Background: Polymyositis (PM) and dermatomyositis (DM) are infrequent diseases. Data on incidence and prevalence are scarce and conflicting. There are no such data in Latin America and in Argentina in particular.
Objectives: We undertook to examine the incidence and prevalence of PM/DM in the prepaid health maintenance organization (HMO) of our hospital, in the city of Buenos Aires.
Methods: Members of the HMO between January 1999 and June 2009 were identified from medical records of patients followed up by us at the HMO. Incident cases and prevalence were calculated at the end of the period.
Results: During the study period, 146,747 persons contributed a total of 937,902.6 person-years (mean age was 46.6 [SD, 18.4] years, and 59% were female). Ten incident cases were detected, 7 women and 3 men with a global incidence rate (IR) of 1.07 per 100,000 person-years (95% confidence interval [CI], 0.5-1.84). Three subjects had DM with an IR of 0.32 per 100,000 person-years (95% CI, 0.1-0.99), and 7 had PM with an IR of 0.75 per 100,000 person-years (95% CI, 0.35-0.16). On June 1, 2009, 17 prevalent cases were detected, with a mean age of 48.9 (SD, 17.7) years; 76% were female, representing a prevalence of 17.4 per 100,000 persons (95% CI, 10.1-27.8). Among the 17 patients with idiopathic inflammatory myopathy, 10 patients had DM, with a prevalence of 10.22 per 100,000 persons (95% CI, 4.9-18.8), and 7 had PM (prevalence, 7.2 per 100,000 persons [95% CI, 2.9-14.7]).
Conclusions: It is difficult to compare studies from different populations and using different ascertainment techniques. These first data from Latin America are in general agreement with many studies.