Causes of Hypernatremia
Hypernatremia (serum sodium >145 mmol/L) is primarily caused by either inadequate water intake, excessive water loss, or excessive sodium intake, with inadequate water intake being the most common etiology in clinical practice 1.
Pathophysiological Classification of Hypernatremia
1. Water Loss (Hypovolemic Hypernatremia)
Renal water losses:
- Diabetes insipidus (central or nephrogenic)
- Osmotic diuresis (hyperglycemia, mannitol, urea)
- Diuretic use (especially loop diuretics)
- Post-obstructive diuresis
- Intrinsic renal disease
Extra-renal water losses:
- Gastrointestinal losses (vomiting, diarrhea, fistulas)
- Respiratory losses (tachypnea, mechanical ventilation)
- Skin losses (fever, burns, excessive sweating)
- High output enterostomies
2. Sodium Gain (Hypervolemic Hypernatremia)
- Iatrogenic sodium administration (hypertonic saline, sodium bicarbonate)
- Excessive sodium chloride intake
- Primary hyperaldosteronism
- Cushing's syndrome
- Salt water drowning
3. Combined Water Loss and Inadequate Intake (Euvolemic Hypernatremia)
- Impaired thirst mechanism
- Limited access to water (elderly, debilitated patients)
- Altered mental status preventing water intake
- Inadequate water prescription in hospitalized patients 1
Special Populations and Risk Factors
Neonates and Infants
- Hypernatremia in neonates and very low birth weight infants is often iatrogenic
- Common causes include:
- Incorrect replacement of transepidermal water loss (TEWL)
- Inadequate water intake
- Excessive sodium intake during transition phase 2
Cirrhotic Patients
- While cirrhotic patients more commonly develop hyponatremia, hypernatremia can occur due to:
- Excessive diuretic use
- Gastrointestinal losses
- Inadequate fluid replacement 2
Diagnostic Approach to Hypernatremia
When evaluating hypernatremia, a systematic approach is recommended:
- Exclude pseudohypernatremia
- Confirm glucose-corrected sodium concentrations
- Determine extracellular volume status (hypovolemic, euvolemic, hypervolemic)
- Measure urine sodium levels and osmolality
- Assess urine volume
- Check ongoing urinary electrolyte free water clearance
- Determine arginine vasopressin/copeptin levels when appropriate
- Assess other associated electrolyte disorders 3
Clinical Implications and Management Considerations
Hypernatremia causes neurologic dysfunction due to cellular dehydration and brain cell shrinkage. Symptoms range from mild (thirst, lethargy, irritability) to severe (seizures, coma, intracranial hemorrhage) 1.
Treatment principles include:
- Identifying and addressing the underlying cause
- Replacing water deficit gradually
- Monitoring serum sodium frequently during correction
- Aiming for a sodium reduction rate of 10-15 mmol/L/24h to prevent cerebral edema, seizures, and neurological injury 2, 1
Prevention Strategies
Understanding the root causes of hypernatremia is essential for prevention:
- Ensure adequate fluid prescription in hospitalized patients
- Monitor ongoing fluid losses
- Pay special attention to high-risk patients (elderly, infants, critically ill)
- Regular assessment of electrolytes in patients receiving parenteral nutrition or diuretics 1, 4
By identifying the specific cause of hypernatremia using this classification approach, clinicians can implement targeted interventions to correct the underlying disorder and safely normalize serum sodium levels.