封入体筋炎とアミロイド

Sporadic Inclusion Body Myositis and Amyloid

封入体筋炎は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。骨格筋には縁取り空胞と呼ばれる特徴的な組織変化や炎症細胞浸潤を伴うと同時にアミロイドの沈着が認められる。免疫学的治療に反応せず難治性である。経過は進行性で5～10年で車椅子生活となるが，嚥下障害や転倒・骨折に注意が必要である。わが国でも患者数が増加しており，2013年に厚生労働省の研究班により新しい診断基準が作成された。

Abstract

Sporadic inclusion body myositis (sIBM) is an intractable and progressive skeletal muscle disease of unknown etiology and without effective treatment. While the etiology is still unknown, however, genetic factors, aging, life style, and environmental factors may be involved. Muscle biopsy typically reveals endomysial inflammation, invasion of mononuclear cells into non-necrotic fibers and rimmed vacuoles, suggesting inflammation and degeneration co-exist as part of the pathomechanism. Recent studies implicate amyloid beta accumulation, defects of proteolysis, and immune system abnormalities.

The clinical course is slow with chronic worsening. Diagnosis of sIBM is usually made 5 years after onset. Muscle weakness and atrophy in the quadriceps, wrist flexor and finger flexors are the 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. Elucidation of the pathomechanism of sIBM is the most important to therapy.