## Diagnosis of Hyponatremia in Malignancy **Key Point:** The clinical triad of hyponatremia + low serum osmolality + inappropriately high urine osmolality is pathognomonic for SIADH. ### Pathophysiology of SIADH SIADH occurs when ADH (vasopressin) is secreted autonomously or ectopically, independent of osmotic or hemodynamic regulation. In small-cell lung cancer (SCLC), malignant cells produce ADH directly, leading to: 1. Excessive water reabsorption in collecting ducts 2. Dilution of serum sodium and osmolality 3. Paradoxically concentrated urine (high osmolality despite low serum osmolality) ### Diagnostic Criteria for SIADH | Feature | SIADH | Primary Polydipsia | Adrenal Insufficiency | |---------|-------|-------------------|----------------------| | Serum osmolality | Low (<280) | Low | Low | | Urine osmolality | High (>200) | Low (<200) | Variable | | Urine Na+ | Normal/high | Low | High | | ADH level | Elevated | Suppressed | Elevated | | Response to fluid restriction | No improvement | Improves | No improvement | **High-Yield:** SIADH is the most common cause of hyponatremia in hospitalized patients and the most frequent paraneoplastic syndrome in SCLC (occurs in 10–15% of cases). ### Why SIADH in SCLC? - SCLC cells produce ectopic ADH (vasopressin) - Accounts for >50% of all SIADH cases in malignancy - Other cancers: gastric, pancreatic, bladder, prostate - Non-malignant causes: CNS disease, pulmonary infection, medications (SSRIs, carbamazepine) **Clinical Pearl:** The key discriminator is the inappropriately high urine osmolality *despite* low serum osmolality — this is the hallmark of SIADH and distinguishes it from other causes of hyponatremia. **Mnemonic: SIADH causes** — **SCLC, CNS, Chest** (SCLC, CNS disease, Chest infections like TB/pneumonia), plus Carbamazepine and other drugs.
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