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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: adhesions from previous abdominal surgery (~60%). Then hernias (~20%), malignancy, Crohn's stricture
- Classic tetrad: colicky abdominal pain, vomiting (early and profuse), abdominal distension, absolute constipation (no flatus or faeces)
- Abdominal X-ray: dilated small bowel loops (>3 cm), valvulae conniventes visible, no gas in large bowel. CT is gold standard
- Initial management: "drip and suck" — IV fluids, NG tube decompression, catheter, analgesia
- Operate if: signs of strangulation (peritonitis, tachycardia, rising lactate, fever), complete obstruction not resolving with conservative management, or closed-loop obstruction
Overview
Small bowel obstruction (SBO) is a common surgical emergency caused by mechanical blockage of the small intestinal lumen. The commonest cause in developed countries is adhesions from previous surgery (~60%), followed by incarcerated hernias (~20%), Crohn's disease strictures, and less commonly by tumours, volvulus, intussusception, foreign bodies, or gallstone ileus. Obstruction leads to proximal bowel dilatation, increased intraluminal pressure, fluid and electrolyte sequestration into the bowel lumen (third-space losses), and risk of strangulation (vascular compromise → ischaemia → necrosis → perforation).
Epidemiology
SBO accounts for approximately 20% of all emergency surgical admissions. Adhesional SBO is increasingly common as more abdominal surgeries are performed. Approximately 70% of adhesional SBO resolves with conservative management; 30% require surgery. Strangulation carries a mortality rate of ~25% if not promptly treated. Recurrence rate for adhesional SBO is approximately 30% over 10 years.
Clinical Features
Symptoms
Colicky central abdominal pain (comes in waves corresponding to peristalsis)
Profuse vomiting (early and bilious — the more proximal the obstruction, the earlier and more profuse the vomiting)
Absolute constipation (no passage of flatus or faeces) — late feature once obstruction is complete
Abdominal distension (degree depends on level of obstruction — greater in distal SBO)
Continuous severe pain replacing colicky pain (suggests strangulation)
Signs
Abdominal distension (may be minimal in proximal/jejunal obstruction)
Visible peristalsis
High-pitched "tinkling" bowel sounds with rushes
Tenderness with localised guarding (strangulation)
Surgical scars (adhesion risk)
Check hernial orifices (inguinal, femoral, umbilical) — commonly missed cause
Tachycardia, fever, peritonitis (strangulation/perforation)
Investigations
First-line
Abdominal X-rayDilated small bowel loops (>3 cm), valvulae conniventes (cross the full width of the lumen), multiple fluid levels on erect film, paucity of distal gas
BloodsFBC, U&Es (dehydration, electrolyte derangement), lactate (raised = ischaemia/strangulation), CRP, VBG (metabolic derangement)
Second-line
CT abdomen/pelvis with IV contrastGold standard — identifies site, cause, and degree of obstruction. Transition point. Signs of strangulation (reduced wall enhancement, mesenteric haziness, free fluid, pneumatosis)
Water-soluble contrast follow-through (Gastrografin)Diagnostic AND therapeutic — appearance of contrast in colon at 4–24 hours predicts resolution with conservative management. Also has osmotic effect that promotes resolution
Specialist
CT angiographyIf mesenteric ischaemia suspected
Management
Bologna Guidelines and ASGBI guidance1
Conservative ("drip and suck")
- IV fluid resuscitation (correct dehydration and electrolyte abnormalities)
- NG tube decompression (on free drainage or low intermittent suction)
- Urinary catheter (monitor fluid balance)
- Analgesia (IV paracetamol ± opioid)
- VTE prophylaxis
- Nil by mouth
- Gastrografin challenge: if adhesional SBO not resolving at 24–48 h — both diagnostic and therapeutic
2
Surgical (operative)
- Indications for surgery: signs of strangulation (peritonitis, tachycardia, rising lactate, fever), irreducible hernia, closed-loop obstruction, failure to resolve with conservative management (48–72 h)
- Adhesiolysis (division of adhesions) — laparoscopic or open
- Bowel resection if non-viable segment identified
- Hernia repair if obstructed hernia is the cause
Complications
- Strangulation: Vascular compromise → bowel ischaemia → necrosis → perforation. Mortality ~25%
- Perforation and peritonitis: From ischaemic necrosis or caecal blow-out from closed-loop obstruction
- Dehydration and electrolyte derangement: Significant third-space fluid loss — metabolic alkalosis (from vomiting) or metabolic acidosis (from ischaemia)
- Aspiration: From vomiting — NG decompression reduces this risk
- Recurrence: ~30% adhesional SBO recurrence over 10 years
UKMLA Exam Tips
- 1ALWAYS examine hernial orifices in ANY patient with vomiting and abdominal pain — obstructed hernia is a commonly missed diagnosis
- 2Valvulae conniventes (plicae circulares) cross FULL width of lumen = small bowel. Haustral folds are INCOMPLETE = large bowel
- 3SBO causes: Adhesions > Hernias > Crohn's > Malignancy. "AHCM" or just remember adhesions are number one
- 4Strangulation signs: continuous (not colicky) pain, peritonitis, tachycardia, fever, raised lactate → urgent surgery
- 5Gastrografin reaching colon by 4–24 h = safe to continue conservative management. If not → likely needs surgery
- 6Proximal obstruction: early profuse vomiting, minimal distension. Distal obstruction: later vomiting, more distension
practicetest your knowledge on small bowel obstructionApply what you've learnt with UKMLA-style questions from the iatroX Q-Bank — gastroenterology and beyond.
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