Abstracts

Rationale: The use of microelectrodes has allowed researchers to monitor brain activity at the sub-millimeter scale, allowing for the study of possible seizure generators in more detail. Microepileptic discharges (epileptic discharges detected on microelectrodes but not on clinical electrodes) resembling high-frequency oscillations, periodic spikes, and seizures have been identified, however it is unclear how they originate, and why these activities are confined to a small area in the brain. We sought to understand the nature of these microepileptic events in the brain and whether they reflect clinically relevant information.

Methods: We obtained continuous recordings using research electrodes (Neuroport array and PMT Micro-Macro depth electrodes) from patients with medical refractory epilepsy who underwent presurgical evaluation with intracranial electrodes. Microepileptic events were classified into two subgroups of (a) microseizures, where the events exhibited similar dynamics to seizures whereby a transient change, evolving over time before ending abruptly, occurred in a small number of microelectrode channels and (b) microepileptic periodic discharges, during which a run of activity similar to interictal spiking was visible on at least one channel of the microelectrode array. The rate of event occurrence, their spatial extent, their relation to the occurrence of clinical seizures, and the rate of multi units on each microelectrode were analyzed.

Results: Data from four patients (three with the Neuroport array and one with the PMT Micro-Macro depth) were reviewed. The microelectrodes of two patients were situated within clinically identified seizure onset areas. In 5539 minutes of recordings we identified 55 microseizures and 28 microepileptic periodic discharges. Only a subset of microelectrode contacts exhibited microepileptic events and the events were seen on average on 6.6±1% of contacts. The identified microseizures were confined to specific electrodes only, although some microepileptic periodic discharges were visible on neighboring microelectrode contacts. We examined the rate of occurrence of microseizues in an hour period before the clinical seizures of each patient. In 22 clinical seizures recorded in the patients, only one was preceded by a microseizure. Interestingly, there was no clear relationship between multiunit neuronal firing and these events even while their overall rate of occurrence was generally higher in the microelectrode portion where the microseizures were seen.

Conclusions: Through our findings, we demonstrate that microepileptic discharges may appear in different forms and that they are confined to specific areas with the smallest being ~500 µm². These events are independent of tissue pathology and they may not propagate to surrounding electrodes. Revealing the specific neural mechanisms underlying these events and understanding why these events are only seen in small areas and do not propagate to the neighboring areas may explain how we can manage seizure spread and eventually offer new treatments for patients with epileptic disorders.

Funding: Please list any funding that was received in support of this abstract.: NIH/NINDS R01-2NS062092 and K24-NS088568, Tiny Blue Dot.