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

#### **Figure 13.**

*This diagram illustrates a case with epileptogenic zone confirmed with direct electrical stimulation. The epileptogenic lesion was the gliosis with T2 hyperintensity in MRI in the temporoparietal junction. Yet, the patient's semiology was often a frontal one. SEEG electrodes were implanted around the gliosis and the dipoles found in MEG. They were also implanted across the superior longitudinal fasciculus. Stimulation triggers rhythmic discharge in the superficial contacts around the gliosis and thus identifies the ictal onset zone. Visual pathway was also identified in the deeper contacts of one adjacent electrode.*

leads and excision of the EZ. Usually, the leads inserted to the proposed EZ would be removed first to provide the surgical exposure. The remaining leads could be left to detect any interictal signals. If they are present, they could serve as a guide to successful disconnection of the EZ once they disappeared. However, one should not compromise the surgical exposure which is critical to adequate excision of the EZ and careful hemostasis. Awake craniotomy might be needed for assessment of the eloquent areas with direct electrical stimulation to avoid neurological deficits after the operation. Awake craniotomy might even need a larger exposure for adequate assessment. Details of awake craniotomy would be out of the scope of this chapter. As the leads have been implanted for 1–2 weeks before the concluding procedure, risk of infection could be higher than usual craniotomy cases. Therefore, intra-operative wound irrigation and post-operative antibiotics might also be helpful.

#### **2.8 Implantation strategy**

SEEG electrodes should be implanted according to the anatomical-clinical-electrophysiological hypothesis [19]. It is also important to place electrodes, apart from the proposed EZ, the symptomatogenic and functional deficit zones.

Temporal lobe epilepsy (TLE), as mentioned, with mesial temporal sclerosis and concordant presurgical workup, could have resection surgery proceeded right away. However, the following four situations might warrant SEEG implantation.

	- a.Orbitofrontal structures (spread via uncinate fasciculus)
	- b.Posterior cortices i.e. parietal and occipital (spread via inferior frontal-occipital fasciculus or inferior longitudinal fasciculus)
	- a.Orbitofrontal structures
	- b.Insular cortex, frontal operculum
	- c.Temporo-parieto-occipital junction

From these one could appreciate in TLE, orbitofrontal structures and insula are two very important structures to have the electrodes stationed. Understanding the connectome with the white matter tracts is also essential as it is often how epileptiform discharges spread (**Figure 14**).

For frontal lobe epilepsy, the epileptiform discharges often spread quickly within the frontal lobe, and it has high connectivity with the extra-frontal structures and the contralateral frontal lobe. SEEG is mainly to compartmentalize the seizure pattern. The frontal operculum could be divided into the pars orbitalis, pars triangularis and opercularis. Orthogonal insertion of the SEEG electrode into the pars orbitalis could provide assessment of the orbitofrontal structures. On the other hand, mesial frontal structures are often underrepresented in scalp EEG. Orthogonal insertion of the SEEG electrodes via middle frontal gyrus often provide assessment of that with the deeper contacts. Therefore, if frontal lobe is of concern and mesial frontal structures must be assessed, a longer electrode might have to be used.

Posterior quadrant epilepsy is a less frequently encountered type. It involves parietal and occipital lobes. Clinical polymorphism is present due to high density of brain connections to the insula and temporal as well as contralateral posterior quadrant. It is often some challenging cases and multi-lobar and bilateral electrodes might have to be used to assess the propagation pathways and assessment of the functional areas such as Wernicke's area, reading, calculation, vision, and face recognition. Tractography study such as diffusion tensor imaging (DTI) might be helpful to look for the ventral and dorsal white matter tracts through the occipital lobe. SEEG electrodes could be implanted according to the white matter tracts and see if the epileptiform discharge propagates from one to another electrode via the pathway.

Insular epilepsy has the semiology mimicking frontal and temporal lobe epilepsy. As mentioned, it is often an important alterative to exclude. Again, it is underrepresented in scalp electrodes. Difficulty to plan trajectories of the SEEG electrodes is often encountered due to the vascular constraints, especially with the orthogonal approach. The comparison of orthogonal and 3D approaches is discussed in previous part.

#### **Figure 14.**

*An illustration of a case of temporal lobe epilepsy semiology with hand and oral automatism followed by postictal drowsiness yet EZ concluded to be in the orbitofrontal structures with spread to temporal lobe via pathways such as uncinate fasciculus or arcuate fasciculus. SEEG concluded the ictal onset zone (yellow) while the PET-MRI confirmed the functional deficit zone (green). Electrodes were also inserted in an orthogonal approach to the insula as it is an important alternative hypothesis to exclude.*
