A 60-year-old male with small cell lung cancer develops confusion. Serum sodium is 120 mEq/L, serum osmolality is 250 mOsm/kg, and urine osmolality is 420 mOsm/kg with normal volume status. What is the diagnosis?

Answer and explanation

Correct answer: A. Syndrome of inappropriate ADH secretion

  1. Step 1: Hypotonic hyponatremia with inappropriately concentrated urine (>100 mOsm/kg) and euvolemic clinical examination indicates syndrome of inappropriate antidiuretic hormone secretion (SIADH).
  2. Step 2: Small cell lung cancer is the classic paraneoplastic malignancy producing ectopic arginine vasopressin.
Answer
Syndrome of inappropriate ADH secretion.
Why not the others: Diabetes insipidus produces hypernatremia with dilute urine (urine osmolality < 300 mOsm/kg). Psychogenic polydipsia produces maximally dilute urine (urine osmolality < 100 mOsm/kg). Hypovolemic hyponatremia exhibits signs of extracellular fluid depletion such as hypotension and dry mucous membranes.

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