Causes and Treatment of Hypernatremia
Hypernatremia is caused by either excessive sodium intake or free water loss, and treatment should focus on correcting the underlying cause while carefully restoring normal serum sodium levels at a rate not exceeding 0.4 mmol/L/hour for chronic cases to prevent neurological complications. 1
Causes of Hypernatremia
Hypernatremia (serum sodium >145 mmol/L) can be classified based on volume status:
1. Hypovolemic Hypernatremia
- Renal losses: Osmotic diuresis, diuretic use, post-obstructive diuresis
- Extra-renal losses: Excessive sweating, diarrhea, burns, respiratory losses
- Clinical signs: Orthostatic hypotension, dry mucous membranes, tachycardia
2. Euvolemic Hypernatremia
- Diabetes insipidus:
- Neurogenic/central: Due to traumatic, vascular, or infectious events affecting ADH production
- Nephrogenic: Due to medication (especially lithium), hypokalemia, or kidney disease
- Excessive insensible losses: Fever, hyperthermia, mechanical ventilation
- Impaired thirst mechanism: Common in elderly or neurologically impaired patients
3. Hypervolemic Hypernatremia
- Iatrogenic: Administration of hypertonic saline or sodium bicarbonate
- Endocrine: Primary hyperaldosteronism
- Excessive sodium intake: Salt tablet overconsumption
Treatment Approach
Step 1: Assess Severity and Chronicity
- Mild: Sodium 146-150 mmol/L
- Moderate: Sodium 151-160 mmol/L
- Severe: Sodium >160 mmol/L
- Acute: Developed within 48 hours
- Chronic: Developed over >48 hours
Step 2: Determine Volume Status and Underlying Cause
Step 3: Calculate Water Deficit
Water deficit (L) = Total body water × [(Current Na⁺/140) - 1] (Total body water ≈ 0.6 × weight in kg for men; 0.5 × weight in kg for women)
Step 4: Correct Hypernatremia Based on Volume Status
For Hypovolemic Hypernatremia:
- First: Restore intravascular volume with isotonic fluids (0.9% saline)
- Then: Administer hypotonic fluids (0.45% saline or 5% dextrose) to correct free water deficit
For Euvolemic Hypernatremia:
- Administer hypotonic fluids (0.45% saline or 5% dextrose)
- For diabetes insipidus:
- Central: Desmopressin (DDAVP)
- Nephrogenic: Treat underlying cause, thiazide diuretics, amiloride, NSAIDs
For Hypervolemic Hypernatremia:
- Loop diuretics to promote sodium excretion
- Hypotonic fluid replacement
- Treat underlying condition (e.g., hyperaldosteronism)
Step 5: Monitor Rate of Correction
- For acute hypernatremia (<48 hours): Correction can be more rapid but still cautious
- For chronic hypernatremia (>48 hours): Correct at a rate not exceeding 0.4 mmol/L/hour or 10 mmol/L/day 1
- Monitor serum sodium every 2-4 hours during initial treatment
Important Considerations
- Avoid overly rapid correction: Too rapid correction of chronic hypernatremia can lead to cerebral edema and neurological damage 1, 2
- Special populations: Elderly and critically ill patients are at higher risk for hypernatremia due to impaired thirst mechanisms and altered mental status 3
- Monitor for complications: Watch for seizures, altered mental status, and cerebral edema during treatment
- Underlying causes: Always identify and treat the underlying cause while correcting sodium levels
Potential Pitfalls
- Failure to recognize diabetes insipidus: Consider this diagnosis in patients with polyuria and hypernatremia
- Overcorrection: Can lead to cerebral edema and neurological damage
- Undercorrection: Persistent hypernatremia can cause cellular dehydration and brain damage
- Ignoring volume status: Treatment approach differs significantly based on volume status
- Medication-induced hypernatremia: Certain medications like lithium can cause nephrogenic diabetes insipidus 1
Hypernatremia, while less common than hyponatremia, carries significant morbidity and mortality, particularly in critically ill patients 3. Careful assessment of volume status, calculation of water deficit, and appropriate rate of correction are essential for successful management.