Mannitol

Summary

Mannitol is an osmotic diuretic that works by increasing the osmolality of the blood. This pulls water from the tissues into the blood, shifting fluid from the interstitial space to the intravascular space. The extra fluid is then excreted in the urine. Through this mechanism, mannitol can decrease intracranial pressure as well as intraocular pressure.

Clinically, mannitol is used to treat glaucoma and elevated intracranial pressure. It can also increase the excretion of drugs in the urine, which can help treat certain drug overdoses.

Mannitol can only be given to patients who are able to urinate. It is contraindicated in conditions like anuria and heart failure. Early side effects of mannitol include pulmonary edema and hyponatremia. Late side effects include hypernatremia and dehydration.

Key Points

• Mannitol
  • Mechanism
    • Osmotic diuretic
      • ↑ serum osmolality
        • Causes fluid shift from interstitium into intravascular space
      • ↑ urine flow (diuresis)
        • Mannitol is filtered by the glomerulus and not reabsorbed in the tubules, causing an increase in osmolarity of tubular fluid
        • This increases the amount of water that is drawn into the tubule, causing diuresis
    • Decreases intracranial pressure (ICP)
      • Due to increased serum osmolality
    • Decreases intraocular pressure (IOP)
      • Due to increased serum osmolality
  • Clinical Use
    • Treats drug overdose (increases urinary drug excretion)
    • Treats elevated intracranial pressure
    • Treats elevated intraocular pressure (glaucoma)
  • Side Effects
    • Dehydration
      • Due to excess urination/diuresis
    • Hyponatremia and/or Hypernatremia
      • Mannitol first pulls water from interstitium into intravascular space, which can cause early hyponatremia.
      • Later, mannitol gets excreted in the urine with excess free water, which can cause late hypernatremia due to the induced diuresis.
    • Pulmonary edema (early)
      • As mannitol pulls water into the blood vessels, the increased intravascular volume can overwhelm the ability of the pulmonary capillaries to handle the fluid load, resulting in leakage into the alveoli and pulmonary edema
    • Contraindicated in HF
      • Due to the above exacerbation of pulmonary edema
    • Contraindicated in anuria
      • Without urine production, fluid keeps accumulating in intravascular space, which can lead to severe pulmonary edema and electrolyte abnormalities (e.g. severe hyponatremia)