## Clinical Context Small-cell lung cancer (SCLC) is a classic malignancy associated with hyponatremia. The patient's presentation—confusion, lethargy, and severe hyponatremia (Na 118 mEq/L)—is consistent with symptomatic hyponatremia from SIADH. ## Why SIADH is Most Common in SCLC **Key Point:** SIADH is the most common cause of hyponatremia in malignancy, and SCLC is the most frequent malignant cause of SIADH overall. SCLC cells produce ectopic antidiuretic hormone (ADH), leading to: - Inappropriate water retention - Dilutional hyponatremia - Low serum osmolality (<280 mOsm/kg) - Urine osmolality inappropriately high (>100 mOsm/kg) - Normal renal and adrenal function ## Diagnostic Criteria for SIADH | Feature | Finding | |---------|----------| | **Serum Na** | <130 mEq/L (often <120) | | **Serum osmolality** | <280 mOsm/kg | | **Urine osmolality** | >100 mOsm/kg (inappropriately concentrated) | | **Urine Na** | Typically >40 mEq/L | | **TSH, cortisol** | Normal | | **Euvolemia** | Present (no edema, normal JVP) | **High-Yield:** SCLC accounts for ~40% of all malignancy-associated SIADH cases. Other malignancies (pancreatic, gastric, bladder, lymphoma) are less common causes. ## Clinical Pearl The severity of hyponatremia (Na 118) and acute symptoms (confusion, lethargy) suggest this is a medical emergency requiring careful hypertonic saline or fluid restriction depending on acuity and volume status. Rapid overcorrection risks osmotic demyelination syndrome. [cite:Harrison 21e Ch 297]
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