A 58-year-old man with small-cell lung cancer presents with confusion and lethargy. Serum sodium is 118 mEq/L. Urine osmolality is 650 mOsm/kg, urine sodium is 85 mEq/L, and plasma osmolality is 245 mOsm/kg. He is euvolemic on clinical examination. What is the most appropriate immediate next step in management?

## Clinical Diagnosis **Key Point:** The clinical and biochemical picture is diagnostic of **Syndrome of Inappropriate Antidiuretic Hormone (SIADH)**: - Hyponatremia (Na 118 mEq/L) with euvolemia - Inappropriately concentrated urine (osmolality 650 mOsm/kg) despite low plasma osmolality (245 mOsm/kg) - Elevated urine sodium (85 mEq/L) — rules out volume depletion - Small-cell lung cancer is a classic SIADH cause (ectopic ADH secretion) ## Why 3% Hypertonic Saline Is the Correct Immediate Next Step **High-Yield:** Per European Clinical Practice Guidelines (Spasovski et al., 2014) and Harrison's Principles of Internal Medicine (21st ed.), **symptomatic hyponatremia with neurological manifestations (confusion, lethargy) requires immediate treatment with 3% hypertonic saline**, regardless of the underlying etiology. - Serum sodium of **118 mEq/L** is severely low (< 120 mEq/L threshold). - **Confusion and lethargy** represent moderate-to-severe neurological symptoms indicating cerebral edema and risk of herniation. - The immediate goal is to raise serum sodium by **4–6 mEq/L in the first 1–2 hours** to relieve acute cerebral edema, then slow the rate to not exceed **8–10 mEq/L per 24 hours** (to avoid osmotic demyelination syndrome). - A rate of **1 mL/kg/hr of 3% saline** is the standard initial infusion rate for symptomatic hyponatremia. **Clinical Pearl:** Fluid restriction is the correct *long-term* management of SIADH, but it acts slowly (over days) and is **not appropriate as the immediate step** when the patient is symptomatic with neurological compromise. Waiting for fluid restriction to work in a confused, lethargic patient risks progression to seizures, coma, or death. ## Management Algorithm for SIADH ``` SIADH Confirmed | v Symptomatic? (confusion, lethargy, seizures, coma) | YES → 3% Hypertonic Saline (1 mL/kg/hr) | → Target: raise Na 4–6 mEq/L in first 1–2 hrs | → Max correction: 8–10 mEq/L per 24 hrs | NO (asymptomatic/mild) → Fluid restriction 500–800 mL/day ``` ## Why Other Options Are Incorrect | Option | Why Wrong | |--------|-----------| | **Fluid restriction** | Correct for *chronic, asymptomatic* SIADH. Too slow for a symptomatic patient with Na 118 mEq/L and neurological symptoms; risks clinical deterioration while awaiting effect. | | **0.9% normal saline** | Contraindicated in SIADH. Urine osmolality (650 mOsm/kg) far exceeds saline osmolality (~308 mOsm/kg); the kidney will retain free water and excrete sodium, paradoxically **worsening** hyponatremia. | | **Demeclocycline** | A second-line agent for **chronic, refractory SIADH**. Onset of action is 5–7 days; completely inappropriate for acute symptomatic management. | **Warning:** Do not confuse SIADH management with hypovolemic hyponatremia (which requires 0.9% saline) or hypervolemic hyponatremia (which requires diuretics + fluid restriction). In SIADH with neurological symptoms, hypertonic saline is always the immediate priority. *Reference: Harrison's Principles of Internal Medicine, 21st ed., Chapter on Fluid and Electrolyte Disorders; Spasovski G et al., Clinical practice guideline on diagnosis and treatment of hyponatraemia, Eur J Endocrinol 2014.*