Lateralization of temporal lobe foci: depth versus subdural electrodes

Abstract

Objectives: Definitive localization of an epileptic focus correlates with a favorable outcome following epilepsy surgery. This study was undertaken to determine the incremental value of data yielded for surgical decision making when using subdural electrodes alone and in addition to depth electrodes for temporal lobe epilepsy.

Methods: Standardized placement for intracranial electrodes included: (1) longitudinal placement of bilateral temporal lobe depth electrodes; (2) bilateral subtemporal subdural strips; and (3) bilateral orbitofrontal subdural strips. Sixty-three events were randomly reviewed for: (1) subdural electrodes alone; and (2) depth electrodes in conjunction with subdural electrodes.

Results: Of the 63 seizures, 54 (85.7%) demonstrated congruent lateralization to ipsilateral subtemporal subdural strip electrodes (based on depth electrode localization) when subdural strip electrodes were utilized alone. In 3 of 22 patients, 7 seizures demonstrated ‘false localization’ on subdural electrode analysis alone when compared with depth recording and post-surgical outcome. For these 3 patients, retrospective review of neuroimaging demonstrated suboptimal ipsilateral placement of subtemporal subdural electrodes with the most mesial electrode lateral to the collateral sulcus. Four additional patients had suboptimal placement of subtemporal subdural electrodes. Two of these 4 patients had congruent localization with subdural electrodes to ipsilateral depth electrodes despite suboptimal placement. Subtemporal subdural electrodes accurately localized for all seizures from the mesial temporal lobe when the mesial electrodes of the subtemporal subdural strip recorded mesial to the collateral sulcus from the parahippocampal region.

Conclusion: We conclude that although there are high concordance rates between subdural and depth electrodes, localization of seizure onset based on subdural strip electrodes alone may result in inaccurate focus identification with potential for possible suboptimal treatment of temporal lobe epilepsy. When subtemporal subdural electrodes provide recording from the parahippocampal region, there is accurate localization of the seizure focus. If suboptimal placement occurs lateral to the collateral sulcus, the electroencephalographer cannot make a definitive identification of the seizure focus.

Introduction

Epilepsy affects 1% of the population with an incidence of 30–50 per 100 000 (Sander and Salinpaa, 1997). Approximately 20% of patients remain refractory to standard antiepileptic drug therapy, and new antiepileptic drugs and alternative forms of therapy have had limited benefit (Dreifuss, 1987;Elwes et al., 1991, Wingkun et al., 1991, Gonzalez-Darder et al., 1992, Uthman et al., 1993). The efficacy of conventional surgery for epilepsy has been well documented (Rasmussen, 1975, Blume et al., 1982, Babb and Brown, 1987, Dreifuss, 1987, Elwes et al., 1991, Wingkun et al., 1991, Awad et al., 1991, Cascino et al., 1992), but it carries the intrinsic risk of morbidity and mortality associated with surgical intervention (Pilcher et al., 1993, Espinosa et al., 1994, Ross et al., 1996). Definitive localization of an epileptic focus correlates with a favorable outcome following epilepsy surgery (Lieb et al., 1981, Spencer et al., 1982). For mesial temporal lobe epilepsy (TLE), scalp electrode videoEEG can demonstrate temporal lobe origin but definitive lateralization may not be possible in all cases, and in some cases may result in false lateralization (Spencer et al., 1985, Sammaritano et al., 1987). Therefore, patients with temporal lobe onset by scalp EEG may require intracranial recording, particularly when neuroimaging does not assist in localization (Spencer et al., 1982, Engel and Ojemann, 1993).

Mesial temporal lobe seizures originate in the hippocampus or amygdalo-perihippocampal region (Falconer, 1974, Gates and Cruz-Rodgriguez, 1990, Mathern et al., 1996, Pringle et al., 1993). Although previous studies have provided information for the utilization of depth and subdural grids for lateralization and localization of the epileptogenic focus in mesial TLE (Lieb et al., 1976, Olivier et al., 1985, Flanigan and Smith, 1987, Spencer et al., 1987, Spencer et al., 1990, Sperling and O'Connor, 1988, Sperling and O'Connor, 1989, So et al., 1989, Luders et al., 1992, Shimuzu et al., 1992, Blatt et al., 1997, Brekelmans et al., 1998), questions persist regarding the optimal means for intracranial monitoring. This study was undertaken to determine the incremental value of data yielded for surgical decision making when using subdural strip electrodes alone and in addition to depth electrodes for TLE.