Pathophysiology and aetiologies of hypernatremia.

Hypernatremia is a disorder of water balance defined by a serum sodium concentration above 145 mEq/L. It reflects a relative deficiency of free water rather than sodium excess. Under physiological conditions, hypothalamic osmoreceptors maintain plasma osmolality through stimulation of arginine vasopressin release- which promotes renal water conservation- and stimulation of thirst, which drives fluid intake. Hypernatremia develops when these defences fail due to impaired vasopressin secretion or action, diminished thirst, or inability to access water. The resultant hyperosmolality causes osmotic efflux of water from brain cells, leading to cerebral shrinkage and neurological dysfunction. Hypernatremia is most often observed in hospitalized patients, particularly the elderly, the critically ill, and those with impaired consciousness, and is associated with substantial morbidity and mortality. Unreplaced water loss, renal or extrarenal, is the predominant cause, whereas sodium overload is a less frequent mechanism. Accurate diagnosis and carefully titrated correction of water deficit are essential to prevent neurological injury.