**2.2 Presurgical workup**

A presurgical workup aims to generate an *anatomical-clinical-electrophysiological (ACE) hypothesis*. MRI, video EEG and neuropsychiatric assessment form the core of it. An MRI with good resolution would offer the *localization* of the epileptogenic lesion, which lays the foundation of good surgical outcome to start with, as lesional epilepsy has a better outcome (by two to three times) than non-lesional epilepsy after neurosurgical intervention [7]. 3-T MRI, which has a higher signal-to-noise ratio, could be more accurate in delineating the lesion as compare with 1.5-T MRI as illustrated in some qualitative studies [8]. An ictal video EEG would be more valuable than an interictal EEG because it offers the appreciation of the clinical semiology with the ictal discharges simultaneously, while interictal EEG could only illustrate the irritative zone. Neuropsychiatric assessment could offer evidence of brain dysfunction secondary to epileptic encephalopathy. For example, frontal lobe epilepsy might cause frontal lobe syndrome such as disinhibition, perseverance, etc. For temporal lobe epilepsy in dominant hemisphere, it causes more verbal memory deficit as compared with the visual memory deficit in the non-dominant hemisphere. It is often how *lateralization* is done in cases such as mesial temporal sclerosis. Bilateral mesial temporal sclerosis is not uncommon.

If these three investigations provide concordant findings, in which an EZ could be concluded, then surgical excision might be proceeded. The most often encountered

example would be temporal lobe epilepsy secondary to mesial temporal sclerosis. Indeed, mesial temporal resection (i.e. amygdalohippocampectomy), via either transtemporal (anterior temporal lobectomy) or transsylvian approach, offer some of the best outcome in terms of seizure control.

Functional images such as interictal PET-CT could provide supportive information but often it is not conclusive due to its lack of spatial resolution. Nowadays, a PET-MRI could be done in the same setting to provide the simultaneous appreciation of anatomical and functional data, which might be more superior than fusing, by software, the interictal PET-CT with the MRI which were done in two settings. Ictal SPECT is also valuable to evaluate the part of brain with the greatest perfusion which is often the EZ. It is the most useful when there is more than one lesion, and the clinician could not conclude which one is the culprit. However, it is technically demanding as the radioactive isotope should be injected once the ictal event starts, in terms of seconds. Also, if the electrophysiological onset precedes the clinical one, the ictal event has started for some time before clinical semiology happens. For lateralization of dominant hemisphere, functional MRI by means of blood oxygen level dependent (BOLD) might be helpful to lateralize the language area. Yet, there could be bilateral activation at times especially in those who were agitated or intellectually disabled. Of note, even functional MRI could lateralize the language area, it has relatively poor spatial resolution as it is all about the adjustment of the threshold in the software. It could not replace the language assessment in awake craniotomy in case of proximity of the EZ to the language areas. Magnetoencephalography (MEG) is to detect dipoles which occur in the EZ. It is similar to SPECT but it is not as time-dependent.

If the above measures (some refer them as level 1) provide discordant findings (**Table 2**), and *lateralization* and *localization* could not be achieved, invasive workup (i.e. level 2) might have to be considered [9]. Wada test involves the endovascular injection of barbiturate to internal carotid artery (ICA) to temporarily suppress the activity of one cerebral hemisphere to see if it causes verbal or visual memory deficits, as mentioned, which then points to left or right-side temporal lobe epilepsy respectively. If ICA Wada test fails to lateralize the dominant hemisphere, then selective posterior cerebral artery (PCA) Wada test could be considered to selectively suppress the activity of the hippocampus [10].

#### **2.3 Invasive EEG**

Here comes to the invasive EEG. If an epilepsy is lateralized and partially localized in the level 1 presurgical workup, craniotomy for putting in subdural grid or depth electrodes (SDE) was the commonest method for invasive EEG. Subdural grid offers


**Table 2.** *Level 1 presurgical workup.*

#### *Stereotactic Electroencephalography (SEEG) DOI: http://dx.doi.org/10.5772/intechopen.110215*

examination of epileptiform discharges across the cerebral convexity while depth electrodes provide the assessment of epileptiform discharges from deep to superficial. Yet, it involves the risks of bleeding and infection, especially in those with prior craniotomy. It might also cause unhabitual seizure events during the recording which might confuse the clinical picture.

