Mortality trends in polymyositis and dermatomyositis: A general population-based study

To evaluate mortality trends in polymyositis (PM) and dermatomyositis (DM) between January 1, 1997, and December 31, 2014. Using an administrative health database from the province of British Columbia, Canada, we identified all patients with incident PM/DM and up to 10 age-, sex-, and index date matched non-PM/DM individuals. Study cohorts for both PM and DM were divided into two subgroups based on the year of diagnosis (i.e., early cohort [1997-2005] and late cohort [2006-2014]). Mortality rates, hazard ratios (HRs), and rate differences were compared between these cohorts. Mortality rates (per 1000 person-years) in the early cohorts for PM and DM patients were higher than those in the late cohorts (for PM: 58.6 vs. 39.4; for DM: 80.6 vs. 51.3), whereas smaller improvements were observed in the comparison cohorts (for non-PM: 15.5 vs. 12.5; for non-DM: 14.1 vs. 11.5). Corresponding to these two time periods, multivariable HRs for PM were 2.4 (95% CI, 1.7 to 3.4) and 2.0 (95% CI, 1.4 to 2.9), respectively (P-value for interaction=0.62). The corresponding absolute mortality rate differences were 32.6 (95% CI, 20.8 to 44.4) and 18.6 (95% CI, 9.2 to 28.0), respectively (P-value for interaction=0.02). Similar results with higher HRs and risk differences were seen in DM. In this general population study, we found a declining excess mortality in PM or DM patients in recent years, although there was a considerable residual premature mortality gap in the late cohort.