Key Facts

Irritable bowel syndrome (IBS) is a disorder of gut-brain interaction (DGBI) characterised by chronic abdominal pain associated with altered bowel habit, without identifiable organic pathology. It is one of the commonest conditions in primary care.

Epidemiology

IBS affects 10-15% of the UK adult population. Female:male ratio is approximately 2:1. Peak onset is 20-40 years. It accounts for 25-50% of gastroenterology outpatient referrals. Only ~30% of sufferers consult a doctor. IBS significantly impairs quality of life — comparable to IBD and depression in impact.

Aetiology

IBS is multifactorial:

• Gut-brain axis dysfunction: altered central processing of visceral signals (visceral hypersensitivity)
• Post-infectious: 10-15% develop post-infectious IBS after gastroenteritis (Campylobacter, Salmonella, norovirus)
• Psychological factors: anxiety, depression, childhood adversity, stress (bidirectional gut-brain interaction)
• Gut microbiome alteration: dysbiosis, small intestinal bacterial overgrowth
• Dietary triggers: FODMAPs (fermentable oligo-, di-, monosaccharides and polyols), lactose, caffeine, alcohol
• Genetic: modest familial aggregation

Pathophysiology

• Visceral hypersensitivity: lowered pain threshold in the gut — rectal balloon distension studies show pain at lower volumes
• Altered motility: accelerated transit in IBS-D, delayed transit in IBS-C
• Mucosal immune activation: increased mast cells, T lymphocytes in mucosa (especially post-infectious IBS)
• Serotonin (5-HT) dysregulation: 95% of body serotonin is in the gut — increased 5-HT in IBS-D, decreased in IBS-C
• Central sensitisation: altered brain processing of gut signals (fMRI studies show increased activation of pain processing areas)

Typical Presentation

• Recurrent abdominal pain/discomfort: often lower abdominal, colicky; related to defaecation (may improve or worsen)
• Altered bowel habit: diarrhoea, constipation, or alternating; urgency; incomplete evacuation
• Bloating/abdominal distension: common, often worse through the day
• **Symptoms often triggered by food, stress, or menstruation

Subtypes (Rome IV)

• IBS-C: hard/lumpy stools ≥25% of bowel movements; loose stools <25%
• IBS-D: loose/watery stools ≥25%; hard stools <25%
• IBS-M: both hard and loose stools ≥25% each

Red Flags (Not IBS — Investigate Further)

• Age >50 with new symptoms
• Unexplained weight loss
• Rectal bleeding
• Family history of bowel/ovarian cancer
• Anaemia
• Nocturnal symptoms waking from sleep
• Palpable abdominal/rectal mass
• Raised inflammatory markers (CRP, calprotectin)

Bedside

• History: Rome IV criteria assessment, dietary triggers, psychological comorbidity, medication review
• Examination: usually normal; mild abdominal tenderness; check for mass, organomegaly

Bloods

• FBC: exclude anaemia
• CRP/ESR: normal in IBS (raised suggests IBD/other organic disease)
• tTG-IgA + total IgA: exclude coeliac disease (NICE CG61)
• TFTs: if clinical suspicion of thyroid disease
• CA-125: if ovarian cancer considered (women >50 with bloating)

Stool Tests

• Faecal calprotectin: <100 μg/g effectively excludes IBD (NPV >95%); key investigation in distinguishing IBS from IBD (NICE DG11)
• FIT (faecal immunochemical test): if colorectal cancer concern
• Stool cultures: if infective/post-infective aetiology

Imaging/Endoscopy

• NOT routinely required for IBS diagnosis in young patients without red flags
• Colonoscopy: if red flags, age >50, raised calprotectin, failed empirical treatment

Special Tests

• SeHCAT scan: bile acid malabsorption (especially IBS-D)
• Hydrogen breath tests: lactose intolerance, SIBO
• Anorectal physiology: if constipation with suspected pelvic floor dyssynergia

Non-pharmacological

• Lifestyle: regular meals, adequate hydration (8 cups/day), regular exercise, stress management
• Dietary advice (first-line):
  • General: reduce caffeine, alcohol, fizzy drinks, high-fat foods, limit fresh fruit to 3 portions/day
  • Low FODMAP diet: supervised by dietitian; elimination for 4-8 weeks then staged reintroduction; evidence of benefit in ~50-75% (Staudacher et al., 2017)
  • Fibre: soluble fibre (ispaghula husk) may help IBS-C; insoluble fibre (bran) often worsens symptoms
• Psychological therapy: CBT (face-to-face or digital), gut-directed hypnotherapy (evidence from Manchester group), mindfulness — effective for refractory IBS

Pharmacological

• IBS-C:
  • Ispaghula husk (Fybogel) 1 sachet BD
  • Linaclotide 290 mcg OD (guanylate cyclase-C agonist — NICE TA)
  • Lubiprostone 8 mcg BD (if linaclotide ineffective)
  • PEG laxatives (macrogol) — less evidence specifically for IBS-C
• IBS-D:
  • Loperamide 2-4 mg PRN (max 16 mg/day)
  • Cholestyramine 4 g BD-QDS trial (bile acid malabsorption)
  • Eluxadoline 100 mg BD (mixed opioid agonist/antagonist — limited UK use)
• Pain/global symptoms:
  • Antispasmodics: mebeverine 135 mg TDS, hyoscine butylbromide 10 mg TDS, peppermint oil capsules 187 mg TDS
  • TCAs (first-line for refractory pain per NICE): amitriptyline 10-30 mg ON — start low, increase slowly; NNT ~4 for IBS
  • SSRIs: fluoxetine 20 mg OD, citalopram 10-20 mg OD — for IBS-D with anxiety/depression; less evidence than TCAs for pain

Surgical/Interventional

• Not indicated for IBS
• Biofeedback for pelvic floor dyssynergia (if contributing to IBS-C)

Referral Criteria

• Gastroenterology: red flags, diagnostic uncertainty, refractory symptoms despite treatment
• Dietitian: FODMAP diet guidance
• Psychological services: CBT, gut-directed hypnotherapy for refractory IBS

IBS is a chronic relapsing condition. Most patients have ongoing symptoms over many years, although severity fluctuates. Approximately 50% of patients improve over time. IBS does not increase mortality or risk of IBD/cancer. Quality of life is significantly impaired — work absenteeism, social isolation, healthcare utilisation are substantial. Low FODMAP diet improves symptoms in 50-75%. TCAs improve global IBS symptoms in ~60% (NNT 4). Psychological therapies have durable benefit.

Rome IV Diagnostic Criteria for IBS

Bristol Stool Chart and IBS Subtypes