Electroencephalogram (EEG)
The EEG test measures electrical activity of the brain (brainwaves), much as the EKG measures electrical activity of the heart. Routine EEG testing takes place in an outpatient laboratory over a period of about an half an hour (plus an hour for registration and set-up time). Wires are pasted onto the scalp and recording signals amplified.
The test is interpreted later that day, to see if there are any abnormalities in the brainwave patterns, such as those, for example, spikes, that tend to be associated with epilepsy. Aside from spikes, slowing of the normal brain rhythms in certain regions of brain, or all over, may also give clues to abnormalities.
About half of people with epilepsy have EEG abnormalities between seizures, but the other half have normal EEGs between seizures (pickup of abnormalities increases with more that one EEG). Therefore, the routine EEG is helpful if it shows something abnormal, but a normal EEG does not rule epilepsy. Rarely will a routine EEG capture a seizure during the 30 - 60 minutes of actual recording.
In some circumstances, doctors request a prolonged outpatient EEG recording for 4 - 8 hours, often with simultaneous video-recording. Another related test is the ambulatory 24 hour EEG recording, with wires attached to the scalp run to a purse-sized recording box on a shoulder strap. The EEG patterns are saved over 24 hours of normal outpatient activity. During this time, the patient keeps a log, and pushes an event button to mark the occurrence of a typical symptom.
During a seizure, the EEG demonstrates a rhythmical build-up of electrical activity. The place at which this activity begins can help to identify the seizure focus. Unless seizure activity is very frequent, a prolonged EEG recording session may be required to capture a seizure. Fortunately, diagnosis and treatment with medicines (as opposed to surgery) usually does not require recording of a seizure.
EEG in isolation never establishes a diagnosis of epilepsy, since a few percent of the normal population has EEG spikes. A history of seizure-like events would be required to diagnose epilepsy. Contrarily, the EEG cannot rule out epilepsy, since EEG can be normal between seizures in people with epilepsy. If a patient has a good story for seizures, a negative EEG should not discourage the clinician from treating the patient for those seizures. Therefore, the EEG is a adjunctive test, helpful to add additional support to a diagnosis of epilepsy, and to help to classify and localize the type of seizures.
A clinician will refer a patient for specialized EEG testing when prolonged recording or correlation of EEG and behavior are required. Inpatient Video-EEG monitoring is done in patients who are being evaluated for possible curative seizure surgery. Video monitoring may also be done in patients where there is a question as to whether the patient is suffering from epilepsy or one of the imitators of epilepsy.
In such a procedure, the EEG is left attached for several days to the patient who can wander freely around the room on a cable. A TV camera records behavior. Medications may be discontinued to provoke seizures for analysis. The most important thing to learn from this is what type of electrical activity is present at the start of a seizure and where in the brain it occurs. Ambulatory EEG monitoring also can be accomplished in the outpatient setting, with special cassette recorders.