Causes of Hypernatremia (High Sodium)
Primary Mechanisms
Hypernatremia reflects an imbalance in water balance, most commonly resulting from increased loss of free water relative to sodium excretion, rather than excessive sodium intake. 1, 2
Water Loss (Most Common)
- Renal water losses occur in diabetes insipidus (central or nephrogenic), osmotic diuresis, or use of loop diuretics 1, 2
- Extrarenal water losses result from gastrointestinal losses (diarrhea, vomiting), excessive sweating, burns, or respiratory losses in mechanically ventilated patients 2, 3
- Impaired water intake due to altered mental status, sedation, intubation, impaired thirst mechanism, or lack of access to water is particularly common in ICU patients 4, 2
Sodium Gain (Less Common)
- Administration of hypertonic saline solutions, sodium bicarbonate, or hypertonic feeding formulas 1, 2
- Excessive sodium-containing IV fluids in critically ill patients 2
- Salt poisoning or ingestion (rare) 1
High-Risk Populations
- Critically ill ICU patients are at particularly high risk due to inability to control free water intake from sedation, intubation, altered mental status, and fluid restriction 2, 3
- Elderly patients with impaired thirst mechanisms or limited access to water 4
- Patients with diabetes insipidus who cannot concentrate urine appropriately 1, 4
Diagnostic Approach
Volume status assessment and urine osmolality are the key diagnostic tools. 1
- Urine osmolality >300 mOsm/kg suggests extrarenal water losses or sodium gain 1
- Urine osmolality <300 mOsm/kg indicates renal water losses (diabetes insipidus) 1
- Assess for signs of volume depletion (hypotension, tachycardia, poor skin turgor) versus volume overload 2
Treatment Principles
The primary treatment involves replacing free water deficit while addressing the underlying cause, with correction rate being critical to avoid cerebral edema. 1, 4
Correction Rate Guidelines
- For chronic hypernatremia (>48 hours): Do not reduce sodium by more than 8-10 mmol/L per day to prevent osmotic demyelination syndrome 1, 2
- For acute hypernatremia (<24 hours): More rapid correction is safe, and hemodialysis can be considered for severe cases 1
- Frequent laboratory monitoring (every 2-4 hours initially) is essential to ensure appropriate response 2, 3
Fluid Replacement Strategy
- Hypotonic fluids (5% dextrose in water or 0.45% saline) are preferred for free water replacement 1, 4
- Oral free water guided by thirst is ideal when possible 2
- Parenteral fluid replacement is usually necessary in critically ill patients 2
- Various calculations for estimating free water deficit can guide initial therapy, though frequent monitoring is more important than the specific formula used 2
Special Considerations
- In diabetes insipidus: Desmopressin (DDAVP) administration is indicated alongside fluid replacement 1
- When starting renal replacement therapy: Be cautious of rapid sodium drops in patients with chronic hypernatremia 1
- In ICU patients: Free water requirements should be routinely assessed and judicious electrolyte-free water prescribed for those at risk 2
Clinical Consequences
- Central nervous system dysfunction including confusion, altered consciousness, or coma 1, 2
- Pronounced thirst in awake patients 1
- Risk of hemorrhagic complications or death from vascular stretching and rupture in severe cases 2
- Hypernatremia is an independent risk factor for increased mortality in critically ill patients 3