The Clinical Study of Hyponatremia in the Elderly, Part Three Plasma ADH Secretion in Dilutional Hyponatremia

Abstract

In the previous reports, we described that hyponatremia in the elderly with unknown etiology could be devided into two subgroups according to urinary sodium concentration. The pathophysiological findings of the first subgroup of hyponatremia with urinary sodium concentration over 50mEq/l were compatible with those of SIADH, namely, dilutional hyponatremia except for the presence of mild renal dysfunction. The purpose of this study was to clarify the ADH secretion in this group. In the normal aged (n=5), serum osmolality and plasma ADH levels were decreased after water load, but plasma renin activity was not changed. The delta plasma ADH/delta serum osmolality were higher (0.50±0.19) than the values reported in the young subjects. This result suggests that the normal aged have a hypersensitivity for ADH secretion by serum osmolality. Plasma ADH levels in the aged hyponatremia (serum Na≤125mEq/l, urinary Na>50mEq/l), 6.6±1.8pg/ml, were significantly higher than those in controls (4.6±1.7pg/ml, p<0.02). Plasma ADH/PRA ratio showed no significant difference between the hyponatremia and the controls. However, plasma ADH/serum osmolality in the hyponatremia showed significantly higher value (2.68±0.82) than those in controls (1.57±0.59, p<0.01). According to the threshold theory, plasma-ADH/serum osmolality ratio indicates the sensitivity of the osmotic regulation in ADH secretion. Therefore, the higher plasma ADH/serum osmolality ratio in the aged dilutional hyponatremia suggests that the dysfunction in osmotic regulation play an important role in the pathogenesis of this morbid state. Furthermore, we have examined the effect of hypertonic saline with or without furosemide in the aged dilutional hyponatremia. Serum Na, serum osmolality and plasma ADH were increased by the treatment. Serious determination of plasma ADH, serum Na and osmolality in the course of the treatment, suggested that not only the sensitivity but also the set point of the regulating system in ADH secretion might be disturbed.