the manitol works in two phases.
in the first one he is a volume expander and there is a dilutional hyponatremia .if the pt is already hyponatremic thus will lower the na+ further and as you know the most freqent cause of cerebral edema is na<120 meq/l.
in the second phase he will dehydrate the brain and the vascular compartment.today the tendency to restrict fluids is replaced by euvolemia.no more dehydration.
overdehydration will cause a lot of troubles(acidosis,atn,)
thank you for the attention and i will be glad to participate in the forum.