Genesis and pathophysiology of lower gastrointestinal bleeding.

The incidence of lower gastrointestinal bleeding (LGB) is estimated to be 20-30 per 100,000 of the adult population at risk, which is clearly correlated with increasing age. The problem of LGB is identification of the bleeding source. LGB stops spontaneously in 80% of cases, but 10% of bleeding sources cannot be identified, and rebleeding occurs in 25%. The quality of LGB--hematochezia, melena, or occult bleeding--may point to the region of the bleeding source, as patient age is correlated with specific pathologies. In many patients, LGB is a leading symptom of a chronic disorder. Most acute peranal bleeding arises from the colon (80%) with colonic diverticula and angiodysplastic lesions as the most frequent reasons. In 5% of cases, LGB is caused by disorders of the small bowel, in most cases due to small-bowel tumors or to Meckel's diverticulum in younger patients. In 15-20%, acute peranal bleeding is caused by lesions in the upper gastrointestinal tract. The intensity of LGB determines the urgency of identification of the bleeding source; however, chronic occult blood loss superimposed by melena may place the patient at risk as early as a patient with hematochezia. Therefore, prompt resuscitation is required in many LGB patients before diagnostic procedures are initiated.