The skin is one of the most common extraintestinal organ system affected in patients with
inflammatory bowel disease (IBD), including both
Crohn's disease and
ulcerative colitis. The
skin manifestations associated with IBD are polymorphic and can be classified into 4 categories according to their pathophysiology: (1) specific, (2) reactive, (3) associated, and (4) induced by IBD treatment. Cutaneous manifestations are regarded as specific if they share with IBD the same granulomatous histopathological pattern: perianal or metastatic
Crohn's disease (commonly presenting with
abscesses,
fistulas or
hidradenitis suppurativa-like features) is the prototype of this setting. Reactive cutaneous manifestations are different from IBD in the histopathology but have close physiopathological links:
pyoderma gangrenosum, a neutrophil-mediated autoinflammatory
skin disease typically manifesting as painful
ulcers, is the paradigm of this group. Among the cutaneous diseases associated with IBD, the most commonly seen are
erythema nodosum, a form of
panniculitis most commonly involving bilateral pretibial areas, and
psoriasis, a T helper 1/T helper 17-mediated erythematous squamous inflammatory disease. Finally, the number of cutaneous adverse reactions because of IBD
therapies is progressively increasing. The most frequent drug-induced cutaneous manifestations are
psoriasis-like,
eczema-like, and
lichenoid eruptions, as well as
cutaneous lupus erythematosus for biologics, and nonmelanoma
skin cancer, mainly basal cell and
squamous cell carcinomas for thiopurines.