4.1 Introduction

Stereoelectroencephalography (SEEG) method was developed in France by Jean Talairach and Jean Bancaud in 1950s and has been mostly used in Europe as the method of choice for invasive localization in refractory focal epilepsy. ¹ It has more recently been adopted as an alternative to subdural grids and strips for seizure localization in North America. Large-volume epilepsy surgery centers now offer both subdural electrode and SEEG for seizure localization. Depth electrode implantation is a well-recognized means of localizing seizure foci. ² Different methods exist to accomplish implantation. Stereotactic implantation of parenchymal electrodes has progressed significantly in recent years. The aim of this chapter is to detail the different methods of implantation with a specific focus on percutaneous stereotactic EEG placement. Subdural grids and strips placement is covered elsewhere in detail. Of note, the two methods can be combined in their usage and indications for each have been covered in detail in other studies. ³ This chapter has been constructed chronologically to mirror the planning and operative execution of electrode implantation. Variations to technique and complications are discussed at relevant points in this chronology. Among the different methods discussed in this chapter are (1) robot-based approaches, (2) frame-based stereotaxy, and (3) frameless approaches to electrode placement.

4.2 Preoperative Planning

The recommendation for pursuing SEEG depth electrode implantation is finalized during a weekly, multidisciplinary epilepsy conference. During the conference, a detailed review of each case is conducted including the benefit of possible invasive monitoring. A previous study on the subject articulated our opinions on the division of electrode implantation and subdural strip implantation. ⁴ A portion of the planning process is initiated as targets are proposed given the semiology of the patient’s seizures.

The targets for stereotactic implant are left to the discretion of the neurologist and neurosurgeon in each individual case, though a few stereotactic implants have well recognized targets for paradigmatic seizure types.

Examples of these include (1) unilateral temporal implantation (▶Fig. 4.1); (2) bilateral temporal implantation; (3) frontal lobe implantation; and (4) perirolandic implantation (▶Fig. 4.2).

4.3 Operative Process

Once decided, all the SEEG procedures are scheduled for the operating room in short course. An approximate total of 16 electrodes is targeted during planning. The majority of published data uses this number as the total. Additional electrodes may contribute to brain shift and swelling, affecting the precision of placement. Three accepted strategies exist for implantation: (1) robot guidance; (2) precision aiming device (Brain Lab, Vario-Guide); (3) frame-based approach. ▶Table 4.1 summarizes a comparison of the three strategies. The first approach is the standard method employed at our institution; however, each of these strategies is discussed later. Of note, the majority of published data employ either frame-based approaches or robot guidance for electrode insertion. Another crucial difference involves calculation of distance to target, which is described following bolt placement.

Robotic guidance is used in place of a manual stereotactic arm to orient instruments along planned trajectory. The use of the robot is detailed below and has demonstrated favorable results in lesion localization with improved operative times at our institution. Precision aiming devices, such as Vario-Guide (Brain Lab product), may also be used to direct electrode placement. Some groups have used this method for electrode placement, but there are a number of limitations. In our experience, the stability of these systems is not sufficient to ensure adequate accuracy of electrode placement. Second, the frameless stereotactic guidance arms are often limited in the number of angles that can be achieved for insertion especially for inferior electrodes in the middle fossa. Third, the entry point at the skin and dura must be aligned with the end target, as the system employs a locking mechanism that limits adjustment to misaligned skin incision.

Frame-based approaches are further subclassified by the type of frame employed. Frame types include Integra CRW frame and Leksell G frame. Though both may be used for insertion, the CRW frame requires stereotactic software capable of applying the “Mohawk” configuration for placement of the arc. The CRW frame does offer increased freedom in fixation placement and, therefore, less issue with electrode collusion. The Leksell G frame does not require the additional configuration but does require the use of an adapter piece. The frame may be used in either the Mohawk or standard configuration. The standard configuration limits lateral approaches for electrode implantation and requires an additional coordinate (total of five) per electrode. In the case of a lateral insertion technique with the Leksell frame, an “L” piece has to be outfitted (▶Fig. 4.3). This piece is no longer available commercially but may be custom fabricated. If employed, the lateral insertion technique allows selection of Y and Z coordinates without need for arc measurement. The trajectory is purely lateral to medial and is doubly fast when compared to arc-based insertion techniques. Additionally, this technique allows for a lower probability of making a coordinate error. Given these benefits, the Leksell frame, with use of an L piece, is our recommendation for frame-based approach to electrode placement. Distance calculation for freebase methods is described in ▶Fig. 4.4. For all methods, the distance between the target location and the direct measurement must be noted in the planning software.

No matter the strategy for implantation, preoperative imaging is mandatory. At our institution, a navigation protocol MRI scan with contrast is principally used for operative planning in nonlesional cases, with the goal of good visualization of arteries and veins. Images are transferred to a stereotactic neuronavigation software and trajectories are calculated to avoid cortical venous injury and minimize cortical disruption. If the imaging study is completed prior to surgery, the patient is discharged home and returns on the day of surgery.

Other institutions have discussed the use of CT angiogram or formal cerebral angiogram prior to the case to minimize the risk of vascular disruption. The improvements in MR imaging and the outcomes from our initial series without dedicated angiographic imaging suggest that an MR is sufficient for planning purposes. Double dose contrast is not formally necessary.