01Purpose and principlesWhat the assessment is for and the core concepts behind it.
Large-bowel obstruction is a failure of onward colonic transit caused by a physical blockage or twist. The useful diagnostic endpoint is more precise than “dilated bowel”: identify where flow stops, why it stops, whether the affected bowel remains viable and which intervention can relieve it. A tumour may obstruct gradually until a previously compensated narrowing becomes complete. Volvulus can compromise the mesenteric circulation as well as the lumen, making delay dangerous even before dramatic biochemical abnormalities appear. Distension, pain, vomiting and altered stool passage provide the starting pattern, but none separately defines the diagnosis.
Assessment and resuscitation occur together. Ask about the last normal bowel movement, flatus, previous attacks, cancer symptoms, abdominal surgery, medication changes and baseline function. Examine hydration and perfusion before concentrating on the abdomen. Distension may obscure local signs, especially in frail patients or people unable to describe pain clearly. A rectal examination can identify an accessible lesion, faecal loading or an empty vault, but an empty rectum does not distinguish cancer from volvulus. The radiology request should communicate the suspected mechanism, clinical deterioration and the question of threatened bowel, so the report addresses the decisions that cannot wait.
Key points
- Obstruction with shock, guarding or suspected bowel infarction requires immediate surgical assessment while resuscitation proceeds; imaging must not postpone necessary source control.
- Contrast-enhanced CT of the abdomen and pelvis is the principal investigation for suspected malignant obstruction and uncertain or complicated volvulus.
- Read the transition point, proximal colon, caecum, mesenteric vessels and bowel enhancement together; a normal lactate cannot establish bowel viability.
- A competent ileocaecal valve can trap a distal obstruction within the colon; small-bowel dilatation may occur when the valve permits backflow.
- A plain film can quickly recognise a familiar stable sigmoid volvulus, but it cannot reliably determine the cause or exclude ischaemia.
- Exclude a mechanical lesion and inflammatory or infective toxic dilatation before calling a distended colon acute pseudo-obstruction.
02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.
Progressive distension, colicky pain and failure to pass flatus suggest obstruction. Vomiting can be late in large-bowel disease, and passage of liquid stool or residual distal contents does not exclude a developing blockage. Establish the trend and severity rather than requiring every textbook symptom.
Tachycardia, hypotension, reduced urine output, confusion, fever or cool peripheral perfusion indicate a patient who needs urgent resuscitation and senior review. Dehydration can coexist with sepsis. Interpret these signs against premorbid observations, medication effects and the speed of deterioration.
Persistent focal pain, new caecal tenderness, guarding or rebound changes a distension problem into possible ischaemia or perforation. Request immediate surgical reassessment even if an earlier examination was reassuring. Analgesia is appropriate, but reassess the abdomen and physiology after treatment.
Prior similar decompressions and a very distended tympanic abdomen suggest sigmoid volvulus; weight loss or anaemia may suggest malignancy. A recent major operation, severe medical illness or electrolyte disturbance raises ACPO as a possibility. These are probability modifiers and never replace exclusion of an obstructing lesion.
Review scars, hernial sites, rectal findings, hydration and respiratory restriction from distension. Establish the patient’s capacity, baseline independence and treatment preferences early, using collateral history where necessary. These findings influence the safest intervention as well as the diagnostic interpretation.
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
Urgent contrast-enhanced CT abdomen and pelvisFirst step - Why
- Identify a transition, its cause, bowel perfusion and complications in a patient stable enough for scanning.
- Interpretation and limitations
- Follow the colon from rectum to caecum. Describe collapsed distal bowel, the obstructing mass or twisted mesentery, proximal calibre and wall enhancement. Free gas, extraluminal contents, reduced enhancement or concerning free fluid must be interpreted with the clinical state and communicated directly to the surgeon.
- 02
Selective plain abdominal radiograph - Why
- Recognise a characteristic volvulus quickly when the clinical situation makes that useful.
- Interpretation and limitations
- A greatly enlarged loop with opposed medial walls can produce a coffee-bean appearance. This is a clue to configuration, not proof of viability. An equivocal film should lead promptly to CT; suspected malignant obstruction does not require a preliminary film before CT.
- 03
Blood count, electrolytes, renal function and inflammatory markers - Why
- Measure physiological consequences and prepare safely for contrast, anaesthesia and treatment.
- Interpretation and limitations
- Anaemia may support chronic cancer-related blood loss, while leukocytosis can accompany inflammation or infarction. Renal impairment and potassium abnormalities need correction and influence prescribing. Normal inflammatory markers early in the illness do not justify ignoring a deteriorating examination.
- 04
Blood gas and serial lactate - Why
- Assess acid-base disturbance and perfusion while following the response to resuscitation.
- Interpretation and limitations
- A rising lactate or acidosis adds urgency but has several possible causes. A normal value cannot exclude focal bowel ischaemia, particularly before advanced necrosis or systemic hypoperfusion. Use the trajectory alongside pain, haemodynamics, urine output and imaging rather than as a clearance test.
- 05
Targeted endoscopy after imaging and surgical discussion - Why
- Resolve uncertainty about a stable distal lesion or undertake an appropriate therapeutic procedure.
- Interpretation and limitations
- Endoscopy can obtain tissue and decompress selected sigmoid volvulus or malignant obstruction. It is not a routine diagnostic step in peritonitis or suspected infarction. Histological confirmation is valuable, but must not postpone emergency treatment when the structural diagnosis and need for surgery are already clear.
