Question
Mannitol
Answer
Mannitol is the prototype osmotic diuretic.
MOA: freely filtered at the glomerulus but poorly reabsorbed by the tubule, mannitol raises the osmolality of the tubular fluid, osmotically retaining water throughout the nephron and reducing net water (and to a lesser extent Na⁺) reabsorption — a fundamentally different mechanism from transporter-blocking diuretics.
Uses: reduction of raised intracranial pressure (cerebral edema — draws water out of brain tissue across the blood-brain barrier), reduction of raised intraocular pressure (acute angle-closure glaucoma, pre-operatively), prophylaxis of acute renal failure in high-risk settings (maintains urine flow), forced diuresis in certain poisonings.
Adverse effects: transient intravascular volume expansion before diuresis begins — can precipitate acute pulmonary edema in a patient with reduced cardiac reserve (contraindicated in anuria/severe heart failure for this reason), dehydration and hypernatremia with excessive use, headache.