Diagnosing Epilepsy: EEG's Limits

Topic Overview

Electroencephalography (EEG) is thought to be the most useful test in confirming a diagnosis of epilepsy, but it is not foolproof.

• Some people with abnormal EEG results do not have epilepsy. This is not common.
• About 50% of people with epilepsy will have normal results on their first EEG.^{footnote 1} If epilepsy is still suspected, a follow-up EEG may be done. This second test may be a sleep-deprived EEG, in which the test is done after you have been forced to stay awake for a longer period of time than usual. A sleep-deprived EEG can sometimes reveal abnormalities that did not show up on the regular EEG.
• From 10% to 40% of people with epilepsy will have normal EEG results even after having several EEG tests done.^{footnote 1}

Video and EEG monitoring records seizures on videotape and computer so that the doctor can see what happens just before, during, and right after a seizure occurs. The video records what you are doing while the EEG records the electrical activity occurring in your brain. This type of monitoring may be used:

• When your medical history and repeated EEGs are not enough to figure out what kinds of seizures you are having. Simultaneous video and EEG recording can provide important clues about what type of seizure you have had.
• To evaluate your condition before you have epilepsy surgery.
• To diagnose seizures that are not from epilepsy, such as psychogenic seizures.