Suspected bowel obstruction

ACR AC Suspected Small-Bowel Obstruction (2020); WSES ASBO 2017

The question is almost never "is there an obstruction" alone. It is level, cause, and whether the bowel is compromised — and only the last of those three changes what happens in the next hour.

Distension, vomiting, colicky pain and absent flatus, commonly with previous abdominal surgery or a hernia.

Referenced decision support — confirm against your local protocol.

Decision support, not a directive. Protocols and timings shown are typical published ones — your local protocol takes precedence, and the vetting radiologist decides.

The request in front of you

Everything is optional. Leave a field alone and the answer assumes nothing — the verdict updates as you go.

Study requested

What the referrer actually asked for. It is evidence of intent, not a constraint on the right answer.

Contrast as written

What the request form says, not what it should say.

Age

Pick a band, or type an exact age if it matters.

years

Pregnancy status

Renal risk factors

The question a vetter can actually answer from the request. An explicit “none known” is a real answer, and it removes checks rather than deferring them.

Previous contrast reaction

The pathway — tap anything already done

Marking a study complete moves the answer on. A patient arrives partway through a pathway far more often than at the start of one.

Accept as requested

CT Abdomen and Pelvis — Portal Venous Phase

CT Abdomen and Pelvis

What we'd amend, and why

Intravenous contrast is not there to show the dilatation, which is visible without it. It is there to show bowel wall enhancement, and reduced or absent mural enhancement is the imaging evidence of strangulation that turns a conservative admission into an emergency laparotomy. The retained fluid in obstructed loops acts as its own neutral luminal contrast, which is why oral agents are unnecessary and counterproductive here.

Nothing needs resolving before this goes ahead. Routine checks below are owned downstream.

How was this decided?

pathwayadult — Adults
rulerule-contrast-reaction-premed — Prior contrast reaction and elective premedication; checked by Nurse before the scan
rulerule-metformin — Metformin and iodinated contrast; checked by Radiographer at the scanner
rulerule-paeds-dose — Child-sized technique and contrast dose; checked by Radiographer at the scanner
rulerule-pregnancy-ionising — Pregnancy status before an ionising exposure; checked by Radiographer at the scanner
rulerule-renal-iodinated — Kidney function and intravenous iodinated contrast; checked by Radiographer at the scanner
rulerule-iv-access — Intravenous access adequate for the planned injection; checked by Radiographer at the scanner

Decision support only. Local protocol takes precedence.

Handled at the scanner(4)nothing for you to do

Settled and owned downstream. Each returns to a radiologist only on the stated trigger.

Metformin and iodinated contrast
Confirm whether the patient takes metformin or a metformin-containing combination, and if so whether ACR Category II applies (eGFR below 30, known or suspected AKI, or an arterial catheter study likely to cause renal embolisation). If Category I — that is, no AKI and eGFR at or above 30 — no action of any kind is needed.
Radiographer at the scannerAt the scanner
Flags back if: The patient takes metformin AND meets ACR Category II — eGFR below 30 mL/min/1.73 m2, known or suspected acute kidney injury, or an arterial catheter procedure with likely renal arterial embolisation. Metformin plus a normal or mildly reduced eGFR is explicitly NOT a flag-back: there is no need to stop metformin before or after intravenous iodinated contrast in Category I patients, and no need to re-check creatinine afterwards.
Child-sized technique and contrast dose
Confirm that a size- or weight-based protocol is selected — child-sized kV and mAs against size-based diagnostic reference ranges — and that contrast volume is calculated by weight rather than taken from an adult default. Weight-based iodinated contrast volumes of roughly 1.5–2.0 mL/kg are widely used in paediatric CT.
Radiographer at the scannerAt the scanner
Flags back if: No paediatric or size-based protocol exists on the scanner for the requested examination, or the requested coverage or number of phases exceeds what the clinical question needs — for example a multiphase study where a single phase answers it, or whole-body coverage for a focal question.
Pregnancy status before an ionising exposure
Make the pregnancy enquiry immediately before the exposure and record the answer. In the UK this is a statutory operator duty discharged at the time of exposure under the employer’s written procedures required by IR(ME)R 2017 — it is not something the vetting radiologist can perform or pre-empt, and a request is complete without it.
Radiographer at the scannerAt the scanner
Flags back if: The patient states that she is, or may be, pregnant AND the uterus is in or near the primary beam. The exposure is then paused for re-justification by the IR(ME)R practitioner before it proceeds.
Intravenous access adequate for the planned injection
Site and test a cannula that supports the protocol flow rate, preferring an antecubital or large forearm vein, and observe the injection for extravasation. A 20-gauge or larger cannula is preferred for flow rates of 3 mL/s or more.
Radiographer at the scannerAt the scanner
Flags back if: No cannula can be sited that supports the protocol flow rate — for example only a 22-gauge hand or foot cannula for a CT angiogram needing 4–5 mL/s; or the only available access is a central line or port that is not labelled power-injectable; or an extravasation occurs.

