September 4, 2020

What is an EEG and what does it show?

About Epilepsy

An EEG is a test that can help find out if you have epilepsy (and other conditions). Read about the different types of EEG and what EEGs show.

Has your doctor booked you in for an EEG appointment? This kind of test might look a little differentYou might be wondering how it will feel, what happens during an EEG and how long it will last. Your doctor might also have booked you in for a special type of EEG such as an ambulatory EEG or a sleep-deprived EEG.  

Here is everything you need to know before you go. 

What is an EEG?

An EEG is a recording of electrical activity in your brain. It is short for Electroencephalogram and it is one of the ways that doctors diagnose epilepsy. 

Your brain is continually producing patterns of electrical activity and an EEG is able to record some of those signals. If the patterns are unusual, this might be a sign of epilepsy. 

EEG tests are mainly used for diagnosing epilepsy, but they might also be used when:

  • You are preparing for brain surgery
  • Your doctor wants to learn more about your type of epilepsy
  • Your doctor is thinking of reducing or stopping your epilepsy medication
  • It might also show you actually have a brain condition that is not epilepsy
Man wearing common type of EEG headwear


What EEG’s do (and don’t) show

An EEG shows what electrical activity is happening in your brain at the time of the test. However, if you do not have any epileptic activity during the appointment, the test will come back as ‘normal’. This happens with about half of all people with epilepsy who have EEG tests. 

That does not mean you don’t have epilepsy, but your doctor will need more information to diagnose your seizures. This might come from:

  • Videos or detailed accounts of your seizures
  • Seizure diaries - you can use Epsy to record when and where seizures happened
  • Physical examinations
  • MRI scans
  • Family history

Learn more: What happens in your brain during a seizure?

What happens during an EEG?

When you have an EEG test you will visit a brain doctor called a neuropsychologist. During the appointment they will:

  • Clean your scalp (the skin on your head)
  • Attach electrodes to your scalp using a sticky paste. An electrode is a small round metal disk which can sense electrical activity in your brain
  • The electrodes are attached by wires to a machine which shows the electrical activity
  • They then analyze the results

EEG’s are not painful and you will normally feel perfectly comfortable. Your doctor will tell you how to prepare for the test before you go. 

How long does an EEG take?

An EEG usually takes between 30 minutes and one hour. Sometimes, you might need to wear the device for longer with certain types of EEG (see below). 

What are the different types of EEG?

There are several types of EEG. Some of the most common are:

  1. Routine EEG

These last 30 minutes to one hour. You might be asked to breathe deeply, look at a flashing light or open and close your eyes. 

  1. Sleep-deprived EEG

The electrical activity in our brains is different when we are asleep or awake. In a sleep-deprived EEG you will be asked to not sleep the night before the test so you are more likely to fall asleep during the day. This means the neuropsychologist can look at your brain’s electrical patterns when you are sleeping. 

  1. Ambulatory EEG

A routine EEG might be too short to diagnose epilepsy. In an ambulatory EEG, you wear a kind of EEG hat for a few hours or days at a time. This makes it more likely that the EEG will record when you are actually having a seizure. 

Learning about your seizures

EEGs help doctors diagnose your epilepsy, and are one of the many tools that can help them understand your seizures better. While the different types of EEG might seem a little strange at first, they will mean you get the best treatment possible in the end.