What is an EEG and what does it show?

  • September 3, 2020
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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

What does seizure activity on an EEG look like?

Everyone’s brain produces ‘waves’ of electrical activity that can be seen on an EEG. But for people with epilepsy, the kinds of waves that happen are often very noticeable. If you have epilepsy, the EEG may record:

  • A series of rapid spikes in electrical activity
  • Spike waves, when there are rapid spikes then long waves

The treatment team who are doing the test will be trained to analyze seizure activity on an EEG, to help with diagnosing your seizures.

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?

How far back can an EEG detect a seizure?

Even if you don’t have a seizure during the EEG, the test may still be able to identify unusual brainwave patterns (known as epileptiform activity) for several hours or even days after your last seizure. 

Everybody is different, but one study found that EEG can identify epileptiform activity in about half of people in the first 16 hours after a seizure. After more than 16 hours, it becomes harder to ‘spot’ the activity.

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. 

Seizures with a normal EEG and MRI

As noted above, roughly half of people with epilepsy who have an EEG get a ‘normal’ reading. This is because the EEG can only record what’s happening at the time of the test itself. If you don’t happen to have any epileptiform brain activity at that time, then it won’t show up. 

Don’t worry - this doesn’t mean your seizures aren’t ‘real’. It just means more investigation is needed. 

If no epileptiform brain waves show up on a regular EEG, then your doctor might recommend other kinds of test to learn more (such as a sleep-deprived EEG or an ambulatory EEG). They may also recommend things like MRI scans, CT scans, or blood tests which can also help with diagnosing epilepsy. 

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. 

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