A closer look at EEG

When someone has had seizures, and it is thought that they might have epilepsy, there are various tests that their specialist might ask for. Two of these tests are the electroencephalogram (EEG) and MRI.

Before we look at EEG in detail, we need to understand that neither of the tests will say for certain whether the person has epilepsy or not. But these tests, alongside other information, might help the specialist to decide if epilepsy is a likely cause of the seizures.  

Electroencephalographs - EEGs

Not all seizures are due to epilepsy. There are other medical conditions that might cause someone to have a seizure for example, diabetes. The difference between epileptic seizures and other seizures is that epileptic seizures are caused by a disruption in the way the brain is working. The fact that epileptic seizures always start in the brain is important when considering the EEG. An EEG looks at what is happening in the brain – the activity of the brain cells. It does not look at the structure of the brain (how the brain is made up).  

‘Brain waves’ and electrical signals

Brain cells (neurones) work by sending nerve impulses from one cell to another to transfer messages around the brain and the body. These messages, called action potentials, happen due to changes in the electrical charge of the cells. So when the brain is ‘working’ cells communicate using electrical signals, and when they do this they ‘give off’ electricity. It is this electrical activity, sometimes called ‘brain waves’, that is picked up on by EEG.

The electrical signals from the brain are picked up by small electrodes (about one centimetre across), which are placed on the person’s head. The electrodes only record the electrical activity of the brain; they do not give out electricity. The electrodes cannot pick up the electrical signals from individual neurones – the cells are far too small and the electrical charge is also too small. Instead, they record the electrical activity from small areas of the brain. The EEG shows brain function, and looks for the presence or absence of specific brain activity in specific areas of the brain. The EEG cannot interpret what the messages are ‘saying’ (or what you are thinking!), only that brain activity is happening.

Because the electrical signals are still quite small, they are amplified (made stronger) so that they can be recorded. The activity is recorded on an electroencephalograph (recording machine) either on paper or more usually on computer.  

Neurones – the scientific name for nerve cells. The brain is made up of millions of neurones. Neurones control all of the body’s functions by communicating using electrical signals.

The 10-20 system

When someone has an EEG, a number of electrodes are used (usually 25-30). These electrodes are put in specific positions on the person’s head, which means that different electrodes can record the activity from different and known areas of the brain. When the technician or doctor is looking at the results of the EEG, they can tell what brain activity is happening, and in which particular part of the brain it is happening in.

To work out where to put the electrodes, a special arrangement called the 10-20 system is used. Each electrode is put either 10 or 20 per cent of the total distance between specific points on the head, done by measuring the person’s head and marking the position with a soft pencil. Each electrode has a number; all the odd numbers are on the left side of the head, and the even numbers on the right. The electrodes also have a letter, depending on the area of brain that it is recording from: F for frontal lobe, T for temporal lobe, P for parietal and O for occipital lobes. The letter Z is used for the line of electrodes sited on the midline of the head.  

Frontal lobes – the area at the front of the brain, behind the forehead. The frontal lobes are responsible for voluntary movement (movement you decide you want to do, for example, walking and talking), conscious thought (for example, deciding you want a cup of tea), learning, speech and your personality.

Temporal lobes – the areas of the brain at the side of the head, above your ears. The temporal lobes are responsible for making memories and remembering, and emotions (such as feeling happy or sad). They are also involved in speech, hearing and perception (how we see the world around us).

Parietal lobes – the area of the brain at the top of your head behind your frontal lobes. The parietal lobes control how we feel and understand sensations. They also control how we judge spatial relationships (such as the distance between two objects), our coordination and our ability to read, write and do maths.

Occipital lobes – the area at the back of the brain: at the back of your head. The occipital lobes are responsible for our sense of sight: receiving information from our eyes and translating it into what we see around us.

What do the readings show?

The EEG records the electrical activity of the brain. The type of brain activity that happens depends on many different things: if the person is awake or in different stages of sleep, what they are doing, and if their eyes are open or closed. Some activity seen in ‘well’ children, would not be expected to be seen in healthy adults.

What’s in a wave?

The EEG recording shows different types of brain waves. A wave is any type of brain activity, which appears as a ‘wave’ shape on the EEG recording. There are different names for the brain waves which are put into bands according to their frequency or number of ‘waves’ per second (see below), and each type of wave looks different on the EEG. Some brain waves happen at particular times or in different areas of the brain.

  • Alpha waves are at a frequency of 8-13 waves per second, and are the typical waves seen in adults who are relaxed with their eyes closed. These waves are clearest in the occipital lobes (the part of the brain responsible for our sight and seeing).
  • Beta waves are at frequencies greater than 13 per second. These are often seen in people who are awake, with their eyes open or closed. They are often seen in the frontal lobes (responsible for conscious thought and movement) and in central areas of the brain.
  • Theta waves happen between the frequencies of 4-7 waves per second, and are also called slow activity. Theta waves occur during sleep and in young children. They are not obvious in adults who are awake.
  • Delta waves are at frequencies up to 4 waves per second. These are the slowest type of wave but have the highest amplitude (strongest signal). Delta waves are common in children under one year. They also happen during some parts of sleep.
  • Gamma waves are at frequencies of 26-100 waves per second.
  • Spikes are very fast waves and are called spikes because of their shape on the EEG. Each lasts less than 80 milliseconds (less than 1/12th of a second) and may be followed by slow delta waves. Spikes clearly stand out from other brain activity on the EEG.
  • Polyspikes are a series of spikes that happen quickly.
  • Spike waves happen when one or more brief spikes are followed by a slow wave, and this happens three times per second.
  • Sharp waves happen over 80-200 milliseconds.  

What does it all mean?

So EEGs show what is happening in the brain during the test, but why is this helpful in epilepsy? Epileptic seizures are caused by disturbed brain activity – which simply means that the normal activity of the brain (and therefore the normal EEG reading) is suddenly interrupted and changes. When a seizure happens during an EEG, the normal pattern of brain activity that is seen on the EEG reading changes, and different brain activity can be seen.

In focal seizures the change in brain activity can be seen only on the electrodes on the part of the brain the seizure is happening in. The readings from the rest of the electrodes remain the same as ‘normal’ brain activity (when a seizure is not happening). In generalised seizures the altered activity can be picked up by all the electrodes, from all parts of the brain.  

Focal seizures – (previously called partial seizures). These are seizures that happen in, and affect, only part of the brain (not both sides of the brain) and start from a 'focal point' in the brain. What happens in these seizures varies depending on which part of the brain is affected and what that part of the brain normally does.

Generalised seizures  – seizures that happen in, and affect, both sides of the brain from the start. There are many different types of generalised seizures but they all involve the person becoming unconscious, even just for a few seconds, and they won't remember the seizure itself. The most well-known generalised seizure is the tonic clonic (convulsive) seizure.

Sounds simple doesn’t it?

Although this all sounds simple, it is not quite so easy! Usually when someone has a seizure, the seizure does not cause a typical pattern on the EEG, it just causes a change in the EEG (although there are some types of epilepsy that have typical EEG patterns associated with them). This means that sometimes the EEG is described as ‘abnormal’ (that is ‘not normal’ brain activity) but does not ‘prove’ that the person has epilepsy.

To complicate this further, some people have ‘abnormal’ EEGs but do not have epilepsy. Also, many people who do have epilepsy will only have ‘abnormal’ activity on the EEG if they have a seizure at the time the test is happening.

This is why having an EEG cannot diagnose epilepsy, and why EEGs are used alongside other tests and investigations.

Information reviewed November 2019

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A closer look at EEG leaflet

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