Grid Electrodes for EEG Monitoring

Grid electrodes, like the ones shown here, are placed on the surface of the brain to record electrical activity and help determine where seizures start and whether the person is a candidate for epilepsy surgery. (Images on this page provided courtesy of M. Z. Koubeissi.)

Finding out if you or a loved one could have epilepsy surgery requires a number of tests. The different tests help you and your epilepsy team learn as much as possible about your seizures, what parts of the brain are involved, and if surgery can be done safely.  Finding the seizure onset, or the area in the brain where seizures start is most important. If seizures start from one area and that area can be removed safely, then a type of surgery called a resection can be done. 

Why is Video EEG telemetry done?

EEG telemetry is routinely done to record a person’s EEG over a number of days. The goal of this testing is to record a person’s EEG when a seizure occurs – this shows the electrical activity in the brain ideally at the beginning, during, and after a seizure. If surgery is being considered, video and audio recording is also done with the EEG, so the epilepsy team can see and hear what occurs during a seizure. All this information together helps determine the type of seizure, where it may begin in the brain, and where it may spread or other brain areas that may be involved. This testing may also be known by other terms, for example continuous EEG monitoring or long-term EEG monitoring.

When are scalp electrodes used with video EEG telemetry?

When a person is considering epilepsy surgery, scalp electrodes are first used to record the EEG or brainwaves. These electrodes are attached to the scalp of the head with a paste. Wires then connect to electronic equipment that records and analyzes the EEG. This information can then be viewed on a computer. Ideally, the EEG results, together with results of brain scans (such as MRI or PET) and other data can locate where seizures may come from and if surgery is possible and safe to perform. One example of this is a person with temporal lobe epilepsy with complex partial seizures. Tests results show seizure activity starting in the tip of a temporal lobe and an MRI shows scar tissue (called mesial temporal sclerosis) in the same area.

What happens if scalp electrodes can’t find where seizures come from?

In some people, seizures can’t be well localized using scalp electrodes. The beginning of seizures may not be seen or it may look like more than one brain area is involved. For example, EEG tests may show that seizures involve one of the frontal lobes of the brain, but other tests suggest seizures in a temporal lobe. Or seizure activity may be seen on both sides of the brain. In these situations, we need to know if seizures do come from both sides or if they start on one side but spread quickly to the other side. Surgery can not be considered in either of these situations unless more information is obtained.

When seizures can’t be easily localized, "invasive electrodes" are placed under the skull and on the surface of the brain or into certain brain areas.

There are two main types of invasive electrodes, strips or grids and depth electrodes. These electrodes can pick up electrical activity of the brain much better than scalp electrodes. They can also be used to map how the brain works near where the seizures arise ("brain mapping").

How are invasive electrodes placed?

Grid Electrodes for EEG Monitoring

Surgery is needed to place these invasive electrodes, then the EEG is monitored to record the person during seizures. The surgery to place the electrodes may take only a few hours, but the monitoring may take a number of days or a few weeks.

After the monitoring period, the electrodes are removed. Depending on the type of electrode used and results of testing, surgery may or may not be done at this time. It’s not unusual for a person to be in the hospital for three weeks or so for this procedure and the EEG testing.

What are depth electrodes and when are they used?

Grid Electrodes for EEG Monitoring

Depth electrodes (shown in image to the right) are used if seizures may occur in deeper areas of the brain and not on the surface of the brain. They are thin electrodes that look like a needle. Each electrode is able to record from a number of sites along the electrode. The number of electrodes used will vary depending on what test results suggest for each person. For example, a person could have two to four electrodes placed on each side of the brain. They are placed in specific areas of the brain that have been determined based on scalp EEG monitoring, MRI scans, and other tests.

Surgery is needed to place these electrodes. The person is given general anesthesia so they sleep during the surgery. A frame is then placed on the person’s head to help the surgeon find the exact place to put the electrodes. Very small holes are made in the skull and the electrodes are placed through these. Once in place, the electrodes are secured into the bone of the skull. A head dressing is placed over the electrodes and the wires that lead to the EEG monitoring equipment.

How are subdural strips or grids placed and when are they used?

If the seizure focus is thought to be on the surface of the brain, electrodes called strips or grids are used. They can record electrical activity from many points on the strip or grid. A strip may look like a rectangle, like the size of a small bandaid. A grid is usually shaped like a square and can cover larger areas of the brain.

Surgery is a bit different to place the strips or grids. A larger opening in the skull is made (called a craniotomy) and the strips or grid are placed on the brain. Small thin wires attached to the strips or grids connect to the EEG equipment and are wrapped in the head dressing. The person then goes to the epilepsy monitoring unit to have seizures recorded over a number of days. 

In addition to recording seizures, strip, grid, and depth electrodes are also used to map brain function by a procedure known as brain mapping and electrical stimulation. During this procedure, each point on the strip, grid, or depth electrode is stimulated by delivery of a brief tiny electrical current and the person is observed for changes in movement, strength, language, and vision. A map is drawn based on the location of each of the points stimulated to understand the function of the part of the brain being considered for surgery to make sure it is safe to remove. Once enough information is recorded, the electrodes are removed and a decision regarding surgery can be made.

What are the risks of having invasive electrodes placed?

Depth electrodes have a very small risk of complications. Smaller holes are made in the skull so people may recover quicker and have less pain or discomfort than after grids or strips. Bleeding in the brain and infection are possible complications but happen infrequently. Since surgery to place strips or grids needs a larger opening in the brain, the potential for complications and headaches after surgery is slightly greater.

Summary

Finding out if a person can have epilepsy surgery may take time and many tests. Some people may need surgery to place electrodes (depth, grids, or strips) on the surface or into certain brain areas. This testing can help find where seizures start when other tests can’t. While surgery may have a small chance for serious risks, the testing can give much more detailed information about seizure location than scalp electrodes. People considering epilepsy surgery should be seen at an epilepsy center to consider all options during the presurgical work-up.

Authored By: 
Mohamad Z. Koubeissi MD | Surgery and Devices Editor
Patricia O. Shafer RN MN | Associate Editor / Community Manager
Authored Date: 
Wednesday, July 8, 2015
Reviewed By: 
Joseph I. Sirven MD | Editor-in-Chief of epilepsy.com
Reviewed Date: 
Wednesday, July 8, 2015