Hyponatremia (Low Blood
Sodium Level)

Hyponatremia (Low Blood Sodium Level): Overview

Hyponatremia is a lower-than-normal concentration of sodium in the blood.  Sodium is an electrolyte that helps with nerve and muscle function, and also helps to maintain blood pressure.

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About 40% of the body's sodium is contained in bone.  Approximately 2-5% occurs within organs and cells and the remaining 55% is in blood plasma and other extracellular fluids.  Sodium must be maintained at a specific concentration in the blood and the fluid surrounding the body's cells for the body to function properly.  The body maintains a balance of sodium in the blood by matching the amount of sodium we take in with the amount excreted (put out) by the kidneys.  Hyponatremia occurs when the level of sodium in the blood becomes diluted by too much water intake.

The body continually regulates its handling of sodium.  When dietary sodium is too high or low, the intestines and kidneys respond to adjust concentrations to normal.  During the course of a day, the intestines absorb dietary sodium while the kidneys excrete a nearly equal amount of sodium into the urine.  If a low sodium diet is consumed, the intestines increase their efficiency of sodium absorption, and the kidneys reduce its release into urine.

Causes and Development; Contributing Risk Factors

Hyponatremia can be caused by the following:

  • Kidney disorders in which the kidneys have difficulty eliminating fluids.
  • Disorders in organs that control the body's regulation of sodium or water.  The adrenal gland secretes a hormone called aldosterone that travels to the kidney, where it causes the kidney to retain sodium by not excreting it into the urine.  Addison's disease causes hyponatremia as a result of low levels of aldosterone due to damage to the adrenal gland.  The hypothalamus and pituitary gland are also involved in sodium regulation by making and releasing vasopressin, known as anti-diuretic hormone, into the bloodstream.  Like aldosterone, vasopressin acts in the kidney, but it causes it to reduce the amount of water released into urine.  With more vasopressin production, the body conserves water, resulting in a lower concentration of plasma sodium.  Certain types of cancer cells produce vasopressin, leading to hyponatremia.
  • Abnormal consumption or excretion of dietary sodium or water.
  • Diuretic drugs used to treat high blood pressure.  These drugs make the kidneys produce more urine, which can wash away too much sodium, especially when the patient is following a low sodium diet.  This is especially of concern in elderly patients, who have a reduced ability to regulate the concentrations of various nutrients in the bloodstream.  Diuretic drugs that frequently cause hyponatremia include furosemide (Lasix), bumetanide (Bumex), and most commonly, the thiazides.  Diuretics enhance the excretion of sodium into the urine, with the goal of correcting high blood pressure.  However, too much sodium excretion can result in hyponatremia.  Usually only mild hyponatremia occurs in patients taking diuretics, but when combined with a low sodium diet or with the excessive drinking of water, severe hyponatremia can develop.
  • Some psychiatric disorders cause people to drink extremely large quantities of water, which can result in hyponatremia.
  • Drinking excess water sometimes causes hyponatremia, because the absorption of water into the bloodstream can dilute the sodium in the blood.  This cause of hyponatremia is rare, but has been found in psychotic patients who compulsively drink more than 20 liters of water per day.
  • Receiving too much fluid intravenously.
  • Excessive drinking of beer, which is mainly water and low in sodium, can also produce hyponatremia when combined with a poor diet.
  • Maintenance of a low salt diet for many months.
  • Severe and prolonged diarrhea can also cause hyponatremia.  Severe diarrhea, causing the daily output of 8-10 liters of fluid from the large intestines, results in the loss of large amounts of water, sodium, and various nutrients.  Some diarrheal diseases release particularly large quantities of sodium and are therefore most likely to cause hyponatremia.
  • Prolonged vomiting.
  • Excessive sweat loss during a race on a hot day can present a challenge to the body to conserve adequate sodium levels.  Marathon running, under certain conditions, leads to hyponatremia.  Races of 25-50 miles can result in the loss of great quantities (8 to 10 liters) of sweat, which contains both sodium and water.  Studies show that about 30% of marathon runners experience mild hyponatremia during a race.  But runners who consume only pure water during a race can develop severe hyponatremia because the drinking water dilutes the sodium in the bloodstream.  Such runners may experience neurological disorders as a result of the severe hyponatremia and require emergency treatment.
  • Hypothalamus and pituitary disorders.
  • Certain types of cancer.

Hyponatremia is more likely to occur in people whose kidneys do not function properly, as well as in those with heart failure, cirrhosis of the liver, and Addison's disease, in which underactive adrenal glands excrete too much sodium.

Signs and Symptoms

Because the brain is very sensitive to sodium levels, low sodium causes symptoms including confusion and lethargy.  The patient may feel nauseated, and experience muscle twitching, which can progress to seizures.  Eventually, severe hyponatremia can lead to coma and death.

Symptoms of moderate hyponatremia include tiredness, disorientation, headache, muscle cramps, and nausea.  Severe hyponatremia can lead to seizures and coma.  These neurological symptoms are thought to result from the movement of water into brain cells, causing them to swell and disrupt their functioning.

Diagnosis and Tests

In most cases of hyponatremia, doctors are primarily concerned with discovering the underlying disease causing the decline in plasma sodium levels.

Hyponatremia is diagnosed by acquiring a blood sample, preparing plasma, and using a sodium-sensitive electrode for measuring the concentration of sodium ions.  Normal blood sodium levels are 136 to 145 milliequivalents per liter (mEq/L) of blood.  A patient with hyponatremia will have a blood sodium level lower than 136 mEq/L.

Unless the cause is obvious, a variety of tests are subsequently run to determine if sodium was lost from the urine, diarrhea, or from vomiting.  Tests are also used to determine abnormalities in aldosterone or vasopressin levels.  The patient's diet and use of diuretics must also be considered.

Treatment and Prevention

Severe hyponatremia can be treated by infusing a solution of 5% sodium chloride in water into the bloodstream.  Moderate hyponatremia due to use of diuretics or an abnormal increase in vasopressin is often treated by instructions to drink less water each day.  Hyponatremia due to adrenal gland insufficiency is treated with hormone injections.

When hyponatremia is severe, it is considered a medical emergency that is treated in a hospital.  Very low plasma sodium levels can result in seizures and coma.  The doctor will slowly increase the blood sodium levels with intravenous (IV) fluids.  Additional treatment depends upon the underlying cause of hyponatremia.

If you take diuretics, make sure you have your blood sodium levels checked on a regular basis.  If you are sick and symptoms include vomiting or diarrhea, follow your doctor's recommendations for replacing lost fluids with clear liquids or electrolyte replacement fluids.  People who run marathons and train intensely should drink electrolyte replacement fluids to keep sodium levels balanced.

Prognosis; Complications

Hyponatremia is just one manifestation of a variety of disorders.  While hyponatremia can easily be corrected, the prognosis for the underlying condition that causes it varies.

Death that occurs during hyponatremia is usually due to other features of the disease rather than to the hyponatremia itself.

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Recommendations for Hyponatremia (Low Blood Sodium Level):


Increased Salt Consumption

The concentration of sodium in the blood plasma depends on the total amount of sodium and water in the circulatory system.  Hyponatremia, can be corrected either by increasing sodium or by decreasing body water.

Reduced Water Consumption

Mild hyponatremia is treated by reducing water intake and monitoring the use of diuretics.

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