膠原病内科医が診る筋炎

Idiopathic Inflammatory Myopathies from the Viewpoint of Rheumatologists

はじめに

　四肢・体幹の骨格筋に単核球の浸潤を認め，筋痛や筋力低下などの筋障害をきたし，原因が明らかでないものを特発性炎症性筋疾患という¹⁾。特発性炎症性筋疾患には，多発筋炎（polymyositis：PM）や皮膚筋炎（dermatomyositis：DM）のほかに封入体筋炎（inclusion body myositis）も含まれる。特発性炎症性筋疾患の中でも膠原病内科医が扱う筋炎はPMとDMが主である。

　PM/DMにおいて，個々の患者により初発症状が異なる。例えば，皮疹が主たる症状の患者は皮膚科を受診する。また，手足に力が入りくいなど筋力低下を主症状とする患者は神経内科を受診する。さらに，発熱，関節痛，咳など全身に多彩な症状を認める場合や抗核抗体が陽性の場合には膠原病内科を紹介される。このように，PM/DMの筋炎患者は，主となる自覚症状やプライマリケア医の判断により，どの科を受診するのか異なってくる。このような背景から，当然，皮膚科医，神経内科医，膠原病内科医の各科で診る筋炎患者のポピュレーションにはバイアスがかかり，各専門医でPM/DMをみる視点が異なってくる。

　本稿では，当施設でのデータもまじえて，膠原病内科を受診するPM/DM患者の臨床像の特徴，PM/DMを診断・治療を行ううえで膠原病内科医はどのような視点で診療を行っているのか，また皮膚科医や神経内科医とどのように連携をとっていくべきかについて述べていく。

Abstract

　Idiopathic inflammatory myopathies (IIMs) are a group of inflammatory muscle disorders of unknown etiology; these include polymyositis (PM), dermatomyositis (DM), and inclusion body myositis. Extra-muscular manifestations such as dermatitis, arthritis, interstitial lung disease (ILD), cardiomyopathy, and enteropathy are occasional complications in patients with PM/DM. Several myositis-specific autoantibodies (MSAs) have been discovered in IIMs; these can help predict clinical characteristics, response to treatment, and prognosis. For example, anti-aminoacyl-tRNA synthetase (ARS) antibodies, including Jo-1 antibody (Ab) and anti-melanoma differentiation-associated gene 5 (MDA-5) Ab, have been associated with the manifestation of ILD in PM and DM. Anti-MDA5 Ab-associated ILD has a 1-year survival rate of 50-60%; however, short-term prognosis is relatively good in anti-ARS Ab-associated ILD. Fatal outcome occurs remarkably often within the first 6 months of anti-MDA5 Ab-associated ILD. Therefore, intensive treatment should be administered to patients harboring anti-MDA-5 Ab or showing hyperferritinemia in ILD with DM. In addition, corticosteroid occasionally induces myopathy, which is an issue arising in PM/DM treatment. Some experts recommend combination therapy of corticosteroid and an immunosuppressive agent as a first-line treatment for myositis in PM/DM. Methotrexate and azathioprine are commonly used immunosuppressive agents for myositis in western countries. Immunosuppressive agents are steroid-sparing, serving to mitigate corticosteroid-related side effects, thus making combination therapy an effective treatment option. Preventing the progression of physical dysfunction is of prime importance to patients with PM/DM. Dermatologists, neurologists, and rheumatologists should therefore work together to care for these patients before muscular and extra-muscular involvement develop progressively and irreversibly.