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This is a clinician-written, evidence-based summary aligned to the 2026 MLA Content Map. It is intended for medical students and junior doctors preparing for the UKMLA. Always cross-reference with NICE guidance, local protocols, and clinical judgement.
The Bottom Line
- Commonest cause: colorectal cancer (~60%). Then volvulus (~20%), diverticular stricture
- Presents more gradually than SBO: progressive distension, absolute constipation, late vomiting (faeculent if at all)
- Risk of caecal perforation if ileocaecal valve is competent (closed-loop obstruction) — caecal diameter >9 cm = imminent perforation risk
- CT abdomen/pelvis with contrast: gold standard for diagnosis, site, cause, and complications
- Management depends on cause: colonic stenting (bridge to surgery) vs emergency colectomy ± stoma
Overview
Large bowel obstruction (LBO) results from mechanical blockage of the colon or rectum. The most common cause is colorectal cancer (~60%), followed by sigmoid/caecal volvulus (~20%) and diverticular stricture. Unlike SBO, LBO tends to present more insidiously with progressive abdominal distension and absolute constipation; vomiting is late and may be faeculent. A critical consideration is the competence of the ileocaecal valve: if competent, a closed-loop obstruction develops with no retrograde decompression, leading to progressive caecal dilatation and risk of perforation (caecal diameter >9 cm on imaging is concerning).
Epidemiology
LBO accounts for approximately 25% of all bowel obstructions. Approximately 15–20% of colorectal cancers present as emergency LBO. Sigmoid volvulus is the most common type of colonic volvulus, more common in elderly, institutionalised, or psychiatrically medicated patients. Caecal volvulus is less common but typically requires surgery. Overall mortality from emergency LBO is significant (~15–20%), partly due to the advanced age and comorbidity of affected patients.
Clinical Features
Symptoms
Progressive abdominal distension (dominant symptom)
Absolute constipation (no faeces or flatus)
Colicky lower abdominal pain
Vomiting is LATE (faeculent — implies long-standing obstruction)
Previous change in bowel habit or rectal bleeding (if underlying malignancy)
Weight loss (if malignancy)
Signs
Marked abdominal distension
Tympanic (resonant) percussion
Tinkling or absent bowel sounds
Empty rectum on DRE (obstruction is proximal)
Palpable mass (tumour)
Peritonism (indicates perforation or ischaemia)
Investigations
First-line
Abdominal X-rayDilated large bowel (>6 cm colon, >9 cm caecum), peripheral colonic gas with haustral folds (incomplete across lumen). May show "coffee bean" sign in sigmoid volvulus
BloodsFBC, U&Es, lactate (strangulation), CRP, group and save
Second-line
CT abdomen/pelvis with contrastGold standard — identifies cause (tumour, volvulus, stricture), site of obstruction (transition point), and complications (perforation, ischaemia). Helps surgical planning
Specialist
Water-soluble contrast enemaIf CT equivocal — can demonstrate level and nature of obstruction. Also helps differentiate pseudo-obstruction from mechanical LBO
Management
ACPGBI and NICE NG151 guidelines1
Initial resuscitation
- IV fluids, catheter, NG tube if vomiting
- Correct electrolyte abnormalities
- VTE prophylaxis and analgesia
2
Cancer-related LBO
- Colonic stenting (self-expanding metallic stent): bridge to elective surgery in left-sided obstruction — avoids emergency surgery and stoma in many cases
- Emergency Hartmann procedure: left-sided obstruction with perforation/peritonitis — sigmoid resection with end colostomy (reversal later)
- Right hemicolectomy with primary anastomosis: right-sided obstruction — generally safe as primary procedure
- Subtotal colectomy: if perforation with proximal dilatation or synchronous tumours
3
Sigmoid volvulus
- Flexible sigmoidoscopy with flatus tube insertion: first-line decompression for sigmoid volvulus (success rate ~80%)
- Elective sigmoid colectomy: definitive treatment after successful decompression (high recurrence rate ~60% without surgery)
- Emergency laparotomy: if signs of gangrene, perforation, or failed endoscopic decompression
4
Caecal volvulus
- Cannot be decompressed endoscopically — requires surgical intervention
- Right hemicolectomy: definitive treatment
Complications
- Caecal perforation: If ileocaecal valve competent (closed-loop) — caecal wall tension increases. Caecal diameter >9 cm = imminent perforation risk
- Bowel ischaemia and gangrene: From vascular compromise in volvulus or closed-loop obstruction
- Faecal peritonitis: From perforation — very high mortality
- Sepsis: Bacterial translocation from distended, ischaemic bowel
UKMLA Exam Tips
- 1LBO: late vomiting, marked distension. SBO: early vomiting, less distension
- 2Caecal diameter >9 cm on AXR = high risk of perforation — surgical emergency
- 3Sigmoid volvulus: "coffee bean" sign on AXR. Decompress with sigmoidoscopy + flatus tube, then elective resection
- 4Competent ileocaecal valve = CLOSED LOOP obstruction → progressive caecal dilatation → perforation risk
- 5Pseudo-obstruction (Ogilvie syndrome): clinical picture of LBO but NO mechanical cause. Treat with neostigmine
- 6Hartmann procedure = sigmoid resection + end colostomy + rectal stump closure — standard emergency operation for obstructed/perforated sigmoid cancer
practicetest your knowledge on large bowel obstructionApply what you've learnt with UKMLA-style questions from the iatroX Q-Bank — gastroenterology and beyond.
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