What is the most likely cause of very heavy rectal bleeding in a 63-year-old man?

Most Likely Cause of Very Heavy Rectal Bleeding in a 63-Year-Old Man

Diverticulosis is the most likely cause of very heavy rectal bleeding in this 63-year-old man, accounting for 20-41% of acute lower gastrointestinal bleeding cases in this age group. 1

Epidemiologic Evidence Supporting Diverticulosis

The patient's age of 63 years places him squarely in the highest-risk demographic for diverticular bleeding:

• Diverticulosis represents the single most common cause of acute lower GI bleeding in patients aged 63-77 years, with prevalence rates of 20-41% across multiple large studies 1
• The incidence of diverticular bleeding increases dramatically with age, showing a >200-fold increase from age 20 to 80 years 1
• Diverticulosis prevalence reaches 50% in patients over 60 years, making it the dominant pathology in this age bracket 2
• Diverticular bleeding typically presents as massive, painless rectal hemorrhage, which matches the clinical description of "very heavy" bleeding 3

Comparative Likelihood of Other Diagnoses

Angiodysplasia is the second most common cause but significantly less likely:

• Angiodysplasia accounts for only 2-40% of lower GI bleeding cases, with most studies showing rates of 3-15% 1
• While angiodysplasia does increase with age, it remains substantially less common than diverticulosis in the 60-70 year age range 1

Colon cancer/polyps are even less likely as a cause of massive bleeding:

• Cancer and polyps account for only 6-27% of acute lower GI bleeding cases 1
• Importantly, cancer typically presents with chronic, intermittent bleeding rather than acute massive hemorrhage 1
• While cancer must be excluded (6% risk in patients over 50 with rectal bleeding), it rarely causes the hemodynamically significant bleeding pattern described 4

Diverticulitis is not a cause of heavy rectal bleeding:

• Diverticulitis presents with abdominal pain, fever, and inflammatory symptoms—not isolated massive bleeding 1
• The question appears to conflate diverticulitis (inflammation) with diverticulosis (bleeding from diverticula), which are distinct clinical entities

Clinical Context and Natural History

• Approximately 80-85% of lower GI bleeding stops spontaneously, but 50% of patients with diverticular bleeding require blood transfusion 1, 2
• The mortality rate for acute lower GI bleeding in this age group is 2-4% 1
• Diverticular bleeding resolves spontaneously in approximately 80% of cases, though rebleeding rates range from 18-53% 3, 2

Critical Diagnostic Caveat

• 10-15% of patients presenting with severe hematochezia actually have an upper GI source identified on endoscopy, which must be excluded first 1
• For hemodynamically unstable patients with very heavy bleeding, CT angiography should be performed first to rapidly localize the bleeding site 4
• Colonoscopy within 12-48 hours after rapid bowel preparation remains the gold standard for definitive diagnosis and potential therapeutic intervention 3

The combination of the patient's age (63 years), presentation with very heavy bleeding, and epidemiologic data from multiple large studies makes diverticulosis the clear most likely diagnosis, with angiodysplasia as a distant second possibility and cancer as an important but less likely differential that must still be excluded.