[Hyponatremia: from physiopathology to practice]

Hypotonic hyponatremia is the most common electrolyte abnormality encountered in hospitalized patients. It is often asymptomatic but associated with increased mortality and morbidity. Prompt recognition of the underlying cause using a systematic physiology based approach and careful evaluation the chronicity of the hyponatremia is mandatory for an optimal management. One should first document hypotonicity, and then assess the renal response to hypotonicity to exclude water intoxication, and the extracellular volumes. The further step will identify hyponatremia due to volemic stimulation of vasopressin associated to extracellular dehydration (corrected by isotonic saline infusion) or to oedematous states. After exclusion of hypocorticism and hypothyroidism, one would conclude to inappropriate secretion of antidiuretic hormone whose etiology would have to be established. The use of hypertonic saline solutions should be restricted to the treatment of acute and severe hyponatremia with evidence of brain damage. Chronic hyponatremia should be correct slowly to avoid the risk of osmotic demyelination syndrome. Water restriction is commonly recommended in inappropriate secretion of antidiuretic hormone or in hypervolemia with a questionable effectiveness. The recent development of vasopressin receptor antagonists (vaptans) will modify our therapeutic approaches. Yet, further studies are needed to document their additional impact on morbidity and mortality.