Research

Our research group has recently developed a methodology for measuring laryngeal responses as an indicator of VNS-induced A-fiber activation by recording surface laryngeal motor evoked potentials (LMEPs). LMEPs serve as markers of efferent A-fiber activation, specifically from the recurrent laryngeal nerve, which is a branch of the vagus nerve, and can be recorded using two surface electrodes placed on the ventral side of the neck. Currently, there is no information available on the following: (i) the recovery of the vagus nerve post-surgery, (ii) the evolution of VNS-induced EEG desynchronization after surgery, and (iii) changes in LMEPs after implantation. EEG was recorded 1-2 months prior to surgery (V1), and LMEP and EEG recordings were conducted 2 weeks post-surgery (V2-LMEP recording), as well as 1 month (V3), 3 months (V4), and 6 months (V5) after VNS implantation. EEG connectivity analysis is performed across various frequency bands (delta, theta, alpha, beta, and broadband), using the weighted phase lag index (wPLI) to measure functional brain connectivity across different scalp regions.

Vagus nerve stimulation (VNS) is an adjunctive treatment for refractory epilepsy. Although widely used, many patients still experience insufficient decrease of seizure frequency. Light, particularly blue light, can stimulate alertness, attention, and cognition by modulating anatomical targets common to the vagal afferent network. This project aims to explore the potential synergy between blue-enriched light and VNS, that may arise through their combined action on the locus coeruleus (LC), by investigating a series of VNS biomarkers, such as pupil size and EEG.

The purpose of this project is to analyse epileptogenic networks non-invasively through resting-state functional MRI and a microstructural analysis approach based on diffusion: Micro Fingerprinting. Resting-state functional MRI will identify grey matter regions involved in epileptogenesis while the microstructural analysis will identify white matter tracts connecting those regions, on one part, as well as analyse the intrinsic characteristics of those tracts.

In the 1^st phase, we will compare this analysis to electrophysiological network analysis performed in patients undergoing invasive EEG.  In the 2^nd phase, we will follow the subgroup of this cohort that will undergo curative epilepsy surgery and analyze the functional outcome at 1 year. We aim to identify  identify prognostic factors of the non-invasive network analysis correlating with better epileptic control. Finally, in the 3^rd phase, we will recruit a new cohort of drug-resistant epileptic patient and perform the non-invasive network analysis upfront (before discussing potential invasive EEG) and  determine the impact of this analysis on the therapeutic strategy of the multidisciplinary staff meeting.

The purpose of this retrospective study is to assess how recent techniques of EEG signal processing can help in managing children with severe, drug-resistant epilepsy in order to improve:

• Localization of the epileptic focus during the non-invasive presurgical work-up
• Our understanding of the epilepsy-related brain dysfunction leading to cognitive and developmental issues, opening the window for possible therapeutic implications

For the first point, we will use a technique called ESI (electrical source imaging), performing a co-registration of the interictal and ictal EEG data with the patient’s own pre-operative MRI, to estimate the brain region (at a sublobar level) with the highest probability to be the generator of the EEG signal (electrical dipole). The novelty of this study is the application of the technique in a very challenging population, such as young children (< 7 years at surgery) for which visual EEG analysis can be often misleading. ESI results will be compared with the resection zone (assessed by co-registration with post-operative MRI) and surgical outcome (ILAE class) at 1-year post-intervention.

The second part of the study will be performed on a cohort of children with DEEs (developmental and epileptic encephalopathies). Functional connectivity analysis using different metrics will be applied on awake and sleep EEG before and after treatment (namely steroids) and results will be compared with those obtained in other 2 groups (“non-epileptic, typically developing children” and “epileptic, non-encephalopathic children”). For more details about functional connectivity, refer to the specific section

A key aspect of our research involved the exploration of response biomarkers to deepen our current knowledge about the biological prerequisites linked to therapeutic response. In this context, electrophysiological recordings of Laryngeal Motor Evoked Potentials (LMEPs) and multimodal Magnetic Resonance Imaging (MRI) were used to develop response biomarkers. In particular, this research delved into the structural and functional characteristics of the locus coeruleus, a small brainstem nucleus involved in the antiseizure effects of VNS, as well as the structural attributes of thalamocortical tracts in patients with drug-resistant epilepsy who were implanted with a VNS device.

Finally, an industrial part of our research aimed at developing a system, the Optical Communication Device (OCD), to communicate remotely (from the command room) with the fully MRI-compatible Synergia Medical optoelectronic neurostimulator in an MRI environment. The use of the OCD will ultimately help in the realization of functional MRI studies, aiming at better understanding the acute effects of VNS administration and optimizing this therapy by tuning the stimulation parameters to maximize the recruitment of structures involved in the antiseizure effects.

The auditory oddball task, a common neuropsychological method, is employed to generate a measurable arousal response. This response may be quantified through the P300, an event-related potential, and the pupil dilation response, both of which reflect neural activity. Our study aims to compare the direct effects of VNS on modulating these responses, offering insights into how VNS influences arousal and how this effect differs between responders and non-responders. Additionally, we assess the long-term effects of VNS by analyzing changes in these biomarkers before and after VNS implantation.

