Hyponatremia: a diagnostic problem.

Hyponatremia, a common clinical problem, may result from a wide variety of causes. Treatment can vary from fluid restriction to rehydration, depending on the pathogenesis of the disorder. Essential to evaluation of hyponatremic patients is correct estimation of the status of the extracellular fluid volume, because this determines the type of therapy: a patient who is hypovolemic will not benefit from fluid restriction; a euvolemic patient should not be "rehydrated." To distinguish between these two conditions requires a searching history and clinical examination, including measurement of the blood pressure both while the subject is standing and lying. Not all patients with the classical biochemical features of the syndrome of inappropriate secretion of vasopressin (low plasma [Na+] and osmolality, high urinary [Na+] and osmolality) will have the disorder, because these same features may also be present in other hyponatremic disorders, including hypovolemic hyponatremia.