What is Cerebral Salt Wasting (CSW)?
Cerebral salt wasting is a syndrome of hypovolemic hyponatremia caused by renal sodium loss (natriuresis) and excessive urine output (diuresis), typically occurring after central nervous system injury. 1, 2
Clinical Definition and Pathophysiology
CSW represents a distinct clinical entity characterized by three essential features 3:
- Hyponatremia (serum sodium <131 mmol/L)
- Hypovolemia (extracellular fluid volume depletion)
- Increased urine output with elevated urinary sodium excretion
The underlying mechanism involves abnormal elevations in circulating natriuretic peptides (particularly atrial natriuretic peptide and brain natriuretic peptide), which drive renal sodium and water loss 2, 4. This occurs despite appropriate—not inappropriate—ADH secretion, distinguishing it fundamentally from SIADH 5.
Clinical Context and Associated Conditions
CSW occurs most commonly in neurosurgical patients, with prevalence rates as high as 50% in this population 1. The syndrome is particularly associated with:
- Subarachnoid hemorrhage (most common)
- Traumatic brain injury
- Intracranial surgery
- Brain tumors
- Central nervous system infections
Critical caveat: CSW can occur without cerebral disease, leading some experts to propose the term "renal salt wasting" as more accurate 5, 6.
Distinguishing CSW from SIADH
The key diagnostic challenge lies in differentiating CSW from SIADH, as both present with hyponatremia and elevated urine sodium. The critical distinguishing feature is volume status 1, 2:
CSW presents with:
- Hypovolemia (negative fluid balance, decreased CVP, clinical dehydration)
- Persistent hypouricemia and elevated fractional excretion of urate even after sodium correction
- Elevated natriuretic peptides (BNP, ANP)
- Decreased aldosterone and plasma renin activity
SIADH presents with:
- Euvolemia or mild hypervolemia
- Hypouricemia that corrects with water restriction
- Normal natriuretic peptides
Important pitfall: Clinical assessment of volume status is accurate in only 50% of cases 6. When available, invasive monitoring (CVP) and measurement of fractional excretion of urate can help differentiate, though hormone levels (ADH, natriuretic peptides) are not routinely recommended 1.
Clinical Significance and Outcomes
Misdiagnosis carries serious consequences because treatment approaches are diametrically opposed 1:
- CSW requires aggressive fluid and sodium replacement
- SIADH requires fluid restriction
A retrospective study of 134 SAH patients demonstrated that fluid restriction in hyponatremic patients (mistakenly treated as SIADH) resulted in cerebral infarction in 21 of 26 patients (81%) 1. This underscores why accurate diagnosis is critical for preventing morbidity and mortality.
Diagnostic Workup
When serum sodium falls below 131 mmol/L, evaluation should include 1:
- Serum and urine osmolality
- Urine electrolytes (particularly 24-hour urine sodium)
- Serum uric acid and fractional excretion of urate
- Clinical assessment of extracellular fluid volume status (orthostatic vital signs, skin turgor, mucous membranes, CVP if available)
- Brain natriuretic peptide (if available, though not required)
The diagnosis requires demonstrating hypovolemia in the setting of renal sodium wasting with hypotonic hyponatremia 1, 2.