How should hyponatremia be evaluated and managed, distinguishing acute versus chronic and symptomatic versus asymptomatic cases, and what are the appropriate treatment options?

Hyponatremia: Evaluation and Management

Initial Assessment and Classification

Hyponatremia (serum sodium <135 mmol/L) requires immediate evaluation based on symptom severity and onset timing, with treatment urgency determined by clinical presentation rather than sodium level alone. 1

Critical First Steps

• Measure serum and urine osmolality, urine sodium, and assess extracellular fluid volume status to determine the underlying cause 1
• Determine acuity: acute (<48 hours) versus chronic (>48 hours), as this fundamentally changes correction rates and demyelination risk 1, 2
• Assess symptom severity: severe symptoms (seizures, coma, altered consciousness) constitute a medical emergency requiring immediate hypertonic saline regardless of sodium level 3, 4

Volume Status Assessment

Physical examination alone has poor accuracy (sensitivity 41%, specificity 80%) 1, but look for:

• Hypovolemic signs: orthostatic hypotension, dry mucous membranes, decreased skin turgor, flat neck veins 1, 5
• Hypervolemic signs: peripheral edema, ascites, jugular venous distention, pulmonary congestion 1, 6
• Euvolemic: absence of both hypovolemic and hypervolemic findings 1

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Treatment Based on Symptom Severity

Severe Symptomatic Hyponatremia (Medical Emergency)

For patients with seizures, coma, altered consciousness, or respiratory distress, administer 3% hypertonic saline immediately with a target correction of 6 mmol/L over 6 hours or until severe symptoms resolve. 1, 3, 4

• Dosing: 100 mL boluses of 3% saline over 10 minutes, repeatable up to 3 times at 10-minute intervals 1
• Maximum correction: Never exceed 8 mmol/L in any 24-hour period to prevent osmotic demyelination syndrome 1, 3, 4, 2
• Monitoring: Check serum sodium every 2 hours during active correction 1, 3
• ICU admission: Required for close monitoring during treatment 1

Critical safety point: Acute hyponatremia (<48 hours) can be corrected more rapidly without demyelination risk, but chronic hyponatremia (>48 hours) requires strict adherence to the 8 mmol/L/24-hour limit 2, 7

Moderate Symptomatic Hyponatremia

Patients with nausea, vomiting, confusion, headache, or gait instability warrant hospital admission with monitored correction. 3, 8

• Approach: More gradual correction than severe cases, still respecting 8 mmol/L/24-hour maximum 1
• Monitoring: Check sodium every 4-6 hours after symptom resolution 1

Asymptomatic or Mild Hyponatremia

Even mild chronic hyponatremia (130-135 mmol/L) is not benign and increases mortality 60-fold (11.2% vs 0.19%), fall risk (21% vs 5%), and causes cognitive impairment. 1, 8, 4

• Management: Treat underlying cause, more conservative correction based on etiology 3
• Do not ignore: Sodium <131 mmol/L warrants full workup 1

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Treatment Based on Volume Status and Etiology

Hypovolemic Hyponatremia

Discontinue diuretics and administer isotonic saline (0.9% NaCl) for volume repletion. 1

• Urine sodium <30 mmol/L predicts good response to saline (positive predictive value 71-100%) 1
• Correction rate: 4-8 mmol/L per day, maximum 8 mmol/L in 24 hours 1
• High-risk patients (cirrhosis, alcoholism, malnutrition): limit to 4-6 mmol/L per day 1, 2, 7

Euvolemic Hyponatremia (SIADH)

Fluid restriction to 1 L/day is the cornerstone of treatment for SIADH. 1, 3

• If no response: Add oral sodium chloride 100 mEq three times daily 1
• Severe symptomatic cases: Use 3% hypertonic saline as above 1
• Pharmacologic options for resistant cases: Vasopressin receptor antagonists (tolvaptan 15 mg daily, titrate to 30-60 mg), urea, demeclocycline, lithium 1, 4

Diagnostic criteria for SIADH: Hypotonic hyponatremia, inappropriately concentrated urine (>100 mOsm/kg), urine sodium >20-40 mmol/L, clinical euvolemia, normal renal/thyroid/adrenal function 1

Hypervolemic Hyponatremia (Heart Failure, Cirrhosis)

