Clinical and Pathological Features of Immune-Mediated Necrotising Myopathies in a Single-Centre Muscle Biopsy Cohort

Abstract Objective Immune-mediated necrotising myopathy (IMNM) is a recently entitled novel subset of idiopathic inflammatory myopathies (IIM) characterized by significant elevated creatine kinase (CK) level, muscle weakness and predominant muscle fibre necrosis in muscle biopsy. This study aimed to investigate the clinical and pathological characteristics of patients with IMNM in our single-centre muscle biopsy cohort. Methods A total of 860 patients who had muscle biopsy reports in our centre from May 2008 to December 2017 were enrolled in this study. IMNM was diagnosed in according with 2018 European Neuromuscular Centre (ENMC) clinicopathological diagnostic criteria for IMNM. Results The muscle biopsy cohort consisted of 531 patients with IIM (61.7%), 253 patients with non-IIM (29.4%), and 76 undiagnosed patients (8.8%). Among IIM patients, polymyositis (PM), dermatomyositis(DM), amyopathic dermatomyositis, juvenile DM, and inclusion body myositis were 182(21.2%), 236(27.4%), 83(9.7%), 18(2.1%) and 3(0.3%), respectively. In PM subgroup, 59 patients met serological and pathological characteristics of IMNM according to 2018 ENMC criteria including 29 anti-SRP-positive patients,10 anti-HMGCR-positive patients and 20 MSA-negative patients. Limb girdle muscular dystrophy (LGMD) 2B and lipid storage myopathy (LSM) were 29 and 16 respectively, which present similar manifestations of IMNM with elevated CK levels and muscle weakness among non-IIM group. IMNM patients had older age of onset (mean: 42.25 vs 21.66 and 24.56, p<0.0001), shorter duration of diseases (mean: 22.56 vs 66.69 and 48.94, p<0.0001) and more frequent of dysphagia (33.9% vs 3.4% and 6.3%, p<0.0001) compare to patients with LGMD 2B and LSM. Muscle biopsy from IMNM patients showed frequent muscle fibre necrosis (96.6% vs 72.4% and 56.3%, p<0.0001), overexpression of MHC-I on sarcolemma (81.4% vs 37.9% and 12.9%, p<0.0001) and CD4+ T cell endomysial infiltration (89.9% vs 53.6% and 50%, p<0.0001) compared with LGMD 2B and LSM patients. Conclusions It is easy to distinguish IMNM from other subtype of IIM according to clinical symptoms and MSAs profiles. However, distinguishing IMNM from disorders clinically similar non-IIM need to combine with clinical, serological and pathological features.