Hypernatremia is most often due to unreplaced water that is lost from the gastrointestinal tract (vomiting or osmotic diarrhea), skin (sweat), or the urine (diabetes insipidus or an osmotic diuresis due to glucosuria in uncontrolled diabetes mellitus or increased urea excretion resulting from catabolism or recovery from renal failure) (table 1) [1,2]. Hypernatremia due to water loss is called dehydration. This is different from hypovolemia, in which both salt and water are lost . (See "General principles of disorders of water balance (hyponatremia and hypernatremia) and sodium balance (hypovolemia and edema)".)
Excessive water loss seldom leads to hypernatremia because the resulting increase in plasma osmolality stimulates thirst (figure 1), which leads to increased intake of fluids that lowers the serum sodium into the normal range. Thus, in patients who have access to water, hypernatremia primarily occurs in those who are unable to sense thirst or respond to thirst normally. This is most commonly seen in infants and in adults with impaired mental status, particularly the elderly . Elderly patients may also have a diminished thirst response to osmotic stimulation via an unknown mechanism [5,6].
Less commonly, hypernatremia results from the administration of salt in excess of water as can occur with hypertonic sodium bicarbonate therapy during a cardiac arrest, inadvertent intravenous administration of hypertonic saline during therapeutic abortion, or salt ingestion. (See "Etiology and evaluation of hypernatremia", section on 'Sodium overload'.)
This topic will focus on the treatment of hypernatremia induced by water loss, which is the most common cause. The treatment of hypernatremia in patients with impaired thirst, with or without diabetes insipidus, and with primary sodium overload will also be reviewed. The causes and evaluation of patients with hypernatremia and the treatment of central and nephrogenic diabetes insipidus are discussed elsewhere. (See "Etiology and evaluation of hypernatremia" and "Treatment of central diabetes insipidus" and "Treatment of nephrogenic diabetes insipidus".)
APPROACH TO THERAPY
Correction of hypernatremia requires the administration of dilute fluids to both correct the water deficit and replace ongoing water losses, and also, when appropriate, interventions to limit further water loss.