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Health condition · Clinically reviewed

Ischaemic colitis, the watershed injury behind sudden left-sided pain and bloody diarrhoea.

The commonest form of intestinal ischaemia. Most cases settle with supportive care in one to two weeks - a small number need urgent colorectal surgery, and knowing which is which is the whole point.

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Why trust this guide

  • 01

    Clinically reviewed

    Written by our editorial team and reviewed by a registered UK clinician before publication.

  • 02

    Sourced from guidance

    Checked against BSG, AGA and peer-reviewed sources you can see at the end.

  • 03

    Current for 2026

    Reflects modern UK practice on CT-first imaging, selective antibiotics and the small subset who need urgent colorectal surgery.

Key facts

Ischaemic colitis at a glance.

The essentials, in plain English - what it is, who gets it, how it presents and how it is treated in the UK today.

  • What it is

    Reduced blood flow to the colon causing mucosal inflammation and injury - the commonest form of intestinal ischaemia.

  • Who it affects

    Typically older adults with cardiovascular risk factors, but also seen after low-flow states, aortic surgery and in long-distance runners.

  • Where it hits

    The watershed areas of the colon - splenic flexure and rectosigmoid - are most vulnerable to hypoperfusion.

  • Classic story

    Sudden left-sided abdominal pain followed by bloody diarrhoea and an urgent need to open the bowels.

  • First-line imaging

    CT of the abdomen and pelvis is the gold standard - fast, sensitive and identifies complications.

  • Outlook

    Most mild cases settle with supportive care in one to two weeks - a small number need urgent surgery.

Why this guide matters

Most settles quickly. A few do not.

Ischaemic colitis sits between the everyday and the emergency. The three points below shape everything else on this page.

  • CT first, endoscopy second

    A CT of the abdomen and pelvis is the fastest, most sensitive first look - endoscopy confirms the pattern once the acute picture is clear.

  • Antibiotics are selective

    Most mild disease settles on bowel rest and fluids alone. Antibiotics are reserved for moderate-to-severe cases and systemic upset.

  • Know the surgical triggers

    Peritonitis, perforation, gangrene or a failure to improve after 48 to 72 hours are the moments to escalate to colorectal surgery.

How the diagnosis is made

From the first cramp to a settled plan.

The steps a UK acute team and gastroenterologist will normally follow, in order - so you know what to expect and why.

  1. 01

    Assessing

    Focused history and risk factors

    Recent hypotension, cardiac disease, atrial fibrillation, aortic surgery, colonoscopy, medications, cocaine or amphetamine use and any prior clotting problems.

  2. 02

    Assessing

    Abdominal examination

    Localised tenderness is common - peritonism or systemic toxicity is a red flag for transmural ischaemia and urgent surgical review.

  3. 03

    Assessing

    Bloods including lactate

    Full blood count, CRP, U and E and a lactate - a rising lactate raises concern about severity and needs specialist input.

  4. 04

    Confirming

    CT of the abdomen and pelvis

    Gold-standard first imaging - see our patient guide to private CT. Shows bowel wall thickening, thumbprinting and complications.

  5. 05

    Confirming

    Flexible sigmoidoscopy or colonoscopy

    Segmental haemorrhagic or oedematous mucosa is characteristic - see our guide to colonoscopy. Biopsies help exclude mimics.

  6. 06

    Confirming

    Rule out infective colitis

    Stool cultures and C. difficile testing to exclude an infective mimic before settling on ischaemic colitis as the diagnosis.

  7. 07

    Planning

    Multidisciplinary review

    A gastroenterology, vascular and colorectal MDT decides on conservative versus surgical management and works up the underlying cause.

Typical timeline: from admission to a settled plan within 24 to 72 hours.

Symptoms

What ischaemic colitis actually feels like.

The classic mix of sudden left-sided pain, bloody diarrhoea and urgent defecation. And the features that mean it is time to escalate.

