[Hypo- and hypernatremia]

Hyponatremia is an electrolyte disorder that is defined by a serum sodium concentration of less than 135 mmol/L. Hyponatremia occurs at a high incidence. It is commonly associated with mild to moderate mental impairment. Hypoosmolar hyponatremia occurs in the setting of plasma volume deficiency ("hypovolemia", e. g. after gastrointestinal fluid loss), liver cirrhosis and cardiac failure ("hypervolemic" hyponatremia) and syndrome of inappropriate antidiuretic hormone secretion ("euvolemic" hyponatremia). Excessive antidiuretic hormone and continued fluid intake are the pathogenetic causes of these hyponatremias. Whereas hypovolemic hyponatremia is best corrected by isotonic saline, conventional proposals for euvolemic and hypervolemic hyponatremia consist of the following: fluid restriction, lithium carbonate, demeclocycline, urea and loop diuretic. None of these nonspecific treatments is entirely satisfactory. Recently a new class of pharmacological agents - orally available vasopressin antagonists, collectively called vaptans - have been described. A number of clinical trials using vaptans have been performed already. They showed vaptans to be effective, specific and safe in the treatment of euvolemic and hypervolemic hyponatremia. In Europe the vaptanes are currently certified exclusively for the treatment of euvolemic hyponatremia. Hypernatremia is caused by a relative deficit of free water and often occurs in elderly patients, who have an impaired thirst mechanism or are unable to ask for water. The cornerstone of treatment is administration of free water to correct the relative water deficit.