The abnormal rhythm can happen at any age and is so fast that the heart cannot pump out blood as well as it should. As a result, the person suddenly faints or passes out during an attack. There may be little or no warning and these episodes are characteristically caused by sudden exercise, fear or noise. Sometimes the heart spontaneously recovers back to a normal rhythm, but not always, and therefore attacks are potentially life threatening.
Because of its rarity, and because the patient may appear to fit if the loss of blood supply to their brain lasts for sufficient length of time, attacks may be mistaken for the much commoner condition of epilepsy.