Cerebral Salt-Wasting Syndrome is a condition for which the mechanism of onset is unclear. It is attributed to an imbalance in renal sensitivity to body water and sodium levels resulting in excess sodium loss in urine. The stimulus to this occurring is linked to a combination of increased sympathetic nervous system activity resulting in increased renal perfusion pressures or the release of natriuretic factors (e.g. brain natriuretic peptide or urodilatin) from the injured brain.
As a result the kidneys may excrete excessive amounts of sodium despite levels being already depleted leading to hyponatraemia. Urate (or uric acid) excretion may also be increased, thus body or blood levels of urate may be reduced. Syndrome of Inappropriate Antidiuretic Hormone (SIADH) is also associated with low urate levels or hypouricaemia. However the levels tend to return to normal once treated in SIADH. Thus uric acid levels may help in discrimination between Cerebral Salt-Wasting Syndrome and SIADH.
Cerebral Salt-Wasting Syndrome is relatively infrequently diagnosed, however it should be considered in patients, who have been assumed to have SIADH and who are not responding to treatment. Failure to do so in a patient with hyponatremia following brain injury could lead to inappropriate therapy with fluid restriction.
Features of Cerebral Salt-Wasting Syndrome similar to SIADH include:
- Hyponatraemia.
- Increased urinary sodium.
- Urine osmolarity may be higher than appropriate when taking low plasma osmolarity into account.
Discriminating features of Cerebral Salt-Wasting Syndrome from SIADH:
- Persisting hyponatraemia despite treatment for SIADH.
- Continuing negative fluid balance (urine output greater than fluid intake).
- Clinically dehydrated.
- Persisting low uric acid levels (measurement of fractional excretion of uric acid and phosphate may be required).
Treatment Options for Cerebral Salt-Wasting Syndrome include:
- Intravenous hypertonic saline.
- Fludrocortisone, following discussion with the metabolic team.
NOTE: You should seek senior medical advice or consult your employing health authority’s specific management protocols for investigation and management of this condition.