Depth versus subdural electrode studies for unlocalized epilepsy

Abstract

It is accepted that precise localization of the epileptic focus or zone is imperative for planning surgical treatment in uncontrolled epilepsy (1). It is equally and widely held that the place from which spontaneous electrical seizures arise is the area being sought (2). Localization of abnormal cerebral function is helpful but not definitive in this regard. The accuracy and the manner of electrically recording seizures has therefore been a field of continuing investigation. Surface EEG provides sampling of many cerebral areas, which is important, but it attenuates cortical signals by factors of up to 58 to 1 (3). Besides, artifact limits its utility for accurate localization of the area of seizure onset (4). Accordingly, various progressively more invasive means of recording and localizing spontaneous seizure onset have been developed. These include nasopharyngeal electrodes, sphenoidal electrodes, electrocorticography, depth electrodes, and subdural or epidural electrodes. Because of their suitability for bilateral chronic recording of multiple cerebral areas, depth and subdural electrodes are most commonly employed to localize the focus in difficult diagnostic problems considered for resective surgery. The preference for depth electrodes or subdural electrodes should be based on available data that responds to the following questions: (a) What is the yield of localization by depth or subdural electrodes in difficult localizing problems? (b) What is the accuracy of localization by either technique? (c) Is one technique particularly suited to investigation in temporal lobe epilepsy? Extratemporal epilepsy? Neocortical epilepsy? Lesional patients? (d) Are morbidity and mortality comparable?