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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.
Melena vs. hematochezia
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 hemorrhage in the stomach or duodenum (upper gastrointestinal tract), for example by a peptic ulcer.
Diagnosis
If a source in the upper GI tract is suspected, an upper endoscopy is usually performed to diagnose the cause. Lower GI bleeding sources may be detected by tagged red blood cell scan or mesenteric angiogram.
Causes
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.
It is usually a medical emergency because it is a sign that a patient/animal is losing blood into the gastro-intestinal tract. This bleeding may be torrential and difficult to control.
A less serious, self-limiting case of melena can occur in newborns two to three days after delivery, due to swallowed maternal blood.
See also
"Melena"