Characteristics and outcomes of overlap myositis: a comparative multigroup cohort study in adults from the MyoCite cohort.

Overlap myositis (OM), an important subset of idiopathic inflammatory myopathies (IIM), is being increasingly recognized with wider myositis-specific autoantibody (MSA) testing. We studied the differences in clinical characteristics and long-term outcomes of OM with Dermatomyositis (DM), Polymyositis (PM), anti-synthetase syndrome (ASSD), and Cancer-associated IIM (CAM). Data from the MyoCite registry (Dec2017-May2020), a prospective dataset of IIM was extracted for the clinical profile, and MSAs, immunosuppressants received, disease activity (relapses and incomplete response), and treatment-related (drugs ADRs and infections) adverse events (DRAE and TRAE) were collected and analyzed between groups. Of 214 adults (58-OM,89-DM,27-ASSD,33-PM,7-CAM), OM had a greater female preponderance (13.5:1). Raynaud's and sclerodactyly were the prime distinguishing features of OM. OM could be distinguished from PM by frequent arthritis (OR-3.2) and infrequent dysphagia (OR-0.17); DM with greater nephritis (OR-20), infrequent dysphagia (OR-0.24) and rashes (OR-0.02); and ASSD by infrequent ILD (OR-0.07), and mechanic's hand (OR-0.05). 50% fulfilled the classification criteria for ASSD in the absence of MSA testing. ANA was positive more often (PM/DM: OR-6.7) and anti-Ro52 (OR-4.5) frequent in OM. Baseline serum creatinine and acute phase reactants were higher. OM received lower glucocorticoids (0 mg/kg, p < 0.001). Overall, 90% and 84% of OM at 12 and 24 months, respectively, achieved remission, with similar DRAE and TRAE as other IIM subsets. OM can be misdiagnosed as ASSD in the absence of MSA testing. Raynaud's, sclerodactyly, and a positive ANA may identify OM and prevent overtreatment.