Stereotactic EEG (SEEG) involves the minimally invasive approach to implant multiple electrodes with twist drill to cover different areas of a cerebral hemisphere. Bilateral implantation is also feasible. Simultaneous deep and superficial assessment of multiple sites is one of its greatest advantages [11]. The spatial and temporal relationship of the epileptiform discharges as in a 3D coordinate system could be delineated. The epileptic network could be determined. It causes less unhabitual seizure event. In the series by McGonigal A, *et al*., SEEG localizes the EZ in non-lesional epilepsy cases as effective as in lesional ones [12]. In the meta-analysis published by Mullin, *et al.*, the bleeding and infection rate was found to be 1% and 0.8% respectively [13]. In the meta-analysis by Arya R. *et al.*, the rate was 4% and 2.3% respectively [14]. In some lesional cases with anatomical discordance (e.g., right temporal lobe epilepsy with a deeper pathology hypothalamic hamartoma), or when there are multiple pathologies (e.g., polymicrogyria, periventricular nodular heterotopia), SEEG could be helpful to identify or confirm the true pathology that is responsible for the epileptogenesis [15]. At the moment, there is no head-to-head series to compare the seizure control rate between SEEG and SDE. According to the review by Katz JS *et al.*, there is no definite superiority of SEEG vs. SDE (Engel class I rate 56–68% vs. 30–70%) with reference to different case series [16].

There are three ways to implant SEEG, namely frameless, frame-based and robotic-assisted stereotactic navigation. Frameless navigation system makes use of guidance by neuroimaging with surface matching registration. Frame-based approach involves the application of stereotactic frame to patient's head. Fusion of post-frame CT scan with the initial planning MRI will provide the coordinates with reference to the stereotactic frame to navigate the intended trajectory. Robotic-assisted stereotactic navigation also involves registration but it differs by having robot to bring neurosurgeons to the intended trajectory, which could speed up the procedure when there are multiple electrodes to be inserted, as in the case of SEEG. In the meta-analysis performed by Vejay N. Vakharia *et al.* in 2017, the mean error of the entry point and target point was 2.45 and 2.89 mm for frameless approach, 1.43 and 1.93 mm for frame-based approach, and 1.17 and 1.71 mm for robotic-assisted approach [17]. Despite the apparently smaller mean error in frame-based and robotic-assisted approaches, there was high heterogeneity among studies and the parameters used were different (Euclidean distance vs. lateral deviation). This precluded meaningful comparison of the different approaches.

#### **2.4 SEEG planning**

The flow of the SEEG planning and implantation procedures is as follows. At the combined epilepsy meeting, the findings of the presurgical workup are presented and the plan for SEEG is confirmed in selected cases with discordant findings. Multidisciplinary discussion to generate the anatomical-clinical-electrophysiological hypothesis would be essential. Implantation of electrodes would depend on the best hypothesis generated and important alternative hypothesis to be rejected. A 2D grid coordinate system (aka Talairach Stereotactic System) might be helpful for communication among the team (**Figure 2**). Fine-cut MRI brain with contrast would be

**Figure 2.** *Talairach stereotactic system.*

used for stereotactic navigation and trajectory planning. A CT cerebral angiogram would be useful to appreciate the bony architecture as well as the cortical and Sylvian vessels. MRI with cerebral angiogram (MRA) fusing with a plain CT brain is also an alternative to reduce contrast use.

In the planning software, we could put the trajectories in an orthogonal manner i.e., perpendicular to mid-sagittal plane of the brain. It is similar to the Talairach approach, yet the latter made use of 2D angiography (digital subtraction angiography). The patient would be positioned laterally during the implantation procedure. Orthogonal approach has the advantage of easier interpretation of the spread of the epileptiform discharges as the 3D system could be more regular in shape if all the electrodes are parallel to each other. However, the more peripheral the SEEG electrodes are, there could be greater deviation as the electrode direction is not perpendicular to the skull and the brain surface which could lead to deviation towards the periphery. Therefore, the region of interest of the procedure must be determined and marked during the planning and the setup of the navigation system. Of course, the region of interest is often the proposed EZ. On the other hand, orthogonal insertion to insula could be difficult. Insula is often the important alternative to exclude in both frontal and temporal epilepsy. Anatomically it is covered by frontal, temporal, and parietal operculum with Sylvian fissure and middle cerebral artery (MCA) branches on the surface. Therefore, the orthogonal insertion to insula region needs great care not to pass through important vasculature. However, it allows the simultaneous assessment of both operculum and the insula on the same electrode. The other way to implant the SEEG would be a 3D approach, adjusting to the skull shape at the entry site (**Figures 3** and **4**) and to avoid MCA branches in Sylvian fissure in case the insula is one of the targets (**Figure 5**). Yet, as the electrodes are not parallel to each other, interpretation could be more difficult.

