11PAn advanced clinico-sero-histological model to improve differentiation among idiopathic inflammatory myopathy subgroups

Idiopathic inflammatory myopathies (IIM) are a group of rare autoimmune disorders. The identification of myositis-specific autoantibodies and overlap with other connective tissue diseases suggests distinct subgroups. This study aims to establish a novel classification system for IIM, integrating clinical, serological, and histopathological profiles. This retrospective cohort study included 156 definitive IIM patients with complete data, collected between October 2019 and December 2023. Patients were classified into subgroups: dermatomyositis (DM), polymyositis (PM), inclusion body myositis (IBM), immune-mediated necrotizing myopathy (IMNM), anti-synthetase syndrome (ASS), and overlap myositis (OM). A refined classification scheme was developed using unsupervised multiple correspondence analysis and hierarchical clustering based on 58 variables. Of the 156 participants (60.9% female, median age 38 years), six clusters emerged. Cluster 1 (n=59) was mainly DM (n=45, 76%, pa=1e-05 (OR=152.7, 95%CI [33.3-700.5])), Cluster 2 (n=33) consisted of PM (n=19, 58%, pa=1e-05 (OR=13.8, 95%CI [5.5-34.9])), Cluster 3 (n=19) was primarily IBM (n=14, 74%, pa=1e-05 (OR=125.1, 95%CI [27-579.7])), Cluster 4 (n=21) was mostly OM (n=9, 43%, pa=0.0162 (OR=3.7, 95%CI [1.4-153.7])), Cluster 5 (n=16) was IMNM (n=11, 69%, pa=1e-05 (OR=41.8, 95%CI [11.4-0.01]), and Cluster 6 (n=8) was ASS (n=5, 63%, pa=1e-04 (OR=33.6, 95%CI [6.6-169.7]). K-fold cross-validation of the classification tree achieved the highest area under the curve value (0.85) at a complexity parameter (cp) of 0.02. The clinico-sero-pathological classification system identified six IIM subgroups: dermatomyositis, polymyositis, inclusion body myositis, immune-mediated necrotizing myopathy, antisynthetase syndrome, and overlap myositis, warranting external validation.