A 45-year-old man has serum sodium 122 mEq/L found incidentally. He has no symptoms. BP 128/80 mmHg, no oedema, skin turgor normal, urine osmolality 520 mOsm/kg, urine sodium 55 mEq/L, serum osmolality 258 mOsm/kg. He takes no diuretics and has no history of cardiac, renal, or liver disease. The MOST likely diagnosis and the CORRECT approach to correction is:

• A Hypovolaemic hyponatraemia; 0.9% normal saline at 1 mL/kg/hour
• B SIADH (syndrome of inappropriate ADH secretion); fluid restriction to 800–1000 mL/day; correct sodium by no more than 8–10 mEq/L in 24 hours to avoid osmotic demyelination ✓
• C Pseudohyponatraemia; no treatment needed
• D Cerebral salt wasting; hypertonic saline and fludrocortisone

Explanation

Euvolaemic hyponatraemia with high urine osmolality (>100 mOsm/kg, here 520), high urine sodium (>30 mEq/L, here 55), low serum osmolality (258), no oedema, and normal renal/cardiac/adrenal function defines SIADH. Fluid restriction (800–1000 mL/day) is first-line. The rate of correction should not exceed 8–10 mEq/L in the first 24 hours and 18 mEq/L in the first 48 hours to prevent osmotic demyelination syndrome (central pontine myelinolysis), which is particularly dangerous when chronic hyponatraemia (>48 hours) is corrected too rapidly. Vaptans (tolvaptan) are second-line for refractory SIADH.

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

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Written and medically reviewed by the StethoPrep medical team.