One complication of transmural inflammation with Crohn disease is fistula formation. Seen here is a fissure extending through mucosa into the submucosa toward the muscular wall, which eventually will form a fistulous tract. Fistulae can form between loops of bowel, bladder, and even skin. With colonic involvement, perirectal fistulae are common.

Use of biologic agents such as adalimumab and infliximab, which are monoclonal antibodies targeting tumor necrosis factor (TNF), has improved therapy for Crohn disease. A biologic agent plus the immunosuppressant azathioprine can be effective in diminishing the inflammation and tissue destruction, with reduction in complications.