*Stereotactic Electroencephalography (SEEG) DOI: http://dx.doi.org/10.5772/intechopen.110215*

**Figure 3.** *Comparison of orthogonal (left) with 3D (right) approach.*

#### **Figure 4.**

*3D reconstruction from real case MRI to illustrate the orthogonal approach of SEEG placement. (Left: anteriorposterior, right: lateral-oblique) This is a case of right temporal non-lesional epilepsy with bilateral temporal EEG onset and MEG dipoles with semiology of cephalic aura, bilateral hearing loss followed by left side twitching.*

#### **Figure 5.**

*This is an anterior-posterior view of the 3D-reconstructed brain parenchyma. Blue line represents the 3D approach which avoids the Sylvian fissure while the red line represents the orthogonal approach which punctures through the Sylvian fissure.*

The planning strategy in different epilepsy would be discussed in later part. During the planning, entry sites and target sites must be determined. Then the length from dura to target should be measured. Many of the planning software could customize the lead contact size and intervals. With reference to the insert of the electrodes, one could decide the length of different electrodes to be used. After planning the trajectories in the planning software, either frameless stereotactic navigation system or robotic-assisted system could be used directly. Frame-based approach would require a post-frame CT.

#### **2.5 SEEG implantation**

Patient is then put under general anesthesia and intubation. Head is secured by clamp and navigation system is set. Skin is prepared and draped. Adjustment of the guiding system is performed (**Figure 6**). The length inserted would be calculated with reference to the different parts of the system (**Figure 7**). Skin-dura distance is measured in navigation system (**Figure 8**). Stopper-skin distance is measured intraoperatively. Adding the two up we would have the length of drill needed to puncture the skull. Stab wound is made on the scalp. Drill with length marked by stopper is brought in. Skull is drilled (**Figure 9**). Sometimes 1–2 mm more might be needed to puncture the inner table. A sense of give-way under careful control is often the sign

*Stereotactic Electroencephalography (SEEG) DOI: http://dx.doi.org/10.5772/intechopen.110215*

**Figure 6.** *Guiding system is adjusted by frameless stereotactic navigation.*

**Figure 7.**

*Blue arrows = skin-dura distance. Red arrows = stopper to skin distance. Blue + red = length of drill to puncture the skull. Purple arrows = dura-target distance. Green arrows = bolt-dura distance. Purple + green = length of electrode.*

of complete puncture. Dura and pia are cauterized and punctured by monopolar (**Figure 10**). The bolt is anchored to the skull. Length of electrode is needed is the sum of dura-target distance and the bolt-dura distance (i.e., from bolt to target). The electrode with length marked by the cap is inserted with the cap screwed to the bolt (**Figure 11**). Opposition of the scalp wounds might be needed to prevent CSF leak. The whole procedure is repeated in different electrodes. In case of bilateral

#### **Figure 8.**

*Electrodes are planned in orthogonal manner. Skin-dura distance could be measured.*

**Figure 9.** *Stopper-dura distance is set. Bone drilling.*

implantation, the patient would be repositioned in the contralateral side, in lateral position, with navigation system set again. It should be reminded that the EEG signals recorded intraoperatively might be affected by the general anesthesia. Patient would be transferred back to the ward for monitoring of the SEEG signals (**Figure 12**). In author's center, we put patients on empirical antibiotics when the patient is under the

*Stereotactic Electroencephalography (SEEG) DOI: http://dx.doi.org/10.5772/intechopen.110215*

**Figure 10.** *Dura cauterization and puncture.*

**Figure 11.** *Bolt anchoring and electrode insertion.*

SEEG monitoring. Antiepileptics would be withheld according to the patient's semiology and seizure frequency. CSF leak is what one should carefully watch out for.
