effect of temporary impairment of blood circulation to a part of the body. The term is most often used as a synonym for fainting, which is caused by insufficient blood flow to the brain as a result of a fall in blood pressure.

Fainting tends to be preceded first by paleness, nausea, and sweating and then by dilatation of the pupils, yawning, deeper and more rapid breathing, and a rapid heartbeat. The faint usually lasts from a fraction of a minute to several minutes and may be followed by headache, confusion, nervousness, and a feeling of weakness. It is usually prompted by fear, anxiety, or pain.

Carotid sinus syncope, sometimes called the tight-collar syndrome, also causes brief unconsciousness from impaired blood flow to the brain. Unlike the ordinary faint, this syncope is not preceded by pallor, nausea, and sweating. (The carotid sinus is a widened portion of the carotid artery where there are nerve endings sensitive to pressure; when they are stimulated, the heart is slowed, blood vessels dilate, and blood pressure consequently falls, causing, in turn, reduction in blood flow to the brain.) Pressure on the carotid sinuses by a tight collar, by turning the head to the side, in swallowing, or even in shaving the side of the neck over the carotid sinus may be sufficient to cause the syncope, or it may occur spontaneously. This syncope may be used diagnostically, since faintness upon massage of one carotid sinus may suggest a narrowed carotid or basilar artery on the opposite side of the neck.

Syncope involving temporary unconsciousness may also be caused by any of a number of organic (physical) diseases or disorders, such as aortic stenosis, heart failure, and a low level of sugar in the blood.

Local syncope is whitening, weakness, coldness, and numbness of a small area of the body, especially the fingers, as a result of diminished blood flow to the part. It is associated with Raynaud's disease (Raynaud syndrome).