A Brief Story of Mannitol

Mannitol remains a standard therapy for reducing intracranial pressure. The initial dose of mannitol is 1 g/kg and can be given emergently without intracranial pressure monitoring as long as the patient is normotensive.

Mannitol is particularly useful if the patient is showing any signs or symptoms of impending herniation. Subsequent dosing to a serum osmolality up to 320 mosm/l is usually recommended as needed. Dosing to higher levels has not been shown to improve outcome and increases the risk of acute renal failure.

Mannitol reduces intracranial pressure through two mechanisms. The immediate mechanism is expansion of intravascular volume which reduces blood viscosity. This results in an increase in cerebral blood flow in the areas of the brain where cerebral autoregulation remains intact and ICP falls. The second mechanism, which occurs later, involves the establishment of osmotic gradients between the serum plasma and the brain cells. This ultimately decreases intracellular volume and reduces intracranial pressure.

Since mannitol also functions as a diuretic there is always a risk of reducing blood pressure and therefore cerebral perfusion pressure if the patient is not adequately volume resuscitated. For this reason, hypertonic saline has gained favor as an osmotic agent for increased ICP since it is less likely to cause hypotension.