Differential Diagnosis of Acute Lower Gastrointestinal Bleeding
The differential diagnosis of acute lower GI bleeding is age-dependent and anatomically organized, with diverticular disease and angiodysplasia being the most common causes in adults, while hemorrhoids dominate in younger patients.
Age-Stratified Differential Diagnosis
Younger Patients (<50 years)
- Anorectal sources are most common in this age group 1
- Hemorrhoids (28% of all cases) - significantly more prevalent in patients under 50 1
- Inflammatory bowel disease (IBD)
- Infectious colitis
- Ischemic colitis (less common in this age group)
Older Patients (>70 years)
- Left colonic sources predominate 1
- Diverticular disease (19% of cases) - significantly more common in elderly patients 1
- Angiodysplasia - second most common cause in Western populations 2, 3
- Colorectal malignancy (10% of cases) 1
- Ischemic colitis
- Post-polypectomy bleeding
Anatomic Classification of Causes
Colonic Sources
Structural Lesions:
- Diverticular bleeding - most common cause in Western populations 2, 4
- Angiodysplasia - second most common, particularly in elderly 2, 3
- Colorectal neoplasms (10% of cases) - both malignant and benign polyps 1
- Post-polypectomy bleeding
Inflammatory/Ischemic:
- Colitis (infectious, inflammatory, ischemic) - leading cause in Indian studies 4
- Ischemic colitis - associated with higher mortality risk 2
- Radiation colitis
Anorectal Sources
- Hemorrhoids - most common overall cause (28%), especially in younger patients 1
- Anal fissures
- Rectal ulcers
Upper GI Sources Masquerading as Lower GI Bleeding
Critical consideration: 11-15% of patients presenting with apparent lower GI bleeding actually have an upper GI source 5. This is particularly important in:
- Patients with hemodynamic instability and brisk rectal bleeding
- History of peptic ulcer disease or portal hypertension
- Elevated blood urea/creatinine ratio
- Antiplatelet drug use 5
Geographic Variations
Western populations: Diverticular disease is the leading cause 2, 3
Indian/Asian populations: Colitis and hemorrhoids are the leading causes 4
High-Risk Causes Associated with Increased Mortality
The following diagnoses carry significantly worse prognosis 2:
- Intestinal ischemia - highest mortality risk
- Bleeding from malignancy
- Aortoenteric fistula - rare but catastrophic, may require direct surgical intervention 5
- Bleeding in the context of severe comorbidities
Diagnostic Yield by Investigation
Colonoscopy: 42-90% diagnostic yield (wide variation due to inclusion of presumed sources like diverticulosis) 5
CT Angiography: 49.7% diagnostic yield in national audit, with sensitivity 79-95% and specificity 95-100% 5
Nuclear medicine scintigraphy: 60-93% sensitivity for intermittent or slow bleeding 5
Video capsule endoscopy: 50-72% diagnostic yield for obscure overt bleeding after negative upper and lower endoscopy 5
Critical Diagnostic Pitfalls
Assuming anorectal source in unstable patients: Bright red blood with hemodynamic instability may indicate brisk upper GI bleeding 5
Missing upper GI sources: Always consider gastroscopy if no source found on initial CTA in unstable patients 5
Attributing bleeding to incidental findings: Diverticulosis or hemorrhoids may be present but not actively bleeding - look for stigmata of recent hemorrhage
Age bias: Don't assume hemorrhoids in elderly or malignancy in young patients without proper investigation
17% of cases remain undiagnosed even after comprehensive investigation 1
Causes Requiring Specific Mention
Small bowel bleeding is now recognized as a distinct entity and technically not "lower GI bleeding" by modern definitions 2, 4. However, it must be considered when upper and lower endoscopy are negative.
Post-procedural bleeding: Increasingly recognized with widespread colonoscopy screening programs 2
Drug-induced: Anticoagulants and antiplatelets don't cause bleeding per se but unmask underlying lesions and worsen severity 5, 4