• Definition

    Hyponatremia is a metabolic condition in which there is not enough sodium (salt) in the body fluids outside the cells.

    Alternative Names

    Dilutional hyponatremia; Euvolemic hyponatremia; Hypervolemic hyponatremia; Hypovolemic hyponatremia

    Causes, incidence, and risk factors

    Sodium is found mostly in the body fluids outside the cells. It is very important for maintaining blood pressure. Sodium is also needed for nerves and muscles to work properly.

    When the amount of sodium in fluids outside cells drops, water moves into the cells to balance the levels. This causes the cells to swell with too much water. Although most cells can handle this swelling, brain cells cannot, because the skull bones confine them. Brain swelling causes most of the symptoms of hyponatremia.

    In hyponatremia, the imbalance of water to salt is caused by one of three conditions:

    • Euvolemic hyponatremia -- total body water increases, but the body's sodium content stays the same
    • Hypervolemic hyponatremia -- both sodium and water content in the body increase, but the water gain is greater
    • Hypovolemic hyponatremia -- water and sodium are both lost from the body, but the sodium loss is greater

    Hyponatremia is the most common electrolyte disorder in the United States.

    Causes of hyponatremia include:

    • Burns
    • Congestive heart failure
    • Diarrhea
    • Diuretic medications, which increase urine output
    • Kidney diseases
    • Liver cirrhosis
    • Syndrome of inappropriate antidiuretic hormone secretion (SIADH)
    • Sweating
    • Vomiting