封入体筋炎（inclusion body myositis:IBM）は，核内あるいは細胞質内に好酸性の封入体様構造物を認める炎症性筋疾患である．比較的高齡者に多く，治療抵抗性で緩徐に進行する．筋力低下は初期から遠位筋も罹患することもあり，血清CK上昇は軽度である．筋病理像は，炎症細胞浸澗を認めるが軽度であり，縁取り空胞（rimmed vacuoles）をもつ変性筋線維を認めるのが特徴である．IBMは炎症性変化と同時に筋の変性を認めるため，他の炎症性筋疾患とは異なる病態をもつと考えられている．
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