The EEG Test

'The EEG is the most important clinical tool in evaluating patients with suspected seizures.'

Figure at left:EEG recording of a normal brain showing no unusual activity

Apart from the patient history and the neurological exam, the EEG (electroencephalograph) is the most influential tool in the diagnosis of seizures and epilepsy. It provides a record of ongoing electrical activity in the brain.

An EEG machine is a recording device connected by wires to electrodes pasted at key points on the patient’s head. The electrodes pick up signals produced by electrical discharge of neurons in the related areas of the brain; the amplified signal from each electrode causes pens writing on a moving belt of paper to jump—similar to the action of a seismograph when an earthquake occurs.

The resulting EEG tracing, with its record of electrical discharge, provides a record of activity in key areas of the brain during the period of the test. Excessive discharge (of the type that, if large enough, might cause a seizure) may show up as a sharp spike or series of spikes; some patterns (such as the 3-per-second spike and wave of absence seizures) are unique to particular forms of epilepsy.

EEG recordings of patients while awake are made with the eyes open and with the eyes closed. A flashing light is used to assess whether the patient is photosensitive—that is, if he or she will have a seizure in response to the stimulus of a flashing light.

Figure at left:EEG recording of an absence seizure showing the distinctive 3-per-second spike and wave discharge

Hyperventilation (rapid over breathing) is another common trigger for seizures and is also a feature of an EEG assessment. Almost all patients with typical absence seizures who are not receiving antiepilepsy medication will have the characteristic 3-per-second spike wave EEG pattern during hyperventilation. Patients may be asked to go to sleep during the test because EEG abnormalities are more likely to show up during sleep.

If standard recordings do not produce evidence of seizures, 24-hour EEGs, or portable home EEG monitoring devices may be used. Nasopharyngeal and sphenoidal electrodes (long wires inserted through the nose or inserted into the jaw muscle) may produce information unobtainable from regular recordings. Grid or depth electrodes may be implanted in the brain in a surgical procedure when patients are being evaluated for epilepsy surgery and it is vital to get precise information on where the seizure site is located.

If the type and cause of the seizures are unclear, a type of evaluation known as intensive monitoring may be undertaken. In this procedure, people are videotaped during an EEG recording session. The combined image of EEG tracings and visible behavior helps the physician diagnose the epilepsy and identify affected areas of the brain. Intensive closed circuit TV and EEG monitoring of this type also helps distinguish between true epileptic seizures caused by electrical discharge and non-epileptic seizures caused by psychological factors.

Various ictal (seizure) and interictal (between seizure) EEG patterns correspond to specific seizure types and types of epilepsy, although the correlation varies. While the EEG is almost always abnormal during a seizure, it may be normal between seizures. Thus, lack of interictal EEG abnormalities does not exclude a diagnosis of epilepsy. However, at some time, most epilepsy patients have abnormal EEG discharges. In contrast, some persons with EEGs that show epilepsy-like activity never have seizures. Thus physicians interpret EEG results within the context of other information they are gathering.

Despite some limitations, however, the EEG remains the most important clinical tool in evaluating patients with suspected seizures.