Hyponatremia and natriuresis following subarachnoid hemorrhage in a monkey model.

A monkey model of subarachnoid hemorrhage (SAH) was used to study both the incidence of hyponatremia and natriuresis and the associated changes in antidiuretic hormone (ADH) secretion and salt and water balance. Following SAH, seven of nine monkeys became natriuretic and hyponatremic. The natriuretic period lasted an average of 4.4 +/- 0.4 days. The mean nadir of serum sodium content was 125.7 +/- 1.6 mEq/liter, and occurred on the average on the 5th day following SAH. The sodium balance after SAH was negative as compared to the preoperative positive sodium balance (p less than 0.001). The plasma vasopressin level was usually elevated for a day following surgery, but there was no significant difference in the levels during the preoperative period and during the period of natriuresis following SAH. The daily urine output and aldosterone levels were not significantly different, and the plasma volume was slightly, but not significantly, decreased after SAH. Four of the animals that had a hyponatremic and natriuretic response following SAH showed a normal regulation of vasopressin in response to both a water challenge and hypertonic saline challenge. The three monkeys that underwent sham procedures did not become hyponatremic and natriuretic postoperatively. The sham-operated monkeys did not show significant differences in their plasma vasopressin levels, urine volume, plasma volume, and aldosterone levels following surgery. These observations are more consistent with primary natriuresis as the cause of hyponatremia rather than the syndrome of inappropriate secretion of ADH. The cause of the renal loss of sodium is not known, but the possibility of a brain natriuretic factor or an alteration in the neural control of the kidney should be considered.