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EEG tests and epilepsy

An EEG (electroencephalogram) may be done in hospital in an outpatient clinic by a highly trained specialist called a clinical neurophysiologist. Sometimes it may be done at home. You will be shown how to do this.

Your doctor might ask you to have an EEG test if:

  • They think you might have epilepsy or
  • You have epilepsy, and they need to know more about it or
  • They are unsure whether or not your seizures are epilepsy or
  • You are being considered for epilepsy surgery or
  • They want to withdraw your epilepsy medicines

Sometimes an EEG is done to check whether someone is in non-convulsive status epilepticus. Non-convulsive status epilepticus can change a person’s level of awareness or cause confusion, but might not be recognised as a seizure without an EEG.

The results of an EEG can help doctors to make the right diagnosis and decide on the best treatment. They should always be interpreted by someone who specialises in reading EEG results. This is because reading an EEG incorrectly is one of the most common reasons for people to be given the wrong diagnosis.

What does an EEG test do?

Your brain constantly produces tiny electrical signals. During an EEG test, small sensors, called electrodes, are placed on your scalp. They are attached using a special glue or paste. These are connected by wires to an EEG recording machine. The electrodes pick up the electrical signals from your brain and record them on a computer. 

The electrodes only pick up the electrical signals. They don’t affect your brain and they don’t cause you any pain.

The electrical signals look like wavy lines and these wavy lines show your brainwave patterns. The EEG test can only show your brainwave patterns at the time the test is carried out. At different times, your brainwave patterns may be different.

Most people’s brainwave patterns look similar to other people’s. Sometimes the EEG test shows that a person has different brainwave patterns to other people. These are caused by unusual electrical activity in their brain. They can sometimes, but not always, show that the person has epilepsy.

What information does an EEG test give?

An EEG test gives information about the electrical activity that is happening in your brain at the time the test is carried out.

With many types of epilepsy, you only have unusual electrical activity in your brain when you are having a seizure. The rest of the time your brain activity is normal. So, if your EEG test doesn’t show any unusual activity, it usually means that there is no epileptic activity in your brain at the time the test is being done. This doesn’t prove that you don’t have epileptic activity in your brain at other times. And it doesn’t mean that you don’t have epilepsy.

People with some types of epilepsy have unusual electrical activity in their brain all the time, even when they are not having a seizure. When they have an EEG test, the results can show certain brainwave patterns that doctors recognise. This information is very helpful for doctors when they are making a diagnosis.

A small number of people have unusual EEG test results, even though they never have seizures and they don’t have epilepsy. These could be caused by other medical conditions, problems with their vision, or brain damage. So, an EEG that shows unusual brainwave patterns doesn’t always mean that you have epilepsy.

Can an EEG test show what type of seizures I have?

When an EEG test picks up unusual electrical activity, it shows the areas of your brain where it is coming from. And it can also show up some types of seizure. But it might not show up some focal (partial) seizures unless they involve a lack of awareness.

Can an EEG test show if there is any damage in my brain?

An EEG test only gives information about the electrical activity in your brain. It doesn’t show if there’s any damage or physical abnormalities in your brain.

Will an EEG test cause me to have a seizure?

There’s a very small risk that you could have a seizure during an EEG test. This could be caused by looking at a flashing light or breathing deeply. These activities are usually part of the test.

Your doctor might ask you to reduce your epilepsy medicine or have less sleep than usual before you have some types of EEG tests. This would also increase the risk that you would have a seizure around the time of having the test.

Will I have to give my consent (permission) for an EEG test to be done?

You will usually have to give your consent to having an EEG test done. This is because there is a risk of having a seizure during the test. A video recording is usually taken as part of the EEG test. Your consent will also be needed for this.

Depending on your hospital, you might be asked to give your consent:

  • By your consultant when they refer you to have an EEG or
  • By post, if you are sent a consent form with the appointment letter or
  • At the hospital, just before the test is carried out

If you give your consent, but then change your mind, you can withdraw your consent at any time.

