The Stereotactic Technique in the SEEG Method





Introduction



SEEG is a stereotaxic method that embraces a stereotactic technique


In the SEEG method, as the main goal is to acquire precise anatomical localization of inter-ictal and ictal activities, and to provide reliable seizure electro-clinical correlations, effort should be concentrated on time and space resolution of seizure analyses. A precise SEEG stereotactic technique is essential to accomplish a successful evaluation.


An important step in the SEEG method is to define a strategy for electrode placement. The electrode implantation must be designed to question specific hypotheses of localization, as well as checking functional constraints and definition of surgical resection boundaries. The electrode implantation plan will depend on four data categories that better characterize the patient’s epileptic seizures :



  • •

    Ictal data: video-EEG recording of seizures with analysis of electro-clinical correlations, completed by an early injection ictal SPECT, whenever possible.


  • •

    Interictal data: interictal electromagnetic activity from video-EEG and MEG or high-resolution EEG; PET; neuropsychological and neurological assessment.


  • •

    Structural/morphological data: high resolution and technically adequate MRI (T1 and FLAIR images, reformatted in three different orientations) with image processing when available.


  • •

    Functional data provided by Wada test and fMRI and MEG studies



A basic principle in the formulation of hypotheses is to maintain independence for each category while deliberating about localization and extension of the hypothetical EZ. The independence among categories will circumvent inherited bias associated with the non-invasive methods. As a practical example, not all the MRI identifiable lesions are equivalent in terms of epileptogenicity and their relevance in the hypothesis build up are very variable. In a case of a lesion suggestive of focal cortical dysplasia whose location on MRI is discordant from seizure semiology, the question directed to SEEG could be about epileptogenicity of the MRI visible lesion and its spatial limits, and the hypotheses being about possible different sizes of EZ. In other types of malformations (like polymicrogyria or nodular heterotopia) the question could be if part of the lesion(s) if any is (are) epileptogenic or contributory to the organization of the seizure patterns. In non-developmental vascular, degenerative lesions or traumatic lesions, as encephalomalacia or hippocampal sclerosis, which epileptogenic networks are the consequence of the lesion(s) (if any) or still related to it. In MRI-negative epilepsies, implantation will mainly rely upon electro-clinical data. Careful and undogmatic attention must be exercise regarding subtle lesions with unclear or incomplete imaging features, as the so-called “pseudo-lesions”: normal variants or illusions caused by poor segmentation or low MRI quality that create false MRI interpretations, which can generate erroneous implantation plans and suboptimal guided resections. MRI visible lesions that are not epileptogenic or are related to parts of the epileptogenic zone are perhaps one of the main causes of failure epilepsy surgery.


This chapter is intended to provide guidance and suggestions regarding the technical aspects of the SEEG method. As in any aspect related to SEEG, individual nuances must be considered and carefully incorporated into the SEEG plan. No guidance and suggestions are absolute.


Planning electrode implantations using electroclinical correlations



The implantation plan starts with careful and weighted interpretation of non-invasive data. Not all non-invasive data has the same weight when formulating the hypothesis of implantation


As a general principle, SEEG implantations are design to target key areas of the hypothetical epileptic network, taking into account the cortical volume to be covered. A highly restricted focal implantation will be inefficient and likely excessive because signal information derived from electrodes located in the same gyrus will be roughly similar. Consequently, the (surgical) boundaries of epileptogenic regions are impossible to delineate (because the temporo-spatial ictal dynamics would be impossible to decipher) if some electrodes were not placed remotely in “non-epileptogenic” regions.


Two other important aspects will guide appropriate SEEG implantations: (i) functional “mapping” and (ii) “sentinel” electrodes, to help defining corticectomy limits. The SEEG implantation must be designed to localize the epileptogenic zone, but also to provide the anatomical boundaries of the possible resection. The extent of the epileptogenic zone and the interface with possible eloquent cortical areas are equally important goals for the SEEG explorations. For example, in the above cases, if the hypothetical temporal-perisylvian epilepsy is in the dominant hemisphere, some electrodes will be added at the periphery of the core hypothesis electrodes, that is, in the posterior superior temporal and supra-marginal gyri as well as in the posterior inferior frontal gyrus on the one hand, in the anterior and mid-cingulate gyrus on the other hand.


