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OverviewAetiologyPathophysiologyClinical features and red flagsInvestigationsDifferential diagnosisManagementComplicationsMonitoring and follow-upSpecial situationsCommon pitfallsQuestionsSources
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Educational draft · awaiting clinical reviewThe full textbook explains uncertainty but does not replace live national or local guidance, specialist advice, or current prescribing information.

Small-bowel obstruction

Gastroenterology and hepatology · UK scope · Sources checked 27 Aug 2026 · Clinical review pending

RapidFull
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Time-critical presentation

Continuous or disproportionate pain, focal tenderness, guarding, tachycardia, fever, acidosis, shock, a closed-loop CT pattern or an irreducible tender hernia suggests strangulation or perforation. Keep the patient nil by mouth, resuscitate, decompress when indicated and obtain immediate senior surgical and theatre review; do not wait for peritonism to develop.

Overview

Recognise small-bowel obstruction and strangulation, resuscitate while defining cause and transition on contrast CT, and select non-operative management only when a safe monitored adhesional pathway is appropriate with immediate senior surgical, radiology and local contrast-protocol input.

Mechanical obstruction causes proximal accumulation of gas and fluid, vomiting, third-space loss and eventually venous congestion, arterial compromise and perforation. The clinical pattern depends on level: proximal obstruction vomits earlier with less distension, while distal obstruction produces marked distension and later faeculent vomiting. Partial obstruction can still pass flatus. Ileus is diffuse dysmotility without a mechanical transition and has different causes and imaging.

Prior laparotomy raises adhesion probability but is not permission to stop thinking. A tender irreducible hernia may be the whole diagnosis. A virgin abdomen raises hernia, malignancy, inflammatory stricture and gallstone ileus, but adhesions can still occur. In postoperative Roux-en-Y anatomy, internal hernia may be intermittent and high risk. Examine scars, groins and stomas and retrieve operation records.

Practice

Two practice questions

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Gastroenterology and hepatologyOriginal SBA

Closed-loop danger

CT in a patient with small-bowel obstruction shows two transition points, mesenteric swirl and reduced bowel-wall enhancement. What is the best next step?

Choose one answer, then submit
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Sources and review status5 sources · checked 27 Aug 2026 · clinical review pending+
Sources

Sources and review status

National guidance is shown before implementation-dependent detail. Typical adult dose examples remain subject to patient factors, contraindications and the live BNF or specialist protocol. Source check completed 27 Aug 2026; clinical approval remains outstanding.

  • NICE CKS: Acute abdominal pain investigationsUK primary acute-abdomen investigation context, including obstruction and lactate.
  • RCS Annals: Gastrografin in small-bowel obstructionCurrent UK emergency-surgery evidence and protocol variability.
  • RCR and RCEM emergency contrast CT statementDo not delay life-saving contrast CT for routine renal testing.
  • NICE CG174: Intravenous fluid therapyResuscitation, replacement and monitoring principles.
  • BNF: Intravenous fluidsCurrent UK fluid and electrolyte prescribing context.

Editorial contact: Doctor’s Passport editorial team. Source-check date is separate from clinical approval. Report a content problem · Content changes

Authoring stateComplete draft
Non-operative management is an active surgical decision. It requires absence of peritonitis, strangulation, closed loop and a cause needing correction; repeated examinations, fluid replacement, urine monitoring and a time-limited endpoint are mandatory. Failure to improve, worsening pain, rising support needs or unfavourable contrast progression triggers surgery. Frailty, pregnancy and cancer change decisions but do not erase ischaemia risk.

Key points

  • Small-bowel obstruction presents with colicky pain, vomiting, distension and obstipation, but diarrhoea or stool passage can continue early or in partial obstruction.
  • Adhesions are common after surgery, but always examine hernial orifices and consider tumour, Crohn's stricture, volvulus, intussusception, gallstone ileus and radiation disease.
  • Continuous pain, fever, tachycardia, peritonism, acidosis or CT reduced enhancement, closed loop, mesenteric oedema, pneumatosis or free fluid raises strangulation and requires urgent operation.
  • A normal lactate does not exclude early ischaemia, and a raised lactate may result from dehydration; serial physiology and contrast CT matter.
  • Initial care is nil by mouth, intravenous access and balanced crystalloid, electrolyte and renal correction, analgesia, antiemetic, nasogastric decompression when indicated and early surgical review.
  • Contrast-enhanced CT usually confirms the transition point, cause, closed loop and bowel viability; a plain radiograph cannot safely answer all of these.
  • Selected uncomplicated adhesive obstruction can undergo a closely monitored non-operative trial; hernia, malignancy, closed loop or ischaemia follows a different pathway.
  • Water-soluble oral contrast can predict resolution and may be therapeutic in selected adhesional obstruction, but aspiration and hyperosmolar fluid-shift risks require a local protocol.