Will having a seizure during an EEG test affect my right to drive?

If you hold a driving licence, having a seizure could mean that you have to stop driving until you have been seizure free for 12 months.

If you are concerned about the risk of having a seizure, talk to the doctor who has asked you to have the test.

Will I have to have more than one EEG test?

There are several ways an EEG test can be done.

If you have an EEG test that doesn’t show any unusual electrical activity in your brain, your doctor might ask you to have another. It can be helpful, if possible, to have an EEG test at times when you are more likely to have a seizure. For example, this might be early in the morning. For some women, it might be around the time of having a period.

Standard EEG tests

You will usually have a standard EEG test at an outpatient’s appointment at the hospital. During the test, you sit or lie down. You may be asked to breathe deeply for some minutes and also to look at a flashing light. These activities can change the electrical activity in your brain, and this will show on the computer. This can help the doctor to make a diagnosis.

You will be asked to keep as still as possible during the test. Any movement can change the electrical activity in your brain, which can affect the results

Routine EEG recordings usually take 20 to 40 minutes, although a typical appointment will last about an hour, including some preparation time at the beginning and some time at the end. Other types of EEG recording may take longer. You can go home as soon as the test has been done.

Sleep EEG tests

Your doctor might ask you to have an EEG test while you are asleep. This could be because your seizures happen when you are asleep. Or you may have had a standard EEG test when you were awake, but it didn’t show any unusual electrical activity. When you are asleep, your brainwave patterns change and may show more unusual electrical activity. There are also some types of seizure that mainly happen during sleep.

A sleep EEG test is usually done in hospital, using a standard EEG machine. Before the test, you may be given some medicine to make you go to sleep. The test lasts for one to two hours and you usually go home once you have woken up.

Sleep EEGs can be particularly useful when epilepsy is suspected in children under 5. This is because there are some types of epilepsy which are common in young children, where seizures mainly happen in sleep. Examples are the epilepsy syndromes autosomal dominant nocturnal frontal lobe epilepsy and electrical status epilepticus during slow wave sleep (ESESS).

For some older people, a sleep EEG can also be useful as brainwave patterns related to focal seizures are more likely to be seen in sleep. Focal seizures are the most common type of seizure in older people.

Sleep-deprived EEG tests

A sleep-deprived EEG test is done when you have had less sleep than usual. When you are tired, there is more chance that there will be unusual electrical activity in your brain. Your doctor might ask you to have this test if you have had a standard EEG test, but it didn’t show any unusual electrical activity. A sleep-deprived EEG can show up subtle seizures, including absence, myoclonic or focal (partial) seizures.

Before you have a sleep-deprived EEG test, your doctor may ask you not to go to sleep at all the night before. Or they may ask you to wake up much earlier than you usually do.

The beginning of the sleep-deprived EEG test is the same as a standard EEG test. You may then fall asleep or doze while the EEG is still recording the activity in your brain. The test lasts for a few hours and you usually go home once you have woken up.

Ambulatory EEG tests

Ambulatory means designed for walking. So you can have an ambulatory EEG test while you are moving around. An ambulatory EEG test is designed to record the activity in your brain over a few hours, days or weeks. This means there is more chance that it will pick up unusual electrical activity in your brain, than during a standard or sleep EEG test.

An ambulatory EEG uses electrodes similar to those used on a standard EEG test. However, the electrodes that are attached to your head are plugged in to a small machine that records the results. You can wear the machine on a belt, so you are able to go about your daily business. You don’t usually stay in hospital while the test is being done.

Your doctor will ask you to keep a diary of your activities, such as sleeping and eating, while you are wearing the ambulatory EEG. They will also ask you, or somebody who is with you, to keep a detailed record of any seizures you have. They will then be able to match up what has been happening with the results of your brainwave activity on the EEG test results.

