Which statement best describes the mechanism by which mannitol reduces intracranial pressure?

• A. It increases cerebral blood flow by vasodilation
• B. It increases CSF production
• C. It reduces cerebral edema by drawing water out of brain tissue
• D. It causes diuresis without affecting brain water

Answer: (C)

Mannitol lowers intracranial pressure by creating an osmotic gradient that pulls water out of brain tissue into the bloodstream. As plasma osmolality rises, water moves from the brain’s interstitial space into the vascular compartment, reducing cerebral edema and brain volume within the skull. This decrease in brain water directly lowers intracranial pressure. It isn’t about vasodilation increasing blood flow or about increasing CSF production. While mannitol does cause diuresis, the key effect for ICP reduction is the osmotic movement of water out of the brain, which explains why simply saying it causes diuresis without affecting brain water isn’t accurate.