Fluctuations in EEG band power at subject‐specific timescales over minutes to days explain changes in seizure evolutions

Abstract

Epilepsy is recognised as a dynamic disease, where both seizure susceptibility and seizure characteristics themselves change over time. Specifically, we recently quantified the variable electrographic spatio‐temporal seizure evolutions that exist within individual patients. This variability appears to follow subject‐specific circadian, or longer, timescale modulations. It is therefore important to know whether continuously recorded interictaliEEG features can capture signatures of these modulations over different timescales. In this study, we analyse continuous intracranial electroencephalographic (iEEG) recordings from video‐telemetry units and find fluctuations in iEEG band power over timescales ranging from minutes up to 12 days. As expected and in agreement with previous studies, we find that all subjects show a circadian fluctuation in their iEEG band power. We additionally detect other fluctuations of similar magnitude on subject‐specific timescales. Importantly, we find that a combination of these fluctuations on different timescales can explain changes in seizure evolutions in most subjects above chance level. These results suggest that subject‐specific fluctuations in iEEG band power over timescales of minutes to days may serve as markers of seizure modulating processes. We hope that future study can link these detected fluctuations to their biological driver(s). There is a critical need to better understand seizure modulating processes, as this will enable the development of novel treatment strategies that could minimise the seizure spread, duration or severity and therefore the clinical impact of seizures.

Multiple mechanisms shape the relationship between pathway and duration of focal seizures

“… 17 , 46 However, the factors that modulate seizure durations within the same patient are unknown. We previously hypothesized that preictal variability in brain dynamics 4 or continuous fluctuations in interictal brain dynamics 47 could produce changes in seizure pathways. Likewise, fluctuations in interictal markers 48 such as levels of cortical excitability and inhibition, 49–51 interictal spike rate, 52 , 53 or functional networks 54 could potentially affect seizure duration.…”

A library of quantitative markers of seizure severity

“…Subclinical seizures are also reported to follow a circadian pattern 15 . Recent studies also reported modulations at circadian and longer timescales within many patients in terms of seizure electrographic evolutions 38,39 and other seizure properties 40 . In agreement with previous literature, we found evidence that EEG‐based seizure severity markers are modulated on circadian and longer timescales, although the effect size of the modulation is patient‐specific and weak in some patients.…”

Chronic iEEG recordings and interictal spike rate reveal multiscale temporal modulations in seizure states

“…Second, as with seizure occurrence Leguia et al, 2021;Maturana et al, 2020), different cycles likely interact to produce the observed seizure variability. A predictive model incorporating multiple timescales may be more informative than a single spike rate cycle (Panagiotopoulou et al, 2021).…”

“…Additionally, variability in seizure onset and spread have been linked to preictal and interictal changes in network features (Khambhati et al, 2016), band power (Naftulin et al, 2018), the location of high frequency oscillations (Gliske et al, 2018), and patterns of cortical excitability (Badawy et al, 2009). The same interictal features (network dynamics (Kuhnert et al, 2010;Mitsis et al, 2020), band power (Panagiotopoulou et al, 2021), high frequency activity (Chen et al, 2021), and cortical excitability (Meisel et al, 2015)) have all been shown to vary over circadian and/or multidien cycles. We now show that seizure evolutions also change over the timescales that influence interictal brain dynamics, suggesting that these fluctuations share common mechanisms.…”