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Hyponatremia is an electrolyte disturbance in which the sodium concentration in the serum is lower than normal.Normal serum sodium levels are between 135-145 mEq/L. Hyponatremia is defined as a serum level of less than 135 mEq/L and is considered severe when the serum level is below 125 mEq/L. Sodium is the dominant extracellular cation and cannot freely cross the cell membrane. Its homeostasis is vital to the normal physiologic function of cells.
Hyponatremia is most often a complication of other medical illnesses in which either fluids rich in sodium are lost  or excess water accumulates in the body at a higher rate than can be excreted (for example in congestive heart failure, syndrome of inappropriate antidiuretic hormone, SIADH, or polydipsia).Regarding sodium loss as a cause of hyponatremia, it is important to note that such losses promote hyponatremia in only 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 signal for the release of ADH (anti-diuretic hormone). As a result of ADH-stimulated water retention, blood sodium becomes diluted and hyponatremia results.
The imbalance between sodium and water in blood may occur in three primary ways:
    * Hypervolemic hyponatremia, excess water dilutes the sodium concentration, causing low sodium levels. Hypervolemic hyponatremia is commonly the result of kidney failure, heart failure or liver failure.
    * Euvolemic hyponatremia, normal water levels are combined with low sodium levels. This condition is commonly due to chronic health conditions, cancer or certain medications.
    * Hypovolemic hyponatremia, water and sodium levels are both low. This may occur, for example, when exercising in the heat without replenishing fluid electrolytes or with marked blood loss.
Symptoms of hyponatremia include nausea and vomiting, headache, confusion, lethargy, fatigue, appetite loss, restlessness and irritability, muscle weakness, spasms, or cramps, seizures, and decreased consciousness or coma. The presence and severity of symptoms are associated with the level of serum sodium, with the lowest levels of serum sodium associated with the more prominent and serious symptoms. However, emerging data suggest that mild hyponatremia (serum sodium levels at 131 mEq/L or above) is associated with numerous complications and undiagnosed symptoms.

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