Overview of Vasovagal Syncope

The Most Common Cause of Fainting

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Syncope—also known as fainting—is the medical term for a temporary loss of consciousness. Any episode of syncope is important for at least two reasons. First, syncope can cause injury, so it is important to try to prevent it from recurring. And second, syncope can sometimes be a sign of a serious underlying medical problem. While several medical conditions can lead to syncope, by far the most common type of syncope is vasovagal syncope.

Overview

Vasovagal syncope (also called neurocardiogenic syncope) is a temporary loss of consciousness caused by a neurological reflex that produces either sudden dilation of the blood vessels in the legs, or a very slow heart rate (bradycardia), or both.

Vasovagal syncope accounts for more than half of all episodes of syncope. While doctors often refer to it as a "simple fainting spell," the mechanism of vasovagal syncope actually is not all that simple. And misunderstanding how vasovagal syncope works can lead to problems in making the correct diagnosis or in choosing adequate treatment.

Causes

Vasovagal syncope occurs when something triggers the vasovagal reflex, causing the blood vessels to suddenly dilate. Dilation of the blood vessels causes a significant proportion of the blood volume to pool in the legs.

This pooling is often accompanied by a slowing of the heart rate. As a result, the blood pressure will suddenly drop. If the drop in pressure is enough to rob the brain of oxygen, fainting occurs.

In most people with vasovagal syncope, the dilation of blood vessels appears to be the predominant factor that causes loss of consciousness. In some people, however, the slowing of the heart rate plays a major role.

Common triggers of vasovagal syncope include:

  • Sudden, severe pain
  • Having your blood drawn
  • Being exposed to a traumatic sight or event
  • Extremely difficult urination or defecation
  • A severe coughing spell
  • Hyperventilation
  • Standing motionless for long periods of time
  • Overexerting yourself in hot weather
  • Excessive alcohol or substance use

If a fainting episode follows any of these events, vasovagal syncope is almost certainly the cause.

Symptoms

While the loss of consciousness with vasovagal syncope can be quite sudden, more characteristically it is preceded by a few seconds or a few minutes of warning symptoms. These symptoms are sometimes referred to as a “prodrome” of syncope. Those that occur after you are revived are called "postdromal."

Prodromal Symptoms

These prodromal symptoms of syncope often include:

  • Lightheadedness
  • Ringing or buzzing in the ears
  • Visual disturbances, such as shimmering vision or tunnel vision
  • Sudden sweating.
  • Sudden nausea

These prodromal symptoms are followed by a sensation of "graying out," then finally by loss of consciousness. The time between the onset of prodromal symptoms and actually passing out may be a few minutes, or just a second or two.

If you feel like you’re going to faint, you may be able to abort the episode by lying down with your legs elevated or sitting in a chair with your head between your knees. Wait until you feel better before trying to stand.

Syncope Characteristics

The syncope itself also has several characteristic features: 

  • Vasovagal syncope almost always occurs when the victim is standing, or sitting upright (when blood pooling in the legs can occur), and it virtually never happens while lying down.
  • People who have vasovagal syncope usually regain consciousness after a few seconds, once they have fallen (or, if they're lucky, are helped) to the ground. This is because once on the ground, gravity no longer causes the blood to pool in the legs and the blood pressure improves almost immediately.
  • On the other hand, if the victim is held upright by a well-meaning bystander, the unconsciousness can become very prolonged. This is a potentially dangerous situation because as long as the victim is upright and unconscious, his or her brain is not being adequately perfused with blood.

If you see somebody faint, lay that person on his or her back and elevate the legs about 12 inches above the heart. Loosen any belt, collar, or constrictive clothing. Seek help if the fainting lasts for longer than a few seconds.

Postdromal Symptoms

After an episode of vasovagal syncope, many people will feel terrible for a few hours or even for the next few days, or even longer. During this “postdromal” period they commonly experience extreme fatigue, nausea, dizziness, and loss of appetite. 

It is particularly important to note that, until these lingering (and very annoying) symptoms disappear, people are particularly prone to fainting again—so they need to be particularly alert for the warning symptoms that may indicate that an episode of syncope is imminent. 

Recurrent Syncope

People who have had one or two episodes of vasovagal syncope are frequently able to recognize the warning symptoms, so they will know when another event is about to occur. More importantly, if they do recognize the warning symptoms, they can prevent the blackout simply by lying down and elevating their legs.

On the other hand, trying to "fight off" an impending episode of vasovagal syncope by forcing yourself to remain upright and willing yourself not to faint almost never works out very well.

Older people with vasovagal syncope are more likely to have "atypical" symptoms. Their syncope may occur without any identifiable trigger, and without any warning symptoms. Making the correct diagnosis in these cases can present a real challenge to the doctor.

In general, vasovagal syncope is not itself life-threatening, but injuries that result from falling may be. And if episodes are frequent enough this condition can certainly be quite disruptive to a normal life.

When to See a Doctor

A single fainting episode is not usually serious. However, you should see a doctor if you are pregnant, have recurrent episodes, or experience blurred vision, chest pain, confusion, trouble talking, shortness of breath, or irregular heartbeat prior to fainting.