  • Sudden left-sided pain

    A crampy, sometimes severe pain that comes on quickly - often in the left iliac fossa or lower abdomen.

  • Bloody diarrhoea

    Bright red or maroon blood mixed with loose stool, usually within 24 hours of the initial pain.

  • Urgency to open the bowels

    A strong, sudden urge to defecate - often the tipping point that brings people to hospital.

  • Subacute course

    Symptoms typically build over hours rather than minutes - unlike acute mesenteric ischaemia, which is far more dramatic.

  • Fever or systemic upset

    Low-grade fever, tachycardia or feeling unwell hints at more extensive inflammation and warrants urgent review.

  • Peritonism

    Rigid or board-like tummy, guarding or rebound tenderness - a red flag for full-thickness injury.

  • Recent trigger

    A recent operation, dehydration, low blood pressure, marathon, new medication or an episode of atrial fibrillation is often in the story.

  • Red flag - severe or worsening pain

    Pain out of proportion, rising lactate, sepsis or gangrene means emergency colorectal review, not watchful waiting.

Treatment

How ischaemic colitis is treated in the UK.

Bowel rest, fluids and treating the trigger for most people - selective antibiotics for moderate-to-severe disease, and urgent surgery for the small group with peritonitis or gangrene.

  • Bowel rest and IV fluids

    The mainstay of mild disease - nil by mouth, generous rehydration and careful electrolyte correction while the colon settles.

  • Selective IV antibiotics

    Reserved for moderate-to-severe disease, systemic upset or transmural signs - not routine for every mild episode.

  • Treat the trigger

    Correct hypotension, optimise heart failure, review offending medications and manage atrial fibrillation or a hypercoagulable state.

  • Anticoagulation review

    Weighed carefully - useful when embolic disease or a clotting disorder is confirmed, balanced against ongoing colonic bleeding.

  • Repeat endoscopy

    A follow-up colonoscopy at six to eight weeks confirms healing and rules out an alternative diagnosis such as inflammatory bowel disease.

  • Urgent surgery

    Segmental colectomy for peritonitis, perforation, full-thickness gangrene or failed conservative care - a specialist colorectal decision.

  • Stricture dilation or stenting

    Post-ischaemic strictures can be managed endoscopically or, in selected cases, with resection - guided by symptoms and imaging.

  • Long-term follow-up

    A minority develop chronic segmental colitis and need ongoing gastroenterology input, biologics or, occasionally, elective surgery.

What this guide is based on

The sources behind every claim on this page.

UK and international guidance from specialist societies, current at the time of last review.

Key references

Guidelines and standards we relied on.

A quiet reminder

This guide is for information, not medical advice.

Your GP, gastroenterologist or surgical team knows your history and can tell you which parts apply to you. If in doubt, get seen.

  • British Society of Gastroenterology (BSG). Guidance on colonic ischaemia.

  • American Gastroenterological Association (AGA). Guidelines on the management of colon ischaemia.

  • European Society for Trauma and Emergency Surgery. Consensus on acute mesenteric and colonic ischaemia.

  • NICE. Clinical Knowledge Summaries on lower gastrointestinal bleeding and abdominal pain.

Red flags

When ischaemic colitis needs urgent attention.

Most cases settle in hospital with supportive care. These are the situations that will not - and where the surgical team needs to be involved without delay.

  • Peritonitis

    A rigid, tender abdomen with guarding means full-thickness injury until proven otherwise - immediate surgical review.

  • Rising lactate or sepsis

    Escalating lactate, tachycardia, low blood pressure or confusion points to systemic compromise and needs resuscitation and critical care.

  • Right-sided or pan-colonic disease

    Isolated right-sided ischaemic colitis carries a worse prognosis and a higher chance of needing surgery than classic left-sided disease.

  • Failed conservative care

    No improvement after 48 to 72 hours of optimal medical treatment lowers the threshold for endoscopic reassessment or surgery.

