Management of Hyponatremia
Initial Assessment and Classification
Hyponatremia is defined as serum sodium <135 mmol/L and should be further investigated when levels are <131 mmol/L. 1
Your initial workup must include:
- Serum and urine osmolality 1
- Urine sodium concentration 1
- Uric acid level 1
- Assessment of extracellular fluid volume status (hypovolemic, euvolemic, or hypervolemic) 1, 2
Volume status assessment is critical but challenging—physical examination alone has poor accuracy (sensitivity 41.1%, specificity 80%). 1 Look specifically for:
- Hypovolemic signs: orthostatic hypotension, dry mucous membranes, decreased skin turgor, flat neck veins 1
- Hypervolemic signs: peripheral edema, ascites, jugular venous distention 1
- Euvolemic: absence of both hypovolemic and hypervolemic signs 1
A urine sodium <30 mmol/L predicts response to saline with 71-100% positive predictive value in hypovolemic states. 1 Serum uric acid <4 mg/dL suggests SIADH with 73-100% positive predictive value. 1
Treatment Based on Symptom Severity
Severe Symptomatic Hyponatremia (Seizures, Coma, Altered Mental Status)
For severe symptoms, immediately administer 3% hypertonic saline with a target correction of 6 mmol/L over 6 hours or until severe symptoms resolve. 1, 3
- Give 100 mL boluses of 3% saline over 10 minutes, repeatable up to three times at 10-minute intervals until symptoms improve 1
- Total correction must not exceed 8 mmol/L in 24 hours to prevent osmotic demyelination syndrome 1, 2, 3
- Consider ICU admission for close monitoring 1
- Monitor serum sodium every 2 hours during initial correction 1
Calculate sodium deficit: Desired increase in Na (mEq/L) × (0.5 × ideal body weight in kg) 1
Mild to Moderate Symptomatic or Asymptomatic Hyponatremia
Treatment depends on volume status (see below). The correction rate should not exceed 8 mmol/L in 24 hours. 1, 2
Treatment Based on Volume Status
Hypovolemic Hyponatremia
Discontinue diuretics and administer isotonic saline (0.9% NaCl) for volume repletion. 2, 4
- Initial infusion rate: 15-20 mL/kg/h, then 4-14 mL/kg/h based on response 1
- Urine sodium <30 mmol/L confirms appropriate diagnosis 1
- Once euvolemic, reassess and adjust management 1
Euvolemic Hyponatremia (SIADH)
Fluid restriction to 1 L/day is the cornerstone of treatment for SIADH. 1, 2, 5
- If no response to fluid restriction, add oral sodium chloride 100 mEq three times daily 1
- For resistant cases, consider pharmacological options: 1
Critical distinction: In neurosurgical patients, differentiate SIADH from cerebral salt wasting (CSW), as CSW requires volume and sodium replacement, NOT fluid restriction. 1, 2 CSW is characterized by true hypovolemia with CVP <6 cm H₂O, high urine sodium >20 mmol/L despite volume depletion, and evidence of extracellular volume depletion. 1
Hypervolemic Hyponatremia (Heart Failure, Cirrhosis)
Implement fluid restriction to 1-1.5 L/day for serum sodium <125 mmol/L. 1, 2, 4
- Discontinue diuretics temporarily if sodium <125 mmol/L 2
- In cirrhotic patients, consider albumin infusion alongside fluid restriction 2
- Avoid hypertonic saline unless life-threatening symptoms are present, as it worsens ascites and edema 1, 2
- For persistent severe hyponatremia despite fluid restriction, consider vaptans (tolvaptan) with extreme caution 2, 6
Important: In cirrhosis, it is sodium restriction (not fluid restriction) that results in weight loss, as fluid passively follows sodium. 2
Critical Correction Rate Guidelines
The maximum correction rate is 8 mmol/L in 24 hours for all patients. 1, 2, 3
High-risk patients require even slower correction (4-6 mmol/L per day): 1, 2
- Advanced liver disease
- Alcoholism
- Malnutrition
- Prior encephalopathy
- Severe hyponatremia (<120 mmol/L)
- Hypokalemia, hypophosphatemia
If overcorrection occurs (>8 mmol/L in 24 hours):
- Immediately discontinue current fluids and switch to D5W (5% dextrose in water) 2
- Consider administering desmopressin to slow or reverse the rapid rise 2
- Target relowering to bring total 24-hour correction to ≤8 mmol/L 2
Special Considerations for Neurosurgical Patients
Cerebral Salt Wasting (CSW)
CSW should be treated with replacement of sodium and intravenous fluids, NOT fluid restriction. 1, 2
- For severe symptoms: 3% hypertonic saline plus fludrocortisone (0.1-0.2 mg daily) in ICU 1
- Hydrocortisone may be used to prevent natriuresis in subarachnoid hemorrhage patients 1
- Never use fluid restriction in CSW—this worsens outcomes 1, 2
Subarachnoid Hemorrhage
- Fludrocortisone may be considered in patients at risk of vasospasm 1
- Do not treat with fluid restriction in patients at risk of vasospasm 1
- Hydrocortisone may prevent natriuresis 1
Pharmacological Interventions
Vaptans (Vasopressin Receptor Antagonists)
Tolvaptan is FDA-approved for euvolemic and hypervolemic hyponatremia. 6
- Starting dose: 15 mg once daily, titrate to 30-60 mg based on response 6
- Avoid fluid restriction during first 24 hours to prevent overly rapid correction 6
- Monitor sodium every 8 hours initially 6
- Contraindicated with strong CYP3A inhibitors (ketoconazole increases tolvaptan AUC 5.4-fold) 6
- Use with extreme caution in cirrhosis due to 10% risk of gastrointestinal bleeding (vs. 2% placebo) 6
Monitoring and Prevention of Complications
Monitoring Schedule
- Severe symptoms: Check serum sodium every 2 hours during initial correction 1
- Mild symptoms: Check every 4 hours after symptom resolution 1
- Continue daily monitoring until stable 1
Osmotic Demyelination Syndrome
Watch for signs typically occurring 2-7 days after rapid correction: 1, 2
- Dysarthria
- Dysphagia
- Oculomotor dysfunction
- Quadriparesis
Prevention is key—never exceed 8 mmol/L correction in 24 hours. 1, 2, 3
Common Pitfalls to Avoid
- Overly rapid correction (>8 mmol/L in 24 hours) leading to osmotic demyelination syndrome 1, 2
- Using fluid restriction in cerebral salt wasting—this worsens outcomes 1, 2
- Inadequate monitoring during active correction 1
- Failing to recognize and treat the underlying cause 1
- Using hypertonic saline in hypervolemic hyponatremia without life-threatening symptoms 1, 2
- Ignoring mild hyponatremia (130-135 mmol/L) as clinically insignificant—it increases fall risk (21% vs. 5%) and mortality (60-fold increase) 1, 3
- Using normal saline in SIADH or hypervolemic states—this can worsen hyponatremia 1