Large bowel obstruction on X‑ray

Written by the X-Ray Atlas editorial team from the sources listed below · Medical review pending · Updated 24 October 2026 · Level: Beginner

Large bowel obstruction (LBO) is a blockage of the colon or rectum. It usually develops more slowly than small bowel obstruction, with progressive distension, colicky lower abdominal pain, constipation and, later, vomiting. It mainly affects older people, and the most common cause is colorectal cancer. The danger is perforation, especially of the caecum, which can be lethal. The abdominal X-ray (AXR) often shows the dilated colon clearly, but it cannot reliably tell a mechanical obstruction from colonic pseudo-obstruction, and CT is needed to find the cause and plan treatment. This page covers how to recognise LBO, the role of the ileocaecal valve, caecal size, the causes, and pseudo-obstruction.

1. How to look

Start with the systematic abdominal approach on a supine AP film. Then trace the colon from the caecum in the right iliac fossa, up the ascending colon, across the transverse colon, down the descending colon and into the sigmoid and rectum. Ask:

1. Is the colon dilated, and to what diameter?
2. Where does the gas stop? Is there a transition point, with collapsed bowel beyond it?
3. Is there gas in the rectum?
4. Is the small bowel dilated too?
5. How big is the caecum?
6. Is there free gas, or gas in the bowel wall?

2. Signs of large bowel obstruction

• Dilated colon (more than 6 cm; caecum more than 9 cm), lying peripherally around the abdomen.
• Haustral folds that cross only part of the bowel width, unlike the valvulae conniventes of small bowel. See small bowel obstruction for the comparison.
• Faecal content in the dilated colon, giving a mottled appearance.
• A cut-off: the dilated colon ends at a transition point, with little or no gas beyond it. The level suggests the site, for example dilated colon to the splenic flexure with collapsed descending colon.
• Absent rectal gas supports a complete distal obstruction, but its presence does not exclude one, especially after a rectal examination or enema.

3. The ileocaecal valve

Whether the ileocaecal valve lets gas back into the small bowel shapes the film, and the risk:

• Competent valve (it does not allow reflux; in roughly a quarter or more of patients, figures vary): gas and fluid build up between the obstruction and the valve, creating a closed loop. The colon, and particularly the caecum, distends progressively, and the small bowel stays relatively normal. This is the dangerous pattern, with a high risk of caecal perforation.
• Incompetent valve: the colon decompresses back into the small bowel. Both small and large bowel are dilated, which can mimic small bowel obstruction or ileus, but the caecum is under less pressure.

4. The caecum: when to worry

By Laplace's law, the widest part of the colon has the highest wall tension, so the caecum is where perforation usually happens, even when the obstruction is in the sigmoid colon. Measure the caecum on every LBO film:

• above 9 cm is dilated;
• above about 10–12 cm is the range at which perforation risk is classically said to rise, and should prompt urgent surgical review;
• local tenderness in the right iliac fossa, rapid enlargement on serial films, or gas in the caecal wall suggest impending perforation.

The threshold depends on how quickly the caecum has dilated: chronic dilatation is tolerated better than acute. The numbers are a guide, not a rule.

5. Causes

• Colorectal carcinoma: the most common cause, roughly 60% in classic series, usually in the left colon or sigmoid.
• Volvulus: sigmoid more often than caecal. The appearance is usually distinctive.
• Diverticular stricture from recurrent diverticulitis.
• Hernias, including inguinal hernias containing sigmoid colon.
• Less common causes include extrinsic compression by pelvic tumours, inflammatory strictures, faecal impaction and intussusception.

6. Pseudo-obstruction (Ogilvie syndrome)

Acute colonic pseudo-obstruction is dilatation of the colon without a mechanical blockage. It mostly affects older, unwell patients: after surgery (especially orthopaedic and cardiac), with sepsis, electrolyte disturbance, opiates, or immobility.

• The film shows a dilated colon, often most marked in the caecum and right colon, sometimes with gas all the way to the rectum.
• It cannot be reliably distinguished from mechanical LBO on a plain film. CT is needed to exclude a mechanical cause.
• The same caecal perforation risk applies, and serial films are often used to monitor caecal diameter.

Historically, a water-soluble contrast enema was used to show whether contrast flowed freely past the apparent cut-off (pseudo-obstruction) or stopped at a mechanical lesion. CT has largely replaced it in UK practice, because it also shows the cause, the caecal wall and any perforation, but contrast studies are still used in some centres when CT is equivocal. Whatever the test, the key clinical question is the same: is there a lesion that needs surgery or a stent, or a functional problem that may settle with supportive treatment and decompression?

Also distinguish toxic megacolon, where the colon dilates because of severe colitis. The patient is systemically unwell and the bowel wall looks abnormal.

7. Pitfalls

• Calling pseudo-obstruction or obstruction from the plain film alone: CT is required.
• Not measuring the caecum.
• Faecal loading in constipation can look alarming but does not mean obstruction if the colon is not dilated.
• Overlapping loops of dilated colon can look like a volvulus and vice versa.
• Missing free gas: look for Rigler's sign and the other supine signs, and request an erect chest X-ray. See pneumoperitoneum.

8. Next steps

• CT abdomen and pelvis with intravenous contrast confirms mechanical obstruction, shows the site and cause (and in cancer, staging), and identifies perforation or ischaemia.
• Early surgical review. Management depends on the cause and the patient: options include surgery, endoscopic stenting in selected cancers, and endoscopic decompression for sigmoid volvulus or pseudo-obstruction.
• For pseudo-obstruction: correct reversible factors, stop drugs that slow the bowel where possible, and monitor caecal size. Pharmacological or endoscopic decompression may be used under specialist direction.

Red flags: act now

What to focus on, by role

Role	Priorities
Medical student	Haustra versus valvulae conniventes, peripheral versus central loops, the 3-6-9 rule and the significance of a competent ileocaecal valve.
Nurse	Distension, bowel and stoma output, and escalation of increasing pain, fever or tachycardia.
Junior doctor	Measure the caecum, request CT, and get early surgical review. Look for reversible causes in suspected pseudo-obstruction.
Radiographer	Include the whole colon, the rectum and the diaphragm where possible; serial films may be used to track caecal size.
Advanced / FRCR	Closed-loop obstruction, pseudo-obstruction versus mechanical LBO, and CT correlation.

Practise on abdominal X-rays →

References

1. Radiology Masterclass. Large bowel obstruction. radiologymasterclass.co.uk
2. Radiopaedia. 3-6-9 rule. radiopaedia.org
3. Royal College of Radiologists. iRefer: making the best use of clinical radiology. irefer.org.uk
4. NICE. Colorectal cancer (NG151). nice.org.uk

For education only. Not a substitute for a formal report, senior review or your local policy. Never use this page to make decisions about a real patient.