Melena is a term that describes a particular appearance of blood in the stool. This term describes stools that are black, tarry, and often foul-smelling due to the presence of blood from the upper gastrointestinal tract. Melena is associated with gastrointestinal bleeding, peptic ulcers, gastritis, and other disorders of the upper gastrointestinal tract.
The blood associated with melena gives stool a black, tarry appearance because blood is rich in a molecule called hemoglobin. This molecule is responsible for transporting oxygen through the bloodstream, and each hemoglobin molecule contains an iron atom. As hemoglobin passes through the stomach and gastrointestinal tract, iron undergoes a chemical reaction called oxidation, which changes the color and appearance of blood and stool.
Blood usually passes through the intestines quickly. When blood originates from the lower GI tract, it is usually bright red, because the iron hasn't had time to oxidize yet. This means that the source of oxidized blood is usually the upper gastrointestinal tract. Bright red or brown blood in the stool is called hematochezia and usually originates from the large intestine, rectum, or anus. This type of bleeding is most often caused by hemorrhoids. In rare cases, rapid-onset massive bleeding in the stomach can also lead to bright red blood in the stool.
The appearance of blood in the stool is often not a cause for alarm, but even so, a doctor should be consulted as soon as possible. The doctor will first request a medical history, which may include questions related to medications used, diet, gastrointestinal health, and the presence of symptoms other than blood in the stool. The doctor may also perform a physical exam of the abdomen and rectum to determine if there are any physical signs present that may provide diagnostic clues.
Diagnostic tests that may be performed include blood tests, stool cultures, and x-rays. Testing the stool is important because in certain specific situations, the stool can take on the appearance of melena, even if no bleeding is present. For example, eating black licorice, eating blueberries, or taking iron pills can cause stool to have a similar appearance. Eating tomatoes or beets regularly can also make your stool look red. The diagnostic process may also include a colonoscopy, a test that allows the doctor to closely examine the digestive tract for growths.
People experiencing one or more episodes of melena may be advised to modify their diet to prevent further bleeding, depending on the cause. For example, a diet high in natural fiber and low in saturated fat and alcohol might be recommended to reduce constipation and hemorrhoids. Medications may be prescribed if the bleeding has been caused by an ulcer. Some types of medication, such as over-the-counter anti-inflammatory medications, can cause upper gastrointestinal bleeding, so people using these medications may be encouraged to reduce their intake.