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Occasionally, someone will actually have a seizure during the EEG recording and this can confirm the diagnosis of epilepsy and the type of seizure which has occurred. However, during most routine EEGs people do not have seizures. This is referred to as an ‘inter-ictal EEG’ - that is, an EEG between (inter) attacks (ictal).

An inter-ictal EEG will often be normal, even in someone who definitely has epilepsy. This is because the electrical disturbance in the brain which occurs in epilepsy comes and goes, and since a routine EEG only gives quite a short sample of what is happening in the brain over 30 minutes or so, any abnormalities could very well fail to show up in the recording. However, the combination of a waking EEG and a sleep EEG will reveal abnormalities in about 80 per cent of adults with epilepsy and in an even higher proportion of children, and this goes up to over 90 per cent with repeated waking and sleep EEGs. For this reason, even if earlier recordings have shown nothing out of the ordinary, your doctor may still want to repeat the test at a later stage.

If abnormalities are revealed by the EEG, their location in the brain can provide useful information about the kind of epilepsy a person has.

Why are EEGs performed when people are already being treated for epilepsy?
Doctors quite often request EEGs so that they can monitor the progress of people with epilepsy, and in particular, many doctors will request an EEG if there is a change for the worse in someone’s epilepsy. This will often fail to give any new information, but occasionally something important is discovered. If someone with epilepsy has had no seizures for a number of years, the question may arise as to whether they could try coming off the drugs to see if they will remain free of attacks without them. This is an important decision and should be taken only after careful discussion with the doctor treating the epilepsy. If the drugs are to be stopped, this needs to be done in slow steps and under careful medical supervision as there is always a risk that the seizures may return, and very severe seizures may sometimes occur if this is done too quickly.

Can the EEG help predict the risk of seizures returning if medication is withdrawn?
In adults, the EEG is not a particularly reliable indicator of the risk of this happening. Other information, such as the type of epilepsy, whether there has been a specific injury to the brain, such as a head injury or brain haemorrhage, and how long the seizures had been occurring before they were brought under control, may be more valuable in predicting the risk of a return of seizures if the drugs are stopped.

With children the value of the EEG in predicting the risk of a relapse if they stop taking anti-epileptic drugs is better. If epileptiform activity is seen in the EEG in children, the risk of seizures returning if the drugs are stopped is, unfortunately, quite high.