Mannitol is an osmotic diuretic that works in two phases. Firstly, mannitol in the bloodstream creates a hypertonic solution that draws water out of the surrounding tissues and into the bloodstream. Next, mannitol is excreted by the kidneys, where it acts as a diuretic to osmotically draw water out of the blood and into the filtrate to increase urine production.

The net effect of mannitol is to draw water out of cells into blood, and then eventually into the urine. Mannitol is therefore useful in treating elevated intracranial pressure.

However, there are a couple adverse effects of mannitol. Firstly, because mannitol creates a temporary increase in intravenous fluid volume before it is excreted, it may lead to pulmonary edema in individuals with congestive heart failure. Secondly, mannitol use over time can cause hypernatremia through increased loss or diuresis of water in urine! The risk of hypernatremia is even more common in patients who are anuric, since mannitol pooling in the kidney will cause greater and greater amounts of water to be pulled out of the blood.

Key Points

  • Mannitol
    • Mechanism:
      • Increases hypertonic fluid
        • Increases osmolarity of the urine/serum and draws water out of cells. This enables mannitol to reduce tissue edema
          • Mannitol in the serum is not absorbed into cells. By increasing serum osmolarity, it can be used to reduce edema (i.e. cerebral edema) by drawing water out of the edematous tissue.
      • Osmotic diuretic
        • Mannitol is excreted by the kidney and not reabsorbed. After being filtered by the glomerulus, it raises the osmolarity of tubular fluid, increasing the amount of water that is drawn into the tubule, causing diuresis.
    • Indications:
      • Elevated ICP
        • By increasing serum osmolarity, mannitol pulls water from the brain into the blood, reducing brain water content, bulk, and therefore ICP.
        • Mannitol is an intravenous osmotic diuretic used primarily to lower intracranial pressure and treat acute glaucoma.
      • Glaucoma
        • Elevated intraocular pressure
      • Increase urine flow
        • Used in oliguric states to encourage urine production
      • Used to measure ECF
    • Adverse effects:
      • Pulmonary edema (early)
        • Mannitol is not used to treat heart failure as it can cause an initial rise in extracellular fluid volume (due to hypertonicity), exacerbating pulmonary congestion and peripheral fluid retention.
        • Therefore contraindicated in congestive heart failure
        • Mannitol is not given in states of volume overload (ex. CHF) due to this fact.
      • Hypernatremia (late)
        • As mannitol works, it first increases the intravascular free water content which can worsen electrolyte abnormalities including hyponatremia. In the second phase of action, mannitol gets excreted in the urine with excess free water, which can cause hypernatremia due to the induced diuresis.
        • If the patient cannot produce urine (anuria), mannitol can build up quickly causing life-threatening hypernatremia!
        • Contraindicated in states of anuria