Background and objective: Epidemiologic studies of polymyositis-dermatomyositis incidence in Spain are lacking. The aim of the study was to determine the incidence rates of dermatomyositis-polymyositis and their distribution in our country.
Material and method: Observational and descriptive study. Data was obtained from hospital discharge code (CMBD) during the period 1997-2004. Sex, region and age were analysed. Raw and standardized incidence rates for each region studied were calculated as new cases/million population/year with 95% confidence interval (CI). Trend odds ratio (OR) was calculated by means of a logistic regression.
Results: Global incidence rate in polymyositis-dermatomyositis was 8.9 new cases/million population/year (CI 95%, 8.6-9.2); 3.9 (CI 95%, 3.7-4.1) for polymyositis and 4.9 (CI 95%, 4.7-5.2) for dermatomyositis. Rate differences between both diseases reached statistical significance (p < 0.001). Annual incidence rate decreased significantly across the period studied in both diseases with a trend OR of 0.95 (CI 95%, 0.93-0.97; p < 0.001) in dermatomyositis and 0.96 (CI 95%, 0.93-0.97; p < 0.001) in polymyositis. Both diseases were significantly more frequent in females (p < 0.001). The incidence rates for each region varies between 2.2 and 10.6 cases/million population/year in polymyositis and between 2.9 and 8.6 cases/million population/year in dermatomyositis.
Conclusions: Incidence of dermatomyositis-polymyositis in Spain is similar to other countries. Decreased incidence observed across the period and the higher incidence of dermatomyositis could be explained because of a better diagnosis of these entities.