Background/objective Patients with polymyositis (PM) and dermatomyositis (DM) may have an increased risk of venous thromboembolism (VTE); however, no general population data are available to date. The purpose of this study was to estimate the future risk and time trends of new VTE (deep venous thrombosis (DVT) or pulmonary embolism (PE)) in individuals with incident PM/DM at the general population level.

Methods We assembled a retrospective cohort of all patients with incident PM/DM in British Columbia and a corresponding comparison cohort of up to 10 age-matched, sex-matched and entry-time-matched individuals from the general population. We calculated incidence rate ratios (IRR) for VTE, DVT and PE and stratified by disease duration. We calculated HRs adjusting for relevant confounders.

Results Among 752 cases with inflammatory myopathies, 443 had PM (58% female, mean age 60 years) and 355 had DM (65% female, mean age 56 years); 46 subjects developed both diseases. The corresponding IRRs (95% CI) for VTE, DVT and PE in PM were 8.14 (4.62 to 13.99), 6.16 (2.50 to 13.92) and 9.42 (4.59 to 18.70), respectively. Overall, the highest IRRs for VTE, DVT and PE were observed in the first year after PM diagnosis (25.25, 9.19 and 38.74, respectively). Fully adjusted HRs for VTE, DVT and PE remained statistically significant (7.0 (3.34 to 14.64), 6.16 (2.07 to 18.35), 7.23 (2.86 to 18.29), respectively). Similar trends were seen in DM.

Conclusions These findings provide the first general population-based evidence that patients with PM/DM have an increased risk of VTE. Increased vigilance of this serious but preventable complication is recommended.