantibodies.