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Page last reviewed: 13/07/2011

An electroencephalogram (EEG) is a painless test that records brain activity.

An EEG is a snapshot in time and informs the doctor reporting it what is happening to your brain during the EEG. Along with a detailed clinical history it is used to support a diagnosis of epilepsy.

How it works

When the brain cells send messages to each other, they produce tiny electrical signals. In an EEG test, electrodes (flat metal discs) are placed onto your scalp using a sticky substance. These electrodes pick up the electrical signals from your brain and send them to an EEG machine, which will record the signals as wavy lines onto paper or on a computer.

The EEG rarely causes any side effects and takes 30 to 45 minutes (see How an EEG is performed for more information).

Why it is used

The pattern of electrical activity is useful for diagnosing a number of conditions that affect the brain, particularly epilepsy. EEGs are also used to investigate other conditions that may affect brain function, such as dementia, infection of the brain, head injuries, brain tumours or bleeding on the brain (haemorrhage).

The EEG may help to show your doctor the section of your brain that is not working as it should. It will also help them to decide which treatment may be most suitable for you.

EEGs are also sometimes used for patients in a coma to help determine their level of brain function. For more information, see Why an EEG is carried out.

Page last reviewed: 13/07/2011


An electroencephalogram (EEG) is mainly used to help diagnose epilepsy. Epilepsy is a condition that involves abnormal brain activity, which causes people to have seizures. Seizures occur when the nerve cells in your brain are unable to transmit messages effectively.

By monitoring the electrical activity in your brain, your doctor should be able to diagnose the type of epilepsy you have and identify any factors that may be triggering your seizures. This will help your  doctor to prescribe the most effective type of medication for your condition.

A very small number of people with epilepsy coming from a specific part of their brain are able to have surgery to remove the damaged part of brain tissue that causes their seizures. A special prolonged EEG which is carried out as an inpatient in a designated epilepsy monitoring unit will help to identify the specific area of damaged brain tissue that needs to be removed.

For more information, see Health A-Z: epilepsy.

Patients in a coma

EEGs are also used for some patients who are deeply unconscious (in a coma).

In this situation, an EEG can help to determine whether that person has reversible brain damage that can be treated, or whether the damage is so severe that there is no brain function left at all. This can sometimes help in deciding whether someone should be kept alive by using a life-support machine.

If an EEG machine shows that a person is brain dead, a life-support machine will not be able to improve that person's condition.

Other conditions

The EEG is sometimes used to investigate other conditions that may affect brain function, including:

  • dementia
  • head injury
  • vertigo (sensation that you are spinning even when still)
  • brain tumour
  • brain abscess (collection of infected fluid on the brain)
  • encephalitis (inflammation of the brain)
  • brain haemorrhage (bleeding in the brain)
  • cerebral infarct (brain tissue that has died because of blocked blood flow)
  • sleep disorders

Page last reviewed: 13/07/2011

An EEG is painless and you will feel perfectly comfortable during the procedure. The recording lasts for about 30-45 minutes and the whole procedure takes 1-1½ hours.

An EEG is carried out in hospital and you can usually go home on the same day.

General procedure

Around 20 electrodes (small metal discs) are attached to specific places on your scalp after the skin underneath has been cleansed. They are connected to an EEG machine 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. Very occasionally, the cleaning liquid or paste may cause temporary skin irritation.

You will usually be lying down or sitting in a chair and will be encouraged to relax. Your doctor may ask you to close your eyes and then open them for short periods. You will be asked to breathe deeply (hyperventilate) for approximately three minutes. This over-breathing will usually produce a change in your brain's electrical activity and show up any abnormal signals.

At the end of the recording, you are often shown a light that flashes at different rates. This is done to test the reactivity of your brain to the light. Some people who are sensitive to flashing lights may show abnormal activity in the EEG. With some conditions, such as epilepsy, flickering lights may provoke seizures.

Often, simultaneous video monitoring may be done during the EEG recording, so other activity can be very closely correlated to the EEG tracing. You may also have an event button connected to the EEG machine so that times when you think you are having an epileptic attack, can be easily identified.

After a routine  EEG, the electrode jelly is removed and you can go home straight away and carry out activities as usual.

Sleep EEG

A sleep EEG is done while you are asleep. It may be carried out if a routine EEG did not shown any epileptic activity, or to test for sleep disorders. Brainwave patterns change dramatically during sleep and sleep can make the electrical patterns between seizures (fits) more obvious, so a sleep EEG can provide a lot of information.

You may be deprived of sleep before you have the EEG.

The sleep EEG may be carried out at a sleep centre, in your home or during an overnight stay in hospital. If it is being done to diagnose a sleep disorder, the tracing will be taken together with heart rate, airflow, respiration, oxygen saturation and limb movement.

Sleep-deprived EEG

This test is carried out in the EEG department and usually takes place early in the morning. Before a sleep-deprived EEG you will be asked not to sleep for a number of hours. The EEG department staff will instruct you of how long you must be awake for.

The procedure is exactly ths same as for a routine EEG, execpt that you will be asked to relax and to try to go to sleep during the test, however you will usually be asked not to drive and to be accompanied home with a responsible adult following the EEG.

Ambulatory EEG

An ambulatory EEG means recording your brain activity throughout the day and night, over a period of one or more days. A small, portable EEG recorder is attached to your belt.

While your brainwaves are being recorded, you will be asked to keep an account of your daily activities, which can later be matched up to the traces on your EEG.

Page last reviewed: 13/07/2011

You should discuss the result of your EEG with your neurologist  or epilepsy nurse who will correctly interpret the result for you.

Negative result

If the result of an EEG is negative, the test showed that you have a normal pattern of brain activity.

For example, in healthy brains, a brainwave pattern called the alpha rhythm should display 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.

Many EEGs will have a negative result because it is sometimes difficult to record the times when your brain activity is abnormal. For example, people who have epilepsy will often have a negative EEG result because their brain activity only alters during an epileptic fit.

This is why further EEG testing may have to be carried out, such as a sleep deprived EEG or an ambulatory EEG, to help doctors record your brain activity while you are having a seizure.

Positive result

A positive result means that your EEG detected abnormal electrical brain activity. Some people with certain types of epilepsy may have abnormal brain activity between attacks, not just when they have seizures.

  A small number of people who do not have epilepsy may also have a positive EEG result. This may be an indication that you have a different condition that is affecting your brain activity, such as encephalitis (inflammation of the brain) or it may be not be a significant finding, this is why you should discuss the result of the EEG with a specialist doctor or nurse.

Your EEG results will help your doctor determine the best course of treatment for you. The results may help you to identify the causes and triggers of any fits you may be experiencing. Preventative action can then be taken.

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

Content provided by NHS Choices www.nhs.uk and adapted for Ireland by the Health A-Z.

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