What is the management approach for a patient with true hyponatremia, characterized by serum osmolality less than 280 mOsm/kg?

Management of True Hyponatremia

For true hyponatremia (serum osmolality <280 mOsm/kg), management is determined by volume status and symptom severity, with the critical principle being that correction must never exceed 8 mmol/L in 24 hours to prevent osmotic demyelination syndrome. 1

Initial Assessment and Classification

Determine volume status through physical examination, though recognize this has limited accuracy (sensitivity 41.1%, specificity 80%) 1:

• Hypovolemic signs: orthostatic hypotension, dry mucous membranes, decreased skin turgor, flat neck veins 1
• Euvolemic signs: normal volume status without edema or dehydration 1
• Hypervolemic signs: peripheral edema, ascites, jugular venous distention 1

Obtain urine sodium and osmolality 1:

• Urine sodium <30 mmol/L suggests hypovolemic hyponatremia (71-100% positive predictive value for saline responsiveness) 1
• Urine sodium >20-40 mmol/L with urine osmolality >300 mOsm/kg suggests SIADH 1

Management Based on Symptom Severity

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

Administer 3% hypertonic saline immediately with a target correction of 6 mmol/L over 6 hours or until severe symptoms resolve. 1, 2

• Give 100 mL boluses of 3% saline over 10 minutes, repeatable up to three times at 10-minute intervals 1
• Monitor serum sodium every 2 hours during initial correction 1
• Total correction must not exceed 8 mmol/L in 24 hours 1, 2
• For high-risk patients (cirrhosis, alcoholism, malnutrition), limit to 4-6 mmol/L per day 1

Mild to Moderate Symptoms or Asymptomatic

Treatment is based on volume status:

Management Based on Volume Status

Hypovolemic Hyponatremia

Discontinue diuretics immediately and administer isotonic saline (0.9% NaCl) for volume repletion. 1, 3

• Initial infusion rate: 15-20 mL/kg/h, then 4-14 mL/kg/h based on response 1
• Urine sodium <30 mmol/L predicts good response to saline 1
• Correction rate: 4-8 mmol/L per day, maximum 8 mmol/L in 24 hours 1

Euvolemic Hyponatremia (SIADH)

Fluid restriction to 1 L/day is the cornerstone of treatment. 1, 2

• If no response to fluid restriction, add oral sodium chloride 100 mEq three times daily 1
• For persistent cases, consider vasopressin receptor antagonists (tolvaptan 15 mg once daily, titrate to 30-60 mg) 1, 4
• Urea can be used as an alternative pharmacological option 1
• For severe symptoms, use 3% hypertonic saline with careful monitoring 1

Hypervolemic Hyponatremia (Heart Failure, Cirrhosis)

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

• Discontinue diuretics temporarily if sodium <125 mmol/L 1
• For cirrhotic patients, consider albumin infusion alongside fluid restriction 1
• Avoid hypertonic saline unless life-threatening symptoms are present, as it worsens ascites and edema 1
• Tolvaptan may be considered for persistent hyponatremia despite fluid restriction, but use with extreme caution in cirrhosis due to increased gastrointestinal bleeding risk (10% vs 2% placebo) 1, 4

Special Considerations for Neurosurgical Patients

Distinguish between SIADH and cerebral salt wasting (CSW), as they require opposite treatments. 1, 5

• SIADH: euvolemic, treat with fluid restriction 1
• CSW: hypovolemic with CVP <6 cm H₂O, treat with volume and sodium replacement (normal or hypertonic saline), never fluid restriction 1
• For severe CSW symptoms, use 3% hypertonic saline plus fludrocortisone 0.1-0.2 mg daily 1
• In subarachnoid hemorrhage patients at risk of vasospasm, avoid fluid restriction and consider fludrocortisone or hydrocortisone 1

Critical Correction Rate Guidelines

The maximum correction rate is 8 mmol/L in 24 hours for average-risk patients. 1, 2

• For high-risk patients (advanced liver disease, alcoholism, malnutrition, severe hyponatremia, prior encephalopathy): limit to 4-6 mmol/L per day 1
• Calculate sodium deficit: Desired increase in Na (mEq/L) × (0.5 × ideal body weight in kg) 1
• If overcorrection occurs, immediately discontinue current fluids, switch to D5W, and consider desmopressin 1

Monitoring Requirements

• Severe symptoms: check serum sodium every 2 hours initially 1
• Mild symptoms: check every 4 hours after symptom resolution 1
• Chronic hyponatremia: monitor daily to ensure correction doesn't exceed limits 1
• Watch for osmotic demyelination syndrome signs (dysarthria, dysphagia, oculomotor dysfunction, quadriparesis) typically occurring 2-7 days after rapid correction 1

Common Pitfalls to Avoid

• Never correct chronic hyponatremia faster than 8 mmol/L in 24 hours - this causes osmotic demyelination syndrome 1, 2
• Never use fluid restriction in cerebral salt wasting - this worsens outcomes 1
• Never use hypertonic saline in hypervolemic hyponatremia without life-threatening symptoms 1
• Don't ignore mild hyponatremia (130-135 mmol/L) - it increases fall risk (21% vs 5%) and mortality (60-fold increase with Na <130 mmol/L) 1, 6
• Avoid hypotonic fluids (lactated Ringer's) in any hyponatremia - they worsen the condition 1