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. Diagnosis
In addition to the mechanism described above, a number of other medical conditions may cause syncope. Making the correct diagnosis for loss of consciousness is difficult. The core of the diagnosis of vasovagal syncope rests upon a clear description of a typical pattern of triggers, symptoms, and time course.It is pertinent to differentiate lightheadedness, seizures, vertigo, and low blood sugar as other causes.
Medications which may be effective include: CNS stimulants fludrocortisone, midodrine, SSRIs such as paroxetine or sertraline, disopyramide, and, in health-care settings where a syncope is anticipated, atropine or epinephrine (adrenaline). For people with the cardioinhibitory form of vasovagal syncope, implantation of a permanent pacemaker may be beneficial or even curative.Types of long-term therapy for vasovagal syncope include
Preload agents
Vasoconstrictors
Anticholinergic agents
Negative cardiac inotropes
Central agents
Mechanical device
Discontinuation of medications known to lower blood pressure may be helpful, but stopping antihypertensive drugs can also be dangerous in some people. Taking antihypertensive drugs may worsen the syncope, as the hypertension may have been the bodys way to compensate for the low blood pressure. Prognosis
Brief periods of unconsciousness usually cause no lasting harm to health. Reflex syncope can occur in otherwise healthy individuals, and has many possible causes, often trivial ones such as prolonged standing with the legs locked.The main danger of vasovagal syncope (or dizzy spells from vertigo) is the risk of injury by falling while unconscious. Medication therapy could possibly prevent future vasovagal responses; however, for some individuals medication is ineffective and they will continue to have fainting episodes. See also
Orthostatic hypotension
Orthostatic intolerance
Postural orthostatic tachycardia syndrome
Roemheld Syndrome


== References ==