Which is more common in aneurysmal Subarachnoid Hemorrhage (SAH), Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) or Cerebral Salt Wasting (CSW)?

Cerebral Salt Wasting is More Common Than SIADH in Aneurysmal Subarachnoid Hemorrhage

SIADH is more common than cerebral salt wasting in patients with aneurysmal subarachnoid hemorrhage, with SIADH accounting for approximately 75% of hyponatremia cases compared to CSW at 12-23%. 1, 2

Epidemiology and Prevalence

• Hyponatremia occurs in 10-30% of patients with aneurysmal subarachnoid hemorrhage (aSAH) 3
• In a retrospective study of 316 patients with aSAH:
  • 59.2% developed hyponatremia (serum sodium <135 mmol/L)
  • 15.2% developed moderate to severe hyponatremia (serum sodium <130 mmol/L)
  • Among those with moderate to severe hyponatremia:
    • 35.4% were diagnosed with SIADH
    • 22.9% were diagnosed with CSW 1
• A more recent study of 335 patients found that among those who developed hyponatremia:
  • 75% had SIADH
  • 12% had CSW
  • 13% did not fit either diagnosis 2

Clinical Features and Risk Factors

Cerebral Salt Wasting (CSW)

• More common in patients with:
  • Poor clinical grade aSAH
  • Ruptured anterior communicating artery aneurysms
  • Hydrocephalus 3
• Typically occurs between days 3-10 after aSAH, with peak incidence on days 7-8 4
• Characterized by:
  • Hyponatremia
  • Excessive natriuresis
  • Volume contraction
  • Low central venous pressure (CVP <6 cm H₂O) 3, 5

SIADH

• More common in anterior circulation aneurysms (90% of hyponatremia cases) compared to posterior circulation (75%) 2
• Characterized by:
  • Hyponatremia
  • Inappropriately high urine osmolality (>500 mosm/kg)
  • Inappropriately high urinary sodium (>20 mEq/L)
  • Normal volume status (CVP 6-10 cm H₂O) 5

Diagnostic Approach

To differentiate between SIADH and CSW:

1. Assess volume status:

   • CSW: Signs of hypovolemia (orthostatic hypotension, tachycardia, dry mucous membranes)
   • SIADH: Euvolemic with no edema, normal vital signs 5

2. Laboratory evaluation:

   • Serum sodium and osmolality
   • Urine sodium and osmolality
   • Serum uric acid (<4 mg/dL suggests SIADH, but may also be seen in CSW) 5

3. Hemodynamic monitoring:

   • Central venous pressure measurement can help distinguish:
     • CSW: CVP <6 cm H₂O
     • SIADH: CVP 6-10 cm H₂O 5

Management Implications

For Cerebral Salt Wasting:

• Aggressive volume resuscitation with isotonic or hypertonic saline 3
• Mineralocorticoids (fludrocortisone) to correct negative sodium balance 3
• Avoid fluid restriction which can worsen cerebral ischemia 3

For SIADH:

• May be treated with fluid restriction, urea, diuretics, lithium, or demeclocycline 3
• Hypertonic saline for symptomatic hyponatremia 5

Clinical Significance

• Hyponatremia is associated with longer hospital stays (15.7 ± 1.9 vs. 9.6 ± 1.1 days) compared to eunatremic patients 1
• CSW may be an independent risk factor for poor outcome in aSAH patients 3
• Correct treatment approach is critical as:
  • Fluid restriction (appropriate for SIADH) can worsen outcomes in CSW
  • Volume expansion (appropriate for CSW) can worsen hyponatremia in SIADH 6

Treatment Recommendations

1. For CSW:

   • Isotonic or hypertonic saline for volume expansion 3
   • Fludrocortisone (Class I evidence) for patients at risk of vasospasm 3
   • Hydrocortisone to prevent natriuresis 3

2. For SIADH:

   • Avoid fluid restriction in aSAH patients at risk of vasospasm (Class II evidence) 3
   • Consider urea, diuretics, or demeclocycline 3

3. For both conditions:

   • Avoid increasing serum sodium by >10 mmol/L/day to prevent osmotic demyelination syndrome (Class III evidence) 3, 5
   • Monitor sodium levels closely during correction 5

Conclusion

While both SIADH and CSW can cause hyponatremia in aSAH patients, current evidence indicates that SIADH is more common, accounting for approximately 75% of hyponatremia cases compared to 12-23% for CSW.