Classification of idiopathic inflammatory myopathies: pathology perspectives

Purpose of review Idiopathic inflammatory myopathies (IIM) are rare diseases with heterogenous clinicopathological features. In recent years, new classification systems considering various combinations of clinical, serological, and pathological information have been proposed. This review summarizes recent clinicoseropathological development in major subgroups of IIM. Recent findings Considering clinicoseropathological features, IIM are suggestively classified into four major subgroups: dermatomyositis, immune-mediated necrotizing myopathy (IMNM), antisynthetase syndrome (ASS), and inclusion body myositis (IBM). Many historically diagnosed polymyositis have been mainly reclassified as IBM, IMNM, and ASS. Different types of myositis-specific antibodies (MSA) suggest distinct clinicopathological subsets of IIM. Excluding IBM, at least one-third of the IIMs have no known associated MSA. MSA are crucial for IIM classification but can be negative. Thus, IIM should be universally classified using stepwise or integrated information on clinical, serological, and pathological findings.