筋炎・壊死性ミオパチーの筋病理—筋炎と筋ジストロフィーは病理で鑑別できるか

Clinicopathological Features of Myositis and Necrotizing Myopathy: How to Distinguish between Myositis and Muscular Dystrophy on Muscle Pathology

特発性炎症性筋疾患は，近年，特に脳神経内科分野を中心に，病理所見および血清学的所見を重視し，皮膚筋炎，抗合成酵素症候群，封入体筋炎，免疫介在性壊死性ミオパチーの4疾患を主なサブタイプとする分類が用いられるようになっている。その中で免疫介在性壊死性ミオパチーは，特に小児例や慢性に経過する例で，筋ジストロフィーとの鑑別が問題となる。しかし，適切な免疫染色用いた筋病理診断を臨床情報と組み合わせることで，鑑別が可能である。本稿では病理所見を中心に筋ジストロフィーと鑑別が必要な特発性炎症性筋疾患について概説する。

Abstract

In the field of neurology, idiopathic inflammatory myopathy has been classified into four sub-categories, namely, dermatomyositis, anti-synthetase syndrome, inclusion body myositis, and immune-mediated necrotizing myopathy (IMNM), based upon histological and serological findings. Among them, IMNM has features similar to muscular dystrophy, and it may thus be difficult to differentiate between these two conditions, not only clinically but also pathologically, especially in chronic cases and pediatric patients. This is partly because the main pathological feature of both IMNM and muscular dystrophy is myofiber necrosis and regeneration. Furthermore, IMNM patients with anti-SRP antibodies tend to have more prominent muscle atrophy, especially in the shoulder girdle, which mimics the muscle involvement pattern in facioscapulohumeral muscular dystrophy. Some IMNM patients with anti-HMGCR antibodies have onset in their childhood or even in infancy, and may be misdiagnosed with muscular dystrophy. On the other hand, some muscular dystrophies have been reported to show more prominent lymphocyte infiltration than others, which may also mislead muscle pathologists. Nevertheless, these conditions can be distinguished using the relevant histological evaluations, including muscle immunohistochemistry for the MHC and C5b-9 antigens, in addition to appropriate clinical and laboratory examinations including muscle MRI and autoantibody testing.