Wake slow waves in focal human epilepsy impact network activity and cognition

Interictal network dysfunction and cognitive impairment in epilepsy

Epilepsy is diagnosed when neural networks become capable of generating excessive or hypersynchronous activity patterns that result in observable seizures. In many cases, epilepsy is associated with cognitive comorbidities that persist between seizures and negatively impact quality of life. Dysregulation of the coordinated physiological network interactions that are required for cognitive function has been implicated in mediating these enduring symptoms, but the causal mechanisms are often elusive. Here, we provide an overview of neural network abnormalities with the potential to contribute to cognitive dysfunction in epilepsy. We examine these pathological interactions across spatial and temporal scales, additionally highlighting the dynamics that arise in response to the brain's intrinsic capacity for plasticity. Understanding these processes will facilitate development of network-level interventions to address cognitive comorbidities that remain undertreated by currently available epilepsy therapeutics.

Mechanistic insights into the interaction between epilepsy and sleep

Epidemiological evidence has demonstrated associations between sleep and epilepsy, but we lack a mechanistic understanding of these associations. If sleep affects the pathophysiology of epilepsy and the risk of seizures, as suggested by correlative evidence, then understanding these effects could provide crucial insight into the basic mechanisms that underlie the development of epilepsy and the generation of seizures. In this Review, we provide in-depth discussion of the associations between epilepsy and sleep at the cellular, network and system levels and consider the mechanistic underpinnings of these associations. We also discuss the clinical relevance of these associations, highlighting how they could contribute to improvements in the management of epilepsy. A better understanding of the mechanisms that govern the interactions between epilepsy and sleep could guide further research and the development of novel approaches to the management of epilepsy.

Widespread decoupling of spindles and slow waves in temporal lobe epilepsy

OBJECTIVE

Memory impairment is common in people with temporal lobe epilepsy (TLE). Recent studies in healthy subjects showed a positive correlation between sleep spindles coupled to slow waves (SWs) and memory performance. We aimed to determine differences in spindle-SW coupling in TLE patients compared to healthy controls using combined high-density electroencephalography and polysomnography.

METHODS

The study population consisted of 20 patients (12 female, 36.5 ± 9.9 years old) with unilateral drug-resistant TLE (10 left temporal) and 20 age- and sex-matched controls (12 female, 31.2 ± 6.3 years old). Spindles (10-16 Hz, .5-3 s) and SWs (.5-4 Hz) were automatically detected during all N2 and N3 epochs using validated detectors. Coupling of spindles with SWs was defined as overlap between both detected events.

RESULTS

Coupled spindle-SW rates (per minute) were globally reduced in patients with TLE compared to healthy controls (median = .18 [interquartile range (IQR) = .08-.36] vs. .35 [IQR = .24-.46], p = .014, d = -.46). This reduction was also found for coupled fast spindle (12-16 Hz)-SW (.06 [IQR = .02-.13] vs. .18 [IQR = .07-.25], p = .013, d = -.46) and slow spindle (10-12 Hz)-SW rates (.11 [IQR = .04-.23] vs. .19 [IQR = .13-.27], p = .034, d = -.40). Within TLE patients, there was no local difference between the coupling rates in the lobe with the epileptic focus compared to the contralateral side (.09 [IQR = .02-.13] vs. .07 [IQR = .02-.13], p = .18). The effect size of the reduction was stronger in early than late sleep for both N2 and N3 sleep (early N2 d = -.50 vs. late N2 d = -.39; early N3 d = -.53 vs. late N3 d = -.47).

SIGNIFICANCE

Despite a focal epileptic generator, patients with unilateral TLE showed a widespread decoupling between sleep spindles and SWs that was most prominent in early sleep. As coupling was shown to be associated with neuropsychological performance in healthy people, this global decoupling may constitute one potential mechanism of poor memory performance in people with TLE.

Sleep links hippocampal propensity for epileptiform activity to its viscerosensory inputs

The development of a seizure relies on two factors. One is the existence of an overexcitable neuronal network and the other is a trigger that switches normal activity of that network into a paroxysmal state. While mechanisms of local overexcitation have been the focus of many studies, the process of triggering remains poorly understood. We suggest that, apart from the known exteroceptive sources of reflex epilepsy such as visual, auditory or olfactory signals, there is a range of interoceptive triggers, which are relevant for seizure development in Temporal Lobe Epilepsy (TLE). The hypothesis proposed here aims to explain the prevalence of epileptic activity in sleep and in drowsiness states and to provide a detailed mechanism of seizures triggered by interoceptive signals.

Working memory deficit in patients with focal epilepsy is associated with higher interictal theta connectivity

OBJECTIVE

Interictal cognitive disturbances are frequent in patients with focal epilepsies and the links with alteration of resting state brain oscillations are not well known. Changes in theta oscillations, may contribute to cognitive impairment. This study aimed to investigate whether changes in theta activity are related to cognitive disturbances.

METHODS

Retrospective data of 23 patients with temporal/frontal lobe epilepsy were included. Theta connectivity, power and interictal spikes rate from five-minute interictal resting state stereoelectroencephalography datasets were computed. Cognitive performances were assessed by Wechsler Intelligence Scale (WAIS-IV) and Weschler Memory Scale (WMS-III). Linear regression was performed to evaluate effect of interictal activity and seizure related parameters on cognitive scores.

RESULTS

WAIS-IV working memory score in patients with epilepsy showed negative correlation with frontotemporal theta connectivity (F(1,17) = 5,239, p = 0,036, R2 = 0,200, β = -0,497). Moreover, theta connectivity was correlated with mesial temporal spike rate and theta power (F(2,17) = 10,967, p = 0,001, adj.R2 = 0,540).

