[Hypo-osmolar hyponatremic syndromes: physiopathology and clinical aspects]

Hyponatremia in virtually all patients results from water retention due to an inability to excrete ingested water. In most cases, this defect represents the persistent secretion of ADH (such as in effective circulating volume depletion, and in the syndrome of inappropriate ADH secretion), although free water excretion can also be limited in disorders in which ADH levels may be appropriately suppressed (such as in advanced renal failure, and in primary polydipsia). The symptoms of hyponatremia primarily reflect neurologic dysfunction induced by cerebral edema and are related both to the severity and to the rapidity of reductions in the plasma sodium concentration. The degree of cerebral edema which occurs in acute hyponatremia is much less with chronic hyponatremia, because the brain cells lose solutes, leading to the osmotic movement of water out the cells and less brain swelling. In general, hyponatremia is corrected acutely by giving Na+ to patients who are volume-depleted and by restricting water intake in patients who are normovolemic or edematous. The optimal rate of correction should be defined to prevent the risk of central demyelinating lesions.