Abstracts

Rationale: Preoperative scalp spike frequency is a strong predictor of surgical outcome in patients with mesial temporal lobe epilepsy (MTLE). It is presumed that scalp spike frequency likely reflect the extent of seizure progression from mesial to lateral temporal cortex. Therefore, patients with frequent scalp interictal spikes are associated with poor surgical outcomes. However, evidence supporting this hypothesis is largely lacking. The objective of our study is to determine the value of scalp spike frequency as a potential EEG marker in gauging the extent of epileptogenesis in temporal neocortex in patients MTLE. Methods: We recorded simultaneously scalp and intracranial EEG spikes using 26 channels of scalp EEG with sub-temporal supplementary electrodes and 46 to 98 channels of intracranial EEG in 5 patients with medically-intractable temporal lobe epilepsy. Subdural electrodes were implanted extensively on the anterior through mid-temporal lobe. The scalp and intracranial interictal spikes were analyzed for their location and frequency during the first-hour sleep on the 1st day of intracranial study. The intracranial EEG correlates of scalp EEG spikes were then determined. Results: Mesial temporal seizure onset was recorded in all the 5 patients. A total of 462 ECoG interictal spikes were identified in mesiobasal temporal cortex, and none of these ECoG spikes were recordable on scalp EEG. A total of 1639 ECoG spikes were identified in the lateral temporal neocortex with or without mesiobasal spiking source and only 117 of these ECoG spikes (8%) were recordable on scalp EEG. The percentage of ECoG spikes recordable on scalp EEG is dependent upon their cortical source area and synchrony, and is significantly variable among the 5 patients. Conclusions: Basomesial temporal spikes need recruiting sufficient amount of lateral temporal cortex in order to be recordable on the scalp EEG. While scalp interictal spike mainly correlates with the spiking source in temporal neocortex, scalp spike frequency may not reflect the frequency of ECoG spikes in temporal neocortex. Therefore, ECoG spike frequency is more reliable than scalp spike frequency in gauging the severity of epileptogenesis in temporal neocortex.