2020 年 37 巻 2 号 p. 115-122
Immune–mediated necrotizing myopathy (IMNM) is a subgroup of idiopathic inflammatory myopathies, pathologically characterized by myofiber necrosis with little lymphocytic infiltration. Autoantibodies toward signal recognition particle (SRP) and 3–hydroxy–3–methylglutaryl–coenzyme A reductase (HMGCR) are two major autoantibodies specifically associated with IMNM. Anti–SRP autoantibody–associated IMNM patients are more likely to present with severe muscle weakness and atrophy, cervical muscle weakness, dysphagia, and respiratory insufficiency ; anti–HMGCR autoantibody–associated IMNM patients show a higher proportion of statin use, especially in elderly patients. It is occasionally challenging to distinguish from non–inflammatory myopathies such as muscular dystrophy without autoantibody testing when patients show a relatively slowly progressive course. RNA immunoprecipitation is the original detection method for anti–SRP antibodies, but it requires complicated techniques and is not suitable for clinical use. Therefore, immunoassays using recombinant 54kDa subunit protein of the SRP complex, e.g. immunodot assay and enzyme–linked immunosorbent assay, are commonly used in clinical settings. However, it should be kept in mind that false–negative results can occur in the assays because anti–SRP antibodies occasionally recognize epitopes other than the 54kDa subunit. The pathomechanism has been regarded to involve complement–mediated immunity. In light of recent advances in the pathogenesis, complement–targeting therapy for refractory patients is being discussed.