Hypotonic hyponatremia: therapeutic and controversial aspects

Inappropriately slow or excessive correction of severe (< 115 mEq/l) hyponatremia could induce a high morbidity and mortality rate. In acute (< 48 hr) hyponatremia, there is an increased risk for death and permanent brain damage due to grand mal seizure and respiratory arrest. Indeed, if correction is delayed, the ability of brain to adapt to hyponatremia by limiting the amount of brain swelling is not sufficient and brain stem herniation occurs. Menstruant women are particularly likely to experience these complications. In chronic (> 48 hr) hyponatremia, the extrusion of intracerebral osmolytes decreases the brain size which returns to an almost normal volume. In this situation, an excessive correction (> 15-20 mEq/l/24 h) will lead to brain dehydration and brain demyelination also called "central pontine myelinolysis" or "osmotic demyelination syndrome" (ODS) could develop. Asymptomatic patients with chronic hyponatremia are particularly at risk to develop brain demyelination, therefore, they must be corrected cautiously with frequent monitoring of the natremia and with a magnitude of correction not exceeding 15 mEq/l/24 h. The different therapeutic approach regarding to the origin of the hyponatremia are considered.