DefinitionHyponatremia is a metabolic condition in which there is not enough sodium (salt) in the body fluids outside the cells.Alternative NamesDilutional hyponatremia; Euvolemic hyponatremia; Hypervolemic hyponatremia; Hypovolemic hyponatremiaCauses, incidence, and risk factorsSodium is found mostly in 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 imbal