http://circ.ahajournals.org/content/111/22/2997

The reflex syncopes are a group of disorders that occur because of a sudden failure of the ANS to maintain adequate vascular tone during orthostatic stress, resulting in hypotension (frequently associated with bradycardia) that results in cerebral hypoperfusion and loss of consciousness. The 2 most frequent types of reflex syncope are neurocardiogenic (vasovagal) syncope and carotid sinus syndrome. The other types of reflex syncopes are often referred to as situational because they are often associated with specific activities or conditions. Neurocardiogenic syncope (NCS) can be quite varied in presentation. 

It tends to occur in younger patients and tends to exhibit 3 distinct phases that consist of a distinct prodrome (usually lightheadedness, nausea, diaphoresis, or visual changes) followed by a sudden loss of consciousness. Recovery is usually quite rapid, and postictal states are rare. However, nearly one third of patients (most commonly older adults) will experience few, if any, prodromal symptoms, and loss of consciousness occurs suddenly, with little warning. The cause of neurocardiogenic syncope is unclear. It can be brought on by pain, emotional distress, or by prolonged standing (especially in very warm environments), yet many episodes occur without any specific provocation. Although still the subject of considerable debate, it is presently believed that the pathophysiology of NCS often is related to prolonged orthostatic stress. 

This results in an increased amount of peripheral venous pooling to a point great enough that the venous return to the heart falls so precipitously that a significant increase in ventricular inotropy occurs. This hypercontractile state is believed to activate mechanoreceptors that would normally fire only during stretch. The sudden increase in neural traffic to the medulla is thought to mimic the conditions seen in hypertension, thus causing an apparent “paradoxic” decline in sympathetic activity that results in hypotension, bradycardia, and ultimately syncope. It should be remembered, however, that other stimuli (for example fear, strong emotion, or epileptic discharges) can elicit nearly identical responses, which raises the possibility that these patients may have an inherent susceptibility to these events. During head-upright tilt-table testing, these individuals will demonstrate a relatively sudden fall in blood pressure that is often (but not always) followed by a decline in heart rate (sometimes to the point of asystole, which is when the heart stops beating- usually for just a few seconds).