Abstracts

Rationale: There is mounting evidence that seizures are a manifestation of aberrant network function involving disseminated cortical and subcortical brain regions. Identification of patient-specific epileptogenic networks is essential for potentially modulating these networks to decrease seizure probability. These networks have been sought to be characterized by a variety of non-invasive methods including fMRI, MEG, PET. However an optimal method for delineating these networks would be to study the flow of in vivo electrical information between different regions of the network. Cortico-cortical evoked potentials (CCEPs) in response to external low frequency electrical stimulation have been used to characterize these networks. However, these artificial currents may not reflect the true endogenous connectivity especially in atypical seizure networks. Responses to endogenous inter-ictal spikes may provide an indirect measure of functional connectivity because studies have shown similar neuronal firing patterns during endogenous inter-ictal spikes and CCEPs.Methods: In order to estimate the intensity and direction of information flow between these regions, intracranial recordings in patients with frequent inter-ictal discharges during pre-surgical evaluation for medically refractive epilepsy were used. We estimated cerebral connectivity using evoked potentials from endogenous inter-ictal spikes (eCCEPs) to identify and visualize functional connectivity among brain regions. Three hundred spontaneously occurring epileptiform spike discharges were used to align recordings from other cortical regions in two patients. On-going work is focused on expanding these analyses to a larger patient cohort and correlating eCCEP measures with other metrics of network connectivity.Results: We found that majority of electrodes that showed significant averaged eCCEPs overlay regions exhibiting early spread patterns during ictal events. Additionally, distant regions also showed large responses suggestive of the aberrant connectivity expected in these patients.Conclusions: In comparison to the interventional approach of CCEPs, which is time-consuming, determined by clinical needs, and may result in seizure production, evoked potentials from endogenous spikes utilizes the routinely collected inter-ictal data. Our analyses suggests that this strategy could enable us to delineate the regions of early and late seizure spread using eCCEPs. This work is supported by a grant from NSF (1406556)