CONCLUSIONS

Patients with focal epilepsy often encounter compromised cognitive functions, particularly notable in the domain of working memory. This impairment might be attributed to physiological mechanisms involving increased theta connectivity within the frontotemporal regions and interictal spiking.

SIGNIFICANCE

Our study highlights the relation between theta connectivity and working memory impairments in patients with focal epilepsy.

Timing Mechanisms for Circadian Seizures

For centuries, epileptic seizures have been noticed to recur with temporal regularity, suggesting that an underlying biological rhythm may play a crucial role in their timing. In this review, we propose to adopt the framework of chronobiology to study the circadian timing of seizures. We first review observations made on seizure timing in patients with epilepsy and animal models of the disorder. We then present the existing chronobiology paradigm to disentangle intertwined circadian and sleep-wake timing mechanisms. In the light of this framework, we review the existing evidence for specific timing mechanisms in specific epilepsy syndromes and highlight that current knowledge is far from sufficient. We propose that individual seizure chronotypes may result from an interplay between independent timing mechanisms. We conclude with a research agenda to help solve the urgency of ticking seizures.

Sleep-like cortical dynamics during wakefulness and their network effects following brain injury

By connecting old and recent notions, different spatial scales, and research domains, we introduce a novel framework on the consequences of brain injury focusing on a key role of slow waves. We argue that the long-standing finding of EEG slow waves after brain injury reflects the intrusion of sleep-like cortical dynamics during wakefulness; we illustrate how these dynamics are generated and how they can lead to functional network disruption and behavioral impairment. Finally, we outline a scenario whereby post-injury slow waves can be modulated to reawaken parts of the brain that have fallen asleep to optimize rehabilitation strategies and promote recovery.

Sleepiness and the transition from wakefulness to sleep

The transition from wakefulness to sleep is a progressive process that is reflected in the gradual loss of responsiveness, an alteration of cognitive functions, and a drastic shift in brain dynamics. These changes do not occur all at once. The sleep onset period (SOP) refers here to this period of transition between wakefulness and sleep. For example, although transitions of brain activity at sleep onset can occur within seconds in a given brain region, these changes occur at different time points across the brain, resulting in a SOP that can last several minutes. Likewise, the transition to sleep impacts cognitive and behavioral levels in a graded and staged fashion. It is often accompanied and preceded by a sensation of drowsiness and the subjective feeling of a need for sleep, also associated with specific physiological and behavioral signatures. To better characterize fluctuations in vigilance and the SOP, a multidimensional approach is thus warranted. Such a multidimensional approach could mitigate important limitations in the current classification of sleep, leading ultimately to better diagnoses and treatments of individuals with sleep and/or vigilance disorders. These insights could also be translated in real-life settings to either facilitate sleep onset in individuals with sleep difficulties or, on the contrary, prevent or control inappropriate sleep onsets.

Closed-loop electrical stimulation to prevent focal epilepsy progression and long-term memory impairment

Interictal epileptiform discharges (IEDs) are ubiquitously expressed in epileptic networks and disrupt cognitive functions. It is unclear whether addressing IED-induced dysfunction could improve epilepsy outcomes as most therapeutics target seizures. We show in a model of progressive hippocampal epilepsy that IEDs produce pathological oscillatory coupling which is associated with prolonged, hypersynchronous neural spiking in synaptically connected cortex and expands the brain territory capable of generating IEDs. A similar relationship between IED-mediated oscillatory coupling and temporal organization of IEDs across brain regions was identified in human subjects with refractory focal epilepsy. Spatiotemporally targeted closed-loop electrical stimulation triggered on hippocampal IED occurrence eliminated the abnormal cortical activity patterns, preventing spread of the epileptic network and ameliorating long-term spatial memory deficits in rodents. These findings suggest that stimulation-based network interventions that normalize interictal dynamics may be an effective treatment of epilepsy and its comorbidities, with a low barrier to clinical translation.

One-Sentence Summary

Targeted closed-loop electrical stimulation prevents spread of the epileptic network and ameliorates long-term spatial memory deficits.

Widespread slow oscillations support interictal epileptiform discharge networks in focal epilepsy

Interictal epileptiform discharges (IEDs) often co-occur across spatially-separated cortical regions, forming IED networks. However, the factors prompting IED propagation remain unelucidated. We hypothesized that slow oscillations (SOs) might facilitate IED propagation. Here, the amplitude and phase synchronization of SOs preceding propagating and non-propagating IEDs were compared in 22 patients with focal epilepsy undergoing intracranial electroencephalography (EEG) evaluation. Intracranial channels were categorized into the irritative zone (IZ) and normal zone (NOZ) regarding the presence of IEDs. During wakefulness, we found that pre-IED SOs within the IZ exhibited higher amplitudes for propagating IEDs than non-propagating IEDs (delta band: p = 0.001, theta band: p < 0.001). This increase in SOs was also concurrently observed in the NOZ (delta band: p = 0.04). Similarly, the inter-channel phase synchronization of SOs prior to propagating IEDs was higher than those preceding non-propagating IEDs in the IZ (delta band: p = 0.04). Through sliding window analysis, we observed that SOs preceding propagating IEDs progressively increased in amplitude and phase synchronization, while those preceding non-propagating IEDs remained relatively stable. Significant differences in amplitude occurred approximately 1150 ms before IEDs. During non-rapid eye movement (NREM) sleep, SOs on scalp recordings also showed higher amplitudes before intracranial propagating IEDs than before non-propagating IEDs (delta band: p = 0.006). Furthermore, the analysis of IED density around sleep SOs revealed that only high-amplitude sleep SOs demonstrated correlation with IED propagation. Overall, our study highlights that transient but widely distributed SOs are associated with IED propagation as well as generation in focal epilepsy during sleep and wakefulness, providing new insight into the EEG substrate supporting IED networks.