New estimates of the incidence and prevalence of dermatomyositis (DM) in the US are higher than previously reported, with standard mortality ratios 3 times higher than those in the general population, according to study results published in Arthritis Care & Research.

Dermatomyositis is an autoimmune disease that causes a skin rash and muscle inflammation and can lead to muscle weakness. However, the incidence, prevalence, and mortality of DM using established classification criteria have not been well studied.

The current population-based cohort study was conducted to better understand the epidemiology of DM.


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New cases of DM diagnosed between January 1995 and December 2019, using International Classification of Diseases (ICD) 9 or 10 codes, were identified. Each case was manually reviewed for DM using the European Alliance of Associations for Rheumatology/American College of Rheumatology (EULAR/ACR) 2017 DM classification criteria, subspecialist physician diagnosis, and the Bohan & Peter 1975 criteria.

The researchers calculated incidence and prevalence (as of January 1, 2015) that was age- and sex-adjusted to the US, non-Hispanic, White, 2000 population. Standardized mortality ratios were calculated by comparing survival of the DM cohort with expected survival rates.

A total of 40 cases of incident DM verified by 1 of the 3 criteria were identified; 29 patients resided in Olmstead County. Of these, 26 (90%) were women and 12 (41%) had clinically amyopathic DM (a type of DM without muscle weakness). The median follow-up time was 8.2 years.

The overall adjusted incidence of DM was 1.1 per 100,000 person-years (95% CI, 0.7-1.5 per 100,000 person-years), which corresponded to 2858 new cases of DM in the US each year. The prevalence of DM was 13 per 100,000 person-years (95% CI, 6-19 per 100,000 person-years), which corresponded to 34,061 individuals in the US with DM. The standardized mortality ratio was significantly higher than expected for individuals with DM (3.1; 95% CI, 1.1-6.8), but not for those with clinically amyopathic DM (1.1; 95% CI, 0.2-3.3). The positive predictive value of 2 or more ICD 9 or 10 DM codes for validated DM was 40 of 82 (49%).

Limitations of the study included the limited geographical region that lacked racial and ethnic diversity; the reliance on clinically diagnosed DM that may have excluded mild cases of DM; potentially nonuniform documentation of DM characteristics by different providers; missing antibody data for early DM cases; a small sample size; and the exclusion of patients with juvenile DM.

The researchers concluded, “This population-based study using validated DM criteria found incidence and prevalence on the higher end of previous reports, and mortality of myopathic DM significantly elevated compared to peers. Future, larger studies of validated DM cases should examine incidence and mortality over time and causes of death to better understand disease pathogenesis and treatment.”

Disclosure: Some study authors declared affiliations with biotech, pharmaceutical, and/or device companies. Please see the original reference for a full list of disclosures.

Reference

Kronzer VL, Kimbrough BA, Crowson CS, Davis III JM, Holmqvist M, Ernste FC. Incidence, prevalence, and mortality of dermatomyositis: a population-based cohort study. Arthritis Care Res. Published online September 21, 2021. doi:10.1002/ACR.24786