Hyponatremia

Last reviewed:December 2019

Last updated:January 2019

Summary

Defined as a serum sodium concentration of <135 mEq/L.

Most common electrolyte disorder encountered in clinical practice.

Can occur in settings of volume depletion, volume overload, or euvolemia.

Serum osmolality, urine osmolality, and urine sodium concentration help to determine the underlying cause.

Cerebral edema is a medical emergency and occurs more frequently when hyponatremia develops over <48 hours. Patients should be treated promptly with hypertonic 3% saline.

Other treatment depends on the underlying cause and may include fluid restriction and stopping causative medications.

Overly rapid correction of serum sodium concentration can lead to the life-threatening complication of myelinolysis and should be avoided.

Definition

Hyponatremia is defined as a serum sodium concentration of <135 mEq/L (normal serum sodium concentration is in the range of 135-145 mEq/L). It is a disorder of water balance reflected by an excess of total body water relative to electrolytes (total body sodium and potassium) leading to low plasma osmolality (i.e., <275 mOsm/kg).[1] Hyponatremia is generally caused by an increase in renal water reabsorption due to release of vasopressin (arginine vasopressin also known as antidiuretic hormone) along with water intake, and can occur in situations of volume depletion, volume overload, or normal volume. (It should be noted that loss of sodium is minor compared with gains in water in most types of hyponatremia.)

This topic principally focuses on hypotonic (hypo-osmolar) hyponatremia, the most common type of hyponatremia.