Toxic megacolon

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Toxic megacolon is the endpoint of severe colonic inflammation and neuromuscular failure. The colon becomes atonic and non-obstructively dilated while inflammatory mediators and bacterial translocation drive fever, tachycardia, hypotension, altered consciousness and organ dysfunction. Ulcerative colitis is a classic cause, but Crohn colitis and infections—especially Clostridioides difficile—also cause the syndrome. Mechanical large-bowel obstruction and acute colonic pseudo-obstruction cause dilatation by different mechanisms and need active distinction.

Management protects the colon from further pressure, restores physiology and treats the cause while preparing for surgery. Endoscopic distension, bowel preparation, antimotility drugs and opioids can worsen an already paralysed colon. A limited sigmoidoscopy may sometimes provide essential diagnostic tissue in stable severe colitis, but full colonoscopy is inappropriate. Subtotal colectomy with end ileostomy is the usual emergency operation when medical control fails or perforation, haemorrhage or deterioration occurs; restorative pouch surgery is deferred.

Key points

Toxic megacolon combines acute severe colitis and systemic toxicity with non-obstructive colonic dilatation; transverse-colon diameter above 5.5 cm on plain radiography is a practical criterion, but dilatation alone is not the diagnosis.
Examine repeatedly for distension and peritonism, obtain FBC, CRP, renal profile, electrolytes, albumin, lactate, cultures, C difficile tests and serial abdominal imaging.
Stop loperamide, codeine, other opioids and anticholinergic drugs because impaired motility can worsen dilatation; avoid bowel preparation, barium enema and full colonoscopy.
In ulcerative colitis use high-dose IV corticosteroid while joint medical-surgical review continues; identify and treat C difficile or another infection rather than assuming every toxic colon is IBD.
Perforation, peritonitis, uncontrolled haemorrhage or physiological deterioration requires urgent subtotal colectomy with end ileostomy, usually preserving the rectum in the acute operation.
A stable initial observation is not reassurance: measure abdominal findings, physiology, lactate, stool and colonic diameter often enough to detect a narrowing operative window.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Inflammatory bowel disease

Acute severe ulcerative colitis is a major cause, while extensive Crohn colitis can produce the same toxic dilatation syndrome.

Severe infectious colitis

C difficile and other invasive pathogens can inflame and paralyse the colon, especially after antibiotics, hospital exposure or immunosuppression.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Neuromuscular paralysis
Deep inflammatory mediators impair smooth-muscle contraction and enteric nerve function, causing an atonic colon that accumulates gas and fluid.
2
Wall stress and ischaemia
Increasing radius raises mural tension, while inflammation and hypoperfusion reduce tissue strength, creating progressive risk of necrosis and perforation.
3
Systemic inflammatory spillover
Barrier failure, inflammatory cytokines and bacterial translocation together produce fever, vasodilatation, capillary leak and eventually progressive multi-organ dysfunction.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Systemic toxicity

Fever, pulse elevation, hypotension, dehydration, confusion, anaemia and raised inflammatory markers show that colonic disease has become a whole-patient emergency.

Acute colonic dilatation

Marked transverse or right-colon enlargement on radiograph or CT accompanies distension and tenderness; serial change is more informative than one measurement.

Inflamed rather than obstructed colon

Toxic megacolon is non-obstructive dilatation arising from severe colitis, so imaging must exclude a distal cancer, volvulus or other mechanical transition point.

Impending perforation

Increasing pain, guarding, rebound, absent bowel sounds, acidosis, lactate rise or free gas indicates wall failure and requires urgent operation.

Possible infectious trigger

Recent antibiotics, hospital contact and immunosuppression raise concern for C difficile, while travel and exposure history may suggest another enteric pathogen.

Red flags requiring action

Generalised guarding, rebound, free intraperitoneal gas or a rapidly rising lactate suggests perforation or bowel ischaemia and requires immediate operative source control.
Increasing colonic diameter, new loss of bowel sounds, worsening pain or progressive distension despite treatment indicates failure even before laboratory markers deteriorate.
Hypotension, oliguria, confusion, hypoxaemia or escalating vasopressor need signals systemic decompensation and removes time for prolonged medical rescue.
Ongoing severe colorectal haemorrhage with transfusion requirement is an independent indication for urgent surgical control.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

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
Contrast CT abdomen and pelvisFirst stepPreferred
Why
preferred initial imaging when clinically suspected toxic megacolon can be assessed without unsafe transfer; define perforation, ischaemia, collection, mechanical obstruction and alternative pathology.
Interpretation and limitations
CT is preferred for the initial assessment when feasible because it identifies complications and alternative causes. An unstable peritonitic patient should not have surgical source control delayed for imaging.
02
Serial abdominal radiographs
Why
follow established colonic dilatation at the bedside with minimal transfer; compare calibre and look for progression or free intraperitoneal gas.
Interpretation and limitations
A transverse-colon diameter above 5.5 cm on plain radiography supports megacolon when acute colitis and systemic toxicity are present. Serial films can monitor dilatation; worsening diameter or mucosal islands suggests failing control.
03
FBC, CRP, U&E, magnesium, albumin and blood gas
Why
measure inflammation, anaemia, electrolyte loss, organ dysfunction and tissue hypoperfusion.
Interpretation and limitations
Falling haemoglobin, potassium depletion, renal injury or rising lactate identifies loss of reserve; trends must trigger action rather than merely populate a chart.
04
Stool culture and C difficile assay
Why
identify a treatable infection causing or worsening the toxic colitis.
Interpretation and limitations
A positive C difficile result changes antimicrobial and infection-control management but does not remove the need for urgent surgical observation.
05
Limited flexible sigmoidoscopy when stable
Why
assess distal colitis and obtain tissue only when the result will change immediate treatment.
Interpretation and limitations
Use minimal insufflation and stop early; full colonoscopy and bowel preparation risk perforation in a dilated severely inflamed colon.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Mechanical large-bowel obstruction

