Vasovagal syncope (also called cardioneurogenic syncope) is a condition in which temporary loss of consciousness (syncope) occurs due to 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 is by far the most common cause of syncope and probably accounts for more than 50% of all episodes. While doctors often refer to it as a "simple fainting spell," the mechanism of vasovagal syncope actually is not particularly simple. In fact, many doctors seem not to grasp how vasovagal syncope occurs, which too often leads to problems in making the correct diagnosis or in determining adequate treatment.
What Causes Vasovagal Syncope?
Vasovagal syncope occurs when, due to a reflex, the blood vessels in the legs suddenly dilate, causing 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. The blood pressure subsequently drops and fainting occurs.
The reflex responsible for vasovagal syncope works like this: First, a person is exposed to some 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 in the legs, 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 to produce a drop in the heart rate.
In most people with vasovagal syncope, the dilation of blood vessels appears to be the predominant mechanism that causes loss of consciousness. In some people, however, the slowing of the heart rate plays the major role.
The "trigger" that initiates a vasovagal reflex can be any of a number of different things. Pain is a particularly common cause of fainting. Other common triggering events include having your blood drawn, being exposed to the sight of blood, having difficult urination or defecation, severe coughing, painful swallowing, receiving upsetting news, sudden fright or standing motionless for long periods of time. (This is why soldiers standing at parade rest, or singers in a choir, will sometimes faint.) In fact, if fainting follows any of these events, vasovagal syncope is extremely likely to be the cause.
What Are the Symptoms of Vasovagal Syncope?
While the loss of consciousness with vasovagal syncope can be quite sudden, more typically it is preceded by a few seconds or a few minutes of warning symptoms. These warning symptoms often include lightheadedness, ringing in the ears, visual disturbances, sweating and/or nausea. This is followed by a sensation of "graying out," followed immediately by loss of consciousness.
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 when 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. When somebody passes out, the right thing to do is to 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.
After an episode of vasovagal syncope, many individuals will feel terrible for a few hours or even for the next day or two. During this time they commonly experience extreme fatigue, nausea, dizziness and loss of appetite. Until these symptoms disappear they are particularly prone to fainting again.
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 important, 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.) Further, if they can avoid actually blacking out, they can usually also avoid the period of feeling "sick" that often follows such and 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 individuals with vasovagal syncope are more likely to have an "atypical" form of this condition, in which syncope may occur without any identifiable trigger and without any warning symptoms. Making the correct diagnosis in these individuals 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.
Who Gets Vasovagal Syncope?
The reflex that causes vasovagal syncope can occur to some extent in everyone, so almost anyone can have an episode as long as a strong enough triggering event occurs. Indeed, it is likely that most people will have a fainting episode at least once in 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 often faint even with relatively mild triggering events. These individuals tend to relate histories of syncope dating back to adolescence. They frequently will describe several episodes, often with several different kinds of triggering events.
In some people, vasovagal syncope is so frequent and so difficult to treat that they become virtually disabled by it. These individuals often have a form of dysautonomia (imbalance of the autonomic nervous system) that makes them very prone to the 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.
In fact, the several-hour period of symptoms that can follow an episode of vasovagal syncope (mentioned above) strongly mimics many of the symptoms experienced chronically by people with dysautonomia.
How Is Vasovagal Syncope Diagnosed?
Doctors who are good 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 in their patients with this condition; as a result, they end up doing unnecessary tests and procedures 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 thus in helping the doctor sort through the likely possibilities.
While in most cases the diagnosis of vasovagal syncope ought to be made 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 history is not typical for vasovagal syncope or if it is difficult to distinguish between vasovagal syncope and orthostatic hypotension.
How Is Vasovagal Syncope Treated?There are several approaches to treating vasovagal syncope, and while the condition cannot be "cured," in the vast majority of people with vasovagal syncope it can be controlled sufficiently to allow for a completely normal life.
Task Force for the Diagnosis and Management of Syncope, European Society of Cardiology (ESC), European Heart Rhythm Association (EHRA), et al. Guidelines for the diagnosis and management of syncope (version 2009). Eur Heart J 2009; 30:2631.
Chen-Scarabelli C, Scarabelli TM. Neurocardiogenic syncope. BMJ 2004; 329:336.