Urine Output and Sodium Concentration in Cerebral Salt Wasting vs SIADH
In cerebral salt wasting (CSW), urine output is typically increased (polyuria) with high urinary sodium concentration, while in syndrome of inappropriate antidiuretic hormone secretion (SIADH), urine output is decreased (oliguria) with inappropriately high urinary sodium concentration. 1, 2
Key Differences in Laboratory Parameters
Cerebral Salt Wasting (CSW)
- Urine Output: Increased (polyuria) - typically >1 cc/kg/hour or >2000-2600 mL/24 hours 3, 2
- Urine Sodium: Markedly elevated - approximately 394 ± 369 mmol/24 hours (significantly higher than normal) 2
- Urine Osmolality: High relative to serum osmolality 1
- Volume Status: Hypovolemic with evidence of extracellular volume depletion (hypotension, tachycardia, dry mucous membranes) 1, 4
Syndrome of Inappropriate ADH (SIADH)
- Urine Output: Decreased (oliguria) - typically around 745 ± 298 mL/24 hours 2
- Urine Sodium: Inappropriately elevated for hyponatremia but lower than CSW - approximately 51 ± 25 mmol/24 hours 2
- Urine Osmolality: Inappropriately high (>500 mosm/kg) relative to serum osmolality 1, 5
- Volume Status: Euvolemic (normal volume status) 1, 5
Diagnostic Approach
Clinical Assessment
- Evaluate extracellular fluid volume status - this is the critical differentiating factor 1
- Check central venous pressure (CVP): CSW typically has CVP <6 cm H₂O while SIADH has CVP 6-10 cm H₂O 1
- Look for signs of volume depletion in CSW: hypotension, tachycardia, dry mucous membranes 1, 6
Laboratory Evaluation
- Measure 24-hour urine volume and sodium excretion 2
- Check serum uric acid: levels <4 mg/dL have a positive predictive value of 73-100% for SIADH (though may also be present in CSW) 1
- Monitor fractional excretion of urate: persistently elevated in CSW but normalizes after correction of hyponatremia in SIADH 4
Treatment Implications
The distinction between CSW and SIADH is crucial as their treatments are diametrically opposed 2:
- CSW Treatment: Volume and sodium replacement with normal or hypertonic saline; fludrocortisone may be beneficial 1, 6
- SIADH Treatment: Fluid restriction (1L/day); may consider urea, diuretics, lithium, demeclocycline 1, 5
Common Pitfalls
- Misdiagnosing CSW as SIADH can lead to inappropriate fluid restriction, worsening the hypovolemia in CSW 1, 4
- Both conditions present with hyponatremia and elevated urine sodium, making volume status assessment critical 1
- CSW is more common than previously recognized, especially in neurosurgical patients 4
- CSW may occur without cerebral disease, leading some to suggest renaming it "renal salt wasting" 4
Special Considerations
- In subarachnoid hemorrhage patients at risk for vasospasm, fluid restriction should be avoided 1
- Fludrocortisone and hydrocortisone have shown benefit in managing CSW in neurosurgical patients 1, 6
- Correction of sodium should not exceed 8 mmol/L in 24 hours to prevent osmotic demyelination syndrome 1
Understanding these differences in urine output and sodium concentration is essential for accurate diagnosis and appropriate management of these two distinct but often confused conditions.