Regardless of the trigger, the mechanism of syncope is similar in the various vasovagal syncope syndromes. The nucleus tractus solitarii of the brainstem is activated directly or indirectly by the triggering stimulus, resulting in simultaneous enhancement of parasympathetic nervous system (vagal) tone and withdrawal of sympathetic nervous system tone. This results in a spectrum of hemodynamic responses: # On one end of the spectrum is the cardioinhibitory response, characterized by a drop in heart rate (negative chronotropic effect) and in contractility (negative inotropic effect) leading to a decrease in cardiac output that is significant enough to result in a loss of consciousness. It is thought that this response results primarily from enhancement in parasympathetic tone. # On the other end of the spectrum is the vasodepressor response, caused by a drop in blood pressure (to as low as 80/20) without much change in heart rate. This phenomenon occurs due to dilation of the blood vessels, probably as a result of withdrawal of sympathetic nervous system tone. # The majority of people with vasovagal syncope have a mixed response somewhere between these two ends of the spectrum. One account for these physiological responses is the Bezold-Jarisch reflex. Vasovagal syncope may be part of an evolved response, specifically, the fight-or-flight response.
Reflex syncope
Hypotonic-hyporesponsive episode
Heat syncope
Laughter-induced syncope
Reflex asystolic syncope
Panic attack
Hair-grooming syncope
Postural orthostatic tachycardia syndrome