Toxic megacolon and colitis on X-ray

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

Toxic megacolon is a life-threatening complication of severe colitis, in which the inflamed colon dilates and the patient becomes systemically toxic. It occurs most often in ulcerative colitis, less often in Crohn's colitis, and in severe infective colitis, notably Clostridioides difficile. Its main danger is perforation, which carries a high mortality. The diagnosis needs both a dilated colon and systemic toxicity: a large colon on its own is not toxic megacolon. The abdominal X-ray (AXR) remains a key early test in acute severe colitis, both to detect dilatation and to estimate the extent of disease. This page covers the measurements, the signs of colitis on the film, the mimics and the urgency.

1. Definition and who gets it

Toxic megacolon combines two things:

1. Colonic dilatation, classically a transverse colon diameter above 6 cm on the AXR (some guidance, including the British Society of Gastroenterology in the context of acute severe ulcerative colitis, uses 5.5 cm). [VARIES]
2. Systemic toxicity. The classic Jalan criteria (1969) require features such as fever, tachycardia, a raised white cell count or anaemia, together with dehydration, altered consciousness, electrolyte disturbance or hypotension.

Causes include:

• inflammatory bowel disease, especially ulcerative colitis, often early in the disease or during a first severe attack;
• infective colitis, including C. difficile, Salmonella, Shigella, Campylobacter and cytomegalovirus (CMV) in immunosuppressed patients;
• ischaemic colitis, less commonly.

Precipitants include antimotility drugs (for example loperamide, opiates), anticholinergic drugs, electrolyte disturbance (low potassium) and colonoscopy or barium studies during a severe attack.

2. How to look and measure

Use the systematic abdominal approach on a supine AP film. On a supine film, gas rises to the most anterior part of the colon, which is the transverse colon, so this is the segment that is measured.

In acute severe colitis, serial AXRs are often used to track the diameter; increasing dilatation is a warning sign even below the threshold.

The AXR is one of the few situations where a plain abdominal film is still recommended routinely on admission: in acute severe ulcerative colitis it provides a baseline diameter, shows the extent of disease and looks for free gas. Pair it with an erect chest X-ray when perforation is a concern, because free gas can be very hard to see on a supine film of a distended abdomen.

3. Signs of severe colitis on the film

• Loss of haustra: the colon becomes featureless, with smooth walls, in the dilated segment.
• Mucosal islands: small irregular soft-tissue nodules projecting into the gas-filled lumen. These are islands of oedematous, surviving mucosa (inflammatory pseudopolyps) between areas of deep ulceration. Their presence in acute severe UC suggests deep ulceration and is associated with a poorer response to medical treatment.
• Thumbprinting: thickened, oedematous haustral folds that indent the lumen like the impression of thumbs. Seen in severe colitis of many causes.
• Thickened bowel wall, visible where gas outlines both sides of it.
• Absence of faecal residue in the inflamed segment.
• Pneumatosis (gas in the bowel wall) and free gas indicate impending or actual perforation.

4. Estimating the extent of colitis

Faeces generally do not stay in actively inflamed colon. So on the AXR:

• Faecal residue in a segment of colon usually means that segment is not severely inflamed.
• Absence of faeces, with a gas-filled, featureless or thick-walled colon, suggests active colitis in that segment.

This is a rough guide only, and endoscopy (often a limited, careful flexible sigmoidoscopy in acute severe disease) and CT give a more accurate picture.

In chronic ulcerative colitis, the colon may become shortened and narrowed with complete loss of haustra, the "lead-pipe" colon. This is a feature of long-standing disease, not of acute toxicity.

5. Other colitides on the plain film

• Ischaemic colitis: thumbprinting, typically at the splenic flexure and other watershed areas between arterial territories, in older patients with vascular disease. Gas in the bowel wall or portal venous gas is ominous.
• Infective colitis (including C. difficile, which can cause pseudomembranous colitis): thickened walls and thumbprinting, sometimes pancolonic, and sometimes with marked dilatation.
• Neutropenic colitis (typhlitis): mainly affects the caecum and right colon in neutropenic patients.

The plain film rarely distinguishes these. Clinical context, stool tests, CT and endoscopy do.

6. Mimics

• Pseudo-obstruction (Ogilvie syndrome): a dilated colon in an unwell patient, but without colitis. Haustra are usually preserved and the wall is normal. See large bowel obstruction.
• Mechanical large bowel obstruction: a transition point with collapsed colon beyond it.
• Volvulus, with its characteristic loop shape.
• Chronic megacolon (for example in long-standing constipation or neurological disease): marked dilatation, usually with faeces, in a patient who is not toxic.

7. Next steps

• Urgent joint medical and surgical care. BSG guidance for acute severe ulcerative colitis recommends early involvement of the colorectal surgical team.
• Stop precipitating drugs (antimotility, opiate and anticholinergic drugs), correct electrolytes, especially potassium, and give intravenous fluids.
• Treat the cause: intravenous corticosteroids and rescue therapy in IBD, as directed by gastroenterology; specific treatment for C. difficile.
• Imaging: serial AXRs to monitor colonic diameter, and CT when perforation, abscess or another diagnosis is suspected.
• Surgery (usually colectomy) is needed if the patient deteriorates, fails to respond, or perforates.

Red flags: act now

What to focus on, by role

Role	Priorities
Medical student	Definition (dilatation plus toxicity), the transverse colon threshold, mucosal islands and thumbprinting, and why the transverse colon is measured.
Nurse	Stool chart, observations, abdominal distension and the danger of antimotility drugs in colitis. Escalate fever, tachycardia or new distension.
Junior doctor	Measure the transverse colon on the admission AXR in acute colitis, look for free gas, and involve gastroenterology and surgery early.
Radiographer	Supine AP film including the whole colon and diaphragm; consistent technique on serial films so measurements are comparable.
Advanced / FRCR	Mucosal islands, extent assessment, ischaemic and infective colitis patterns, and CT features of perforation.

Practise on abdominal X-rays →

References

1. Lamb CA, et al. British Society of Gastroenterology consensus guidelines on the management of inflammatory bowel disease in adults. Gut 2019;68(Suppl 3):s1–s106. bsg.org.uk
2. Radiopaedia. Toxic megacolon. radiopaedia.org
3. Jalan KN, et al. An experience of ulcerative colitis. I. Toxic dilation in 55 cases. Gastroenterology 1969;57(1):68–82.
4. Royal College of Radiologists. iRefer. irefer.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.