{{infobox disease | name	Dehydration | Image Cholera rehydration nurses.jpg| Caption Nurses encourage a patient to drink an oral rehydration solution to reduce dehydration he acquired from cholera.| ICD10 | ICD9 | }}

In physiology and medicine, dehydration (''hypohydration'') is defined as the excessive loss of body fluid. It is literally the removal of water () from an object; however, in physiological terms, it entails a deficiency of fluid within an organism. Dehydration of skin and mucous membranes can be called medical dryness.

There are three types of dehydration: hypotonic or hyponatremic (primarily a loss of electrolytes, sodium in particular), hypertonic or hypernatremic (primarily a loss of water), and isotonic or isonatremic (equal loss of water and electrolytes). In humans, the most commonly seen type of dehydration by far is isotonic (isonatraemic) dehydration which effectively equates with hypovolemia, but the distinction of isotonic from hypotonic or hypertonic dehydration may be important when treating people who become dehydrated. Physiologically, dehydration, despite the name, does not simply mean loss of water, as water and solutes (mainly sodium) are usually lost in roughly equal quantities to how they exist in blood plasma. In hypotonic dehydration, intravascular water shifts to the extravascular space, exaggerating intravascular volume depletion for a given amount of total body water loss. Neurological complications can occur in hypotonic and hypertonic states. The former can lead to seizures, while the latter can lead to osmotic cerebral edema upon rapid rehydration.

Definition

Hypovolemia is specifically a decrease in volume of blood plasma. Furthermore, hypovolemia defines water deficiency only in terms of volume rather than specifically water.

Signs and symptoms

Symptoms may include headaches similar to what is experienced during a hangover, a sudden episode of visual snow, decreased blood pressure (hypotension), and dizziness or fainting when standing up due to orthostatic hypotension. Untreated dehydration generally results in delirium, unconsciousness, swelling of the tongue and, in extreme cases, death.

Dehydration symptoms generally become noticeable after 2% of one's normal water volume has been lost. Initially, one experiences thirst and discomfort, possibly along with loss of appetite and dry skin. This can be followed by constipation. Athletes may suffer a loss of performance of up to 30% and experience flushing, low endurance, rapid heart rates, elevated body temperatures, and rapid onset of fatigue.

Symptoms of mild dehydration include thirst, decreased urine volume, abnormally dark urine, unexplained tiredness, irritability, lack of tears when crying, headache, dry mouth, dizziness when standing due to orthostatic hypotension, and in some cases can cause insomnia. Other possible symptoms include cloudy urine and stinging during urination. Blood tests may show hyperalbuminemia. Mild dehydration also has been shown to negatively impact people’s moods. Experiments by the USDA's Agricultural Research Service has shown that dehydration is associated with confusion, fatigue, and negative moods. Mild dehydration, which includes water losses between 1% and 2%, observed in the experiment are comparable to mild dehydration experienced by people in their everyday lives.

In moderate to severe dehydration, there may be no urine output at all. Other symptoms in these states include lethargy or extreme sleepiness, seizures, sunken fontanel (soft spot) in infants, fainting, and sunken eyes.

The symptoms become increasingly severe with greater water loss. One's heart and respiration rates begin to increase to compensate for decreased plasma volume and blood pressure, while body temperature may rise because of decreased sweating. At around 5% to 6% water loss, one may become groggy or sleepy, experience headaches or nausea, and may feel tingling in one's limbs (paresthesia). With 10% to 15% fluid loss, muscles may become spastic, skin may shrivel and wrinkle (decreased skin turgor), vision may dim, urination will be greatly reduced and may become painful, and delirium may begin. Losses greater than 15% are usually fatal.

In people over age 50, the body’s thirst sensation diminishes and continues diminishing with age. Many senior citizens suffer symptoms of dehydration. Dehydration along with hyperthermia results in the elderly dying suddenly during extreme hot weather.

Diseases of the gastrointestinal tract can lead to dehydration in various ways. Often, dehydration becomes the major problem in an otherwise self-limited illness. Fluid loss may even be severe enough to become life-threatening.

In numerous studies of terminally ill patients who have chosen to die, it has been shown that deaths by terminal dehydration are generally peaceful, and are not associated with suffering when supplemented with adequate pain medication.

Differential diagnosis

In humans, dehydration can be caused by a wide range of diseases and states that impair water homeostasis in the body. These include:

External or stress-related causes

* Prolonged physical activity with sweating without consuming adequate water, especially in a hot and/or dry environment

* Prolonged exposure to dry air, e.g., in high-flying airplanes (5%–12% relative humidity)

* Blood loss or hypotension due to physical trauma

* Diarrhea

* Hyperthermia

* Shock (hypovolemic)

* Vomiting

* Burns

* Lacrimation

* Use of methamphetamine, amphetamine, caffeine and other stimulants

* Excessive consumption of alcoholic beverages

Infectious diseases

* Cholera

* Gastroenteritis

* Shigellosis

* Yellow fever

Malnutrition

* Electrolyte disturbance

** Hypernatremia (also caused by dehydration)

** Hyponatremia, especially from restricted salt diets

* Fasting

Recent rapid weight loss may reflect progressive depletion of fluid volume (the loss of 1 L of fluid results in a weight loss of 1 kg (2.2 lb)).

