[Hyponatremia and syndrome of inappropriate ADH secretion (SIADH)].

The syndrome of inappropriate ADH secretion (SIADH), also termed ''syndrome of inappropriate antidiuresis (SIAD)'', is an often unrecognized cause of hypotonic hyponatremia, arising from ectopic release of ADH in lung cancer or as a side effect of various drugs. In SIADH, hyponatremia results from selectively impaired water excretion by the kidney, whereas the external Na+ balance is normally regulated. Despite the increase in total body water, only a slight reduction of urine output and modest edema are usually seen. Renal function and acid-base balance are generally preserved, while subclinical neurological impairment may occasionally become life-threatening, when hyponatremia has an abrupt onset. The major clinical variants of SIADH are reviewed here, with particular emphasis on causes, iatrogenic complications and hospital-acquired hyponatremia. Effective treatment of SIADH is based on water restriction, hypertonic saline plus loop diuretics, or aquaretics. Worsening of hyponatremia may result from parenteral isotonic fluid administration, emphasizing the importance of an early diagnosis and careful follow-up of these patients.