What causes hypernatremia?

Causes of Hypernatremia

Hypernatremia (serum sodium >145 mmol/L) is primarily caused by water deficit relative to sodium content, resulting from either inadequate water intake, excessive water loss, or excessive sodium intake. 1

Pathophysiological Classification

1. Water Loss Mechanisms

• Inadequate water intake - often seen in patients with altered mental status, infants, elderly, or those without access to water 2
• Increased insensible losses - fever, burns, tachypnea, or mechanical ventilation 3
• Renal water losses:
  • Diabetes insipidus (central or nephrogenic) - characterized by large volumes of dilute urine 4
  • Osmotic diuresis (hyperglycemia, mannitol administration) 2
  • Polyuric phase of acute tubular necrosis 3
• Gastrointestinal losses - diarrhea, vomiting, fistulas, or drainage tubes 1
• Skin losses - excessive sweating, particularly in endurance athletes 1

2. Sodium Gain Mechanisms

• Iatrogenic causes - most common in clinical settings:
  • Incorrect replacement of transepidermal water loss, especially in very low birth weight infants 1
  • Excessive sodium administration in parenteral nutrition 1
  • Administration of hypertonic saline or sodium bicarbonate 5
  • Excessive sodium-containing antibiotics or other medications 3

3. Combined Mechanisms

• Refeeding syndrome - shifts of electrolytes during nutritional repletion 1
• Reset osmostat - altered set point for antidiuretic hormone release 5

Clinical Context-Specific Causes

In Hospitalized Patients

• Inadequate fluid prescription - most common iatrogenic cause 1
• Unrecognized increased losses - fever, tachypnea, or gastrointestinal losses 2
• Impaired thirst mechanism or access to water - sedated, intubated, or cognitively impaired patients 4

In Pediatric Patients

• Incorrect replacement of transepidermal water loss in premature infants 1
• Inadequate water intake relative to breast milk or formula sodium content 1
• Diarrheal illness with hypotonic fluid losses 6

In Patients with Liver Disease

• Excessive diuretic use causing hypovolemic hypernatremia 1
• Administration of hypertonic sodium solutions for treatment of hyponatremia 1

In Athletes

• Excessive sweating without adequate water replacement 1
• Inappropriate hydration strategies during endurance events 1

Diagnostic Approach

Essential Assessments

• Volume status evaluation - to distinguish between hypovolemic, euvolemic, and hypervolemic hypernatremia 3
• Urine osmolality and sodium - helps determine if kidneys are appropriately concentrating urine 1
• Calculation of free water deficit - guides replacement therapy 4

Management Considerations

• Rate of correction should not exceed 10-15 mmol/L/24h for chronic hypernatremia (>48h) to avoid cerebral edema, seizures, and neurological injury 1
• Acute hypernatremia (<24h) may be corrected more rapidly, sometimes with hemodialysis in severe cases 4
• Treatment must address underlying cause while simultaneously correcting water deficit 2

Common Pitfalls

• Failure to recognize hypernatremia as a serious condition with high mortality 5
• Overly rapid correction leading to cerebral edema and neurological complications 1
• Inadequate monitoring during correction of hypernatremia 3
• Overlooking ongoing losses during treatment 4

Hypernatremia represents a fundamental disorder of water balance that requires prompt recognition and careful management to prevent serious neurological complications and reduce mortality.