Hypernatremia Causes
Hypernatremia (serum sodium >145 mmol/L) most commonly results from inadequate water intake or excessive water loss relative to sodium, rather than from primary sodium excess. 1
Primary Mechanisms
Water Loss (Most Common)
Excessive water loss is the predominant cause of hypernatremia, occurring through multiple routes:
- Transepidermal water loss (TEWL) - particularly in very low birth weight infants (VLBWI) during the transition phase, where inadequate replacement of insensible losses leads to iatrogenic hypernatremia 1
- Gastrointestinal losses - voluminous diarrhea causing hypotonic fluid depletion 1
- Renal water losses - nephrogenic diabetes insipidus or other renal concentrating defects where the kidneys cannot adequately conserve water 1
- Severe burns - causing massive evaporative water losses 1
Inadequate Water Intake
Impaired access to water or inability to respond to thirst represents a critical cause:
- Impaired thirst mechanism - common in elderly patients or those with neurologic impairment 2
- Lack of access to water - patients who are bedridden, hospitalized, or otherwise unable to obtain fluids independently 2
- Transient diabetes insipidus - particularly in neurosurgical patients following brain injury 1
Excessive Sodium Intake (Rare)
Primary sodium excess is an uncommon but important iatrogenic cause:
- Inadvertent excessive sodium administration during parenteral nutrition, especially in neonates 1
- Inappropriate formula preparation - excessive solute load from incorrectly concentrated infant formula 3
- Hypertonic saline administration - particularly when isotonic fluids are given to patients with renal concentrating defects 1
Clinical Context-Specific Causes
Neonatal/Pediatric Population
In VLBWI and neonates, hypernatremia is often iatrogenic:
- Incorrect replacement of TEWL during the first days of life 1
- Inadequate water intake relative to insensible losses 1
- Excessive sodium intake through parenteral nutrition 1
Neurosurgical Patients
Central causes predominate in this population:
- Transient diabetes insipidus following neurosurgery or traumatic brain injury 1
- Impaired thirst mechanism from hypothalamic injury 2
Critical Pitfall to Avoid
Administering isotonic maintenance fluids to patients with significant renal concentrating defects (such as nephrogenic diabetes insipidus) will cause or worsen hypernatremia, as these patients require hypotonic fluid replacement. 1 This represents a dangerous exception to the general pediatric guideline favoring isotonic maintenance fluids.
Associated Morbidity and Mortality
Hypernatremic dehydration carries the highest morbidity and mortality rate among all types of dehydration, primarily due to CNS dysfunction. 4 Rapid correction poses additional risk, as overly fast reduction of serum sodium can cause cerebral edema, seizures, and neurological injury. 1 A reduction rate of 10-15 mmol/L per 24 hours is recommended for established hypernatremia. 1