Cerebral salt washing
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cerebral salt wasting syndrome hyponatremia differentiation SIADH management
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https://www.ncbi.nlm.nih.gov/books/NBK534855
Cerebral salt wasting (CSW) is a syndrome of renal loss of sodium and water after a central nervous system insult, producing hypovolemia and hypotonic hyponatremia.
It is reported with subarachnoid hemorrhage, traumatic brain injury, CNS infection, tumors, and after neurosurgery. Its existence and frequency are debated, so it is generally a diagnosis of exclusion. [Frameworks for Internal Medicine, p. 509] [Schwartz's Principles of Surgery, p. 126]
Typical findings
- Low serum sodium and serum osmolality
- Inappropriately high urine sodium, often with concentrated urine
- Polyuria and negative fluid balance
- Clinical volume depletion: hypotension, tachycardia, dry mucosa, weight loss, raised hematocrit or urea
- Often hypouricemia with increased fractional excretion of urate
CSW vs SIADH
| Feature | Cerebral salt wasting | SIADH |
|---|---|---|
| Effective circulating volume | Low - hypovolemic | Usually euvolemic |
| Primary issue | Renal sodium and water loss | Excess water retention from ADH |
| Urine sodium | High | High |
| Serum sodium | Low | Low |
| Treatment | Give sodium and volume | Restrict free water |
| Effect of fluid restriction | Can worsen shock/hypovolemia | Usually improves sodium |
Both can occur after brain injury and look similar on initial laboratory studies. Careful volume assessment, exclusion of adrenal insufficiency, thyroid disease, diuretics, kidney disease, and other causes of hyponatremia are needed. After serum sodium correction, a persistently elevated fractional excretion of urate supports CSW/renal salt wasting, whereas it tends to normalize in SIADH. A 2026 systematic review examined these markers, mainly in neurologically ill children (PMID 41058069).
Treatment
Management is inpatient and guided by serial sodium, urine output, fluid balance, and neurologic status:
- Treat the underlying CNS condition.
- Replace intravascular volume and sodium, commonly with 0.9% saline.
- If severe symptomatic hyponatremia, such as seizures or coma, use carefully monitored 3% hypertonic saline.
- Oral salt supplementation may be used once suitable.
- Fludrocortisone is sometimes used for persistent renal sodium loss, with monitoring for hypertension, fluid overload, and hypokalemia.
- Avoid fluid restriction if CSW is suspected, because it can worsen hypovolemia. [Frameworks for Internal Medicine, p. 509]
Sodium must be corrected cautiously, because overly rapid correction can cause osmotic demyelination. Current reference guidance commonly limits the rise to less than 8 mmol/L in 24 hours, with individual targets based on chronicity and risk factors (NCBI clinical reference).
Urgent issue: confusion, vomiting, seizures, reduced consciousness, or severe headache with hyponatremia requires emergency assessment.
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