Hyponatremia or decreased serum sodium occurs when serum sodium concentration is less than 136 meq/liter. Sodium is the main ion in the extracellular fluid compartment. Its metabolism is intertwined with water metabolism.
Reduction in serum sodium can be classified into three categorizes:
1) Hypovolemic hyponatremia- which is the reduction in both serum sodium and total body water.
2) Noromovolemic hyponatremia- where sodium is low but total body water is normal.
3) Hypervolemic hyponatremia- where sodium is low but total body water is increased.
1) Gastrointestinal- diarrhea and vomiting, where fluid and sodium are lost
2) Kidney diseases- interstitial nephritis, polycystic kidney disease, medullary cystic disease, diuretics
3) Third spacing- in these situations, sodium and water are lost into different compartments of the body- such as:
d. Intestinal obstruction
1) medications- barbiturates, carbamazepine
2) adrenal disorders
4) SIADH or inappropriate secretion of antidiuretic hormone
5) Primary polydipsia- or drinking excess water
8) Post operative conditions
1) heart failure
3) chronic kidney diseases such as nephrotic syndrome
Hyponatremia in AIDS
It is seen in about 50% of hospitalized AIDS patients. The etiology is unknown.