A patient with subarachnoid hemorrhage develops progressive lethargy and coma; hyponatremia is most likely due to which condition?

• A. SIADH
• B. Cerebral salt-wasting syndrome
• C. Adrenal insufficiency
• D. Hypernatremia

Answer: (B)

Cerebral salt-wasting syndrome explains hyponatremia after a subarachnoid hemorrhage because brain injury triggers renal loss of sodium with accompanying free water loss, producing a hypovolemic state. The kidneys dump sodium inappropriately (high urine sodium) and the urine is concentrated, which lowers serum sodium and leads to lethargy or coma as the brain becomes more sensitive to changes in osmolality. This contrasts with SIADH, where hyponatremia occurs with euvolemia or mild hypervolemia and the patient is not volume-depleted, so treatment would be fluid restriction rather than saline. Adrenal insufficiency can cause hyponatremia but is not the typical pattern following SAH, and hypernatremia would not produce hyponatremia.