Abstracts

Rationale:
Chronic intracranial electroencephalography (iEEG) is performed for the patients with drug-resistant epilepsy whose epileptic focus was not clearly identified by non-invasive pre-surgical evaluations. Implantation of subdural electrodes under craniotomy is still widely performed in Japan in spite of the recent introduction of less invasive therapeutic devices and robot-assisted stereotactic implantation system for stereo-electroencephalography, partly because of the limited availability of new devices. For exploration of deep-seated activities, stereotactic implantation of depth electrodes is frequently combined with subdural electrodes. This study is aimed to clarify the efficacy and limitation of combined implantation of subdural and depth electrodes for intractable epilepsy patients, and to identify the appropriate candidates for surgery.
Method:
This study included 57 patients with drug-resistant epilepsy who underwent combined implantation of subdural and depth electrodes for chronic iEEG before resective epilepsy surgery, and who obtained minimum 100 days post-operative follow-up. The mean age at the timing of electrode implantation was 18.1 years (ranged from 3 to 54 years). The mean duration of follow-up was 627 days (ranged from 103 days up to 1268 days). Depth electrodes were stereotactically implanted before the opening of dura matter after craniotomy, before subdural electrodes are implanted under direct vision. The clinical information including characteristics of the patients and their epilepsy, locations of epileptic foci, details of electrode implantation, post-operative seizure outcomes, and etiologies of epilepsy were collected retrospectively from our database. We compared the detectability and morphological patterns of early ictal iEEG change (EIIC) between subdural and depth electrodes, and also investigated the association of the detectability of EIIC and low-voltage fast activity (LVFA) to the clinical factors including the extent of magnetic resonance imaging (MRI) lesion, the concordance between fluorodeoxyglucose-position emission tomography (FDG-PET) abnormality and MRI lesion, the etiology, seizure outcome, and the location of suspected epilepsy focus.
Results:
The detectability of EIIC in the subdural electrodes was 86.0%, and in the depth electrodes was 75.4%, respectively. No clear superiority of subdural or depth electrodes to the other was observed in the detection of EIIC. EIIC was detected by depth electrodes alone in 8 patients including all the 3 patients with insulo-opercular foci and two patients with TLE. EIIC was less detected with depth electrodes in ulegyria than in the other etiologies. There was no association of the detection of EIIC and LVFA to seizure outcome. Suspected TLE patients who required electrode implantation had poorer seizure outcome than extra-TLE patients.
Conclusion:
The added value of depth electrode to subdural electrodes is limited in terms of detection of EIIC. However, additional depth electrode is useful in detecting EIIC from the apparently deep structures such as the insula and mesial temporal structures. Depth electrodes should be carefully used in consideration of its efficacy depending on the etiology to avoid injudicious usage, and careful indication of our approach is necessary to the suspected TLE patients.
Funding:
:None