04Clinical next stepsHow the result changes management or prompts escalation.
01Immediate assessmentFrom obstruction to a timely decisionFirst stepA patient presents with marked distension and possible large-bowel obstruction.+
- 1Call the surgical team, keep the patient nil by mouth, establish intravenous access and assess shock, peritonitis and aspiration risk. Resuscitate and provide appropriate analgesia while urgent intervention is organised.
- 2For a stable patient, obtain contrast-enhanced CT promptly and ask explicitly about the transition point, cause, caecal distension and bowel viability. If shock or peritonitis already mandates surgery, do not create a mandatory CT-first delay.
- 3Review the images with radiology when the written report leaves a management question unresolved. A tumour, sigmoid twist, caecal volvulus and ACPO require different decompression strategies.
- 4EscalationDocument the selected intervention, its urgency and a named reassessment plan. Continued observation needs a defensible diagnosis, physiological stability and explicit triggers for escalation.
02Worked caseA scan that changes the operationA 69-year-old has three days of distension, no flatus and increasing right-sided pain.+
- 1Initial assessment finds pulse 112, blood pressure 108/66 and right-sided tenderness without guarding. Intravenous access, fluids, nil-by-mouth care and surgical review begin while blood tests and CT are arranged.
- 2CT demonstrates an obstructing descending-colon tumour, a distended caecum with patchy reduced enhancement, and little small-bowel dilatation. Lactate is 1.8 mmol/L, but the imaging and persistent focal pain remain concerning.
- 3The consultant interprets the proximal colon as threatened in a closed compartment. The team chooses urgent operative assessment instead of a stent or further outpatient cancer investigations.
- 4At laparotomy the caecum is nonviable and the distal tumour remains obstructing. Subtotal colectomy removes both problems; reconstruction is selected from the actual physiological state and tissue quality, with consent already covering a possible stoma.
- 5Postoperatively, serial perfusion measures and abdominal assessments improve. The operation and pathology findings are recorded, and staging and colorectal multidisciplinary review follow recovery. The initial normal-range lactate never overruled the evidence that led to source control.
03Diagnostic reconciliationWhen the findings do not fit neatlyCT shows colonic dilatation but the reported cause remains uncertain.+
- 1Revisit the study with a radiologist to decide whether there is a fixed transition, a twist or an apparent functional calibre change. Confirm whether the entire distal colon has been adequately assessed.
- 2Check for recent surgery, drugs, electrolyte abnormalities and medical precipitants, while looking separately for diarrhoea, colitis and systemic toxicity. These contextual findings guide the differential but do not independently prove ACPO.
- 3Choose additional imaging or selective endoscopy only to answer the unresolved question and only when the patient remains suitable. Repeat clinical examination immediately if symptoms intensify rather than waiting for a planned investigation slot.
05Risks, monitoring and follow-upComplications, safety checks and further assessment.
- Follow pain location, abdominal tenderness and distension alongside pulse, blood pressure, respiratory rate, mental state and urine output. A change from intermittent discomfort to persistent focal pain needs fresh assessment.
- Repeat renal function, electrolytes and perfusion investigations according to severity and response. A reassuring single measurement cannot substitute for showing that resuscitation has improved the patient.
- Check that the CT report has been acted upon: record the obstructing site, bowel viability concerns, proposed intervention and responsible senior clinician. A scan filed without a management decision does not complete investigation.
- After decompression or surgery, verify actual symptom relief and physiological improvement. Persistent distension, pain or sepsis requires reconsideration of incomplete relief, a second lesion, ischaemia or a procedure-related complication.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
Calibre is contextual
The caecum is vulnerable because increasing radius raises wall tension at a given pressure. Diameter therefore informs risk, but there is no universal number below which obstruction is safe. Speed of expansion, duration, wall appearance and the patient’s condition determine whether observation remains appropriate.
Read both ends
A distal tumour plus a competent ileocaecal valve can produce a closed-loop large-bowel obstruction. If the valve is incompetent, gas and fluid may pass into the small bowel. Neither a dilated small bowel nor its absence excludes a distal colonic cause, so trace the actual transition.
Pneumatosis needs interpretation
Gas within the caecal wall can be a concerning finding, but it is not by itself a histological diagnosis of irreversible infarction. Reduced enhancement, pain, peritoneal signs, systemic deterioration and other CT features matter. Equally, uncertain imaging should not neutralise a convincing clinical picture of threatened bowel.
Different tests answer different questions
A film may identify loop configuration, CT can define the cause and complications, and endoscopy can inspect accessible mucosa or deliver decompression. Calling a single investigation definitive for every presentation hides these differences. Select the test that changes the next decision without creating avoidable delay.
07Common pitfallsFrequent interpretation and management errors.
- 01
Waiting for faeculent vomiting or complete obstipation can delay recognition; proximal decompression through an incompetent valve and residual distal contents alter the presentation.
- 02
Using a low lactate as evidence against bowel ischaemia mistakes a systemic perfusion marker for a direct measurement of every segment of colon.
- 03
Assuming all postoperative colonic dilatation is ileus or ACPO can miss a mechanical lesion; establish the anatomy before using a prokinetic.
- 04
Ordering repeated plain films in an undiagnosed, worsening obstruction can consume the time needed for CT interpretation and definitive treatment.