Hyponatremia: pathophysiology, differential diagnosis and new aspects of treatment.

Hyponatremia is the most frequent electrolyte disorder in clinical medicine. It is usually attributable to primary vasopressin excess, causing the syndrome of inappropriate antidiuresis (SIAD), or to secondary vasopressin stimulation, involving a baroreceptor mechanism. The latter is regularly found in the hyponatremia of liver cirrhosis, cardiac failure and volume contraction. In the first kind of setting the concentrations of creatinine, urea and urate in plasma will be low because of the associated volume expanded state. In the second type of setting they will be elevated because of the circulatory compromise of these patients. The hyponatremia of SIAD may be treated by water restriction, furosemide and substitution of the inadvertent sodium losses by giving 3% NaCl. Baroreceptor hyponatremia is best treated by fluid restriction together with judiciously administered saline. In correcting severe chronic hyponatremia, the rate of correction should not exceed 1 mM/l/h and the corrected serum sodium concentration should not be higher than 130 mM/l. In the foreseeable future oral non-peptide oral vasopressin antagonists will become available. They are expected to become new tools for the treatment of hyponatremia.