Fainting (syncope) is a defence mechanism used by the brain. If the supply of blood and oxygen to the brain becomes too low, the brain will immediately take any available blood and oxygen from the rest of the body at the expense of other bodily functions.
Your breathing rate will be increased (hyperventilation) as your brain tries to increase its supply of oxygen. Your heart rate will also increase, to pump more blood from the rest of your body to your brain.
The increase in heart rate will lead to a loss of blood pressure in the rest of your body (hypotension) as blood is diverted to your brain. The combination of hyperventilation and hypotension can then lead to a temporary loss of consciousness and muscle weakness, resulting in fainting.
There are different types of fainting which are caused by the blood supply to the brain being interrupted for different reasons. The different types of fainting are explained below.
Neurally mediated syncope
Neurally mediated syncope is also called reflex syncope. Neurally means that it is related to the nervous system, which is made up of your brain, nerves and spinal cord. There are three types of neurally mediated syncope. They are:
- vasovagal syncope
- situational syncope
- carotid sinus syndrome
These are explained in more detail below.
Vasovagal syncope
Vasovagal syncope occurs when something triggers a temporary malfunction in your autonomic nervous system. The autonomic nervous system is the part of the nervous system responsible for regulating many of the body’s automatic functions, such as heartbeat and blood pressure.
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
Situational syncope
Situational syncope is also a type of fainting related to the nervous system. It occurs when a bodily function or activity places a sudden strain on your autonomic nervous system. Possible causes of situational syncope include:
- coughing
- sneezing
- swallowing
- laughing
- passing stools
- urinating
- 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.
Carotid sinus syndrome
The carotid sinus is a collection of sensors in the carotid artery. The carotid artery is located in your neck and is the main artery that supplies blood to your brain. The carotid sinus helps to regulate the flow of blood through the carotid artery into your brain.
Some people can develop a hypersensitive carotid sinus. This means that any physical stimulation of the carotid sinus can cause the sensors suddenly to trigger a 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.
Orthostatic hypotension
Orthostatic hypotension is a condition in which your blood pressure falls suddenly when you stand up, which can cause fainting. 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.
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 effect 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.
Cardiac syncope
Cardiac syncope is where the blood supply to the brain is interrupted due to an underlying problem with your heart. 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.
