The Clinical Study of Hyponatremia in the Elderly. The Incidence and the Characterization of Hyponatremia with Unknown Etiology

Abstract

Hyponatremia in the elderly patients was characterized by the difficulties in its diagnosis and treatment.
The purpose of this study was to classify the hyponatremia in the elderly patients.
A total of 78 cases of hyponatremia was obsurbed in 525 consecutive aged autopsied (14.6%). Among these hyponatremia 25 cases (32%) had an apparent cause of hyponatremia, such as heart failure, renal failure or suction of body fluids. On the other hands, the cause of hyponatremia in the remainder (41 cases) was obscure and wasting diseases such as cerebrovascular accident (39.0%), malignancy (31.4%) and pneumonia (51.2%) were frequently observed as a underlying disorders.
Subsequently, the group with hyponatremia of unknown etiology was examined. According to their urinary sodium concentration, they could be devided into two subgroups, one with urinary sodium concentration over 50mEq/L and the other with that below 50mEq/L. The first group of hyponatremia with urinary sodium concentration over 50mEq/L was characterized as follows.
1. excessive sodium loss to urine in spite of hyponatremia (108.0±5.22mEq/day)
2. high urinary Na/K ratio (4.0±0.57)
3. increased total plasma volume (65.0±2.0ml/Kg)
4. low plasma renin activity (PRA) (1.3±0.21/ngml/hr.)
The other group of hyponatremia with urinary sodium concentration below 50mEq/L was characterized as follows.
1. high serum BUN levels (32.0±5.09mg/dl)
2. high amount of daily urinary potassium excretion (35.7±5.38mEq/day)
3. low urinary Na/K ratio (1.6±0.22)
4. heigh level of PRA (3.7±0.65ng/ml/hr.)
The supressed PRA and low serum sodium concentration in the first group might be explained by the increased plasma volume. Therefore, the first group of hyponatremia with urinary sodium concentration over 50mEq/L showed the characteristics compatible with dilutional hyponatremia.
In the patients of hyponatremia, with urinary sodium concentration below 50mEq/L, the systolic blood pressure during hyponatremia (135±13.0mmHg) was significantly lower than that before developing hyponatremia (175±6.5mmHg). (P<0.02) This significant fall of systolic blood pressure posibly induced by the decreased plasma volume or by the loss of sodium from the body fluids might be related to the increased PRA.
Therefore the latter group of hyponatremia with urinary sodium concentration below 50mEq/L showed the characteristics compatible with volume depletion.
In conclution, the measurement of the urinary sodium concentration is a simple and helpful tool for the evaluation of the hyponatremia.