Mannitol

Mannitol Pharmacology

1. Mechanism of Action

Mannitol is an osmotic diuretic.

• It is freely filtered at the glomerulus and not reabsorbed in renal tubules.
• This increases tubular fluid osmolarity, especially in the proximal tubule and descending limb of the loop of Henle.
• The increased osmolarity prevents water reabsorption through osmotic pressure.

↑ Urine Flow

↓ Intracranial Pressure

↓ Intraocular Pressure

Key Concept: Mannitol pulls water from tissues into the intravascular space, which is then excreted by the kidneys.

2. Clinical Uses

Elevated Intracranial Pressure (ICP)

Used in cerebral edema (Head injury, Brain tumors, Intracranial hemorrhage).

Elevated Intraocular Pressure (IOP)

Used in acute glaucoma attacks before surgery.

Drug Overdose

Promotes forced diuresis to increase toxin elimination.

Acute Renal Failure Prevention

Used to maintain urine flow in rhabdomyolysis or hemolysis.

3. Adverse Effects

• Dehydration: Due to excessive water loss.
• Sodium Disturbances:
  • Hyponatremia (early): Dilutional effect from water shift into plasma.
  • Hypernatremia (later): Excessive water loss in urine.
• Pulmonary Edema: Initial expansion of intravascular volume can be dangerous in heart failure patients.

4. Contraindications

Anuria

Accumulation worsens fluid overload if urine production is zero.

Heart Failure

Volume expansion may precipitate pulmonary edema.

Exam / USMLE Pearls

• Does NOT cross an intact blood-brain barrier (BBB).
• If BBB is disrupted, mannitol may enter brain tissue and worsen cerebral edema.
• Acts mainly in proximal tubule and descending limb of loop of Henle.
• Given IV only (poor oral absorption).

Quick Memory Trick

MANNITOL

"Moves Water Out"

Moves water from brain/eye

Acute glaucoma

Neuro (↓ ICP)

Natrium changes (Na⁺ disturbances)

Intravascular expansion

Tubular osmolarity ↑

Osmotic diuretic

Lung edema risk

Interactive Quiz

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