A 58-year-old man with small-cell lung cancer presents to the emergency department with confusion and lethargy. His wife reports he has been increasingly drowsy over the past 3 days. Vital signs: BP 128/82 mmHg, HR 88/min, RR 20/min, Temperature 37.2°C. Serum sodium is 118 mEq/L (normal 135–145), serum osmolality 245 mOsm/kg (normal 280–295), and urine osmolality 680 mOsm/kg. Urine sodium is 65 mEq/L. Thyroid function tests and adrenal function are normal. What is the most likely diagnosis?

Explanation

## Diagnosis: SIADH ### Clinical Presentation This patient presents with **symptomatic hyponatremia** (confusion, lethargy) in the setting of small-cell lung cancer, a classic malignancy associated with ectopic ADH production. ### Diagnostic Criteria for SIADH **Key Point:** SIADH is diagnosed when all of the following are present: 1. Hyponatremia (Na⁺ < 135 mEq/L) — **118 mEq/L ✓** 2. Low serum osmolality (< 280 mOsm/kg) — **245 mOsm/kg ✓** 3. Inappropriately high urine osmolality (> 100 mOsm/kg despite low serum osmolality) — **680 mOsm/kg ✓** 4. Urine sodium typically > 40 mEq/L (reflects euvolemia) — **65 mEq/L ✓** 5. Normal thyroid and adrenal function — **Both normal ✓** 6. No diuretic use or volume depletion — **Not present ✓** ### Pathophysiology Small-cell lung cancer produces ectopic ADH, leading to: - Increased renal water reabsorption in the collecting duct - Dilutional hyponatremia despite euvolemia (normal BP, no edema) - Concentrated urine (high osmolality) in the face of dilute plasma ### High-Yield Mnemonic **SIADH causes:** **CHIMPANZEES** - **C**ancer (lung, pancreas, bladder, lymphoma) - **H**ead injury, meningitis, encephalitis - **I**nfections (pneumonia, TB, meningitis) - **M**edications (SSRIs, carbamazepine, vincristine, NSAIDs) - **P**ulmonary disease (pneumonia, positive pressure ventilation) - **A**cute porphyria - **N**europsychiatric conditions - **Z**oster (varicella) - **E**ndocrine (hypothyroidism — ruled out here) - **E**xcretion (renal failure) - **S**tress, surgery ### Clinical Pearl **Symptomatic hyponatremia** (Na⁺ < 120 mEq/L with neurological signs) is a medical emergency requiring cautious hypertonic saline (3% NaCl) with careful rate control to avoid osmotic demyelination syndrome. [cite:Harrison 21e Ch 297]