Medicine chapters
Irritable bowel syndrome
What irritable bowel syndrome is, how it is diagnosed positively with the Rome IV criteria, which red flags need investigation, and how symptoms are treated by subtype.
- IBS is a common functional bowel disorder: recurrent abdominal pain linked to defecation or a change in stool frequency or form, with no structural disease.
- Diagnose it positively with Rome IV and a few simple tests (FBC, CRP, coeliac serology, faecal calprotectin); investigate further only if there are red flags such as rectal bleeding, weight loss, nocturnal symptoms, anaemia or onset in older age.
- Treat with explanation, diet (fibre for IBS-C, low FODMAP for bloating), antispasmodics for pain, loperamide for IBS-D, laxatives for IBS-C, and low-dose tricyclics for persistent pain.
On this page
Overview
Of all the functional gut disorders, irritable bowel syndrome (IBS) is seen most often. "Functional" means the patient has real symptoms but no structural, inflammatory or biochemical abnormality on routine tests.
IBS is common in young adults, especially women, and accounts for a large share of gastroenterology outpatient work. It is not dangerous and does not lead to cancer, but it can badly affect quality of life. The doctor's main jobs are to make a confident positive diagnosis, not miss organic disease, and treat the dominant symptom.
Causes and mechanisms
There is no single cause. IBS is best seen as a disorder of brain–gut interaction.
- Visceral hypersensitivity: the gut feels normal stretching as painful (a low pain threshold to balloon distension).
- Altered motility: faster transit in IBS-D, slower in IBS-C.
- Brain–gut axis: stress, anxiety and depression change gut sensation and motility, and gut symptoms worsen mood.
- Post-infectious IBS: starts after an episode of gastroenteritis in a minority of patients; risk is higher in women and with severe or long infections.
- Changes in the gut microbiota, mild immune activation, and bile acid malabsorption in some patients.
- Triggers: psychological stress, life events, abuse, anxiety and depressive disorders, certain foods (fermentable carbohydrates), pelvic surgery, antibiotics.
Clinical features
Gut symptoms
- Recurrent abdominal pain or cramps, often in the lower abdomen, related to defecation (usually eased, sometimes worsened).
- Altered bowel habit: diarrhoea, constipation, or both alternating.
- Bloating and visible distension, worse through the day.
- Passage of mucus; sense of incomplete evacuation; urgency.
Non-gut symptoms (common)
- Fatigue, headache, backache, poor sleep.
- Urinary frequency and urgency.
- Dysmenorrhoea, dyspareunia.
- Associated with fibromyalgia, chronic fatigue syndrome, temporomandibular joint pain and anxiety or depression.
Examination
Usually normal, apart from mild tenderness. A normal examination is reassuring but does not on its own exclude disease.
Diagnosis
Rome IV criteria
Recurrent abdominal pain, on average on 1 or more days each week, over the past 3 months, associated with two or more of:
- Related to defecation.
- Change in frequency of stool.
- Change in form (appearance) of stool.
Rome IV in three words: "Pain + Poo + Pattern": pain linked to defecation, change in frequency, change in form.
Rome IV is about pain. Painless chronic diarrhoea is functional diarrhoea (or organic disease), not IBS; bloating alone without pain is functional bloating.
Subtypes (by the main stool form on abnormal days)
| Subtype | Stool pattern |
|---|---|
| IBS-C (constipation) | Hard or lumpy stools in more than 25% and loose stools in less than 25% of bowel movements |
| IBS-D (diarrhoea) | Loose or watery stools in more than 25% and hard stools in less than 25% |
| IBS-M (mixed) | Both hard and loose stools in more than 25% |
| IBS-U (unclassified) | Does not fit the above |
Basic tests (in all patients)
- Full blood count (anaemia), ESR/CRP (inflammation).
- Coeliac serology (tissue transglutaminase IgA).
- Faecal calprotectin in diarrhoea, to exclude IBD.
- In Pakistan, consider stool microscopy for parasites such as Giardia and amoebae, and thyroid tests where relevant.
A normal faecal calprotectin makes IBD very unlikely and supports IBS in a patient with diarrhoea.
If these are normal and there are no red flags, the diagnosis of IBS can be made without colonoscopy.
Red flags (alarm features): investigate further
- Rectal bleeding.
- Unintentional weight loss.
- Nocturnal diarrhoea or pain that wakes the patient.
- Fever.
- Iron deficiency anaemia, raised inflammatory markers or raised calprotectin.
- Abdominal or rectal mass.
- New symptoms in an older patient (bowel cancer is more likely).
- Family history of bowel or ovarian cancer, coeliac disease or IBD.
- Large-volume diarrhoea or features of malabsorption.
Nocturnal symptoms, bleeding and weight loss do not fit IBS. They point to organic disease such as IBD, colon cancer, coeliac disease, microscopic colitis or infection.
IBS is a positive diagnosis made on symptom criteria plus a few simple tests; it is not simply what remains after endless investigation.
