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Syncope and Its Causes

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Updated April 10, 2014

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

What Is Syncope?

Syncope (sin-co-pee) is a temporary loss of consciousness, commonly referred to as fainting, or passing out. It is a fairly common event -- a majority of people pass out at least once in their lives. Still, syncope is always a potentially serious problem, since any loss of consciousness can produce injury.

Furthermore, syncope may be a sign of a serious medical problem. So, if you have syncope, you should be evaluated by your doctor to determine its cause.

Why Does Syncope Occur?

Syncope occurs when the blood flow to the brain is markedly reduced for as few as five or six seconds. The brain's blood flow can be interrupted for a number of different reasons. Fortunately, in most cases, syncope is a transient and benign condition, so usually -- as long as the syncopal episode itself has not caused serious injury -- the event will have no long-term significance.

However, sometimes syncope is a sign that a dangerous or even life-threatening underlying medical condition may be present. This is why, if you have an episode of syncope, it is important for you to consult with your doctor.

What Conditions Can Cause Syncope?

Syncope can result from a large variety of medical conditions. In fact, the sheer number of possibilities can make the evaluation of syncope a difficult and even intimidating prospect for doctors who do not approach the problem in an organized, systematic way.

Doctors who are adept at evaluating syncope usually classify the potential causes into two general categories -- syncope due to cardiac causes, and syncope due to non-cardiac causes. Virtually all of the potentially life-threatening kinds of syncope are in the cardiac category. Fortunately,the cardiac causes of syncope are often relatively easy to identify with a careful general evaluation -- essentially, by taking a good medical history and performing a thorough physical examination. And once a cardiac cause has been ruled out, you and your doctor can be reasonably assured that whatever the cause turns out to be, you are very unlikely to have a life-threatening condition -- and the remainder of your evaluation can be conducted without any sense of extreme urgency.

The Cardiac Causes of Syncope

Roughly one in four people with syncope will prove to have a cardiac cause. Two general types of heart problems can produce syncope -- heart problems that obstruct the flow of blood through the heart, and cardiac arrhythmias.

Obstructive cardiac disease:

An obstruction to the blood flowing through the heart can cause a drop in the amount of blood being pumped by the heart, thus robbing the brain of sufficient blood flow. Several heart disorders can partially block the flow of blood through the heart. These include:

Cardiac arrhythmias:

Cardiac arrhythmias are a much more common cause of syncope than are obstructive cardiac conditions. Unfortunately, any arrhythmia that can cause syncope can also cause sudden death, if the arrhythmia persists for several minutes.

Arrhythmias can reduce cardiac function in two different ways -- by making the heart rate too slow, or by making it too rapid.

Slow heart arrhythmias (called bradycardia) can produce syncope simply by causing the heart to beat so slowly that the brain does not get enough blood flow. If bradycardia is found to be the cause of syncope, and if the bradycardia is thought to be likely to recur, effective treatment can be provided by inserting a cardiac pacemaker.

Rapid heart arrhythmias (called tachycardia) can also cause syncope by making the heart beat so rapidly that it can no longer pump effectively. While there are many different kinds of tachycardia, the ones that most commonly cause syncope are ventricular tachycardia and ventricular fibrillation. These particular arrhythmias are life-threatening, and commonly produce sudden death.

A careful medical history and physical examination -- along with an electrocardiogram (ECG) -- should give your doctor enough clues to judge whether any of these cardiac conditions is a likely cause of your syncope. And if a heart condition is thought to be likely, an immediate, focused cardiac evaluation is needed, which may include an echocardiogram, a stress test, or other diagnostic procedures. Your doctor may even decide that you should be hospitalized for your own protection until a diagnosis is pinned down, and appropriate treatment is given.

Fortunately, however, in the large majority of cases, a good initial medical evaluation rules out a cardiac cause of syncope. Your doctor can then turn his or her attention to the non-cardiac causes.

The Non-Cardiac Causes of Syncope

There are three general categories of non-cardiac disorders that cause syncope -- neurological causes, metabolic causes, and vasomotor causes.

