Long-term outcomes of idiopathic inflammatory myopathies: a large-scale longitudinal cohort study

To investigate mortality, patient-reported outcomes (PROs), and drug-free remission (DFR) in a large well-characterised cohort of idiopathic inflammatory myopathies (IIMs).

This study retrospectively enrolled 1854 patients with IIMs. Follow-up lasted up to 20 years. Mortality was analysed using the standardised mortality ratio (SMR) and Kaplan-Meier survival analysis. PROs and DFR rates were examined in the survivors at the end of follow-up.

Of 1854 patients, 348 (18.8 %) died during follow-up, with an overall SMR of 6.82 (95 % confidence interval [CI] 6.11-7.54). Subgroup analysis revealed the highest SMRs in dermatomyositis (DM), followed by antisynthetase syndrome (ASS), and immune-mediated necrotising myopathy, while SMRs in patients with polymyositis indicated no significant mortality difference from general population. Patients with anti-MDA5-positive DM exhibited higher SMRs than those with other IIM serotypes. Respiratory failure was the leading cause of death among patients with IIMs. Patients with DM had the lowest survival rates within the initial nine years of disease duration, whereas patients with ASS exhibited significantly reduced survival after nine years. At the end of follow-up, 17.1 % of patients achieved DFR (cumulative 3-, 5-, and 10-year DFR rates of 6.1 %, 14.9 %, and 29.3 %, respectively). Patients with DM presented with better PROs and higher DFR rates than those with other IIM subtypes.

Our data indicated increased mortality in patients with IIM compared with the general population and provided an important foundational understanding of IIMs. These findings emphasise the heterogeneity in the long-term outcomes across IIM subtypes, DM's acute nature, and ASS's progressive course.