A 58-year-old man with small cell lung cancer presents to the emergency department with confusion, headache, and nausea. His wife reports he has been drinking excessive amounts of water over the past 3 days. On examination, he is alert but disoriented, blood pressure is 128/82 mmHg, and there is no peripheral edema. Laboratory investigations reveal: serum sodium 118 mEq/L (normal 135–145), serum osmolality 245 mOsm/kg (normal 280–295), urine osmolality 580 mOsm/kg, and urine sodium 65 mEq/L. What is the primary mechanism responsible for this patient's hyponatremia?

Explanation

## Diagnosis: Syndrome of Inappropriate ADH Secretion (SIADH) ### Clinical Presentation Analysis **Key Point:** The combination of hyponatremia with inappropriately concentrated urine (high urine osmolality despite low serum osmolality) in a patient with small cell lung cancer is pathognomonic for SIADH. ### Laboratory Interpretation | Parameter | Patient Value | Normal Range | Interpretation | |-----------|---------------|--------------|----------------| | Serum Na⁺ | 118 mEq/L | 135–145 | Severe hyponatremia | | Serum osmolality | 245 mOsm/kg | 280–295 | Hypoosmolar | | Urine osmolality | 580 mOsm/kg | 50–1200 | Inappropriately concentrated | | Urine Na⁺ | 65 mEq/L | Variable | Adequate for excretion | | BP | 128/82 (normal) | — | Euvolemic state | ### Mechanism of SIADH 1. **Ectopic ADH production** by small cell lung cancer cells 2. **Continuous ADH secretion** independent of serum osmolality feedback 3. **Increased aquaporin-2 channels** in collecting duct principal cells → enhanced water reabsorption 4. **Net water retention** → dilutional hyponatremia 5. **Urine remains concentrated** because ADH is persistently high (kidneys cannot "turn off" reabsorption) **High-Yield:** The hallmark of SIADH is **euvolemic hyponatremia with inappropriately high urine osmolality**. The patient is not edematous (euvolemic), ruling out heart failure or cirrhosis. ### Pathophysiology Diagram ```mermaid flowchart TD A[Small cell lung cancer]:::outcome --> B[Ectopic ADH secretion]:::action B --> C[Continuous ADH despite low serum osmolality]:::action C --> D[Increased aquaporin-2 in collecting duct]:::action D --> E[Enhanced water reabsorption]:::action E --> F[Dilutional hyponatremia]:::outcome F --> G[High urine osmolality persists]:::outcome G --> H[Inappropriate ADH = SIADH]:::outcome ``` **Clinical Pearl:** In SIADH, the kidneys are functioning normally—the problem is the persistent ADH signal. The kidneys *should* dilute urine when serum osmolality is low, but ADH prevents this. ### Why Urine Osmolality Is High ADH acts on V2 receptors → ↑ cAMP → ↑ aquaporin-2 insertion → water reabsorption in collecting duct. Even though the patient drinks water and serum osmolality drops, the continuous ADH keeps the collecting duct permeable, so urine remains concentrated. [cite:Guyton and Hall Ch 29]