A 68-year-old man with small-cell lung cancer presents with severe hyponatraemia (Na⁺ 118 mEq/L), confusion, and headache. Urine osmolality is 580 mOsm/kg and urine sodium is 95 mEq/L. Serum osmolality is 245 mOsm/kg. What is the most appropriate immediate management?

Explanation

## Clinical Scenario Analysis This patient has **symptomatic hyponatraemia** secondary to SIADH (syndrome of inappropriate antidiuretic hormone secretion): - Serum Na⁺ 118 mEq/L (severe; normal 135–145) - Serum osmolality 245 mOsm/kg (hypoosmolar; normal 280–295) - Urine osmolality 580 mOsm/kg (inappropriately concentrated despite hypo-osmolality) - Urine sodium 95 mEq/L (high, indicating euvolaemia) - **Neurological symptoms** (confusion, headache) = cerebral oedema from acute hyponatraemia ## Management Principle **Key Point:** Symptomatic hyponatraemia with neurological manifestations is a medical emergency requiring **rapid partial correction** with hypertonic saline to raise serum Na⁺ by 4–6 mEq/L acutely and prevent seizures and cerebral herniation. **High-Yield:** The rate of correction matters: - **Acute symptomatic hyponatraemia** (< 48 hrs): Correct at **8–10 mEq/L in first 24 hrs** to relieve symptoms - **Chronic hyponatraemia** (> 48 hrs): Correct at **≤ 8 mEq/L per 24 hrs** to avoid osmotic demyelination syndrome (ODS) - This patient's presentation is acute (symptoms present), so faster correction is justified ## Why Hypertonic Saline (3%) is Correct 1. **Symptomatic hyponatraemia requires hypertonic saline**, not fluid restriction 2. **3% NaCl** provides 513 mEq/L Na⁺ (vs 154 mEq/L in 0.9% saline) 3. **Infusion rate of 1 mL/kg/hr** achieves controlled rise of ~4–6 mEq/L in first 1–2 hours, relieving cerebral oedema without overcorrection 4. Once symptoms resolve (usually after 4–6 mEq/L rise), switch to fluid restriction (500 mL/day) to manage the underlying SIADH **Clinical Pearl:** The goal in acute symptomatic hyponatraemia is to raise Na⁺ just enough to stop seizures and confusion — not to normalize it immediately. Overcorrection (> 12 mEq/L in 24 hrs) risks osmotic demyelination syndrome, especially in chronic cases. ## Treatment Algorithm for Hyponatraemia ```mermaid flowchart TD A[Hyponatraemia Na+ < 130]:::outcome --> B{Symptoms?}:::decision B -->|Yes: seizures, confusion, coma| C[Symptomatic]:::urgent B -->|No| D[Asymptomatic]:::outcome C --> E{Acute < 48 hrs or Chronic?}:::decision E -->|Acute| F[3% NaCl at 1 mL/kg/hr]:::action E -->|Chronic| G[Slow correction: 3% NaCl cautiously]:::action D --> H{Volume status?}:::decision H -->|Euvolaemic SIADH| I[Fluid restriction 500 mL/day]:::action H -->|Hypovolaemic| J[0.9% NaCl + treat cause]:::action H -->|Hypervolaemic| K[Loop diuretic + fluid restriction]:::action F --> L[Monitor Na+ q 2-4 hrs]:::action L --> M{Na+ risen 4-6 mEq/L?}:::decision M -->|Yes| N[Switch to fluid restriction]:::action M -->|No| O[Continue 3% NaCl]:::action ``` **Mnemonic: SIADH causes** — **SIADH**: - **S**mall-cell lung cancer (most common malignancy) - **I**nfections (pneumonia, TB, meningitis) - **A**cute CNS disease (head injury, subarachnoid haemorrhage, encephalitis) - **D**rugs (carbamazepine, SSRIs, vincristine, cyclophosphamide) - **H**ypothyroidism, Hyponatraemia (chronic) ## Why Other Options Are Wrong | Option | Why Incorrect | |--------|---------------| | **Fluid restriction alone** | Appropriate for asymptomatic SIADH, but this patient is **symptomatic** with neurological signs requiring urgent Na⁺ elevation. Fluid restriction alone is too slow. | | **Normal saline (0.9%)** | Contains only 154 mEq/L Na⁺; in SIADH (where ADH is already high), 0.9% saline is hypotonic relative to urine and worsens hyponatraemia. Hypertonic (3%) saline is essential. | | **Desmopressin (DDAVP)** | Worsens hyponatraemia by increasing ADH effect. Used in central diabetes insipidus (low ADH), not SIADH (high ADH). Contraindicated here. |