Expressing hypotheses for designing the implantation is a critical step in SEEG method. The main hypothesis of localization must be falsifiable by the others of lower order and vice-versa. The process requires the generation of alternatives hypotheses, making clear in anatomical terms, the diverse interpretations of the multimodal data collected during phase I. Not only the electro-clinical data, but also the imaging and neuropsychological data will be included in the discussion. The optimal way to organize it is to sort out data by their specific place in the epileptogenic process. Therefore, electro-clinical data allied with ictal SPECT depict the ictal state/network whereas EEG/MEG and PET the inter-ictal state, and MRI, EEG and PET the lesional area. So, the hypotheses do not only infer the possible ictal networks but also their relations with MRI or PET images as well. Therefore, electrode implantation implements the whole hypothetical framework in stereotaxic space.


The stereotactic space defined by Talairach



The indirect targeting, informed by a pre-defined stereotactic space coordinate system, supplements the direct targeting, which is provided by MR images. Both are complementary and necessary


Over the years, several brain stereotactic coordinate spaces were developed and applied in different scientific and clinic scenarios. Historically, the Talairach coordinate system has been applied in the SEEG method since the method’s inception in the late 50’s. , ,


In the process of designing the plan for a SEEG implantation, the formulation of the implantation hypotheses is translated to an anatomical stereotactic space. This step is extremely important because corresponds to the first stage in the transition between the theoretical phase to the surgical application of the principles of SEEG and generated hypotheses. In this “transition period,” a universal and equally understood communication path between surgeons and epileptologists takes place. The stereotactic space, which is defined by stereotactic coordinates, corresponds to the communication path, or the “common language,” in this phase of the SEEG planning, translating the electro-clinical correlations to the stereotaxic space. Equally important, the “common language” provided by the stereotaxic space concept allows a direct comparison across subjects, of patterns of intracortical electrical recordings and structural and functional connectivity. Such comparative analysis improves basic knowledge and concepts of neurobiology that will be apply in several neuroscience subfields.


On an individual basis, despite great progression in medical imaging techniques, they still lack adequate localization of functional cortical areas. Sulci and gyri are clearly identified in modern MRI but accurate identification of key anatomo-functional features are rarely achieved without a system of reference. A combination of direct visualization of structures with indirect localization that is based on a reference system is likely the most adequate method to understand and guide the SEEG implantations and its related guided resections.


The three-dimensional proportional system designed by Talairach and colleagues allows for a practical and precise exploration of key cortical structures. At first impression, cerebral sulci and gyri are so variable across individuals that they would be unfit for any possibility of common localization based on universal reference points. However, there are major lines of cortical enfolding that provide the cortex its most general morphology and permits the collection of so-called “normalized” data.


Three reference lines constitute the basis of the Talairach three-dimensional proportional grid system: the AC/PC, the VAC, the VPC and midline:



  • 1.

    The AC-PC line: The line passes through the superior edge of the anterior commissure and the inferior edge of the posterior commissure. It follows a path essentially parallel to the hypothalamic sulcus, separating the thalamic from the subthalamic region.


  • 2.

    The VAC (vertical AC) line: The line is passing through the posterior/ventricular margin of the anterior commissure, orthogonal to the AC-PC plane.


  • 3.

    The VPC (vertical PC) line: The line is passing through the anterior/ventricular margin of the posterior commissure, orthogonal to the AC-PC plane.


  • 4.

