What is an EEG?
An electroencephalogram (EEG) is a test that measures the electrical signals your brain cells use to communicate. It can help diagnose epilepsy and other brain conditions, like a stroke or sleep disorders, by detecting abnormal patterns in your brain waves. During the test, small sensors called electrodes are attached to your scalp to measure your brain activity, which appears as a graph on a screen. In some cases, electrodes are placed in brain tissue after minimally invasive surgery.
Penn Medicine has teams of experts who conduct neurodiagnostic assessments, like EEGs. Our providers have specialized training in EEG testing and analysis. We’re also part of many research studies that shape clinical practice, including trials looking for noninvasive ways to map the brain. Because abnormal EEG patterns can be subtle, the combination of our extensive expertise and leading technology means we can provide an accurate and precise diagnosis that guides your treatment plan.
Types of EEG testing
All EEGs work by recording your brain waves to look for abnormal activity. There are several ways this can be done, and the approaches may be combined to get the most accurate results.
The most common type, a routine EEG may be one of the first tests done to check for epilepsy or another disorder. This test might also be used after a diagnosis to monitor your condition or changes in medication.
During an evoked potential test, a trigger is used to provoke a seizure or seizure-like activity in your brain so that it can be recorded. Triggers may include looking at flashing lights or taking quick breaths. Your care team is there to provide treatment right away if you have a seizure during the test.
A prolonged EEG lasts several hours and records your brain activity while you’re awake and asleep. It may be done in a health care center or while you’re at home. Testing over a longer period increases the chances of abnormal activity being recorded.
Your care team may take a video recording of you during your EEG. This helps your doctor see your movements and behavior during abnormal brain activity. A long-term video EEG takes place in a health care center for several days. You might need a long-term test to determine if you have epilepsy or another condition, or to record how often you have seizures, see how well antiseizure drugs are working, or assess whether you’re a candidate for epilepsy surgery.
An ambulatory EEG takes place over a few days while you’re at home and doing your usual activities. If you have seizures, an ambulatory EEG can help your doctor learn how often they occur, whether you need medication, or if your current medication can be tapered. Sometimes ambulatory EEGs are recorded on video with equipment you set up at home.
Stereoelectroencephalography (stereo-EEG) requires minimally invasive surgery to create small holes in your skull so that electrodes can be placed temporarily into your brain tissue. This type of EEG is done if you have epilepsy that hasn’t responded to medications. Placing electrodes in your brain can help your doctor determine where your seizures begin and whether you’re a candidate for epilepsy surgery. A stereo-EEG can also map the areas of your brain to help plan surgery.
Why your doctor might recommend an EEG
If you’ve had seizures or if your doctor suspects you have epilepsy, an EEG can show what type of seizures you’re having and what triggers them. The test may also be used to help diagnose other conditions that cause changes in your brain waves. These include sleep disorders, brain tumors, brain damage after a head injury, stroke, brain infections, and diseases like encephalopathy. If you’ve already been diagnosed with epilepsy or another brain condition, EEGs can help your doctor monitor the disease and make treatment decisions.
What happens during an EEG
To prepare for an EEG, your care team will give you full instructions, which may include stopping certain medications. You’ll also need to avoid caffeine for eight hours before your appointment. Wash your hair with shampoo the night before, but don’t use conditioners or styling products. You might also be asked to limit your sleep, which may increase the chances of recording abnormal brain activity during your test.
When you arrive for your EEG, you’ll lie on a reclining chair or bed. A member of your care team will place several small discs called electrodes on your scalp, using medical glue to hold them in place. You’ll lie still as the electrodes pick up your brain activity and a machine records it as wavy lines on a screen. For an evoked potential EEG, you might need to take deep, fast breaths or to look at flashing lights. The test may be recorded on video.
A routine or evoked potential EEG takes around 20 to 40 minutes, while a prolonged EEG takes an hour or longer. If you’re having a long-term video EEG, the test will take several days, and you’ll stay in the hospital during that time.
For an ambulatory EEG, you can go home after the electrodes are attached to your scalp. You’ll be given a recorder to wear and optional video equipment to observe and track any seizures. You can do your daily activities during the test, which lasts one to three days.
A stereo-EEG is done in a hospital under general anesthesia, so you’ll be asleep during the surgery. After the anesthesia takes effect, your care team will place a frame on your head to keep it stable and secure. Your surgeon makes small incisions in your scalp and will drill tiny holes in your skull. Small electrodes will then be placed through the holes to reach brain areas where your seizures may begin. Imaging tests may be done before and during the procedure to offer guidance. Afterward, your care team will remove the head frame and put a bandage or soft wrap around your head. Your care team will take brain scans to see the exact locations of the electrodes. Surgery can last several hours.
Afterward, you’ll go to an epilepsy monitoring unit where the electrodes will be connected to a device that records your brain activity. A computer creates 3D images that show where your seizures begin and spread. You’ll be in the hospital for about a week or longer for this type of EEG while seizure activity is recorded. When monitoring is complete, your doctor will remove the electrodes and close the incisions with stitches under local anesthesia. You’ll be awake but numb during this procedure.
What to expect after an EEG
After a noninvasive EEG, you’ll need to wash your hair to remove the glue that held the electrodes in place. You can go back to your daily activities immediately after the test.
After surgery for a stereo-EEG, you may have mild headaches or jaw pain that can be treated. Your care team will give you wound care instructions. Recovery may take one or two weeks.
Are there risks to having an EEG?
A noninvasive EEG is safe and painless. Some people have mild skin irritation for a few hours from the adhesive, but generally an EEG doesn’t cause any side effects. Because a stereo-EEG involves surgery, there’s a low risk of bleeding and infection.
Understanding your EEG results
The results of your EEG will be analyzed by an expert and sent to your doctor, who will discuss the findings with you. If the EEG recorded a seizure, your doctor may be able to tell you what type of seizure you had. To help with a diagnosis, you may also need other tests, like imaging tests.
The results of a stereo-EEG will help your doctor determine if epilepsy surgery is right for you. Your care team will discuss your results and treatment options with you during a follow-up appointment.
Highly experienced in all EEG testing
Penn Medicine offers the full range of EEG tests to establish a clear and accurate diagnosis so you can receive treatments tailored to your needs. We also work to help you feel comfortable during your EEG, whether you’re here for a routine test, long-term EEG monitoring, or an advanced stereo-EEG.
If you have a seizure disorder, like epilepsy, our specialists are established leaders and researchers in this field. When you come here, you benefit from:
- State-of-the-art long-term epilepsy monitoring: If you need extended, round-the-clock EEG testing, Penn’s Epilepsy Monitoring Unit (EMU) provides a comfortable environment with experienced, specially trained staff. Our advanced monitoring includes high-definition video with night vision.
- Neurosurgeons for the most advanced procedures: Our providers have many years of experience conducting EEGs and perform a significant number of stereo-EEGs each year using advanced technology. Neurosurgeons at our Comprehensive Epilepsy Center use the Robotic Stereotactic Assistance (ROSA) system, which allows them to precisely place electrodes in areas of the brain being tested. This robotic guidance allows our surgeons to complete stereo-EEGs more quickly and with less risk.
- Ongoing research: Our research helps inform our approach to EEG testing. We’re currently studying new and innovative methods for EEG monitoring. Penn researchers have also been collecting data for a national intracranial EEG database for more than a decade. Combining and studying data from several epilepsy centers around the nation helps us improve the diagnosis and treatment of epilepsy.
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