Syndrome of inappropriate antidiuretic hormone secretion (SIADH) must be distinguished from cerebral salt wasting (CSW) in a patient with hyponatraemia following subarachnoid haemorrhage. Which single finding best differentiates SIADH from CSW?

• A Serum osmolality: <275 mOsm/kg in SIADH, normal in CSW
• B Urine osmolality: >100 mOsm/kg in SIADH vs. <100 mOsm/kg in CSW
• C Urine sodium: >20 mEq/L in both; volume status: euvolemia in SIADH vs. hypovolemia in CSW ✓
• D Serum potassium: hyperkalemia in SIADH vs. hypokalemia in CSW

Explanation

Both SIADH and cerebral salt wasting cause hyponatraemia with concentrated urine (urine Na >40 mEq/L, urine osmolality >serum osmolality). The critical differentiator is volume status: SIADH is associated with euvolemia (or mild hypervolemia), while CSW features true hypovolemia (low CVP, orthostatic hypotension, elevated urea, contracted plasma volume) from excessive renal sodium loss. This distinction is clinically vital because treatment is opposite: SIADH requires fluid restriction, while CSW requires aggressive sodium and fluid replacement — restricting fluids in CSW worsens hypovolaemia.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.