It is associated with increased mortality, morbidity and length of hospital stay in patients presenting with a range of conditions. Hyponatremia is therefore both common and important.
Hyponatremia results from the inability of the kidney to excrete a water load or excess water intake. Water intake depends upon thirst mechanism. Thirst is stimulated by increase in osmolality.
Thirst is sensed by osmoreceptors located in the hypothalamus and leads to the release of anti-diuretic hormone (vasopressin) from the posterior pituitary. Anti-diuretic hormone acts on the V2 receptors located at the basolateral aspect of the collecting duct cells and leads to increased aquaporin expression on the luminal aspect of the collecting duct cells which increases water absorption and abolishes thirst.
The most common causes of hyponatremia are the syndrome of inappropriate anti-diuresis (SIAD), diuretic use, polydipsia, adrenal insufficiency, hypovolemia, heart failure, and liver cirrhosis (the latter two are often collectively referred to as “hypervolemic hyponatremia”).
The most common classification system for hyponatremia is based on volume status:
*hypovolemic (decreased total body water with greater decrease in sodium level)
*euvolemic (increased total body water with normal sodium level)
*hypervolemic (increased total body water compared with sodium)
Hyponatremia – serum sodium concentration less than normal
