In the PICU, a 4-year-old with meningococcal meningitis on mechanical ventilation develops hyponatraemia (Na 122 mEq/L) with concentrated urine (urine Na 65 mEq/L), urine osmolality > plasma osmolality, and no evidence of oedema or volume depletion. What is the most likely diagnosis?

• A Syndrome of inappropriate antidiuretic hormone secretion (SIADH) ✓
• B Cerebral salt wasting syndrome
• C Diabetes insipidus
• D Hypothyroidism-induced hyponatraemia

Explanation

SIADH is characterised by euvolaemic hyponatraemia with inappropriately concentrated urine (urine osmolality > plasma osmolality) and elevated urine sodium (>20–40 mEq/L) in the absence of oedema, volume depletion, renal disease, adrenal insufficiency, or hypothyroidism. Meningitis is a classic trigger of SIADH. Cerebral salt wasting also produces hyponatraemia with high urine sodium, but it is accompanied by volume depletion (negative sodium balance, haemoconcentration) — distinguishing it is clinically critical. Diabetes insipidus causes hypernatraemia with dilute urine.

Reference: Ghai Essential Pediatrics, 10th ed.

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Written and medically reviewed by the StethoPrep medical team.