Crohn Disease
Crohn disease can involve any part of the gastrointestinal tract from mouth to anus, most often the terminal ileum, with characteristic skip lesions of diseased bowel separated by normal segments. Inflammation is transmural, producing a cobblestone mucosa, deep fissures, non-caseating granulomas, and complications such as fistulas, abscesses, and strictures. Patients typically present with crampy abdominal pain, non-bloody diarrhea, weight loss, and sometimes a palpable right lower quadrant mass.
Ulcerative Colitis
Ulcerative colitis is limited to the colon and rectum, begins at the rectum, and extends proximally in a continuous fashion without skip areas. Inflammation is confined to the mucosa and submucosa, producing friable mucosa, crypt abscesses, and pseudopolyps. The hallmark presentation is bloody diarrhea with mucus, tenesmus, and lower abdominal cramping. Complications include toxic megacolon, perforation, and a substantially increased risk of colorectal cancer.
Extraintestinal Manifestations
Both diseases share extraintestinal features that are frequently tested: joint involvement (peripheral arthritis, ankylosing spondylitis, sacroiliitis), skin lesions (erythema nodosum, pyoderma gangrenosum), eye disease (uveitis, episcleritis), and hepatobiliary disease. Primary sclerosing cholangitis is strongly associated with ulcerative colitis, while gallstones and kidney stones are more common in Crohn disease because of terminal ileal disease.
Diagnosis and Treatment
Diagnosis relies on colonoscopy with biopsy, supported by imaging and inflammatory markers; perinuclear ANCA is more common in UC and anti-Saccharomyces cerevisiae antibodies in Crohn. Treatment follows a step-up strategy: aminosalicylates (5-ASA) for mild disease, corticosteroids for flares, and immunomodulators (azathioprine, methotrexate) and anti-TNF or other biologics for moderate to severe disease. Colectomy is curative for ulcerative colitis but not for Crohn.