2017 年 33 巻 6 号 p. 622-626
Immune–mediated necrotizing myopathy (IMNM) is a subgroup of idiopathic inflammatory myopathies, characterized by usually subacutely progressive course, symmetrical and proximal muscle weakness and atrophy, highly elevated serum creatine kinase levels, and muscle fiber necrosis and regeneration with little or no lymphocytic inflammatory infiltration on muscle pathology. Two major autoantibodies, anti–signal recognition particle (SRP) and anti–3–hydroxy–3–methylglutaryl–coenzyme A reductase (HMGCR) antibodies, are known to be associated with IMNM. IMNM with those antibodies were initially reported in adults but are now known to be seen also in children. Anti–SRP antibodies seem to be more frequently seen in IMNM pateints than anti–HMGCR antibodies. Although anti–HMGCR necrotizing myopathy was first found in patients who were taking statins, majority of patients do not have history of statin use. Myositis associated with anti–aminoacyl tRNA synthetases (ARS) is pathologically characterized by perifascicular necrotizing myopathy with perimysial alkaline phosphatase activity. Clinically, patients typically develop triad of interstitial lung disease, mechanic's hands, and myositis, which is sometimes called anti–synthetase syndrome or anti–ARS syndrome. Of note, necrotizing muscle pathology can also be observed in a variety of other conditions, including myositis associated with anti–mitochondria M2 autoantibodies, malignancy, viral infections (HIV and hepatitis C), and connective tissue diseases. In addition, muscular dystrophy, such as dysferlinopathy or facioscapulohumeral muscular dystrophy, may also display similar muscle pathology. Not surprisingly, it can be clinically difficult to distinguish IMNM from muscular dystrophy especially in children. As IMNM is treatable by immunosuppressive therapy, it should not be misdiagnosed. Immunohistochemical analysis including MHC class I and C5b–9 may be useful as cytoplsmic upregulation of MHC class I in non–necrotic fibers and sarcolemmal deposition of membrane attack complex (C5b–9) are often observed in IMNM but not in muscular dystrophy. This review focuses on muscle pathological features of IMNM.