Differential diagnosis
| Condition | Clue | Test |
|---|---|---|
| Coeliac disease | Diarrhoea, iron or folate deficiency | Anti-tTG IgA, duodenal biopsy |
| IBD | Blood, nocturnal stools, raised CRP | Calprotectin, colonoscopy |
| Bowel cancer | Older age, bleeding, anaemia, weight loss | Colonoscopy |
| Ovarian cancer | Older woman with new, persistent bloating | CA-125, pelvic ultrasound |
| Microscopic colitis | Older woman, watery diarrhoea, NSAIDs/PPIs | Colonic biopsies |
| Bile acid diarrhoea | After cholecystectomy or ileal disease | SeHCAT scan, response to a bile acid binder |
| Lactose intolerance | Symptoms after milk | Exclusion diet |
| Giardiasis / amoebiasis | Travel, local exposure | Stool tests |
| Thyroid disease | Hyper- (diarrhoea) or hypo- (constipation) | TFTs |
Management
1. Explanation and reassurance
- Make a positive diagnosis; explain the brain–gut model and that IBS is real but not dangerous.
- Avoid repeated unnecessary tests, which increase anxiety.
- A good doctor–patient relationship improves outcome.
2. Lifestyle and diet
- Regular meals, adequate fluids, exercise, stress management.
- Reduce caffeine, alcohol, fizzy drinks and fatty or spicy food if they trigger symptoms.
- IBS-C: increase soluble fibre (ispaghula). Insoluble fibre (bran) often worsens bloating.
- Bloating and wind: a dietitian-led low FODMAP diet (restricts fermentable sugars in onions, garlic, beans, wheat, some fruits and sweeteners).
- Probiotics may help some patients.
3. Drugs according to the main symptom
| Symptom | First-line | Second-line |
|---|---|---|
| Pain / cramps | Antispasmodics: mebeverine, hyoscine butylbromide, peppermint oil, dicycloverine | Low-dose tricyclic antidepressant (e.g. amitriptyline at night) |
| Diarrhoea (IBS-D) | Loperamide | Low-dose tricyclic; rifaximin; eluxadoline; alosetron (restricted) |
| Constipation (IBS-C) | Soluble fibre, osmotic laxative (macrogol) | Linaclotide; prucalopride; an SSRI may suit IBS-C |
| Psychological distress | CBT, hypnotherapy, psychotherapy | Antidepressants |
- Avoid lactulose in IBS-C if bloating is a problem; it ferments and adds gas.
- Avoid opioids (such as codeine) for pain; they worsen constipation and can cause narcotic bowel.
- Tricyclics suit IBS-D (they slow the gut); SSRIs suit IBS-C.
Tricyclics are given at low doses for their pain-modulating effect, not as antidepressants, and they are constipating, so they are better for IBS-D than IBS-C.
4. Psychological therapy
For symptoms that persist despite diet and drugs: cognitive behavioural therapy, gut-directed hypnotherapy or psychotherapy.
Prognosis
IBS is chronic and relapsing but benign: it does not shorten life or cause cancer. Symptoms often wax and wane with stress. A change in the pattern of symptoms, especially in an older patient, should prompt reassessment.
Expect: the definition of a functional disorder, Rome IV criteria (pain at least weekly over 3 months with two of: related to defecation, change in frequency, change in form), subtypes by the 25% rule, female predominance and post-infectious onset, red flags that need colonoscopy, faecal calprotectin to exclude IBD, coeliac serology, and treatment by symptom: antispasmodics for pain, loperamide for diarrhoea, fibre and laxatives for constipation, low FODMAP for bloating, and low-dose tricyclics for persistent pain.
Quick revision
- Functional disorder; mechanism is visceral hypersensitivity plus altered motility.
- Commoner in young women; may follow gastroenteritis.
- Rome IV: pain at least 1 day/week for 3 months + 2 of defecation-related, frequency change, form change.
- Subtypes: IBS-C, IBS-D, IBS-M by the 25% rule.
- Tests: FBC, CRP, coeliac serology, faecal calprotectin.
- Red flags: bleeding, weight loss, nocturnal symptoms, anaemia, fever, older age, family history.
- Pain: mebeverine, hyoscine, peppermint oil; then low-dose amitriptyline.
- IBS-D: loperamide. IBS-C: soluble fibre, macrogol, linaclotide.
- Bloating: low FODMAP diet; avoid bran and lactulose.
Written by The House Job from standard references (Kumar and Clark's Clinical Medicine).
FAQ
Can IBS be diagnosed without colonoscopy?
Yes. In a young patient who meets the Rome IV criteria, has no red flags and has normal blood count, CRP, coeliac serology and faecal calprotectin, IBS can be diagnosed positively.
Does IBS cause nocturnal diarrhoea?
Usually not. Diarrhoea or pain that wakes the patient at night suggests organic disease such as IBD or infection and needs investigation.
Which antidepressant is used in IBS and why?
Low-dose amitriptyline, a tricyclic, reduces visceral pain and slows the gut, so it is most useful in IBS with diarrhoea.
What is a low FODMAP diet?
A dietitian-guided diet that cuts down fermentable sugars and sugar alcohols (for example in onions, garlic, beans, wheat and some fruits) to reduce bloating and wind.