Intravenous mannitol is used in ____________?

Correct Answer: D. Glaucoma

Mannitol is a non-metabolizable osmotic diuretic that creates an osmotic gradient, drawing fluid from the intracellular and interstitial spaces into the intravascular compartment. In acute angle-closure glaucoma, elevated intraocular pressure (IOP) results from impaired aqueous humor drainage. Mannitol rapidly reduces IOP by decreasing aqueous humor production and increasing its reabsorption through the osmotic effect—it draws fluid from the vitreous humor into the blood, shrinking the vitreous volume and lowering IOP. This is a first-line emergency intervention in acute glaucoma (along with topical beta-blockers, prostaglandin analogs, and carbonic anhydrase inhibitors) to prevent permanent blindness. The IV route ensures rapid onset (15–30 minutes) and peak effect within 30–60 minutes. Mannitol does not accumulate in the eye (unlike oral glycerol) and is preferred in acute presentations requiring immediate IOP reduction before definitive laser peripheral iridotomy.

Why the other options are wrong

A. Acute kidney injury with anuria — This is wrong because mannitol is contraindicated in established anuria (urine output <0.5 mL/kg/hr). In oliguric/anuric AKI, mannitol cannot be filtered by the glomerulus and accumulates in the blood, worsening hypervolemia, pulmonary edema, and hypertension. Mannitol is used only in the polyuric phase of AKI or in early AKI to prevent progression to anuria—never after anuria is established. This is a classic NBE trap pairing mannitol with AKI without specifying the phase. B. Pulmonary edema — This is wrong because mannitol is contraindicated in pulmonary edema. Although mannitol is a diuretic, it initially expands intravascular volume by drawing fluid from tissues into the bloodstream. In pulmonary edema (especially cardiogenic), this transient hypervolemia worsens pulmonary congestion and hypoxemia before diuresis occurs. Loop diuretics (furosemide) are the DOC for pulmonary edema because they reduce preload directly. Mannitol may be used in cerebral edema (not pulmonary), another common NBE confusion point. C. Congestive cardiac failure — This is wrong because mannitol is contraindicated in CCF. The osmotic diuresis and initial plasma expansion increase cardiac preload and afterload, precipitating acute decompensation, arrhythmias, and cardiogenic shock in failing hearts. Loop diuretics (furosemide, torsemide) are the standard therapy for CCF. Mannitol may cause hyperkalemia and hyponatremia in CCF patients on ACE inhibitors/ARBs, compounding electrolyte derangement.

High-Yield Facts

Mnemonics

MANNITOL uses (Osmotic Diuretics) Mannitol → Acute glaucoma, Neurosurgery (cerebral edema), Nephrotoxic drug overdose (early AKI), Increased ICP, Transurethral resection syndrome, Oliguria prevention, Low sodium states. Remember: NOT in anuria or pulmonary edema. Glaucoma Emergency Rx (ABCA) Acetazolamide (IV/oral), Beta-blockers (topical), Carbonic anhydrase inhibitors, Approximately 30% IOP reduction needed. Add Mannitol IV if IOP >40 mmHg or vision-threatening.

NBE Trap

NBE pairs mannitol with AKI to trap students who remember "mannitol is used in AKI" without recalling the critical distinction: polyuric phase only, never in anuria. Similarly, confusing mannitol (osmotic) with loop diuretics (DOC in pulmonary edema/CCF) is a common error.

Clinical Pearl

In Indian emergency departments, IV mannitol 0.25–1 g/kg is the rapid IOP-lowering agent of choice in acute angle-closure glaucoma before laser iridotomy can be arranged. However, always check urine output and serum osmolality—if osmolality >320 mOsm/L or anuria is present, mannitol is contraindicated and may precipitate acute tubular necrosis.

_Reference: KD Tripathi Pharmacology Ch. 14 (Diuretics); Harrison Ch. 328 (Glaucoma); Robbins Ch. 28 (Kidney pathology)_