2023 年 40 巻 3 号 p. 335-339
Historically, the diagnosis of myositis with skin rash fell under the classification of dermatomyositis, while myositis without skin rash were classified as polymyositis. However, with advances in pathology, autoimmune myositis is now classified into four categories : inclusion body myositis (IBM), immune–mediated necrotizing myopathy (IMNM), dermatomyositis (DM), and anti–synthetase syndrome (ASS). All four exhibit their own characteristic muscle pathology findings. The pathological features of IBM are non–necrotic fibers surrounded and invaded by CD8+ T cells and the expression of both major histocompatibility complex (MHC) class I and MHC class II. In addition to the inflammatory infiltrates, fibers with rimmed vacuoles are seen. Immunohistochemical staining reflects the pathological aggregate of TDP–43 and p62. In IMNM, muscle pathology shows many necrotic and regenerating fibers. On the other hand, lymphocytic infiltration is very mild. In chronic cases, endomysial fibrosis is increased, making the distinction between IMNM and muscular dystrophy difficult. The expression of MHC class I is relatively weak but class II is usually not observed. The circumferential deposit of membrane attack complex (MAC) on the sarcolemma is also observed, which suggests that the complement activation pathway is related to the mechanism of the disease. In addition, p62 immunohistochemical staining shows a diffuse expression on the sarcoplasm, reflecting chaperone–assisted selective autophagy. The muscle pathology of dermatomyositis is characterized by the presence of punched–out vacuoles and perifascicular atrophy. Atrophic muscle fibers show dark staining of NADH–TR and decreased cytochrome c oxidase activity. Microinfarction within the fascicles is sometimes seen. The intensity of MHC class I expression is sometimes increased on the edge of the fascicles. The presence of MAC deposits on the endomysial capillaries is a specific feature. MxA expression is a highly sensitive and specific marker for dermatomyositis and is thought to be related to interferon type I. In ASS, necrotic and regenerating fibers are mainly seen at the edge of the fascicles (perifascicular necrosis). Fragmentation of the perimysial connective tissue and alkaline phosphatase enzymatic activity in the perimysium are also seen. An increase in both MHC class I and II expression have been observed, especially on the fibers located at the edge of the fascicles. Understanding the classification of myositis is important because of its impact in the selection of the treatment options that are appropriate for each condition.
