Abstracts

Rationale: Intracranial EEG is widely used for localization of the epileptogenic zone in patients with drug resistant focal epilepsy, often preceded by an extensive, noninvasive workup. Since each intracranial electrode captures activity from a small fraction of the brain, the procedure is used to sample key regions of interest and test existing hypotheses, while the greater part of the brain remains unsampled. As a result, the insights gained rely heavily on precise electrode placement, and may fail to encompass the epileptic network in its entirety.

We and others have shown that scalp correlates of intracranial epileptiform activity can be revealed by performing spike-locked averaging (SLA) utilizing data from intracranial recordings. This method uses the intracranial sharp wave to synchronize averaged epochs, decreasing noise and uncovering scalp activity, which may reflect the intracranial spike used or other, distant, time locked activity. Here, we hypothesized that by evaluating the scalp fields of this averaged sharp activity and subjecting it to electric source imaging, new sources may be identified, beyond the scope measured by intracranial electrodes.

Methods: Simultaneous EEG recordings from 21 scalp electrodes and intracranial electrodes from two patients with drug resistant epilepsy were analyzed. Over 500 intracranial interictal spike and wave complexes were manually identified and annotated per patient, and then clustered using principal component analysis into unique subgroups. Both intracranial and scalp EEG then underwent SLA, resulting in a 2 second timeframe containing the averaged spike. Source localization using sLORETA was performed on the scalp component of the averaged spikes, at 4 timepoint during the sharp wave (positive peak, negative peak, positive half peak, negative half peak).

Results: In both patients, SLA revealed a scalp sharp and slow wave which was not observed prior to averaging in different intracranial spike populations. In the first patient, 2 main intracranial spike populations involving the right temporal operculum and right anterior cingulate were averaged, and scalp-based sources were traced to the frontal and temporal poles, as well as temporo-parietal and basal temporal regions. Notably, some of these sources were distant from the site of the intracranial recording electrodes (Figure 1).

In the second patient, SLA of a population of intracranial spikes involving left posterior subtemporal and the left hippocampus resulted in an ipsilaterally distributed sharp wave on scalp EEG, with sourced in the ipsilateral occipital, frontal and temporal regions.

Conclusions: Averaged scalp spikes may be traced to sources distant from their time locked intracranial counterparts, expanding hypotheses regarding the extent of the epileptic network. While their clinical context has yet to be tested, these sources may facilitate further intracranial testing.

Funding: Please list any funding that was received in support of this abstract.: No funding was received.