A 62-year-old woman with a 40-pack-year smoking history presents to the emergency department with acute confusion, headache, and polyuria. Laboratory investigations reveal serum sodium of 118 mEq/L (normal 135–145), serum osmolality of 250 mOsm/kg (normal 280–300), and urine osmolality of 580 mOsm/kg with inappropriately elevated urine sodium. Chest CT shows a 3.5 cm left hilar mass with mediastinal lymphadenopathy. Bronchoscopic biopsy confirms small cell lung cancer. What is the mechanism of the electrolyte abnormality in this patient?

Explanation

## Mechanism: Ectopic ADH Production (SIADH) ### Clinical Presentation of SIADH **Key Point:** The patient presents with hyponatremia (Na⁺ 118 mEq/L), low serum osmolality (250 mOsm/kg), and **inappropriately high urine osmolality (580 mOsm/kg)** — the hallmark of SIADH. **High-Yield:** In SIADH, the kidneys retain free water despite low serum osmolality because ADH suppresses aquaporin-2 channels in the collecting duct, causing concentrated urine and dilute serum. ### Diagnostic Criteria for SIADH | Feature | Finding | Significance | |---------|---------|---------------| | Serum osmolality | <280 mOsm/kg | Hyposmolar | | Urine osmolality | >100 mOsm/kg (here 580) | Inappropriately concentrated | | Serum Na⁺ | <135 mEq/L (here 118) | Hyponatremia | | Urine Na⁺ | Elevated (>40 mEq/L) | Renal sodium wasting | | TSH, cortisol | Normal | Rules out hypothyroidism, adrenal insufficiency | | Volume status | Euvolemic | No edema, normal BP | ### SCLC and Paraneoplastic SIADH **Clinical Pearl:** SIADH occurs in 10–15% of SCLC patients, making it the **most common paraneoplastic syndrome** associated with this malignancy. SCLC cells produce and secrete ADH (vasopressin) ectopically. **Mnemonic: Malignancies causing SIADH — "SCLC Leads All"** - **S**mall cell lung cancer (most common) - **C**arcinomas (gastric, pancreatic, bladder, prostate) - **L**eukemia/Lymphoma - **C**arcinoid tumors ### Pathophysiology 1. SCLC cells produce ADH peptide 2. ADH acts on V~2~ receptors in collecting duct principal cells 3. Aquaporin-2 water channels increase → free water reabsorption 4. Serum osmolality drops, Na⁺ diluted → hyponatremia 5. Urine becomes concentrated despite low serum osmolality ### Clinical Manifestations **Warning:** Acute hyponatremia (<120 mEq/L) causes neurological symptoms: - Confusion, headache (as in this case) - Seizures (if Na⁺ <115 mEq/L) - Cerebral edema, coma, death (if severe and rapid) ### Management **Key Point:** Treatment depends on acuity and severity: 1. **Acute symptomatic hyponatremia:** Hypertonic saline (3%) at 1–2 mL/kg/hr; target correction 4–6 mEq/L/hr (max 10–12 mEq/L/day) to avoid osmotic demyelination syndrome 2. **Chronic/asymptomatic:** Fluid restriction (500–1000 mL/day) 3. **Definitive:** Treat underlying SCLC with chemotherapy (cisplatin + etoposide) 4. **Refractory cases:** Vaptans (tolvaptan — V~2~ receptor antagonist) [cite:Harrison 21e Ch 108; Robbins 10e Ch 15]