[Pathogenesis of hyponatremia observed in the treatment of acute subarachnoid hemorrhage].

The cause of hyponatremia following subarachnoid hemorrhage (SAH) has been understood as an inappropriate secretion of antidiuretic hormone (SIADH). Whereas, water restriction for the management of this condition sometimes induces a severe dehydration, resulting in vasospasm. To clarify the pathogenesis of hyponatremia following SAH, we measured the daily sodium and water balance with the plasma concentration of atrial natriuretic peptide (ANP), antidiuretic hormone (ADH), and plasma renin activity (PRA) in seventeen cases after subarachnoid hemorrhage. Although the patients received an adequate amount of fluid (more than 4080ml/day; daily average in seventeen cases) and sodium (more than 277 mEq/day; daily average in seventeen cases), eight out of the seventeen cases showed transient hyponatremia of a slight degree beginning on 8.8 days after SAH. ANP values were elevated markedly in fifteen out of the seventeen cases, remaining high during the first two weeks following SAH. ADH values were elevated remarkably in eight out of the seventeen cases. However, these values declined immediately to a normal range within two days following SAH. PRA were increased or came within the normal range, suggesting the lack of water retention. Overall sodium balance and water balance did not differ significantly between hyponatremia cases and normonatremia ones, whereas, sodium balance in acute phase was significantly negative, associated with marked natriuresis in patients with hyponatremia. These correlations suggested that hyponatremia after SAH is the result of natriuresis by an increased ANP rather than ADH. In conclusion, a greater replenishment of water and sodium is required to avoid hyponatremia with dehydration. This technique may be helpful for the prevention of vasospasms following SAH.