Implement fluid restriction to 1-1.5 L/day for serum sodium <125 mmol/L. 1, 3, 6

• Discontinue diuretics temporarily if sodium <125 mmol/L 1
• Cirrhotic patients: Consider albumin infusion (8 g per liter of ascites removed) alongside fluid restriction 1
• Avoid hypertonic saline unless life-threatening symptoms present, as it worsens ascites and edema 1
• Correction rate: 4-6 mmol/L per day maximum for cirrhotic patients 1

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Special Populations: Neurosurgical Patients

Distinguishing SIADH from Cerebral Salt Wasting (CSW)

In neurosurgical patients, cerebral salt wasting is more common than SIADH and requires opposite treatment—this distinction is critical. 1, 3, 5

SIADH characteristics: 1, 5

• Euvolemic state
• Urine sodium >20-40 mmol/L
• Urine osmolality >300 mOsm/kg
• Treatment: Fluid restriction

Cerebral Salt Wasting characteristics: 1, 5

• True hypovolemia (orthostatic hypotension, tachycardia, dry mucous membranes)
• Urine sodium >20 mmol/L despite volume depletion
• Central venous pressure <6 cm H₂O
• Treatment: Volume and sodium replacement with isotonic or hypertonic saline

CSW Management

For cerebral salt wasting, treatment focuses on aggressive volume and sodium replacement—fluid restriction worsens outcomes and can be fatal. 1, 5

• Severe symptoms: 3% hypertonic saline plus fludrocortisone (0.1-0.2 mg daily) in ICU 1
• Subarachnoid hemorrhage patients at risk of vasospasm: Never use fluid restriction; consider fludrocortisone to prevent vasospasm and hydrocortisone to prevent natriuresis 1, 3

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Critical Correction Rate Guidelines

Standard-Risk Patients

Maximum correction: 8 mmol/L in any 24-hour period. 1, 3, 4, 2

• Target rate: 4-8 mmol/L per day, not exceeding 10-12 mmol/L in 24 hours 1
• For severe symptoms: Correct 6 mmol/L over first 6 hours, then only 2 mmol/L additional in next 18 hours 1

High-Risk Patients

Patients with advanced liver disease, chronic alcoholism, malnutrition, severe hyponatremia, or prior encephalopathy require even slower correction: 4-6 mmol/L per day, maximum 8 mmol/L in 24 hours. 1, 3, 2, 7

• Liver transplant recipients: Risk of osmotic demyelination syndrome is 0.5-1.5% despite careful management 1
• Rationale: These populations have exceptionally high susceptibility to osmotic demyelination 2, 7

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Management of Overcorrection

If sodium correction exceeds 8 mmol/L in 24 hours, immediately discontinue current fluids and switch to D5W (5% dextrose in water). 1

• Consider desmopressin to slow or reverse the rapid rise in serum sodium 1, 2
• Goal: Bring total 24-hour correction back to ≤8 mmol/L from baseline 1

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Monitoring Protocol

During Active Correction

• Severe symptoms: Check serum sodium every 2 hours 1, 3
• After symptom resolution: Every 4-6 hours 1
• Once stable: Daily monitoring for at least 7-10 days (14 days for neurosurgical procedures) 3

Watch for Osmotic Demyelination Syndrome

Signs typically appear 2-7 days after rapid correction: dysarthria, dysphagia, oculomotor dysfunction, quadriparesis. 1, 2

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Common Pitfalls to Avoid

Critical errors that worsen outcomes: 1, 3

• Overly rapid correction exceeding 8 mmol/L in 24 hours—causes osmotic demyelination syndrome 1, 2, 7
• Using fluid restriction in cerebral salt wasting—worsens hypovolemia and cerebral ischemia 1, 5
• Fluid restriction in subarachnoid hemorrhage patients at risk of vasospasm—increases ischemic complications 1
• Inadequate monitoring during active correction—leads to overcorrection 1
• Failing to recognize and treat underlying cause—hyponatremia recurs 1
• Using hypertonic saline in hypervolemic hyponatremia without life-threatening symptoms—worsens fluid overload 1
• Ignoring mild hyponatremia (130-135 mmol/L) as clinically insignificant—associated with increased mortality and falls 1, 8, 4