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Water intoxication, also known as water poisoning or dilutional hyponatremia, is a potentially fatal disturbance in brain functions that results when the normal balance of electrolytes in the body is pushed outside safe limits by over-hydration.
Under normal circumstances, accidentally consuming too much water is exceptionally rare. Nearly all deaths related to water intoxication in normal individuals have resulted either from water drinking contests in which individuals attempt to consume large amounts of water, or long bouts of intensive exercise during which electrolytes are not properly replenished, yet huge amounts of fluid are still consumed. Moreover, water cure, a method of torture in which the victim is forced to consume excessive amounts of water, can cause water intoxication.
Water, just like any other substance, can be considered a poison when over-consumed in a specific period of time. Water intoxication mostly occurs when water is being consumed in a high quantity without giving the body the proper nutrients it needs to be healthy.
Excess of body water may also be a result of a medical condition or improper treatment; see "hyponatremia" for some examples. Water is considered the least toxic chemical compound, with a LD50 of 90 g/kg or more in rats.
Low body mass (infants)
It can be very easy for children under one year old to absorb too much water, especially if the child is under nine months old. Because of their small body mass, it is easy to take in a large amount of water relative to body mass and total body sodium stores.
Marathon runners are susceptible to water intoxication if they drink too much while running. This is caused when sodium levels drop below 135 mmol/L when athletes consume large amounts of fluid. This has been noted to be the result of the encouragement of excessive fluid replacement by various guidelines. This is largely been identified in marathon runners as a dilutional hyponatremia. Medical personnel at marathon events are trained to suspect water intoxication immediately when runners collapse or show signs of confusion.
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. People using drugs such as MDMA (often referred to colloquially as "Ecstasy") may overexert themselves, perspire heavily, and then drink large amounts of water to rehydrate, leading to electrolyte imbalance and water intoxication – this is compounded by MDMA use increasing the levels of antidiuretic hormone (ADH), decreasing the amount of water lost through urination. Even people 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.
Psychogenic polydipsia is the psychiatric condition in which patients feel compelled to drink large quantities of water, thus putting them at risk of water intoxication. This condition can be especially dangerous if the patient also exhibits other psychiatric indications (as is often the case), as the care-takers might misinterpret the hyponatremic symptoms.
When an unconscious person is being fed intravenously (for example, total parenteral nutrition) or via a nasogastric tube the fluids given must be carefully balanced in composition to match fluids and electrolytes lost. These fluids are typically hypertonic, and so water is often co-administered. If the electrolytes are not monitored (even in an ambulatory patient) either hypernatremia or hyponatremia may result.
Some neurological/psychiatric medications (Oxcarbazepine, among others) have been found to cause hyponatremia in some patients. Patients with diabetes insipidus are particularly vulnerable due to rapid fluid processing.
At the onset of this condition, fluid outside the cells has an excessively low amount of solutes (such as sodium (hyponatremia) and other electrolytes) in comparison to that inside the cells causing the fluid to shift through (via osmosis) into the cells to balance its concentration. This causes the cells to swell. In the brain, this swelling increases intracranial pressure (ICP). It is this increase in pressure which leads to the first observable symptoms of water intoxication: headache, personality changes, changes in behavior, confusion, irritability, and drowsiness. These are sometimes followed by difficulty breathing during exertion, muscle weakness & pain, twitching, or cramping, nausea, vomiting, thirst, and a dulled ability to perceive and interpret sensory information. As the condition persists, papillary and vital signs may result including bradycardia and widened pulse pressure. The cells in the brain may swell to the point where blood flow is interrupted resulting in cerebral edema. Swollen brain cells may also apply pressure to the brain stem causing central nervous system dysfunction. Both cerebral edema and interference with the central nervous system are dangerous and could result in seizures, brain damage, coma or death.
Water intoxication can be prevented if a person's intake of water does not grossly exceed his or her losses. Healthy kidneys are able to excrete approximately 0.8 to 1 litre of fluid water (0.21 - 0.26 gallons) per hour. However, stress (from prolonged physical exertion), as well as disease states, can greatly reduce this amount.
Mild intoxication may remain asymptomatic and require only fluid restriction. In more severe cases, treatment consists of:
- Diuretics to increase urination, which are most effective for excess blood volume.
- Vasopressin receptor antagonists
- 1991: Andy Warhol 5 years after his death, Warhol's family publicly accused the hospital where he has his gallbladder removed, of causing his death by water intoxication administered post-operatively. A claimed autopsy weight of 150, with his weight being 128 when admitted, was cited as evidence that too much fluid had been given.
- October 24, 1995: Anna Wood, a 15-year-old Australian schoolgirl who died from the effects of water intoxication secondary to use of MDMA (commonly known as ecstasy).
