The elusive pathophysiology of neurally mediated syncope.

Syncope is defined as a sudden transient loss of consciousness and postural tone due to cerebral hypoperfusion. Although no permanent medical sequelae should result from syncope itself, isolated or recurrent events are often dramatic and disrupt the lifestyle of affected individuals. Syncope is a common clinical problem that affects up to 3.5% of the general population.1 Strikingly, in close to 40% of cases, the exact cause of syncope remains elusive, and '30% of affected patients will experience recurrent episodes.1 Neurally mediated syncope (NMS) is a common type of syncope (Figure 1); clinical descriptions of it have been present in the medical literature for .100 years. Despite its prevalence, significant gaps in our understanding of its pathophysiology and treatment remain. The purpose of this review is to critically evaluate proposed theories that attempt to explain the pathophysiological mechanisms of NMS.