A 58-year-old woman with a history of colorectal cancer (treated 3 years ago) presents with progressive abdominal distension, constipation, and colicky pain for 3 days. On examination, the abdomen is distended with visible peristalsis. Abdominal X-ray shows a dilated colon with a transition point at the sigmoid region. What is the most appropriate next investigation to confirm the diagnosis and assess resectability?

Explanation

## Investigation of Choice in Large Bowel Obstruction with Malignancy **Key Point:** CT abdomen and pelvis with IV and oral contrast is the investigation of choice in suspected malignant large bowel obstruction because it confirms the diagnosis, determines the level and cause, assesses for metastases, and evaluates resectability. ### Why CT Over Other Modalities? ```mermaid flowchart TD A[Large bowel obstruction suspected]:::outcome --> B{Cause likely malignant?}:::decision B -->|Yes or high suspicion| C[CT abdomen/pelvis with IV + oral contrast]:::action C --> D[Confirms obstruction]:::outcome C --> E[Identifies tumor]:::outcome C --> F[Assesses metastases]:::outcome C --> G[Evaluates resectability]:::outcome B -->|No, simple obstruction| H[Contrast enema or colonoscopy]:::action ``` ### CT Advantages in Malignant Obstruction | Parameter | CT | Contrast Enema | Sigmoidoscopy | |-----------|----|----|---| | **Confirms obstruction** | ✓ | ✓ | ✗ | | **Identifies tumor** | ✓ (tissue density) | ✗ (outline only) | ✓ (direct visualization) | | **Assesses metastases** | ✓ (liver, peritoneum, nodes) | ✗ | ✗ | | **Evaluates resectability** | ✓ (invasion, vascular involvement) | ✗ | ✗ | | **Safe in obstruction** | ✓ | ⚠ (risk of perforation) | ⚠ (risk of perforation) | | **Determines level** | ✓ | ✓ | ✗ (limited reach) | **High-Yield:** CT is superior because it provides: 1. **Tissue characterization** — distinguishes tumor from stricture or volvulus 2. **Staging information** — identifies liver metastases, peritoneal involvement, lymph node enlargement 3. **Surgical planning** — assesses invasion into adjacent organs, vascular encasement, resectability 4. **Safety** — avoids the risk of perforation associated with contrast enema or endoscopy in obstructed colon ### Why Not Contrast Enema? **Warning:** Contrast enema is contraindicated in complete large bowel obstruction because: - Risk of **colonic perforation** (especially in cecum, which is at highest risk due to Laplace's law) - Does not provide tissue characterization or staging information - Cannot assess for metastases or vascular invasion - In malignancy, does not determine resectability **Clinical Pearl:** Water-soluble contrast enema may be used therapeutically in uncomplicated sigmoid volvulus (non-malignant), but in suspected malignant obstruction, CT is mandatory. ### Why Not Sigmoidoscopy? - **Limited reach** — cannot assess proximal colon or determine if obstruction is due to tumor - **Risk of perforation** — contraindicated in complete obstruction - **No staging information** — does not assess for metastases or resectability - Should be reserved for diagnostic purposes in partial obstruction or after resolution of acute obstruction **Tip:** In malignant large bowel obstruction, CT is the single investigation that answers all critical questions: diagnosis, cause, stage, and resectability. It guides the decision between primary resection, colostomy, or neoadjuvant therapy.