Syncope — pronounced “sink–oh–pee” — is the sudden loss of consciousness. It used to be called a swoon. Today we may call it a fainting spell, but the medical term is syncope. It describes a sudden, short loss of consciousness. You become unresponsive, lose muscle tone and may fall. Its occurrence is fairly common and increases with age. Most episodes are not harmful and you regain consciousness without help, although injuries may occur with falls.

The specialists in Lurie Children's Heart Center treat syncope. Learn more.


The causes of syncope are broadly divided into heart-related (cardiac-related) and non-heart-related (non-cardiac-related) causes. Syncope due to cardiac-related causes is more serious and increases your risk of death slightly. The outlook for most people with syncope, however, is excellent. Other causes of loss of consciousness include neurologic disorders such as seizures. In the beginning, both seizures and syncope may look similar.

Non-heart-related Causes of Syncope

The major non-cardiac causes are anxiety, overbreathing (hyperventilation), low blood sugar and decreased oxygen in the blood.

Hyperventilation syncope usually occurs with emotional stress or anxiety. The anxiety causes overbreathing, which alters your body chemistry enough to cause you to faint. Low blood sugar from diabetic treatment or other causes does the same. Faints also occur because not enough oxygen is getting to the brain. A rare condition called subclavian steal may cause unconsciousness when the arms are exercised vigorously. Some medicines can cause syncope. Often, in older adults, syncope may result following meals, especially when the person stands upright after a meal. This is because much blood pools in the abdomen as it absorbs nutrients from the meal.

Heart-related Causes of Syncope

Syncope from cardiac causes is due to a sudden lack of adequate blood flow to the brain resulting from a reduced output of blood from the heart.

Reflex syncope usually occurs in stressful or frightening situations. For example, some people faint at the sight of blood. The fainting is due to nerve impulses that expand (dilate) blood vessels in the lower body and slow the heart rate. Pooling of blood in the lower portion of the body causes the heart to temporarily pump less blood to the brain. The medical term for this kind of faint is called vasovagal syncope. 

There are other less common causes of reflex syncope. Spells of severe coughing may result in a faint. Some people have a tendency to faint with urination or defecation. Temporary pressure on a sensitive spot in the neck causes fainting in some people. As people age, the nervous system's ability to regulate blood pressure becomes less sensitive. Rising from a lying to a standing position or prolonged standing may result in low blood pressure. This condition, called orthostatic hypotension, can cause syncope. 

Cardiac syncope occurs when the heart is not able to pump blood properly. This can happen for a number of reasons. The left ventricular pumping chamber may be blocked by thickened heart muscle as seen in a disease called hypertrophic cardiomyopathy or by a narrowed heart valve as seen in valvar aortic stenosis.

Some heart attacks immediately damage the pumping muscle and cause syncope. Other types of heart diseases cause weakening of the heart muscle known as a “dilated cardiomyopathy” which may also cause syncope.

Rhythm abnormalities may also cause fainting. In some cases, the heart beats so rapidly that too little blood gets to the brain. On the other hand, very slow heart rates due to heart block or pacemaker failure also cause faints. Sudden unconsciousness without any warning symptoms raises the suspicion of a heart rhythm abnormality as the cause.


Your/your child's doctor will ask you about the events surrounding the faint because they may provide a clue as to its cause. A medication history is very important since drugs, singly or in combination, are a common cause of syncope. A routine physical examination will detect most causes of blockage to cardiac outflow. 

Most laboratory tests don't help much. An electrocardiogram (a recording of the heart's electrical activity) may indicate heart abnormalities. A 24-hour recording of the electrocardiogram sometimes identifies spells of very fast or very slow heart rates. An echocardiogram may detect the presence of a dilated cardiomyopathy not noted on the examination.

A tilt table test helps to diagnose reflex syncope. In this test, a person's blood pressure is measured while the table on which they’re lying is progressively tilted toward the head-up position. Even with all these tests, the exact cause for syncope may not be discovered. 


Treatment depends on the most likely cause of syncope.  Heart-related problems can often be repaired. Heart valves causing blockage can be replaced. Pacemakers can be inserted to correct very slow heart rates.

The non-heart-related causes are treated individually. A careful review of medications and their dosages can eliminate drugs as a cause for syncope. Getting up slowly when lying or sitting may also help.

If your child experiences, syncope, call your child's doctor, especially if your child have had a recent change in medicines.

Related Specialties & Services

Read about the specialty areas and services that treat Syncope.