A 72-year-old woman with small-cell lung cancer (SCLC) is admitted with lethargy, nausea, and a single episode of seizure. Serum sodium 118 mEq/L, serum osmolality 252 mOsm/kg, urine osmolality 680 mOsm/kg, urine sodium 78 mEq/L, normal renal and adrenal function. Chest imaging shows a 4 cm left hilar mass. What is the most appropriate initial management for her electrolyte abnormality?

Explanation

## Clinical Diagnosis: SIADH (Syndrome of Inappropriate Antidiuretic Hormone) with Symptomatic Hyponatraemia ### Diagnostic Criteria for SIADH | Criterion | Finding | Status | |-----------|---------|--------| | Hyponatraemia | 118 mEq/L | ✓ Present | | Hypoosmolality | 252 mOsm/kg | ✓ Present | | Urine osmolality inappropriately high | 680 mOsm/kg (>100) | ✓ Present | | Urine sodium | 78 mEq/L | ✓ Elevated (euvolaemia) | | Normal renal/adrenal function | Stated | ✓ Present | | Euvolaemic state | Clinical context | ✓ Likely | **High-Yield:** All five diagnostic criteria for SIADH are met. The concentrated urine (680 mOsm/kg) in the setting of low serum osmolality (252) is pathognomonic—ADH is being secreted despite osmotic suppression. ### Why This Patient Requires Hypertonic Saline **Key Point:** This patient has **symptomatic hyponatraemia** (seizure, altered mental status). Symptomatic hyponatraemia is a medical emergency requiring rapid partial correction to prevent cerebral oedema and further seizures. **Clinical Pearl:** The rate of sodium correction is critical: - **Too fast (>12 mEq/L in 24 hours)** → osmotic demyelination syndrome (central pontine myelinolysis) - **Too slow (<4 mEq/L in 24 hours)** → persistent cerebral oedema, seizures, death - **Target: 8–10 mEq/L in 24 hours** for acute symptomatic hyponatraemia ### Mechanism of Hypertonic Saline 1. Hypertonic saline (3% = 513 mEq/L) provides sodium directly 2. Raises serum osmolality, causing water to shift out of neurons 3. Reduces cerebral oedema and seizure risk 4. Allows time for definitive treatment (e.g., vaptans, fluid restriction) to take effect ### Calculation of Sodium Requirement $$\text{Na}^+ \text{ deficit} = 0.6 \times \text{BW (kg)} \times (\text{desired Na}^+ - \text{current Na}^+)$$ For a 60 kg woman aiming to raise Na⁺ from 118 to 126 mEq/L (8 mEq/L rise): $$\text{Deficit} = 0.6 \times 60 \times 8 = 288 \text{ mEq}$$ 3% saline contains 513 mEq/L → approximately 560 mL needed over 24 hours (≈ 23 mL/hour or 1–2 mL/kg/hour). **Mnemonic:** **SIADH + Symptoms = Hypertonic saline**. Fluid restriction is for chronic asymptomatic SIADH. ### Why Each Distractor is Wrong **Option 1 (Fluid restriction alone):** - Appropriate for chronic asymptomatic SIADH - **NOT appropriate for acute symptomatic hyponatraemia** (seizure) - Takes days to weeks to correct sodium; patient may have recurrent seizures **Option 3 (Desmopressin):** - DDAVP is a synthetic ADH analogue - Contraindicated in SIADH (ADH is already elevated) - Would worsen hyponatraemia **Option 4 (Normal saline):** - 0.9% saline = 154 mEq/L (hypotonic relative to 3% saline) - In SIADH, kidneys reabsorb water from any fluid, even normal saline - May paradoxically worsen hyponatraemia ("urine is more concentrated than saline") - Only indicated if patient is hypovolaemic (not the case here) ### Definitive Management of SIADH 1. **Acute symptomatic:** Hypertonic saline (3%) as above 2. **Chronic asymptomatic:** Fluid restriction (500–800 mL/day) 3. **Refractory:** Vaptans (tolvaptan, conivaptan—V2 receptor antagonists) or urea 4. **Underlying cause:** Treat SCLC (chemotherapy, radiation)