Understanding Irritable Bowel Syndrome (IBS) for PLAB 2
- Ann Augustin
- 2 minutes ago
- 3 min read
🔹 1. IBS vs IBD — The Fundamental Difference
Feature | IBD (Inflammatory Bowel Disease) | IBS (Irritable Bowel Syndrome) |
Nature | Structural & inflammatory disease (autoimmune) | Functional bowel disorder (no visible inflammation) |
Examples | Crohn’s disease, Ulcerative colitis | IBS-C, IBS-D, IBS-M |
Pathology | Mucosal damage, ulceration | Normal mucosa, abnormal gut–brain signalling |
Investigations | Raised CRP, positive fecal calprotectin, abnormal colonoscopy | Normal investigations |
Treatment | Anti-inflammatory & immunosuppressive drugs | Symptom-directed, gut–brain regulation |
🧠 Easy memory:
IBD = Inflammation
IBS = Irritation + Sensitivity
🔹 2. What Causes IBS?
IBS is a disorder of gut–brain interaction, not a structural disease.
Mechanisms
Visceral hypersensitivity – the gut wall is over-sensitive to stretch or gas.
Altered motility – spasms → diarrhoea or constipation.
Post-infectious changes – gut nerves stay “on edge” after gastroenteritis.
Microbiome imbalance – dysbiosis → fermentation and bloating.
Psychological stress – anxiety or low mood amplifies gut signals via the brain–gut axis.
🔹 3. Why Avoid Sorbitol and Lactulose?
Both are poorly absorbed sugars → osmotic agents that pull water into the bowel and ferment in the colon.
Substance | Mechanism | Why It Worsens IBS |
Sorbitol (in sugar-free foods) | Poorly absorbed sugar alcohol → fermented by bacteria | Produces gas and bloating |
Lactulose (synthetic disaccharide laxative) | Fermented by colonic bacteria → gas + acid + water | Causes cramps and distension |
🔸 IBS patients have visceral hypersensitivity, so even normal gas volumes cause discomfort.
🔸 These agents are FODMAPs, which are avoided in low-FODMAP diets.
🧠 Remember:
Sorbitol & Lactulose = “Gas + Water = Disaster” for IBS
🔹 4. Safe Laxative Alternatives — Why Macrogol Is Preferred
Feature | Lactulose | Macrogol (PEG) |
Type | Fermentable sugar | Non-fermentable polymer |
Mechanism | Fermented → gas + acid draws water | Directly binds water molecules (no fermentation) |
Side-effects | Bloating, cramps, flatulence | Gentle, minimal gas |
Use in IBS-C | ❌ Avoid | ✅ Preferred |
💡 How it differs: Macrogol retains water without fermentation, giving a gentle, gas-free stool-softening effect — ideal for IBS patients.
🔹 5. Pharmacological Management of IBS
IBS treatment is symptom-based and escalated stepwise.
IBS Type | Common Symptoms | Drug Options | How They Help |
IBS-C | Hard, infrequent stools | 💊 Macrogol / PEG – draws water gently 💊 Prucalopride – stimulates peristalsis 💊 Linaclotide – increases secretion & reduces pain | Soften stool, improve motility, ease pain |
IBS-D | Loose frequent stools | 💊 Loperamide – slows gut transit 💊 Bile-acid binders – for bile acid diarrhoea | Reduces frequency & urgency |
Pain-predominant IBS | Cramping, bloating | 💊 Antispasmodics – mebeverine, hyoscine (Buscopan) 💊 Peppermint oil capsules | Relax smooth muscle, ease cramps |
Persistent or mixed IBS | Pain ± altered bowel habits | 💊 Low-dose amitriptyline (10–25 mg HS) 💊 SSRIs if anxiety/depression | Modulate gut–brain axis & reduce visceral pain |
🔹 6. Linaclotide — The New Targeted Option for IBS-C
Aspect | Description |
Class | Guanylate cyclase-C (GC-C) receptor agonist |
Mechanism | ↑ cGMP → ↑ chloride & bicarbonate secretion → water follows → soft stool |
Additional effect | cGMP reduces pain signalling → less bloating & discomfort |
Dose | Once daily, 30 min before food |
Side-effects | Diarrhoea, mild distension |
Avoid if | Bowel obstruction |
🧠 Mnemonic:
“Linaclotide lubricates & lessens pain.”
🔹 7. Non-Drug Management — Always First-Line
Approach | Mechanism / Benefit |
Low-FODMAP diet | Reduces fermentable sugars (lactose, sorbitol, fructose) → ↓ gas/bloating |
Regular meals & hydration | Prevents motility swings |
Soluble fibre (Ispaghula husk) | Normalises stool form |
Limit caffeine/alcohol | Reduces spasms |
Stress management / CBT / relaxation | Improves gut–brain control |
Probiotics (4-week trial) | Restores microbiota balance |
🔹 8. Patient-Friendly Explanation (PLAB 2-Style)
“You have a sensitive bowel rather than an inflamed one. It tends to over-react to things like stress or certain foods.We’ll start by adjusting your diet and lifestyle — small, regular meals, avoiding foods that cause gas like beans or fizzy drinks.If constipation is a problem, we’ll use gentle laxatives such as macrogol to draw water in without causing bloating. If that’s not enough, a medicine called linaclotide can help soften the stool and reduce pain. Remember, this condition isn’t dangerous, but it can be uncomfortable — and our goal is to help you manage it effectively.”
🔹 9. Quick Revision Table
Category | Example | Mechanism | IBS Use |
Osmotic laxative | Macrogol | Holds water in stool | IBS-C |
Fermentable laxative | Lactulose | Fermented → gas | ❌ Avoid |
Antispasmodic | Mebeverine / Buscopan | Smooth-muscle relaxant | Pain relief |
Anti-diarrhoeal | Loperamide | Slows transit | IBS-D |
Secretagogue | Linaclotide | ↑ fluid & ↓ pain | IBS-C |
Antidepressant (low dose) | Amitriptyline | Gut-brain modulation | Persistent pain |
Diet | Low-FODMAP | ↓ Fermentation | All types |
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