A 35-year-old patient undergoing elective knee arthroscopy is found to have serum sodium of 123 mEq/L preoperatively. She has been drinking 4–5 litres of water daily. What is the MOST likely diagnosis and the recommended rate of sodium correction?

• A SIADH; correct at 1–2 mEq/L/hr freely
• B Primary polydipsia with hypovolaemic hyponatraemia; correct rapidly with hypertonic saline
• C Hypervolaemic hyponatraemia due to heart failure; correct at any rate with fluid restriction alone
• D Psychogenic polydipsia causing euvolaemic hyponatraemia; correct ≤10–12 mEq/L in 24 hours to prevent osmotic demyelination ✓

Explanation

Excessive water intake (psychogenic polydipsia) causes euvolaemic dilutional hyponatraemia. The critical principle is that chronic hyponatraemia (>48 hours) must be corrected slowly — no more than 10–12 mEq/L in 24 hours (or 18 mEq/L in 48 hours) — because rapid correction risks osmotic demyelination syndrome (central pontine myelinolysis), particularly in malnourished or alcoholic patients. Elective surgery should be postponed until sodium is safely corrected to >130 mEq/L.

Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.

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