What are the causes of hypernatremia?

Causes of Hypernatremia

Hypernatremia (serum sodium >145 mmol/L) is primarily caused by inadequate water intake or increased water loss relative to sodium, with less common causes being excessive sodium intake. 1

Classification by Volume Status

1. Hypovolemic Hypernatremia (Water and Sodium Loss, with Greater Water Loss)

• Renal Losses:

  • Osmotic diuresis (e.g., uncontrolled diabetes, mannitol)
  • Diuretic use (especially loop diuretics)
  • Post-obstructive diuresis
  • Intrinsic renal disease

• Extra-renal Losses:

  • Gastrointestinal losses (vomiting, diarrhea, fistulas)
  • Skin losses (excessive sweating, burns, severe dermatitis)
  • Respiratory losses (tachypnea, mechanical ventilation)
  • High output enterostomies 1, 2

2. Euvolemic Hypernatremia (Pure Water Loss)

• Central Diabetes Insipidus: Deficiency of antidiuretic hormone (ADH)
• Nephrogenic Diabetes Insipidus: Kidney resistance to ADH action 3
• Insensible losses: Fever, hyperthermia, burns
• Inadequate water intake: Common in elderly, infants, or patients with altered mental status 1, 4

3. Hypervolemic Hypernatremia (Sodium Gain)

• Iatrogenic sodium administration (hypertonic saline, sodium bicarbonate)
• Excessive sodium ingestion (salt tablets, seawater ingestion)
• Primary hyperaldosteronism
• Cushing's syndrome
• Excessive administration of sodium-containing antibiotics 5, 2

Risk Factors and Special Populations

• Elderly patients: Decreased thirst perception, impaired access to water
• Infants: Inability to communicate thirst, dependency on caregivers for fluid
• Critically ill patients: Multiple risk factors including impaired thirst, inability to access water
• Patients with neurological disorders: Impaired thirst mechanism
• Patients receiving diuretics: Especially in cirrhotic patients 3, 1
• Patients on mechanical ventilation: Increased insensible losses
• Patients with diabetes insipidus: Congenital or acquired forms 3

Pathophysiological Mechanisms

Hypernatremia develops when there is:

1. Impaired thirst mechanism or restricted access to water: Most common cause in clinical practice 1
2. Excessive water loss: Through renal or extra-renal routes
3. Inadequate ADH secretion or action: Leading to polyuria and concentrated hypernatremia 3
4. Iatrogenic causes: Incorrect fluid replacement, especially in hospitalized patients 1

Clinical Pearls and Pitfalls

• Always assess volume status when evaluating hypernatremia, as this guides treatment approach
• Hypernatremia that develops over >48 hours should be corrected slowly (8-10 mmol/L/day) to prevent cerebral edema 6
• In congenital nephrogenic diabetes insipidus, avoid salt-containing solutions (especially 0.9% NaCl) during hypernatremic dehydration as they worsen the condition 3
• Hospital-acquired hypernatremia is often preventable through careful monitoring of fluid balance and regular electrolyte assessment 1
• The severity of symptoms correlates with both the degree of hypernatremia and the rapidity of onset 5

Understanding the specific cause of hypernatremia is crucial for appropriate management and prevention of recurrence. The diagnostic approach should include assessment of volume status, urine osmolality, and urine sodium to differentiate between the various etiologies.