Video-telemetry tests

Hospital video-telemetry

During a video-telemetry test (vEEG), you need to stay in hospital. A vEEG involves wearing an ambulatory EEG. At the same time, all your movements are recorded by a video camera. The test is usually carried out over a few days. Sometimes your epilepsy medicine is reduced or withdrawn. This is to increase the chances that you will have a seizure that can be recorded.

After the test, doctors can watch the video to see any seizures that you had. They can also look at the EEG results for the time you were having the seizure. This will tell them about any changes to your brainwave patterns at the time of the seizures.

Home video-telemetry

Home video-telemetry can be done at home in a similar way to an ambulatory EEG. You will be shown how to set up the equipment, which will include a video recorder.

You will usually have had other types of EEG tests before you are considered for a vEEG. Here are some examples of why your doctor might ask you to have a vEEG:

  • It’s not clear what type of seizures you have or
  • Your epilepsy medicine isn’t working well or
  • There’s a possibility that your seizures are not caused by epilepsy, but something else or
  • You’re being assessed for epilepsy surgery

Invasive EEG-telemetry

Some people who are being considered for surgery will have invasive EEG-telemetry (iEEG). A neurosurgeon will do an operation to place the EEG electrodes directly onto the surface of the brain or into the brain. The electrodes are called ‘strip’ or ‘grid’ or ‘stereo’ electrodes.

The reason for iEEG is to find out where exactly your seizures are coming from. Another part of this test is called ‘cortical mapping’. Cortical mapping is done to see exactly which part of your brain is responsible for things like your memory or speech. It is done to reduce the risk of complications after surgery.  

If you have any other questions before an EEG test, you could ask your family doctor, epilepsy specialist, epilepsy specialist nurse or the person who will carry out the test.

If you would like to see this information with references, visit the Advice and Information references section of our website. If you are unable to access the internet, please contact our Epilepsy Action Helpline freephone on 0808 800 5050.

Code: 
F064.04

Our thanks

Epilepsy Action would like to thank Dr Aline Russell, Consultant Clinical Neurophysiologist, William Quarriers Scottish Epilepsy Centre, UK, for their contribution.

Dr Russell has declared no conflict of interest.

This information has been produced under the terms of The Information Standard.

  • Updated May 2017
    To be reviewed May 2020

Comments: read the 19 comments or add yours

Comments

I can't believe you have all of this USEFUL and ACCURATE information, yet you're asking for money at the end. This is a critical test to be preformed. It's all ridiculous because just like they said with most people (not myself luckily) EEG's come out NORMAL with people with epilepsy. Blowing money to the wind.

Submitted by Stan on

Hello Stan

Many thanks for your comment. We believe what we do is an important contribution to improving the lives of people with epilepsy. As an organisation which does not receive any government funding , we need to request donations as we want to continue to help to people with epilepsy.

Karen
Epilepsy Action Advice & Information Team

Submitted by Karen, Epilepsy... on

My 15 year old daughter is waiting for an EEG & MRI as she has had 4 seizures. She is type 1 diabetic but her sugars have been ok shortly before the last two seizures. We have been monitoring her closely and checking her blood sugars during the night. She has some rapid drops in blood sugar which ar unexplained. We are running her levels high at the moment, as she can rapidly drop by 10. Can epilepsy affect her blood sugar in this way?

Submitted by Janet on

Hi Janet
Thank you for your question. It must be difficult trying to understand if one medical condition is affecting the other. From our resources we are not aware of epilepsy having an effect on blood sugar levels.

Maybe your daughter's diabetes team will know if there is a connection.

Regards

Diane Wallace
Epilepsy Action Advice and Information Team

Submitted by Diane, Epilepsy... on

My14 year old son has started having apparent Focal Seizures within the last 2 years. He has Right Hemiplegic Cerebral Palsy which was caused by a brain haemorrhage at birth and Autism, his consultant has asked for an EEG however, I don't think my son will tolerate anyone touching his head, let alone remaining still for this. Are there other procedures that will help instead? and also would his brain patterns be different because of the Autism and Hemiplegia?