Aetiology

01

Postoperative adhesions

Fibrous bands after abdominal surgery are a common cause, kinking or trapping bowel without a visible mass.

02

Hernia, tumour and inflammatory stricture

External or internal hernias, neoplasia and Crohn-related fibrosis create focal transition points and may compromise blood supply.

03

Volvulus and intraluminal causes

Twisting, intussusception, gallstone ileus, bezoar and foreign material produce less common mechanical obstruction with cause-specific imaging features.

Pathophysiology

  1. 1
    Proximal fluid and gas accumulation

    Continued secretion and swallowed air distend bowel above the blockage, causing colicky pain, vomiting and third-space loss.

  2. 2
    Closed-loop vascular compromise

    Obstruction at two points traps a segment, rapidly impairing venous drainage and then arterial inflow as pressure rises.

  3. 3
    Necrosis and contamination

    Strangulated bowel loses barrier integrity, progresses to infarction and perforation, and releases bacteria and contents into the peritoneum.

Clinical features and red flags

Typical mechanical pattern

Intermittent colicky central pain, vomiting, distension, high-pitched or changing bowel sounds and reduced flatus support obstruction, with level and completeness modifying the sequence.

Strangulation or perforationRed flag

Pain becomes continuous, localised or disproportionate, with tachycardia, fever, guarding, acidosis, shock or free air. This requires immediate senior surgical and theatre planning.

Closed-loop obstructionRed flag

Rapid severe pain and CT C- or U-shaped loop with two transition points, radial mesenteric vessels or swirl has high ischaemia risk even before peritonism.

Incarcerated herniaRed flag

A tender irreducible groin, incisional, parastomal or umbilical lump with obstruction is a source-control emergency; avoid forceful reduction when strangulation is plausible.

High proximal obstructionRed flag

Early profuse vomiting with little distension can cause severe chloride, potassium and volume loss; aspiration risk and metabolic alkalosis may dominate.

Investigations

  1. 01
    Contrast-enhanced CT abdomen and pelvisFirst step
    Why
    Confirm mechanical obstruction, level and cause, identify closed loop, hernia, tumour, inflammatory disease and signs of ischaemia or perforation.
    Interpretation and limitations
    Reduced mural enhancement, mesenteric congestion, pneumatosis, portal venous gas and free fluid increase concern, but no single sign replaces clinical assessment.
  2. 02
    Serial blood gas and lactate
    Why
    Assess perfusion, acid-base disturbance and trend during resuscitation alongside FBC, renal profile, CRP and electrolytes.
    Interpretation and limitations
    A normal early lactate does not exclude threatened bowel; a falling value after fluids does not negate a closed loop.
  3. 03
    Abdominal radiograph
    Why
    Provide a rapid overview where CT is delayed or to track selected contrast protocols, showing dilated central loops and fluid levels.
    Interpretation and limitations
    Sensitivity and cause assessment are limited. A non-diagnostic film does not exclude obstruction or ischaemia.
  4. 04
    Water-soluble contrast challenge
    Why
    In a selected stable adhesional obstruction, administer contrast by the local protocol and image its progression to colon.
    Interpretation and limitations
    Colonic arrival predicts non-operative resolution. Failure supports surgery, but the test is unsafe with aspiration risk, perforation or suspected strangulation.
  5. 05
    Hernia and operation assessment
    Why
    Examine all groins, scars and stomas, review previous operations and define altered anatomy before imaging and decompression.
    Interpretation and limitations
    A palpable source can accelerate treatment; absent scars do not rule out adhesions or congenital bands.
  6. 06
    Pregnancy and pre-operative tests
    Why
    Check pregnancy where relevant, group and save or crossmatch, coagulation and ECG according to urgency and comorbidity.
    Interpretation and limitations
    Pregnancy modifies imaging but does not justify unsafe diagnostic delay; senior radiology and obstetric input selects the fastest adequate test.

Differential diagnosis

01

Paralytic ileus

Diffuse bowel dilatation without a transition after surgery, sepsis or electrolyte disturbance suggests motility failure rather than focal blockage.

02

Large-bowel obstruction

Predominant colonic dilatation and a distal colonic transition point identify LBO, although an incompetent ileocaecal valve can dilate small bowel too.

03

Gastroenteritis or functional pain

Prominent diarrhoea, sick contacts and no transition favour infection, while functional symptoms lack persistent vomiting, distension and obstructive imaging.

