Fainting (syncope) is caused by a temporary reduction in blood flow to the brain.
The blood flow to the brain can be interrupted for a number of reasons. The different causes of fainting are explained below.
Autonomic nervous system malfunction
Fainting is most commonly caused by a temporary malfunction in the autonomic nervous system. This type of fainting is sometimes called a 'neurally mediated syncope'.
The autonomic nervous system consists of the brain, nerves and spinal cord. It regulates automatic bodily functions, such as heart rate and blood pressure.
An external trigger, such as an unpleasant sight, heat or sudden pain, can temporarily cause the autonomic nervous system to stop working properly, resulting in a fall in blood pressure and fainting.
It may also cause your heartbeat to slow down or pause for a few seconds, causing a temporary interruption to the brain's blood supply (vasovagal syncope).
Coughing, sneezing or laughing can sometimes place a sudden strain on the autonomic nervous system, which can also cause you to faint (situational syncope).
The autonomic nervous system can also sometimes respond abnormally to upright posture. Normally, when you stand your heart rate remains the same. However, standing upright can occasionally cause a person's heart rate to increase by more than 30 beats a minute, resulting in symptoms such as light-headedness, breathlessness, palpitations and fainting. This is known as postural orthostatic tachycardia (POTs).
Low blood pressure
Fainting can also be caused by a fall in blood pressure when you stand up. This is called orthostatic hypotension and it tends to affect older people, particularly those over 65 years of age. It's a common cause of falls in older people.
When you stand up after sitting or lying down, gravity pulls blood down into your legs, which reduces your blood pressure. The nervous system usually counteracts this by making your heart beat faster and narrowing your blood vessels. This stabilises your blood pressure.
However, in cases of orthostatic hypotension this doesn't happen, leading to the brain's blood supply being interrupted, causing you to faint.
Possible triggers of orthostatic hypotension include:
dehydration - if you're dehydrated, the amount of fluid in your blood will be reduced and your blood pressure will decrease; this makes it harder for your nervous system to stabilise your blood pressure and increases your risk of fainting
diabetes - uncontrolled diabetes causes frequent urination which can lead to dehydration; excess blood sugar levels can also damage the nerves that help regulate blood pressure
medication - some medicines can cause orthostatic hypotension; these include diuretics, beta-blockers and some types of antidepressants
neurological conditions - conditions that affect the nervous system, such as Parkinson's disease, can trigger orthostatic hypotension in some people
Heart problems can also interrupt the brain's blood supply and cause fainting. This type of fainting is called cardiac syncope.
The risk of developing cardiac syncope increases with age. You're also at increased risk if you have:
- narrowed or blocked blood vessels to the heart (coronary artery disease)
- chest pain (angina)
- had a heart attack in the past
- weakened heart chambers (ventricular dysfunction)
- structural problems with the muscles of the heart (cardiomyopathy)
- an abnormal electrocardiogram (a test used to check for abnormal heart rhythms)
- repeated episodes of fainting that come on suddenly without warning
See your GP if you think fainting is due to a heart problem.
Reflex anoxic seizures (RAS)
Reflex anoxic seizures (RAS) is a type of fainting that occurs when the heart briefly pauses due to excessive activity of the vagus nerve.
The vagus nerve is one of 12 nerves in your head. It runs down the side of your head, passes through your neck and into your chest and abdomen.
RAS tends to be more common in small children and often occurs when they're upset.
The website of the Syncope Trust and Reflex Anoxic Seizures (STARS) has more information about RAS.