If further, or longer duration EEGs are done, the yield increases. It must therefore be clearly understood that the EEG does not prove, nor disprove the diagnosis of epilepsy. There is one important exception to this, and this is with a type of epilepsy called non-convulsive status epilepticus. This may present with bizarre or confused behaviour with semi-purposeful, almost automatic movements. It may be difficult to decide whether this behaviour is epilepsy, but if it is, the EEG helps make the diagnosis

The EEG also is not a good guide to either the activity or prognosis of epilepsy. There is one type of epilepsy, however, in which the EEG is particularly useful—this is typical absence epilepsy (petit mal). In this epilepsy syndrome the frequent seizures may be so brief and subtle that some time may elapse before they are recognized. In children with typical absences, the EEG almost always shows a seizure discharge, which may be induced by hyperventilation, and even more easily after deprivation of sleep.

The EEG is usually not helpful in identifying a cause. Occasionally, however, the EEG may show marked differences between the two sides of the brain, such as a slow wave discharge arising from one particular area. This suggests the presence of a structural abnormality as the cause of the patient’s epilepsy. However, structural abnormalities are best investigated by imaging techniques (brain-scans). These are, after the EEG, the most commonly used investigation in epilepsy.

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