How do you differentiate and manage Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) and Cerebral Salt Wasting (CSW)?

Differentiating SIADH and Cerebral Salt Wasting

The key difference between SIADH and CSW is volume status: SIADH patients are euvolemic while CSW patients are hypovolemic, requiring fundamentally different treatment approaches. 1, 2, 3

Diagnostic Criteria

Clinical Features

• SIADH: Euvolemic hyponatremia with normal skin turgor, no orthostatic hypotension, and no edema 2, 3
• CSW: Hypovolemic hyponatremia with signs of dehydration, orthostatic hypotension, dry mucous membranes, and unquenchable thirst 2, 4

Laboratory Findings

Parameter	SIADH	CSW
Serum sodium	<135 mmol/L	<135 mmol/L
Serum osmolality	<275 mosm/kg	<275 mosm/kg
Urine sodium	>20 mmol/L	>20 mmol/L
Urine osmolality	>500 mosm/kg (inappropriately high)	>500 mosm/kg
Volume status	Euvolemic	Hypovolemic
Central venous pressure	6-10 cm H₂O	<6 cm H₂O
NT-proBNP	<125 pg/ml	>125 pg/ml
Serum uric acid	<4 mg/dL (improves after correction)	<4 mg/dL (persists after correction)

1, 2, 3, 5, 6

Pathophysiology

• SIADH: Excessive ADH secretion causing inappropriate water retention and dilutional hyponatremia 3
• CSW: Excessive natriuresis due to natriuretic peptide release, leading to volume depletion and secondary hyponatremia 4, 7

Treatment Algorithm

For SIADH:

1. Fluid restriction to 1 L/day as cornerstone of treatment for mild/asymptomatic cases 1, 3
2. For severe symptoms (seizures, coma):
   • 3% hypertonic saline with goal to correct 6 mmol/L over 6 hours or until severe symptoms resolve 1, 3
   • Monitor sodium every 2 hours during initial correction 1
3. For resistant cases:
   • Oral sodium chloride 100 mEq three times daily 1
   • Consider vasopressin receptor antagonists (tolvaptan) for euvolemic hyponatremia 8
   • Alternative options: demeclocycline, urea, or lithium 3

For CSW:

1. Volume and sodium replacement as primary treatment 1, 4
2. Normal saline or hypertonic saline based on severity 1, 4
3. For severe cases:
   • ICU admission with 3% hypertonic saline 1
   • Consider fludrocortisone to reduce natriuresis 1, 4
4. Avoid fluid restriction as it can worsen outcomes 1, 3

Critical Monitoring Parameters

• Maximum correction rate: 8 mmol/L in 24 hours (4-6 mmol/L for high-risk patients) 1, 3
• Monitor for signs of osmotic demyelination syndrome (dysarthria, dysphagia, quadriparesis) 1
• For severe symptoms: check sodium levels every 2 hours initially 1

Special Considerations

• In neurosurgical patients, CSW is more common than SIADH 1, 5
• Subarachnoid hemorrhage patients at risk for vasospasm should not receive fluid restriction 1, 3
• NT-proBNP >125 pg/ml has high sensitivity (87.5%) and specificity (93.3%) for diagnosing CSW 5
• Persistent hypouricemia after hyponatremia correction suggests CSW rather than SIADH 6, 9

Common Pitfalls to Avoid

• Misdiagnosing volume status leading to inappropriate treatment 1, 9
• Using fluid restriction in CSW (worsens outcomes) 1, 3
• Overly rapid correction causing osmotic demyelination syndrome 1, 3
• Inadequate monitoring during active correction 1
• Failing to recognize and treat the underlying cause 1, 3

By carefully assessing volume status and using appropriate laboratory tests, clinicians can differentiate between these two conditions and implement the correct treatment strategy to improve patient outcomes.