Lower GI bleeding often presents as a medical emergency. Like other medical emergencies, optimum patient care requires careful assessment and resuscitation. The history and physical findings provide important clues to the etiology and are critical for determining the severity and location of the bleeding site.

Lower GI bleeding can be classified arbitrarily as major or minor. Patients presenting with the passage of significant amounts of bright red blood per rectum and hemodynamic compromise have major GI blood loss and are at risk of life-threatening hypovolemia. Be wary of the patient who may have stabilized temporarily or received intravenous fluids prior to a full clinical assessment. Historical clues to a major bleed include the occurrence of syncope or presyncope prior to seeking medical care. The vital signs, with particular attention paid to postural changes, are crucial to assessing severity. The passage of bright red blood per rectum almost always orginates from the colon. However, it is important to remember that brisk bleeding from a site in the upper GI tract may masquerade as a major lower GI bleed. In contrast to the patient with major lower GI bleeding is the patient who describes the passage of bright red blood per rectum as blood on the tissue paper or on the outside of formed stool in the absence of other symptoms. Such patients, whose general physical examination is normal, usually have a minor lower GI bleed. Most often this is due to local perianal pathology.

In the clinical setting of a major lower GI bleed with the passage of bright red blood per rectum and hemodynamic compromise, there are a number of important clues that may raise the suspicion of an upper GI source. These include a past history or symptoms of peptic ulcer disease, NSAID use, prior abdominal aneurysm repair, alcohol abuse and coexisting liver disease. Unfortunately, the lack of upper GI symptoms does not exclude peptic ulcer disease, as a number of duodenal ulcers present as major GI bleeds without a previous typical ulcer history. On physical examination, the finding of hypovolemic shock, particularly in a young person, should trigger immediate consideration of a proximal source of bleeding. Features of chronic liver disease and portal hypertension suggest varices as a possible cause. Most major upper GI bleeding, even in a young person, is accompanied by a transient rise in the BUN (blood urea nitrogen), whereas this is not typical in a lower GI bleed unless there is renal comorbidity.

When an upper GI source is considered, several actions are necessary. A nasogastric tube returning bloody gastric aspirate positively identifies a proximal source of bleeding, but a negative aspirate may not. A negative aspirate will exclude significant bleeding from the esophagus or stomach but may fail to identify bleeding from the duodenum. Even aspirates with bile staining and no blood may fail to identify 5–10% of bleeding duodenal ulcers. When an upper GI source cannot be excluded with confidence, urgent upper endoscopy is required.

Another potentially confusing scenario involves the patient presenting with melena. Melena results from the digestion of blood as it travels through the GI tract, and almost always originates from the upper GI tract. However, occasionally transit of blood from a bleeding right colon is sufficiently slow that stool can appear as melena or melena mixed with dark red blood.

Positive fecal occult blood tests are another clue to lower gastrointestinal bleeding. Many results prove to be false positives; testing should be done with patients on a controlled diet (no red meat, vitamin C or aspirin) to minimize this possibility. Occult positive stools can result from bleeding sites in either the upper or lower GI tract.

Angiodysplasia and diverticular bleeding are the two most common causes of major lower GI bleeding, accounting for up to 60–70% of cases.

Angiodysplastic lesions result from dilation and tortuosity of submucosal veins associated with small arteriovenous communication with submucosal arterioles. These lesions are typically multiple, less than 5 mm in diameter, and are most commonly found in the right colon and cecum. The pathogenesis of these lesions is unknown but they occur most commonly in elderly patients and differ from congenital vascular lesions. Diverticula are located predominantly in the left colon, but angiographic studies have shown that those in the right colon bleed more frequently. The pathophysiology underlying diverticular bleeding is also uncertain but is thought to result from rupture of arteries that penetrate the dome of the diverticulum.

A number of other possible but less common causes exist (Table 6), but many of these more typically present with minor lower GI bleeding and a clinical picture dominated by other features such as diarrhea. Angiodysplasia, unlike diverticular bleeding, can also present with minor chronic GI bleeding, and may even present as chronic anemia secondary to microscopic blood loss. In contrast to angiodysplasia and diverticular bleeding, which are relatively painless, bleeding secondary to colonic ischemia is typically preceded by minutes to hours of significant abdominal pain. Abdominal x-rays may demonstrate thumb-printing, but this finding is neither specific nor sensitive.

Most major lower GI bleeding will stop without intervention and can be investigated electively, but up to 25% will continue to bleed and require immediate investigation and treatment (Figure 3). After resuscitation, the next priority is to identify the site of bleeding. Radionuclide scanning using technetium-labeled red blood cells is least invasive and readily available in most centers, but interpretation is fraught with false negative and positive results. Although angiography is less available and more invasive, it is more accurate and has the advantage of therapeutic intervention with embolization of the arteriole feeding the bleeding lesion. Colonoscopy can also be attempted to identify the bleeding lesion, and if angiodysplasia is evident it can be treated with electrocautery. However, unless the rate of bleeding is relatively slow, ongoing bleeding usually obscures the lumen, making it difficult to identify the responsible lesion and technically difficult to advance the colonoscope to the site of bleeding. In some cases, continuing bleeding (requiring transfusions of 6–10 units of blood) requires either urgent angiography with embolization or surgical resection with a subtotal colectomy.

Minor bleeding from the lower GI tract is a common complaint and requires a careful approach (Figure 3) to differentiate minor pathology such as hemorrhoids and fissures from serious problems such as colonic tumors. Patients may notice blood only on the outside of formed stool or on the tissue paper, suggesting that the blood originates from the anal canal or the rectosigmoid region. Alternatively, some patients notice that the blood is mixed in the stool, suggesting that bleeding is more proximal within the colon.

Hemorrhoids are the commonest cause of minor bleeding (Table 7), but even when the history is very suggestive, proctoscopic or sigmoidoscopic assessment should be carried out to ensure that a rectal lesion such as proctitis or a tumor is not mimicking this presentation. Patients with ulcerative proctitis often have frequent bowel movements but pass only bright red blood and mucus on many occasions. Radiation proctitis can present shortly after radiotherapy treatment but is often delayed by many months or years. This condition results from chronic inflammation within the blood vessels, called endarteritis obliterans, and this indolent process underlies the delayed presentation.