Adding or repositioning intracranial electrodes during presurgical assessment of neocortical epilepsy: electrographic seizure pattern and surgical outcome.

OBJECT: The aim of this study was to investigate changes in electroencephalography (EEG) patterns obtained from added or repositioned electrodes after those initially implanted had failed to indicate the true local ictal onset zone. The authors focused on the following matters: rationale for adding or repositioning electrodes, topographic and frequency characteristics of ictal onset before and after adding or repositioning electrodes, the effect of the procedures, and the relationship between changes in intracranial EEG onset patterns and surgical outcomes.

METHODS: Of 183 patients with intracranial recordings, 18 experienced repositioning of existing or implanting of additional electrodes 7 or 10 days later. All patients underwent resection and were followed up for more than 1 year. In particular, the relationship between surgical outcome and distribution/frequency of intracranial seizure onset was analyzed. Results of noninvasive presurgical evaluations in patients who had undergone single and double invasive studies were also evaluated. By adding or repositioning electrodes, a new ictal onset zone was revealed in 13 patients. In another four, the second evaluation led to a change in defining the resection margin. Ictal onset in the partially sampled area, simultaneous or independent onset in two separate areas, and onset in the distal end of the electrode strip or grid were common reasons for failing to localize the ictal onset zone during the initial evaluation. Seven of 11 patients who were ultimately found to have a focal ictal onset zone on the second evaluation became seizure free after the operation. Only one of six patients with a regional ictal onset zone identified on the second evaluation became seizure free. There was no relationship between the frequency of the ictal rhythm and surgical outcome. Note, however, that surgical outcome was more favorable in patients who had undergone a single invasive study than in those who had undergone double invasive studies. The patients who needed a second evaluation had less localizing information and less concordant results on presurgical evaluations. When comparing nonlesional cases, surgical outcomes were not significantly different among patients with a single invasive study and those with double invasive studies. No additional morbidity or death occurred during the second study.

CONCLUSIONS: The addition or reposition of intracranial electrodes with a short-term interval should be considered in selected patients. Spatial restriction of the ictal onset rhythm identified on repeated evaluation is the most important predictor of a good surgical outcome.