Objectives To investigate the association between infection or respiratory tract disease and future risk of developing idiopathic inflammatory myopathy (IIM).
Methods A case–control study was performed using Swedish nationwide registers. Adults with newly diagnosed IIM were identified (2002–2011) from the National Patient Register (NPR) and the Swedish Rheumatology Register (n=957). Controls were matched by age, sex and place of residence (n=9476). Outpatient visits and hospitalisations preceding IIM diagnosis indicating infection or respiratory disease were identified from NPR. Conditional logistic regression models were used to calculate OR and 95% CI. Sensitivity analyses were performed by varying the exposure definition, adjusting for previous healthcare consumption and excluding individuals with connective tissue disease, IIM lung phenotype or IIM-associated cancer.
Results Preceding infections were more common in IIM cases compared with controls (13% vs 9%) and were associated with an increased risk of IIM (OR 1.5, 95% CI 1.2 to 1.9). Gastrointestinal and respiratory tract infections were associated with an increased risk of IIM while cutaneous infections were not.
Preceding respiratory tract disease was present in 10% of IIM cases and 4% of controls (OR 2.3, 95% CI 1.8 to 3.0). Both upper and lower respiratory tract diseases were associated with an increased risk of IIM.
Variations in exposure and outcome definitions did not greatly affect the results.
Conclusions Infections and respiratory tract diseases are associated with an increased risk of IIM which suggests that the triggering of the immune system may take place outside the skeletal muscle.
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Contributors Planning of the study: JS, EA, MH, IL. Analyses: JS. Interpretation of results: JS, EA, MH, IL. Writing: JS, EA, MH, IL.
Funding This study was supported by grants from the Swedish Society of Medicine, Swedish Research Council K2014-52X-14045-14-3, the Swedish Rheumatism Association, King Gustaf V 80 Year Foundation, Konung Gustaf V:s och Drottning Victorias Frimurarstiftelse, Funds at the Karolinska Institutet (KID) and through the regional agreement on medical training and clinical research (ALF) between Stockholm County Council and Karolinska Institutet.
Competing interests IL has received honoraria from Bristol Myers Squibb and is currently receiving a research grant from Bristol Myers Squibb and from Astra Zeneca.
Ethics approval Ethics Committee at Karolinska Institutet.