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An electroencephalogram (EEG) is a recording of brain activity.

The brain's cells produce tiny electrical signals when they send messages to each other. During an EEG test, small electrodes are placed on to your scalp.

They pick up your brain's electrical signals and send them to a machine called an electroencephalograph, which records the signals as wavy lines on to a computer screen or paper.

An EEG is painless, takes 30-45 minutes and rarely causes any side effects.

Read more about what happens during an EEG.

When an EEG is used

The pattern of electrical activity produced on an EEG can be used to help diagnose a number of conditions that affect the brain.

An EEG is mainly used to diagnose and manage epilepsy (a condition that causes repeated brain seizures). However, it can also be used to investigate other conditions that affect brain function, including:

EEGs can also be used to diagnose and manage sleep disorders such as insomnia.

An EEG can identify areas of the brain that are not working properly. This helps doctors to make decisions about the type of treatment that is most suitable for you.

EEGs are also sometimes used to determine the level of brain function in people who are in a coma.

Read more about why an EEG is used and EEG results.

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Why should it be done?

Electroencephalography is often used to diagnose and manage epilepsy.

In someone with epilepsy, the electrical impulses in their brain are disrupted. This causes them to have seizures, which are known as epileptic fits. Epileptic fits occur when the nerve cells in the brain are unable to transmit messages effectively.

Monitoring your brain's electrical activity will enable your doctor to identify the type of epilepsy you have and what may be triggering your fits. This will help them to prescribe the most effective type of medication for your condition.

In some cases of epilepsy, surgery can be used to remove the damaged area of the brain. An EEG can be used to help identify the area of damaged brain tissue that needs to be removed.

Read more about epilepsy and how it is treated.

Patients in a coma

EEGs can also be used to monitor the brain activity of people in a coma. An EEG can be used to determine whether a person has reversible brain damage that can be treated, or whether the damage is so severe that there is little or no brain function at all.

This information can help to decide whether someone should be kept alive using a life-support machine. If the results of an EEG indicate that a person is brain dead, a life-support machine will not improve their condition.

Other conditions

EEGs can also be used to investigate a number of other conditions that affect brain function including:

  • dementia – a group of symptoms that are responsible for the decline of brain function
  • head injury 
  • vertigo – the sensation that you are moving even when you are standing still
  • brain tumour – an abnormal and uncontrollable growth of cells in the brain 
  • brain abscess – a pus-filled swelling in the brain that is caused by an infection
  • encephalitis – inflammation of the brain tissue that is sometimes caused by an infection
  • brain haemorrhage – bleeding in the brain
  • cerebral infarct – brain tissue that has died due to a blockage in blood flow
  • sleep disorders
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How is it performed?

An EEG is usually carried out as an outpatient procedure, which means you will be able to go home the same day.

The procedure is painless and you should feel comfortable throughout. The recording lasts for about 30-45 minutes and the whole procedure takes 1-1.5 hours.

Routine EEG

Around 20 electrodes (small metal discs) will be attached to specific areas on your scalp after the skin has been cleansed. The electrodes will be connected to an EEG machine (encephalograph) by wires.

The electrodes are stuck to your scalp using a special paste. Electrode jelly may also be applied to improve the conduction of electrical signals from your brain. In rare cases, the cleaning liquid or paste may cause temporary skin irritation.

You will either be lying or sitting down during the procedure so that you can relax. Your doctor may ask you to close and open your eyes for short periods. They may also ask you to breathe deeply for about three minutes. These actions may produce changes in your brain's electrical activity and highlight any abnormal signals.

At the end of the recording, you may be shown a light that flashes at different rates. This will test your brain's reaction to the light. If you are sensitive to flashing lights, the EEG may show abnormal activity. In some conditions, such as epilepsy, the flickering lights may provoke seizures (epileptic fits).

Simultaneous video monitoring may also be used during the EEG recording so that other activity can be observed and linked to the EEG tracing. If you have epilepsy, there may also be a button connected to the EEG machine that you will be asked to press when you think you are having an epileptic attack.

After the EEG has been completed, you will be able to go home and can carry out activities as usual.

Sleep EEG

A sleep EEG will be carried out while you are asleep. It may be carried out if a routine EEG does not show any epileptic activity or to test for sleep disorders.

When you are asleep, your brainwave patterns change significantly and can make the electrical patterns between epileptic fits more obvious. Sleep EEGs can therefore provide a lot of useful information.

A sleep EEG is usually carried out in a hospital EEG department, but it may also be carried out at a specialist sleep clinic or, on rare occasions, in your home. You may be deprived of sleep before having the EEG.

If the procedure is being used to help diagnose a sleep disorder, the EEG tracing will be recorded along with your heart rate, airflow, respiration, oxygen saturation and limb movement.

Ambulatory EEG

An ambulatory EEG involves recording your brain activity throughout the day and night, over a period of one or more days. You will be given a small, portable EEG recorder that can be clipped on to your clothing.

While your brain activity is recorded, you will be asked to keep a record of your daily activities so that it can later be matched with your EEG reading.

Read more about your EEG results.

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A normal EEG reading indicates that you have a normal pattern of brain activity.

For example, if your brain is working normally, a brainwave pattern called an alpha rhythm should be seen when you are sitting quietly with your eyes closed. When you open your eyes, the alpha pattern should either disappear altogether or become less prominent.

EEG results are often normal because recording a person's brain activity during the times it is abnormal is difficult. For example, someone with epilepsy will often have a normal EEG result because their brain activity only alters during an epileptic fit.

Therefore, further EEG testing, such as an ambulatory EEG, may need to be carried out. An ambulatory EEG involves recording your brain activity throughout the day and night, over a period of one or more days. This will enable doctors to assess your brain activity while you are having a seizure.

Abnormal EEG

An abnormal EEG reading occurs when abnormal electrical brain activity is detected. Some people with epilepsy may have abnormal brain activity between attacks, not just when they are having seizures.

A small number of people who do not have epilepsy may also have an abnormal EEG result. This may indicate that you have a different condition that is affecting your brain activity, such as encephalitis (brain inflammation that is often caused by infection).

Your EEG results will help your doctor decide which course of treatment is best for you. If you have epilepsy, the results may also help identify what is triggering your seizures so that you can take preventative measures.

For example, if your EEG results show that your epilepsy is photosensitive (triggered by flashing lights), you can avoid situations where there is likely to be bright, flashing lights, such as nightclubs, discos and flickering television screens.

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