The effect of mannitol on cerebral white matter water content.

The authors have studied the effect of a low-dose (0.28 gm/kg) bolus infusion of mannitol on brain water in man. In eight patients with severe head injury, small pieces of subcortical white matter were taken at craniotomy both before and after infusion of mannitol. The tissue specific gravity was measured using a graduated specific-gravity column, and from it the brain water content was calculated. White matter specific gravity rose from a mean (+/- standard error of the mean) of 1.0325 +/- 0.0012 before mannitol infusion to 1.0352 +/- 0.0011 after mannitol administration, and the brain water content fell from a mean of 80.94% +/- 2.5% to 75.28% +/- 2.3%. The differences were significant (p less than 0.01). This study shows that, after head injury in man, mannitol increases the white matter specific gravity and probably does so by reducing brain water.