Neurally mediated syncope and syncope due to autonomic failure: differences and similarities.

Syncope is a transient loss of consciousness and postural tone caused by a global reduction of blood flow to the brain. Abnormalities in autonomic cardiovascular control can impair blood supply to the brain and produce syncope in two different disorders: autonomic failure and neurally mediated syncope. In autonomic failure, sympathetic efferent activity is chronically impaired so that vasoconstriction is deficient, upon standing blood pressure always falls (i.e., orthostatic hypotension), and syncope or presyncope occurs. Conversely, in neurally mediated syncope, the failure of sympathetic efferent vasoconstrictor traffic (and hypotension) occurs episodically and in response to a trigger. Between syncopal episodes, patients with neurally mediated syncope have normal blood pressure and orthostatic tolerance. This article reviews the characteristics of autonomic failure and describes in more detail the pathophysiology, diagnosis, and treatment of neurally mediated syncope.