Abstracts

The purpose of a comprehensive surgical evaluation for patients with medically intractable focal epilepsy is to estimate the epileptogenic zone as precisely as possible. The gold standard methodology has been video-EEG monitoring. We have developed a potentially new adjunctive technique that may also help to identify or confirm the video-EEG findings. Cortical evoked potentials to direct cortical stimulation, here labeled as cortico-cortical evoked potential (CCEPs), is a technique to investigate functional and anatomical relationships between two different cortical areas. Several previous studies showed that CCEPs presented specific response patterns in the epileptogenic mesial temporal lobe. We have hypothesized that the CCEPs are modified in the neocortical area close to the epileptogenic zone in patients with focal epilepsy.
The study subjects included 7 patients with focal epilepsy who were undergoing invasive EEG monitoring. CCEPs were recorded from subdural or depth electrodes in two different settings: ictal-onset zone stimulation and control stimulation. Electrode pair for the stimulation consisted of two adjacent electrodes that were overlapping the initial EEG seizure-onset zone for the ictal-onset zone stimulation, and those that were separate from the onset zone, but confined in the same anatomical region for the control stimiulation. A single constant-current square-pulse electrical stimulation from 1mA to 15mA was applied through the electrode pair with alternating polarity at 1Hz. The evoked responses were recorded from 12 to 30 electrodes around the stimulated pair, and averaged for 10 to 200 sweeps. The results were compared between the ictal onset zone and control conditions.
CCEPs were recorded from 16 series of the stimulation in all 7 patients. The waveforms consisted of two parts. The initial phase was considered to be baseline drift by stimulus artifacts. This potential had the same polarity and widespread distribution over multiple electrodes with smooth decay, typically sustained from the stimulus for 6-10ms. The second phase was regional cortical responses. This potential showed a negative peak in the electrodes adjacent to the stimulated area between 10 and 60ms. In 2 patients who were stimulated on the parahippocampal gyrus and amygdala, multiple peak responses were seen up to 100ms. The CCEPs at latency between 10 and 100ms were clearly larger in the ictal-onset zone stimulation than the control stimulation in 4 patients. The control stimulation showed greater responses than the ictal onset zone stimulation in 1 patient. Overriding artifacts precluded us to compare the responses in the other 2 patients.
CCEPs tends to be high in amplitude in the vicinity of the seizure onset zone. CCEPs may be useful measure for a local epileptogenicity, though it depends on regional excitability as well as normal anatomical and functional connectivity around the stimulated cortex.