Prevalence of autoimmune inflammatory myopathy in the first nations population of Alberta, Canada

To estimate the population-based prevalence of autoimmune inflammatory myopathy (AIM) in Alberta, Canada, with a specific focus on rates in the First Nations population.

Physician billing claims and hospitalization data for the province of Alberta (1994-2007) were used to estimate the probability of having AIM (i.e., polymyositis or dermatomyositis) based on 3 case definitions. A latent class Bayesian hierarchical regression model was employed to account for the imperfect sensitivity and specificity of billing and hospitalization data in case ascertainment. We accounted for demographic factors of sex, age group, and location of residence (urban or rural) in estimating the prevalence rates within the First Nations and non-First Nations populations.

The overall prevalence of AIM was 25.0 per 100,000 persons (95% credible interval [95% CrI] 13.4-49.0) in the First Nations population and 33.8 (95% CrI 28.9-39.6) in the non-First Nations population. For both groups, prevalence was increased in women relative to men, rural women relative to urban women, and in those age >45 years.

Unlike other rheumatic diseases such as rheumatoid arthritis, systemic lupus erythematosus, and systemic sclerosis, we did not detect an increased prevalence of AIM in Alberta's First Nations population relative to the non-First Nations population. Potential limitations include coding errors, underidentification of First Nations members, and recognized differences in access to care for the First Nations population.