Worth asking the referrer (3)

None of these hold the request up. They sharpen the protocol or the plan that follows.

Has the patient had previous abdominal or pelvic surgery?
It shifts the pre-test probability towards adhesions and frames the search for a transition point, and it is the single most useful line a referrer can add.
How old is the patient?
This is the one fact that changes the whole answer rather than refining it. Under one year the first question is malrotation and the study is an upper GI contrast examination; in an adult it is strangulation and the study is CT. There is no protocol adjustment that bridges those two, so an unstated age defaults to the adult pathway and that default is wrong for a child.
In an infant: is the vomiting bilious?
Bilious vomiting in an infant means the obstruction is beyond the ampulla and puts malrotation with midgut volvulus at the top of the list — a diagnosis measured in hours. Non-bilious vomiting in a well infant of a few weeks is a different question altogether, and is handled by the pyloric stenosis card.

Pathways

Big forks are separate pathways; the first whose conditions match is the one used.

Pregnant

Role	Study & protocol	Why this answers the question
First line	MRI abdomen and pelvis MRI abdomen/pelvis — non-contrast rapid protocol	Heavily T2-weighted sequences make fluid-filled dilated loops and the collapsed distal segment obvious without radiation, and the transition point can usually be followed on coronal images. The gravid uterus displaces bowel, which degrades ultrasound but does not defeat MRI.
Second line	CT Abdomen and Pelvis CT Abdomen and Pelvis — Portal Venous Phase	Where MRI is unavailable or the patient is too unwell to lie still, CT is performed. Delay in recognising strangulation harms the pregnancy more than the exposure does, and the decision should be recorded as deliberate.

Infants and toddlers

Role	Study & protocol	Why this answers the question
First line	Abdominal radiograph Abdominal radiograph — supine usually appropriate	The film is not looking for the obstruction, which is usually clinically obvious. It is separating a high obstruction from a low one, and that single distinction decides which contrast study follows: a few dilated loops send the infant to the upper GI study, many dilated loops to the enema. It takes minutes at the cot side and it does not delay anything.
Second line	Upper GI contrast study Upper GI contrast study usually appropriate	Bilious vomiting in an infant is malrotation with midgut volvulus until the duodenojejunal flexure has been shown to sit where it should. This is second on the ladder only in the sense that a film is quicker to obtain — it is not second in urgency. A volvulus can infarct the whole midgut inside a few hours, and the infant who loses it is left short-gut for life, so the study is arranged immediately and out of hours rather than added to a list.
Reasonable alternative	Paediatric abdominal ultrasound Paediatric abdominal ultrasound	Ultrasound answers several of the competing diagnoses in one sitting — the hypertrophied pylorus, an ileocolic intussusception, and the whirlpool of mesentery and the inverted superior mesenteric artery and vein relationship of malrotation. It is the reasonable first study where it can be done straight away by someone who does paediatric work. What it cannot do is exclude malrotation: a normal ultrasound does not remove the need for the upper GI study, and treating it as if it did is the way this diagnosis gets missed.
Problem solving	Contrast or air enema Air or contrast enema reduction	Where the film shows a low obstruction — many dilated loops, no distal gas — the enema is both the diagnostic study and, in some of these babies, the treatment. It distinguishes Hirschsprung disease, meconium ileus, ileal atresia and small left colon syndrome, and it is performed with surgical cover because perforation and irreducibility both need a surgeon immediately.

Bilious vomiting is the discriminator that matters and it belongs on the request in those words. Non-bilious vomiting in a well infant of four to eight weeks is a pyloric question and has its own card.

Children and adolescents

Role	Study & protocol	Why this answers the question
First line	Abdominal radiograph Abdominal radiograph — supine	A child is not a small adult for this question. The common causes here — adhesions from previous surgery, an incarcerated hernia, intussusception, appendicitis with an ileus — are largely reachable without cross-sectional imaging, and the radiograph establishes whether there is mechanical obstruction at all before anyone commits a child to a CT dose.
Second line	Paediatric abdominal ultrasound Paediatric abdominal ultrasound	Ultrasound reaches the intussusception, the appendix and the incarcerated hernia, and in thin children it will often show dilated loops and a transition point directly. It carries no dose, needs no cannula and no sedation, and in a district hospital it is usually available faster than a paediatric CT list.
Problem solving	Upper GI contrast study Upper GI contrast study	Malrotation is not only a neonatal diagnosis. A child with intermittent bilious vomiting and no surgical history can be presenting late with an unfixed midgut, and the upper GI study is what settles it.
Problem solving	CT Abdomen and Pelvis CT Abdomen and Pelvis — Portal Venous Phase	CT has a place in the older child, and refusing it outright would be its own kind of harm: the adolescent with a previous laparotomy and a rising lactate needs the same strangulation question answered as an adult does, and no radiograph or ultrasound answers it. It is here rather than first because the sequence matters — the dose to a child is what the earlier rungs are buying down, and it is only worth spending once the cheaper studies have failed to explain a child who is not settling. Scan on a paediatric protocol with weight-banded kV and mAs and a single portal-venous pass; a second phase in a child is almost never justified.

