Mannitol reduces brain edema primarily by which mechanism?

• A. Decreasing cerebral blood flow
• B. Increasing plasma osmolality, drawing water from brain to blood
• C. Blocking inflammatory mediators
• D. Vasoconstriction of cerebral vessels

Answer: (B)

Mannitol reduces brain edema by creating an osmotic gradient that pulls water out of swollen brain tissue into the bloodstream. When given intravenously, mannitol raises plasma osmolality, so water moves from the brain’s interstitial spaces into the intravascular compartment, decreasing brain volume and lowering intracranial pressure. This osmotic effect is the primary way it reduces edema, rather than changing cerebral blood flow, blocking inflammation, or vasoconstricting cerebral vessels. (Note: the effectiveness depends on the integrity of the blood-brain barrier; with a severely disrupted barrier, the effect can be diminished or reversed.)