Mannitol is infused to reduce intracranial pressure in a trauma patient. Several days into therapy the benefit wanes and intracranial pressure rises above baseline. The most likely explanation is:

• A Disruption of the blood brain barrier permitting mannitol entry into brain tissue ✓
• B Tachyphylaxis of cerebral vascular osmoreceptors
• C Depletion of renal medullary tonicity reducing water excretion
• D Rebound CSF overproduction by choroid plexus

Explanation

Mannitol lowers intracranial pressure only if the blood brain barrier is intact, drawing water out of brain parenchyma osmotically. Where the barrier is disrupted, mannitol enters brain tissue, and once plasma levels fall during elimination, mannitol trapped in brain pulls water back in, causing rebound elevation of intracranial pressure. Repeated dosing in barrier disruption is therefore counterproductive.

Reference: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 13th ed.

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Written and medically reviewed by the StethoPrep medical team.