Background/Purpose: Previous epidemiologic studies defined dermatomyositis (DM) using international codes of diseases (ICD) codes or Bohan & Peter 1975 criteria, and most were not population-based. Therefore, we aimed to determine the population-based incidence, prevalence, and mortality of DM, all using the new European League Against Rheumatism (EULAR)/American College of Rheumatology (ACR) 2017 DM classification criteria to define patients with DM.

Methods: This population-based cohort study included incident DM from 1/1/1995 and 12/31/2019 among all residents in a single geographical location. We used ICD codes for DM (710.3, M33.0-1, M33.9) and polymyositis (710.4, M33.2) to identify individuals with possible DM, and manually reviewed all to determine if they met criteria for DM by EULAR/ACR criteria (primary), rheumatologist/dermatologist physician diagnosis, and/or Bohan & Peter 1975 criteria. We defined index date of DM as date of EULAR/ACR 2017 criteria fulfilment. For incidence and prevalence, we age- and sex-adjusted our estimates to the United States White year 2000 population and provided 95% confidence intervals (CI). We estimated prevalence on 1/1/2015. For mortality calculations, we compared the survival of our cohort to expected survival rates from local life tables by age, sex, and year using standardized mortality ratios.

Results: We identified 40 cases of verified DM meeting at least one of the three DM criteria. Of those, 29 cases were incident DM in the study geographical location from 1995 to 2019 (mean age 57 years, 90% female). Median follow-up time was 8.2 years with interquartile range (IQR) 3.5 to 16.3 years. Of the 29, 17 (59%) had clinically myopathic DM, and 12 (41%) had clinically amyopathic DM (CADM). After age- and sex-adjusting to the US White 2000 population, the overall incidence of DM was 1.1 (95% CI 0.7 to 1.5) per 100,000 person-years (Table 1), and prevalence was 13 (95% CI 6 to 20) per 100,000 (Table 2). During the study period, 6 individuals with myopathic DM and 3 with CADM died (overall 10-year survival 69%; 95% CI 48-96%). Mortality of this DM cohort was higher than expected for their age-, sex-, and year-matched peers (Figure 1). The standardized mortality ratio was significantly elevated among the myopathic DM cases (3.1, 95% CI 1.1 to 6.8) but not CADM cases (1.1, 95% CI 0.2 to 3.3).

Conclusion: This population-based study using validated DM classification criteria found incidence and prevalence on the higher end of previous reports. Though confidence intervals were wide, mortality was significantly elevated for myopathic DM but not for CADM.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/incidence-prevalence-and-mortality-of-dermatomyositis-a-population-based-cohort/