Hypernatremia reflects a net water loss or a hypertonic
sodium gain, with inevitable hyperosmolality. Severe symptoms are usually evident only with acute and large increases in plasma
sodium concentrations to above 158-160 mmol/l. Importantly, the sensation of intense thirst that protects against severe
hypernatremia in health may be absent or reduced in patients with altered mental status or with hypothalamic lesions affecting their sense of thirst and in infants and elderly people. Non-specific symptoms such as
anorexia,
muscle weakness,
restlessness,
nausea, and
vomiting tend to occur early. More serious signs follow, with altered mental status,
lethargy, irritability, stupor, and
coma. Acute brain shrinkage can induce vascular
rupture, with cerebral
bleeding and
subarachnoid hemorrhage. However, in the vast majority of cases, the onset of hypertonicity is low enough to allow the brain to adapt and thereby to minimize cerebral
dehydration. Organic osmolytes accumulated during the adaptation to
hypernatremia are slow to leave the cell during
rehydration. Therefore, if the
hypernatremia is corrected too rapidly,
cerebral edema results as the relatively more hypertonic ICF accumulates water. To be safe, the rate of correction should not exceed 12 mEq/liter/day.