A patient has serum sodium 119 mmol/L, serum osmolality 264 mOsm/kg, urine sodium 60 mmol/L, and urine osmolality 250 mOsm/kg; what is the most likely diagnosis and appropriate initial management?

Diagnosis: SIADH (Syndrome of Inappropriate Antidiuretic Hormone Secretion)

This patient has SIADH, characterized by hypotonic hyponatremia (serum Na 119 mmol/L, serum osmolality 264 mOsm/kg) with inappropriately concentrated urine (urine osmolality 249 mOsm/kg) and elevated urine sodium (60 mmol/L) despite low serum osmolality. 1

Diagnostic Reasoning

The laboratory pattern is diagnostic for SIADH:

• Serum sodium 119 mmol/L with serum osmolality 264 mOsm/kg confirms hypotonic hyponatremia 1
• Urine osmolality 249 mOsm/kg is inappropriately elevated for the degree of serum hypo-osmolality—in normal physiology, urine osmolality should be <100 mOsm/kg when serum osmolality is this low 1, 2
• Urine sodium 60 mmol/L is markedly elevated (>40 mmol/L), indicating continued renal sodium excretion despite hyponatremia, which is pathognomonic for SIADH 3, 2, 4
• Very high urine sodium (>130 mmol/L in severe cases) predicts poor response to fluid restriction and suggests the need for more aggressive therapy 5

The euvolemic state (assumed based on typical SIADH presentation) distinguishes this from hypovolemic hyponatremia (where urine sodium would be <30 mmol/L) and hypervolemic states (where edema/ascites would be present) 1, 4.

Initial Management Strategy

For Severe Symptomatic Hyponatremia (Seizures, Altered Mental Status, Coma)

If the patient has severe neurological symptoms, immediately administer 3% hypertonic saline with a target correction of 6 mmol/L over 6 hours or until symptoms resolve, with total correction not exceeding 8 mmol/L in 24 hours. 1

• Check serum sodium every 2 hours during initial correction 1
• ICU admission is required for close monitoring 1

For Asymptomatic or Mildly Symptomatic Hyponatremia

Fluid restriction to 1 L/day (or <800 mL/day for refractory cases) is the cornerstone of treatment for SIADH. 1, 2

• If no response to fluid restriction after 24-48 hours, add oral sodium chloride 100 mEq three times daily 1
• Monitor serum sodium every 4-6 hours initially, then daily 1

Alternative Pharmacological Options for Resistant Cases

• Urea 30-80 g/day (divided into 2-3 doses orally or as 30% IV solution) induces osmotic diuresis and is highly effective for rapid correction when combined with fluid restriction and sodium supplementation 6
• Vasopressin receptor antagonists (tolvaptan 15 mg once daily, titrate to 30-60 mg) are effective for euvolemic hyponatremia but should be used with caution to avoid overcorrection 1
• Loop diuretics, demeclocycline, or lithium are additional options for refractory SIADH 1

Critical Correction Rate Guidelines

Never exceed 8 mmol/L correction in any 24-hour period to prevent osmotic demyelination syndrome. 1

• For high-risk patients (advanced liver disease, alcoholism, malnutrition, prior encephalopathy), limit correction to 4-6 mmol/L per day 1
• Osmotic demyelination syndrome typically manifests 2-7 days after rapid correction with dysarthria, dysphagia, oculomotor dysfunction, and quadriparesis 1

If Overcorrection Occurs

• Immediately discontinue hypertonic saline and switch to D5W (5% dextrose in water) 1
• Consider administering desmopressin to slow or reverse the rapid sodium rise 1
• Target is to bring total 24-hour correction back to ≤8 mmol/L from baseline 1

Workup to Identify Underlying Cause

• Medication review: SSRIs, carbamazepine, NSAIDs, opioids, chemotherapy agents (platinum-based, vinca alkaloids) are common culprits 1
• Chest X-ray: to identify pulmonary causes (pneumonia, tuberculosis) or small cell lung cancer 1
• TSH and cortisol: to exclude hypothyroidism and adrenal insufficiency, which must be ruled out before confirming SIADH 1, 3
• CT head: if CNS pathology suspected (trauma, infection, hemorrhage, tumor) 1

Common Pitfalls to Avoid

• Do not administer isotonic saline (0.9% NaCl) in SIADH—this will worsen hyponatremia because the urine osmolality exceeds that of normal saline, resulting in net free water retention 3
• Do not use fluid restriction in cerebral salt wasting (CSW)—this is a distinct entity in neurosurgical patients requiring volume and sodium replacement, not restriction 1
• Do not ignore mild hyponatremia (130-135 mmol/L)—even mild SIADH increases fall risk (21% vs 5%) and mortality (60-fold increase with Na <130 mmol/L) 1
• Do not correct chronic hyponatremia faster than 8 mmol/L in 24 hours—this causes irreversible osmotic demyelination syndrome 1

Monitoring Protocol

• Severe symptoms: serum sodium every 2 hours during initial correction 1
• Mild symptoms: serum sodium every 4-6 hours initially, then daily 1
• Daily weights and fluid balance to assess response to fluid restriction 1
• Watch for signs of osmotic demyelination syndrome 2-7 days after correction 1