[Clinical and pathological characteristics of immune mediated necrotizing myopathy]

To investigate the clinical and pathological features of immune-mediated necrotic myopathies (IMNM) with different myositis-specific antibodies (MSAs).

In the study, 104 IMNM patients who met any of the following three criteria were selected from idiopathic inflammatory myopathy patients who had MSAs results and underwent muscle biopsy from 2008 to 2018 in China-Japan Friendship Hospital: (1) Anti-signal recognition particle (SRP) antibody positive; (2) Anti-3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR) antibody positive; (3) MSAs negative and consistent with the pathological diagnostic criteria of IMNM defined by the European Neuromuscular Centre in 2004. The clinical, laboratory and muscle pathological information of the IMNM patients were retrospectively collected and compared in anti-SRP, anti-HMGCR and MSAs negative groups.

Of 104 IMNM patients, 47 patients (45.2%) were positive for anti-SRP antibody, 23 (22.1%) were positive for anti-HMGCR antibody, and 34 (32.7%) were negative for MSAs. The common symptoms of IMNM patients were muscle weakness (92.3%), elevated serum creatine kinase level (92.3%), dysphagia (33.7%) and interstitial lung diseases (ILD) (49.5%). The anti-HMGCR-positive patients were more frequent to have "V" sign (30.4% vs. 4.3% and 5.9%, P<0.01) as compared with the anti-SRP-positive and MSAs-negative patients. The incidence of ILD in the anti-SRP-positive patients was higher than that in the anti-HMGCR-positive and MSAs negative patients (64.4% vs. 34.8% and 29.0%, P<0.01). The prevalence of the patients combined with other connective tissue diseases in MSAs-negative IMNM was higher than that in the other two groups (32.4% vs. 8.5% and 4.3%, P<0.01). 93.3% of the anti-SRP-positive patients were found with antinuclear antibody positivity, higher than those of the anti-HMGCR-positive and MSAs-negative patients (93.3% vs. 36.4% and 58.8%, P<0.001). The common pathological features of IMNM were muscle fibre necrosis (94.2%), regeneration (67.3%) and phagocytosis (65.4%), overexpression of major histocompatibility complex1 on sarcolemma (78.8%), infiltration of CD4+ T cells (81.7%) and CD68+ macrophage (79.8%) and expression of membrane attack complex (MAC) (77.8%). The endomysial infiltration of CD4+ T cells and CD68+ macrophage and MAC expression on sarcolemma in the MSAs-negative group were more common than that in the anti-SRP and anti-HMGCR groups (88.2% vs. 57.4% and 60.9%, 91.2% vs. 59.1% and 38.1%, 76.5% vs. 45.5% and 42.9%, respectively, P<0.01).

There is heterogeneity in anti-SRP-positive, anti-HMGCR-positive or MSAs-negative patients. The detection of MSAs and performing of muscle biopsy are useful for distinguishing different types of IMNM.