A 3-year-old post-operative child develops low urine output, lethargy, and a serum sodium of 124 mEq/L after receiving 0.18% saline with dextrose at maintenance rates. What is the most likely cause of this hyponatraemia?

• A Hypotonic fluid-induced dilutional hyponatraemia ✓
• B Syndrome of inappropriate antidiuretic hormone (SIADH)
• C Cerebral salt-wasting syndrome
• D Addison's disease

Explanation

Hypotonic maintenance fluids (0.18% saline = 30 mEq/L sodium) cause iatrogenic dilutional hyponatraemia particularly in post-operative children who have elevated ADH secretion as a stress response. This combination of hypotonic fluid infusion plus non-osmotic ADH release leads to free water retention and potentially dangerous hyponatraemia. WHO and NICE now recommend isotonic (0.9%) saline-based maintenance fluids in hospitalized children to prevent this complication. SIADH would show concentrated urine; Addison's would be accompanied by hyperkalemia and hypotension.

Reference: Ghai Essential Pediatrics, 10th ed.

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