Melena: Definition, Causes, and Clinical Significance
Melena is the passage of black, tarry stools resulting from the digestion of blood in the gastrointestinal tract, typically requiring at least 50-100 mL of blood from the upper GI tract. 1
Definition and Appearance
- Melena presents as black, tarry stools due to blood being altered as it passes through the intestines
- This occurs when hemoglobin is degraded by digestive enzymes and intestinal bacteria
- The characteristic appearance results from the conversion of hemoglobin to hematin or other hemochromes
Common Sources of Melena
Melena typically indicates bleeding from the upper gastrointestinal tract (above the ligament of Treitz), with the most common causes being:
| Cause | Approximate Frequency |
|---|---|
| Peptic ulcer disease | 35-50% |
| Gastroduodenal erosions | 8-15% |
| Esophagitis | 5-15% |
| Esophageal varices | 5-10% |
| Mallory-Weiss tears | 15% |
| Vascular malformations | 1% |
| [2,1] |
Clinical Significance and Diagnostic Value
- Melena strongly suggests upper GI bleeding with a likelihood ratio of 5.1-5.9 based on patient-reported history and 25 when confirmed on examination 3
- The presence of melena doubles the odds of finding a bleeding site within the proximal small intestine in patients with obscure GI bleeding 4
- Patients who present with both hematemesis and melena typically have more severe bleeding than those with melena alone 2
- In pediatric patients, the most common cause of melena is duodenal ulcer, followed by gastric ulcer, esophagitis, and esophageal varices 5
Differential Diagnosis
It's important to note that certain substances can cause black stools that mimic melena but aren't related to bleeding:
- Medications: Iron supplements, bismuth subsalicylate (Pepto-Bismol)
- Foods: Licorice, blueberries, charcoal
- True melena can be distinguished from pseudomelena by testing for occult blood
Clinical Pearls
- A serum urea nitrogen:creatinine ratio >30 increases the likelihood of upper GI bleeding (LR 7.5) 3
- Blood clots in stool significantly decrease the likelihood of upper GI bleeding (LR 0.05) 3
- While melena typically indicates upper GI bleeding, approximately 5% of cases may originate from the small intestine 5, 4
- The diagnostic yield of colonoscopy to investigate melena after a non-diagnostic EGD (esophagogastroduodenoscopy) is relatively low (4.8%) 6
Risk Factors for Upper GI Bleeding Presenting as Melena
- Advanced age
- Anticoagulant use
- High-dose NSAID use
- H. pylori infection
- Alcohol consumption
- Smoking
- Pre-existing liver disease 1
Upper GI bleeding ceases spontaneously in approximately 75-85% of cases, but mortality remains significant, highlighting the importance of prompt recognition and appropriate management of melena as a clinical sign 1.