**3.3 Presurgical evaluation**

The first objective is to identify the epileptogenic zone, EZ by various invasive and non-invasive modalities of investigations. The more sophisticated or invasive approach will also depend on the clarity of structural identifiable pathologies in neuroimaging and the link with the clinical semiology.

The second objective, after screening of the potential epileptogenic zone, is to develop strategies to safeguard the lesion can be safely resected with no significant physical or cognitive sequelae.

The ultimate goal in presurgical evaluation is to identify the concept of "Six cortical zones" (**Figure 1**).

From pragmatic point of view, detailed interview of patient and patient's family and friends who can give detailed witness history and past background is mandatory. The interview should aim for recapitulating all relevant past history and probably risk factor or etiological factors. The latter will also give insights to prognosis of the epileptic disorder with respect to surgical treatment. A good example will be a case of post encephalitic epilepsy will render surgical intervention less successful [27].

There should be a multi-disciplinary team and the respective investigations should include neuroimaging, psychiatric, neuropsychological and electrophysiological assessment. A tertiary level or above epilepsy centre should have the available epilepsy surgery presurgical investigations of in two different levels:


**Figure 1.** *Six cortical zones.*

### **3.4 Neurophysiology**

The neurophysiological evaluation includes interictal and ictal EEG sampling, which can be attained by non-invasive or invasive means in a long term recording manner.

The inter-ictal EEG will provide important hint to lateralization or localization of EZ. This is particularly true in cases of TLE solely unilateral anterior temporal spike is a strong predictor of post-operative seizure freedom [28]. Anyhow, it is not uncommon to have unilateral MTS with bitemporal interictal epileptiform discharges found [29]. Another interictal EEG pattern with good localizing value is short bursts of low-voltage, high frequency oscillations associated with focal cortical dysplasia [30].

Conceptually, the video EEG recording will capture the habitual seizures and the ictal EEG discharge and the lateralization and localization of the ictal onset zone can be deduced from analysis of adequate number of captured events.

After combined analysis of ictal and interictal EEG data, the irritative and ictal onset zones can be estimated [31].

Invasive recording is indicated when there is a hypothesis of epileptogenic zone that is not fully supported by the non- invasive diagnostic modalities results. These difficult scenarios are especially found in the non-lesional cases [32, 33].

### **3.5 Structural neuroimaging**

Magnetic resonance imaging (MRI) of brain constitutes the basic, yet the most important investigation of choice in presurgical evaluation. It is particularly true in some epileptic disorder like temporal lobe epilepsy, of which mesial temporal sclerosis is the pathological substrate, got its unique radiological-anatomical correlates:

#### **Figure 2.**

*Patient was regarded to be non-lesional epilepsy initially in 1.5 T MRI (a) and found to have cortical dysplasia in left temporal stem in 3 T MRI (b). Another epilepsy patient had very subtle lesion in right subependymal region in 1.5 T MRI (c) and confirmed to be subependymal heterotopia by 3 T MRI (d).*

The MRI features of hippocampal sclerosis include (1) hippocampal atrophy on T1, (2) increased signal on T2-weighted images or fluid-attenuated inversion recovery (FLAIR) sequences, and (3) decreased signal on inversion recovery sequences [34, 35].

The detection of these abnormalities should be carried out with optimized imaging techniques, which include angulated coronal sections obtained perpendicular to the long axis of the hippocampal structures.

For the extratemporal substrates, MRI can also define hemimegalencephaly, schizencephaly, and focal subcortical heterotopia. Focal cortical dysplasia is the most common developmental pathology in children with extratemporal lobe seizures, and there is an international classification to define the underlying histopathology and foretell the outlook of surgical success [36].

3 T MRI system has better signal-to-noise ratio, spatial and tissue contrast resolution than a 1.5 T system. Studies have shown that for initially nonlesional cases scanned by 1.5 T system with standard MRI brain protocol, more than half had new findings after rescanned by 3 T MRI system with multichannel phased-array coils (**Figure 2**).

The recommended MRI epilepsy protocol includes:

1.Volume acquisition T1W sequence acquired in oblique coronal orientation, orthogonal to long axis of hippocampus, covers whole brain in 0.9–1 mm partition


Diffusion tensor imaging (DTI) and tractography can be used for fiber tracking and noninvasive structural network mapping and is an optional imaging sequence to aid preoperative planning for surgical trajectory. Recent study reported identification of significant diffusion abnormalities of tract sections in ipsilateral dorsal fornix and contralateral parahippocampal white matter bundle in patients with poor postoperative seizure control. Though more studies are warranted to make conclusion, these results may help in understanding the mechanism of postoperative persistent seizure and may act as imaging prognosticator for operation outcome.

However, there are pathological substrates that go beyond the detection of MRI analysis. As a result, multi-modality imaging of the brain will come into play [37–39].

There are some functional neuroimaging modalities, namely PET, SPECT, fMRI and magnetoencephalography (MEG). Some of these scans can be co-registered with MRI to give more detailed structural-functional correlated imaging analysis. They will aid the localization of epileptogenic zone, and the sensitivity will largely depend on the epileptic syndrome. MEG helps to localize the epileptogenic zone and delineate the relationship between the suspected abnormality and the relevant regions in the brain. The placement of invasive electrodes can be guided by the MEG findings. A MEG-guided review of MRI may reveal subtle abnormalities and permit a precise surgical excision of the irritative zone. MEG is also indicated in patients with multiple intracerebral lesions, such as multiple cavernomas, in whom a sole epileptogenic lesion may be identified for lesionectomy [40–43].

With such information, the indication of further invasive studies will also be justified (**Figure 3**).

**Figure 3.** *Algorithm of workup of refractory epilepsy.*