    The m idline: This is the midline interhemispheric sagittal plane



Because of individual variations in heights, length and width of human brains, these measurements, measure in millimeters, are only applicable to one individual. This become more evident in anatomical areas that are distant from the AC-PC line. Consequently, in order to overcome the limitation, a proportional system is applied. The three-dimensional proportional system is established according to the maximal dimensions of the brain in the three planes of space. The system adapts to brains of all dimensions. The proportional localization system marks off the distances separating the basal lines and the more peripheral cortical areas by the definition of limiting planes, which are defined by the following points of reference.



  • 1.

    The highest points in the parietal lobes.


  • 2.

    The most posterior point of the occipital cortex


  • 3.

    The lowest point of the temporal lobe


  • 4.

    The most anterior point of the frontal cortex


  • 5.

    The most lateral point of the parietal-temporal cortex.



The total volume is then divided by horizontal lines, above the AC-PC, in eights, below AC-PC in quarters, anterior to VAC is quarters and posterior to VPC in quarters.


The proportional grid system allows the localization of cortical and subcortical structures with relative precision in all three stereotaxic planes ( Fig. 3.1 ).




Fig. 3.1


The Talairach stereotactic space. MRI T1 images in various orientations (A, coronal; B, axial; C, sagittal) with the interpolation of the Talairach grid stereotactic grid system. Panel D depicts the 3D MRI T1 digital reconstruction within the stereotactic space.


SEEG implantation patterns



The hypothesis of implantation is primordial


As discussed above, the hypotheses are typically generated during a multidisciplinary patient management conference based on the results of non-invasive tests and the appropriate interpretation of these results. It is important to consider that the SEEG exploration has as the primordial goal, the anatomical localization of the epileptogenic zone, but also its anatomical limits and interface with potential eloquent areas. With this premise in mind, SEEG depth electrodes should sample the anatomical lesion (if identified) and surrounding tissue, the more likely structure(s) of ictal onset, the early and late spread regions, and the possible functional networks (cognitive, sensorial-motor, language, visual, etc) involved in seizure semiology or in the hypothetical surgical resection plan.


A three dimensional “conceptualization” of the network is an essential component of the pre-surgical implantation strategy. The conceptualization will consider the three-dimensional aspects of depth electrode recordings, which despite a limited contact size (which is largely compensated by the interpolation process made possible by the electrophysiological methodology: frequencies, spatial and temporal analyses), enable an accurate sampling of the structures along its trajectory, from the entry site to the final impact point. Importantly, the trajectory of the SEEG electrodes is equally important than the target or entry point areas. Consequently, the investigation may include lateral and mesial surfaces of the different lobes, deep-seated cortices as the depths of sulci, insula, posterior areas in the inter-hemispheric cortical surface, etc. The implantation should also consider the different cortical cytoarchitectonic areas involved in seizure organization patterns and their likely connectivity to other cortical and subcortical areas. It is important to emphasize that the implantation strategy is not to map lobes or lobules, but epileptogenic networks, which may involve cortical structures localized in multiple lobes. Furthermore, the exploration should take into consideration possible alternative hypotheses of localization. , ,


Excessive number of electrodes must be avoided


The aim to obtain all the possible information from the SEEG exploration should not be pursued at the expense of an excessive number of electrodes, which will likely increase the morbidity of the implantation. In general, implantations that exceed 15 depth electrodes are rare and the requirement of excessive number of electrodes may indicate a not so well formulated pre-implantation hypothesis framework. In addition, the possible involvement of “eloquent” regions in the ictal discharge requires their judicious coverage, with the two-fold goal to assess their role in the seizure organization and to define the boundaries of a safe surgical resection.


The SEEG implantation patterns are based on a tailored strategy of exploration. In consequence, standard implantations for specific areas and lobes are difficult to conceptualize since SEEG evaluations are individualized, varying from patient to patient. Consequently, patterns of standard SEEG coverage should be applied with extreme caution and reservation. Nevertheless, a few “typical” patterns of coverage are observed.


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Mar 2, 2025 | Posted by in NEUROSURGERY | Comments Off on The Stereotactic Technique in the SEEG Method

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