Acute lower gastrointestinal (GI) bleeding includes a wide clinical spectrum, ranging from minute bleeding to massive hemorrhage with hemodynamic instability. Lower GI bleeding is approximately one fifth as common as upper GI bleeding and accounts for approximately 20 to 30 hospitalizations per 100,000 adults per year.   The incidence of lower GI bleeding increases with age.
In the West, the 2 most common causes of acute lower GI bleeding, resulting in significant blood loss, are colonic diverticular disease and angiodysplasia. Patients with severe bleeding or significant comorbid states require rapid identification and aggressive resuscitation. Hemodynamically insignificant bleeding may frequently result from hemorrhoids and colonic neoplasms. Rare causes of bleeding include solitary rectal ulcer, vasculitis, and endometriosis.
Colonoscopy is the mainstay of the evaluation in patients in whom anorectal or upper GI causes have been ruled out. This is performed to localize the bleeding source and to enable hemostasis. Endoscopic hemostasis is successful in most cases. Mesenteric angiography or nuclear imaging is only performed in patients in whom colonoscopy is not feasible or where there is persistent bleeding and a negative colonoscopy. The source of bleeding cannot be definitively identified in up to 25% of patients.  
Chief of Gastroenterology
Clinical Assistant, Professor of Medicine
University of New Mexico
PR declares that he has no competing interests.
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