Description

• Defined as serum sodium (Na) concentration >145 mEq/L, which usually represents a state of hypertonicity (1),(2)
• Na concentration reflects balance between total body water (TBW) and total body Na. Hypernatremia occurs from deficit of water relative to Na.
• Dehydration refers to hypernatremia from water loss.
• Hypovolemia refers to concomitant water and salt loss.
• Hypernatremia commonly results from net water loss or, more rarely, from primary Na gain (1).
• Hypernatremia will not develop in patients with intact thirst mechanisms who are able to access water.

Epidemiology

Incidence

• More common in elderly and very young
• Occurs in 1% of hospitalized elderly patients (3)
• Seen in about 9% of ICU patients (3)

Etiology and Pathophysiology

• Due to the powerful effect of the thirst mechanism, hypernatremia typically occurs only in patients who cannot readily access water such as infants, intubated patients, and those with altered mental status or patients with hypodipsia (4).
• Water loss out of proportion to salt loss is the most common cause of hypernatremia. The following conditions lead to excessive water loss:
  • Transdermal loss such as burns or excessive sweating (e.g., fever, infants under radiant heaters, heat exposure, extreme exercise)
  • Urinary loss
    • Nephrogenic diabetes insipidus (DI) (congenital or due to renal dysfunction, hypercalcemia, hypokalemia, medication-related, e.g., lithium)
    • Central DI (due to head trauma, stroke, meningitis) (3)
    • Osmotic diuresis: glucose, urea, and mannitol
    • Post-ATN diuresis
  • Gastrointestinal loss
    • Osmotic diarrhea: lactulose, malabsorption, and some types of infectious diarrhea
    • Enterocutaneous fistula
    • Vomiting, NG suction
• Disorders of the thirst mechanism can result in hypernatremia due to reduced water intake (e.g., intracranial lesions, primary hypodipsia, chronic volume expansion in mineralocorticoid excess).
• Excess Na (increase in total body Na) less commonly leads to hypernatremia. The following condition may result in excessive total body Na:
  • IV infusion of hypertonic NaCl or NaHCO₃ during treatment of brain injury, metabolic acidosis, or hyperkalemia (3)
  • Sea water ingestion
  • Excessive use of NaHCO₃ antacid
  • Incorrect infant formula preparation, tube feeding
  • Excessive Na in dialysate solutions
• With acute hypernatremia, the rapid decrease in brain volume can cause rupture of the cerebral veins, leading to focal intracerebral and subarachnoid hemorrhages and possibly irreversible neurologic damage (2).

Genetics
Some forms of DI may be hereditary.

Risk Factors

• Infants/children
• Elderly patients (may also have a diminished thirst response to osmotic stimulation via an unknown mechanism)
• Patients who are intubated/have altered mental status
• Acute gastrointestinal illness
• Poorly controlled diabetes mellitus
• Prior brain injury
• Surgery
• Diuretic therapy, especially loop diuretics
• Lithium treatment

General Prevention

• Treatment/prevention of underlying cause
• Properly prepare infant formula and never add salt to any commercial infant formula.
• Keep patients well hydrated.

Commonly Associated Conditions

• Gastroenteritis
• Altered mental status
• Burns
• Head injury