In hyponatremia, the level of sodium in blood is too low.
A low sodium level has many causes, including consumption of too many fluids, kidney failure, heart failure, cirrhosis, and use of diuretics.
Symptoms result from brain dysfunction.
At first, people become sluggish and confused, and if hyponatremia worsens, they may have muscle twitches and seizures and become progressively unresponsive.
The diagnosis is based on blood tests to measure the sodium level.
Restricting fluids and stopping use of diuretics can help, but severe hyponatremia is an emergency requiring use of medications, intravenous fluids, or both.
(See also Overview of Electrolytes and Overview of Sodium's Role in the Body.)
Causes of Hyponatremia
Hyponatremia occurs when the body contains too little sodium for the amount of fluid it contains. The body may have too much, too little, or a normal amount of fluid. In all cases, however, sodium is diluted. For example, people with severe vomiting or diarrhea lose sodium. If they replace their fluid losses with just water, sodium is diluted.
Disorders, such as kidney disorders (for example, glomerulonephritis) and other disorders (for example, cirrhosis, and heart failure), can cause the body to retain sodium and fluid. Often the body retains more fluid than sodium, which means the sodium is diluted.
Certain conditions may cause people to drink too much water (polydipsia), which can contribute to the development of hyponatremia.
Thiazide diuretics (sometimes called water pills) are a common cause of hyponatremia. These medications increase sodium excretion, which increases water excretion. Thiazide diuretics are usually well-tolerated but can cause hyponatremia in people prone to low sodium, particularly older adults.
Role of vasopressin
Vasopressin (also called antidiuretic hormone) is a substance naturally produced in the body that helps regulate the amount of water in the body by controlling how much water is excreted by the kidneys. Vasopressin decreases water excretion by the kidneys, which retains more water in the body and dilutes the sodium. The pituitary gland produces and releases vasopressin when the blood volume (amount of fluid in the blood vessels) or blood pressure goes down or when levels of electrolytes (such as sodium) become too high.
Pain, stress, exercise, a low blood sugar level, and certain disorders of the heart, thyroid gland, kidneys, or adrenal glands can stimulate the release of vasopressin from the pituitary gland. The following are some of the medications that stimulate the release of vasopressin or enhance its action at the kidney:
Antipsychotic and antidepressant medications
Chlorpropamide (which lowers the blood sugar level)
Methylenedioxymethamphetamine (MDMA; also called ecstasy), a drug that causes excitement and disinhibition (a loosening of control over behaviors), causes people to drink excess water or other beverages and increases vasopressin secretion, causing hyponatremia.
A frequent cause of hyponatremia is the syndrome of inappropriate secretion of antidiuretic hormone (SIADH), in which vasopressin is inappropriately secreted in a variety of other situations (such as certain cancers, infections, and brain disorders).
Other causes of hyponatremia include
Addison disease (underactive adrenal glands)
Blockage of the small intestine
Brain disorders such as head injury, bleeding, stroke, infections, or tumors
Burns, if severe
Cirrhosis (formation of scar tissue in the liver)
Consumption of too much water, as occurs in some mental health disorders
Diarrhea
Hypothyroidism (underactive thyroid gland)
Kidney disorders
Peritonitis (inflammation of the abdominal cavity)
Syndrome of inappropriate secretion of antidiuretic hormone (SIADH)
Vomiting
Symptoms of Hyponatremia
The brain is particularly sensitive to changes in the sodium level in blood. Therefore, symptoms of brain dysfunction, such as sluggishness (lethargy) and confusion, occur first. If the sodium level in blood falls quickly, symptoms tend to develop rapidly and be more severe. Older adults are more likely to have severe symptoms.
As hyponatremia becomes more severe, muscle twitching and seizures may occur. People may become unresponsive, aroused only by vigorous stimulation (stupor), and eventually cannot be aroused (coma). Death may follow.
Diagnosis of Hyponatremia
Measurement of sodium level in the blood
Hyponatremia is diagnosed by measuring the sodium level in blood. Determining the cause is more complex. Doctors consider the person’s circumstances, including other disorders present and medications or illicit drugs taken. Blood and urine tests are done to evaluate the amount of fluid in the body, the concentration of blood, and content of urine.
Treatment of Hyponatremia
Restricting fluid intake
Mild hyponatremia can be treated by restricting fluid intake to less than 1 quart (about 1 liter) per day. If a diuretic or other medication is the cause, the dose is reduced or the medication is stopped. If the cause is a disorder, it is treated.
Occasionally, people are given a sodium solution intravenously, a diuretic to increase excretion of fluid, or both, usually slowly, over several days. These treatments can correct the sodium level.
Some people, especially those with the syndrome of inappropriate secretion of antidiuretic hormone, need long-term treatment for hyponatremia. Fluid restriction alone is often not enough to prevent recurrence of hyponatremia. Salt tablets can be used in people with mild to moderate chronic hyponatremia. Depending on the cause of hyponatremia and the amount of fluid in the person' body, several medications are available for treatment.
Severe hyponatremia is an emergency. To treat it, doctors slowly increase the level of sodium in the blood with intravenous fluids and sometimes with a diuretic. Medications called vaptans, which block vasopressin receptors and prevent the kidneys from responding tovasopressin, are sometimes needed. Increasing the sodium level too rapidly can result in severe and often permanent brain damage.
