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.
Hyponatremia can be caused by the following:
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.
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.
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.
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.
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.
Mild hyponatremia is treated by reducing water intake and monitoring the use of diuretics.