Dilutional hyponatremia; Euvolemic hyponatremia; Hypervolemic hyponatremia; Hypovolemic hyponatremia

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.�

When sodium levels drop in the fluids outside the cells, water will seep into the cells in an attempt to balance the concentration of salt outside the cells. The cells will swell as a result of the excess water. While most cells can accommodate this swelling, brain cells cannot, because the skull confines them. Therefore, most symptoms of hyponatremia will result from brain swelling.

Common symptoms include:

• Loss of appetite
• Nausea
• Vomiting
• Headache
• Restlessness
• Fatigue
• Irritability
• Abnormal mental status
  • Possible coma
  • Hallucinations
  • Consciousness, decreased
  • Confusion
• Convulsions
• Muscle weakness
• Muscle spasms or cramps

• Serum sodium
• Serum osmolality
• Urine osmolality
• Urine sodium

In addition to laboratory studies, a complete physical examination will be done to find the underlying cause of this condition. During this examination, your doctor may order additional tests.

The cause of hyponatremia must be treated, especially in the case of a� malignancy where radiation, chemotherapy, or surgical removal of the tumor may correct the sodium imbalance. Other treatments vary, depending on the type of hyponatremia.

Treatments to correct hyponatremia may include:

• Intravenous (IV) fluids
• Supplemental�oxygen, through a mask or a ventilator
• Water and salt restriction
• Medication to combat symptoms such as seizures

The outcome is related to the underlying disease or condition. In general, acute hyponatremia, one that occurs in less than 48 hours, is more dangerous. When sodium levels fall gradually over a period of days or weeks (chronic hyponatremia), the brain cells have time to compensate and swelling is minimal.

• Possible coma
• Brain herniation
• Death

Hyponatremia can be a life-threatening emergency! Call your health care provider if symptoms of hyponatremia occur.

Prompt treatment of underlying conditions may be helpful. If you are involved in any demanding athletic activity, it is important to drink fluids that contain electrolytes (sports drinks�). Drinking only water while engaging in challenging athletic events can easily lead to acute hyponatremia.

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.