Hyponatremia

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Small cell carcinoma

Hyponatremia

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.

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Inhyponatremia, 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 bodyincrease, but the water gain is greater.
Euvolemic hyponatremia -- there is an increasein 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:

Craig S. Hyponatremia. Available at: http://www.emedicine.com/EMERG/topic275.htm. Accessed 10/27/2005.

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.