According to Kenneth McQuaid, M.D., chief of the gastroenterology section of San Francisco Veterans Administration Medical Center, the causes of bloody stool depend on how severe the bleeding is and your age. Even so, there are many potential causes. If you are younger than 50 years of age, the most common causes of bloody stool are inflammatory bowel disease and anorectal disease. If you are older, then the most common causes are diverticulosis, malignancy and vascular malformations.
Crohn's Disease
Inflammatory bowel disease (IBD) includes Crohn's disease and ulcerative colitis. In both of these diseases, the intestines (bowels) are inflamed. With Crohn's disease, one part of the intestines is inflamed, then the next part is not affected by the disease, but the next part has inflammation, and so on. In other words, the intestines have what is referred to as skip lesions, because the inflammation skips areas of the intestines. If you have Crohn's disease, you will usually experience non-bloody, watery diarrhea. If your rectum is also affected, however, you will have bloody diarrhea.
Ulcerative Colitis
Skip lesions are not present with this inflammatory bowel disease. With ulcerative colitis, the inflammation begins at the rectum and then moves upward. A continuous area of inflammation can reach as far as the small intestines. Because the rectum is always affected, bloody diarrhea results.
Anorectal Disease
This refers to diseases in the anus and rectum, including hemorrhoids and anal fissures. With internal hemorrhoids, bleeding usually develops after a bowel movement. Sometimes blood is visible in the toilet bowl or on toilet tissue. An anal fissure is a tear in the anus usually caused by a hard or large bowel movement. Bleeding is common, but unlike internal hemorrhoids, anal fissures are painful.
Diverticulosis
This is the name given to small herniations in the intestinal wall. The herniations get filled with feces, which are held there, unable to be eliminated with the rest of a bowel movement. This causes the herniations to become inflamed, and also causes bloody stools. As explained by Dr. McQuaid, diverticula are usually present in the left colon, but any bleeding that develops is typically due to diverticula located in the right colon. The bleeding is painless and starts suddenly.
Malignancy
Cancer of the right colon usually causes occult (hidden) blood loss that continues for quite some time. The bowel movement in the right colon is still quite soft, though, so cancer there is not initially obstructive or painful. That's why colon cancer in the right colon can grow silently. Cancer of the left colon, however, is obstructive, because the stool is already formed. In this location, cancer causes constipation and bloody stool.
Vascular Malformations
These malformations are abnormal arteries and veins that are very dilated and twisted. They are usually found in the intestines and are painless, but do cause bloody stool. The bleeding, however, is intermittent and does not cause any hemorrhaging.
References
- "Current Medical Diagnosis and Treatment 2009"; Stephen McPhee, M.D., Maxine Papadakis, M.D.; 2009
- "Robbins Pathologic Basis of Disease"; Ramzi Cotran, M.D., Vinay Kumar, M.D., Stanley Robbins, M.D.; 1994
- The Merck Manual: Anal Fissure
