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In medicine, melena or melaena refers to the black, "tarry" feces that are associated with gastrointestinal hemorrhage. The black color is caused by oxidation of the iron in hemoglobin during its passage through the ileum and colon.
Bleeding originating from the lower GI tract (such as the sigmoid colon and rectum) is generally associated with the passage of bright red blood, or hematochezia, particularly when brisk. Blood acts as a cathartic agent in the intestine, promoting its prompt passage. Only blood that originates from a high source (such as the small intestine), or bleeding from a lower source that occurs slowly enough to allow for oxidation, is associated with melena. For this reason, melena is often associated with blood in the stomach or duodenum (upper gastrointestinal tract), for example by a peptic ulcer. A rough estimate is that it takes about 14 hours for blood to be broken down within the intestinal lumen; therefore if transit time is less than 14 hours the patient will have hematochezia, and if greater than 14 hours the patient will exhibit melena. One often-stated rule of thumb is that melena only occurs if the source of bleeding is above the ligament of Treitz although, as noted below, exceptions occur with enough frequency to render it unreliable.
Patients sometimes present with signs of anemia or those due to low blood pressure. Very often, however, aside from the melena itself, there are no other symptoms. The presence of blood must be confirmed with either a positive hemoccult slide on rectal exam, frank blood on the examining finger, or a positive stool guaiac from the lab. If a source in the upper GI tract is suspected, an upper endoscopy can be performed to diagnose the cause. Lower GI bleeding sources usually present with hematochezia or frank blood. A test with poor sensitivity/specificity that may detect the source of bleeding is the tagged red blood cell scan, whereas mesenteric angiogram is the gold standard.
The most common cause of melena is peptic ulcer disease. Any other cause of bleeding from the upper gastro-intestinal tract, or even the ascending colon, can also cause melena. Melena may also be a sign of drug overdose if a patient is taking anti-coagulants, such as warfarin. It is also caused by tumours especially malignant tumors affecting the esophagous, more commonly the stomach & less commonly the small intestine due to the bleeding surface of them. However,the most prominent and helpful sign in these cases of malignant tumours is haematemesis. It may also accompany hemorrhagic blood diseases (e.g. purpura & hemophilia). Other medical causes of melena include bleeding ulcers, gastritis, esophageal varices, and Mallory-Weiss syndrome.
Some causes of "false" melena include Iron supplements, Pepto-Bismol, Maalox, and Lead. Although blood swallowed as a result of a nose bleed (epistaxis) can lead to melena, it is obviously not due to bleeding in the gastro-intestinal tract.
Melena is often not a medical emergency because the bleeding is slow. Urgent care however is required to rule out serious causes and prevent potentially life-threatening emergencies.
A less serious, self-limiting case of melena can occur in newborns two to three days after delivery, due to swallowed maternal blood.