Fainting – a loss of consciousness – due to a fall in BLOOD PRESSURE. This may be because the cardiac output has fallen or because the PERIPHERAL RESISTANCE has decreased. The simple faint or vaso-vagal attack (so called because it is mediated by the vagus nerve which supplies the heart) is a result of a failure to maintain an adequate venous return of blood to the heart. This is likely to occur after prolonged periods of standing, particularly when standing still or if sit is very hot. It can also result from an unpleasant or painful experience. Pallor, sweating and a slow pulse are characteristic. Recovery is immediate when the venous return is restored by lying flat.

Syncope can also result when the venous return to the heart is impaired as a result of a rise in intrathoracic pressure. This may happen after prolonged vigorous coughing – the so-called COUGH SYNCOPE – or when elderly men with prostatic hypertrophy strain to empty their bladder. This is known as micturition syncope. Syncope is particularly likely to occur when the arterial blood pressure is unusually low. This may result from overtreatment of HYPERTENSION with drugs or it may be the result of diseases, such as ADDISON'S DISEASE, which are associated with low blood pressures. It is important that syncope be distinguished from EPILEPSY.