Neurological Causes:

Neurological conditions turn out to an infrequent cause of syncope, accounting for only about 1% of syncopal episodes. There are only three neurological conditions that are likely to produce syncope:

A neurological condition that is often confused with syncope is epilepsy. The confusion arises because an epileptic seizure, like syncope, also causes a transient loss of consciousness. But seizures are usually not caused by a disruption of the flow of blood to the brain -- instead, they are caused by abnormal electrical activity in the brain. A doctor will usually suspect that a seizure disorder is the cause of a loss of consciousness when the episode is accompanied by the typical involuntary movements that usually occur with a seizure. A firm diagnosis of epilepsy can usually be confirmed by an electroencephalogram (EEG) -- a test that records the brain's electrical activity. It is important to do the necessary testing and get this diagnosis right, since the treatment of seizure disorders is very different from the treatment of syncope. Unfortunately, people with benign forms of syncope (usually young people) are all too often misdiagnosed as having a seizure disorder.

Metabolic Causes:

Fewer than 1% of people with syncope turn out to have metabolic causes. Metabolic causes of syncope include hypoxia (reduced oxygen in the blood, which is never seen unless some severe and obvious lung or heart disease is present); hyperventilation, which occurs in severe anxiety reactions or panic attacks; and severe hypoglycemia (low blood sugar), which should be suspected in diabetics, especially those taking insulin.

Vasomotor Causes:

For the great majority of people who have syncope, this is where the money is. Vasomotor syncope occurs when the body's complex mechanisms that maintain a normal blood pressure fail (either transiently or chronically), leading to a drop in blood pressure, which subsequently robs the brain of its blood supply.

There are two general types of vasomotor syncope -- orthostatic hypotension and vasovagal (or cardioneurogenic) syncope.

Orthostatic hypotension

Normally when you stand up, the blood vessels in your legs constrict, which helps to keep the blood from "pooling" in your legs, and thus to maintain a normal blood pressure. In people who have orthostatic hypotension, for one of several reasons, a normal blood pressure cannot be maintained when standing up. If their blood pressure drops far enough, they can lose consciousness when they are upright. Orthostatic hypotension is most often seen in older people, and is most commonly caused by prescription drugs. But it can also be caused by diabetes, Parkinson's disease, and in several other medical disorders.

A related condition is postural orthostatic tachycardia syndrome, or POTS. POTS is different from orthostatic hypotension in that: a) it is seen almost exclusively in young people (generally people under the age of 45), and b) while POTS may also produce low blood pressure, the chief problem is an extremely rapid heart rate upon standing. People who have POTS experience many symptoms when they stand up, most often including palpitations, dizziness and weakness -- and roughly 40% of them will experience syncope at least once.

Vasovagal (cardioneurogenic) syncope

Vasovagal syncope (also known as cardioneurogenic syncope) is the most common cause of syncope, probably accounting for more than 80% of all syncopal episodes. It is caused by the sudden dilation of blood vessels in the legs in response to an exaggerated neurological reflex. If you have syncope, and have no history of heart disease or an elevated risk of heart disease, odds are high that you will turn out to have vasovagal syncope -- in which case you should learn as much about this condition as you can.

The Next Step - How Syncope Should Be Evaluated

Now that we have reviewed the broad spectrum of medical conditions that can cause syncope, if you or a loved one have syncope it would be useful to know how doctors ought to sort through all the possibilities, and efficiently get to the correct diagnosis. Sources

Strickberger SA, Benson DW, Biaggioni I, et al. AHA/ACCF Scientific Statement on the evaluation of syncope: from the American Heart Association Councils on Clinical Cardiology, Cardiovascular Nursing, Cardiovascular Disease in the Young, and Stroke, and the Quality of Care and Outcomes Research Interdisciplinary Working Group; and the American College of Cardiology Foundation: in collaboration with the Heart Rhythm Society: endorsed by the American Autonomic Society. Circulation 2006; 113:316.

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