Ulcerative Colitis versus Crohn's Disease, Animation

(USMLE topics) UC vs CD: How to differentiate symptoms, pathophysiology; differential diagnosis.

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Voice by: Marty Henne
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Ulcerative colitis and Crohn’s disease are the 2 major forms of inflammatory bowel disease. Both conditions are characterized by chronic inflammation of the gastrointestinal tract, or GI tract. Ulcerative colitis is limited only to the large intestine. It usually starts from the rectum, and may remain confined to that location; or, it can spread upward in a continuous fashion, affecting parts, or all, of the colon.
Crohn disease, on the other hand, can occur anywhere along the GI tract, but it usually involves the small intestine. Most commonly affected are the end of the small bowel, the ileum, and the beginning of the colon. Crohn disease is not continuous: inflamed segments are usually interrupted by normal healthy tissues. The rectum is always affected in ulcerative colitis, but is usually spared in Crohn’s disease.
Ulcerative colitis only involves the inner lining of the intestinal wall, while Crohn’s disease extends to its entire thickness. In imaging studies, the mucosa in Crohn’s disease has a patchy appearance, with alternating regions of diseased and normal tissues; the bowel wall is affected asymmetrically. In ulcerative colitis, mucosal inflammation is diffuse and uniform; the bowel wall is affected symmetrically. Thickening of the intestinal wall, bowel obstructions and fistulas are unique to Crohn’s disease.
Both conditions result from an inappropriate response of the immune system, but their mechanisms are likely different. Ulcerative colitis is associated with abnormally high levels of helper T-cell type 2, Th-2, while Crohn’s disease has more type 1 (Th1) activities.
Age of onset and risk factors are similar for both conditions, with a notable exception of smoking, which exacerbates Crohn’s disease but has a protective effect on ulcerative colitis.
The 2 disorders share many common symptoms such as diarrhea, abdominal pain and extra-intestinal manifestations. However, Crohn’s disease symptoms can be more variable depending on the affected locations, and often include anal fistulas and abscesses. Intestinal bleeding is much more common in ulcerative colitis, while malnutrition and weight loss is more common in Crohn’s disease.
When Crohn’s disease involves only the colon, the symptoms can be indistinguishable from ulcerative colitis. Fortunately, because treatment is similar, no distinction is necessary unless surgery is considered. This is because surgical removal of the diseased colon is curative for ulcerative colitis, but only provides temporary relief for Crohn’s disease.