Background and aims: Epidemiological studies have established an association between psoriasis and inflammatory bowel disease [IBD], i.e. ulcerative colitis [UC] and Crohn's disease [CD], but results are inconsistent. The aim of this study was therefore to quantify the prevalences and association between IBD and psoriasis.

Methods: PubMed, Web of Science, and EMBASE were searched from database inception through April 2018 for studies reporting data on psoriasis among patients with IBD and vice versa. Meta-analysis was performed to estimate, respectively, the prevalences and association between IBD and psoriasis. Data extraction was according to the PRISMA guideline, and quality assessment was made using the Newcastle-Ottawa Scale. The main outcomes were the proportion of psoriasis patients with IBD and vice versa, as well as the association (odds ratio [OR]) of IBD in psoriasis and psoriasis in IBD, respectively.

Results: Based on quantitative analysis of 93 studies, the prevalence of psoriasis in CD and in UC was 3.6% (95% confidence interval [CI] 3.1%-4.6%) and 2.8% [95% CI 2.0%-3.8%] respectively. The prevalence of CD and UC was 0.7% [95% CI 0.2%-1.3%] and 0.5% [95% CI 0.3%-0.8%], respectively, among patients with psoriasis. Presence of CD or UC was significantly associated with psoriasis, with OR 2.0 [95% CI 1.4-2.9] and OR 1.5 [95% CI 1.2-2.0], respectively. Presence of psoriasis was significantly associated with CD: OR 2.2 [95% CI 1.6-3.1] and with UC: OR 1.6 [95% CI 1.3-2.0].

Conclusions: We found significant bidirectional associations between psoriasis and IBD, warranting increased awareness among clinicians in the diagnostic process, especially in children and adolescents with IBD. Last, this study showed an increased frequency of paradoxical psoriasis in patients treated with biologics.