Comparison of combined versus subdural or intracerebral electrodes alone in presurgical focus localization

Purpose: The yield of subdural versus intracerebral electrodes for ictal lo calization remains a point of controversy. We assessed the relative sensiti vity of these two types of electrodes per case. Methods: Eighty-three intracranial recordings obtained from 82 patients wer e retrospectively reviewed to establish which type of electrode performed b est in which patients and which seizure types. Results: Sixty (73%) of 82 patients had temporal lobe seizure onsets, eight frontal, nine widespread or multifocal/multilobar or both, whereas in five , seizure onset was not localized. Exclusive use of intracerebral electrode s would have been sufficient for accurate localization of the seizure-onset zone in all 35 patients with strictly mesial temporal seizure onsets. In o nly 20 (57%) of these 35 patients, the same decision would have been reache d with exclusive use of subdural electrodes. In wide-spread neocortical and mesial temporal seizures (n = 25), yield of both electrode types was at ab out the same level, but neither was sufficient to identify the zone of icta l onset on its own. In frontal or multilobar seizures (n = 22), yield of su bdural electrodes was slightly better then that of the intracerebral electr odes, but was not sufficient in all cases. Conclusions: This study indicates that, depending on the characteristics of the seizure disorder, exclusive use of either intracerebral or subdural el ectrodes may easily result in erroneous diagnosis because of insufficient s ampling of the brain. These findings are in contrast with other studies emp hasizing the high yield of reliable EEG findings in evaluations with a sing le type of electrode and corroborate the results of one of our previous stu dies.