Abstracts

Rationale:
In previous work1, we found evidence for ultra-low amplitude sharp activity that occurred immediately after the start of the postictal state – underneath apparent complete suppression and initially only visible at high gain. Figure 1 shows examples of this early postictal sharp activity (EPSA); note sharp activity (A1, A2) is only observed when the same EEG segment is viewed under high gain (A) and not under the lower gain used for clinical review (B). Figure 2 shows evolution of EPSA over the first 30 seconds from the start of the postictal state – only visible under high gain (A). EPSA was superimposed on a mixed frequency background that also included gamma power. Postictal gamma was reported by Bateman et al2. The origin of postictal epileptiform-like activity and its relationship to the interictal irritative zone (IIZ) are unknown. Here we explore the hypothesis that EPSA represents a re-emergence of the IIZ, with the predication that areas active in EPSA correspond to areas active interictally.
Method:
A total number of 12 focal to bilateral tonic clonic seizures (FTBTC) from 12 patients undergoing sEEG as part of a presurgical evaluation will be included in the analysis. We compare the first 60 seconds from the start of the post-ictal state (SPIS) with a 5-minute pre-ictal baseline. All sharp discharges from each EEG segment are documented. For the postictal segment, the EEG is reviewed at ultra-high gain (2-5μV/mm).  The pre-ictal EEG is reviewed using the standard gain settings of 50-100 μV/mm. The latency of occurrence of sharp discharges from each channel is noted in the postictal segment. For each site of epileptiform activity, the following are calculated: the total number of IEDs, number of IEDs per minute, and percentage from each channel of the total IEDs recorded. Results •A comparison of the EPSA and IIZ revealed partial overlap between the two, with some areas that produce sharp activity immediately postictally being interictally silent. Specifically, we found that only 33.1% of the EPSA sites were interictally active. These sites accounted for 35.1% of the total sharp discharges in EPSA. Furthermore, not all IIZ sites were represented in EPSA, with 42.9% of IIZ sites producing sharp activity in the first 60 seconds postictally. •There is reduced irritative activity from the seizure onset zone (SOZ) immediately postictally compared to IIZ. The average percentage of all sharp activity from the SOZ in IIZ was 35.1% versus 0.3% in EPSA. There were 10 IEDs/minute in the SOZ region interictally compared to 0.5 sharps/minute from the SOZ in EPSA (n=2).
Conclusion:
•The data to-date indicates that EPSA is not merely a re-emergence of the interictal irritative zone. Not surprisingly, we also found a suppression of sharp activity from the SOZ in the immediate postictal period. •Although the finding of suppressed irritability from the SOZ immediately postictally is not surprising, it may partially explain the differences in sharp activity between IIZ and EPSA. The suppression of epileptiform activity from SOZ postictally may transiently disinhibit other potential irritative areas, accounting for some of the new EPSA sites not present in the IIZ. •An alternative interesting possibility is that some of the sharp activity of EPSA may not be truly epileptiform, but rather IED-like activity that is occurring as part of the mechanism of transition from postictal suppression back to higher amplitude interictal EEG activity.  References: •Karkar KM et al. Probing the postictal stereotactic EEG for localizing information after focal to bilateral tonic-clonic seizures. AES Abst. 2.011, 2019. •Bateman LM et al. Postictal clinical and electrographic activity following intracranially recorded bilateral tonic-clonic seizures. Epilepsia 2019, 60(1): 74-84. 
Funding:
:NA