Vasovagal (Neurocardiogenic) Syncope

The Most Common Cause Of Fainting

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Fainting. Betty Dupuis/E+/Getty Images

Syncope — commonly called 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 of Vasovagal Syncope

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 of Vasovagal Syncope

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 blood pooling is often accompanied by a slowing of the heart rate.

As a result, the blood pressure suddenly drops. If the fall in flood pressure is enough to rob the brain of its needed blood flow, fainting occurs.

The vasovagal reflex works like this: First, a person is exposed to some noxious stimulus (such as a painful needle stick in a finger) that triggers the reflex.

The triggering event stimulates certain nerves (the pain nerves of the finger), which send an electrical signal to the vasomotor center in the brainstem, the portion of the brain that determines the body's vascular (blood vessel) tone. The vasomotor center, in response, sends signals to the blood vessels, causing them to dilate. This produces blood pooling, which leads to syncope. The same stimulation of the brainstem may also send signals to the heart (through the  vagus nerve ) to produce a drop in the heart rate.

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.

What can trigger vasovagal syncope? The "trigger" that initiates a vasovagal reflex can be any of a number of different things. Common triggers include:

  • Pain, especially pain that is acute in onset.
  • Having your blood drawn.
  • Being exposed to the sight of blood, or to other noxious sights.
  • Having difficult urination or defecation.
  • Severe spell of coughing. 
  • Painful swallowing.
  • Receiving upsetting or shocking news.
  • Sudden fright.
  • Standing motionless for long periods of time. (This is why soldiers standing at parade rest, or singers in a choir, will sometimes faint.) 

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

    Symptoms That Accompany or Precede Vasovagal Syncope

    While the loss of consciousness with vasovagal syncope can be quite sudden, more charateristically it is preceded by a few seconds or a few minutes of  warning symptoms. These warning symptoms are sometimes referred to as a “prodrome” of syncope.

    These prodromal symptoms 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.

    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.

    The bottom line is that if you see somebody faint, you should help them get their head down and elevate their legs. Holding them in an upright position — even if you add the additional "treatment" of yelling in their ears or slapping them — is not helpful, and may be harmful.

    ”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) postdromal 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. 

    People who have already had vasovagal 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. (Stopping an episode is not possible with most other forms of syncope.) Furthermore, if they can avoid actually blacking out, they can usually also avoid the lingering post-vagal period that often follows such an episode.

    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.

    Who Is Affected?

    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 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 also often have other persistent symptoms typical of the dysautonomias, such as abdominal bloating or cramps, diarrhea, constipation, extreme fatigue and various aches and pains.

    Diagnosing Vasovagal Syncope

    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. (When you are dehydrated, dilation of the blood vessels in the legs is more likely to produce a significant drop in your blood pressure.)

    Given these characteristic features and the situational nature of this condition, doctors should be able to make the correct diagnosis in the vast majority of patients simply by asking right questions and listening carefully to the answers. Unfortunately, too many doctors fail to take a careful enough medical history; as a result, they end up doing unnecessary tests and procedures looking for cardiac or neurological disease.

    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.

    Treating Vasovagal Syncope

    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: education, medication, exercise and pacemakers. Of these, education is by far the most effective for the majority of people.

    1) Educating Yourself About Vasovagal Syncope

    People who are prone to vasovagal syncope need to know five important facts about this condition, which we have already discussed. In summary, these are:

    1. Vasovagal syncope is produced by a reflex that causes sudden dilation of the blood vessels in the legs, causing the blood to pool in the lower extremities. 
    2. Any condition that causes a bit of dehydration (which reduces blood volume) will make you more prone to have a syncopal episode.
    3. Prodromal symptoms offer an important warning that syncope is likely to happen momentarily. 
    4. If you lie down and elevate your legs when you experience the prodrome, you can prevent the syncopal episode.
    5. People will often have occasional periods of days or weeks in which they are particularly prone to vasovagal episodes. This may occur, for instance, after a viral illness, or after periods of prolonged stress or sleep deprivation. You need to be particularly alert to prodromal symptoms during such times.

