Irritable bowel syndrome (IBS)

Definition

Irritable bowel syndrome (IBS) is a chronic functional gastrointestinal disorder characterized by recurrent abdominal pain or discomfort associated with altered bowel habits, in the absence of structural or biochemical abnormalities. It is considered a disorder of the gut–brain interaction, with symptoms resulting from a combination of altered gastrointestinal motility, visceral hypersensitivity, immune activation, and psychosocial factors.

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Epidemiology

• Affects approximately 10–15% of the global population.

• More common in females than males, with a peak incidence between ages 20 and 40 years.

• Significant impact on quality of life and work productivity.

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Etiology and Pathophysiology

The exact cause of IBS is multifactorial and incompletely understood. Contributing factors include:

1. Altered gut motility – hypermotility in diarrhea-predominant IBS, hypomotility in constipation-predominant IBS.

2. Visceral hypersensitivity – heightened pain perception to normal intestinal distension.

3. Post-infectious IBS – following gastroenteritis.

4. Gut microbiota alterations – dysbiosis affecting fermentation and immune function.

5. Psychosocial stress and CNS dysregulation – abnormalities in brain–gut axis signaling.

6. Low-grade inflammation – increased immune cell activity in intestinal mucosa.

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Classification (Rome IV Criteria)

Based on predominant bowel habit:

1. IBS with constipation (IBS-C) – hard or lumpy stools ≥25% of bowel movements; loose stools <25%.

2. IBS with diarrhea (IBS-D) – loose or watery stools ≥25%; hard stools <25%.

3. Mixed IBS (IBS-M) – both hard and loose stools ≥25%.

4. Unclassified IBS (IBS-U) – do not fit other subtypes.

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Clinical Features

Core Symptoms

• Recurrent abdominal pain or discomfort ≥1 day per week in the last 3 months, associated with ≥2 of:

  • Related to defecation.

  • Associated with change in stool frequency.

  • Associated with change in stool form.

Other Symptoms

• Bloating, distension.

• Mucus in stool.

• Urgency in IBS-D.

• Sensation of incomplete evacuation in IBS-C.

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Red Flag Symptoms (suggest alternative diagnosis)

• Unexplained weight loss.

• Rectal bleeding or positive fecal occult blood.

• Nocturnal diarrhea.

• Anemia.

• Family history of colorectal cancer, inflammatory bowel disease, or celiac disease.

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Diagnosis

• Clinical – based on Rome IV criteria after excluding organic disease.

• Investigations (to exclude other causes):

  • Full blood count, C-reactive protein/ESR.

  • Stool tests (occult blood, calprotectin, ova and parasites if indicated).

  • Celiac serology (tTG-IgA) in IBS-D or IBS-M.

  • Colonoscopy in patients with alarm features or >50 years.

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Treatment

Management is individualized based on IBS subtype, symptom severity, and psychosocial factors. It includes lifestyle modification, dietary therapy, pharmacological treatment, and psychological interventions.

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1. Lifestyle and Dietary Modification

• Regular meals, adequate hydration.

• Avoid trigger foods (e.g., caffeine, alcohol, fatty foods, gas-producing foods).

• Low FODMAP diet trial (short-term).

• Gradual increase in soluble fiber (psyllium) in IBS-C.

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2. Pharmacological Management

For IBS with Constipation (IBS-C)

• Osmotic laxatives:

  • Polyethylene glycol (PEG) – 17 g dissolved in water once daily; may titrate up.

• Chloride channel activators:

  • Lubiprostone – 8 mcg orally twice daily with food.

• Guanylate cyclase-C agonists:

  • Linaclotide – 290 mcg orally once daily, 30 minutes before first meal.

• Serotonin 5-HT4 agonists (where available):

  • Prucalopride – 2 mg orally once daily (1 mg in elderly).

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For IBS with Diarrhea (IBS-D)

• Antimotility agents:

  • Loperamide – 2–4 mg orally initially, then 2 mg after each loose stool (max 16 mg/day).

• Bile acid sequestrants (for bile acid malabsorption):

  • Cholestyramine – 4 g orally once or twice daily; adjust as needed.

• Non-absorbable antibiotics:

  • Rifaximin – 550 mg orally three times daily for 14 days; may repeat up to two times.

• 5-HT3 antagonists (for severe IBS-D in women without constipation):

  • Alosetron – 0.5 mg orally twice daily, may increase to 1 mg twice daily if tolerated.

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For Abdominal Pain/Bloating

• Antispasmodics:

  • Hyoscine butylbromide – 10–20 mg orally three to five times daily.

  • Dicyclomine – 20 mg orally four times daily.

• Peppermint oil capsules – 0.2–0.4 mL enteric-coated capsules orally three times daily before meals.

• Tricyclic antidepressants (low dose) for visceral pain modulation:

  • Amitriptyline – 10–25 mg orally at bedtime, titrate as needed.

• SSRIs (if comorbid depression/anxiety and IBS-C):

  • Fluoxetine – 20 mg orally once daily.

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3. Psychological Therapies

• Cognitive behavioural therapy (CBT).

• Gut-directed hypnotherapy.

• Stress management programs.

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4. Probiotics

• Certain strains (e.g., Bifidobacterium infantis) may help bloating and pain; dosing varies by product.

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Prognosis

• IBS is chronic but non-progressive, without increased mortality risk.

• Symptoms fluctuate and may improve with appropriate management.