Long Term Outcomes in Idiopathic Inflammatory Myositis: An Observational Epidemiologic Study over 15 Years.

Background: We report a longitudinal observational cohort of idiopathic inflammatory myositis (IIM) focusing on the long-term clinical outcome and associated parameters.

Methods: IIM patients were classified as per Bohan and Peter criteria. In those with ≥ 24 months of follow-up; the treatment response, functional outcomes, and damage at last follow-up were recorded. Complete clinical response and clinical remission as defined by Oddis et al., was used to define outcomes at last follow-up.

Results: The cohort consists of 175 patients, mean age 40.9 (+12.6) years, M:F 1:3.3; and the major subsets were dermatomyositis (44.6%), overlap myositis (25.7%), antisynthetase syndrome (6.3%), polymyositis (14.3%), and juvenile DM/OM (8.6%). Ninety-four patients have followed up for 24 months or more, with the median (IQR) of 65(35,100.7) months. Of them, 74.1% and 11.8% had complete and partial clinical responses respectively at the last follow-up. In our cohort 40.2% were off-steroids and 13.8% were in clinical remission at the last follow-up. Complete clinical response was associated with better functional outcomes and lesser damage as determined by HAQ-DI of 0[OR10.9; 95%CI (3.3,160)], MRS [OR 3.2; 95%CI (1.4,7.3)] and lesser MDI [OR 1.7; 95% CI (1.1,2.7)] respectively as compared to partial response (unadjusted analysis). Baseline parameters and IIM subsets did not significantly influence the functional outcome and damage. The mortality rate in our cohort is 24/175 (13.7%), the disease-specific mortality rate being 9.1%. Large majority of deaths were early, associated with active disease.

Conclusion: We report good long-term outcomes in all major myositis subsets. Partial clinical response to treatment is associated with worse functional outcomes and damage accrual. Death occurs early in association with active disease.