Dermatomyositis incidence, prevalence higher than previously reported
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Using the most recent EULAR/American College of Rheumatology criteria, researchers at the Mayo Clinic and the Karolinska Institute determined that the incidence and prevalence of dermatomyositis are higher than previously reported.
“This study was essential because dermatomyositis is an understudied yet devastating disease,” Vanessa L. Kronzer, MD, MSCI, of the Mayo Clinic, in Rochester, Minnesota, told Healio Rheumatology. “In order to better predict, diagnose and treat it, we need to understand it, starting with epidemiology. No previous population-based studies had calculated dermatomyositis incidence, prevalence, or mortality using the new EULAR/ACR 2017 dermatomyositis definition. The Rochester Epidemiology Project (REP) is a great resource we have here at Mayo to do that.”
To examine the incidence, prevalence and mortality of dermatomyositis based on the EULAR/ACR criteria, Kronzer and colleagues conducted a retrospective, population-based cohort study within the Rochester Epidemiology Project. According to the researchers, the project is a longitudinal cohort of more than 500,000 people who have lived and received care in Olmstead County, Minnesota, from 1966 to the present day. As part of the project, death data are collected from electronic health records, state death certificates and the National Death Index.
For their study, Kronzer and colleagues included all patients with incident diagnosed dermatomyositis between Jan. 1, 1995, and Dec. 31, 2019. The researchers manually reviewed all individuals with at least one code for dermatomyositis or polymyositis to determine if they met the EULAR/ACR criteria, a subspecialty physician diagnosis, and/or the Bohan & Peter criteria.
Incidence and prevalence estimates were calculated for Jan. 1, 2015, and adjusted based on age and sex figures in the U.S. non-Hispanic white population in 2000. In addition, they compared observed vs. expected mortality, using standardized mortality ratios, adjusting for age, sex and year.
According to the researchers, who published their findings in Arthritis Care & Research, there were 40 cases of verified dermatomyositis, with 29 incident cases in Olmsted County from 1995 to 2019. Mean age was 57 years, 90% were female, and 41% demonstrated clinically amyopathic disease. The median follow-up time was 8.2 years.
The overall adjusted incidence of dermatomyositis was 1.1 (95% CI, 0.7-1.5) per 100,000 person-years. This estimate “falls on the upper end of previous reports,” the researchers wrote, which have ranged from 0.1 to 1.5 per 100,000 person-years. In addition, the prevalence was 13 (95% CI, 6-19) per 100,000, compared with previous reports that have ranged from 1.2 to 9.2 per 100,000.
Meanwhile, the standardized mortality ratio was significantly elevated among myopathic dermatomyositis cases (3.1; 95% CI, 1.1-6.8) but not in clinically amyopathic cases (1.1; 95% CI, 0.2-3.3).
The positive predictive value of two or more dermatomyositis codes was 40/82, or 49%.
“The key takeaways are first that dermatomyositis incidence and prevalence are rare but higher than previously reported, perhaps because we captured more amyopathic — skin-only — cases, or perhaps because it’s becoming more common over time,” Kronzer said. “Second, mortality of people with dermatomyositis was over three-fold greater than peers, though this seemed to be true mainly for those with muscle involvement.”
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