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Blood in the Stool (Rectal Bleeding) (cont.)

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Abnormal collections of enlarged blood vessels frequently occur just under the inner lining of the colon. These abnormal vessels are called angiodysplasias. Angiodysplasias usually can be seen easily during colonoscopy as bright red, spider-like lesions just beneath the colon's lining. Although angiodysplasias may occur anywhere in the colon, they are most common in the right or ascending colon. The cause of angiodysplasias is unknown, but they occur with increasing frequency as people grow older. Bleeding from angiodysplasias is painless and can result in bright red, dark red, maroon, or black stools. Angiodysplasias also can cause occult bleeding and iron deficiency anemia.

Colitis and proctitis

Colitis means inflammation of the colon. Proctitis means inflammation of the rectum. Several different diseases can cause colitis and proctitis. These include bacterial or viral infection, ulcerative colitis or proctitis, Crohn's colitis, ischemic colitis, and radiation colitis or proctitis.

Ulcerative colitis, ulcerative proctitis, and Crohn's colitis are chronic inflammatory diseases of the colon due to overactivity of the body's immune system. These diseases can cause abdominal pain, diarrhea, and bloody diarrhea (diarrhea mixed with blood). Occasionally, moderate or severe rectal bleeding may occur. The bleeding originates from ulcerations in the colon.

Like ulcerative colitis and Crohn's colitis, infections - bacterial and, less commonly, viral -- can inflame the colon, leading to abdominal pain, diarrhea, and even bloody diarrhea. Rarely, infections may cause moderate or severe rectal bleeding. Examples of infections causing rectal bleeding include Salmonella, Shigella, Campylobacter, C. difficile, E. Coli O157:H7, and cytomegalovirus (the last in individuals with HIV infection).

Ischemic colitis is inflammation of the colon that is caused when the supply of blood to the colon is reduced suddenly. This is most often due to a blood clot that obstructs a small artery supplying blood to a portion of the colon. The most common part of the colon affected by ischemic colitis is the splenic flexure (the part of the colon where the transverse colon joins the left colon). The sudden reduction in the flow of blood can lead to ulceration of the colon and cause sudden onset of severe lower abdominal, cramping pain followed by rectal bleeding. The amount of blood lost during an episode of ischemic colitis usually is small. Rectal bleeding and the abdominal pain of ischemic colitis usually subside on their own after several days. The colonic ulcers usually heal after a few more days.

Radiation treatment for cancers of the abdomen can cause radiation colitis acutely, but permanent changes to the inner lining of the colon and the colonic blood vessels may occur, which can result in bleeding many years after treatment. A common example is radiation proctitis that results from pelvic radiation for the treatment of prostate cancer. Rectal bleeding from radiation proctitis usually is mild, but occasionally can be chronic enough to cause anemia.

Medically Reviewed by a Doctor on 2/10/2015

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Blood in the Stool - Experience Question: Please describe your experience with blood in the stool.
Blood in the Stool - Cause Question: What was the cause of blood in your stool?
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Blood in the Stool - Other Causes Question: Which one of these were the cause of your case of rectal bleeding, and please describe your experience.
Blood in the Stool - Infection Question: What infection caused your case of rectal bleeding, and how was it treated?
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