Hyponatremia

Definition

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

Alternative Names

Dilutional hyponatremia; Euvolemic hyponatremia; Hypervolemic hyponatremia; Hypovolemic hyponatremia

Causes

Sodium circulates 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 sodium levels drop in the fluids outside the cells, water will seep into the cells to balance the salt levels. The cells will swell as a result of the excess water. Although most cells can handle this swelling, brain cells cannot, because the skull confines 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 sodium content remains the same
• Hypervolemic hyponatremia -- both sodium and water content in the body increase, but 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
• Drugs that increase urine output (diuretics)
• Kidney diseases
• Liver cirrhosis
• Syndrome of inappropriate antidiuretic hormone secretion (SIADH)
• Vomiting and diarrhea

References

Braunwald E, Fauci AS, Kasper DL, et al., eds. Hyponatremia. In Harrison's Principles of Internal Medicine. 15th ed. New York, NY: McGraw-Hill; 2001:274-76.