A population-based case–control study published in the September issue of Clinical Gastroenterology and Hepatology associates asthma with later development of Crohn’s disease (CD) and with ulcerative colitis (UC).
Although the etiology of asthma and inflammatory bowel diseases (IBD) are not well understood, they both involve complex interactions among genetic and environmental factors and the intestinal microbiota. IBD and asthma are each more common among children who are not breastfed or exposed to antibiotics as infants, and associate with variants in genes that regulate inflammation. Previous studies have associated IBD with respiratory disorders, including asthma and chronic obstructive pulmonary disease.
M. Ellen Kuenzig et al performed a case–control study to examine the effect of asthma on later development of CD or UC using health databases from Alberta, Canada. Cases of CD and UC were identified using a validated ICD-9 and -10 coding algorithm; controls had no IBD-related codes and were identified with an age-stratified random sample. Diagnoses of asthma were also determined using coding algorithms, and had to precede diagnoses of IBD. The authors analyzed data from 3087 CD cases, 2377 UC cases, and 402,800 controls.
Overall, 14.5% and 12.8% of individuals with CD and UC, respectively, had a diagnosis of asthma prior to the diagnosis of IBD, compared to 9.8% of controls (P < .0001). Asthma was associated with increased odds of incident CD (adjusted odds ratio, 1.45) regardless of patient age. Asthma associated with a diagnosis of UC at an age of 16 years or younger (adjusted odds ratio, 1.49), and a diagnosis of UC at 40 years or older (adjusted odds ratio, 1.57). However, asthma did not increase the risk of a diagnosis of UC between the ages of 17 and 40 years.
As the prevalence of current smoking increased, the odds ratio for the association between asthma and Crohn’s disease decreased (see figure). The authors did not expect adjusting for smoking status to have a substantial impact on their findings.
People with asthma therefore appear to be at higher risk for later developing IBD. Kuenzig et al state that patients who present to their allergists with chronic gastrointestinal symptoms might benefit from priority referral to a gastroenterologist.
In an editorial that accompanies the article, Edward L. Barnes and Michael D. Kappelman write that the strength of the study was its population-based design, capturing of 99% of individuals residing in a province of more than 4 million. The ability to link data sets and analyze individuals over long periods of time are important for determining the relationship between pre-existing asthma and incident cases of UC or CD.
Are these associations caused by shared pathogenic mechanisms, common environmental risk factors, or risks related to asthma treatment?
Intestines and lungs each derive from the primitive foregut and have similar structures, including a mucous-coated epithelial layer. Asthma and IBD have been associated with defects in epithelial barriers, alterations in the innate immune response to microbes, and defects in inflammatory and immune responses to environmental factors and pathogens. It is not clear how asthma and IBD are linked, but they share many mechanisms of pathogenesis.
This is an interesting area for future research. Kuenzig et al state that studies to investigate the mechanisms connecting asthma to UC should be age specific.