Abstracts

Rationale: The mechanism underlying initiation and development of epileptic seizures, ictogenesis, is unclear. In prior work, we presented a new approach to visualizing EEG activity, the pulsogram, showing three discrete stages of seizure development (induction, reverberant, paroxysmal) in optogenetically induced mice. Here, we adapted the pulsogram to examine the ictogenesis of spontaneous seizures from the intrahippocampal kainic acid (IHKA) model of MTLE. We demonstrate that ictogenesis in murine MTLE seizures also undergoes a similar reverberant increase in excitation prior to paroxysmal ictal activity.

Methods: We examined intracranial recordings from a homozygous PV-Cre mouse following IHKA injection. Kainic acid (20mM, 50nL) was stereotaxically injected into the hippocampal CA1 layer and electrodes were implanted bilaterally, targeting the CA1 and dentate gyrus (DG). EEG of the animal was monitored over thirteen 24-hour sessions for 62 days. A four-channel pulsogram was constructed for each identified seizure, presenting EEG as a time-vs-time heatmap, wherein vertical snippets of EEG are aligned to the peak of spontaneous spiking events and stacked in chronological order along the x-axis. Spiking events were identified as EEG activity with voltage greater than seven times the median absolute deviation.

Results: We define the reverberant phase (RP) as the pre-ictal period characterized by a delayed jump in activity, or secondary discharge, visualized as distinct horizontal banding on the pulsogram (Fig 1). Of the 31 temporal lobe seizures recorded, the RP was present in at least one recording channel for all 29 seizures analyzed (two were excluded due to excessive noise in the EEG); 96.55% of ipsilateral CA1 (iCA1), 65.52% of contralateral CA1 (cCA1), 82.76% of iDG, and 44.83% of cDG recordings showed evidence of an RP. RP length (mean = 11.9160s, SD = 4.3501s) and onset were consistent across all channels (F = 0.5257, one-way ANOVA; p > 0.05, Wilcoxon signed-rank test). Similarly, paroxysmal phase (PP) length (19.5138s, SD = 5.5910s) and onset were consistent across all channels (F = 0.8205, one-way ANOVA; p >0.05, Wilcoxon signed-rank test). RP length was independent of seizure severity on the Racine scale (ρ = -0.1073, p = 0.768, Spearman’s rank correlation). Direct comparison of RP and PP lengths demonstrated independence between the two phases (r = -0.0712, Pearson’s correlation coefficient).

Conclusions: Our results suggest the pulsogram can reliably detect stages of ictogenesis among spontaneous and recurrent seizures recorded in animals with MTLE. Analysis of the reverberant phase specifically indicates a fixed period preceding ictal activity, most evident in iCA1 and iDG, that can be used to predict spontaneous seizures prior to onset regardless of severity. Closed-loop neuromodulation therapies for epilepsy reliant on EEG detection of seizure onset, i.e. responsive neurostimulation, may benefit from use of the pulsogram and reverberant phase, allowing for earlier detection and prevention of seizures.

Funding: N/A