    The best way to prevent vasovagal syncope is to avoid situations that produce the syncope. If you understand the five facts just listed, the following guidelines become immediately apparent.

    • First and foremost, avoid dehydration. Dehydration (and therefore syncope) most commonly occurs after strenuous exercise, after any kind of illness, and early in the morning after an overnight fast. Coffee, tea and diuretic drugs can also produce dehydration. If you are in a situation where dehydration is likely, you should take pains to rehydrate yourself. You should drink plenty of fluids in general, but if you think you may be dehydrated this is particularly important. And you  must  avoid prolonged standing while you are dehydrated.
    • Pay close attention to any prodromal symptoms you may experience. These prodromal symptoms — typically visual disturbances, buzzing in the ears, lightheadedness, sweating, and/or nausea — tend to vary from person to person. But people who have experienced syncopal episodes usually have an excellent idea of what prodromal symptoms they should expect.
    • It is important to recognize your prodromal symptoms, because if you do, you can avoid syncope by lying down and elevating your legs. On the other hand, if you choose to ignore the warning symptoms, you are likely to pass out in the frozen foods section, and the grocery store manager will insist (for well-founded legal reasons) on shipping you to the E.R. in an ambulance, where you'll be subjected to a battery of unnecessary tests and probably an overnight stay in the hospital. Another advantage of avoiding actual syncope by lying down is that when you do, you will also avoid the prolonged period of postdromal illness that often follows a vasovagal episode.
    • Some people have been able to abort an episode of vasovagal syncope (or more often, delay it long enough to reach a convenient spot to lie down) 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. Such exercises may include leg-crossing while tensing the legs, abdominal area and buttocks; tensing the arms with clenched fists; leg pumping; or squeezing rubber balls.
    • People who are prone to vasovagal syncope may experience periods of time, lasting days or weeks, in which syncope is particularly prone to occur. These "sensitive periods" often seem to happen for no identifiable reason. Sometimes, however, they may be related to a viral illness, to menstrual cycles, to periods of being fatigued or run down; or to gastrointestinal, urinary or gynecological problems. If you have had one or two recent syncopal episodes, it pays to be particularly vigilant for prodromal symptoms that might herald another episode. It also pays during these times to make special efforts to remain well hydrated by drinking plenty of fluids.

    2) Drug Therapy

    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 recent 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 at least some help include midodrine (a drug that tends to limit the dilation of blood vessels), disopyramide (Norpace, an antiarrhythmic drug that also has some vagal-blocking properties), serotonin re-uptake inhibitors (drugs in the Prozac category) and theopylline (a drug most commonly used to treat asthma).

    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.

    3) Exercise Therapy

    Many people who have dysautonomia also display a tendency to have vasovagal syncope; indeed, it seems likely that many people who have frequent vasovagal syncope (instead of single, isolated episodes), actually have a form of dysautonomia. Since some dysautonomias are known to respond favorably to exercise training(which can improve vascular function and "readjust" the autonomic nervous system), some experts have suggested that exercise might similarly benefit people with vasovagal syncope. And indeed, the limited data that are currently available suggest this is the case. So, if you have recurrent vasovagal syncope, you should discuss aerobic exercise training (such as walking, jogging or bicycling) with your doctor.

    4) Pacemaker Therapy

    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, in many, if not most people with vasovagal syncope, it is the pooling of blood in the legs, and not the slow heart rate, that produces syncope.

    Still, in some people with vasovagal syncope, the drop in heart rate is the predominant reason for passing out. In these people, pacemakers can indeed reduce the frequency of syncope episodes.

    Currently, pacemakers are recommended for people with vasovagal syncope only if: a) significant slowing of the heart rate is documented during episodes, either during a  tilt-table test or during ambulatory ECG monitoring, and b) avoidance behavior (that is, taking the steps described above to avoid or abort vasovagal syncope) is ineffective.

    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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