Fainting (syncope) is caused by a temporary reduction in blood flow to the brain.
There are different types of fainting, because blood flow to the brain can be interrupted for different reasons.
The different types of fainting are explained below.
Fainting and the nervous system
The most common type of fainting is caused by a temporary malfunction in the autonomic nervous system. This type of fainting is called neurally mediated syncope.
The autonomic nervous system is made of the brain, nerves and spinal cord, and regulates a number of automatic bodily functions, including heart beat and maintenance of blood pressure.
There are three types of neurally mediated syncope. These are explained in more detail below.
Fainting caused by an external trigger
Fainting can occur when an external experience or circumstance triggers a temporary malfunction in your autonomic nervous system. This kind of fainting is called vasovagal syncope.
The malfunction in the autonomic nervous system causes a drop in your blood pressure and a reduction in your heartbeat. This leads to a temporary interruption to your brain’s blood supply.
Vasovagal syncope may be caused by:
- sudden exposure to an unpleasant sight or experience, such as the sight of blood
- standing for long periods of time
- spending a long time in hot or stuffy environments
- a sudden intense episode of stress, emotional upset, fear or anxiety
- a sudden feeling of pain
Fainting and bodily functions
Fainting can occur when a bodily function or activity places a sudden strain on the autonomic nervous system. This kind of fainting is called situational syncope.
Possible causes of situational syncope include:
- passing stools
- some physical activities, such as weightlifting, although this could also be a sign of an underlying heart problem (see cardiac syncope, below)
Situational syncope may also occur after eating a meal or exercising.
Fainting and the carotid sinus
The carotid sinus is a collection of sensors in the carotid artery. It helps to regulate the flow of blood through the main artery in the neck, called the carotid artery, and into the brain.
Some people can develop a hypersensitive carotid sinus. This means that any physical stimulation of the carotid sinus can cause a sudden drop in blood pressure, resulting in fainting. This condition is known as carotid sinus syndrome.
Examples of physical stimulation that may affect your carotid sinus include:
- turning your head to one side
- wearing a tight collar
- shaving over the part of your neck that contains the carotid sinus
Carotid sinus syndrome is very rare in people who are under 40 years of age. It is most common among older men.
Fainting and low blood pressure
Fainting can occur when your blood pressure falls suddenly as you stand up. This drop in blood pressure as you stand up is called orthostatic hypotension.
It is more common in older people, and is the reason for fainting in 1 person in 10 aged 60 years or over who faint. It is a common cause of falls in elderly people. See falls for more information.
When you stand up after sitting or lying down, gravity naturally draws your blood down into your legs, reducing your blood pressure. Usually, the nervous system counteracts this by making your heart beat faster and narrowing your blood vessels, both of which will stabilise your blood pressure.
However, in cases of orthostatic hypotension, something interrupts the process of stabilisation. This means that if you stand up suddenly, the blood supply to your brain is interrupted, causing you to faint.
Some possible causes of orthostatic hypotension are explained below.
- Dehydration (low blood volume). If you become severely dehydrated (where the normal water content of your body is reduced), the amount of fluid in your blood will reduce along with your blood pressure. This will make it much harder for your nervous system to stabilise your blood pressure, increasing your chances of fainting. This can be caused by heavy bleeding, vomiting or diarrhoea.
- Diabetes. Untreated diabetes (a condition where there is too much glucose in the blood) causes frequent urination, which can lead to dehydration. Excess glucose in the blood can also damage the nerves that help to regulate your blood pressure.
- Medication. Some medicines can cause orthostatic hypotension in some people. These include diuretics, which increase the production and flow of urine from the body, beta-blockers and some types of antidepressants.
- Neurological conditions. Some health conditions that affect the nervous system, such as Parkinson’s disease, can trigger orthostatic hypotension in some people.
Fainting and the heart
Fainting can occur when the blood supply to the brain is interrupted due to an underlying problem with your heart. This type of fainting is called cardiac syncope.
If you suspect that you have experienced fainting as a result of a heart problem, you should see a GP.
Possible causes of cardiac syncope include:
- abnormal heart rhythms (arrhythmias)
- narrowing of the heart valves (stenosis)
- a heart attack, which is a medical emergency when the blood supply to the heart is suddenly blocked
Cardiac syncope becomes more common as people get older. For example, it is estimated that a third of people aged 60 years or over who have fainted may have fainted as a result of a heart problem.
Pretending to faint
Rarely, people can pretend to faint. This is called psychogenic syncope.
Psychogenic syncope is more common among young people. In some cases, it is associated with long-term stress or trauma.
Health professionals may investigate the possibility of psychogenic syncope when no other cause for fainting episodes can be established.