Fainting is most commonly caused by a temporary glitch in the autonomic nervous system. This is sometimes known as neurally mediated syncope.
The autonomic nervous system is made up of the brain, nerves and spinal cord. It regulates automatic bodily functions, such as heart rate and blood pressure.
An external trigger can temporarily cause the autonomic nervous system to stop working properly, resulting in a fall in blood pressure and fainting.
The trigger may also cause your heartbeat to slow down or pause for a few seconds, resulting in a temporary interruption to the brain's blood supply. This is called vasovagal syncope.
The trigger may be:
Low blood pressure when you stand up
Fainting can also be caused by a fall in blood pressure when you stand up. This is called orthostatic hypotension, and tends to affect older people, particularly those aged over 65. 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 and 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 makes you urinate frequently, which can lead to dehydration; excess blood sugar levels can also damage the nerves that help regulate blood pressure
- medication – any medication for high blood pressure and any antidepressant can cause orthostatic hypotension
- 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 heart 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 that checks for abnormal heart rhythms
- repeated episodes of fainting that come on suddenly without warning
See your GP as soon as possible if you think your fainting is related to a heart problem.
Reflex anoxic seizures (RAS)
A reflex anoxic seizure (RAS) is a type of fainting that mainly occurs in young children. It's caused by an involuntary slowing of the heart rate, to the extent that the heart actually stops beating for 5-30 seconds.
The child will often open their mouth as if they're going to cry, but make no sound before turning pale grey and losing consciousness.
They'll either become limp – or, more often, stiff – with their eyes rolling upwards and their fingers clawed. Their body may also jerk a few times.
The seizure usually lasts less than a minute. Afterwards, the child will regain consciousness, but may appear sleepy and confused for a few hours.
Reflex anoxic seizures can be frightening to witness, but they aren't dangerous and don't harm the child.
The seizures will become less frequent as the child gets older and usually disappear by the time they're four or five years of age.
Read more about reflex anoxic seizures.