Hypernatremia: Definition and Initial Management
Hypernatremia is defined as a serum sodium concentration above 145 mEq/L and represents a relative deficiency of free water rather than sodium excess; initial management requires immediate assessment of volume status, calculation of free water deficit, and cautious correction with hypotonic fluids at a rate not exceeding 10-15 mmol/L per 24 hours for chronic cases to prevent cerebral edema. 1, 2, 3
Pathophysiology and Clinical Significance
Hypernatremia reflects an imbalance where total body water decreases relative to sodium content, invariably causing plasma hyperosmolality 2, 4. This hyperosmolality triggers osmotic water efflux from brain cells, leading to cerebral shrinkage and neurological dysfunction 4. The condition develops when normal protective mechanisms fail—specifically impaired vasopressin secretion or action, diminished thirst, or inability to access water 4, 5.
The disorder is most commonly encountered in critically ill ICU patients, the elderly, and those with impaired consciousness, where it carries substantial morbidity and mortality 2, 4. In ICU settings, 10-30% of tube-fed neurosurgical patients develop hypernatremia 1. Patients are at particularly high risk due to sedation, intubation, altered mental status, and fluid restriction that prevents them from controlling free water intake 2.
Diagnostic Approach
Step 1: Confirm True Hypernatremia
Exclude pseudohypernatremia and calculate glucose-corrected sodium concentration (add 1.6 mEq/L to sodium for each 100 mg/dL glucose above 100 mg/dL) 6, 5.
Step 2: Assess Volume Status
Determine extracellular volume status through physical examination 5:
- Hypovolemic hypernatremia: Hypotension, tachycardia, dry mucous membranes, decreased skin turgor, flat neck veins
- Euvolemic hypernatremia: Normal volume status
- Hypervolemic hypernatremia: Edema, elevated jugular venous pressure (rare, usually iatrogenic from sodium-containing fluids)
Step 3: Evaluate Urine Studies
- Urine osmolality and volume: High urine osmolality (>600-800 mOsm/kg) with low volume suggests extrarenal water loss; low urine osmolality (<300 mOsm/kg) with high volume indicates diabetes insipidus 5, 7
- Urine sodium: Helps differentiate renal from extrarenal losses 5
Step 4: Identify Underlying Cause
Common etiologies include 2, 4, 5:
- Unreplaced water losses (most common): Diarrhea, vomiting, excessive sweating, burns
- Renal water losses: Diabetes insipidus (central or nephrogenic), osmotic diuresis
- Impaired access to water: Sedation, intubation, altered mental status, physical restraints
- Iatrogenic: Sodium-containing fluids, tube feeding without adequate free water supplementation
Initial Management Strategy
Calculate Free Water Deficit
Use the formula: Water deficit (L) = 0.6 × body weight (kg) × [(current Na/140) - 1] 2, 5. This provides an estimate for initial fluid replacement, though frequent monitoring is essential to adjust therapy 2.
Determine Correction Rate Based on Chronicity
For chronic hypernatremia (>48 hours):
- Maximum correction rate: 10-15 mmol/L per 24 hours 1, 3, 5
- Slower correction prevents cerebral edema, seizures, and neurological injury 1, 3
- Monitor serum sodium every 4-6 hours during active correction 2
For acute hypernatremia (<24-48 hours):
- More rapid correction is permissible without significant risk of neurological complications 1
- Hemodialysis can be considered for very rapid normalization in acute cases 3
Select Appropriate Replacement Fluid
Primary choice: Electrolyte-free water 2, 7
- Oral free water guided by thirst is ideal when feasible 2
- Parenteral D5W (5% dextrose in water) for patients unable to take oral fluids 2
Alternative: Hypotonic saline solutions 2, 7
- 0.45% NaCl (half-normal saline) can be used in some circumstances
- 0.18% NaCl (quarter-normal saline) provides more free water
Avoid isotonic fluids (0.9% NaCl) in patients with renal concentrating defects, as this will worsen hypernatremia 1.
Address Underlying Cause Simultaneously
- Diabetes insipidus: Administer desmopressin (Minirin) 3
- Hypovolemia: Provide initial volume resuscitation with isotonic fluids before switching to hypotonic replacement 7
- Excessive losses: Replace ongoing losses and ensure adequate free water access 2
Monitoring and Safety Considerations
Frequent Laboratory Monitoring
Check serum sodium every 4-6 hours during active correction to ensure appropriate response and prevent overcorrection 2, 3. Both undercorrection and overcorrection are associated with poor prognosis 1.
Watch for Complications
- Cerebral edema from rapid correction: Manifests as confusion, seizures, neurological deterioration 1, 3
- Osmotic demyelination: Can occur if chronic hypernatremia is corrected too rapidly (>10-15 mmol/L/24h) 3
- Hemorrhagic complications: From vascular stretching and rupture in severe, rapidly developing cases 2
Adjust Renal Replacement Therapy Carefully
When initiating dialysis in patients with chronic hypernatremia, avoid rapid sodium drops by adjusting dialysate composition 3.
Common Pitfalls to Avoid
- Correcting chronic hypernatremia too rapidly (>10-15 mmol/L/24h) induces cerebral edema and seizures 1, 3
- Using isotonic saline in patients with impaired renal water excretion, which worsens hypernatremia 1
- Failing to replace ongoing water losses in addition to calculated deficit 2
- Inadequate monitoring frequency, missing overcorrection or undercorrection 1, 2
- Not addressing the underlying cause while focusing solely on sodium correction 3, 5
Special Populations
Neonates and very low birth weight infants:
- Require particularly careful attention to etiology through assessment of intravascular volume and hydration status 1
- Symptomatic hypovolemia requires volume replacement first 1
- Use slow correction rate of 10-15 mmol/L/24h with daily weight and electrolyte monitoring 1
Neurosurgical patients on tube feeding: