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.