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Pinpointing the Epileptogenic Zone with S-EEG

When Precision Counts: Using Stereoencephalography to Better Localize the Epileptogenic Zone Before Resection

In patients with intractable focal epilepsy, surgery to remove the portion of the brain associated with the onset of seizure can eliminate or greatly reduce the number of seizures a patient experiences. For surgery to be successful, the epileptogenic zone (EZ), that area of the brain generating the seizure and the removal of which will cause the seizures to cease, must be precisely localized.

In some patients, noninvasive tests provide sufficiently consistent findings to localize the EZ and proceed directly to surgery. Noninvasive tests include imaging such as magnetic resonance imaging (MRI), positron emission tomography, and single-photon emission computed tomography; scalp electroencephalography (EEG); careful observation of the semiology of the seizure (ie, clinical manifestations of the seizure providing clues to its area of onset) and functional neurological testing to identify any deficits in memory, attention, concentration, speech or language. However, in patients with conflicting or inconclusive findings on initial noninvasive studies, invasive intracranial EEG, where the electrodes are placed inside the brain, may be required to better delineate the EZ.

FIGURE 1. Stereotactic head frame.

In America, the preferred means for invasive EEG has been the placement of subdural electrodes, especially a grid or strip of superficial electrodes or individual depth electrodes. The grid of electrodes requires craniotomy (the opening of the skull) for placement. In Europe, however, stereoencephalography (S-EEG), which does not require craniotomy, has long been the norm, and recent articles suggesting improved outcomes with S-EEG¹ have drawn new interest by American epileptologists and neurosurgeons. Jonathan C. Edwards, M.D., Director of the Comprehensive Epilepsy Center, recently traveled to Montreal for a special training session with European experts in S-EEG and, in collaboration with neurosurgeons Steven S. Glazier, M.D., Director of Pediatric Neurosurgery and Surgical Director of the Comprehensive Epilepsy Center, and William A. Vandergrift III, M.D., is offering this new means of localizing the EZ before surgery to adult patients in South Carolina.

Stereo-EEG allows for the precise placement of depth electrodes to monitor the activity of deeper structures of the brain that could be involved in the EZ or in the propagation of the seizure. Very careful measurements are made of the areas of interest on high-resolution MRI using software specialized for the purpose.

The patient is then placed into the stereotactic head frame (Figure 1) so that the spatial coordinates can be marked and the actual trajectories of the electrodes to be implanted can be planned. In the operating room, the neurosurgeon bores a series of 2-mm holes in the skull using a twist drill; into each a single electrode is inserted and secured with a bolt. Often a robot or other computerized sys- tem programmed with the spatial coordinates and electrode trajectories places the electrodes.

Once implanted, invasive intracranial electrodes (whether conventional or S-EEG) are left in place until the patient has experienced several seizures so that electrophysiological data can be obtained to localize the EZ and plan for surgery. The electrodes can be used not only to monitor brain activity but to stimulate given areas of the brain to determine the functions that they control. For instance, if a given portion of the brain is stimulated and a finger moves, then that portion of the brain is critical for that function. By stimulating a number of electrodes, a functional map of the brain can be created to help guide surgery.

Although rare complications with S-EEG are brain hemorrhage and infection, advocates suggest that, compared with conventional intracranial studies, S-EEG has a lower complication rate, requires a shorter recovery time after implantation, and is less likely to cause headaches due to the reduced bulk of the electrodes vs their grid counterparts. Further studies are required to test these claims.

FIGURE 2. Placement of electrodes in conventional intracranial studies (top) vs S-EEG (bottom).

Unlike more traditional electrodes, which are placed symmetrically to provide as full a picture of brain function as possible, the electrodes in S-EEG are placed very strategically to test and clarify the hypothesis that has been devised based on noninvasive findings as to the location of the EZ (Figure 2). If the decision has been made to proceed to invasive EEG, however, that hypothesis will likely not be clear-cut and there could be two or more regions suspected of being the EZ. One advantage of S-EEG is that, because the placement of the electrodes is less invasive, requiring only a series of small bore holes instead of a craniotomy, multiple regions can be sampled with electrodes. In other words, S-EEG can be used to exclude areas from possible EZ involvement, even if they lie at some distance from the area of primary interest. For instance, if the source of the seizure is thought to be the middle part of the temporal lobe but a good mimicker would be the base of the frontal lobe or the back of the temporal lobe, then, with S-EEG, electrodes could be put in all three places. Such would not have been possible with the traditional EEG because of the surgical trauma involved with the more invasive procedure.

Another advantage of S-EEG is a better, more definitive sampling of the propagation pathway of the seizure. With traditional EEG, if one area showed electrophysiological activity shortly after another, it would not necessarily indicate propagation of the seizure from one area to the next but could simply reflect that one area was being sampled better than the other. With S-EEG, electrodes are placed all along the projected propagation pathway, allowing each step to be tracked so that the source of the onset vs the point to which it propagates can be much more easily discerned.

The long, thin electrodes used in S-EEG and the precision of their stereotactic placement allow them to explore the depths of the gyri and solci (the vertical convolutions of the brain and the fissures that divide them), thus sampling the deep brain more effectively than more conventional EEG.

Although complete removal of a lesion identified on MRI most highly correlates with a good outcome, epilepsy is probably best not defined by purely anatomic means but rather requires a more dynamic understanding of the interplay between the various regions of the brain. Certain areas of the brain are thought to seize together as part of what is known as the epileptogenic network. The ability of S-EEG to sample both deeper tissue and areas of the brain at some distance from one another means that all areas of the brain suspected to be involved in this network can be sampled at the same time. If areas beyond the EZ itself are involved, it might prompt the surgeon to perform a regional rather than a focal resection.

Finally, the precision of S-EEG may allow for resections to proceed in areas adjacent to eloquent zones, those areas of the brain tied to critical neurological functions like language skills, memory, or motor skills. Typically, a patient with an EZ in close proximity to such eloquent areas would not be considered a candidate for surgery because of the danger of lasting neurological deficits. If, however, the eloquent areas could be pinpointed exactly so that they could be avoided, resection might still be possible, meaning that a treatment option would remain open to more patients.

When precision counts, as it does when identifying and removing the portion of the brain that is responsible for seizures while ensuring that essential neurological function is not lost, S-EEG offers neurosurgeons and neurophysiologists at MUSC’s Comprehensive Epilepsy Center an important new tool to achieve excellent outcomes in patients with intractable focal epilepsy.


¹ Cossu M, Cardinale F, Castana L, et al. Stereoencephalography in the presurgical evaluation of focal epilepsy: a retrospective analysis of 215 procedures. Neurosurgery. 2005; 57:706-718. doi: 10.1227/01.NEU.0000176656.33523.1e

This article originally appeared in the January/February 2013 issue of Progressnotes.