Intermittent hypematremia following hypothalamic surgery or trauma is usually attributed to the triphasic dysfunction of vasopressin release (diabetes insipidus, inappropriate vasopressin release, and diabetes insipidus). A 39-year-old patient had hypodipsia and intermittent hypematremia following hypothalamic surgery for a chromophobe adenoma. Mean arterial pressure fell by 25 percent during orthostasis testing and was associated with an increase in vasopressin levels from 1.3 μU/ml to 12 μU/ml. Plasma renin activity and aldosterone increased from 1.1 to 16 ng/ml per hour and from 6.7 to 39 ng/dl, respectively, and remained elevated for three and a half hours after tilt testing. Hypertonic saline infusion, on the other hand, increased serum osmolality from 290 to 304 mOsm/kg but did not result in a significant rise in vasopressin levels (all were less than 1 μU/ml). These results are consistent with a selective dysfunction of the osmoreceptor pathways of vasopressin release and intact volume receptor-mediated pathways. Patients with intermittent hypematremia following hypothalamic surgery or trauma should be questioned specifically regarding thirst. If it is impaired or absent, these patients should be watched carefully, not only for the development of triphasic dysfunction of vasopressin release, but also for a selective osmoreceptor dysfunction associated with thirst deficits as found in patients with "essential hypernatremia.".
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