EEG (Electroencephalogram)

The factual information on this page has been approved by the EEG department of a UK hospital.

• Introduction
• The EEG Test
• During the test
• Medication
• Interpretation of the EEG
• Children and the EEG
• Other ways the EEG is used:
  • Sleep EEGs
  • Sleep Deprived EEGs
  • The ambulatory EEG
  • Video Telemetry
• Conclusion

Introduction

EEG is short for electroencephalogram. It is the name of a commonly performed and important investigation that most people suspected of having epilepsy will have. This test is only one part of the investigation into diagnosing epilepsy. Further information on how epilepsy is diagnosed can be obtained through Epilepsy Action.

The EEG test

Generally, an EEG is carried out at an outpatients appointment at the hospital. The EEG records brainwave patterns from the continuous tiny electrical signals coming from the brain. The EEG machine then magnifies the size of these signals, without affecting the brain, so that the information can be recorded on paper or computer. This can then be interpreted by the EEG consultant (neurophysiologist).

During the Test

The EEG, which usually lasts around one hour, is painless, with the minimum of discomfort. You will be asked to sit or lie down on a couch and to relax. It is important to keep still during the EEG as body movement can be recorded, making it difficult for the specialist to interpret the results of the test. Electrodes are placed on the scalp by way of a cap or a special glue or paste and connected by wires to the EEG machine.

During the session you may be asked to open and close your eyes. You may also be asked to breathe deeply for some minutes, because this could reveal or increase abnormal brainwave patterns. You may also be asked to look at a flashing light to show if this triggers epileptic activity. If the flashing light produces an abnormal pattern during the EEG, the light is immediately switched off by the staff, so there is little risk of further epileptic activity developing.

Medication

There is generally no need to stop your anti-epileptic drugs before having an EEG, unless your doctor advises you to do so. Never stop your medication except under medical supervision, as this could trigger uncontrolled seizures.

Interpretation of the EEG

The EEG gives information about how the brain works, but not about its structure. A normal EEG does not exclude epilepsy because between seizures the brainwave patterns could be normal. An abnormal EEG does not necessarily indicate that someone has epilepsy, because abnormal patterns can occur for other reasons.

During an outpatient's standard EEG there may be no abnormal activity occurring in the brain, so several EEGs may be needed. Sometimes it can be helpful to record the EEG at times when seizures are more likely (such as, in some women, before the menstrual cycle).

If you have generalised seizures (absences, primary tonic-clonic seizures, myoclonic jerks), EEG abnormality is usually seen on both sides of the brain.

If you have partial (focal) epilepsy, abnormality may be seen in one or more specific areas of the brain and there may also be some abnormality of the background rhythms.

Some people have epileptic activity in their brain that does not show itself as a seizure but which may be picked up on an EEG. In this case, it may be necessary to repeat EEGs from time to time, so that doctors can see what progress is being made and decide on treatment.

Children and the EEG

The interpretation of a child's EEG recording is more difficult than an adult's, because the EEG changes during the childhood years. An adult pattern is usually developed by the age of 15 years. As the EEG pattern in infants and children can vary considerably, an abnormal EEG can be over-reported by those unfamiliar with the EEG patterns of children. The phrase ‘compatible with epilepsy' (similar EEG patterns to those seen in epileptic seizures) is often used. However, it is said by some professionals that this is meaningless, as any finding could be ‘compatible with epilepsy'. This highlights the need for a professional interpretation from a neurophysiologist, experienced in interpreting EEGs in children.

Other ways in which the EEG is used to detect epileptic activity

Sleep EEG

If an EEG recorded during waking has not shown any epileptic activity, an EEG during sleep may be recommended. This is because everyone's brainwave patterns change dramatically during sleep. Sleep can also make the brainwave patterns between seizures more obvious. This extra information can help with diagnosis.

A sleep EEG is also usually recommended when someone has seizures during sleep only. It may be carried out in hospital, or at home using an ambulatory EEG (see below).

Sleep Deprived EEG

Depriving someone of sleep can cause changes in the electrical activity of the brain. Recording these changes on an EEG can provide the doctors with important information. Sleep-deprived EEGs are often used when a routine EEG has failed to show anything useful.

The ambulatory EEG

The diagnosis of epilepsy is made from an account of the seizures given to the doctor personally and/or by someone who has actually seen the seizures. The results of any tests are also taken into account. Where there is doubt about the nature of the seizures it is possible to record the EEG over a period of one or more days, using a small portable EEG recorder which is worn on a waist belt. Recording can thereby be made during normal daytime activities, and during sleep.

When you have a seizure the EEG recorder will record the event which can be viewed later on a special machine in the EEG laboratory. You will be asked to keep an account of daily activities, so that they can be related to the EEG recording made at the time. This investigation is called ambulatory EEG monitoring.

Video-telemetry

Where there is doubt about a diagnosis of epilepsy, or where the type of seizures someone experiences is unclear, video-telemetry can be helpful. This is a test that uses a video camera linked to an EEG machine. The camera will visually record your movements and at the same time the EEG machine will record your brainwave pattern. Both the video and EEG are stored onto a computer that can be reviewed once the test is finished. The consultant will be able to see any episodes/seizures that you may have had, as well as any changes in your EEG at that time. The test is often carried out over a number of days in order to increase the chances of recording one of your seizures.

Conclusion

While the EEG cannot, in itself, diagnose epilepsy, it is a useful tool in helping with the diagnosis and treatment of epilepsy.

See also:

• Magnetic Resonance Imaging (MRI)
• Diffusion Tensor Imaging (DTI)

Further information about the EEG is available from Epilepsy Action by using the Email Helpline or if you live in the UK, by phoning the Freephone Helpline on 0808 800 5050.