Drug Therapy of Irritable Bowel Syndrome

Drug Therapy of Irritable Bowel Syndrome

1. Definition, classification and the brain–gut framework

• Irritable bowel syndrome (IBS) is a chronic functional GI disorder ("disorder of brain–gut interaction") defined by recurrent abdominal pain associated with altered bowel habit (constipation, diarrhoea, or both at different times), in the absence of a structural or biochemical explanation (G&G 14e Ch.54, p.1098).
• IBS is best understood as a disorder of brain–gut interaction characterised by visceral hypersensitivity plus GI motor abnormality — not a primary inflammatory or structural disease; for most functional disorders treatment "remains empirical and symptom based, reflecting limited understanding of the pathophysiology" (G&G 14e Ch.54, p.1086).
• Prevalence: affects up to 10–15% of the population in the U.S. and most Western countries (G&G 14e Ch.54, p.1098). KDT classifies IBS-type ("spastic") constipation under functional constipation (KDT 8e Ch.49, p.726).
• IBS results from a varying combination of disturbances in visceral motor and sensory function, frequently with significant affective (psychological) disorders — anxiety/depression comorbidity is common and therapeutically relevant (G&G 14e Ch.54, p.1098).
• Subtype classification (the axis that drives drug choice) — IBS is stratified by predominant stool form (G&G 14e Ch.54, p.1098):
  • IBS-C — constipation-predominant.
  • IBS-D — diarrhoea-predominant.
  • IBS-M — mixed bowel habits (alternating).
  • (IBS-U — unsubtyped.)
• Core therapeutic principle: because there is no single disease mechanism, treatment is subtype-directed and symptom-targeted — agents are chosen to correct the predominant bowel disturbance (loosen stool in IBS-C, firm/slow stool in IBS-D) while a separate analgesic/neuromodulatory layer (antispasmodics, low-dose antidepressants) addresses the abdominal pain / visceral hypersensitivity common to all subtypes (G&G 14e Ch.54, pp.1098–9).
• Spastic vs atonic constipation (KDT framing): in spastic constipation (irritable bowel) the stools are "hard, rounded, stone-like and difficult to pass"; the first-choice laxative is ispaghula or any soluble fibre taken over weeks/months, and stimulant purgatives are contraindicated (KDT 8e Ch.49, p.726).