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Intracranial EEG

When scalp EEG and neuroimaging scans do not provide enough information to perform epilepsy surgery, it may be necessary to do a complicated and invasive procedure to record intracranial EEG (using electrodes placed on the brain surface and within the brain) to get an accurate reading of the brain's activity. Recording from intracranial electrodes is also called electrocorticography or ECoG.

There are two types of intracranial EEG:

  • One is called intrapoerative electrocorticography. In this type of ECoG, the electrodes are placed on the brain during a surgical procedure to remove a lesion causing epilepsy and are removed at the end of the surgery. This procedure is only done in the operating room.
  • The other is called extraoperative monitoring. In this type of ECoG, the electrodes are placed in or over the brain where the epileptic focus is thought to reside and are left in and on the brain at the end of the operation. Recordings are then made from the intracranial electrodes while the child is awake and alert, and during seizures. The electrodes are then removed at a later date, usually in about four to five days.

Both types of intracranial EEG recordings require surgery to place the EEG electrodes. The surgeon will open the skull and place electrodes on the surface of the brain in the subdural area, over the first layer of membrane protecting the brain. Depth electrodes may also be placed into deep areas of the brain using very thin wires. Intracranial EEG gives a more accurate reading than scalp EEG, but this is also a complicated and invasive procedure compared to scalp EEG. Results from intracranial EEG can finely pinpoint the epileptogenic region (the area in the brain where seizures begin) so that it can be surgically removed or disconnected while also identifying functional areas of the brain that should be left intact.

When is intraoperative ECoG used?

Intraoperative ECoG is used to guide the surgeon in removing that part of the brain that is causing the epilepsy, particularly when the neurosurgeon is removing a lesion or brain abnormality that is causing the seizures. In this case, the surgeon does the intraoperative ECoG before removing the lesion, to define the extent of the brain causing the seizures, and then again after the removal of the lesion to determine if any residual epileptic activity remains.

When is extraoperative monitoring used?

Extraoperative monitoring is done as preparation for epilepsy surgery in situations where non-invasive techniques have failed to produce enough information to perform the surgery. For example, this procedure might be used if:

  • A surface EEG does not pinpoint the seizure site.
  • Various scans and exams show conflicting information or seizures in more than one region of the brain.
  • Seizures arise from functionally vital areas of the brain.

Types of intracranial electrodes

There are three main types of intracranial EEG, using strip, grid, and depth electrodes.

  • Strip electrodes are multiple electrodes attached to a strip and implanted in the subdural cortical layer of the brain to record electrical activity. They are used when the region to be studied is small.
  • Grid electrodes are multiple electrodes attached to a rectangular grid and implanted in the subdural cortical layer of the brain to record electrical activity. Grid electrodes are used to evaluate larger surface areas.
  • Depth electrodes, which look like a single thin wire, may be used to access structures deep within the brain, such as the amygdala and the hippocampus. 

Intracranial Electrodes
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Preparing for extraoperative monitoring

Since it is a complex and invasive procedure, extraoperative monitoring is only done at medical centers with significant expertise and experience, particularly when dealing with children.

Your child's neurologist and neurosurgeon will explain to you the entire procedure, the operation itself, the following days of continuous EEG recordings, care issues, and the risks involved. You should feel free to ask any questions you have.

The anaesthetist will discuss the procedure with you beforehand and inform you of the preparations required. As with all cases of general anaesthesia, your child will be required to stop eating eight hours before the surgery.

To reduce your child's anxiety, you can explain the procedure to your child in an age-appropriate way, using the information given below. You can also ask your child's doctor or nurse to help explain the procedure to your child.

Before your child comes in for the operation, she may need to stop taking her anti-epileptic medication so that her seizures will return. The treatment team will explain how and when to taper off the medication.

What happens during extraoperative monitoring

Extraoperative monitoring involves two stages. The first stage is an operation to implant the electrodes within the brain. The second stage involves continuous EEG recordings from the brain surface, usually over four to five days, to identify the origin of the seizures.

Stage 1: Implanting the electrodes

The first stage, the operation, begins with general anaesthesia to put the child to sleep. The next step depends on the type of electrode being implanted:

  • Strip electrodes are smaller and may be inserted through burr holes made in the skull.
  • Grid electrodes are larger and require a craniotomy for placement. A craniotomy involves cutting open a portion of the scalp and skull and peeling back the dura membrane to expose the brain so the electrodes can be placed on the brain surface.
  • For depth electrodes, the surgeon uses a thin wire to slide the electrodes through the brain to reach the desired location.

Stage 2: Monitoring

The child will be monitored in the ICU or in a special video/EEG monitoring room with extra nursing care. The child will stay in this special hospital room while her brain activity is continuously monitored on the EEG and her activities are recorded on video. Her brain may also be stimulated with mild electrical impulses via the electrodes to localize areas controlling speech, movement, and sensation. All this should give an accurate assessment of seizures, epileptogenic regions of the brain, and other vital information.

As with any surgery, there is a risk of infection and so antibiotics are usually given while the electrodes are present in the brain. Another possible complication is cerebral edema or swelling of the brain, related to the presence of the electrodes. If this occurs, the swelling may be treated with steroids. If it is severe, the electrodes may be removed.

What happens after extraoperative monitoring

When the doctors have enough information about the origin of the seizures, they will be able to accurately plan their treatment approach. If surgery is decided on, a second operation is done to remove or disconnect the epileptogenic area of the brain and at the same time remove the intracranial electrodes. If not, an operation is done only to remove the electrodes.

By combining the information from the seizure monitoring and the functional mapping, the team can create a brain map that shows where the seizures are coming from and where important function is located in the brain. In this way the team can tell which parts of the brain contributing to the seizures can be safely removed. All of this information will be presented to you and your child as well as the team's recommendation for a surgical strategy.

Elizabeth J. Donner, MD, FRCPC