The physiological conundrum of hyperadrenergic orthostatic intolerance.

Orthostatic hypotension is a well-recognized medical problem in patients whose blood pressure falls dramatically with standing. Much less recognized is the syndrome of orthostatic intolerance. In patients with orthostatic intolerance, there are symptoms evoked by standing, but little actual fall in blood pressure. On the other hand, orthostatic intolerance patients frequently have a brisk tachycardia on standing. It has recently been recognized that many such individuals have a mild dysautonomia which may be brought on by conditions such as an antecedent viral illness, a rheumatologic disorder, or surgery/anesthesia. Recent studies of the hyperadrenergic (elevated plasma norepinephrine) subgroup of orthostatic intolerance is documenting a clinical spectrum including attenuated plasma renin activity and aldosterone, reduced supine blood volume coupled with dynamic orthostatic hypovolemia, elevated plasma norepinephrine and epinephrine, impaired clearance of norepinephrine from the circulation and evidence of partial dysautonomia. The emergence of partial dysautonomia as an important mechanism of orthostatic intolerance may lead to a substantial alteration in therapeutic approach.