Management

01ResuscitateInitial obstruction careFirst stepA mechanical small-bowel obstruction is clinically suspected.+
  1. 1Use ABCDE, provide analgesia and antiemetic, keep nil by mouth, gain intravenous access and correct fluid and electrolyte loss with urine monitoring.
  2. 2Place a nasogastric tube for significant vomiting, proximal dilatation or aspiration risk according to the surgical plan, confirming position before use.
  3. 3Examine hernias and stomas, send bloods and request contrast CT with an explicit obstruction and ischaemia question.
  4. 4Involve the senior surgical team from the outset and prescribe VTE prevention and antibiotics only when indicated.
02OperateStrangulation or closed loopPeritonism, shock, continuous pain or imaging suggests ischaemia, perforation or a closed loop.+
  1. 1EscalationEscalate immediately to consultant surgery, anaesthesia and theatre while continuing resuscitation, broad antibiotics and crossmatch.
  2. 2Do not delay source control for a water-soluble contrast challenge or repeated plain films.
  3. 3At operation release the cause, assess viability and resect non-viable bowel with damage-control or second-look strategy when needed.
  4. 4Plan postoperative critical care and consider intestinal-failure advice after major resection.
03ObserveSelected adhesional obstructionCT supports simple adhesional obstruction without ischaemia, peritonitis, closed loop or another surgically correctable cause.+
  1. 1Continue nasogastric decompression when needed, intravenous replacement, urine and electrolyte monitoring and repeated senior abdominal examination.
  2. 2Use a local water-soluble contrast protocol when aspiration and perforation risks are acceptable.
  3. 3Set a documented time and clinical endpoint for non-operative treatment; worsening physiology or failure of contrast progression prompts surgery.
  4. 4After resolution advance diet cautiously and provide recurrence and urgent return advice.
Key medicines and prescribing safety3 treatments · regimens, roles and cautions+
Restores circulating volume and replaces obstruction-related extracellular fluid loss.

Balanced intravenous crystalloid

Give an initial resuscitation bolus and subsequent replacement guided by physiology, urine, nasogastric loss and NICE CG174; add electrolytes from measured deficits.

Avoid both under-resuscitation and salt or fluid overload. Frailty, heart and kidney disease require smaller reassessed boluses and senior review.

Covers enteric organisms when mucosal compromise or contamination is likely.

Broad-spectrum perioperative antibiotic

Use the current local intra-abdominal sepsis regimen when strangulation, perforation, surgery or systemic infection is suspected, with renal adjustment.

Antibiotics do not release obstruction. Obtain cultures when useful, de-escalate after source control and monitor C. difficile and allergy risk.

Helps predict non-operative resolution and may draw fluid into the lumen to aid transit.

Water-soluble hyperosmolar contrast

Protocol-specific enteral dose after gastric decompression in selected adhesional obstruction, prescribed jointly by surgery and radiology.

Aspiration can cause severe pulmonary injury and hyperosmolarity can worsen dehydration. Avoid in suspected perforation, strangulation or inability to protect the airway.

Complications

01

Strangulation and perforation

Continuous pain, peritonism or CT vascular compromise indicates bowel infarction requiring urgent surgery rather than a non-operative adhesional pathway.

02

Dehydration, electrolyte loss and aspiration

Vomiting and sequestration cause kidney injury and alkalosis, while a distended stomach increases aspiration risk during deterioration or anaesthesia.

03

Sepsis and short bowel

Necrotic perforated intestine causes shock, and extensive resection can leave chronic malabsorption or intestinal failure after extensive surgery.

Monitoring and follow-up

  • Repeat pulse, blood pressure, temperature, respiratory status, pain and abdominal findings at a frequency matched to risk.
  • Measure urine output and fluid balance, replacing nasogastric and other losses with electrolyte-aware prescriptions.
  • Trend creatinine, sodium, potassium, chloride, bicarbonate, magnesium and lactate through resuscitation.
  • Record nasogastric aspirate amount and character, tube position and ongoing indication.
  • Review CT promptly with radiology and document the transition, cause and viability assessment.
  • During conservative treatment, record the decision deadline, passage of flatus or stool, contrast progression and triggers for theatre.
  • After surgery or major resection, monitor for sepsis, ileus, anastomotic failure and short-bowel physiology.

Special situations

Passing stool does not exclude obstruction

Distal contents can empty after a proximal blockage, and partial obstruction permits some transit.

A quiet abdomen can be late

Bowel sounds may become absent with fatigue or ischaemia. Auscultation should never overrule pain, physiology and CT.

Lactate is a late witness

Mesenteric collateral flow and local injury can precede systemic lactate elevation; do not use a normal value as clearance.

The groin is part of the abdomen

Failure to expose and examine hernial orifices misses a readily treatable and sometimes strangulated cause.

Conservative means monitored

Non-operative care is not passive waiting; it needs repeated examination, replacement and a planned point of failure.

Common pitfalls

  1. 01

    Excluding obstruction because the patient passed stool that morning.

  2. 02

    Using a normal lactate to rule out strangulation.

  3. 03

    Failing to examine groins and stomas.

  4. 04

    Calling every postoperative obstruction adhesive before contrast CT.

  5. 05

    Giving hyperosmolar contrast to a vomiting patient with an unprotected airway.

  6. 06

    Continuing conservative management without a deadline or senior reassessment.

Clinical stateAwaiting review
JurisdictionUnited Kingdom