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Hyponatremia

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Definition:

Hyponatremia involves not having enough sodium in the body fluids outside the cells.

Alternative Names:

Dilutional hyponatremia; Euvolemic hyponatremia; Hypervolemic hyponatremia; Hypovolemic hyponatremia

Causes, incidence, and risk factors:

Sodium is the main cation (positive ion) that circulates in the body fluids outside the cells. It is a critical component in blood pressure maintenance. Sodium is also essential for the proper workings of nerves and muscles.

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

• Hypovolemic hyponatremia -- water and sodium are both lost from the body, but the sodium loss is greater.
• Hypervolemic hyponatremia -- both sodium and water content in the body increase, but the water gain is greater.
• Euvolemic hyponatremia -- there is an increase in total body water, but the sodium content remains constant.

Hyponatremia is the most common electrolyte disorder in the United States. It occurs in approximately 1% of patients admitted to the hospital.

Causes of hyponatremia include:

• Burns
• Vomiting and diarrhea
• Use of diuretics ("water pills"), especially of the type known as thiazide diuretics
• Certain kidney diseases
• Liver cirrhosis
• Congestive heart failure
• Syndrome of inappropriate antidiuretic hormone secretion (SIADH)

SIADH is an inability of the body to excrete dilute urine. Common causes of SIADH are various cancers, central nervous system disorders, medications, hypothyroidism (lower-than-normal thyroid-hormone levels), and extremely stressful conditions, including surgery. 

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.

• Review Date: 9/13/2005
• Reviewed By: Robert Mushnick, M.D., Assistant Clinical Professor, SUNY Downstate Health Center, Brooklyn, NY. Review provided by VeriMed Healthcare Network.

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