The normal concentration of sodium in the blood plasma is 136–145 mEq/L (where mEq/L stands for milliequivalents of solute per liter of solvent). Hyponatremia, the most commonly observed electrolyte imbalance, occurs when sodium falls below 130 mEq/L. Plasma sodium levels of 125 mEq/L or less are dangerous and can result in seizures and coma. Such low levels of sodium in the blood are often caused by excessive sweating, overuse of diuretic drugs, and persistent diarrhea. As such, hyponatremia can be caused by over exertion while exercising or playing athletic events, when excessive sweating occurs within the body. Even though it is good to maintain a regular fitness routine for healthy living, allowing the concentration of sodium to deplete in the body can be harmful.
Sodium is a metallic element that usually carries a single positive charge; making it an ion. The sodium ion may be abbreviated as Na+ or simply Na. Sodium can occur as a salt in a crystalline solid. Sodium chloride (NaCl), sodium phosphate (Na2HPO4), and sodium bicarbonate (NaHCO3) are commonly occurring salts. These salts can be dissolved in water or in juices of various foods. Dissolving involves the complete separation of ions, such as sodium and chloride, in common table salt (NaCl).
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. The amount of sodium in blood plasma is typically 140 mEq/L, a much higher amount than is found in intracellular sodium (about 5 mEq/L). This asymmetric distribution of sodium ions is essential for human life. It makes possible proper nerve conduction, the passage of various nutrients into cells, and the maintenance of blood pressure. Sodium is essential for a healthy body. The proper concentration of sodium within the body is equally important.
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.
The concentration of sodium in the blood plasma depends on two things: (1) the total amount of sodium and (2) water in arteries, veins, and capillaries (the circulatory system). The body uses separate mechanisms to regulate sodium and water, but they work together to correct blood pressure when it is too high or low. Too low a concentration of sodium, or hyponatremia, can be corrected either increasing sodium or decreasing body water. The existence of separate mechanisms that regulate sodium concentration account for the fact that there are numerous diseases that can cause hyponatremia, including diseases of the kidney, pituitary gland, and hypothalamus.
Anyone can get hyponataremia, especially if one excessively sweats during a hot day and does not replenish necessary fluids into the body, especially salt. However, a person is more apt to get hyponatremia if they are very young or very old because both groups have problems regulating their fluid intake and expressing their desire for water and food. In 2010, a U.S. study found that 30% of nursing home patients had contracted hyponatremia some time within the year. In addition, females are more likely to get hyponatremia than are men. Hospitalized patients are also at higher risk.
Hyponatremia can be caused by abnormal consumption or excretion of dietary sodium or water and by diseases that impair the body's ability to regulate them. Maintenance of a low salt diet for many months or excessive sweat loss during a race on a hot day can present a challenge to the body to conserve adequate sodium levels. While these conditions alone are not likely to cause hyponatremia, it can occur under special circumstances. For example, hyponatremia often occurs in patients taking diuretic drugs who maintain a low sodium diet. Diuretic drugs that frequently cause hyponatremia include furosemide (Lasix), bumetanide (Bumex), and thiazide diuretics. Diuretics enhance the excretion of sodium into the urine, with the goal of correcting high blood pressure; 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.
Severe and prolonged diarrhea can cause hyponatremia. Severe diarrhea, causing the daily output of around 270–340 oz. (8–10 L) 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 most likely to cause 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 5 gal. (20 L) of water per day. Excessive drinking of beer, which is mainly water and low in sodium, can also produce hyponatremia when combined with a poor diet.
Marathon running, under certain conditions, leads to hyponatremia. Races of 25–50 mi. (40–80 km) can result in the loss of great quantities (2.0–2.6 gal. [8–10 L]) of sweat, which contains both sodium and water. Studies show that about 30% of marathon runners experience mild hyponatremia during a race. 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.
Hyponatremia caused by drinking too much fluid, not by excessive sodium (salt) loss from the body leads to diluted salt levels within the body while these levels remain the same in the brain. To compensate (to move from low areas of salt concentration to high), fluids move to the brain. Because the brain is contained within the skull, the excess fluid causes the brain to swell. This causes the first signs of hyponatremia: headache, nausea, and blurred vision.
