X-ray of bilaterally implanted stereotactic depth electrodes

X-ray of Bilaterally Implanted Stereotactic Depth Electrodes

Anti-seizure medications can successfully treat seizures in two thirds of all people with epilepsy. In the remaining third of people who continue to have seizures despite medications, surgical evaluation is necessitated. 

When is intracranial monitoring needed before surgery?

In some of these people, surgery can be done without invasive intracranial monitoring, i.e., placing electrodes under the skull to accurately find the location of the seizure focus. These are people whose scalp EEG monitoring suggests a specific area of the brain as the seizure focus, and brain imaging and other pre-surgical data are all concordant in pointing to that specific focus. The classical example is the person with the typical seizure manifestations and scalp EEG signatures of temporal lobe epilepsy whose brain MRI reveals mesial temporal sclerosis, a scar tissue in the hippocampus on the same side, indicating concordance of all pre-surgical data and candidacy for surgical resection of the temporal lobe.

In other people, however, non-invasive diagnostic data may not be concordant. For example, a person may have seizure manifestations indicating a right frontal lobe seizure focus, but the EEG shows generalized changes during seizures or even left-sided seizure discharges. Another example is a person whose seizure discharge indicates temporal lobe onset, but brain MRI reveals a lesion elsewhere. In such people, monitoring with intracranial electrodes is often necessary prior to deciding on proper candidacy for surgical removal of the epilepsy focus. 

Besides finding the location in the brain where seizures occur, how else is intracranial monitoring used during the evaluation for epilepsy surgery?

Intracranial monitoring also facilitates mapping of the brain. This means that the same electrodes that monitor the electrical activity of the brain can be used to deliver electrical stimulation to interrupt the function of specific brain regions and assess whether they are responsible for some essential functions such as speech or movement. This mapping is essential for avoiding the surgical removal of brain areas that serve essential functions.

Making a Surgical Plan

Before surgery to implant the intracranial electrodes, a strong pre-surgical hypothesis about the location of the seizure focus is formulated. At major epilepsy centers, a conference is held between the epilepsy specialist, neurosurgeon, neuropsychologist, neuroradiologist, and other specialists to review the non-invasive data. Such data often include scalp-EEG during and in between seizures, functional and anatomic neuroimaging, and neuropsychological testing. The team will then form a strategy for optimal monitoring that will help confirm or rule out the pre-surgical hypothesis.

  • If the seizure focus is believed to be deep, invasive monitoring could be done with depth electrodes. These are thin, needle-like electrodes that are implanted stereotactically, i.e., with the help of a stereotactic frame using pre-calculated coordinates that guarantee accurate targeting of specific deep regions in the brain. 
  • If the seizure focus is believed to be on the surface of the brain, then subdural electrodes are used. Subdural electrodes are either strips or grids of the electrodes that record electrical activity of the superficial cerebral cortex. 

With either type of monitoring, the person is taken from the operating room to the epilepsy monitoring unit for recording intracranial EEG. 

Stereo-EEG versus Subdural Electrodes

Historically, stereo-EEG has been used more frequently in Europe, while subdural electrodes are more commonly used in the U.S. Such trends have recently started to change, with many centers in the U.S. currently using stereo-EEG. 

For presumed superficial seizure foci, subdural electrode monitoring has excellent spatial resolution. However, the superficial cortex constitutes only a small part of the brain regions that may be generating seizures. Recent experience from U.S. epilepsy centers has shown that stereo-EEG monitoring is successful in localizing the epilepsy focus in the majority of people, with a very small rate of complications. 

  • Stereo-EEG requires only small (2.5-mm) drill holes in the skull, unlike subdural grid implantation that requires a craniotomy. 
  • Post-operative pain is much less with stereo-EEG than with grid placements, and the rate of infection is also less. 
  • Because stereo-EEG requires traversing the substance of the brain, bleeding continues to be a potential complication, but fortunately is infrequent. 
    • In European centers with great experience with stereo-EEG, the overall complication rates range from 0.5 to 5%. 
    • To minimize the chances of bleeding, surgeons often use intraoperative angiography to allow visualization of the vessels. 
    • People undergoing stereo EEG are asked to hold antiplatelet or anticoagulation agents for days or weeks prior to surgery.  
    • Intravenous antibiotics are also given before the surgery to minimize the chance of infection.

In summary, stereo-EEG is a relatively safe procedure with a low rate of surgical risks. High-resolution brain MRI and advanced computer software generate accurate coordinates for stereotactic implantation avoiding blood vessels. The decision on which electrode implantation should be done must be individualized per person since the data generated by each type of electrodes are different. 

Authored By: 
Mohamad Koubeissi MD
MD | Surgery and Devices Editor