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Cerebral Salt Wasting — RACP Adult Medicine MCQ

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HardNephrologyCerebral Salt WastingRACP Adult Medicine

A 45-year-old man with recent subarachnoid haemorrhage develops hyponatraemia (sodium 125 mmol/L). He appears hypovolaemic with tachycardia. Urine sodium is 85 mmol/L and urine osmolality is 450 mOsm/kg. He is losing weight and has negative fluid balance. What is the most likely diagnosis?

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Correct answer: C — Cerebral salt wasting

Cerebral salt wasting occurs after CNS insults (especially SAH) and presents with hypovolaemic hyponatraemia with high urine sodium and osmolality. Unlike SIADH (euvolaemic), CSW patients are volume-depleted with negative fluid balance and weight loss. Treatment is isotonic saline replacement, not fluid restriction (which worsens CSW).

Reference: European Hyponatraemia Guidelines 2023; eTG Endocrine 2024