Subdural electrode recording technique

In general, SDE involves a unilateral, or in some cases bilateral , craniotomy for implantation of the subdural grid and or strip electrodes. Surgery is performed under general anesthesia. Typically, a broad exposure of the brain is performed via a hemicraniotomy to facilitate the placement of the grid and strip electrodes. The dura is opened in a standard fashion and electrodes are placed based on the preoperative hypothesis about the localization of the seizure focus and the need for functional mapping (see Fig. 4.1A and D ). The dura is then closed with a dural graft to minimize the mass effect from electrodes. Grid electrodes are embedded in silastic and often spaced at standard intervals. The silastic grid is secured to the surrounding dura prior to closure to secure the electrode in position. Dura is typically closed in a watertight fashion. After dural closure, the bone flap is replaced. There is considerable variation in the technique for bone fixation, though some groups utilize bent cranial plates to generate a hinge that expands the intracranial volume to minimize the mass effect from the electrodes . Electrodes are typically tunneled toward the vertex to minimize CSF leak with a margin of at least 1 cm from the incision edge to minimize CSF leak and risk of infection.

Summary of X-ray ( top panel ) and photo depiction ( bottom panel ) of the different invasive monitoring methods described in this chapter. This includes SDE ( left , A/D), sEEG ( middle , B/E), and hybrid ( right , C/F).

It is important to note that there can be considerable variation in the types of electrodes used and the position of electrodes implanted. For example, variation in electrode spacing ranges from relatively sparse recordings to high-density arrays (see Fig. 4.1D and F for contrast in interelectrode spacing). Also, arrays can be customized for particular brain regions, such as the temporal pole, which is typically difficult to cover with high-density grid electrodes . Additionally, coverage of brain regions beyond the convexity such as the parahippocampal gyrus or interhemispheric fissure can be recorded by using specific exposure and implantation methods .

Indications and contra-indications

The indication for invasive monitoring with SDE is to delineate the EZ when noninvasive tests cannot do this with sufficient accuracy . An alternative indication, particularly in pediatrics, is for functional mapping in a child that would not tolerate an awake craniotomy .

Contra-indications in pediatrics are relative and include (1) a child who may not safely tolerate prolonged invasive monitoring in the epilepsy monitoring unit, (2) inadequate preimplantation hypothesis, and (3) medical comorbidities that would preclude safe invasive monitoring. It is crucial that invasive monitoring of any type is only performed in the setting of well-defined preimplantation hypotheses that would lead to potential surgical resection.

Relative advantages and disadvantages

The advantages and disadvantages of SDE are summarized in Table 4.1 . A major advantage of the SDE technique is that no electrodes are implanted into the brain parenchyma, which may have certain safety advantages (e.g., the risk of intraparenchymal hemorrhage is low). Additionally, the interpretation of SDE can be intuitive given that the spatial conformation of grid electrodes directly maps to the cortical surface, unlike intracerebral recordings whereby electrodes cross sulci are recorded from spatially distinct regions from a single electrode trajectory. Further, though several studies and practices by several groups demonstrate the feasibility of functional stimulation mapping with sEEG , this mapping may be more intuitive with SDE given that adjacent electrode contacts are adjacent to each other on the brain surface .

Disadvantages include the need for a craniotomy and in the case of a bilateral preimplantation hypothesis, bilateral craniotomies. Additionally, special approaches are necessary for recording from mesial regions including the basal temporal, temporal polar, parahippocampal, and interhemispheric regions. Most important, sulcal regions, which make up two-thirds of the cerebral cortex, cannot be recorded using this method.

Stereoelectroencephalography technique

The technique for sEEG implantation in children is described in detail elsewhere . The preoperative phase of the procedure is crucial for planning electrode trajectories that maximize coverage of gray matter that test the predetermined anatomo-electro-clinical hypotheses while avoiding vascular structures. To this end, preoperative imaging is obtained typically in the form of a thin-cut postcontrast MPRAGE T1 MRI and some form of vascular imaging. Contrast enhancement can diminish gray matter definition on the MPRAGE T1, but this can be improved through custom optimization of the imaging parameters. The French sEEG guidelines recommend CT angiogram, digital subtraction angiography, or postcontrast Mr imaging for vascular imaging. The structural (MPRAGE Mr) and vascular imaging are merged and electrode trajectories are then planned. It is our view that if more than 20 electrodes are necessary, then further preoperative testing is necessary to reach a more finite preimplantation hypothesis. Electrode implantations should not be “fishing expeditions.”

