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.
Causes
Hypovolemic hyponatremia
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:
a. Burns
b. Pancreatitis
c. Peritonitis
d. Intestinal obstruction
Normovolemic hyponatremia
1) medications- barbiturates, carbamazepine
2) adrenal disorders
3) hypothyroidism
4) SIADH or inappropriate secretion of antidiuretic hormone
5) Primary polydipsia- or drinking excess water
6) Pain
7) Stress
8) Post operative conditions
Hypervolemic hyponatremia
1) heart failure
2) cirrhosis
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.
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