High Urine Output in CSW vs SIADH
High urine output is expected in Cerebral Salt Wasting (CSW), not in SIADH.
Key Distinguishing Features
Cerebral Salt Wasting (CSW)
CSW is characterized by polyuria and high urine output due to excessive natriuresis and renal sodium losses 1, 2, 3. The pathophysiology involves:
- Increased urine volume (polyuria) as a defining feature of the syndrome 2, 3, 4
- High urinary sodium concentration (typically >20 mmol/L) despite volume depletion 1, 4, 5
- Extracellular volume depletion resulting from the excessive renal sodium and water losses 5, 6
- Clinical signs of hypovolemia including hypotension, tachycardia, dry mucous membranes, and low central venous pressure (<6 cm H₂O) 1, 7
The mechanism involves excessive secretion of natriuretic peptides causing both natriuresis and diuresis, which may provoke volume contraction 1.
SIADH (Syndrome of Inappropriate Antidiuretic Hormone)
SIADH does NOT typically present with high urine output 8, 7. Instead, it features:
- Water retention with inappropriately concentrated urine (>500 mOsm/kg) 8, 7
- Euvolemic state - no clinical signs of volume depletion or overload 1, 8
- Normal to slightly elevated central venous pressure (6-10 cm H₂O) 7, 6
- Inappropriately elevated urinary sodium (>20-40 mEq/L) but without the polyuria seen in CSW 8, 7
Clinical Implications
This distinction is critical because treatment approaches are opposite 1, 3, 5:
- CSW requires aggressive volume and sodium replacement with isotonic or hypertonic saline (0.9% or 3% NaCl), not fluid restriction 1, 2, 3
- SIADH requires fluid restriction (typically 1 L/day) as first-line treatment 1, 8
Common Pitfall
Using fluid restriction in CSW can worsen outcomes and lead to further volume depletion, cerebral ischemia, and poor neurological outcomes 1, 5. Patients with CSW who are incorrectly diagnosed as SIADH and treated with fluid restriction experience clinical deterioration 5.
Diagnostic Approach
To differentiate between these conditions in neurosurgical patients 1, 7, 6:
- Assess volume status carefully: Look for orthostatic hypotension, dry mucous membranes, decreased skin turgor (CSW) vs. euvolemia (SIADH) 1, 7
- Monitor urine output: Polyuria suggests CSW 2, 3, 4
- Check central venous pressure if available: <6 cm H₂O suggests CSW; 6-10 cm H₂O suggests SIADH 7, 6
- Evaluate fractional excretion of urate: Persistently elevated even after sodium correction suggests CSW rather than SIADH 5, 6
CSW is more common than SIADH in neurosurgical patients, particularly those with subarachnoid hemorrhage, traumatic brain injury, or other intracranial pathology 1, 5, 6.