Hyponatremia (British: ''hyponatraemia'') is an electrolyte disturbance (a disturbance of the salts in the blood) in which the sodium (''Natrium'' in Latin) concentration in the plasma is lower than normal (''hypo'' in Greek), specifically below 135 mEq/L. The large majority of cases of hyponatremia occurring in adults result from an excess amount or effect of the water retaining hormone known as Antidiuretic Hormone commonly abbreviated as ADH.
Hyponatremia is most often a complication of other medical illnesses in which either fluids rich in sodium are lost (for example because of diarrhea or vomiting), or excess water accumulates in the body at a higher rate than it can be excreted (for example in polydipsia (rarely) or syndrome of inappropriate antidiuretic hormone, SIADH). Regarding sodium loss as a cause of hyponatremia, it is important to note that such losses promote hyponatremia only in an indirect manner. In particular, hyponatremia occurring in association with sodium loss does not reflect inadequate sodium availability as a result of the losses. Rather, the sodium loss leads to a state of volume depletion, with volume depletion serving as a signal for the release of ADH. As a result of ADH-stimulated water retention, blood sodium becomes diluted and hyponatremia results.
There may also be spurious hyponatremia (pseudohyponatremia or factitious hyponatremia) if other substances expand the serum and dilute the sodium (for example, high blood sugar (hyperglycemia) or if a blood constituent leads to the creation of a sodium-free phase in the blood thereby causing the blood plasma volume to be overestimated (e.g. extreme hypertriglyceridemia).
Hyponatremia can also affect athletes who consume too much fluid during endurance events, people who fast on juice or water for extended periods and people whose dietary sodium intake is chronically insufficient.
The diagnosis of hyponatremia relies mainly on the medical history, clinical examination and blood and urine tests. Treatment can be directed at the cause (for example, corticosteroids in Addison's disease) or involve restriction of water intake, intravenous saline or drugs like diuretics, demeclocycline, urea or vaptans (antidiuretic hormone receptor antagonists). Correcting the salt and fluid balance needs to occur in a controlled fashion, as too rapid correction can lead to severe complications such as heart failure or a sometimes irreversible brain lesion known as central pontine myelinolysis.
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