01Purpose and principlesWhat the assessment is for and the core concepts behind it.
Mechanical obstruction produces proximal accumulation of gas and fluid with distal decompression. The imaging task is anatomical and physiological: identify small or large bowel, locate one or more transition points, define the cause and decide whether perfusion is threatened. Adhesions dominate adult SBO after surgery, while hernia, tumour, Crohn disease, volvulus and intussusception require different responses.
Large-bowel obstruction commonly reflects colorectal malignancy, diverticular stricture or volvulus. The degree of small-bowel dilatation depends on ileocaecal valve competence. A competent valve can trap pressure between the obstructing lesion and valve, progressively dilating the caecum and increasing perforation risk even when small bowel is not prominent.
Clinical assessment and CT complement each other. Early strangulation may have subtle laboratory results, and no single CT sign proves viability. A concerning combination, especially closed loop and reduced enhancement with pain or systemic illness, should drive direct communication and prompt operative judgment.
Key points
- Obstruction imaging must answer level, transition point, severity, cause and complications rather than merely confirm dilated bowel.
- Small bowel is usually central with valvulae crossing the lumen; colon is more peripheral with haustra that do not fully traverse it.
- Plain radiography may support obstruction but CT abdomen and pelvis is usually required in acute adult SBO to define cause and threatened bowel.
- Closed-loop configuration, impaired enhancement, pneumatosis, portal venous gas, mesenteric oedema or free fluid raises concern for strangulation.
- Do not delay surgical escalation for repeat radiographs or oral-contrast transit when the patient has peritonism, shock or CT ischaemia signs.
- Differentiate mechanical obstruction from ileus by seeking a discrete transition, collapsed downstream bowel and a causative lesion, while accepting overlap.
02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.
Dilated central loops with valvulae conniventes crossing the lumen, step-ladder air-fluid levels and relative distal collapse support SBO.
Peripheral haustrated colon dilates proximal to a distal transition; assess caecum, sigmoid configuration, rectum and any distal gas.
A calibre change from dilated proximal bowel to collapsed distal bowel localises mechanical obstruction and directs the search for a cause.
Two adjacent transition points isolate a segment; C- or U-shaped loops, converging mesenteric vessels and twisting raise urgent concern.
Reduced enhancement, wall thickening, mesenteric oedema, haemorrhage, pneumatosis, portal venous gas or free fluid may indicate compromised bowel.
Generalised small- and large-bowel dilatation without a clear transition can follow surgery, sepsis, metabolic disturbance or medication, but overlap demands context.
03Method and interpretationA systematic approach to the test and its findings.
Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.
- 01
Contrast CT abdomen and pelvisFirst step - Why
- Locate transition, define cause and assess bowel wall, mesentery and complications.
- Interpretation and limitations
- Closed loop, ischaemic wall appearances and free fluid can mandate surgery without delay; protocol adapts to renal and haemodynamic context.
- 02
Abdominal radiography - Why
- Provide a rapid overview of bowel gas and gross dilatation when selectively indicated.
- Interpretation and limitations
- Sensitivity and cause determination are limited; fluid-filled or early obstruction and strangulation may be missed.
- 03
Water-soluble contrast study - Why
- Assess transit in selected adhesive SBO managed non-operatively under a surgical protocol.
- Interpretation and limitations
- Failure to reach colon at the protocol endpoint predicts failure of conservative management, but danger signs preclude waiting.
- 04
Serum lactate and blood gas - Why
- Assess perfusion and metabolic consequences alongside imaging.
- Interpretation and limitations
- Elevation supports severity, but a normal early lactate does not exclude strangulated bowel.
- 05
Hernia and stoma examination - Why
- Identify an external mechanical cause that may need immediate treatment.
- Interpretation and limitations
- A tender irreducible hernia with obstruction is urgent even before definitive cross-sectional characterisation.
04Clinical next stepsHow the result changes management or prompts escalation.
01Worked casePostoperative vomiting and distensionFirst stepAn adult with previous laparotomy develops colicky pain, vomiting and abdominal distension.+
- 1Context: assess physiology, peritonism, herniae, last flatus, prior operations and whether pain has become continuous.
- 2Reasoning: use CT to distinguish mechanical obstruction from ileus, locate the transition and look specifically for a closed loop or reduced enhancement.
- 3Outcome: uncomplicated adhesive SBO may enter a supervised non-operative pathway; danger signs prompt immediate operative review.
- 4Verification: monitor symptoms, observations, urine output and laboratory trend, and document passage of contrast or decision for surgery.
02Acute obstructionDefine anatomy and dangerMechanical small- or large-bowel obstruction is suspected clinically.+
- 1Resuscitate, correct electrolyte disturbance, provide analgesia and involve the surgical team while arranging imaging.
- 2Perform appropriately protocoled CT in most acute adults; describe level, transition, severity, cause and complications.
- 3Communicate a closed loop, ischaemia, perforation, obstructing hernia, volvulus or dangerous caecal dilatation immediately.
- 4Choose operative, endoscopic or non-operative management according to cause, viability, physiology and specialist assessment.
03Equivocal radiographResolve ongoing suspicionPlain radiography is normal or nonspecific but obstruction remains clinically plausible.+
- 1Reassess for progression, peritonism, sepsis and an incarcerated hernia rather than using the film as a stopping rule.
- 2Review projection and whether fluid-filled bowel or an early process could be occult.
- 3EscalationEscalate to CT and senior surgical assessment when the result will change urgent care.
- 4Give explicit safety-netting and assign review ownership if immediate cross-sectional imaging is not undertaken.
05Risks, monitoring and follow-upComplications, safety checks and further assessment.
- Repeat clinical examination because transition from colic to continuous pain may signal strangulation.
- Trend pulse, blood pressure, temperature, urine output, renal function, electrolytes, acid-base state and lactate.
- Document nasogastric output, vomiting, distension and passage of flatus or stool in supervised conservative care.
- Review the CT report for transition point, cause and every viability sign, not only the word obstruction.
- Escalate immediately when pain, physiology or laboratory markers worsen despite apparently conservative imaging.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
Distal gas can persist
Partial or early obstruction may retain colonic and rectal gas, so its presence does not rule out a mechanical transition.
Two transitions matter
A closed loop traps a segment at both ends and can strangulate rapidly even when overall bowel dilatation seems modest.
Diameter is insufficient
Absolute calibre helps describe severity but cannot by itself distinguish ileus, chronic dilatation or threatened viability.
Contrast has two roles
Intravenous contrast assesses enhancement and complications; enteric water-soluble contrast is a separate supervised adhesive-SBO strategy.
LBO affects small bowel variably
An incompetent ileocaecal valve decompresses colon into small bowel, whereas a competent valve risks isolated progressive caecal dilatation.
Cause changes treatment
Adhesion, hernia, malignant stricture and volvulus may create similar upstream gas patterns but require distinct operative, endoscopic or oncological routes.
07Common pitfallsFrequent interpretation and management errors.
- 01
Calling ileus without seeking a transition point or mechanical cause.
- 02
Letting a normal radiograph exclude an early or fluid-filled obstruction.
- 03
Reporting obstruction without level, cause or complication assessment.
- 04
Waiting for contrast transit despite peritonism or CT signs of ischaemia.
- 05
Equating passage of stool or distal gas with absence of partial obstruction.