Hyponatremia is defined as a plasma sodium concentration below 135 mmol / L. This is a common occurrence (20-50%) during subarachnoid hemorrhage (SAH). Its appearance is often associated with vasospasm. It is associated with an increase in morbidity and mortality linked to induced neurological disorders. Hyponatremia is caused by two etiologies: the syndrome of inappropriate secretion of anti-diuretic hormone (SIADH), and the cerebral salt wasting syndrome, CSWS. Theoretically, these two entities are differentiated by the patient's volemia; in practice, this parameter is difficult to measure. In addition, the correction of hyponatremia is diametrically opposed according to its mechanism: water restriction in the case of SIADH, sodium intake in the event of CSWS. Urea is offered as a second-line treatment in the event of treatment failure to correct hyponatremia. However, the efficacy of this treatment is based on small, observational, retrospective studies. Moreover, the mechanism of action of urea remains poorly understood: it could be a hyperosmolar effect or passive renal reabsorption of sodium.