Abstracts

RATIONALE: The purpose of this study was to investigate the change of electrographic patterns when the repositioning of implanted electrodes after it failed to catch the true local onset zone. We focused on the following aspects: (a) the reasoning background of the repositioning electrodes, (b) topographic and frequency characteristics of ictal EEG onset before and after the repositioning, (c) the effectiveness of repositioning or additional implanatation of intracranial electrodes on surgical outcome, (d) the relationship between change of intracranial EEG onset pattern and surgical outcome.
METHODS: Of 183 cases with intracranial recordings between 1994 and 2000, 18 cases underwent a second invasive study consisting of repositioning or additional implantation of intracranial electrodes performed seven days after the initial invasive study. All patients underwent resective surgery and followed up for more than a year.
RESULTS: The repositioning of intracranial electrodes identified a new ictal onset zone in 13 patients. In another four cases the second evaluation made the change of decision on the resection margin. No change on the decision of resection was made in one case. The assumption of failure in the localization of ictal onset zone during the initial evaluation were based on the following reasons: (1) ictal onset in the partially sampled area (5 cases), (2) ictal onset in the distal end of strip or grid (4 cases), (3) simultaneous or independent ictal onset in two separate areas (5 cases), (4) widespread ictal onset zone (2 cases), (5) preceding clinical onset before electrographic change (2 cases). Of eight patients with regional ictal onset in the initial evaluation, six had focal ictal onset in the second invasive study. Four of them were seizure free after the operation. Four of five patients with two separate ictal onset zones in the first study, the second invasive study found regional ictal onset zones and only one of them were seizure free after the resection. There was no relationship between the frequency of ictal rhythm and the surgical outcome. There was no additional morbidity or mortality with the second invasive studies.
CONCLUSIONS: The repositioning or addition of intracranial electrodes with one-week interval in the patients who had unsuccessful initial intracranial evaluation allowed good surgical outcome in half of them. Two-thirds of those patients who were ultimately found focal ictal onset zone by the second evaluation became seizure free after the operation. The spatial restriction of ictal rhythm is the important predictor for good surgical outcome. These results support consideration of one-week interval repositioning of intracranial electrodes in selected patients.
[Supported by: Seoul National University Hospital]