What causes fainting?
Fainting or syncope is a transient (temporary) loss of consciousness associated with the patient collapsing to the ground and spontaneous recovery without any interventions. A symptom rather than an illness, the causes range from harmless to life-threatening, often resulting in a challenge to diagnose the cause. 
Approximately one third of all people will experience a syncopal episode during their lifetime. The most common cause is when the blood pressure is too low and the heart doesn’t pump a normal supply of oxygen to the brain.
Syncope results when the brain is temporarily deprived of oxygenated blood. A reduction in cerebral blood flow of 35 per cent or a complete disruption of cerebral perfusion for five to ten seconds results in syncope. Any physiological process that affects cerebral perfusion can result in syncope.
When the reticular activating system (area of the brain responsible for maintaining consciousness) is deprived of oxygen (or nutrients) a loss of consciousness and postural tone (causing collapse) occurs. By falling on to the floor cerebral blood flow is re-established and the patient regains consciousness.
Syncope may be caused by emotional stress, pain, pooling of blood in the legs due to sudden changes in body position, overheating, dehydration, heavy sweating or exhaustion. Syncope may occur during violent coughing episodes as a result of rapid changes in blood pressure. It also may result from several heart, neurological, psychiatric, metabolic and lung disorders. Syncope can also be the result of a side effect of some medicines.
Some causes of syncope which can suggest a serious cause include:
- those occurring with exercise;
- those associated with palpitations or irregularities of the heart; and
- those associated with a family history of recurrent syncope or sudden death.
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