Infections and respiratory tract diseases are linked to a greater risk for idiopathic inflammatory myopathy (IIM), and this finding suggests that setting off the immune system may occur outside of skeletal muscle.Findings were reported online in the Annals of Rheumatoid Disease.
“We believe an early time window is relevant for disease pathogenesis, as immunomodulation preceding development of clinically manifested disease could be initiated years before disease onset,” the authors wrote, “and therefore may be triggered by exposures occurring long before disease diagnosis. As has been demonstrated in other rheumatic diseases.”
The goal of this study was to explore the association between infection or respiratory tract disease and future risk of developing IIM, a group of rare autoimmune disorders the hallmarks of which are weakness and inflammation in skeletal muscles.
Case reports and series have revealed a higher frequency of infections before IIM, but what is lacking are population-based studies exploring infection as a risk factor for IIM.
To perform this population-based, case-control study, Swedish researchers used nationwide registers. The study included 957 newly diagnosed IIM cases in the community and 9476 matched control patients, and took place between 2002 and 2011.
Researchers determined respiratory tract disease and infections from the time before the IIM diagnosis. They then used conditional logistic regression models to calculate odds ratios and 95% CI and performed sensitivity analyses by changing the exposure definition, adjusting for prior healthcare use and excluding people with disease of connective tissue, IIM lung phenotype, or cancer associated with IIM.
Results showed that having had an infection before was more common in IIM cases compared with in control patients (13% vs 9%) and was associated with an increased risk for IIM (odds ratio, 1.5; 95% CI, 1.2-1.9). Skin infections were not associated with an increased risk for IIM, but gastrointestinal and respiratory tract infections were.
Having had a respiratory tract infection before IIM was present in 10% of cases and 4% of controls (odds ratio, 2.3; 95% CI, 1.8-3.0). Both lower and upper respiratory tract infections were linked to greater risk for development of IIM. Changes in definitions of outcome and exposure did not change the results very much.
“Our findings do not provide evidence of a protective effect of infections which has been suggested for autoimmune disease and allergies through the hygiene hypothesis,” the authors wrote. “Rather our findings imply that infections in the gastrointestinal and respiratory tracts can increase the risk of IIM.”
In terms of study limitations, the authors could not rule out that observed associations were caused by residual confounding.
“Smoking has been suggested as a risk factor for a subgroup of IIM with anti-Jo-1 autoantibodies,” they wrote, “and is closely linked to respiratory conditions like [chronic obstructive pulmonary disease], asthma as well as upper respiratory conditions like chronic rhinitis.” However, they added that they do not believe smoking would fully explain the results.
Svensson J, Homqvist M, Lundberg I and Arkema E. Infections and respiratory tract disease as risk factors for idiopathic inflammatory myopathies: a population-based case-control study [published online August 30, 2017]. Ann Rheum Dis. doi: 10.1136/annrheumdis-2017-2111174