Many diseases can cause 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 can partially 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 the heart is pumping, thus robbing the brain of sufficient blood flow. Several heart disorders can partially block the flow of blood through the heart. These include:
- Heart valve disease that produces stenosis, especially aortic stenosis and mitral stenosis
- Hypertrophic cardiomyopathy, which, in some cases, can reduce the flow of blood across the aortic valve
- An obstruction in a major blood vessel near the heart (such as a massive pulmonary embolus)
- Cardiac tumors (such as an atrial myxoma, a benign tumor that can obstruct the mitral valve)
Cardiac arrhythmias are a much more common cause of syncope than are obstructive cardiac conditions. Unfortunately, almost 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.
Either sinus bradycardia or heart block can produce significant bradycardia.
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.
There are three general categories of non-cardiac disorders that cause syncope -- neurological causes, metabolic causes, and vasomotor 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:
- Transient ischemic attacks (TIAs), usually involving the vertebrobasilar arteries. The vertebrobasilar arteries are the blood vessels in the back of the neck, next to the spinal cord, that supply the brainstem — the portion of the brain that, among other things, is responsible for maintaining consciousness. A blockage of blood flow in the vertebrobasilar arteries can cause syncope.
- Subclavian steal syndrome
- Normal pressure hydrocephalus (NPH)
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.
Fewer than 1% of people with syncope turn out to have metabolic conditions as a cause. Metabolic causes of syncope include hypoxia (reduced oxygen in the blood, which almost always means 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.
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.
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.