This is secondary to Syndrome of inappropriate antidiuretic hormone secretion (SIADH) or Cerebral salt wasting syndrome (CSW).
SIADH is much more common than CSW in patients with aSAH. Anterior circulation aneurysms may be associated with a higher rate of SIADH than posterior circulation aneurysms 1).
Hyponatremia and dehydration due to natriuresis after subarachnoid hemorrhage are related to symptomatic vasospasm. Therefore, most institutions are currently targeting euvolemia and eunatremia in subarachnoid hemorrhage patients to avoid complications 2).
Current evidence from a Systematic Review does not demonstrate a benefit of preventative treatment with mineralocorticoids in clinically important outcomes, although a difference cannot be ruled out due to imprecision. Larger well-designed trials are needed to establish the impact of mineralocorticoids and fluid and sodium supplementation strategies on clinically relevant outcomes in the prevention and treatment of hyponatremia in patients with SAH 3).
CSW occurs from increased natriuretic peptide secretion and causes hyponatremia with diuresis and natriuresis, reduces total blood volume and increases risk of vasospasm. SIADH manifests as euvolemic hyponatremia with concentrated urine from excessive ADH secretion. CSW is managed by administering isotonic fluids and fludrocortisone while SIADH is corrected with fluid restriction. Severe and refractory hyponatremia may warrant hypertonic saline administration. Other electrolyte disturbances in these patients include hypomagnesemia, hypokalemia and hypocalcemia 4).