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Osmotic Diuretic Pregnancy: Use with caution — osmotic effect may affect fetal fluid balance; use only for life-threatening raised ICP

Mannitol (Surgical — ICP/Osmotherapy)

Brand names: Mannitol 20%, Osmitrol

Adult dose

Dose: Raised ICP: 0.25–1 g/kg IV over 15–30 min; Renal protection (aortic surgery): 0.5 g/kg IV before aortic cross-clamp; Cerebral oedema: 1 g/kg IV
Route: IV via filter (crystallisation risk — use 5 micron in-line filter)
Frequency: Every 6–8 hours as needed (ICP); single perioperative dose (renal protection)
Max: 2 g/kg per dose; cumulative daily dose limited by serum osmolality (target gap <20 mOsm/kg)
20% mannitol = 200 g/L. Osmotic mechanism draws water from cerebral and soft tissue into intravascular space. Peak ICP effect in 15–30 min. Used in neurosurgery for brain relaxation, raised ICP from trauma/haematoma, and as renal protectant before aortic cross-clamping or during cardiac surgery.

Paediatric dose

Dose: 0.25–1 g/kg
Route: IV over 15–30 min
Frequency: Every 6 hours as needed
Max: 1 g/kg per dose
Paediatric raised ICP: 0.25–0.5 g/kg IV. Monitor serum osmolality and osmolar gap in children.

Dose adjustments

Renal

Avoid in anuric renal failure — osmotic load cannot be excreted; causes hypernatraemia and circulatory overload. Use hypertonic saline as alternative for ICP in renal failure.

Hepatic

No specific adjustment required.

Paediatric weight-based calculator

Paediatric raised ICP: 0.25–0.5 g/kg IV. Monitor serum osmolality and osmolar gap in children.

Clinical pearls

  • Mannitol vs hypertonic saline for raised ICP: both effective. Hypertonic saline increasingly preferred in TBI — does not cause systemic dehydration, maintains MAP, and avoids rebound ICP. NICE NG158 (Head Injury): consider hypertonic saline as first-line if haemodynamically compromised
  • Osmolar gap monitoring: measure serum osmolality before repeat doses; osmolar gap = measured − calculated osmolality; target gap <20 mOsm/kg to avoid mannitol accumulation and renal toxicity
  • Neurosurgical brain relaxation: mannitol infused during craniotomy creates slack brain — improves surgical access. Often combined with head elevation (30°), moderate hyperventilation (PaCO2 4.0–4.5), and positioning

Contraindications

  • Anuria/severe oliguria (accumulation causes circulatory overload)
  • Severe pulmonary oedema
  • Severe dehydration
  • Active intracranial haemorrhage (relative — debate in trauma)

Side effects

  • Initial plasma volume expansion (can worsen cardiac failure before diuresis)
  • Hypernatraemia
  • Hypovolaemia (excessive diuresis)
  • Rebound cerebral oedema (if osmolar gap allowed to fall rapidly)
  • Crystallisation in IV lines (use filter)

Interactions

  • Diuretics (additive dehydration and electrolyte loss)
  • Lithium (increased lithium excretion — monitor levels)
  • Cisplatin (additive nephrotoxicity)

Monitoring

  • Serum osmolality and osmolar gap (before repeat doses)
  • Serum electrolytes (Na, K)
  • Urine output (oliguria = dose omit)
  • ICP monitoring if invasive bolt in situ
  • Fluid balance

Reference: BNFc; BNF 90; NICE NG158 (Head Injury); Neurocritical Care Society Guidelines 2023; ESA Perioperative Guidelines; BNFc. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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