A 52-year-old man with syndrome of inappropriate antidiuretic hormone secretion (SIADH) secondary to small-cell lung cancer presents with severe hyponatraemia (Na⁺ = 118 mEq/L) and altered mental status. Regarding the pathophysiology and management of SIADH, all of the following statements are correct EXCEPT:

Explanation

## SIADH: Pathophysiology and Management ### Definition and Pathophysiology **Key Point:** SIADH is a state of **inappropriate ADH secretion** despite low plasma osmolality. This causes excessive free water reabsorption in the collecting duct, leading to dilutional hyponatraemia. ### Diagnostic Criteria for SIADH | Feature | Finding | |---------|----------| | **Serum osmolality** | Low (< 275 mOsm/kg) | | **Serum sodium** | Low (< 135 mEq/L) | | **Urine osmolality** | Inappropriately high (> 100 mOsm/kg) — should be dilute if plasma osmolality is low | | **Urine sodium** | Usually > 40 mEq/L | | **Plasma ADH** | Elevated despite low osmolality | | **Volume status** | Euvolaemic (not hypovolaemic or hypervolaemic) | **High-Yield:** The hallmark is **inappropriately concentrated urine in the setting of low serum osmolality** — the kidney is reabsorbing water when it should be excreting it. ### Acute vs. Chronic Hyponatraemia Management ```mermaid flowchart TD A["Hyponatraemia in SIADH"]:::outcome --> B{"Symptoms?"}:::decision B -->|"Yes (seizures, altered mental status, coma)"|C["ACUTE SYMPTOMATIC"]:::urgent B -->|"No or mild"|D["CHRONIC/ASYMPTOMATIC"]:::outcome C --> E["3% hypertonic saline IV<br/>Correct by 4-6 mEq/L in first hour<br/>Then 8-10 mEq/L over 24 hrs"]:::action D --> F["Fluid restriction<br/>typically 500-800 mL/day"]:::action E --> G["Monitor for overcorrection<br/>Risk: osmotic demyelination"]:::decision F --> H["Gradual correction over days<br/>Target: 4-8 mEq/L per 24 hrs"]:::action ``` ### Why Option 2 Is WRONG **Warning:** The statement "should be corrected **rapidly**" is dangerous and incorrect. - **Acute symptomatic hyponatraemia** (Na⁺ < 120 mEq/L with seizures, altered mental status, coma) requires **3% hypertonic saline**, BUT correction must be **gradual and controlled**: - Correct by **4–6 mEq/L in the first 1–2 hours** to stop seizures - Then **8–10 mEq/L over the next 24 hours** - **Never exceed 8–10 mEq/L per 24 hours** in chronic hyponatraemia - **Rapid correction** (> 10–12 mEq/L per 24 hrs) causes **osmotic demyelination syndrome (ODS)**, a devastating complication with central pontine and extrapontine myelinolysis, leading to permanent neurological damage or death. **Clinical Pearl:** The brain adapts to chronic hyponatraemia by extruding intracellular osmolytes (taurine, betaine, myo-inositol). Rapid correction causes water to leave brain cells faster than osmolytes can be reabsorbed, causing cell shrinkage and demyelination. ### Correct Management Approach **Acute Symptomatic (Na⁺ < 120 with seizures/coma):** - 3% hypertonic saline IV - Correct 4–6 mEq/L in first 1–2 hours - Then 8–10 mEq/L over next 24 hours - Monitor neurological status and serum sodium every 2–4 hours **Chronic Asymptomatic (Na⁺ 120–130):** - **Fluid restriction** (500–800 mL/day) — first-line - Gradual correction: 4–8 mEq/L per 24 hours - Vaptans (V2 receptor antagonists) if fluid restriction fails - Treat underlying cause (e.g., chemotherapy for SCLC) **High-Yield:** In this patient's case (Na⁺ = 118 with altered mental status), 3% saline is indicated, but correction must be **slow and monitored** — not rapid.