Introduction: What Is an EEG and Why Is It Important?
An EEG, or Electroencephalogram, is a painless, safe test performed to record electrical activity in the brain. It is one of the most significant tools doctors use to diagnose epilepsy, to determine the type of seizure a person has, and to guide treatment options. Although seizures can occur anywhere and at any time, they do not always occur during a short exam. This calls for different types of EEGs, depending on what the doctor is looking for. Let’s explore them!
1. Routine EEG
A routine EEG is usually the first step taken when epilepsy is suspected. Routine EEG’s typically last between 20 to 30 minutes and can be done in a hospital, clinic or neurologist’s office. This is a non-invasive test that records brain wave activity via small metal electrodes placed on the scalp. These electrodes then detect the electrical impulses produced by the brain’s neurons and display them as wavy lines on a computer screen or paper. The patterns of the waves can help doctors identify a variety of neurological conditions. During the test, you can expect to lie down comfortably and possibly be asked to do simple tasks like taking deep, rapid breaths (hyperventilate) or looking at flashing lights to see how the brain responds. In addition to wakefulness, the EEG may capture drowsiness or early sleep stages which may be helpful in your diagnosis.
This EEG is relatively low risk; however, there are some limitations. While it can sometimes pick up signs of epilepsy, it may not show anything abnormal. This is why additional types of EEGs may be needed.
2. Sleep-Deprived EEG
In the case that a routine EEG does not provide clear results, a sleep-deprived EEG may be recommended by a Neurologist. The sleep-deprived EEG is like a routine one, except this is done when you’ve had much less sleep than usual. You may be instructed to sleep only a few hours (or not at all) before the test. This approach is used to detect subtler seizures, like focal seizures or absence seizures. It is especially helpful in unveiling seizure patterns that are more difficult to pick up when you are fully rested. The rationale for this method is that being sleep-deprived can lower the threshold for seizure activity, making it more likely for seizure activity or markers of epilepsy to occur during the EEG.
3. Prolonged EEG
A prolonged EEG is simply a standard EEG that lasts longer than the duration of a routine EEG. These may be used when seizures are infrequent or subtle, making them harder to catch during a shorter test. By testing for several hours, this EEG increases the chance of detecting abnormal brain activity and can even capture seizures. These can be combined with some of the EEGs specified earlier. For example, you could have a Prolonged Sleep-deprived EEG, which may increase your chances of reaching a diagnosis.
4. Ambulatory EEG (At-Home EEG)
To monitor seizures/brain activity for a longer period of time during normal daily activities including sleep, you may have an ambulatory EEG. This EEG records brain activity continuously over one or more days (usually 24 to 72 hours). It can capture seizures during regular activities and while sleeping, which is helpful if seizures happen infrequently or in the middle of the night or in the home environment with preexisting triggers or stress. You can expect to wear a portable EEG while going about your daily routine at home. This involves electrodes being placed on the scalp, which are connected to a small recording device worn on the body. Often patients are asked to keep track of symptoms or events during their test using a diary. The events reported in the diary are useful to your Neurologist to correlate with your brain activity seen on the EEG.
5. Video EEG Monitoring (Inpatient)
Another common monitoring method is video EEG. Video EEG’s can be done for several days in a special hospital room known as the EMU (Epilepsy Monitoring Unit). Here, while an EEG records your brain activity, a camera will record your physical behavior. Neurologists can then refer to the video when analyzing your brain activity in order to ensure proper identification of a potential seizure or electroclinical correlation. Overall, video EEG is especially useful for confirming an epilepsy diagnosis, identifying the specific type of seizures, or localizing your seizures to prepare for epilepsy surgery. This test occurs in a safe hospital environment and allows your Neurologist to lower your medications if needed for your diagnosis. By matching brain activity to physical activity or symptoms, a clearer understanding of what is happening during seizures is provided by this EEG. Outside the EMU, video EEG can be done in the Intensive Care Unit (ICU) or other hospitalized patients as Long-Term Monitoring (LTM). The goal of these tests is to evaluate or rule out seizures as the cause of presenting neurological symptoms.
6. Intracranial EEG (iEEG) or Stereo-EEG (Inpatient)
In more complex cases where seizures are not controlled with medication, surface scalp EEG’s have not clearly identified the seizure focus, and your Neurologist is considering surgery to remove the part of the brain where seizures begin, an Intracranial EEG may be considered. This is an invasive (surgical) procedure where electrodes are placed inside the skull, directly on or within the brain. This commonly involves several depth electrodes during Stereo EEG or may need removal of bone flap to place strips or grids of electrodes on the brain surface. This test offers high spatial and temporal resolution of neural activity. However, due to its invasiveness, it is only performed when absolutely necessary and with close medical supervision in the hospital setting. This test helps your Neurologist determine whether removal of seizure focus or a neurostimulation device to target your seizures is the right approach for your epilepsy.
Final Thoughts: What Patients and Parents Should Know
Ultimately, a Neurologist’s choice of EEG depends on various factors. These include frequency of seizures, what the presented symptoms or seizure behaviors are, results of previous brain imaging or EEGs, and whether epilepsy surgery is being considered. In many cases, more than one type of EEG is used over time to fully understand a patient’s condition.
EEG’s are essential tools in care for epilepsy. They are noninvasive (except intracranial EEG) and are safe for many ages (adults and children). It is important to not feel too discouraged, because not every EEG will show abnormal activity right away. Each test provides valuable information that helps doctors make informed decisions.
If you or your child is scheduled for an EEG, do not hesitate to ask your doctor what to expect and how to prepare. Understanding the different types of the most common EEGs itself can make the experience less stressful and help you feel more at ease in the process.
References:
Shih JJ, Fountain NB, Herman ST, Bagic A, Lado F, Arnold S, Zupanc ML, Riker E, Labiner DM (2018) Indications and methodology for video-electroencephalographic studies in the epilepsy monitoring unit. Epilepsia.
Schomer DL (2021) Ambulatory EEG Monitoring, Reviewing, and Interpreting. J Clin Neurophysiol.
Benbadis SR (2015). What type of EEG (or EEG-video) does your patient need? Expert Rev Neurother.