Hyponatremia may present in many medical conditions and requires careful assessment, treatment, and monitoring in order to prevent consequences of rapid fluid shifts in the brain. Sodium (Na) concentrations are maintained physiologically between 135 to 145 mEq/L by a balance of salt and water intake and excretion. Cells throughout the body respond to different serum concentrations of sodium by shifting water through an osmotic process, but nowhere in the body are the shifts as delicate as in the cells of the brain. If a patient has had an acute manifestations of hyponatremia, often seen during a course of intravenous hydration and demonstrates symptoms of hyponatremic encephalopathy, he or she would most benefit from a rapid correction of the serum sodium level. However, if a patient has developed hyponatremia slowly, he or she could be relatively asymptomatic but be at risk for cerebral demyelination (i.e., central pontine myelinolysis) related to sudden corrections in serum sodium levels. Taking a good history and having an understanding of the acuity of hyponatremia will help protect the patient from any devastating consequences of overly rapid correction.

Patients who are dehydrated, have had a recent operation, or have a contributing pulmonary or central nervous system disorder and have been treated with hypotonic fluids are at risk of developing hyponatremic encephalopathy from rapid fluid shifts resulting in cerebral edema. They often present acutely with lethargy, restlessness, seizure, respiratory arrest, or coma. In these circumstances, rapid treatment with 3% saline under the guidance of a specialist is beneficial. The treatment should be directed at increasing the serum sodium by 1 mEq/L/hr until the patient is alert and free of seizures, the serum sodium has increased by 20 mEq/L or a serum sodium level of 125 mEq/L has been achieved. One mL/kg of 3% sodium chloride will raise the serum sodium by approximately 1 mEq/L. Hypertonic (3% saline) should not be used in asymptomatic hyponatremia due to the risk of rapid fluid shifts.