Incidence of dermatomyositis and clinically amyopathic dermatomyositis: a population-based study in Olmsted County, Minnesota.

OBJECTIVES: To identify new and existing cases of dermatomyositis and its subtypes in Olmsted County, Minnesota, from 1976 through 2007, and to establish a population-based estimate of the incidence and prevalence of dermatomyositis and amyopathic dermatomyositis.
DESIGN: Retrospective population-based study.
SETTING: Community-based epidemiology project. Patients Patients with a diagnosis of dermatomyositis were identified from the Rochester Epidemiology Project. MAIN OUTCOME MEASURES: Incidence of dermatomyositis and clinically amyopathic dermatomyositis and risk of malignancy in clinically amyopathic dermatomyositis.
RESULTS: Of the 29 patients identified, 6 (21%) had the clinically amyopathic subtype of dermatomyositis and 22 (76%) were female. Overall age- and sex-adjusted incidence of dermatomyositis including all subtypes was 9.63 (95% confidence interval [CI], 6.09-13.17) per 1 million persons and 2.08 (95% CI, 0.39-3.77) per 1 million persons for clinically amyopathic dermatomyositis. Age- and sex-adjusted prevalence for all subtypes of dermatomyositis was 21.42 (95% CI, 13.07-29.77) per 100,000 persons. Eight patients (28%) had a malignant condition during the study period; the risk of malignancy (odds ratio) for classic dermatomyositis compared with clinically amyopathic dermatomyositis was 4.61 but was not statistically significant (95% CI, 0.22-96.09) (P=.44).
CONCLUSIONS: Dermatomyositis is a rare disease, and clinically amyopathic dermatomyositis represents an estimated 20% of all dermatomyositis cases. Larger population-based studies are needed to estimate the risk of malignancy associated with subtypes of dermatomyositis, particularly clinically amyopathic dermatomyositis.