American researchers analysed the prevalence and typology of myositis in the United States over the period 1986-2011 from a federal patient registry. They were particularly interested in the geographical distribution of cases among the 1,247 patients who agreed to participate in the survey:
- the study cohort consisted of 464 cases of dermatomyositis, 358 cases of polymyositis and 318 cases of inclusion myositis;
- the geographical distribution was uneven with a higher prevalence in the North-East and North-West;
- there was a correlation between myositis and home proximity to major traffic routes, with pollution possibly being an explanatory variable.
Although not free of bias, these data constitute an original epidemiological approach full of lessons.