Syncope (passing out) is the temporary loss of consciousness because of a sudden decrease in the flow of blood with its supply of oxygen and glucose to the brain. It is a common event in otherwise healthy children and adolescents, but it is important to rule out potential serious causes. Simple syncope can result from stimulation of the autonomic nervous system in the body. It can be caused by sudden pain, anxiety, or emotion. Syncope as a result of standing for a long time, often in a hot environment, is related to blood pooling or collecting in the lower extremities and less blood returning to the brain. A family history of syncope may be helpful for diagnosing a simple faint, but a family history of syncope associated with exercise or sudden death may have greater significance.

Rarely, syncope is cardiac in origin, caused by irregularities in heart function. For example, loss of consciousness may be the result of the irregular rate or rhythm of the heart's beating. Changes in the volume of blood pumped to the brain may also be responsible.

Cardiac syncope is usually associated with virtually no prodromal symptoms (no signs that the syncope was about to occur). It often occurs during exercise. The loss of muscle tone is almost immediate and complete. A history of falling abruptly with injuries is common. After the event, the cardiovascular system often appears normal. Cardiac syncope most commonly occurs in patients with congenital heart disease, Cardiomyopathy (either dilated or hypertrophic-thickened), or rhythm disorders. The rhythm disorder could involve the heart's beating too fast, too slow, or not at all.  Syncope during exercise should always be evaluated for a heart condition.

Studies have shown that 3% of Americans experience at least one episode of syncope during a 26 year period, 30% of whom will have recurrences of the problem. Children are more rarely affected than adults, with incidents occurring in only one tenth of one percent.