EEG Q & A
by Seline Haines, R.EEG.T.
Seline Haines is an EEG Technologist at the Epilepsy Center at Barnes Hospital and a member of the professional advisory board of the Epilepsy Foundation of the St. Louis Region (EFS). This article originally appeared in the newsletter of the EFS.
The first thing I hear from our patients is “How hard is it to get that stuff out of my hair?” As many of you know, it takes a lot of work to finally get rid of the glue. But the EEG is a very important test used for diagnosing epilepsy. EEG recordings help doctors to determine the course of treatment for the individual patient.
What is the EEG?
The EEG (electroencephalogram) displays the electrical activity of the brain. Nerve cells in the brain are constantly creating very small electrical signals, whether a patient is waking or sleeping. The EEG machine contains amplifiers which make these signals, or brainwaves, big enough so we can see them. The electrical signals are picked up by electrodes glued to the scalp, and travel to the amplifiers of the EEG machine and then are either written out on paper or saved on the hard drive of a computer and displayed on the computer’s monitor. There are two electrodes plugged into each amplifier on the EEG machine. The amplifier looks at the two electrode signals coming into it and cancels out signals that are the same. So, the signal that you see on the paper or on the computer screen is actually the difference between the electrical activity picked up by two electrodes. The placement of the electrodes is important because the closer the electrodes are to each other the less differences in their brainwaves. Therefore, if the electrodes are too close, the EEG will look like a straight line instead of showing the brainwaves. This is why the technician measures the head of each patient. Measuring the electrode placements allows the technician to have equal distances between all the electrodes to get clear and symmetrical brainwaves. The technician also has to constantly watch the EEG to make sure the electrodes are working properly and to eliminate any artifact or electrical interference that might occur.
How can the EEG help in the treatment of seizures?
The EEG is still the leading test used to help diagnose seizures. Many people do not have a detectable brain lesion causing their seizures, and tests like the MRI and CT scan show normal brain structure. The EEG, however, can show abnormal electrical function of the brain even when these other tests are normal.
The EEG of persons with epilepsy can be divided into two categories, the interictal and ictal EEG. The interictal (routine) EEG is the EEG recording taken when the patient is not having seizures. Most patients with seizures will have at least one routine EEG. This test is done to look for interictal epiliptiform abnormalities, that is, abnormal activity that can occur in a patient with epilepsy in the absence of an actual seizure. Sometimes the patient is asked to attempt to go to sleep, or to hyperventilate, or a strobe light is flashed in his or her eyes to try to bring out these abnormalities. Finding these abnormalities confirms that the patient has seizures, and helps the doctor determine what type they are.
The ictal EEG is the EEG recording taken during an aura and/or a seizure. This is done when a patient’s seizures fail to respond to treatment, and the doctor wants to confirm the diagnosis, or possibly determine if brain surgery can be used to treat the seizures. Usually, ictal EEG recordings need to be made during closed-circuit television monitoring in an Epilepsy Center. For these tests, the person is usually admitted to the hospital and has their EEG and their video recorded continuously to capture actual seizure events. In our center, it typically takes about 1 or 2 days to confirm a patientUs seizure diagnosis, while it takes an average of 5 days in the hospital to evaluate a patient for epilepsy surgery.
What does the EEG tell the doctor?
The most common interictal EEG abnormality in persons with epilepsy is a spike, which is a burst of electrical activity which stands out from the normal EEG patterns. These spikes can be confined to one area of the brain (focal), or can occur in several areas of the brain independently of each other (multifocal), or come simultaneously from wide areas of the brain (generalized). The pattern of interictal abnormalities can determine if the person has focal or generalized epilepsy. These types of epilepsy have different causes, and may respond to different drugs.
The ictal EEG abnormalities are a little more complicated. These abnormalities can show rhythmic activity but can also show other EEG patterns depending on whether the recording was done from scalp electrodes or from electrodes implanted on the brain itself. The ictal EEG of the person with generalized epilepsy will usually show widespread brain involvement from the onset of the seizure. The ictal EEG of the person with focal epilepsy will usually show the seizure starting from a specific brain area but it can then spread to involve other areas, or even the whole brain. These patterns can tell the doctor which part of the brain is causing the seizure and how much of the brain becomes involved during it.
In summary, the EEG is an important test used in the treatment of epilepsy because it shows abnormal brain signals even if the brain structure is normal. The EEG records the electrical activity of the brain between seizures, and during seizures. By looking at the patterns of the EEG in these different stages the doctor can tell if the patient has seizures, which type of epilepsy the person has, and which part of the brain is causing the seizures. This information can help the doctor to determine which medication is appropriate to treat the seizures. If medications do not help, and a specific area was found to cause the seizures, then the person may be eligible for neurosurgical treatment.