重症筋無力症治療の現状と展望―胸腺手術の観点から

Current Status and Future Prospects of Therapy for Myasthenia Gravis: Considering Thymectomy

はじめに

　重症筋無力症（myasthenia gravis：MG）の臨床における現状は，大きく変化している^1）。その理由として，①アセチルコリン受容体抗体（acetylcholine receptor antibody：AChR Ab）の測定が容易となり，診断が早期にされるとともに診断の正確さが向上した，②ステロイドによる治療が浸透するとともに，その使用方法が改善された，③免疫抑制薬（わが国においては，特にカルシニューリン阻害薬）の併用療法が広がった^2），④胸腺摘除術適応の見直しが進んでいる，⑤MG患者の年齢分布が変化している，といった原因が挙げられる。これら5つの原因はそれぞれが独立しているようで互いに関連している。本稿では，神経内科医の立場からMG治療の現状と展望について，特に胸腺手術に重点を置いてまとめてみたい。

Abstract

　The benefits of thymectomy in non-thymomatous patients with myasthenia gravis (MG) remain controversial.　The first detailed case of thymectomy in a patient with MG was reported in 1939,following which many cases were published.　In 2000,Gronseth and Barohn reported the first meta-analysis of the effectiveness of thymectomy in MG patients without thymoma.　They reviewed 28 papers systematically and reached these conclusions: (1) The benefit of thymectomy in non-thymomatous autoimmune MG has not been conclusively established,and (2) a well-designed controlled trial is essential.　Following this report,Newsom-Davis et al. designed a thymectomy trial for non-thymomatous MG patients receiving prednisone (the MGTX study).　Their study compared extended trans-sternal thymectomy (ETTX) combined with prednisone and prednisone alone groups with the aim to answer 3 questions: (1) Is the former more effective in improving myasthenic weakness? (2) Does the former require a lower total dose of prednisone,and thus decrease the likelihood of concurrent and long-term toxic effects? (3) Does the former enhance patients' quality of life by reducing adverse events and symptoms associated with the therapy?　Currently,67 centers,including our institute,are involved in this study.　In total,106 patients have been enrolled (the recruitment goal is 150).　The patients are scheduled for a 5-year follow-up.　The MGTX study will offer new information on the role of thymectomy in improving the quality of life of patients with MG.