Adults

Matches your inputsDefault

Role	Study & protocol	Why this answers the question
First line	CT Abdomen and Pelvis CT Abdomen and Pelvis — Portal Venous Phase usually appropriate	Intravenous contrast is not there to show the dilatation, which is visible without it. It is there to show bowel wall enhancement, and reduced or absent mural enhancement is the imaging evidence of strangulation that turns a conservative admission into an emergency laparotomy. The retained fluid in obstructed loops acts as its own neutral luminal contrast, which is why oral agents are unnecessary and counterproductive here.
Reasonable alternative	Abdominal radiograph Abdominal radiograph — supine	A supine abdominal film can confirm gross dilatation quickly where CT will be delayed, but it cannot give the level, the cause or the state of the bowel wall, so a normal film in a symptomatic patient does not close the question.
Problem solving	CT Abdomen and Pelvis CT Abdomen and Pelvis — Positive Oral Contrast	The water-soluble contrast challenge is a management tool rather than a diagnostic protocol: contrast reaching the colon within a day of administration predicts resolution without surgery in adhesional obstruction, and the administration is itself mildly therapeutic. Two details keep it from becoming a second unnecessary CT. The follow-up in the published protocols is a plain abdominal radiograph at around eight to twenty-four hours after the contrast is given — a repeat cross-sectional acquisition is not what the challenge needs, and where a CT is used it should be because a new diagnostic question has arisen rather than to look for contrast in the colon. And the challenge belongs after the diagnostic study, in the patient already selected for non-operative management, never as the initial acquisition when strangulation is the concern and never in a patient with peritonism, a rising lactate or a closed loop.

Pitfalls

Adding positive oral contrast to the diagnostic study. It costs an hour of preparation, dilutes nothing useful, and its density obscures the mural enhancement the study was requested to assess.
Accepting a plain film request as the answer in a patient with a rising lactate — it cannot exclude strangulation and delays the scan that can.
Stopping at "dilated small bowel". The vetting note should push for the transition point and the cause, because those are what the surgeon acts on.
Reading a closed-loop configuration as simple adhesional obstruction; the mesenteric swirl and the U-shaped loop are the findings that change urgency.
Vetting a child down the adult pathway because the request form looks the same. Bilious vomiting in an infant is a midgut volvulus until the duodenojejunal flexure is shown to be normal, and the study that shows it is fluoroscopic, not a CT.
Accepting a normal ultrasound as exclusion of malrotation. It is a good screen and a poor rule-out; the upper GI study is what closes the question.
Turning the water-soluble challenge into a second CT. The published protocol follows the contrast with a plain abdominal radiograph at roughly eight to twenty-four hours; scanning again to see whether contrast has reached the colon spends a second dose on a question a film answers.

Priors — what to pull first

Retrieve the operation notes: the site of previous surgery predicts where the transition point will be.
Compare with any recent CT for a known hernia, a stricture or a previously documented adhesive band.

What makes a good request

State the surgical history and whether there is a hernia: an obstruction with no previous surgery has a different differential and a lower threshold for early operation.
Tachycardia, acidosis or a raised lactate turns this into an urgent question about strangulation, and that belongs on the request.

Scoring this once it is done

The classification and risk tools this question ends in.

eGFR (CKD-EPI 2021 Creatinine)

How these studies are acquired

Contrast, phases and timing for every study on the pathways above.

MRI abdomen/pelvis — non-contrast rapid protocol CT Abdomen and Pelvis — Portal Venous Phase Abdominal radiograph — supine Upper GI contrast study Paediatric abdominal ultrasound Air or contrast enema reduction CT Abdomen and Pelvis — Positive Oral Contrast

Confirm locally

CT Abdomen and Pelvis — Portal Venous Phase: timings are typical — confirm against local protocol.
Timings, contrast volumes and rates above are typical published values. Your department's protocol, scanner and patient population decide the actual numbers.
Safety thresholds and premedication policy follow local policy where it differs from the cited guidance.

References

Implemented from the cited published sources. Educational and workflow support only; confirm against current guidelines and local policy before clinical use.