Key Facts

Bowel obstruction is a common surgical emergency. The distinction between small and large bowel obstruction is important as aetiology, management, and prognosis differ significantly.

Epidemiology

SBO accounts for ~80% of all intestinal obstructions and is responsible for ~20% of emergency surgical admissions. LBO accounts for ~20%. Adhesional SBO is the commonest cause overall. Colorectal cancer is the commonest cause of LBO.

Aetiology

Small bowel obstruction:

• Adhesions from previous surgery (~60%) — commonest overall cause
• Hernias (~20% — inguinal, femoral, incisional, internal)
• Malignancy (small bowel tumours, peritoneal carcinomatosis)
• Crohn's disease (stricture)
• Intussusception (children — idiopathic; adults — lead point e.g. polyp/tumour)
• Gallstone ileus (large gallstone through cholecystoenteric fistula → impaction at ileocaecal valve)

Large bowel obstruction:

• Colorectal cancer (~60%)
• Volvulus (~20% — sigmoid > caecal)
• Diverticular stricture (~10%)
• Pseudo-obstruction (Ogilvie syndrome)

Pathophysiology

Mechanical obstruction causes proximal bowel dilatation from accumulated fluid and gas. Intraluminal pressure rises → venous congestion → mucosal oedema → bacterial translocation → ischaemia (strangulation) → necrosis → perforation. Third-space fluid loss causes dehydration and electrolyte disturbance. A closed-loop obstruction (e.g. LBO with competent ileocaecal valve, or volvulus) has the highest perforation risk due to progressive caecal dilatation (>12 cm = high perforation risk).

Small Bowel Obstruction

• Colicky central/periumbilical pain (5-10 minute cycles)
• Vomiting (early — bilious/faeculent if distal)
• Abdominal distension (moderate)
• Absolute constipation (late — initially may still pass flatus/stool)
• Tinkling/high-pitched bowel sounds → absent (late)

Large Bowel Obstruction

• Colicky lower abdominal pain
• Absolute constipation (early — no flatus or stool)
• Marked abdominal distension
• Vomiting (late — may be faeculent)

Red Flags (Strangulation/Ischaemia)

• Constant severe pain (rather than colicky)
• Tachycardia, hypotension, fever
• Peritonism (guarding, rebound)
• Irreducible tender hernia
• Raised lactate, leucocytosis
• Absent bowel sounds
• Rapidly worsening clinical state

Bedside

• Observations: pulse, BP, temperature, urine output (catheter if needed)
• NG tube: decompression (therapeutic and diagnostic — bilious aspirate)
• Hernia sites: examine ALL hernial orifices (inguinal, femoral, incisional, umbilical)

Bloods

• FBC: leucocytosis (strangulation/infection)
• U&Es: dehydration, hypokalaemia (vomiting), AKI
• Lactate: raised with ischaemia/strangulation
• LFTs, amylase/lipase: exclude biliary/pancreatic cause
• VBG: metabolic alkalosis (vomiting) or acidosis (ischaemia)
• Group and save: pre-operative

Imaging

• CT abdomen/pelvis with IV contrast: gold standard — identifies site of obstruction (transition point), cause, and complications (ischaemia, perforation, closed-loop)
  • SBO: dilated small bowel (>3 cm), collapsed distal, transition point
  • LBO: dilated colon (>6 cm; caecum >9 cm), obstructing lesion
• AXR: initial assessment — dilated loops (SBO: central, valvulae conniventes; LBO: peripheral, haustral folds); limited sensitivity/specificity
• Gastrograffin follow-through: for adhesional SBO — therapeutic (osmotic effect) and prognostic; passage to colon on 24-hour film predicts resolution without surgery

Special Tests

• CT angiography: if mesenteric ischaemia suspected
• Colonoscopy: therapeutic for sigmoid volvulus decompression; diagnostic for LBO cause

Non-pharmacological

• "Drip and suck": IV fluid resuscitation + NG decompression
• NBM: bowel rest
• Catheterisation: monitor urine output
• Thromboprophylaxis: LMWH
• Monitoring: serial examinations, bloods (lactate, WCC), repeat imaging if not improving

Pharmacological

• IV fluids: aggressive crystalloid resuscitation (Hartmann's or 0.9% saline with K+ replacement)
• Electrolyte correction: potassium, magnesium
• Analgesia: IV paracetamol + IV opioids (titrated)
• Anti-emetics: ondansetron 4-8 mg IV, cyclizine 50 mg IV
• Gastrograffin 100 mL oral/via NG tube: for adhesional SBO — safe if no strangulation; predict need for surgery (contrast in colon on 24h XR = likely to resolve)
• Neostigmine 2 mg IV (over 5 min with cardiac monitoring): for acute colonic pseudo-obstruction (Ogilvie syndrome) if caecal diameter >12 cm

Surgical/Interventional

• SBO — adhesional: conservative management resolves ~75% within 48-72 hours; surgery if strangulation, failed conservative (no improvement by 72 hours, or no contrast in colon on 24h Gastrograffin)
• SBO — hernia: emergency surgery if strangulated/irreducible
• LBO — cancer: emergency colonic stenting (as bridge to surgery) or emergency surgery (Hartmann's procedure or primary anastomosis ± defunctioning ileostomy)
• LBO — volvulus: see separate entry
• Laparoscopic/open adhesiolysis: for adhesional SBO
• Bowel resection: for ischaemic/gangrenous bowel (with or without anastomosis depending on contamination)

Referral Criteria

• All bowel obstruction: urgent surgical review
• Emergency theatre: strangulation, perforation, closed-loop obstruction
• Colorectal MDT: if LBO from malignancy
• Interventional radiology/endoscopy: colonic stenting

Adhesional SBO: ~75% resolve with conservative management. Mortality for uncomplicated SBO is <5%. Strangulated/ischaemic bowel increases mortality to 10-25%. LBO from colorectal cancer: emergency surgery mortality is 15-20% (vs 2-5% for elective resection). Closed-loop obstruction has the highest perforation risk. Recurrent adhesional SBO occurs in ~15-20% after adhesiolysis. Colonic stenting as bridge to surgery has lower stoma rates than emergency surgery.

SBO vs LBO Comparison

Normal vs Obstructed Bowel Diameters