Mannitol is an osmotic diuretic used in cerebral edema. Which property makes mannitol effective for lowering intracranial pressure?

• A It is actively secreted by brain astrocytes, drawing water out of cerebral tissue
• B It does not cross an intact blood-brain barrier (BBB), creating an osmotic gradient that draws water from brain tissue to plasma, reducing cerebral water content and ICP ✓
• C It is freely filtered but not reabsorbed, increasing tubular osmolality and obligating water excretion from the renal collecting duct
• D It inhibits aquaporin-4 in astrocytes, directly preventing cytotoxic edema formation

Explanation

Mannitol's mechanism in cerebral edema involves two complementary effects: (1) Osmotic effect — mannitol is given intravenously and, because it does not penetrate an intact BBB, it raises plasma osmolality rapidly. This creates an osmotic gradient across the BBB drawing water from interstitial and intracellular brain compartments into the hyperosmolar blood, reducing brain water content and ICP within 20-30 minutes. (2) Rheological effect — mannitol reduces blood viscosity and may improve cerebral microcirculation. However, if the BBB is severely disrupted (e.g., by tumor or abscess), mannitol may cross into brain tissue and paradoxically worsen edema ('rebound effect'), hence repeated high-dose use is cautioned.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.