Having hair that is dirty, oily, or covered with hair spray or other hair preparations. This can cause a problem with the placement of the electrodes.
What To Think About
If the doctor thinks that a person has epilepsy but the EEG is normal, the technologist running the EEG test may have the person look at a flashing light (photic stimulation), breathe fast and deeply (hyperventilation), or sleep during the test. These techniques sometimes show epileptic EEG patterns that did not show up at first. If epilepsy is suspected after an initial EEG, the doctor may repeat the EEG more than once.
An EEG done during a seizure will almost always show abnormal electrical patterns. This makes an EEG useful when a doctor thinks that a person is having psychogenic nonepileptic seizures (also called pseudoseizures), which have no physical cause but can be caused by stress, emotional trauma, or mental illness. Psychogenic seizures do not cause abnormal electrical activity in the brain and will not show abnormal EEG results.
Other tests that may also be done include:
Video EEG. Video EEG records seizures on videotape and on computer so that the doctor can see what happens just before, during, and right after a seizure. This test can be very helpful in finding the specific area of the brain that the seizures may be coming from. It is also helpful in diagnosing psychogenic seizures, which may look like real seizures but do not affect the electrical activity in the brain. Video EEG may be used short-term or long-term:
Short-term monitoring is done on an outpatient basis and may last up to 6 hours.
Long-term monitoring is done in the hospital and may last 3 to 7 days.
Brain mapping. Brain mapping is a fairly new method that is very similar to EEG. With electrodes placed on the person's scalp to transmit the brain's electrical activity, a computer makes a color-coded map of signals from the brain. It is sometimes done to find a specific problem area in the brain that has already shown up on a regular EEG. Doctors are still not certain how brain mapping could be best used.
Ambulatory EEG monitoring. In ambulatory EEG monitoring, the person is able to move around, and the test allows for long periods of time in recording of electrical activity in the brain. Fewer electrodes are attached to the person, and the person carries a small, portable recording unit. The recording may last for a full day or more, and the person is allowed to leave the hospital. Ambulatory EEG monitoring is not as accurate as a regular EEG.