mannitol
osmotic diuretic· Brands: Osmitrol, ResectisolIVPO
Mechanism of action
Mannitol is an osmotic diuretic. It is freely filtered by the glomerulus and not reabsorbed by the renal tubules. The presence of mannitol in the tubular fluid increases osmotic pressure, preventing water reabsorption and producing diuresis. In addition, mannitol increases plasma osmolality, drawing water from tissue spaces into the vascular compartment, which reduces intracranial and intraocular pressure.
Pharmacokinetics
Mannitol is administered intravenously and is not absorbed when given orally. It is distributed throughout the extracellular fluid, is not metabolised, and is excreted unchanged by the kidneys via glomerular filtration. After an intravenous dose, its effect begins within 15 to 30 minutes, peaks at 1 to 2 hours, and lasts 4 to 6 hours. The elimination half-life is approximately 70 minutes, but it is prolonged in renal impairment. Mannitol is not bound to plasma proteins.
Indications & dosing
Increased intracranial pressure due to cerebral oedema
Administer as IV infusion over 30-60 min. Monitor serum osmolality and fluid balance.
- adult · IV: 0.25–1 g/kg every 6-8 hours PRN · Administer as IV infusion over 30-60 min. Monitor serum osmolality and fluid balance.
Acute angle-closure glaucoma (reduction of intraocular pressure)
Administer over 30-60 min. May repeat if needed.
- adult · IV: 1–2 g/kg single dose · Administer over 30-60 min. May repeat if needed.
Diuresis in acute renal failure (off-label)off-label
Test dose; not first-line. Monitor renal function.
- adult · IV: 12.5–25 g single dose · Test dose; not first-line. Monitor renal function.
Contraindications & precautions
- Anuria due to renal failure · Mannitol cannot be excreted and may cause volume overload.
- Severe pulmonary oedema or congestive heart failure · Mannitol expands intravascular volume and may worsen pulmonary oedema.
- Severe dehydration or hypovolaemia · May exacerbate hypovolaemia; use only if fluid resuscitation is prioritized.
- Hypersensitivity to mannitol · Risk of anaphylaxis.
- Monitor fluid balance, serum electrolytes, blood pressure, and urine output during therapy.
- Use with caution in patients with cardiac or renal disease, as mannitol may worsen heart failure or cause renal injury.
- Administer through a large vein to prevent extravasation; local tissue necrosis can occur if extravasated.
- Ensure adequate hydration before and during treatment to avoid dehydration and electrolyte disturbances.
Adverse effects
- Fluid and electrolyte imbalanceserious
- Hypervolaemiaserious
- Dehydration
- Headache
- Nausea
- Pulmonary oedemaserious
- Acute kidney injuryserious
- Anaphylaxisserious
Renal & hepatic adjustment
- eGFR 0–30: Avoid in severe renal impairment (eGFR < 30 mL/min/1.73m²) or anuria due to risk of volume overload and osmotic nephrosis. (avoid)
Monitoring, pregnancy & toxicity
- Serum electrolytes (sodium, potassium) · Before and during therapy; then at least daily
- Urine output · Hourly during infusion
- Fluid balance · Daily
- Serum osmolality · Periodically, especially with prolonged use
- Renal function (creatinine, eGFR) · Baseline and then daily
- Signs of pulmonary oedema or fluid overload · Continuously
- Pregnancy: Mannitol should be used during pregnancy only if clearly needed and when potential benefit outweighs risk. It can cross the placenta and may cause fluid and electrolyte imbalance in the fetus. Short-term use may be acceptable for specific indications such as raised intracranial pressure.
- Toxicity: Fluid and electrolyte imbalance (hyponatraemia/hypernatraemia, hypokalaemia) · Discontinue mannitol; replace fluids and electrolytes based on serum levels and clinical status.
- Toxicity: Pulmonary oedema due to fluid overload · Stop infusion; administer oxygen and use loop diuretics if haemodynamically stable; monitor urine output and respiratory status.
- Toxicity: Osmotic nephrosis (reversible renal injury) · Stop mannitol; maintain adequate hydration; monitor renal function; consider haemodialysis in severe cases.
- Toxicity: Rebound intracranial pressure elevation · Discontinue mannitol; use alternative measures to control intracranial pressure; monitor neurological status.
Clinical notes
Mannitol is used to reduce intracranial pressure in cerebral oedema and intraocular pressure in acute glaucoma. It may also be used to induce diuresis in selected patients with oliguric renal failure or to promote excretion of toxins. It is contraindicated in anuria, severe dehydration, active intracranial bleeding, and pulmonary oedema. Adverse effects include fluid and electrolyte disturbances, headache, nausea, and rises in intracranial pressure as it declines. Mannitol must be given through an inline filter to prevent crystallisation, and extravasation can cause tissue necrosis. Monitor urine output, serum electrolytes, osmolality, and hydration status. Use caution in patients with renal impairment, as accumulation can cause osmotic nephrosis. Doses are usually 0.25 to 2 g/kg given as an intravenous infusion.
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BNF; local formulary