CT Appearance of Large Bowel Obstruction
Large bowel obstruction on CT scan demonstrates dilated colon (typically >6 cm) proximal to a distinct transition point, with collapsed or decompressed bowel distal to the obstruction. 1, 2
Key Diagnostic CT Features
Primary Findings
- Transition point: An abrupt change in bowel caliber where dilated colon meets collapsed bowel, which pinpoints the anatomical location of obstruction 1, 2
- Colonic dilatation: Colon diameter exceeding 6 cm proximal to the obstruction (compared to the 2.5-3 cm threshold used for small bowel) 3, 2
- Collapsed distal bowel: Normal or decompressed colon and rectum distal to the transition point 1, 2
Identifying the Cause
- Mass lesion: Visible in approximately 78% of cases with obstructing carcinoma (14 of 18 patients in one study), appearing as a soft tissue mass at the transition point 1
- Mural thickening: Segmental wall thickening at the obstruction site may indicate malignancy or inflammatory causes 1
- Extraluminal abnormalities: CT excels at demonstrating causes outside the bowel lumen, including peritoneal carcinomatosis, abscesses, or extrinsic compression 3, 2
Technical Imaging Approach
Optimal CT Protocol
- IV contrast: Should be administered unless contraindicated to enhance detection of masses and assess bowel wall perfusion 1
- Oral contrast: Generally not required and may delay diagnosis; the dilated fluid-filled bowel serves as natural contrast 4, 1
- Additional positioning: Prone and/or decubitus views in 75% of cases (33 of 44 patients) help clarify equivocal transition points on supine imaging 1
Diagnostic Performance
CT scanning achieves 91% sensitivity, 91% specificity, 91% positive predictive value, and 91% negative predictive value for mechanical large bowel obstruction, with positive likelihood ratio of 10.1 1. This substantially exceeds plain radiography's limited 60-70% accuracy 5.
High-Risk CT Findings Requiring Urgent Surgery
Watch for these features indicating complications:
- Closed-loop obstruction: Bowel segment obstructed at two points, creating a closed loop at high risk for ischemia 5, 4
- Signs of ischemia: Reduced or absent bowel wall enhancement, mesenteric edema, ascites, pneumatosis intestinalis, or mesenteric venous gas 4, 6
- Perforation: Intraperitoneal free air mandates immediate surgical consultation 4
Common Etiologies Visible on CT
The most frequent causes of large bowel obstruction identifiable on imaging include:
- Colorectal carcinoma: Accounts for the majority of cases (18 of 22 mechanical obstructions in one series), appearing as an obstructing mass 1, 7
- Volvulus: Twisted bowel with characteristic "whirl sign" of mesenteric vessels 5, 2
- Diverticular stricture: Segmental narrowing with wall thickening 2
- Extrinsic masses: Pelvic tumors, endometriosis, or peritoneal carcinomatosis compressing the colon 3, 8
Critical Distinction: Obstruction vs Pseudo-Obstruction
CT is the imaging method of choice to differentiate mechanical obstruction from acute colonic pseudo-obstruction (Ogilvie syndrome), which shows colonic dilatation without a transition point 2. This distinction is crucial as management differs fundamentally—mechanical obstruction often requires surgery while pseudo-obstruction is managed conservatively 2.
Clinical Pitfalls to Avoid
- False negatives: Occur in approximately 9% of cases, sometimes showing only subtle mural thickening rather than obvious obstruction 1
- Adhesions: When no mass or extrinsic cause is visible on CT but a transition point exists, adhesions are the likely cause (responsible for 13 of 15 cases without visible CT abnormality in one study) 3
- Relying on plain films: Six patients with normal plain radiographs had positive CT scans for obstruction, while three patients with false-negative CT scans showed abnormalities on plain films—use both modalities complementarily 3
CT should replace contrast enema as the initial imaging method for suspected large bowel obstruction given its superior diagnostic accuracy and ability to identify the cause and complications in a single examination 1, 2.