封入体筋炎

Sporadic Inclusion Body Myositis

封入体筋炎は中高年に発症する特発性の筋疾患である。左右非対称の筋力低下と筋萎縮が大腿四頭筋や手指・手首屈筋に見られる。骨格筋には縁取り空胞と呼ばれる特徴的な組織変化を生じ炎症細胞浸潤を伴う。炎症性筋疾患に分類されるが，筋の「変性疾患」としての側面を持つ。ステロイドなどの免疫学的治療に反応せず，かえって増悪することもある。厚生労働省研究班による全国調査によるとわが国でも患者数が増加している。

Abstract

Sporadic inclusion body myositis (sIBM) is an intractable and progressive skeletal muscle disease of unknown etiology. Muscle biopsy typically reveals endomysial inflammation, invasion of mononuclear cells into non-necrotic fibers, and rimmed vacuoles, suggesting that inflammation and degeneration co-exist in the pathomechanism. According to a nationwide survey conducted by a research team of the Ministry of Health, Labor, and Welfare, the number of patients is increasing in Japan as well. The clinical progression shows a slow and chronic deterioration. sIBM is usually diagnosed five years after onset. Muscle weakness and atrophy in the quadriceps, wrist flexors, and finger flexors are typical neurological findings of sIBM. Dysphagia and asymmetric weakness are often found. Serum creatine kinase is usually below 2,000 IU/L. sIBM is generally refractory to current therapy, such as steroids or immunosuppressants. Understanding the pathomechanism of sIBM is crucial for developing effective therapeutic strategies.