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Gastroenterology & Nutrition

Toxic megacolon

Toxic megacolon is acute non-obstructive colonic dilatation accompanied by systemic toxicity, usually complicating acute severe ulcerative colitis or severe infection such as C. difficile; it is a time-critical colorectal emergency because the inflamed colon can perforate.

In a nutshell

Toxic megacolon is acute non-obstructive colonic dilatation, classically above about 6 cm, plus systemic toxicity in severe colitis. It is a colorectal emergency: admit, resuscitate, stop antimotility and ileus-promoting medicines, investigate infection and perforation, involve surgery early and treat the underlying acute severe UC or infection. Toxic megacolon, perforation, severe haemorrhage or deterioration requires urgent subtotal colectomy with end ileostomy in the current BSG pathway.

Classic presentation

A person with acute severe ulcerative colitis develops fever, tachycardia, abdominal distension and tenderness; the abdominal radiograph shows non-obstructive transverse-colon dilatation.

Key points

  • The diagnosis requires both non-obstructive colonic dilatation and systemic toxicity; dilatation alone is not toxic megacolon.
  • Use urgent bloods, stool culture/C. difficile testing, abdominal imaging and flexible sigmoidoscopy; avoid full colonoscopy and bowel preparation in a toxic dilated colon.
  • Stop loperamide and other antimotility or anticholinergic medicines, correct electrolytes, provide VTE prophylaxis when safe and involve surgery early.
  • For acute severe UC, use high-dose IV corticosteroids; rescue infliximab or ciclosporin is a specialist option after at least 3 days if appropriate.
  • Toxic megacolon, perforation, severe haemorrhage or deterioration is an indication for subtotal colectomy and end ileostomy; do not delay for a fixed medical-treatment interval.
  • A falling stool frequency may be ominous when the colon is becoming atonic; track the whole clinical picture and serial abdominal findings.

First-line investigation

Urgent abdominal radiograph or suitable non-invasive imaging plus bloods and stool testing for systemic toxicity and an infective trigger.

Management

Resuscitate and involve surgery early

  • Admit as an acute severe colitis complication, correct fluid and electrolytes, monitor closely and involve colorectal surgery, gastroenterology, critical care and stoma care immediately.1,2
  • Stop antimotility and anticholinergic medicines, avoid unnecessary opioids and bowel instrumentation, and provide VTE prophylaxis when not contraindicated.1,6

Treat the underlying cause

  • Use high-dose IV corticosteroids for acute severe UC under the BSG/BNF pathway; investigate and treat C. difficile or another infection using current antimicrobial guidance.1,2,4

Do not delay colectomy for complications

  • Toxic megacolon, perforation, severe haemorrhage or deterioration requires urgent subtotal colectomy with end ileostomy; rescue infliximab or ciclosporin is not a reason to postpone operation.1,13,5

Recover through staged IBD and surgical care

  • After emergency colectomy, manage the ileostomy, rectal stump, nutrition, steroid withdrawal, VTE risk and psychological needs; consider later restorative options only after recovery and shared specialist review.1,5

Exam traps

  • Colonic dilatation without systemic toxicity is not toxic megacolon.
  • Reduced diarrhoea can reflect an atonic, obstructed colon rather than improvement.
  • Avoid loperamide, opioids where possible, anticholinergics, colonoscopy and bowel preparation in a toxic dilated colon.
  • Do not allow rescue medical therapy to delay surgery when toxic megacolon, perforation, severe haemorrhage or deterioration is present.
  • Test for C. difficile and other infection; do not assume every toxic megacolon episode is ulcerative colitis.

Illustrations

Toxic megacolon on abdominal radiographA plain abdominal radiograph showing marked non-obstructive colonic dilatation in severe colitis, with the dilated segment and measurement site labelled and a warning that systemic toxicity is required for the syndrome diagnosis.Hellerhoff, Wikimedia Commons · CC-BY-SA-3.0

Key sources

  1. BSG: British Society of Gastroenterology guidelines on inflammatory bowel disease in adults: 2025 (Gut 2025;74(Suppl 2):s1–s101)Published 23 Jun 2025
  2. NICE NG130: Ulcerative colitis: management (Recommendations 1.2.22–1.2.27 for acute severe ulcerative colitis)Published 2 May 2019
  3. ECCO-ESGAR diagnostic review: toxic megacolon imaging (Established radiographic definition and CT complication assessment)Published 28 Aug 2018
  4. NICE NG199: Clostridioides difficile infection: antimicrobial prescribing (Current antimicrobial prescribing recommendations)Published 23 Jul 2021
  5. Crohn’s & Colitis UK: Surgery for ulcerative colitis (Current patient information on ASUC and toxic megacolon)
  6. Crohn’s & Colitis UK: Ulcerative colitis information (Current patient information on toxic megacolon and emergency symptoms)
  7. BNF: Methylprednisolone (Current prescribing monograph)
  8. BNF: Hydrocortisone (Current prescribing monograph)
  9. BNF: Vancomycin (Current prescribing monograph)
  10. NICE TA163: Infliximab for acute exacerbations of ulcerative colitis (Technology appraisal recommendations)Published 17 Dec 2008
  11. BNF: Infliximab (Current prescribing monograph)
  12. BNF: Ciclosporin (Current prescribing monograph)
  13. ACPGBI consensus guidelines in surgery for inflammatory bowel disease (Surgical consensus guidance on acute severe colitis and toxic megacolon)Published 1 Nov 2018

This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.