It is now known within the medical community that drinking too much water is not healthy and can be harmful to the body. Knowledgeable athletes drink small amounts of fluids on a regular basis when exercising strenuously to prevent too much water from entering the body. General advice is to drink frequently and in small amounts, and avoid feeling thirsty.
Hyponatremia can be acute, developing over 48 hours or less, or chronic, taking days or sometimes weeks to develop. Patient symptoms are often milder if the hyponatremia occurs over time because the body, particularly the brain, has time to compensate for the lower serum (blood) sodium concentration. When hyponatremia occurs rapidly, there is less time for the brain to adjust to the lowered sodium level and clinical manifestations related to cerebral edema are common. If the cerebral edema is severe enough, the result may be herniation of the brainstem and death. Rapid identification and treatment of the hyponatremic state is critical in averting these serious complications.
Symptoms of moderate hyponatremia include:
Severe hyponatremia can lead to seizures, decreased consciousness, and eventually coma. These neurological symptoms are thought to result from the movement of water into brain cells, causing them to swell and disrupt their functioning (cerebral edema).
The symptoms of hyponatremia are the same as from dehydration. If a person has been exercising for long periods and has been drinking a lot of water, then symptoms such as confusion may occur. Hyponatremia could be the problem. Anyone who is confused, has seizures, or passes out, should be immediately transported to the hospital. The last thing to give such a person in this case is more fluids because it will further swell the brain and possibly kill the person. A trained medical professional should treat a person with hyponatremia.
The condition known as hyponatremia is diagnosed with tests that measure the concentration of sodium in the body, along with other related substances. These tests include the measurement of orthostatic vital signs that assess the volume of sodium and other essential substances within the body. In addition, an examination of the cardiopulmonary and neurological systems of the body is important in learning more about the patient's condition with respect to hyponatremia.
Hyponatremia is diagnosed by acquiring a blood sample and then measuring the concentration of sodium ions in the blood. 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 be considered.
The goals of treatment of acute hyponatremia are to raise the sodium level above 120 mEq/L and to raise the sodium level rapidly by 4–6 mEq/L within the first one to two hours of treatment. Patients with severe hyponatremia should be treated with an infusion of hypertonic (3%) saline to raise the serum sodium level by 4–6 mEq/L. Further correction could be dangerous and is avoided in most patients unless serious neurological symptoms are present.
Patients exhibiting mild symptoms as a result of chronic hyponatremia should be treated cautiously to avoid serious and irreversible damage to the central nervous system that could be precipitated by rapid and overcorrection of hyponatremia. The goals of therapy in these patients is slow and cautious correction of serum sodium levels and identification of the cause of the hyponatremia.
Vasopressin receptor antagonists have been found effective for the treatment of hyponatremia. These so-called “vaptan” drugs help to control blood pressure and kidney function in order to stabilize sodium levels back to normal. As of 2017, these vaptan drugs are either in clinical use or clinical trials. Some of them include: conivaptan, nelivaptan, lixivaptan, mozavaptan, relcovaptan, satavaptan, and tolvaptan. The U.S. Food and Drug Administration (FDA) approved conivaptan (Vaprisol) in 2004 for hyponatremia caused by syndrome of inappropriate antidiuretic hormone (SIADH). In addition, lixivaptan was in phase III clinical trials, as of May 2010, for patients with hyponatremia.
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.
Patients who take diuretic medications must be checked regularly for the development of hyponatremia, especially while exercising.
Hyponatremia is more likely to occur in infants and in the elderly. Individuals in these groups are less able to express thirst and are less able to independently regulate their fluid intake. In order to prevent hyponatremia, caregivers of infants and the elderly should be educated regarding the importance of adequate and appropriate fluid intake.
Athletes and people who exercise should take precautions when participating in high-intensity sporting events and activities. One should drink only as much fluid as one loses to sweating during exercise. This amount is generally considered to be about 34 oz. (1 L) of water per hour during extended exercise; however, this amount can vary depending on the intensity of the activity, the surrounding temperature, and other such factors.
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Tom Brody, PhD
Revised by Melinda Granger Oberleitner, RN, DNS, APRN, CNS
Revised by William A. Atkins, BB, BS, MBA