Rheumatoid arthritis (RA) is an autoimmune disease characterized with symmetric synovitis and occasional extra articular involvement; also, some neurologic disorders can be observed during the disease course. Myasthenia gravis (MG) is one of them and it is an autoimmune disease of neuromuscular junction. It is caused by autoantibodies against neuromuscular junction proteins: the nicotinic acetylcholine receptor (AChR) and the muscle specific tyrosine kinase (MuSK). Very few studies have reported the associated autoimmune disorders in MuSK-MG. Here, we present the first patient who has MuSK antibody-positive MG gravis and rheumatoid arthritis. A 53-year-old woman with RA presented with fatigue and fluctuating proximal muscle weakness. Her electroneurophysiological investigation resembled MG. Her AchR antibody level was normal but MuSK antibodies were high. After the acute treatment with plasmapheresis which lead to complete recovery in myasthenic symptoms, she is following with mycophenolate mofetil. Concomitant autoimmune disorders are common in the population. MG should be considered in patients with an autoimmune disorder and developing new neuromuscular weakness.

中文翻译：

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抗 MuSK 抗体阳性重症肌无力与类风湿性关节炎共存

类风湿性关节炎 (RA) 是一种自身免疫性疾病，其特征是对称性滑膜炎和偶发的关节外受累；此外，在病程中可以观察到一些神经系统疾病。重症肌无力（MG）就是其中之一，它是一种神经肌肉接头的自身免疫性疾病。它是由针对神经肌肉接头蛋白的自身抗体引起的：烟碱乙酰胆碱受体 (AChR) 和肌肉特异性酪氨酸激酶 (MuSK)。很少有研究报告 MuSK-MG 中相关的自身免疫性疾病。在这里，我们介绍了第一位患有 MuSK 抗体阳性重症肌无力和类风湿性关节炎的患者。一名患有 RA 的 53 岁女性因疲劳和波动性近端肌肉无力就诊。她的神经电生理检查类似于 MG。她的 AchR 抗体水平正常，但 MuSK 抗体水平很高。在用血浆置换术进行急性治疗后，肌无力症状完全恢复，她继续服用吗替麦考酚酯。伴随的自身免疫性疾病在人群中很常见。患有自身免疫性疾病并出现新的神经肌肉无力的患者应考虑 MG。