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 Dr. 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 done?

Video EEG (electroencephalography) is routinely done over a number of days to record a person’s brain activity when a seizure occurs. Ideally, the EEG shows the electrical activity in the brain at the beginning, during, and after a seizure. 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 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?

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 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. This is commonly done in a person who has temporal lobe epilepsy with focal impaired awareness seizures (complex partial seizures). Tests results that show seizure activity in the tip of a temporal lobe and a spot or scar tissue on an MRI in the same area suggests that surgery may be a good option.

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 cases, we need to know if seizures come from both sides or if they start on one side but spread quickly to the other side. Surgery can not be considered until there is more information.

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 brain activity much better than scalp electrodes. They can also be used to map how the brain works near where the seizures start ("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 are used if seizures may start in deeper areas of the brain and not on the surface. Depth electrodes are thin wires 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 are selected 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, similar to 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 surface of 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.

What else are invasive electrodes used for?

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 with a brief, tiny electrical current. The person is watched for changes in movement, strength, language, and vision. A map is made based on the location of each stimulation point. This helps the team understand the function of those brain areas and if surgery is safe to do. Once enough information is recorded, the electrodes are removed and a decision about 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 often requires many tests done over weeks to months. Some people may need surgery to place electrodes (depth, grids, or strips) on the surface or into the brain. This testing can help find where seizures start when other tests can’t. While electrode placement surgery may have a small chance for serious risks, the data from these invasive electrodes can give much more detailed information about seizure location than scalp electrodes. If your doctor recommends this testing, it is to help ensure the best possible outcome from surgery and to minimize epilepsy surgery risk. People considering epilepsy surgery should be seen at an epilepsy center to consider all treatment options during the presurgical work-up.

Authored By: 
Mohamad Z. Koubeissi MD and Patricia O. Shafer RN, MN
Authored Date: 
10/2018
Reviewed By: 
Joseph I. Sirven MD
on: 
Sunday, October 14, 2018