[Hyponatremia].

Hyponatremia is the most common electrolyte disorder in clinical practice and is associated with increased mortality in hospitalized patients. Sodium is the predominant extracellular cation and, together with its anion chloride, constitutes most of the osmoles in the extracellular space. In the presence of an intact thirst sensation, serum osmolality is set by the mechanisms of salt and water homeostasis to ~280-290 mOsm/kg. Hyponatremia results from excess of free water more often than from lack of salt. Free water clearance is regulated by the antidiuretic hormone and depends on adequate delivery of solute to the diluting segments of the nephron. Analysis of hyponatremia is best performed in a systematic manner, taking into account measured or calculated serum osmolality and the volume state as assessed clinically. Urinary osmolality and sodium concentration may facilitate the differential diagnosis. Therapy of hyponatremia should aim at correcting the underlying disorder. Only in severe, symptomatic hyponatremia should hypertonic saline be infused, in which case care should be taken to avoid complications such as osmotic demyelination.