A tumour, volvulus or stricture produces a transition point and upstream dilatation; diarrhoeal colitis and diffuse inflammatory toxicity may be absent.

Acute colonic pseudo-obstruction

Autonomic dysregulation dilates the colon during severe illness or after surgery without primary destructive colitis, although perforation risk remains.

Simple severe colitis

A patient may meet acute severe colitis criteria without radiological megacolon; the distinction changes procedural risk and operative urgency.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01Worked case: toxic dilatation during ASUCRecognise medical failure and reach emergency source controlFirst stepA 36-year-old receiving intravenous hydrocortisone for acute severe ulcerative colitis develops increasing distension and pain. Temperature is 39.0°C, pulse 126/min, blood pressure 94/58 mmHg and lactate 2.8 mmol/L; radiography shows transverse colon 7.2 cm without free gas.

1Diagnose toxic megacolon from severe colitis, toxicity and non-obstructive 7.2 cm dilatation; call colorectal surgery, anaesthesia and critical care now rather than waiting for the next routine ASUC review.
2Keep nil by mouth, establish large-bore intravenous access, cross-match blood, correct potassium and volume deficits and insert urinary monitoring; stop codeine prescribed overnight and avoid colonoscopy or bowel preparation.
3Send blood cultures, stool culture and C difficile testing, repeat blood gas and obtain careful CT because the patient is sufficiently stable for transfer and perforation or abscess would change the operation.
4CT confirms pan-colonic dilatation and severe colitis without a mechanical lesion or free air, but over four hours pain and tenderness increase and lactate reaches 4.0 mmol/L despite resuscitation; abandon further drug rescue.
5Urgent subtotal colectomy with end ileostomy finds patchy serosal ischaemia but no established perforation. Postoperative lactate falls to 1.6 mmol/L and histology confirms severe ulcerative colitis; document rectal-stump care and later reconstruction discussion.

02Life-threatening C difficile colitisTreat infection while obtaining early surgical control adviceA 74-year-old develops profuse diarrhoea after antibiotics, then abdominal distension, temperature 38.8°C, pulse 118/min, white cells 24×10^9/L and creatinine twice baseline. CT shows 6.8 cm non-obstructive colonic dilatation without perforation and stool is positive for C difficile toxin.

1Isolate with enteric precautions, notify microbiology and colorectal surgery, stop the precipitating antibiotic where possible and review proton-pump inhibitor, opioid and antimotility exposure.
2Under the NICE life-threatening CDI table, start both vancomycin 500 mg orally four times daily and metronidazole 500 mg/100 mL intravenously three times daily, each for 10 days, with immediate microbiology and colorectal input while fluid and electrolyte losses are replaced.
3Chart stool, abdomen, urine output and physiology closely and repeat lactate, white count, creatinine and imaging; ileus may impair delivery to the distal colon and is a reason for specialist antimicrobial advice.
4At twelve hours blood pressure is 82/46 mmHg, lactate is 4.5 mmol/L and guarding develops, so proceed to urgent surgical source control rather than completing a nominal antibiotic trial.
5Subtotal colectomy confirms fulminant pseudomembranous colitis with no cancer. Verify postoperative infection treatment, renal recovery to baseline and environmental cleaning before transfer from isolation.

03Dilatation from mechanical obstructionDo not apply a toxic-colitis label to every enlarged colonAn 81-year-old has progressive constipation, vomiting and distension without diarrhoea. Temperature is 36.8°C and CRP 18 mg/L; CT shows an obstructing sigmoid tumour with caecal dilatation 10 cm and a clear transition point.

1Recognise mechanical large-bowel obstruction rather than toxic megacolon because CT demonstrates an obstructing lesion and systemic inflammatory colitis is absent.
2Resuscitate, keep nil by mouth, decompress the stomach when vomiting is significant and correct renal and electrolyte disturbance while assessing for caecal ischaemia or perforation.
3Stage the visible disease as the emergency permits and discuss resection, diversion or carefully selected stenting according to site, intent, perforation risk and local expertise.
4The patient undergoes subtotal colectomy because the caecum is ischaemic and the remaining colon contains synchronous lesions; pathology confirms sigmoid adenocarcinoma rather than inflammatory megacolon.
5Verify postoperative renal recovery, stoma function, full pathological staging and oncology referral, keeping the causal label accurate for future care.

