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SIADH-related hyponatraemia — MCCQE Part 1 MCQ

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HardFatigueSIADH-related hyponatraemiaMCCQE Part 1

A 70-year-old man with small-cell lung cancer has mild confusion. Sodium is 121 mmol/L, serum osmolality is low, urine osmolality is high and urine sodium is elevated; he is clinically euvolaemic. Which of the following is the most likely diagnosis?

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Correct answer: A — Syndrome of inappropriate antidiuretic hormone secretion

Euvolaemic hypo-osmolar hyponatraemia with inappropriately concentrated urine and high urine sodium suggests SIADH, particularly with small-cell lung cancer. Primary polydipsia usually has maximally dilute urine, and hypovolaemia has clinical volume depletion. Severe symptoms would require hypertonic saline, while chronic mild cases require controlled correction to avoid osmotic demyelination.

Reference: Canadian internal medicine practice; Choosing Wisely Canada laboratory medicine