Patients with Myasthenia Gravis and Thymoma Have Anti-Skeletal Muscle and Anti-Acetylcholine Receptor Antibodies Simultaneously

The frequency and mean titer of anti-skeletal muscle (SM) antibody in myasthenic patients with thymoma (64 cases) were significantly high (frequency, 92%; mean titer, 10.1) in all stages of severity, while in myasthenic patients without thymoma (202 cases), they were significantly low (frequency, 24%; mean titer, 2.2). A strong association was found in myasthenic patients between the occurrence of anti-SM antibodies and the presence of thymoma. No correlation was found between titers of anti-SM antibody and those of anti-acetylcholine receptor (AChR) antibody (r=0.036), although myasthenic patients with thymoma had simultaneously high prevalences of anti-AChR (62/64) and anti-SM (59/64) antibodies. Most of the ocular mysthenic patients with and without thymoma had no or extremely low anti-AChR antibody titers (frequency, 25%, 16/65; mean titer, 1.6pmol/ml). Whereas, in all cases of ocular myasthenic patients with thymoma anti-SM antibodies were significantly high titers (frequency, 100%, 8/8; mean titer, 11.0). Reductions in both anti-AChR and anti-SM antibody titers were observed by the serial measurement after thy(mo)mectomy on most of the 10 myasthenic patients with thymoma.Our data on a large number of myasthenic sera reveal that coexistence of anti-SM and anti-AChR antibodies is strongly linked to myasthenia gravis associated with thymoma. The measurement of anti-SM antibodies is recommended for demonstration of thymoma in myasthenic patients before surgery.