Syncope, or sudden loss of consciousness, usually results after the brain has been deprived of oxygen for about ten seconds.

The most common cause of syncope is an abnormal reaction of the vagus nerve which can slow the heart rate and thus cardiac output and blood supply to the brain. Such a reaction may be due to fright, pain, or trauma.

Other causes of fainting include diseases of the brain, cardiac problems, and a variety of abnormalities of the vessels that cause inadequate blood flow.

Additional diagnoses include arrhythmia, narrowed heart valves, constant posture, abnormal metabolic conditions (including diabetes), dehydration, hysteria, and epilepsy.