medwireNews: A population-based cohort study using the 2017 EULAR/American College of Rheumatology (ACR) criteria for dermatomyositis has identified a higher incidence, prevalence, and associated mortality rate than previously reported.
“[T]hese findings provide an important, foundational understanding of this debilitating disease and highlight key targets for future research, ideally with larger populations,” comment the investigators.
They identified 161 participants of the Rochester Epidemiology Project who had at least one ICD 9 or 10 code for dermatomyositis between 1995 and 2019. Of these, 40 cases of adult-onset dermatomyositis were manually verified using the new EULAR/ACR 2017 criteria, which requires a score of 5.5 points or higher and no muscle biopsy or a score of 6.7 points or higher with a muscle biopsy, as well as the presence of at least one of three skin criteria.
After excluding individuals who either did not reside in Olmsted County in Minnesota, USA, or were diagnosed outside the study period, the researchers pinpointed 29 incident cases of dermatomyositis in the county. Fifty-nine percent of these cases were clinically myopathic disease, while the remaining 41% were clinically amyopathic dermatomyositis (CADM).
The age- and sex-adjusted overall incidence of dermatomyositis was 1.1 per 100,000 person–years, the equivalent of 2858 new cases each year in the USA, report Vanessa Kronzer (Mayo Clinic, Rochester, Minnesota, USA) and co-authors. For myopathic dermatomyositis and CADM, the corresponding incidence rates were 0.7 and 0.5 per 100,000 person–years.
The estimated prevalence of dermatomyositis at the beginning of the study (January 2015) was 13.0 cases per 100,000 person–years, after adjusting for age and sex, which translates into 34,061 cases in the USA. The prevalence of myopathic dermatomyositis and CADM was 5.8 and 7.0 per 100,000 person–years, respectively.
The team notes that “the incidence and prevalence estimates are on the higher end of what has been previously reported, which may reflect either increased capture of CADM cases or a rise in this disease over time.”
During the median follow-up of 8.2 years, nine patients with dermatomyositis died, giving a standardized mortality ratio of 2.0, which was higher than that for the reference Minnesota population. The 5- and 10-year survival rates for people with dermatomyositis were 74% and 69%, respectively, both lower than the expected survival rates for the reference population, at 95% and 89%, respectively.
The study authors highlight that relative to the reference population, the mortality rate was significantly elevated among individuals with myopathic dermatomyositis, but not those with CADM, with standardized mortality ratios of 3.1 and 1.1, respectively.
The researchers also assessed the positive predictive value of ICD codes for dermatomyositis, and found that the codes had a “poor” predictive value for identifying true dermatomyositis cases, at 44% and 49% for one and two codes, respectively.
They acknowledge several limitations of the research, such as the small number of patients and the reliance on clinical diagnoses instead of active surveillance, which may have led to an underestimation of the true incidence and prevalence of disease in the population.
Kronzer et al conclude in Arthritis Care & Research: “Future larger studies of validated [dermatomyositis] cases should examine incidence and mortality over time and causes of death to better understand disease pathogenesis and treatment.”
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Arthritis Care Res 2021; doi:10.1002/acr.24786