What are the differential diagnoses and typical presentations for lower gastrointestinal bleeding?

Lower Gastrointestinal Bleeding: Differential Diagnoses and Clinical Presentations

Lower GI bleeding presents most commonly as hematochezia (bright red blood per rectum), with diverticulosis and hemorrhoids being the leading causes, though you must immediately assess hemodynamic stability using shock index (HR/systolic BP) to guide your diagnostic and therapeutic approach.

Clinical Presentations

LGIB manifests in three distinct patterns that dictate urgency:

• Acute hematochezia (75% of presentations): Bright red blood per rectum, ranging from minor streaking to massive hemorrhage 1
• Melena (10%): Dark, tarry stools suggesting slower bleeding or possible upper GI source 1
• Chronic occult bleeding: Iron-deficiency anemia or fecal occult blood positivity 2

Critical pitfall: 10-15% of patients presenting with severe hematochezia actually have an upper GI source, particularly when hemodynamically unstable 3. Always consider this, especially if shock index >1 after resuscitation 4.

Differential Diagnoses by Frequency

Most Common Causes (Account for >80% of cases):

Colonic sources:

• Diverticulosis: Most frequent cause of acute severe LGIB 2, 3
• Hemorrhoids: Leading cause in many series, particularly for minor bleeding 1
• Angiodysplasia: Second most common after diverticulosis in acute bleeding 2, 3
• Ischemic colitis: Associated with higher mortality risk 2
• Neoplasms (polyps, colorectal cancer): More common with chronic bleeding
• Inflammatory bowel disease/colitis: Various etiologies including infectious, inflammatory 2
• Post-polypectomy bleeding: Iatrogenic cause 2

Less Common Sources (5-15%):

• Anorectal disorders: Fissures, fistulas, proctitis
• Small bowel lesions (5%): Tumors, Meckel's diverticulum (especially in younger patients) 5
• Upper GI sources (15-20%): Presenting as hematochezia when brisk bleeding 5

Rare but Important:

• Appendiceal hemorrhage: Obscure cause requiring high index of suspicion 6

Initial Assessment Algorithm

Step 1: Hemodynamic Stratification (First Priority)

Unstable patients (shock index >1):

• Defined as heart rate divided by systolic BP >1 4
• Requires immediate resuscitation and urgent investigation
• Proceed directly to CT angiography as fastest means to localize bleeding before endoscopic/radiological therapy 4
• If CTA negative, perform immediate upper endoscopy to exclude upper GI source 4

Step 2: Risk Stratification for Stable Patients

Use Oakland score to categorize as major vs. minor bleed 4:

• Oakland score ≤8 points: Minor self-terminating bleed
  • Can be discharged for urgent outpatient investigation if no other admission indications 4
• Oakland score >8 points: Major bleed
  • Admit for inpatient colonoscopy 4

Step 3: Age-Related Considerations

Patient age correlates with specific pathologies 5:

• Younger patients (<50 years): Consider Meckel's diverticulum, inflammatory bowel disease, hemorrhoids
• Elderly patients (>50 years): Diverticulosis, angiodysplasia, ischemia, malignancy predominate 7

Key Clinical Features to Elicit

History elements predicting LGIB:

• Prior history of LGIB (strongest predictor) 7
• Age >50 years 7
• Presence of blood clots per rectum (indicates active, significant bleeding) 7
• Anticoagulation/antiplatelet use
• Comorbidities: cardiovascular disease, chronic kidney disease

Physical examination priorities:

• Orthostatic vital signs
• Shock index calculation (HR/systolic BP)
• Digital rectal examination to assess stool color, presence of masses, hemorrhoids
• Nasogastric aspirate if upper GI source suspected

Prognostic Factors

Factors associated with increased mortality and severe course 2:

• Advanced age
• Significant comorbidities
• Intestinal ischemia as cause
• Hemodynamic instability despite resuscitation
• Active bleeding on presentation

Natural history: 80% of LGIB stops spontaneously, but 25% experience rebleeding and 10% have unidentified sources 5.

Common Pitfalls to Avoid

1. Assuming all hematochezia is lower GI: Always exclude upper GI source in unstable patients 4, 3
2. Underutilizing risk stratification tools: Oakland score prevents unnecessary admissions and identifies high-risk patients 8
3. Delaying CTA in unstable patients: This is the fastest diagnostic modality when shock index >1 4
4. Proceeding to emergency laparotomy prematurely: Exhaust all radiological and endoscopic options first except in exceptional circumstances 4