Risk Factors

The reflex that causes vasovagal syncope can occur to some extent in everyone, so almost anyone can have a vasovagal episode if a sufficiently strong triggering event occurs. Indeed, it is likely that most people will have a fainting episode sometime during their lives.

Vasovagal syncope can occur at any age, but it is much more common in adolescents and young adults than in older people.

Some people are particularly prone to vasovagal episodes and may faint even with relatively mild triggering events. These people tend to have recurrent episodes of syncope, beginning in adolescence. They will often experience syncope following several different kinds of triggering events.

In rare individuals, vasovagal syncope is so frequent and so difficult to treat that they become virtually disabled by it. These people often have a form of dysautonomia (imbalance of the autonomic nervous system) that makes them very prone to the vasovagal reflex that causes this condition. They often have other symptoms typical of the dysautonomias, such as abdominal bloating or cramps, diarrhea, constipation, extreme fatigue, and various aches and pains.

Diagnosis

Doctors who are skilled at correctly diagnosing vasovagal syncope understand that this condition is almost always situational. Vasovagal syncope is particularly likely to occur after a viral illness, after exercise, after a warm shower, or early in the morning—in other words, any time relative dehydration is likely to be present.

Given these characteristic features and the situational nature of this condition, doctors should be able to make the correct diagnosis by simply reviewing the symptoms, medical history, family history, and sequence of events leading up to the fainting.

The physical examination of people with vasovagal syncope is usually completely normal. However, the exam is very helpful in diagnosing the similar conditions of orthostatic hypotension or postural orthostatic tachycardia syndrome (POTS), and can be quite helpful in sorting through the likely possibilities.

While in most cases making the diagnosis of vasovagal syncope ought to be possible by taking a medical history and doing a physical examination, testing is sometimes helpful. In particular, a tilt table study can be helpful if the medical history is not typical for vasovagal syncope, or if it is difficult to distinguish between vasovagal syncope and orthostatic hypotension.

Treatment

People who have a single, isolated episode of vasovagal syncope generally do not require any medical therapy at all.

But if you have had recurrent episodes, you are likely to have even more episodes unless you are effectively treated. And, as anyone with vasovagal syncope knows, these fainting episodes often come at the most inconvenient or impractical times, and can greatly disrupt your life. Fortunately, treatment is usually quite helpful.

There are four general types of therapy for vasovagal syncope: medication, exercise, and pacemakers.

Medications

In some people, vasovagal syncope occurs with disturbing frequency even when all appropriate precautions are taken. For these individuals, drug therapy is often helpful.

In the past, the drugs were most commonly used for vasovagal syncope were the beta blockers, but several studies have failed to show any benefit and these drugs are currently not recommended.

Drugs that have been shown to be of some help include:

While one or more of these drugs will often help reduce episodes of syncope, finding the "right" combination of drugs is usually a matter of trial and error. Patience is required on the part of both the doctor and patient in order to find the best therapy.

Exercise

Many people who have dysautonomia also display a tendency to have vasovagal syncope; indeed, it seems likely that many people with recent vasovagal syncope may actually have a form of dysautonomia. Since some dysautonomias respond favorably to exercise, exercise may likely have a similar benefit to people with vasovagal syncope.

If you have recurrent syncope, be sure to meet with your doctor before embarking on any fitness plan. Depending on the underlying cause, you may need to undergo stress testing and other exams to determine how much exercise you can tolerate.

Interestingly, some people have been able to abort an episode of vasovagal syncope by immediately engaging in muscle-tensing exercises. These exercises apparently reduce blood vessel dilation and increase the amount of blood being returned to the heart. Examples include

  • Crossing your leg while tensing the legs, abdomen, and buttocks
  • Tensing the arms with clenched fists
  • Tensing your leg muscles
  • Squeezing a rubber ball

Pacemakers

Several years ago there was a lot of enthusiasm for using pacemakers to treat vasovagal syncope since vasovagal syncope is usually accompanied by a sudden drop in heart rate. The enthusiasm for pacemaker therapy dropped off rapidly, however, after it was finally noted that many patients with vasovagal syncope who received pacemakers continued to pass out—they just did it without slow heart rates.

As it turns out, it is the pooling of blood in the legs—and not a slow heart rate—that produces syncope in most people. Still, in some, the drop in heart rate may the predominant cause.

A pacemaker should only be explored if the syncope is severe or recurrent, conservative measures have failed, and there is significant slowing of the heart as evidenced by a tilt-table test or ambulatory ECG monitoring.

A Word From Verywell

Vasovagal syncope is a very common condition. Fortunately, it usually occurs in rare, isolated episodes or during a limited period of time. Most people who have vasovagal syncope lead entirely normal lives.

If you have had vasovagal syncope—especially more than one episode—you should learn as much as you can about this condition, including what kinds of things provoke it, how to recognize the warning symptoms and how you might stop an episode.

If you have repeated episodes of syncope despite taking such steps, you should talk to your doctor about whether something more than mere “avoidance therapy” is needed.

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