A 58-year-old woman with a 3-day history of severe colicky abdominal pain, abdominal distension, and constipation presents to the emergency department. Plain abdominal radiograph shows dilated colon with a transition zone and a large rounded opacity visible within the colonic lumen. What is the most common cause of large bowel obstruction in adults?

## Most Common Cause of Large Bowel Obstruction **Key Point:** Colorectal adenocarcinoma is the most common cause of large bowel obstruction in adults, accounting for 40–60% of cases, particularly in patients >50 years of age. ### Epidemiology & Clinical Context **High-Yield:** Large bowel obstruction differs fundamentally from small bowel obstruction in etiology: - **Small bowel:** Adhesions (60–75%) - **Large bowel:** Malignancy (40–60%), volvulus (20–30%), diverticular disease (10–20%), fecal impaction (5–10%) ### Why Colorectal Cancer Is Most Common 1. **Age-related incidence:** Peak incidence in 6th–7th decade; most patients presenting with obstruction are >50 years. 2. **Anatomical predisposition:** Narrower luminal diameter of colon (compared to small bowel) makes even modest tumors obstructive. 3. **Growth pattern:** Annular, stenosing lesions are common and progressively narrow the lumen. 4. **Delayed diagnosis:** Many colorectal cancers are diagnosed at advanced stages when obstruction occurs. ### Plain Radiograph Findings in Large Bowel Obstruction | Finding | Significance | |---------|-------------| | Dilated colon (>6 cm) | Indicates obstruction | | Transition zone | Marks site of obstruction | | Haustra preserved proximal to transition | Suggests functional obstruction (volvulus) | | Haustra lost at transition | Suggests mechanical obstruction (cancer, stricture) | | "Bird's beak" appearance | Pathognomonic for volvulus | | Large rounded opacity in lumen | Suggests fecal impaction or, rarely, foreign body | **Clinical Pearl:** The **closed-loop obstruction** risk in large bowel obstruction is significant. If the ileocecal valve is competent, proximal small bowel may not dilate, masking the severity. If the valve is incompetent, small bowel dilates, increasing perforation risk. ### Comparison of Large Bowel Obstruction Causes | Cause | Frequency | Age Group | Key Imaging Features | Acute vs. Chronic | |-------|-----------|-----------|----------------------|-------------------| | **Colorectal cancer** | 40–60% | >50 years | Irregular narrowing, shouldering, loss of haustra | Usually acute on chronic | | **Sigmoid volvulus** | 20–30% | Elderly, institutionalized | "Bird's beak," "coffee bean," dilated sigmoid | Acute, recurrent | | **Diverticular disease** | 10–20% | >50 years | Stricture with diverticula, spiculated margins | Chronic | | **Fecal impaction** | 5–10% | Elderly, immobile | Large rounded opacity, no transition zone | Acute on chronic | **Mnemonic:** **CVDF** — Common causes of large bowel obstruction: - **C**olorectal cancer ← **Most common** - **V**olvulus - **D**iverticular disease - **F**ecal impaction ### Distinguishing Features on Plain Radiograph ```mermaid flowchart TD A[Large Bowel Obstruction]:::outcome --> B{Transition zone present?}:::decision B -->|Yes| C{Appearance at transition?}:::decision B -->|No| D[Functional obstruction or fecal impaction]:::outcome C -->|Irregular narrowing, shouldering| E[Colorectal cancer]:::action C -->|Bird's beak or coffee bean| F[Sigmoid volvulus]:::action C -->|Stricture with diverticula| G[Diverticular stricture]:::action D --> H{Large rounded opacity?}:::decision H -->|Yes| I[Fecal impaction]:::action H -->|No| J[Pseudo-obstruction or functional disorder]:::outcome ``` **Warning:** Do not mistake sigmoid volvulus for cancer. Volvulus presents acutely in elderly patients with characteristic "bird's beak" or "coffee bean" sign; cancer presents with progressive obstruction and irregular narrowing with shouldering.