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The electrolyte disturbance hyponatremia (British hyponatraemia) exists in humans when the sodium (Natrium in Latin) concentration in the plasma falls below 135 mmol/L. At lower levels water intoxication may result, an urgently dangerous condition. Hyponatremia is an abnormality that can occur in isolation or, as most often is the case, as a complication of other medical illnesses. In the case of other mammals, particularly agricultural animals, different indications are relevant. The following refers to humans; an introduction to sodium deficiency in cattle is appended.SymptomsMost patients with chronic water intoxication are asymptomatic, but may have symptoms related to the underlying cause.Severe hyponatremia may cause osmotic shift of water from the plasma into the brain cells. Typical symptoms include nausea, vomiting, headache and malaise. As the hyponatremia worsens, confusion, diminished reflexes, convulsions, stupor or coma may occur. Since nausea is, itself, a stimulus for the release of ADH, which promotes the retention of water, a positive feedback loop may be created and the potential for a vicious circle of hyponatremia and its symptoms exists.
Blood contains 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 urine, sweat, and water vapor. 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. In other words, the body has too much water and not enough electrolytes.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 that are 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 19 sodium levels below 100 mmol/l (2.3 g/l) frequently result in cerebral edema, seizures, coma, and death within a few hours of drinking the excess water. As with alcohol poisoning, the progression from mild to severe symptoms may occur rapidly as the water continues to enter the body from the intestines or intravenously.A person with healthy kidneys can excrete about 900ml/h (0.24 gal/hr). However, this must be modulated by potential water losses via other routes. For example, a person who is perspiring heavily may lose 1 l/h (0.26 gal/hr) of water through perspiration alone, thereby raising the amount of water that must be consumed before the individual crosses 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 fluctuates over time. Over long periods of deficiencies between electrolyte loss and electrolyte intake, a small deficiency may reach the threshold if continued over many hours because of continual negative net electrolyte intake.Sodium is not the only mineral that can become overdiluted from excessive water intake. Magnesium is also excreted in urine. According to the National Institutes of Health, "magnesium deficiency can cause metabolic changes that may contribute to heart attacks and strokes." Intravenous magnesium is used in cardiac care units for cardiac arrhythmias.
Psychogenic polydipsia is a special form of polydipsia, caused by mental disorders.Clinical presentationThe patient drinks large amounts of water, which raises the pressure of the extracellular medium. As a side effect, the antidiuretic hormone level is lowered. The urine produced by these patients will have a low electrolyte concentration and it will be produced in large quantities (polyuria). If the patient is institutionalised, close monitoring by staff is necessary to control fluid intake. In extreme episodes, the patient's kidneys will be unable to deal with the fluid overload, and weight gain will be noted.Atypical patient profilesWhile psychogenic polydipsia is usually not seen outside the population of those with serious mental disorders, it may occasionally be found among others in the absence of psychosis, although there is no extant research to document this other than anecdotal observations. Such persons typically prefer to possess bottled water that is ice cold, consume water and other fluids at excessive levels, and may be falsely diagnosed as suffering from diabetes insipidus, since the chronic ingestion of excessive water can produce symptoms and diagnostic results that mimic mild diabetes insipidus.