Carbamazepine-induced hyponatremia following meningioma surgery: A case report

Table 3 Differential diagnosis of postoperative hyponatremia

Feature	SIADH	CSWS	Findings in this case
Volume status	Euvolemia or mild hypervolemia	Hypovolemia (effective volume depletion)	Clinically euvolemic; stable hemodynamics; no orthostasis
Urine output	Normal to low (variable)	Often increased (polyuria)	No polyuria documented; no negative fluid balance reported
Serum osmolality	Low	Low	Low (254 mOsm/kg)
Urine osmolality	Inappropriately high (> 100 mOsm/kg)	Often high	High (560 mOsm/kg)
Urine sodium	> 30-40 mmol/L	> 40 mmol/L	High (68 mmol/L)
Response to isotonic saline	Often minimal or may worsen	Improves sodium/volume	Sodium continued to decline despite 0.9% NaCl
Uric acid/FEurate	Often low uric acid; FEurate elevated (may normalize after correction)	Often low uric acid; FEurate elevated and may persist	Low uric acid (150 μmol/L)
Key therapeutic approach	Fluid restriction, treat trigger; hypertonic saline if severe symptoms	Volume and salt repletion, optional fludrocortisone, hypertonic saline if severe symptoms	Trigger removed (CBZ stopped) and hypertonic saline; sodium normalized

CBZ: Carbamazepine; SIADH: Syndrome of inappropriate antidiuretic hormone secretion; CSWS: Central salt wasting syndrome.