Lateralization of temporal lobe foci: depth versus subdural electrodes

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 surged decision making when using subdural electrodes alone and in addition to depth electrodes f or temporal lobe epilepsy. Methods: Standardized placement for intracranial electrodes included: (1) l ongitudinal placement of bilateral temporal lobe depth electrodes; (2) bila teral subtemporal subdural strips; and (3) bilateral orbitofrontal subdural strips. Sixty-three events were randomly reviewed for: (1) subdural electr odes alone; and (2) depth electrodes in conjunction with subdural electrode s. Results: Of the 63 seizures, 54 (85.7%) demonstrated congruent lateralizati on to ipsilateral subtemporal subdural strip electrodes (based on depth ele ctrode 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-surgi cal outcome. For these 3 patients, retrospective review of neuroimaging dem onstrated suboptimal ipsilateral placement of subtemporal subdural electrod es with the most mesial electrode lateral to the collateral sulcus. Four ad ditional patients had suboptimal placement of subtemporal subdural electrod es. Two of these 4 patients had congruent localization with subdural electr odes to ipsilateral depth electrodes despite suboptimal placement. Subtempo ral subdural electrodes accurately localized for all seizures from the mesi al temporal lobe when the mesial electrodes of the subtemporal subdural str ip recorded mesial to the collateral sulcus from the parahippocampal region . Conclusion: We conclude that although there are high concordance rates betw een subdural and depth electrodes, localization of seizure onset based on s ubdural strip electrodes alone may result in inaccurate focus identificatio n with potential for possible suboptimal treatment of temporal lobe epileps y. When subtemporal subdural electrodes provide recording from the parahipp ocampal region, there is accurate localization of the seizure focus. If sub optimal placement occurs lateral to the collateral sulcus, the electroencep halographer cannot make a definitive identification of the seizure focus. ( C) 2001 Elsevier Science Ireland Ltd. All rights reserved.