This project investigates how transcutaneous auricular Vagus Nerve Stimulation (taVNS) modulates Heartbeat Evoked Potentials (HEPs), offering a novel approach to validate the correct stimulation of the auricular branch of the vagus nerve. By examining HEP modulation as a proxy measure, the study aims to confirm effective vagal afferent activation.

This project seeks to identify reliable acute central EEG-based biomarkers that can guide a personalized Vagus Nerve Stimulation (VNS) titration process, potentially improving outcomes for patients with drug-resistant epilepsy (DRE).

Twenty-eight patients with DRE (6 with generalized epilepsy and 22 with focal epilepsy) were recruited for this study. During each session, patients were subjected to a sequence of six VNS stimulation trains, each lasting 18 seconds and ranging from 0.25 mA up to the clinical setting plus 0.25 mA. EEG was recorded using a 64-channel setup. The EEG data was processed to estimate spectral power in different frequency bands and scalp regions and measure phase synchronization across regions.

Responders’ theta power was concentrated in the central region, whereas non-responders exhibited more parieto-occipital theta activity. Responders’ alpha power was predominantly found in the bilateral central-parietal regions, while non-responders showed less activity in these regions. Phase synchronization analysis indicated consistent desynchronization in responders near the clinical intensities, while non-responders maintained stable wPLI ratio values, suggesting potential over- or under-stimulation.

VNS parameters (current intensity, pulse width, pulse frequency, and duty cycle) are periodically adjusted according to clinical response, patient tolerance, and side effects. Studying dose-dependent pupil dilation response (PDR) might help clinicians have a more objective and personalized titration strategy. Since LC-NE system’s activity affects pupil dilation, we study this biomarker to evaluate the central effects of VNS dosage and assess VNS-dosage differences between R and NR.

Central chemoreception is the body’s mechanism for detecting blood CO₂ levels and regulating breathing accordingly. Located in the brainstem, central chemoreceptors sense blood CO₂ indirectly through changes in pH. When plasma CO₂ levels rise, CO₂ crosses the blood-brain barrier into the cerebrospinal fluid, causing a decrease in pH (increased acidity). This increase in acidity stimulates the central chemoreceptors prompting an increase in minute ventilation.

In epilepsy, seizures are often accompanied by respiratory acidosis and central apneas. In severe cases, these apneas can be prolonged, leading to Sudden Unexpected Death in Epilepsy (SUDEP). This suggests that despite elevated blood CO₂ levels, the usual breathing response to increase ventilation is impaired. Therefore, we aim to characterize central chemoreception in epilepsy and hypothesize that it is altered in chronic epileptic conditions.

To address this question, we use various animal models, including:

1. Kainic acid rat model: This model simulates refractory temporal lobe epilepsy characterized by secondarily generalised tonic-clonic seizures.
2. Genetic absence epilepsy rats of Strasbourg (GAERS): These rats have a mutation in the Cacna1h gene, which encodes calcium channels and are characterized by generalized absence seizures

Our research

Our research aims to advance the understanding of vagus nerve activity in relation to seizure onset and associated autonomic symptoms. Indeed, seizures induce multiple autonomic responses, both sympathetic and parasympathetic.  It is known that vagus nerve plays a crucial role in transmitting information between the brain and peripheral organs. Thus, exploiting vagal neural traffic in relation to seizures may offer a novel approach for seizure detection and the development of closed-loop Vagus Nerve Stimulation.  Two rat models  mirroring the electrophysiological and behavioral featuresobserved human epilepsy are used at the Epilepsy and Neurostimualtion Laboratory

Vagus Nerve Activity and Absence seizures

The first model is the Genetic Absence Epilepsy Rat of Strasbourg (GAERS) is a genetic model, characterized by spontaneous occurring absence seizures. Interinstingly, these non-convulsive seizures, marked by transient loss of consciousness, areclinically similar to those experienced by pediatric epilepsy patients. Our initial investigations revealed that vagus nerve activity increases during absence seizures in GAERS, while the parasympathetic response and individual variability in ictal vagus nerve activity modifications decrease with age, despite the occurrence of stable seizure frequency and severity.

Vagus Nerve Activity and Temporal lobe epilepsy

The second model used is induced by intrahippocampal kainic acid injection, which is a model of temporal lobe epilepsy exhibiting spontaneous focal temporal lobe seizures with secondary generalization, which are also accompanied by significant cardiac and respiratory dysfunctions. These disruptions, conveyed via the vagus nerve, are crucial for understanding the systemic impact of epilepsy. In a previous study, we characterized vagus nerve activity during acute kainic acid-induced seizures in anesthetized rats and developed a vagus nerve activity-based algorithm for seizure detection.

Finally, at the Epilepsy Neurostimulation Laboratory, we are committed to further characterize vagus nerve activity in freely moving rats across various seizure types to develop new methods of seizure detection

Our methodology includes

• Surgical implantation of EEG epidural and intrahippocampal electrodes, vagus nerve electrodes, and intrahippocampal cannulae
• Video-EEG/Vagus nerve monitoring
• Photoplethysmography to assess cardiorespiratory parameters in awake, freely moving rats

Currently, this project involves a dedicated team consisting of one postdoc and two PhD students:

• Elise Collard (PhD Student)
• Elena Acedo Reina (PhD Student)
• Dr. Enrique Germany Morrison (Postdoctoral Researcher)