Pharmacologic distinction of different orthostatic hypotension syndromes.

We studied the pressor responses to vasoactive agents in patients with orthostatic hypotension. Greater-than-normal slopes of the stimulus-response curves in patients with multiple system atrophy (Shy-drager syndrome) and idiopathic orthostatic hypotension (IOH) were consistent with deficient reflex modulation. Patients with IOH also had a shift to the left of the plasma norepinephrine-blood pressure curves, suggesting "denervation supersensitivity." This was consistent with the deficient plasma norepinephrine response to tyramine in these patients. Patients in whom orthostatic hypotension is attended by tachycardia are subsensitive to administered catecholamines, and may suffer from deficient effector organ responsivity.