Submitted by Carol on

Hi Carol
Its sounds like having an EEG could be a problem for your son. If he isn’t able to tolerate having an EEG the doctors may look at other options such as a sleep EEG, or they may decide not to do one.  The EEG can be helpful but it’s not always essential. They will do what is best for him.

We don’t know if the EGG patterns will be altered due to your sons other medical conditions, but there are very specific brainwave patterns for epilepsy. His doctor or another charity specialising in his conditions might be able to answer this question. You could try Scope and the National Autistic Society.

I hope all goes well. If we can be of any more help, please feel free to contact us again. You can contact the Helpline team directly  either by email helpline@epilepsy.org.uk  or phone the Epilepsy Action Helpline freephone 0808 800 5050. Our helpline is open Monday to Friday, 8.30am until 5.30pm.

Regards
Diane Wallace
Advice and Information Team

Submitted by Diane, Epilepsy... on

I am 59 and had my first seizure 4 weeks ago when I was completely unconscious and bit into my tongue and lip. I had a urine infection at the time and wonder what the chances are that the infection caused the seizure. For many years I have had feelings of something bad happening and a funny smell but my doctor said it was to do with stress. As the main carer for my father who has Alzeimers losing my licence has been a real blow . Am undergoing all the usual tests and hoping to get my licence back.

Submitted by Kathy clarke on

Hi Kathy
It can be a real shock having a seizure out of the blue like that. Your urine infection may have lowered your seizure threshold enough to make a seizure more likely.

But it may be that your previous feelings and funny smell were in fact auras such as someone may have with temporal lobe epilepsy for example.

Losing your licence can be such a blow. And especially in your situation. Here is all our information about epilepsy and driving. If the neurologist considers you to have had an isolated seizure then it is worth reapplying for your licence after six months. If your neurologist does not think it is isolated you will need to be seizure free for at least 12 months. If you surrender your licence now and DVLA do not revoke it, you would then be able to start the process of reapplying for your licence after 10 months rather than 12. The link takes you to the full information.

If you do get a diagnosis of epilepsy you would be entitled to a free bus pass. This is because you would be refused a driving licence. I understand this would help you little with your father. But it’s worth knowing about.

If we can be of any more help, please feel free to contact us again, either by email or the Epilepsy Helpline freephone 0808 800 5050.

Cherry
Epilepsy Action Advice and information team

Submitted by Cherry, Epileps... on

Thanks for your reply. I have now had all my tests but do not have an appointment with a consultant till march which seems a very long wait to find out what's wrong with me. I can only assume they haven't found anything nasty or they would be seeing me earlier.

Submitted by Kathy clarke on

Can a sleep deprived EEG be affected by patient not falling asleep or dozing due to heavy mental thoughts and play acting in their thoughts. As opposed to relaxing and drifting off? I personally was highly active in my thoughts and using my imagination thinking about old memories from the military asi I was being tested. Non war time. I'm just very imaginative and was bored so just kept my self awake by doing so. Can this affect the accuracy of the test.

Submitted by Jeremy Neff on

Hi Jeremy

It sounds like you have got a very imaginative mind.

It’s the tiredness that helps with this test. .What you were thinking at the time of the test will not affect the EEG recording.

Regards

Diane
Epilepsy Action Advice and Information Team

Submitted by Diane@Epilepsy ... on

Hi, It is about 15 years which I have seizures during the sleep time. I am using phenobarbital and every 2 to 3 months I have seizures. In all of these 15 years, my EEG's was normal or with minimum abnormality. Next week I have to go for an another EEG and I have to prove that I have epilepsy. what should I do before the test and during the survey to Intensify my brain activity signals in order to find out what's wrong with my brain? Regards,

Submitted by Ehsan on

Dear Ehsan

Thank you for your question.

It would be unusual to use the EEG alone to diagnose if someone had epilepsy or not. An EEG only shows what is happening in your brain at the time the test is being done. It is not able to show what has already happened or what is going to happen in the future. So, if there is no epileptic activity in your brain at the time the test is done, the EEG will show clear results. However, EEG tests can sometimes provide useful information to doctors.