Key medicines and prescribing safety3 treatments · regimens, roles and cautions

Treats the underlying severe inflammatory colitis but cannot repair perforation or replace source control when physiology worsens.

Hydrocortisone for ulcerative-colitis toxic megacolon

When toxic megacolon complicates acute severe ulcerative colitis without perforation, give hydrocortisone 100 mg IV every 6 hours; administer each reconstituted injection over 1–10 minutes while urgent surgical assessment continues.

Do not use as treatment for infectious colitis alone. Exclude untreated systemic fungal infection and avoid live vaccine at immunosuppressive doses; monitor infection, glucose, blood pressure, mental state, potassium and fluid balance. Review CYP3A4 modifiers, warfarin and potassium-depleting medicines, and do not prolong treatment because one examination is temporarily unchanged.

Provides luminal C difficile treatment as one component of the NICE life-threatening combination while surgical failure is assessed.

Oral vancomycin for life-threatening C difficile infection

Under specialist advice give vancomycin 500 mg orally every 6 hours WITH metronidazole 500 mg IV every 8 hours, both for 10 days, for life-threatening CDI. Use two 250 mg vancomycin capsules per dose for the selected product; the 2 g/day maximum is not exceeded. This oral preparation treats the colonic lumen and must not be replaced by IV vancomycin for CDI.

Severe intestinal inflammation can increase systemic absorption, especially with renal impairment. In this worked patient with creatinine twice baseline, monitor serum vancomycin concentration and serial renal function; also review nephrotoxic or ototoxic medicines and stop for a severe cutaneous reaction.

Adds systemic anaerobic treatment to oral vancomycin in life-threatening CDI while microbiology and colorectal teams assess urgent source control.

Metronidazole 500 mg/100 mL IV infusion for life-threatening C difficile infection

Give 500 mg intravenously every 8 hours for 10 days as the second component of the NICE life-threatening regimen. Infuse each 100 mL bag over 20–60 minutes and change to an appropriate oral route only when the specialist infection plan permits.

Reduce dose with serum-level monitoring in advanced hepatic insufficiency and monitor renal impairment for metabolite toxicity. Review warfarin and INR, lithium, disulfiram and busulfan. Avoid alcohol before, during and for 72 hours after this IV product; stop and review neurological toxicity or a serious infusion reaction.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Colonic perforation

A thinned ischaemic wall ruptures, releasing faecal contamination and producing peritonitis, septic shock and very high operative risk.

Massive haemorrhage

Diffuse ulceration can cause ongoing blood loss and transfusion dependence even before a discrete bleeding vessel is identified.

Multi-organ failure

Sepsis, hypovolaemia and inflammatory vasodilatation can cause acute kidney injury, respiratory failure, coagulopathy and worsening altered consciousness.

Emergency stoma

Subtotal colectomy usually ends with an ileostomy and retained rectal stump, leaving reconstruction decisions until recovery.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Repeat observations, mental state, urine output, abdominal girth, tenderness and bowel sounds frequently, because deterioration may occur faster than a routine daily review detects.
Trend lactate, blood count, CRP, renal function, potassium, magnesium and albumin and cross-match enough blood for likely surgery. During oral vancomycin in severe inflamed bowel with renal injury, also follow serum vancomycin concentration and nephrotoxic exposure; during IV metronidazole review hepatic, neurological and interaction toxicity.
Compare serial radiographs using the same colonic segment and technique where possible; increasing calibre or free gas must reach the surgical decision maker immediately.
Record all stopped motility-impairing medicines and prevent re-prescription of loperamide, codeine or anticholinergics while dilatation persists.
After colectomy verify pathology, rectal-stump or mucus-fistula management, stoma output, infection treatment and the staged discussion about future reconstruction.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Megacolon needs toxicity

A wide colon without acute colitis and systemic illness may be obstruction or pseudo-obstruction; diameter alone does not establish the syndrome.

Pain can falsely improve

An increasingly silent, paralysed colon may produce less diarrhoea while wall injury worsens, so reduced stool frequency is not automatically recovery.

Avoid diagnostic overreach

Full colonoscopy, barium enema and bowel preparation add luminal pressure and manipulation to a friable dilated colon.

Surgery is time critical

Operating after shock, perforation and severe depletion carries greater risk than colectomy performed when failure first becomes clear.

Cause directs medical treatment

Ulcerative colitis needs anti-inflammatory induction, while C difficile requires targeted antimicrobial therapy and infection-control measures.

11Common pitfallsFrequent interpretation and management errors.

01
Calling colonic dilatation toxic megacolon without demonstrating systemic toxicity and active colitis can obscure mechanical obstruction or pseudo-obstruction.
02
Prescribing loperamide or codeine to make the diarrhoea chart look better can deepen neuromuscular paralysis and delay recognition of failure.
03
Waiting for free gas before asking a surgeon to review misses the safer pre-perforation window for colectomy.
04
Attempting a full colonoscopy to prove severity exposes a thinned inflamed wall to insufflation and perforation.
05
Treating a positive C difficile assay as permission to exclude surgery ignores the possibility of fulminant infection despite correct antibiotics.

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