Water intoxication

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In medicine, water intoxication (also known as hyperhydration or water poisoning) is a potentially fatal disturbance in brain function that results when the normal balance of electrolytes in the body is pushed outside of safe limits by a very rapid intake of water.

Physiology of water intoxication

Water itself is not toxic to the body in any amount. However, body fluids contain electrolytes (particularly sodium compounds, such as sodium chloride) in concentrations that must be held within very narrow limits. Water enters the body orally or intravenously, and leaves the body primarily in the urine and in sweat. If water enters the body more quickly than it can be removed, body fluids are diluted and a potentially dangerous shift in electrolyte balance occurs.

Most water intoxication is caused by hyponatremia, an overdilution of sodium in the blood plasma, which in turn causes an osmotic shift of water from extracellular fluid (outside of cells) to intracellular fluid (within cells). The cells swell as a result of changes in osmotic pressure and may cease to function. When this occurs in the cells of the central nervous system and brain, water intoxication is the result. Additionally, many other cells in the body may undergo cytolysis, wherein cell membranes unable to stand abnormal osmotic pressures rupture, killing the cells. Initial symptoms typically include light-headedness, sometimes accompanied by nausea, vomiting, headache and/or malaise. Plasma sodium levels below 100 mmol/L (2.3g/L) frequently result in cerebral edema, seizures, coma, and death within a few hours of drinking the excess water. As with an alcohol poisoning, the progression from mild to severe symptoms may occur rapidly as the water continues to enter the body from the stomach or intravenously.

A person with two healthy kidneys can rid themselves of about 1.5 litres of water per hour at maximum filtration (other studies find the limit to be as little as 0.9L/hr [link]). Consuming as little as 1.8 liters of water in a single sitting may prove fatal for a person adhering to a low-sodium diet, or 3 liters for a person on a normal diet. However, this must be modulated by potential water losses via other routes. For example, a person who is perspiring heavily may lose 1 L/hr of water through perspiration alone, thereby raising the threshold for water intoxication. The problem is further complicated by the amount of electrolytes lost in urine or sweat, which is variable within a range controlled by the body's regulatory mechanisms. Water intoxication can be prevented by consuming water that is isotonic with water losses, but the exact concentration of electrolytes required is difficult to determine and evolves over time, and the greater the time period involved, the smaller the disparity that may suffice to produce electrolyte imbalance and water intoxication.

Persons at high risk of water intoxication

Certain activities and conditions put a person at elevated risk of water intoxication.

Runners

Marathon runners are susceptible to water intoxication if they drink only water while running. Although sweat is relatively hypotonic compared with body fluids, marathon runners perspire heavily for long periods, potentially causing their sodium levels to drop when they consume large amounts of fluids to quench their thirst. The replacement fluids may not contain sufficient sodium to replace what has been lost, and this puts them at high risk for water intoxication. Medical personnel at marathon events are trained to immediately suspect water intoxication when runners collapse or show signs of confusion. Properly designed electrolyte-replacement drinks and some sports drinks include electrolytes that make them roughly isotonic with sweat, which helps to prevent water intoxication.

Note that overconsumption of sodium (in drinks or food), as well as inadequate intake of water, can cause hypernatremia, a disorder that is nearly the opposite of water intoxication and equally dangerous. Improper use of salt tablets can cause hypernatremia.

Overexertion and heat stress

Any activity or situation that promotes heavy sweating can lead to water intoxication when water is consumed to replace lost fluids. Persons working in extreme heat and/or humidity for long periods must take care to drink and eat in ways that help to maintain electrolyte balance. Persons abusing drugs such as MDMA may overexert themselves, perspire heavily, and then drink large amounts of water to rehydrate, leading to electrolyte imbalance and water intoxication. Even persons who are resting quietly in extreme heat or humidity may run the risk of water intoxication if they drink large amounts of water over short periods for rehydration.

Psychiatric conditions

Psychogenic polydipsia is the psychiatric condition in which patients feel compelled to drink large quantities of water. The condition is often a single symptom in a broader syndrome of psychiatric indications. Patients suffering from psychogenic polydipsia are at high risk of water intoxication, especially as the initial symptoms of lightheadedness and confusion may be misdiagnosed by care-takers as due to other causes.

Unusual water losses in disease

Diarrhea and vomiting can result in very large electrolyte losses, and although drinking water will replace lost water, the lost electrolytes may not be adequately replaced, which can result in water intoxication. Replacement fluids for vomiting and diarrhea should be properly balanced to make them isotonic with the fluids lost in these conditions. Special formulations exist for oral rehydration therapy in these cases.

A great many disorders can affect electrolyte balance, especially disorders of the kidneys. Diuretic therapy, mineralocorticoid deficiency, osmotic diuresis (as in the hyperglycemia of uncontrolled diabetes), and the multiple disorders associated with AIDS are other common causes of electrolyte imbalance, although they do not always produce water intoxication.

Iatrogenic water intoxication

When an unconscious person is being fed intravenously (e.g., total parenteral nutrition) or via a nasogastric tube, the fluids given must be carefully balanced in composition to match fluids and electrolytes lost. If the fluids administered are hypotonic with respect to fluids lost, electrolyte imbalance and water intoxication may result. The latter may not be immediately obvious in an unconscious patient. The electrolyte status of patients on TPN must be monitored carefully even when they are ambulatory.

Famous cases of water intoxication

In a much-publicized case of fraternity hazing, four members of the Chi Tau House at California State University, Chico pled guilty to forcing 21-year-old student Matthew Carrington to drink excessive amounts of water while performing calisthenics in a frigid basement as part of initiation rites on the 2^nd of February 2005[link]. He collapsed and died of heart failure due to water intoxication.

Other fatalities due to water intoxication include Leah Betts [link], Anna Wood [link], 2002 Boston Marathon competitor Cynthia Lucero [link], and Washington, D.C. police officer James McBride [link]. New Zealand race-walker Craig Barrett collapsed during the last kilometer of the 50-km walk in the 1998 Commonwealth Games in a non-fatal case of water intoxication.

Prevention of water intoxication

Water intoxication can be prevented if a person's intake of water and electrolytes closely matches his losses. The body's regulatory mechanisms provide a very generous margin of safety if the two are imbalanced, but some extreme activities (such as heavy, prolonged physical exertion), as well as disease states, can overwhelm or impair these mechanisms. Avoiding situations that provoke extreme or prolonged perspiration and/or drinking fluids that are specially balanced to replace lost electrolytes can help to prevent intoxication. Eating regularly can provide needed electrolytes if only normal water is available for rehydration.

Sports drinks are popular among athletes because they provide the necessary electrolytes to support extended exercise. They help keep the body balanced and carrying the right amount of fluids. However, not all drinks advertised as sports drinks are suitable for this purpose, and professional advice should be sought for potentially risky situations such as those described above.

Note that a person's innate sense of thirst is triggered by overall dehydration, not by changes in electrolytes. Thus, it is possible to develop water intoxication while trying to satisfy thirst, if one drinks a great deal of water over a short period. A dangerous drop in electrolytes, such as the hyponatremia that leads to water intoxication, will not have any effect on thirst.

For people suffering from dehydration due to the heavy perspiration associated with heavy exertion or heat stress, drinking water to rehydrate is much more important than avoiding water intoxication, since the former is extremely common and the latter is rare. One should never avoid drinking water under such conditions; instead, other steps should be taken to ensure that electrolytes are replaced as well, as noted above.

See also

• electrolytes
• hyponatremia
• hypernatremia
• oral rehydration therapy

References

• [The Merck Manual]

 

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