Ryanodine receptor antibodies related to severity of thymoma associated myasthenia gravis.

Ryanodine receptor (RyR) antibodies are detected in about 50% of patients with myasthenia gravis who have a thymoma. The RyR is a calcium release channel involved in the mechanism of excitation-contraction coupling in striated muscle. In this study the severity of myasthenia gravis assessed by a five point disability score was compared between 12 patients with myasthenia gravis, a thymoma, and RyR antibodies and 10 patients with myasthenia gravis and a thymoma but without such antibodies. Symptoms of myasthenia gravis were significantly more severe in patients with RyR antibodies. The mean (SD) disability scores were 3.7(0.5) in patients with antibodies and 2.7 (0.9) in those without at peak of illness, (p = 0.01) and 3.4(1.4) v 1.6(0.7) at the end of an average observation period of five years (p = 0.002). The number of deaths due to myasthenia gravis was five of 12 RyR antibody positive patients, and none of 10 RyR antibody negative patients (p = 0.04). RyR antibody levels correlated positively with severity of myasthenia gravis. The presence of circulating RyR antibodies seems to be associated with a severe form of thymoma associated myasthenia gravis.