* Patient refusal of nutrition and hydration

* Inability to swallow (obstruction of the oesophagus)

Other causes of obligate water loss

Severe hyperglycemia, especially in diabetes mellitus

* Glycosuria

*Uremia

Diabetes insipidus

Acute emergency dehydration event

Foodborne illness

Prevention

Dehydration is best avoided by drinking sufficient water. The greater the amount of water lost through perspiration, the more water must be consumed to replace it and avoid dehydration. Since the body cannot tolerate large deficits or excesses in total body water, consumption of water must be roughly concurrent with the loss (in other words, if one is perspiring, one should also be drinking some water frequently).

For routine activities in which a person is not perspiring to any large degree, drinking when one is thirsty is sufficient to maintain hydration. However, during exercise, relying on thirst alone may be insufficient to prevent dehydration from occurring. This is particularly true in hot environments or for those older than 65. For an exercise session, an accurate determination of how much fluid is necessary to consume during the workout can be made by performing appropriate weight measurements before and after a typical exercise session, to determine how much fluid is lost during the workout.

Drinking water beyond the needs of the body entails little risk when done in moderation, since the kidneys will efficiently remove any excess water through the urine with a large margin of safety.

A person's body, during an average day in a temperate climate such as the United Kingdom, loses approximately 2.5 litres of water. This can be through the lungs as water vapor (about 350ml), through the skin by perspiration (100ml) and by diffusion through the skin (350ml), or through the kidneys as urine (1000-2000ml, about 900ml of which is obligatory water excretion that gets rid of solutes). Some water (about 150-200ml, in the absence of diarrhea) is also lost through the bowels. In warm or humid weather or during heavy exertion, however, the water loss can increase by an order of magnitude or more through perspiration; all of which must be promptly replaced. In extreme cases, the losses may be great enough to exceed the body's ability to absorb water from the gastrointestinal tract; in these cases, it is not possible to drink enough water to stay hydrated, and the only way to avoid dehydration is to either pre-hydrate or find ways to reduce perspiration (through rest, a move to a cooler environment, etc.)

A useful rule of thumb for avoiding dehydration in hot or humid environments or during strenuous activity involves monitoring the frequency and character of urination. If one develops a full bladder at least every 3–5 hours and the urine is only lightly colored or colorless, chances are that dehydration is not occurring; if urine is deeply colored or urination occurs only after many hours or not at all, water intake may not be adequate to maintain proper hydration.

When large amounts of water are being lost through perspiration and concurrently replaced by drinking, maintaining proper electrolyte balance becomes an issue. Drinking fluids that are hypertonic or hypotonic with respect to perspiration may have grave consequences (hyponatremia or hypernatremia, principally) as the total volume of water turnover increases.

If water is being lost through mechanisms such as vomiting or diarrhea, an imbalance can develop very quickly into a medical emergency.

During sports events such as marathons, athletes take frequent water stops and water breaks to avoid dehydration.

Treatment

Rehydration, the treatment for minor dehydration often considered the most effective, is drinking water and stopping fluid loss. Plain water restores only the volume of the blood plasma, inhibiting the thirst mechanism before solute levels can be replenished. Solid foods can contribute to fluid loss from vomiting and diarrhea.

In more severe cases, correction of a dehydrated state is accomplished by the replenishment of necessary water and electrolytes (through oral rehydration therapy or fluid replacement by intravenous therapy). As oral rehydration is less painful, less invasive, less expensive, and easier to provide, it is the treatment of choice for mild dehydration. Solutions used for intravenous rehydration must be isotonic or hypotonic. Pure water injected into the veins will cause the breakdown (lysis) of red blood cells (erythrocytes).

When fresh water is unavailable (e.g., at sea or in a desert), seawater, alcohol, and even urine will worsen the condition.

For severe cases of dehydration where fainting, unconsciousness, or other severely inhibiting symptom is present (the patient is incapable of standing or thinking clearly), emergency attention is required. Fluids containing a proper balance of replacement electrolytes are given orally or intravenously with continuing assessment of electrolyte status; complete resolution is the norm in all but the most extreme cases.

Some research indicates that artificial hydration to alleviate symptoms of dry mouth and thirst in the dying patient may be futile.

References

Ira R. Byock, M.D., "Patient Refusal of Nutrition and Hydration: Walking the Ever-Finer Line." ''American Journal Hospice & Palliative Care'', pp. 8–13. (March/April 1995)

Notes

External links

Dehydration Causes and Symptoms

Water requirements in adults

Definition of dehydration by the U.S. National Institutes of Health's MedlinePlus medical encyclopedia

Rehydration Project at rehydrate.org

About dehydration

Category:Causes of death Category:Electrolyte disturbances Category:Nutrition Category:Symptoms

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