Causes

Fainting is due to a temporary interruption in blood flow to the brain, usually associated with a sudden fall in blood pressure.

Vasovagal syncope

This is caused by a sudden loss of tone in the veins, particularly those of the lower limbs, due to the action of the vagus nerve causing relaxation of the smooth muscle of the walls of the veins. The walls of the veins dilate resulting in a reduced flow of blood back to the heart. Although the heart may beat more forcibly to compensate for this the overall effect may be to reduce cardiac output and the blood pressure falls.

The vasovagal reaction can be brought about by several factors:

• Environmental factors - usually in hot, stuffy, crowded locations
• Physical factors - Standing for a prolonged period without moving the leg muscles causes blood to ‘pool’ in the legs resulting in reduced return blood flow.
• Emotional factors - resulting from stress, worry or an unpleasant experience such as the sight of blood or effects of an injury.
• Illness - tiredness, fever, dehydration, late effects of alcohol
• Postural change - standing up too quickly from lying down can precipitate a faint in susceptible individuals. In some the venous tone does not recover quickly enough to compensate for the sudden change to erect posture causing an abrupt drop in blood pressure.

Vasovagal fainting tends to be an isolated occurrence although some individuals do experience recurrent episodes in certain circumstances.

More serious causes of fainting may relate to problems such as cardiac arrhythmias (irregular heartbeat), heart failure, or disease of the heart valves.

Neurological conditions affecting nervous control of the heart, fall in blood glucose levels from treatment of diabetes mellitus and changes in blood levels of potassium or sodium may also induce a faint.

Drugs used to treat hypertension may cause an excessive fall in blood pressure and may particularly cause postural fainting. Other drugs used for treating circulatory problems or some used for emotional disorders can produce fainting as a side-effect.

Situational syncope

Fainting can also arise in specific situations, for example:

Postural syncope – Occurs on sudden changes of body position, particularly on arising from sitting or lying.

Effort syncope – Due to a sudden drop in blood pressure on exercise, usually associated with cardiac abnormalities such as heart valve obstruction or cardiac arrhythmia (abnormal heart rate)

Swallow syncope – Associated with oesophageal (gullet) disease and due to a reflex stimulation of the vagus nerve.

Cough syncope – Associated with forcible coughing and usually due to compression of the veins in the chest form increased intra-thoracic pressure (Valsalva effect).

Transient Ischaemic Attacks (TIAs) – Fainting can be one of the presenting effects of cerebral ischaemia although other neurological symptoms and signs are usually also apparent.

Situational syncope is often a more serious manifestation of a significant underlying disease and may well be a recurring feature.