The syndrome of inappropriate antidiuresis (SIAD; formerly the syndrome of inappropriate secretion of
antidiuretic hormone) is the most frequent cause of
hyponatremia. A strong association exists between mortality and
hyponatremia, which reflects the severity of the underlying disease. In SIAD,
hyponatremia is associated with normovolaemia but the assessment of extracellular volume can be difficult. Clinical features are mainly neurological and can lead to death but mechanisms of adaptation can limit cerebral oedema. The notion of mild asymptomatic
hyponatremia was questioned by the observation of subclinical neurocognitive impairment, a greater risk of falls and fractures. Aetiologies are classified into six groups:
neurologic disorders,
infections mainly cerebral, meningeal and pulmonary, drugs in particular
antidepressants,
tumors, genetic causes, and idiopathic. Symptomatic acute
hyponatremia is a therapeutic emergency that is not specific of SIAD. When
hyponatremia is asymptomatic, fluid restriction with
salt intake is generally sufficient but
urea can be an alternative. In chronic SIAD, there is currently no recommendation. Fluid restriction is not always feasible;
urea has proved its efficacy, its good tolerance and its long-term harmlessness. Vaptans have demonstrated their good tolerance and their efficacy on the correction of
hyponatremia from SIAD in studies subgroups, for moderate
hyponatremia and asymptomatic patients. In the only study having compared vaptans and
urea, efficacy and tolerance were similar. Because of the cost difference between vaptans and
urea and while waiting for follow-up studies,
urea appears at present as the first-line treatment of
hyponatremia in SIAD.