A vasovagal episode or vasovagal response is a malaise mediated by the vagus nerve. When it leads to fainting, it is called vasovagal syncope, and is the most common type of fainting.
There are a number of different syncope syndromes which all fall under the umbrella of vasovagal syncope. The common element among these conditions is the central mechanism leading to loss of consciousness. The differences among them are in the factors that trigger this mechanism.
Typical triggers for vasovagal episodes include:
- prolonged standing or upright sitting, particularly when standing with legs in a locked position for long periods of time. Avoidance of long-term locking of one's legs in the standing position is taught in the military, as well as marching band and drill team.
- any painful or unpleasant stimuli, such as
- giving a blood donation or watching someone give one
- watching someone experience pain
- watching/experiencing medical procedures
- sight of blood
- occasions of slight discomfort, such as dental and eye examinations
- hyperthermia, a prolonged exposure to heat
- sudden onset of extreme emotions
- nausea or vomiting
- urination ('micturition syncope') or defecation ('defecation syncope')
- abdominal straining or 'bearing down' (as in defecation)
- swallowing ('swallowing syncope') or coughing ('cough syncope')
- random onsets due to nerve malfunctions
- pressing upon certain places on the throat, sinuses, and eyes
- high altitude
- use of certain drugs that affect blood pressure, such as amphetamine
- intense laughter
In people with vasovagal episodes, the episodes are typically recurrent, usually happening when the person is exposed to a specific trigger. The initial episode often occurs when the person is a teenager, then recurs in clusters throughout his or her life. Prior to losing consciousness, the individual frequently experiences a prodrome
of symptoms such as lightheadedness, nausea
, sweating, ringing in the ears, uncomfortable feeling in the heart, weakness and visual disturbances. These last for at least a few seconds before consciousness is lost (if it is lost), which typically happens when the person is sitting up or standing. When sufferers pass out, they fall down (unless this is impeded); and when in this position, effective blood flow to the brain is immediately restored, allowing the person to wake up.
The autonomic nervous system's physiologic state (see below) leading to loss of consciousness may persist for several minutes, so:
- if sufferers try to sit or stand when they wake up, they may pass out again; and
- the person may be nauseated, pale, and sweaty for several minutes.
Pathophysiology and mechanism
Regardless of the trigger, the mechanism of syncope is similar in the various vasovagal syncope syndromes. In it, the nucleus tractus solitarius 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. The drop in heart rate leads to a drop in blood pressure 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 without much change in heart rate. This phenomenon occurs due to vasodilation, 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.
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 one of the most difficult challenges that a physician can face. The core of the diagnosis of vasovagal syncope rests upon a clear description by the patient of a typical pattern of triggers, symptoms, and time course.
It is also pertinent to differentiate lightheadedness, vertigo and hypoglycemia as other causes
In patients with recurrent syncope, diagnostic accuracy can often be improved with one of the following diagnostic tests:
- A tilt table test
- Implantation of an insertable loop recorder
- A Holter monitor or event monitor
- An echocardiogram
- An electrophysiology study
Vasovagal syncope is rarely life-threatening in itself, but may be associated with injury from falling.
Treatment for vasovagal syncope focuses on avoidance of triggers, restoring blood flow to the brain during an impending episode, and measures that interrupt or prevent the pathophysiologic mechanism described above.
- The cornerstone of treatment is avoidance of triggers known to cause syncope in that person. Before known triggering events, the patient may increase consumption of salt and fluids to increase blood volume. Sports and energy drinks may be particularly helpful.
- Discontinuation of medications known to lower blood pressure may be helpful, but stopping antihypertensive drugs can also be dangerous. This process should be managed by an expert.
- Patients should be educated on how to respond to further episodes of syncope, especially if they experience prodromal warning signs: They should lie down and raise their legs; or at least lower their head to increase blood flow to the brain. If the individual has lost consciousness, he or she should be laid down with his or her head turned to the side. Tight clothing should be loosened. If the inciting factor is known, it should be removed if possible (for instance, the cause of pain).
- Wearing graded compression stockings may be helpful.
- There are certain orthostatic training exercises which have been proven to improve symptoms in people with recurrent vasovagal syncope.
- Certain medications may be helpful, but are rarely effective by themselves:
- For people with the cardioinhibitory form of vasovagal syncope, implantation of a permanent pacemaker may be beneficial or even curative.
- Daroff, Robert B. & Carlson, Mark D. (2001). Faintness, Syncope, Dizziness, and Vertigo. In Eugene Braunwald, Anthony S. Fauci, Dennis L. Kasper, Stephen L. Hauser, Dan L. Longo, & J. Larry Jameson (Eds.), Harrison's Principles of Internal Medicine (15th Edition), pp. 111–115. New York: McGraw-Hill these can stand to be attributed with other sources as well.