In the operating room, the patient is placed under general anesthesia with head fixation in a stereotactic frame. A stereotactic technique that is either frame-based or frameless can be performed. Our center utilizes a ROSA robot with bone fiducials for electrode implantation. The skin and bone can be opened with an awl and the bone and dura opened with the drill . Guidance anchors are placed, a stylet pass is made to the target, and the electrode is then inserted and secured to the anchor bolt. Once all of the electrodes are placed, a secure dressing is applied and the patient is taken to the epilepsy monitoring unit after recovering in the perioperative unit.

Indications and contra-indications

The indication for sEEG is to test anatomo-electro-clinical hypotheses generated from the preimplantation phase 1 evaluation. This hypothesis must not be able to be tested without invasive recordings. Unlike SDE, sEEG is not used extensively in pediatrics for language or motor mapping when seizure localization is not needed, though we have used it for this purpose at our center with good results.

Contra-indications are relative and include (1) lack of a well-established hypothesis that can be tested with a reasonable number of electrodes, (2) bone thickness <2 mm (the exact number is debated), and (3) medical or behavioral comorbidities that would prevent safe chronic recordings in the EMU.

Relative advantages and disadvantages

A main advantage of sEEG is the ability to test anatomo-electro-clinical hypotheses in three dimensions with electrodes placed to record from both deep and superficial brain structures. Additionally, sEEG electrodes are implanted without a craniotomy. It is important to note that often a craniotomy is still required for subsequent treatment once the EZ is identified. There’s evidence that sEEG is associated with decreased pain compared to craniotomy methods and decreased utilization of pain medicine .

A disadvantage of sEEG relative to SDE or hybrid methods is that there is relatively sparse spacing between electrode contacts that are aligned across an electrode shaft. Due to this configuration of recording and stimulation electrodes, language and motor mapping may be more difficult or at least less intuitive. For teams new to sEEG, this electrode configuration can also make the interpretation of sEEG recordings a challenge as electrodes that are close to each other on an electrode shaft can record from distinct brain regions. Additionally, sEEG implantation is best performed with specialized equipment, including a stereotactic frame, stereotactic robot, and specialized imaging protocols. The upfront cost of this equipment may be prohibitive for some hospitals depending on resource availability.

Hybrid approach technique

The hybrid approach combines craniotomy-based SDE with depth electrode implantation to target deep brain structures not typically covered by SDE . The rationale is to have the benefits of dense cortical surface coverage while achieving some coverage from deep structures important in epilepsy evaluation such as the amygdala, hippocampus, entorhinal cortex, cingulate, and/or insula . The technique is like SDE in which a large craniotomy is performed under general anesthesia for placement of grid and strip electrodes. After the craniotomy, depth electrodes are implanted using either an image guidance system or a stereotactic frame through an open craniotomy. Robotic assistance can also be utilized for electrode placement. Electrode tunneling and closure are similar to the SDE technique.

Indications and contra-indications

The indications and contra-indications for this procedure are similar to SDE. Again, the indication for this procedure is a testable hypothesis about the localization of the EZ based on noninvasive phase 1 studies. When a mesial EZ is suspected, but an SDE approach is preferred, then a hybrid approach could be indicated to record from mesial structures. In other words, if the hippocampus, amygdala, or insula, for example, are implicated in the EZ, then a hybrid approach could be indicated to record from those deep structures in the setting of an SDE craniotomy.

Relative advantages and disadvantages

The primary advantage of the hybrid approach is the simultaneous advantage of dense surface SDE recordings and selected depth recordings to record from deeper structures. The complication rate of this approach is probably higher, though, in existing comparative metaanalyses, the hybrid approach is often grouped together with SDE . Though theoretically an unlimited number of depth electrodes could be placed, these should be selected carefully to maximize the benefit of each electrode. Further, placement of depth electrodes is probably less accurate than sEEG given that the electrodes are implanted through an open craniotomy that allows for brain shift. Similar to SDE, an additional disadvantage is the necessity of bilateral craniotomies if the focus is not lateralized.