Epidemiologic studies on rheumatic muscle inflammation, myositis

Abstract

Observational study designs are often used in medical research. Swedish national registers have successfully been used as data sources for such observational studies in other rheumatic diseases. The Swedish health care system is tax funded and does together with the ability to link multiple national registers, allowing for longitudinal follow-up, make up a great setting for answering many research questions in a real world setting.

We identified 95 patients with idiopathic inflammatory myopathies (IIM) treated with different biologic therapies between 2000 and 2011 in study 1. All had previously been treated with prednisolone and at least one Disease modifying anti-rheumatic drug (DMARD). Rituximab was the most commonly used drug and even though large variations were seen in treatment length, no conclusions could be made regarding effectiveness.

Between 2007 and 2011 we found an average incidence rate of 11 per 1 000 000 person-years for IIM in Sweden in study 2. We observed a general increase in incidence with age and a peak was observed in the 50-79 year age groups and the highest incidence rate was in the 70-79 year age group. No difference in incidence was observed for different levels of education or population density and even though large variations were seen between Sweden´s 21 counties, no north to south gradient was observed.

In study 3 we conducted a case-control study with IIM as the outcome and respiratory diseases and infections as the exposure. Previous infections were associated with increased future risk to develop IIM, OR 1.5. The risk was elevated for both infections of the respiratory tract and gastrointestinal tract but not for infections of the skin. Respiratory diseases were less common than infections but the relative risk was higher, odds ratio (OR) 2.3, and was elevated for both upper and lower respiratory tract diseases. A dose-response relationship between number of visits indicating exposure increased the risk of IIM for both respiratory diseases and infections.

In Study 4 we used a cohort study design to address if the risk of stroke is increased following IIM diagnosis. We identified 663 newly diagnosed IIM patients and 6673 general population comparators without prior stroke or stroke-related events. We found an increased risk of both ischemic and haemorrhagic stroke (HS), hazard ratio (HR) 1.7 and 2.3 respectively. Because the number of HS were rare we did not further analyse this outcome. For IS, the rate differences were the highest in the oldest age group (≥68) and in men relative risk of IS was highest in the youngest age group (<56 years). When accounting for the competing risk of death, the HR for IS was decreased by 40% and the cumulative incidence was increased in IIM patients compared to the general population directly following diagnosis but they were similar after 10 years.