A 62-year-old man with small cell lung cancer presents with severe hyponatremia (Na⁺ 118 mEq/L) and high urine osmolality despite low serum osmolality. What is the most common cause of this clinical presentation?

Explanation

## Diagnosis: Syndrome of Inappropriate ADH (SIADH) **Key Point:** SIADH is characterized by inappropriate ADH secretion leading to hyponatremia, low serum osmolality, and paradoxically high urine osmolality (>200 mOsm/kg). **High-Yield:** Small cell lung cancer (SCLC) is the **most common malignancy** causing SIADH, accounting for ~80% of cancer-related SIADH cases. The cancer cells directly secrete ADH peptides. ### Pathophysiology Ectopic ADH from SCLC causes: 1. Increased water reabsorption in collecting duct 2. Plasma osmolality decreases (dilutional hyponatremia) 3. Urine becomes inappropriately concentrated despite low serum osmolality 4. Urine sodium typically >40 mEq/L (euvolemic state) ### Diagnostic Criteria for SIADH | Feature | Finding | |---------|----------| | Serum osmolality | <275 mOsm/kg | | Urine osmolality | >200 mOsm/kg | | Serum Na⁺ | <135 mEq/L | | Urine Na⁺ | >40 mEq/L | | TSH, cortisol | Normal | | Volume status | Euvolemic | **Clinical Pearl:** The combination of **malignancy + hyponatremia + high urine osmolality** is pathognomonic for SIADH until proven otherwise. SCLC screening should be considered in any patient with unexplained SIADH. **Mnemonic — SIADH Causes: CHAMPS** - **C**ancers (SCLC, pancreatic, bladder, prostate) - **H**ead injury, hypothyroidism, hypopituitarism - **A**cute illness, antipsychotics, antidepressants - **M**eningitis, malignancy - **P**ulmonary disease (TB, pneumonia, positive pressure ventilation) - **S**tress, surgery [cite:Harrison 21e Ch 295]