  • Perforation on imaging

    Free gas or contained perforation on CT is a surgical emergency - urgent colorectal input is needed.

  • Full-thickness gangrene

    Confirmed gangrene at endoscopy or laparotomy needs prompt resection to prevent overwhelming sepsis.

  • Suspicion of mesenteric ischaemia

    Severe pain out of proportion to examination raises the possibility of small-bowel ischaemia - see our guide to intestinal ischaemia.

  • Late stricture or obstruction

    A healed ischaemic segment can narrow and cause obstruction weeks later - see our guide to intestinal obstruction.

  • Underlying atrial fibrillation

    A new or uncontrolled arrhythmia - see our guide to atrial fibrillation - is a treatable driver of embolic ischaemia.

Living with it

A recoverable condition, with a clear plan.

Four things that make the biggest difference during and after an episode - giving the colon time, treating the trigger, watching for warning signs and staying alert to the small chance of chronic disease.

A quiet reminder

Recovery is usually straightforward.

Most people go home within a week and are back to normal within a month - what matters is treating the trigger so it does not happen again.

  1. 01 Recovery

    Give the colon time

    Most mild episodes settle within one to two weeks - a gradual return to a normal diet, plenty of fluids and gentle activity while symptoms fade.

  2. 02 Cause

    Treat what triggered it

    Optimising blood pressure, heart failure and heart rhythm - and reviewing culprit medications - is what stops another episode.

  3. 03 Watch

    Know the warning signs

    Return quickly if pain worsens, bleeding recurs, fever appears or you cannot keep fluids down - these are the situations that need a hospital, not another day at home.

  4. 04 Chronic

    Chronic colitis is uncommon

    A small minority develop a lingering segmental colitis that needs ongoing gastroenterology input - most people simply recover and move on.

Frequently asked

Everything we get asked about ischaemic colitis.

Quick answers on causes, diagnosis, watershed anatomy, surgery and the risk of recurrence.

  • What is ischaemic colitis?

    Ischaemic colitis is inflammation and injury of the colon caused by reduced blood flow. It is the commonest form of intestinal ischaemia and typically causes sudden left-sided abdominal pain and bloody diarrhoea. Most cases settle with supportive care, though a small number need urgent surgery.

  • Who gets it?

    Ischaemic colitis is most common in older adults with cardiovascular risk factors - atherosclerosis, atrial fibrillation, heart failure or hypertension. It also occurs after low-flow states such as major surgery, sepsis or dehydration, in people with clotting disorders such as factor V Leiden, in sickle cell disease, in long-distance runners and with certain drugs including cocaine, amphetamines, digoxin, NSAIDs and sumatriptan.

  • How is it diagnosed?

    The story of sudden left-sided pain followed by bloody diarrhoea is highly suggestive. A CT of the abdomen and pelvis is the gold-standard first test - it shows bowel wall thickening and rules out other causes. A flexible sigmoidoscopy or colonoscopy with biopsies then confirms the pattern of segmental haemorrhagic mucosa and helps distinguish it from infective colitis or inflammatory bowel disease.

  • Why the splenic flexure and rectosigmoid?

    These are watershed areas where two arterial territories meet, and blood flow drops off quickest when perfusion falls. That is why ischaemic damage most often shows up in a segmental left-sided pattern rather than affecting the whole colon.

  • When is surgery needed?

    Most mild episodes settle with bowel rest, IV fluids and selective antibiotics. Urgent surgery is reserved for peritonitis, perforation, full-thickness gangrene or when conservative care fails after 48 to 72 hours. Colorectal surgeons make that call within a multidisciplinary team.

  • Can it come back?

    Recurrence is possible if the underlying trigger is not addressed - poorly controlled heart disease, ongoing use of a causative drug or an untreated clotting disorder. Optimising cardiovascular risk factors, reviewing medications and managing atrial fibrillation or hypercoagulable states such as factor V Leiden reduce the risk.

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