Orthostatic hypotension. I. Functional and neurogenic causes.

The maintenance of adequate upright BP requires both a baroreceptor-mediated feedback loop and an effective circulating blood volume. Although functional disruptions of these mechanisms are reversible and common, several permanent and often progressive neurologic disorders exist that interfere with necessary reflexes and orthostatic BP control. Multiple system atrophy affects diffuse neurologic systems; autonomic dysfunction causes a failure of peripheral vasoconstriction from defective sympathetic stimulation. Idiopathic orthostatic hypotension is a selective disorder of autonomic nerves; postganglionic neurons cannot release norepinephrine properly and are supersensitive to exogenous pressors. Conversely, excessive sympathetic discharge occurs in sympathicotonic orthostatic hypotension, the pathogenesis and incidence of which are unclear. Any peripheral neuropathy may interfere with sympathetic vasoconstrictor activity and is most commonly seen in diabetes mellitus.