Causes of fainting  

Fainting (syncope) is caused by a temporary reduction in blood flow to the brain.

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 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 – 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, resulting a temporary interruption to the brain's blood supply. This is called vasovagal syncope.

Coughing, sneezing or laughing can sometimes place a sudden strain on the autonomic nervous system, which can also cause you to faint. This is called situational syncope.

The autonomic nervous system can also sometimes respond abnormally to upright posture. Normally, when you sit or stand up, gravity pulls some of your blood down into your trunk (torso) and your hands and feet.

In response, your blood vessels narrow and your heart rate increases slightly to maintain blood flow to the heart and brain, and prevent your blood pressure dropping. This results in a slight increase in blood pressure.

However, occasionally, standing or sitting upright can interrupt the blood supply to the heart and brain. To compensate, the heart races and the body produces noradrenaline (the "fight or flight" hormone).

This is known as postural tachycardia syndrome (PoTS), and can result in symptoms such as dizziness, nausea, sweating, palpitations and fainting.

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 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

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 used to check 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)

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.

Page last reviewed: 04/11/2014

Next review due: 04/11/2016