Irritable bowel syndrome
Description
- A disorder of gut-brain interaction (DGBI) - no longer called "functional"
- Recurrent abdominal pain/discomfort related to defecation, with altered stool frequency or form
- *A POSITIVE clinical diagnosis, not a diagnosis of exclusion - endless investigation reinforces illness behaviour*
Subtypes - by stool form on symptomatic days (Bristol scale)
| Subtype | Definition |
|---|---|
| IBS-C | >25% hard (BSFS 1-2) and <25% loose |
| IBS-D | >25% loose (BSFS 6-7) and <25% hard |
| IBS-M | >25% of both |
| IBS-U | Neither |
- Subtype drives drug choice and often switches over time
Epidemiology
- ~5-10% of adults (Rome IV); higher on the looser Rome V criteria
- F>M ~2:1
- Onset typically <50 - new IBS-type symptoms over 50 need investigation
- ~50-60% have a coexisting anxiety or depressive disorder
- Accounts for ~25-30% of gastroenterology outpatient referrals
Aetiopathogenesis
- Visceral hypersensitivity - lowered pain threshold to rectal distension (the most reproducible abnormality)
- Altered motility - accelerated (IBS-D) or delayed (IBS-C) transit
- Brain-gut axis dysregulation - altered central pain processing; stress, early-life adversity
- Post-infectious IBS - ~10% after bacterial gastroenteritis (Campylobacter, Salmonella, Shigella); low-grade mucosal inflammation, inc enterochromaffin cells
- Microbiome alteration, bile acid malabsorption (~25-30% of "IBS-D"), altered serotonin signalling
- Diet - FODMAPs are fermented -> gas + osmotic load -> distension in a hypersensitive gut
- Genetic: SCN5A, sucrase-isomaltase variants; familial clustering
Diagnosis
Criteria
- Rome V (2026) - abdominal pain OR DISCOMFORT >=3 days/month in the last 3 months, onset >=6 months earlier, associated with >=2 of:
- Related to defecation
- Change in stool frequency
- Change in stool form
- Rome IV (2016) required pain (discomfort excluded) on >=1 day/week. Rome V lowers the frequency threshold and restores "discomfort" -> more patients qualify
Red flags -> investigate, do not diagnose IBS
- Age >=50 with new symptoms
- Rectal bleeding, unexplained iron deficiency anaemia
- Unintentional weight loss
- Nocturnal symptoms waking from sleep
- Progressive symptoms, abdominal or rectal mass, fever
- Family history of CRC, IBD or coeliac disease
Baseline investigations - limited, deliberately
- FBE, CRP, coeliac serology (tTG-IgA + total IgA) in everybody
- Faecal calprotectin if diarrhoea - the single best test to separate IBS from IBD
- Positive iFOBT or any red flag -> colonoscopy
- Constipation-predominant: exclude organic disease with FBE, TFTs, serum calcium, and colonoscopy where age or red flags warrant
- Diarrhoea-predominant: add stool MCS + parasites (Giardia), and consider bile acid malabsorption (SeHCAT, or empirical cholestyramine trial)
- Do NOT routinely order: food-specific IgG, hydrogen breath tests, abdominal imaging, repeated colonoscopy
Management
1. The consultation itself
- Explain the mechanism positively - a real disorder of gut-brain signalling
- Firm diagnosis + therapeutic relationship reduces re-presentation more than any drug
- Identify and address anxiety, depression, prior trauma
2. Diet
- First line: regular meals, limit alcohol/caffeine/fatty and spicy food, adequate fluids, moderate fibre
- Low-FODMAP diet (Monash) - fermentable oligo-, di-, monosaccharides and polyols
- Fructose, lactose, fructans (wheat, onion, garlic), galactans (legumes), polyols (sorbitol, mannitol - "sugar-free" foods)
- Especially effective in IBS-D and bloating - ~50-70% respond
- *Must be dietitian-supervised, 2-6 weeks restriction then STRUCTURED REINTRODUCTION* - long-term restriction harms the microbiome and nutrition
- Soluble fibre (psyllium/ispaghula) helps; INSOLUBLE fibre (bran) makes it worse
3. By predominant symptom
| Symptom | Options |
|---|---|
| Pain/cramping | Antispasmodics - hyoscine butylbromide, mebeverine, peppermint oil |
| Constipation (IBS-C) | Soluble fibre, PEG (macrogol); then linaclotide (PBS-listed for severe IBS-C), lubiprostone, prucalopride |
| Diarrhoea (IBS-D) | Loperamide (titrate, pre-emptive dosing); cholestyramine if bile acid malabsorption; rifaximin (not PBS-subsidised for IBS in Australia) |
| Bloating | Low-FODMAP, peppermint oil, probiotics (trial 4 weeks, stop if no benefit) |
| Global/refractory | Gut-brain neuromodulators |
4. Neuromodulators - for pain, at sub-antidepressant doses
- TCA (amitriptyline 10-30 mg nocte) - preferred in IBS-D (anticholinergic slowing helps); ATLANTIS trial supports it
- SSRI - preferred in IBS-C and where anxiety is prominent
- Explain they are given for visceral pain, not because "it is in your head" - otherwise adherence collapses
5. Psychological therapy - as effective as drugs in refractory disease
- Gut-directed hypnotherapy, CBT, mindfulness
- Nurse- and app-delivered programs (Monash, Nerva) widen access
Avoid
- Opioids - narcotic bowel syndrome; worsens everything
- Repeated normal investigations
- Long-term unsupervised restrictive diets
Associations
- Anxiety, depression, somatisation, prior physical or sexual abuse
- Fibromyalgia, chronic fatigue syndrome, chronic pelvic pain, temporomandibular disorder - shared central sensitisation
- Migraine, interstitial cystitis
- Coeliac disease - 4x more common in IBS-type symptoms; always serology
- Bile acid malabsorption, microscopic colitis, SIBO, giardiasis - the organic mimics of IBS-D
- Post-infectious after bacterial gastroenteritis
- Endometriosis - the classic missed diagnosis in a young woman with cyclical "IBS"
- Medication-induced: metformin, magnesium, SSRIs, opioids
Natural history & complications
- Chronic, relapsing-remitting; does not shorten life and does not progress to organic disease
- Say this explicitly - it is what the patient came for
- Subtype migrates over time (IBS-C <-> IBS-M <-> IBS-D)
- ~30-50% improve substantially over 5 years; a minority remain severely affected
- Risk of misdiagnosis is real - re-evaluate if symptoms change character, new red flags emerge, or there is failure to respond
- Morbidity is from quality of life, work absence, unnecessary surgery (cholecystectomy, hysterectomy, appendicectomy rates are all raised)
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