**4. Presurgical evaluation**

The pre-surgical evaluation is the most crucial aspect for epilepsy surgery; all patients diagnosed with PR should be referred to an epilepsy center for pre-surgical evaluation to confirm the diagnosis of true epilepsy and type of seizure. At this stage of evaluation, around 50% of patients usually have idiopathic generalized seizure, which cannot be treated with resective epilepsy surgery; only patients with focal-onset *Recent Advances in Epilepsy Surgery DOI: http://dx.doi.org/10.5772/intechopen.107856*

seizure with loss of awareness (partial complex seizure) with or without secondary generalized seizure are surgical candidates and can complete the pre-surgical process for investigation. Therefore, the initial evaluation should include proper history, seizure semiology, and EEG. The final goal of pre-surgical evaluation is to define the EZ (region) defining the lateralization and localization of EZ (**Table 2**) [24].

The American Epilepsy Society conference had a consensus about the evaluation protocol through a multidisciplinary methodological approach involving physical exam, scalp video-EEG, telemetry, structural MRI (MRI epilepsy protocol), neuropsychological assessment, neuropsychiatric assessment, social worker, and nursing for patient support network, with realistic expectation of outcome [25]. Additionally parameters include fMRI, Wada test, PET, ictal SPECT, MEG, and intracranial EEG electrodes (**Table 1**) [25, 26].

### **4.1 Neurological and neuropsychological studies**

Neurological and neuropsychological assessment is part of the preoperative evaluation of refractory seizure patients. The assessment should comprise baseline standardized measures of cognitive function in addition to wider measures of behavioral

#### **Table 2.**

*Pre-surgical evaluation parameters of patients with refractory seizure.*

and psychosocial function [27]. The preoperative neuropsychological assessment contributes to seizure lateralization, localization, characterization, and provides predictions of cognitive risk associated with surgery [27].

For future outcome, it is important to include exploration of patient and family expectations from surgical treatment. Neuropsychological changes following surgery are a dynamic process, and it should be an integral part of the postoperative followup. The neuropsychologist plays an important role with postoperative rehabilitation and support of the patient and family members as a part of the multidisciplinary team of epilepsy surgery [28].

#### **4.2 Electrophysiological studies**

#### *4.2.1 Noninvasive EEG monitoring*

The standard procedure after confirming PR patient usually is initiated with video EEG-monitoring for detection of lateralization and localization of seizure focus. Patients are often admitted for several days, depending on their seizure frequency. Most often their AEDs are tapered off in order to capture about 3–5 typical and consistent seizures. There is no consensus on the tapering process, but in general very rapid tapering is avoided to minimize risk of status epilepticus or triggering aberrant seizure onset zones [24, 29].

Scalp-recorded EEG is usually performed for inter-ictal and ictal epileptiform patterns [29]. In the video-EEG monitoring, seizure semiology is recorded, which is important for surgical decision-making [29]. Occasionally when ictal/inter-ictal EEG epileptiform activity discharges are concordant with structural neuroimaging abnormalities, it could be sufficient for surgical localization and treatment [26]. High-density array of EEG electrodes (10–10 electrode placement) may be useful in patient with extra-temporal lobe focal seizure. Deeper seizure foci may not be detected via (scalp) electrodes. Further, scalp EEG recording is usually insufficient in extra-temporal epilepsy or even in non-lesional (normal MRI) temporal lobe epilepsy (TLE). Using extra electrodes placed based on the 10–10 international electrode system may add significant accuracy to scalp recording and in some cases avoid intracranial recording [24].

When seizure onset zone is poorly lateralized due to alternating seizure onset lateralization in bi-temporal asynchrony or frequent bilateral epileptiform discharges, it indicates a less favorable postsurgical seizure outcome [30]. Unilateral hippocampal atrophy on MRI and concordant unilateral interictal spikes are highly predictive of concordant ictal localization [31].

#### *4.2.2 Invasive intracranial EEG*

ECoG, implantation of SDE, is very helpful in detecting, seizure lateralization and localization. The duration of intra cranial-EEG monitoring is determined by seizure frequency, number of seizures needed to make a decision, and the time needed to perform mapping of the eloquent cortex. Interpretation of the data is based on EEG pattern recognition as well as clinical semiology.

It is essential to insert the intracranial EEG electrodes in proper locations at or in close vicinity to the epileptogenic zone although interpretation may be difficult in

some cases since that frequency range may differ from one cerebral area to another due to neurophysiological properties of the anatomical structure and from one etiology to another [24, 32].

## *4.2.3 Stereoelectroencephalography (SEEG)*

Recording EEG signals through surgically implanted depth electrodes provides the best coverage for deeper structures (such as hippocampus, amygdala, and insula) and deep sulci. Depth electrodes in various lengths and number of contacts are implanted using conventional stereotactic technique or by the assistance of stereotactic robotic devices through 2.5 mm diameter drill holes. Risk of infection and intracranial hemorrhage have been reported in 1% of patients. In other series small hemorrhages have been reported in 5.5%, of whom only 0.9% required surgery, and no mortality was reported [33, 34]. The planning of SEEG implantation requires formulating precise hypotheses about the possible epileptogenic zone, seizure onset, and propagation zones to be tested.

#### **4.3 Neuroimaging studies**

Advances in brain imaging technology have substantially improved seizure localization and surgical outcome [30]. All patients with clinical and evidence of focal-onset seizure should have brain imaging, including brain MRI [24].

### *4.3.1 Brain MRI*

MRI is an important noninvasive tool for evaluation of patient with epilepsy that provides two critical data, a potential epileptogenic brain abnormality, and its surrounded anatomy. Whole brain coverage allows for the examination of the lesion location and its relationship to cortical eloquent areas [35]. The main role of brain MRI is to define structural abnormalities that may cause seizure. A high-resolution brain MRI, with epilepsy protocol, is recommended. Epileptogenic lesions in almost one half of those presenting with new-onset seizure are detected. Sensitivity of brain MRI is increased by using epilepsy protocol [36]. The common sequences used by most epilepsy centers include thin section of 1 mm coronal oblique T1 gradient echo, coronal oblique T2 series, high-resolution 3D sequences (sensitive to subtle cortical dysplasia or small tumors), and T2 flair (fluid attenuated inversion recovery) images performed on 3 Tesla or higher MRI systems [24].
