Mannitol: what to check before you give it

What it does and why it is prescribed

Mannitol is an osmotic diuretic. It raises the osmolality of the blood, drawing water out of the brain tissue and across the blood-brain barrier into the vascular space, from where it is excreted by the kidneys. The net effect is a fall in intracranial pressure and a reduction in cerebral oedema.

It is used in traumatic brain injury, stroke with mass effect, and after neurosurgery, wherever swelling is pushing against a fixed cranial vault. It is also used to reduce intraocular pressure before certain ophthalmic procedures, and occasionally to promote diuresis in acute oliguric renal failure, though the ICP indication is the one most tested and most encountered on a neuro unit.

Nursing considerations before giving it

Mannitol is hypertonic and prone to crystallising, especially at higher concentrations or in cold storage. Inspect the bag against a light source before hanging it. If you see crystals, do not administer the bag; warming and re-inspecting is one option, but many units simply discard and obtain a fresh bag rather than risk incomplete redissolution.

Use an in-line filter, or draw up and administer through a filter needle if giving a bolus, because undissolved crystals can occlude a line or, worse, embolise. Give it through a dedicated line where possible, and confirm patent IV access first, since extravasation of a hypertonic solution causes tissue damage.

Check baseline renal function, serum sodium and osmolality before starting, and confirm the order specifies concentration and infusion time, since mannitol is prescribed in different strengths.

What to monitor

Mannitol works fast. Diuresis typically begins within 30 to 60 minutes, so start measuring urine output from the moment the infusion runs and continue hourly. A sudden, large diuresis is expected and is the therapeutic effect, not a side effect to panic over, but it needs to be tracked against intake.

Monitor serum osmolality and electrolytes regularly during therapy, particularly sodium and potassium, since large fluid and electrolyte shifts are the main risk. Many protocols hold or reassess the dose if serum osmolality climbs above roughly 320 mOsm/kg, because the osmotic gradient that protects the brain starts to be outweighed by risk to the kidneys.

Track neurological observations and signs of raised ICP throughout, along with vital signs, since the point of the drug is a measurable neurological response, and a lack of improvement is itself a finding to escalate.

Side effects versus adverse effects

Expected side effects follow directly from the mechanism: a brisk diuresis, transient thirst, and sometimes a mild headache as fluid shifts occur. These are anticipated and managed with fluid balance monitoring rather than stopping the drug.

Adverse effects are different in kind and demand escalation. Watch for signs of fluid overload or pulmonary oedema, since the initial intravascular volume expansion that precedes diuresis can overload a heart with limited reserve. Watch for electrolyte disturbance, particularly hyponatraemia or hypokalaemia, and for acute kidney injury, which can occur with prolonged or high-dose use, especially in patients with pre-existing renal impairment.

What to hold for and when to call

Hold the dose and call the prescriber if crystals persist in the bag after warming, if serum osmolality has risen above the threshold set in your protocol, or if urine output has not increased as expected, since this can signal the kidneys are not able to excrete the load.

Call promptly for signs of pulmonary oedema such as increasing breathlessness, crackles or falling oxygen saturation, for a significant drop in blood pressure, or for a new or worsening neurological deficit, since this may mean the ICP is not responding and other interventions are needed. Anuria is also a reason to stop and escalate, as mannitol accumulating without renal excretion will worsen fluid overload.

Patient teaching

Where the patient is alert enough to participate, explain that they will need to pass urine frequently once the infusion starts and that this is expected, not a sign anything is wrong. Ask them to report any breathlessness, chest tightness or worsening headache promptly.

Reassure family members who see a rapid rise in urine output that this is the drug working as intended, since it can look alarming if unexplained. For patients going home on any oral follow-up related to fluid management, emphasise keeping follow-up bloods for kidney function and electrolytes, since the effects on renal handling of fluid can outlast the infusion itself.