Lateralization of temporal lobe foci: depth versus subdural electrodes

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.

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.

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.

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.