One of the most widely used tests for evaluation of seizures is the EEG. This technique uses electrodes placed on the scalp to measure electrical activity in the brain over time. A typical outpatient recording lasts from 20 to 40 minutes, but it is possible to do recordings for days to weeks when necessary either in the hospital or as an outpatient.

Neurologists who are trained in reading EEGs know the normal patterns of brain waves that occur during sleep and wakefulness. Variations in these patterns can show a region that is damaged (potentially as a source of seizures). The EEG can also show “spikes” or “sharp waves” – particular waves that suggest a risk of seizures. Because these occur randomly, they may or may not be seen on a single brief recording.

Sleep during an EEG allows a more complete evaluation of brain activity. It also increases the chances that an abnormality will be seen if present. Therefore, patients suspected of having epilepsy should have an EEG that includes sleep, particularly, if a waking recording was normal. Often this is done by having patients come to the laboratory sleep deprived (sleeping only a few hours the night before). Alternatively, a longer recording can be obtained either by wearing a device overnight or by staying in a specialized unit in a hospital.

Topic Editor: Carl Bazil, M.D.
Last Reviewed:10/01/05