Emerging evidence suggests that inflammatory bowel disease (IBD) - specifically Crohn’s disease and ulcerative colitis - and psoriasis are strongly associated, sharing common genetic predispositions and immunological mechanisms.
Given their close association, the question is whether IBD causes psoriasis, or psoriasis causes IBD. This large and extremely sophisticated German study was designed to answer exactly that question1.
To this end, researchers used genetic information from 463,772 European individuals. Within this huge sample, researchers identified 12,882 cases of IBD and 5,621 cases of psoriasis. They found that genetic markers for IBD were associated with a higher risk of psoriasis and psoriatic arthritis.
However, no association was found in the opposite direction. In other words, it is IBD that causes psoriasis rather than the reverse. These findings are consistent with a previous study, which showed that the prevalence of psoriasis in those with IBD was 4.2%, whereas the prevalence of IBD in patients with psoriasis, was 1.2%2. It’s worth noting, however, that because psoriasis often goes un-diagnosed, it could be that patients with IBD actually have a higher prevalence of psoriasis than has been reported.
This is an impressive study of great importance, because it answers the question of the direction of the relationship between IBD and psoriasis; it is IBD which causes psoriasis rather than the reverse. Importantly, neither psoriasis nor psoriatic arthritis have any causal relationship with IBD.
Now, obviously, most patients with psoriasis do not have IBD and most with IBD do not have psoriasis, but when they do occur together, this study suggests that the direction is from IBD to psoriasis and not the reverse. The researchers cannot explain why this should be the case, but one might speculate that IBD in some way “primes” inflammatory pathways in the skin, resulting in the characteristic psoriatic plaques.
From a practical perspective, patients with IBD and psoriasis are excellent candidates for a multidisciplinary approach to treatment, involving both gastroenterologists and dermatologists.
- Freuer D, Linseisen J, Meisinger C. Association between inflammatory bowel disease and both psoriasis and psoriatic arthritis: a bidirectional 2-sample Mendelian randomization study. JAMA Dermatol. 2022; 158:1262-1268
- Moona JM, Lee JY et al. Incidence of Psoriasis in Patients with Inflammatory Bowel Disease: A Nationwide Population-Based Matched Cohort Study. Dermatology 2021; 237:330–337
Dr D Ashton March 2023