Sometimes a doctor may arrange for a different type of EEG to see if they can get a better result. For example:

As you are still having seizures it could be the epilepsy medicine you are taking is not right for your epilepsy or the dosage isn’t rights. You could contact the Iranian Epilepsy Association to see what other epilepsy medicines are available in your country.

If we can be of any more help, please feel free to contact us again.

Regards

Diane

Epilepsy Action Helpline Team

Submitted by todd at Epileps... on

I had a video telemetry test at home once. I remember trying to avoid the camera the whole time, and having to sleep on my sister's floor bc I have a bunk bed so I couldn't be filmed up there. :( I've never stayed in hospital, not even for an eeg, even though I've had 6 so far... (one normal, 3 [+] sleep, 1 ambulatory, 1 video telemetry)

Submitted by Ruth Collard-Stayte on

Hi my 10'year old daughter is having an EEG as having absence seizures, at the moment at least 1 or 2 - daily. She is concerned how the EEG is attached as she has thick long hair. Any help would be great thank you :)

Submitted by Ruth on

Hi Ruth
All your daughter will need to do is make sure her hair is clear and free of products. This helps the electrodes to stick to the scalp. Conditioner is good at removing the glue afterwards. The EEG won’t harm her hair or head in anyway and she won’t need any of her hair cutting. There is enough room on our scalps for these things. You might find the general NHS information on EEGs useful.

Also here is a link to a you tube video about a child having an EEG. Your daughter might find it reassuring to watch. I hope it all goes okay.

Regards
Cherry
Epilepsy Action Helpline Team

Submitted by Cherry, Epileps... on

I started having absence seizures afew years back at the age of 41 . Small day dreams i thought . My wife and others observed them as stops in motion and head dropping to the left . Some months later i woke in hospital after a grand mal seizure . Weeks later i had a heart attack . Since then i have had 2 more heart attacks, and a grand mal approx every 3 months . With absence seizures in between . Nearly daily . At the time of my last heart attack i suffered a grandmal immediately prior and woke into chest pains . Which scared me alot . A new neurologist is on my case as the old one retired . She does not listen to anyviews me or my wife have. And blames the siezures on the mental stress of heart attacks . Even though i started with these before and heart problem was shown. Had mri eeg ecg with eeg . Have been put on keppra slowly increased to 3000mg daily but to no effect . Also on 17 other medications due to heart problem and chronic pain . Neuro called me for a sleep dep eeg with video for 3 days i have a phobia of hospitals and dont want to go received a letter yesterday stating if i do not attend they will remove me from nuero and treatments. 1 can i request another neuro to see . 2 i am partly disabled and need help dressing etc .my wife cannot stay at the hospital as we have small children at home but would be needed for help but hospital is too far away from home. Any advice ?

Submitted by Lee on

Hello Lee

 

It sounds as though you have been through a lot over the past few years.

 

I can hear how challenging this situation must be for you. As you will be aware, the video monitoring could give the neurologist some very useful information which could move you closer to getting seizure control. If a neurologist is unsure about whether seizures are caused by electrical activity, or happen for psychological reasons this type of monitoring can help them to give you an accurate diagnosis. This will help you to get the right treatment.

 

You could ask for another opinion if you don’t agree with the treatment suggested. Although you don’t have a legal right to this, your GP or specialist has to consider your request. You may also find it helpful to explain your situation to your GP before you do this, as they may know of ways that they can support you with this.  

 

It would also be worth you getting in touch with the hospital to ask what support they may be able to offer when you are in hospital. They may also be able to tell you about other organisations who may be able to support you with this.

 

Karen

Epilepsy Action Helpline Team

Submitted by rich on

I read the article very carefully and I am not seeing the real benefit of a EEG. The patient is told that they have normal results but are still having seizure; patients told they have abnormal reading but are seizure free...

Submitted by J.D. Cox on