Epidemiology of adult idiopathic inflammatory myopathies in a U.S. managed care plan.

Introduction: Idiopathic inflammatory myopathies have a reported incidence of 0.1 to 1 per 100,000 person-years and prevalence of 0.55 to 6 per 100,000 in the United States.

Methods: We retrospectively examined medical claims for adults aged ≥18 years with myositis (International Classification of Diseases, Ninth Revision, Clinical Modification codes 710.3 [dermatomyositis], 710.4 [polymyositis], and 728.81 [interstitial myositis]) from 2003 to 2008 in a large US managed care database.

Results: The incidence and prevalence cohorts comprised 1,941 and 3,112 subjects, respectively. From 2003 to 2008, the adjusted annual incidence of myositis ranged from 5.8 to 7.9 per 100,000 person-years, and the annual prevalence of myositis ranged from 14.0 to 17.4 per 100,000.

Conclusions: The incidence and prevalence of idiopathic inflammatory myopathies in the managed care plan studied was higher than previously reported in the United States. Because of the limitations inherent in claims analysis, additional research is needed to substantiate these results.