- November 16, 1995: Leah Betts died as the result of drinking too much water, though in the media, her death was initially attributed to taking an ecstasy tablet at her 18th birthday party.
- September 12, 1999: US Air Force basic trainee Micah J. Schindler died of heat stroke, severely complicated by water intoxication, two days after becoming seriously ill during a 5.8 miles (9.3 km) march. The Air Force changed its recruit training procedures as a result.
- June 9, 2002: 4-year-old Cassandra Killpack of Springville, Utah died as a result of water intoxication when her parents forcefully fed her as much as one US gallon (3.8 l) of water in a short period while she was being disciplined. Her mother, Jennette Killpack, was convicted in 2005 of child abuse homicide.
- October 12, 2002: 3-year-old Rosita Gonzalez of Hollywood, Florida died of water intoxication when her babysitter Nancy Gayoso punished her by forcing her to drink three US quarts (2.8 l) of water in a four-hour period. Gayoso was arrested and charged with murder in the first degree on March 10, 2003. After being declared incompetent to stand trial in 2004 and 2005, Gayoso was found competent on March 26, 2007.
- 2003: Walter Dean Jennings, a freshman history major at SUNY Plattsburgh, was pledging the Psi Epsilon Chi "when he was forced to drink urine, stay awake for days and consume vast amounts of alcohol during a 10-day initiation and hazing process." According to PressRepublican.com, "On his last night of pledging the unrecognized fraternity, the 18-year-old was forced to drink gallons of water through a funnel, which caused his brain to swell from water intoxication and ultimately resulted in his death." Eleven members of the fraternity were charged with criminally negligent homicide.
- 2005: In a much-publicized case of fraternity hazing, four members of the Chi Tau House at California State University, Chico pleaded 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 February 2, 2005. He collapsed and died of heart failure due to water intoxication.
- On January 12, 2007, Jennifer Strange, a 28-year-old woman and a mother of three, from Rancho Cordova, California, was found dead in her home by her mother, hours after trying to win one of Nintendo's Wii game consoles. KDND 107.9 "The End" radio station's "Hold Your Wee for a Wii" contest, involved drinking large quantities of water without urinating. A nurse called the radio station to warn them about the danger in which they were putting people, but the disc jockeys rejected the warnings. Lucy Davidson, the winner of the contest, was severely sickened while picking up her prize. Civil charges against the radio station were filed by Jennifer's family, and the family was eventually awarded $16.5 million in the ensuing wrongful death lawsuit. The FCC launched its own investigation to determine if the station violated the terms of its operating license.
- 2008: Jacqueline Henson, a 40-year-old British woman, died after drinking four liters of water in under two hours as part of her LighterLife diet plan.
- Other notable fatalities due to water intoxication include 2002 Boston Marathon competitor Cynthia Lucero, and Washington, D.C. police officer James McBride.
- British actor Anthony Andrews survived a case of water intoxication in 2003. He was performing as Henry Higgins in a revival of the musical My Fair Lady at the time, and consumed up to eight litres of water a day. He was unconscious and in intensive care for three days.
- 2011: Jonathan Paul Dent, 29, became lost during a four-hour walk through Tasmania's Dial Range on April 19, 2011. He called his wife a number of times to say he was lost. Rescue crews found his body in bushland two days later. The coroner found that Mr Dent most likely died from exercise-associated hyponatremia (EAH), a condition caused by drinking too much fluid during prolonged exercise.
- 2012: Savannah Hardin, a 9-year-old Alabama girl, died after being forced to run for hours. 911 was called when she started suffering from seizures and became unresponsive. Three days later she died at the hospital. The cause of death according to her death certificate was "seizure disorder due to hyponatremia."
- 2013: Luana Priscyla Fernandes Soares, a 21-year-old Brazilian woman, died during a radio contest in Brazil, where her group drank 54 liters (14 gallons) of water mixed with Yerba Mate (a plant used in South American to make a tea-like beverage), an average of 5.4 liters/1.4 gallons per person. She collapsed one hour after the contest began. She suffered a stroke during the event and died two days later in the hospital.
- 2013: 20-year-old Dutch student Lisa Nooij died from water intoxication four days after using MDMA on a festival. 
- Hyperkalemia / Hypokalemia
- Hypermagnesemia / Hypomagnesemia
- Hypernatremia / Hyponatremia
- Oral rehydration therapy
- Water urticaria
- Dihydrogen monoxide hoax
- Electrolyte imbalance
- List of unusual deaths
- The dose makes the poison
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-  — see to Section 11: Toxicological Information for the LD50 verification
- Water Intoxication in Infants
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- What Is Diabetes Insipidus?
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- Strange but True: Drinking Too Much Water Can Kill
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