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

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

Common symptoms include:

• Abnormal mental status
  • Confusion
  • Decreased consciousness
  • Hallucinations
  • Possible coma
• Convulsions
• Fatigue
• Headache
• Irritability
• Loss of appetite
• Muscle spasms or cramps
• Muscle weakness
• Nausea
• Restlessness
• Vomiting
• Anorexia nervosa

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

A complete physical examination will also be done to find the cause of this condition. During this examination, your doctor may order other tests.

The cause of hyponatremia must be treated, especially in the case of cancer where radiation, chemotherapy, or surgery to remove the tumor may correct the sodium imbalance. Other treatments depend on the type of hyponatremia.

Treatments to correct hyponatremia may include:

• Intravenous (IV) fluids
• Medication for symptoms such as seizures
• Oxygen through a mask or a breathing machine
• Water and salt restriction

The outcome depends on the condition that is causing the problem. In general, acute hyponatremia, which occurs in less than 48 hours, is more dangerous. When sodium levels fall slowly over a period of days or weeks (chronic hyponatremia), the brain cells have time to adjust and swelling is minimal.

• Brain herniation
• Death
• Possible coma

Hyponatremia can be a life-threatening emergency. Call your health care provider if you have symptoms of this condition.

Treating the condition that is causing hyponatremia can help. If you play any demanding sports, drink fluids that contain electrolytes (“sports drinks”). Drinking only water while you take part in high-energy athletic events can lead to acute hyponatremia.

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.