Mannitol (Osmotic Diuretic — Renal/Neurological)

How it works

It is freely filtered at the glomerulus but not reabsorbed, raising tubular fluid osmolality and obliging water and electrolytes to remain in the lumen; intravascularly it draws fluid out of the brain and other tissues down an osmotic gradient.

Prescribing in practice

• Contraindicated in established anuria, severe heart failure or pulmonary oedema because the initial intravascular volume expansion can precipitate fluid overload; give a test dose if renal function is uncertain.
• Administer through an in-line filter as crystals can form, and inspect the solution for crystallisation before use, warming if necessary.
• Repeated or excessive dosing can paradoxically worsen renal function and cause dangerous hyperosmolality, so monitor and bound cumulative exposure.

Monitoring

Monitor serum and urine osmolality, electrolytes, renal function, fluid balance and, where relevant, intracranial pressure during therapy.

Counselling the patient

• This is given by a drip in hospital under close monitoring.
• Tell staff promptly if you become breathless or notice swelling.

Evidence & guidelines

Its osmotic action is long established in neurosurgical and ophthalmic practice, and current prescribing references and the SPC define its narrow place in therapy.