Abstracts

Rationale: Scalp EEG plays an important role in the diagnosis and localization of epilepsy. The dynamic relation between scalp and intracranial EEG has been investigated previously. Using simultaneous scalp and intracranial EEG recording, it has been demonstrated that the vast majority of scalp EEG spikes originate from the sources between 10 and 30 cm2 of cortical area.  However, the proportion of intracranial spikes recorded on scalp EEG has not been determined.  The purpose of this study is to determine the sensitivity of scalp EEG in detecting the intracranial interictal spikes and to further understand the role of scalp EEG in the diagnosis and localization of mesial temporal lobe epilepsy.  Methods: We recorded simultaneously scalp and intracranial EEG in 20 surgery candidates with mesial temporal lobe epilepsy. Intracranial EEG was recorded from both hippocampus and temporal neocortex using depth and/or subdural electrodes during invasive EEG studies. Scalp EEG was recorded using international 10-20 system. Intracranial interictal epileptiform discharges were identified during the first hour of sleep on the first night of recording. The interictal discharges located in the hippocampus and temporal neocortex were reviewed by visual inspection. The number of intracranial interictal discharges that generated scalp EEG interictal epileptiform discharges was determined.  Results: Hippocampal interictal spiking rate varied widely among the 20 subjects from 101 to 2187 spikes/hour. Hippocampal spikes intermittently propagate to basal, anterior and lateral temporal cortex. Hippocampal spikes without neocortical propagation did not generate identifiable scalp EEG spikes (Figure 1). Approximately 6% (670/11180) hippocampal spikes with sufficient neocortical propagation generated scalp interictal spikes. Hippocampal spikes with a limited neocortical propagation to basal temporal and/or anterior cortex may correlate with small sharp spikes (SSS) and sharp transients on scalp EEG, which are not commonly considered epileptiform discharges (Figure 2).  Conclusions: Our study demonstrates a very low sensitivity of scalp EEG in detecting intracranial interictal spikes in patients with mTLE. Hippocampal interictal spikes without neocortical propagation are not recordable on the scalp EEG. Scalp interictal spikes primarily reflect neocortical epileptic activity. The frequency of scalp interictal spikes may reflect the degree of temporal neocortical epileptic progression in mTLE.  Funding: NA