Hyponatremia is the most frequent
electrolyte disorder and the syndrome of inappropriate
antidiuretic hormone secretion (
SIADH) accounts for approximately one-third of all cases. In the diagnosis of
SIADH it is important to ascertain the euvolemic state of extracellular fluid volume, both clinically and by laboratory measurements.
SIADH should be treated to cure symptoms. While this is undisputed in the presence of grave or advanced symptoms, the clinical role and the indications for treatment in the presence of mild to moderate symptoms are currently unclear. Therapeutic modalities include nonspecific measures and means (fluid restriction, hypertonic saline,
urea,
demeclocycline), with fluid restriction and hypertonic saline commonly used. Recently
vasopressin receptor antagonists, called vaptans, have been introduced as specific and direct
therapy of
SIADH. Although clinical experience with vaptans is limited at this time, they appear advantageous to patients because there is no need for fluid restriction and the correction of
hyponatremia can be achieved comfortably and within a short time. Vaptans also appear to be beneficial for physicians and staff because of their efficiency and reliability. The side effects are thirst,
polydipsia and frequency of urination. In any
therapy of chronic
SIADH it is important to limit the daily increase of serum
sodium to less than 8-10 mmol/liter because higher correction rates have been associated with osmotic
demyelination. In the case of vaptan treatment, the first 24 h are critical for prevention of an overly rapid correction of
hyponatremia and the serum
sodium should be measured after 0, 6, 24 and 48 h of treatment. Discontinuation of any vaptan
therapy for longer than 5 or 6 days should be monitored to prevent hyponatremic relapse. It may be necessary to taper the vaptan dose or restrict fluid intake or both.