A 22-year-old man develops polyuria and polydipsia after a head injury from a road traffic accident. Serum sodium is 152 mEq/L, urine osmolality is 90 mOsm/kg, and plasma ADH is undetectable. After administration of desmopressin, urine osmolality rises to 450 mOsm/kg. What is the diagnosis?

• A Nephrogenic diabetes insipidus
• B Primary polydipsia
• C Central diabetes insipidus ✓
• D Syndrome of inappropriate ADH secretion

Explanation

Trauma to the skull base can damage the hypothalamohypophyseal tract, causing ADH deficiency. The dilute urine with high serum sodium confirms inability to concentrate urine. A clear rise in urine osmolality above 50 percent after desmopressin shows that the renal collecting ducts and V2 receptors are intact, which establishes central DI. In nephrogenic DI the kidney cannot respond, so desmopressin produces no meaningful concentration of urine.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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