Overhydration occurs when the body takes in more water than it excretes and its normal sodium level is diluted. This can result in digestive problems, behavioral changes, brain damage, seizures, or coma. An adult whose heart, kidneys, and pituitary gland are functioning properly would have to drink more than two gallons of water a day to develop water intoxication. This condition is most common in persons whose kidney function is impaired and may occur when doctors, nurses, or other healthcare professionals administer greater amounts of water-producing fluids and medications than the person's body can excrete.
Infants seem to be at greater risk for developing overhydration. The Centers for Disease Control and Prevention has declared that babies are especially susceptible to oral overhydration during the first month of life, when the kidneys' filtering mechanism is too immature to excrete fluid as rapidly as older infants do. Breast milk or formula provides all the fluids a healthy baby needs. Water should be given slowly, sparingly, and only during extremely hot weather. Overhydration, which has been cited as a hazard of infant swimming lessons, occurs whenever a baby drinks too much water, excretes too little fluid, or consumes and retains too much water.
Overhydration is the most common electrolyte imbalance in hospitals, occurring in about 2 percent of all people.
Causes and symptoms
Drinking too much water rarely causes overhydration when the body's systems are working normally. People with heart, kidney, or liver disease are more likely to develop overhydration because their kidneys are unable to excrete water normally. It may be necessary for people with these disorders to restrict the amount of water they drink and/or adjust the amount of salt in their diets.
Since the brain is the organ most susceptible to overhydration, a change in behavior is usually the first symptom of water intoxication. The person may become confused, drowsy, or inattentive. Shouting and delirium are common. Symptoms of overhydration may include blurred vision, muscle cramps and twitching, paralysis on one side of the body, poor coordination, nausea and vomiting, rapid breathing, sudden weight gain, and weakness. The person's complexion is normal or flushed. Blood pressure is sometimes higher than normal, but elevations may not be noticed even when the degree of water intoxication is serious.
Overhydration can cause acidosis (a condition in which blood and body tissues have an abnormally high acid content), anemia, cyanosis (a condition that occurs when oxygen levels in the blood drop sharply), hemorrhage, and shock. The brain is the organ most vulnerable to the effects of overhydration. If excess fluid levels accumulate gradually, the brain may be able to adapt to them, and the person will have only a few symptoms. If the condition develops rapidly, confusion, seizures, and coma are likely to occur.
When to call the doctor
A doctor should be called when a person becomes confused, drowsy, or inattentive. Persons should also consider calling a doctor when a person experiences blurred vision, muscle cramps and twitching, paralysis on one side of the body, poor coordination, nausea and vomiting, rapid breathing, sudden weight gain, or weakness.
Before treatment can begin, a doctor must determine whether an individual's symptoms are due to overhydration, in which excess water is found within and outside cells, or excess blood volume, in which high sodium levels prevent the body from storing excess water inside the cells. Overhydration is characterized by excess water both within and around the body's cells, while excess blood volume occurs when the body has too much sodium and cannot move water to reservoirs within the cells. In cases of overhydration, symptoms of fluid accumulation do not usually occur. On the other hand, in cases of excess blood volume, fluid tends to accumulate around cells in the lower legs, abdomen, and chest. Overhydration can occur alone or in conjunction with excess blood volume, and differentiating between these two conditions may be difficult.
Mild overhydration can generally be corrected by following a doctor's instructions to limit fluid intake. In more serious cases, diuretics may be prescribed to increase urination, although these drugs tend to be most effective in the treatment of excess blood volume. Identifying and treating any underlying condition (such as impaired heart or kidney function) is a priority, and fluid restrictions are a critical component of every treatment plan.
In people with severe neurologic symptoms, fluid imbalances must be corrected without delay. A powerful diuretic and fluids to restore normal sodium concentrations are administered rapidly at first. When the person has absorbed 50 percent of the therapeutic substances, blood levels are measured. Therapy is continued at a more moderate pace in order to prevent brain damage as a result of sudden changes in blood chemistry.
Mild water intoxication is usually corrected by drinking less than a quart of water a day for several days. Untreated water intoxication can be fatal, but this outcome is quite rare.
Cyanosis—A bluish tinge to the skin that can occur when the blood oxygen level drops too low.
Electrolytes—Salts and minerals that produce electrically charged particles (ions) in body fluids. Common human electrolytes are sodium chloride, potassium, calcium, and sodium bicarbonate. Electrolytes control the fluid balance of the body and are important in muscle contraction, energy generation, and almost all major biochemical reactions in the body.
Shock—A medical emergency in which the organs and tissues of the body are not receiving an adequate flow of blood. This deprives the organs and tissues of oxygen and allows the build-up of waste products. Shock can be caused by certain diseases, serious injury, or blood loss.
People should be careful not to drink excessive amounts of water. Persons with impaired kidney function must exert extra caution.
Chronic illness, malnutrition, a tendency to retain water, and kidney diseases and disorders increase the likelihood of a person's becoming overhydrated. Infants and the elderly seem to be at increased risk for overhydration, as are people with certain mental disorders or alcoholism.
Greenbaum, Larry A. "Electrolytes and Acid-Base Disorders." In Nelson Textbook of Pediatrics, 17th ed. Edited by Richard E. Behrman et al. Philadelphia: Saunders, 2003, pp. 191–241.
Horne, Mima M. Pocket Guide to Fluid, Electrolyte, and Acid-Base Balance, 5th ed. Amsterdam: Elsevier Science, 2004.
Kokko, Juha P. "Fluids and Electrolytes." In Cecil Textbook of Medicine, 22nd ed. Edited by Lee Goldman et al. Philadelphia: Saunders, 2003, pp. 669–87.
Singer, Gary G., and Barry M. Brenner. "Fluid and Electrolyte Disturbances." In Harrison's Principles of Internal Medicine, 15th ed. Edited by Eugene Braunwald et al. New York: McGraw-Hill, 2001, pp. 271–82.
American College of Sports Medicine. 401 W. Michigan St., Indianapolis, IN 46202–3233. Web site: <www.acsm.org/>.
"Dehydration and fluid maintenance." National Guideline Clearinghouse. Available online at <www.guideline.gov/summary/summary.aspx?doc_id=3305> (accessed January 7, 2005).
"Overhydration." Merck Manual. Available online at <www.merck.com/mmhe/sec12/ch158/ch158c.html> (accessed January 7, 2005).
"Renal Subsystem." Virtual Naval Hospital. Available online at <www.vnh.org/EWSurg/ch10/10RenalSubsystem.html> (accessed January 7, 2005).
L. Fleming Fallon, Jr., MD, DrPH