Psoriasis and erythema nodosum: two comorbidities of inflammatory bowel diseases

Extra-intestinal manifestations are a relatively common complications of Inflammatory Bowel Diseases (IBD) and skin is one of the organs most commonly affected. Cutaneous findings in IBD patients may be related to different pathogenetic mechanisms and in some cases the etiologic link has not been fully elucidated. In particular, this is the case of psoriasis and erythema nodosum, two of the most frequent skin diseases observed in IBD patients. Aim of this paper was to review the epidemiology and the possible pathogenetic mechanisms implicated in the occurrence of these two dermatosis. In particular, an association between IBD and psoriasis has been observed in several epidemiological studies: psoriasis occurs in about 1-2% of the general population, compared with 3-11% of patients with IBD. Several studies have also evaluated the prevalence of IBD in psoriatic patients, with contrasting results. A common pathogenic pathways between these two conditions seems to be sustained by the responsiveness to therapy with biological treatments, such as anti-Tumor Necrosis Factor (TNF)-alpha agents and ustekinumab (a monoclonal antibody against p40 subunit common to IL-12 and IL-23). On the other hand, although usually idiopathic in half of the patients, erythema nodosum has been associated with a variety of disorders and conditions and IBD accounts for 1-4% of cases.