This is a protective mechanism that reduces the degree of cerebral edema it begins on the first day and is complete within several days. Brain adapts itself to hyponatremia by generation of idiogenic osmoles. The serum sodium concentration is usually above 120meq/L. Syndrome of inappropriate ADH release described below (SIADH).Īcute hyponatremia is characterized by onset of symptoms 48 h should be considered “chronic.” Most patients have chronic hyponatremia. As a result there is water retentionĪbnormal ADH secretion e.g. The reduction in tissue perfusion is sensed by baroreceptors at three sites: (i) In the carotid sinus and aortic arch that regulate sympathetic activity and, with significant volume depletion, the release of antidiuretic hormone (ii) In the glomerular afferent arterioles that regulate the activity of the renin-angiotensin system and (iii) in the atria and ventricles that regulate the release of natriuretic peptides. Effective arterial blood volume depletion occurs by two mechanisms: True volume depletion and in edematous patients with heart failure or cirrhosis in whom tissue perfusion is reduced because of a low cardiac output or arterial vasodilation, respectively. Normal but persistent ADH secretion-In volume depletion the effect of decreased volume counteracts the effect of hypoosmolality and ADH stimulation continues to occur. Hyponatremia occurs if there is persistent ADH stimulation which is seen in following situations. Antidiuretic 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. Thirst is sensed by osmoreceptors located in the hypothalamus and leads to the release of anti-diuretic hormone (vasopressin) from the posterior pituitary. Thirst is stimulated by increase in osmolality. Water intake depends upon thirst mechanism. Hyponatremia results from the inability of the kidney to excrete a water load or excess water intake.
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