Mechanisms of Diarrhoea

Expansion

• Osmotic - unabsorbed solute holds water in the lumen. Stops with fasting. Causes: lactase deficiency, lactulose, magnesium salts, coeliac disease. High stool osmotic gap
• Secretory - active secretion of chloride and water. Continues when fasting and is typically large volume. Causes: cholera toxin and enterotoxigenic E coli (raising cyclic AMP), VIPoma, bile salt malabsorption, some laxatives. Low stool osmotic gap
• Inflammatory (exudative) - mucosal damage with blood, pus and protein loss, often with tenesmus and fever. Causes: inflammatory bowel disease, invasive infection, ischaemia, radiation
• Motility related - reduced transit time gives less contact for absorption, as in thyrotoxicosis, irritable bowel syndrome and post-vagotomy states; or stasis causing bacterial overgrowth

Stool osmotic gap = 290 - 2 x (stool sodium + stool potassium). Above 100 suggests osmotic; below 50 suggests secretory.

Cholera illustrates why oral rehydration works: the toxin switches on chloride secretion but leaves the sodium-glucose cotransporter intact, so glucose-containing solution still drives absorption.