Fluid Management in Neurosurgery

I have been spending a lot of time in neurosurgery rooms this last week. Yesterday, I watched a craniotomy for a resection and removal of a supratentorial brain tumor. I noted how many different types of fluids (keppra, mannitol and 3% saline) we had hanging for the patient which I rarely see in other surgical procedures. I thought fluid management in Neurosurgery would be a good, brief topic. 

Mannitol

Mannitol, a sugar alcohol, accomplishes the goal of decreasing intracranial pressure (ICP) in two ways: an immediate effect due to plasma expansion from simply adding the mannitol and a delayed effect secondary to osmosis (mannitol is hypertonic). The early plasma expansion reduces blood viscosity which will increase the intravascular volume and cardiac output. Together, these effects result in an increase of cerebral blood flow and compensatory cerebral vasoconstriction of the cerebral arteries (autoregulation), resulting in a reduction in ICP. Mannitol also establishes an osmotic gradient between plasma and brain cells, drawing water from the cerebral space into the intravascular space, causing a decrease in cerebral swelling and a reduction in ICP. A side affect of mannitol, however, is osmotic diuresis. Mannitol is freely filtered by the kidney tubules but not reabsorbed and so, through osmosis, free fluid is pulled into the tubules and lost in the urine. 

3% Saline

Hypertonic saline will reduce ICP in a similar way that mannitol does through osmosis and expanding the intravascular volume, but it has recently been found to have some extra beneficial effects. In addition to an osmotic action, hypertonic saline has autoregulatory effects. In particular, hypertonic saline relaxes arteriolar vascular smooth muscle and will improve cerebral microcirculatory flow. Another added benefit is that hypertonic saline does not cause an osmotic diuresis but it can, however, cause hypernatremia. 

Keppra

Antiseizure medications may be given before neurosurgery to prevent seizures post-operatively. I found that there is an interesting discussion going on in the neurosurgery world currently about whether keppra is superior to the traditional phenytoin for supratentorial craniotomies. I read an article from the American Epilepsy Society that found both medications to have similar efficacy in reducing post op seizures and epilepsy but keppra was associated with fewer adverse effects than phenytoin. This study was a retrospective study and the article admitted that a randomized controlled trial with a placebo would be more informative but concluded that, "Until a definitive study is performed, it may be reasonable to use levetiracetam (keppra) instead of phenytoin for seizure prophylaxis after supratentorial craniotomy—if the practitioner chooses to use AED prophylaxis at all in this setting."(Efficacy and Tolerability of Levetiracetam Versus Phenytoin after Supratentorial Neurosurgery Milligan TA, Hurwitz S, Bromfield EB.)