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Fainting is a temporary loss of consciousness that happens when the brain does not receive enough oxygen. It comes on suddenly, only lasts for a short time and you recover fully within a short time. It is also often called a blackout. The medical term is syncope. It isn't the same thing as a seizure which usually causes jerking.

It is important to seek medical attention if you experience faints. Faints may be caused by a serious problem. However, this is very unusual unless you are aged over 40 or they have happened while you were lying down or during exercise. The most common causes are mentioned below.

When you faint, you become unconscious for a few seconds. It is also called passing out or blacking out. The medical term for this is syncope. You may feel sick and sweaty first or pass out with no warning at all.

When you pass out, you fall to the ground. It isn't the same thing as a seizure which usually makes you jerk. You come around after a few seconds and feel back to normal. Some people feel very tired after they've come around. Usually, fainting happens for a reason, like when you're in pain or have been standing for a long time in a hot place.

Fainting happens because the brain needs a constant supply of oxygen. If that supply falls below a certain level, we fall to the ground, which makes it easier for the more oxygen-rich blood to reach the brain.

Fainting is common at all ages and affects up to 4 in 10 people at least once in their lives. Most people never get medical help. Most people (95%) have their first attack of syncope before they're 40 years old.

If you have it for the first time after 40, it is more likely to be due to a serious underlying problem. The most common cause is a common faint, also called neurally mediated syncope (NMS).

Common faints usually happen for the first time in teenage years and affect girls more than boys. In older people, fainting is more likely to be due to an underlying heart problem, low blood pressure or as a side-effect of medication.

Here are some of the more common causes of faints:

Common faint (NMS): this is also known as vasovagal syncope. It is the most common cause of fainting. NMS can occur in various situations. These include:

  • Fear.
  • Severe pain or emotional distress.
  • After extreme exercise.
  • After prolonged standing, especially in hot places (which is why soldiers on parade may faint).
  • When wearing tight collars that constrict the neck.

During attacks, you may look pale and feel sweaty. Your eyes will usually stay open.

Orthostatic hypotension: this is a fall in blood pressure on standing up, which can cause fainting. It can occur:

  • Due to medication prescribed to lower blood pressure.
  • During being sick (vomiting) or experiencing runny stools (diarrhoea) and other reasons for having a lack of fluid in the body (being dehydrated).
  • As a result of neurological diseases such as Parkinson's disease and peripheral neuropathy.
  • After a big meal.

Cardiac syncope: this occurs due to an underlying heart problem. There may be a family history of sudden death. The faint may be preceded by chest pain or the sensation of having a 'thumping heart' (palpitations) and may happen during exercise. 

The doctor will want to know more about what you were doing when you blacked out.

  • You will be asked whether you've recently started new medication.
  • Try to remember whether you had any warning before you blacked out.
  • Did anyone see you fall? (If so, ask them to speak to the doctor if possible).
  • How did you feel when you came around?

These details will help the doctor to make a diagnosis. Your doctor will examine you. They will check your heart, including your blood pressure when sitting and standing and your pulse. You may be asked to have a heart tracing (an electrocardiogram, or ECG). You may have blood tests for anaemia and diabetes. Further tests of your heart and nervous system may be necessary.

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The most common cause of blacking out is fainting. Other causes include epileptic seizures, syncope due to anxiety (psychogenic pseudosyncope) and other rare causes of faints.

Other causes of blacking out may be due to low blood sugar (hypoglycaemia) and lack of oxygen (hypoxia) from a variety of causes. It may be due to over-breathing (hyperventilation) but this is rare.

You may also black out after a fall or blow to the head or due to excess alcohol or street drugs.

Strokes and mini strokes (transient ischaemic attacks) can also result in a blackout.

Prolonged blackout, confusion after the event, incomplete recovery and tongue biting all suggest that the cause is not a simple faint.

Treatment will depend on the likely cause of your blackout. You may be asked to keep a diary of your faints, including what you were doing when each happened. Most people will only need to see their GP but you may be referred for further investigation and treatment at a hospital.

  • Lie down flat with your legs up on a chair or against a wall or sit down on the ground with your head between your knees. Do not just sit on a chair.
  • Squatting down on your heels can be very effective and is less noticeable in public.
  • When feeling better, get up carefully. If symptoms return, resume the position.

If you faint again:

  • Discuss with your doctor stopping any medication that may be responsible.
  • Avoid alcohol.
  • Drink more fluids such as water or soft drinks.
  • Wear support stockings.
  • Do leg crossing and arm tensing exercises.

The Driver and Vehicle Licensing Agency (DVLA) does not have to be informed of a simple faint but greater restrictions apply if the situation is more complicated or if diagnosis is less clear.

If in doubt, contact the DVLA.

You should call an ambulance if you:

  • Have a blackout while exercising or lying down.
  • Have a family history of sudden and unexplained deaths.
  • Experience chest pain or the sensation of a 'thumping heart' (palpitations).

If the attack happens again or you do not feel completely back to normal, you should also seek urgent medical attention. In all other cases, you should see your GP. If you have lots of attacks, or you hurt yourself because of the faints, your GP may want you to see a specialist. They may also want you to see a specialist if your faints could affect your driving.

You will need to find the underlying cause and try to address it if possible. Common faints are by far the most common cause. Many people who faint know when it tends to happen and how to avoid attacks.

Outlook (prognosis) depends on the underlying cause but is generally very good. In young people, when the blackouts are not associated with any heart or nervous system problem, there is nothing to worry about. In older people, there may be a risk to their health but this is due to the underlying condition and the risks from falling.

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