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Both children and adults with atopic dermatitis have an increased risk of inflammatory bowel disease (IBD), according to a population-based cohort study.

In fully adjusted models, kids with atopic dermatitis had a 44% increased risk of IBD (HR 1.44, 95% CI 1.31-1.58), while adults had a 34% increased risk (HR 1.34, 95% CI 1.27-1.40) compared with controls without atopic dermatitis, reported Joel M. Gelfand, MD, MSCE, of the University of Pennsylvania Perelman School of Medicine in Philadelphia, and colleagues.

Breaking it down by subtype, kids had an increased risk of Crohn's disease (HR 1.74, 95% CI 1.54-1.97), but only those with severe atopic dermatitis had an increased risk of ulcerative colitis (HR 1.65, 95% CI 1.02-2.67), whereas adults had increased risks for both Crohn's (HR 1.36, 95% CI 1.26-1.47) and ulcerative colitis (HR 1.32, 95% CI 1.24-1.41), the authors detailed in .

"The association between AD [atopic dermatitis] and IBD may be explained by shared genetic and environmental factors, immune cell activation, and alterations in skin and gut microbiota," Gelfand and team wrote.

"Studies that examine the association between AD and IBD are important because they shed light on common pathophysiologic mechanisms and because, with the advent of targeted therapeutic approaches, they could influence treatment selection," they added.

In addition to age and IBD subtype, the severity of atopic dermatitis played a role in the increased risk of IBD.

For instance, in children, incidence rates for IBD and Crohn's disease were higher across atopic dermatitis severity groups (range 0.18-0.95 for IBD and 0.10-0.91 for Crohn's) compared with controls (range 0.15-0.16 for IBD and 0.07-0.08 for Crohn's). Incidence rates for ulcerative colitis were similar between controls (range 0.07-0.08) and patients who had mild atopic dermatitis (range 0.06-0.09), but were higher in patients with moderate (range 0.13-0.23) and severe (range 0.16-0.41) disease.

In adults, incidence rates for IBD were higher in patients with atopic dermatitis compared with controls (range 0.39-0.41), with higher rates among those with greater severity (mild: range 0.47-0.53; moderate: range 0.58-0.66; and severe: range 0.94-1.25). For Crohn's disease and ulcerative colitis, higher incidence rates were seen across atopic dermatitis groups compared with controls.

Severe atopic dermatitis was associated with the highest increased risks of IBD (HR 2.27, 95% CI 1.96-2.64), Crohn's disease (HR 3.50, 95% CI 2.91-4.20), and ulcerative colitis (HR 2.40, 95% CI, 2.00-2.88) in adults.

"The finding that this risk increases with worsening severity of AD suggests a possible causal association," Gelfand and colleagues concluded.

Atopic dermatitis has been associated with IBD, but have shown . "Although an increased prevalence of IBD has been observed with AD, others have found an association among ," the authors noted, adding that studies looking at ulcerative colitis and Crohn's disease in patients with atopic dermatitis "have shown ."

"While there does seem to be some inconsistencies in the association between IBD and AD, there are enough studies showing an association to state that the finding was not unexpected," Alison Ehrlich, MD, MHS, a dermatologist at FoxHall Dermatology and Research Center in Washington, D.C., told MedPage Today.

"Dermatologists should consider screening questions for IBD when making a diagnosis of AD," she said, adding that patients who meet the criteria for systemic treatment with immunomodulating medications for both AD and IBD may benefit from a team approach between the dermatologist and gastroenterologist.

This study used data from January 1994 through February 2015 from the Health Improvement Network, a U.K. electronic health records database in which general practitioners are the primary point of care for patients. Patients with atopic dermatitis were matched with up to five controls without it based on age, practice, and index date.

A total of 1,809,029 pediatric controls were matched to 409,431 children with atopic dermatitis (93.2% mild, 5.5% moderate, and 1.3% severe). This cohort ranged in median age from 4 to 5 years, and the majority were boys.

For adults, 2,678,888 controls were matched to 625,083 patients with atopic dermatitis (65.7% mild, 31.4% moderate, and 2.9% severe). They ranged in median age from 45 to 50, and were predominantly women.

Gelfand and team used treatments as a proxy for atopic dermatitis severity, which they said "makes it difficult to separate the effects of treatment exposure from those of AD severity." Other limitations included the potential for surveillance bias and residual confounding.

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