EEG (electroencephalogram)

About electroencephalogram (EEG)

An EEG is used to monitor electrical brain activity to help diagnose conditions affecting your brain.

Conditions, such as epilepsy, can affect certain parts of your brain. An EEG can help to show the area from where the electrical impulses come from and if they have changed in any way from what is usually expected.

A series of small electrodes are placed at different points on your scalp to measure the electrical activity naturally occurring in your brain. The electrodes are attached to wires, which are linked to a computer. The computer records the signals as wavy lines and these represent your brainwave patterns. The results of your EEG are looked at by a neurophysiologist (a doctor specialising in conditions of the brain and nervous system). He or she will look at the wavy lines to identify any unusual patterns.

The doctor that looks at your EEG results will take into account the condition you’re being tested for, such as epilepsy encephalitis (inflammation of your brain). With epilepsy, for example, if you have a normal EEG, this doesn’t always mean that you don’t have epilepsy. And, vice versa, if you have an abnormal EEG, this doesn’t always mean that you do have epilepsy.

The procedure is painless and safe. An EEG doesn’t involve passing electricity down the wires to your brain.

Preparing for an electroencephalogram (EEG)

If your doctor suspects that you may have epilepsy, he or she will recommend you have an EEG.

An EEG is carried out in hospital by a technician trained in the procedure and interpreted by a neurophysiologist.

Your doctor will explain how to prepare for your EEG. Your hair should be clean and dry and you shouldn’t use any hair products before the test.

Don't stop taking any medication before the test unless you're advised to do so. If you’re having a sleep EEG, your doctor may give you a sedative. This relieves anxiety and helps you to relax and fall asleep.

The technician will discuss with you what will happen before, during and after your procedure, and any pain you might have. This is your opportunity to understand what will happen, and you can help yourself by preparing questions to ask about the risks, benefits and any alternatives to the procedure. This will help you to be informed, so you can give your consent for the procedure to go ahead. You may be asked to do so by signing a consent form.

What happens during an electroencephalogram (EEG)

The whole procedure, including attaching the electrodes, takes about 90 minutes. The EEG recording itself usually takes about 30 minutes.

You will sit or lie down and small electrodes are placed on your scalp. You may have 30 or more electrodes put on. The electrodes may be stuck onto your scalp using gel. This is done so that the electrodes are as close to your skin as possible to get an accurate reading.

The electrodes record and increase (amplify) the size of the electrical signals in your brain. These signals are passed down the wires to the electronic equipment that provides a reading.

You will need to keep still during the EEG. During the test, you may be asked to do certain things, such as breathe deeply, open and close your eyes or look at flashing lights. These activities can stimulate your brain activity.

Sleep-deprived and sleep EEG

This type of EEG helps to check if your brain activity changes when you’re tired or asleep. You may be asked to stay awake the night before your test or to wake up very early on the day of your test. You may be given medicines to help you fall asleep. A sleep EEG can take a few hours to complete.

Ambulatory EEG

An ambulatory EEG records your brain activity while you’re moving around. It can be used to record your brain activity over 24 hours or longer. The electrodes are attached to a small machine which you can wear on a belt around your waist. Your doctor will ask you to keep a diary of your daily activities to identify any situations that may cause your brainwave patterns to change.

Video telemetry EEG

You may need to stay in hospital for up to two weeks for this type of EEG. The aim of this test is to try and record a seizure on video as well as an EEG reading. You will wear an ambulatory EEG and will be videoed around the clock. The visual monitoring can help find out what type of seizures you’re having, what may be triggering them and whether your medicines are working.

What to expect afterwards

The results of your EEG will be discussed with you once the recording has been analysed. This can take a few weeks, so your results will probably be sent to a neurologist (a doctor specialising in conditions that affect the nervous system) or, if it's for your child, a paediatrician (a doctor who specialises in children's health), who will discuss them with you at your next appointment.

If you have an EEG as an outpatient procedure, you will be able to return home afterwards.

You will need to arrange for someone to drive you home if you have been given a sedative or had a sleep-deprived EEG test.

Recovering from an electroencephalogram (EEG)

You should be able to resume your usual activities when you get home.

Sedation temporarily affects your co-ordination and reasoning skills, so you must not drive, drink alcohol, operate machinery or sign legal documents for 24 hours afterwards. If you’re in any doubt about driving, contact your motor insurer so that you’re aware of their recommendations, and always follow your doctor’s advice.

What are the risks?

EEG is a safe procedure and there are very few risks.

Side-effects

Side-effects are the unwanted but mostly temporary effects you may get after having the procedure.

You may have some glue on your scalp, which should wash away with shampoo. If you were asked to stay awake the night before the test, you may feel tired.

Complications

Complications are when problems occur during or after the procedure.

There is a chance you may have a seizure during the EEG, especially if you’re asked to breathe rapidly or flashing lights are used.

I’m having an electroencephalogram (EEG) test to monitor my brain activity. How will the doctor know there is something wrong?

Answer

Your doctor will look at the patterns of your brain activity. Any unusual patterns can show there is something wrong.

Explanation

Conditions, such as epilepsy, can affect certain parts of your brain. The EEG monitors electrical activity in your brain and records it as wavy lines. Your doctor is trained at reading these brainwave patterns. Most people have similar brainwave patterns. If your brainwave pattern looks different to a normal pattern, it can indicate that you may have epilepsy.

Around one in 10 people with epilepsy don’t have any unusual brain activity, and some people may show unusual brain activity despite having no history of seizures. For this reason, your doctor will use the results of your test, your medical history and symptoms to diagnose your condition.

Why do I need to stay awake before a sleep electroencephalogram (EEG)?

Answer

You will usually be asked to stay awake the night before having a sleep EEG so that it’s easier for you to fall asleep when you have the test.

Explanation

Your brain activity changes when you’re asleep so sometimes this is the best time to pick up unusual activity. Your doctor may suggest a sleep EEG if the standard EEG doesn’t show any unusual activity or if your seizures occur when you’re sleeping.

If you’re having a sleep EEG, your doctor will ask you to stay awake the night before your test so that it’s easier for you to sleep during the procedure.

What happens during a video telemetry electroencephalogram (EEG)?

Answer

When you stay in a hospital room a video recording will be taken so that doctors can review your activity and link this to your EEG results.

Explanation

There will be a camera in your room in the hospital. While the camera records what you're doing, you will wear a small device around your waist that will monitor the activity in your brain. Your brain activity can be associated with your actions and this helps your doctor find out exactly what happens before, during and immediately after you have a seizure and understand whether this is due to epileptic activity in your brain.

Seeing how your body reacts during a seizure can help your doctor understand which part of your brain is affected. Video telemetry EEG can also be used to find out if your medicines are working. It can also be used to try and find out whether you might benefit from having surgery for epilepsy. Video telemetry can play an in important role in helping diagnose epilepsy in babies.