Chronic vagus nerve stimulation increases the latency of the thalamocortical somatosensory evoked potential

Vagus nerve stimulation for the treatment of epilepsy: things to note on the protocols, the effects and the mechanisms of action

Epilepsy is a chronic brain disorder that is characterized by repetitive un-triggered seizures that occur severally within 24 h or more. Non-pharmacological methods for the management of epilepsy were discussed. The non-pharmacological methods include the vagus nerve stimulation (VNS) which is subdivided into invasive and non-invasive techniques. For the non-invasive techniques, the auricular VNS, stimulation of the cervical branch of vagus nerve in the neck, manual massage of the neck, and respiratory vagal nerve stimulation were discussed. Similarly, the stimulation parameters used and the mechanisms of actions through which VNS improves seizures were also discussed. Use of VNS to reduce seizure frequency has come a long way. However, considering the cost and side effects of the invasive method, non-invasive techniques should be given a renewed attention. In particular, respiratory vagal nerve stimulation should be considered. In doing this, the patients should for instance carry out slow-deep breathing exercise 6 to 8 times every 3 h during the waking hours. Slow-deep breathing can be carried out by the patients on their own; therefore this can serve as a form of self-management.HIGHLIGHTSEpilepsy can interfere with the patients' ability to carry out their daily activities and ultimately affect their quality of life.Medications are used to manage epilepsy; but they often have their serious side effects.Vagus nerve stimulation (VNS) is gaining ground especially in the management of refractory epilepsy.The VNS is administered through either the invasive or the non-invasive methodsThe invasive method of VNS like the medication has potential side effects, and can be costly.The non-invasive method includes auricular VNS, stimulation of the neck muscles and skin and respiratory vagal nerve stimulation via slow-deep breathing exercises.The respiratory vagal nerve stimulation via slow-deep breathing exercises seems easy to administer even by the patients themselves.Consequently, it is our opinion that patients with epilepsy be made to carry out slow-deep breathing exercise 6-8 times every 3 h during the waking hours.

From synaptic dysfunction to atypical emotional processing in autism

Autism spectrum disorder (ASD) is a complex neurodevelopmental condition mainly characterized by social impairments and repetitive behaviors. Among these core symptoms, a notable aspect of ASD is the presence of emotional complexities, including high rates of anxiety disorders. The inherent heterogeneity of ASD poses a unique challenge in understanding its etiological origins, yet the utilization of diverse animal models replicating ASD traits has enabled researchers to dissect the intricate relationship between autism and atypical emotional processing. In this review, we delve into the general findings about the neural circuits underpinning one of the most extensively researched and evolutionarily conserved emotional states: fear and anxiety. Additionally, we explore how distinct ASD animal models exhibit various anxiety phenotypes, making them a crucial tool for dissecting ASD's multifaceted nature. Overall, to a proper display of fear response, it is crucial to properly process and integrate sensorial and visceral cues to the fear-induced stimuli. ASD individuals exhibit altered sensory processing, possibly contributing to the emergence of atypical phobias, a prevailing anxiety disorder manifested in this population. Moreover, these individuals display distinctive alterations in a pivotal fear and anxiety processing hub, the amygdala. By examining the neurobiological mechanisms underlying fear and anxiety regulation, we can gain insights into the factors contributing to the distinctive emotional profile observed in individuals with ASD. Such insights hold the potential to pave the way for more targeted interventions and therapies that address the emotional challenges faced by individuals within the autism spectrum.

Acute Effect of Vagus Nerve Stimulation in Patients with Drug-Resistant Epilepsy: A Preliminary Exploration via Stereoelectroencephalogram

As the pathophysiological mechanisms of vagus nerve stimulation (VNS) causing individual differences in the vagal ascending network remains unclear, stereoelectroencephalography (SEEG) provides a unique platform to explore the brain networks affected by VNS and helps to understand the anti-seizure mechanism of VNS more comprehensively. This study presents a preliminary exploration of the acute effect of VNS. SEEG signals were collected to assess the acute effect of VNS on neural synchronization in patients with drug-resistant epilepsy, especially in epileptogenic networks. The results show that the better the efficacy of VNS, the wider the spread of desynchronization assessed by weighted phase lag index at a high frequency band caused by VNS. Future studies should focus on the association between the change in synchronization and the efficacy of VNS, exploring the possibility of synchronization as a biomarker for patient screening and parameter programming.

Connectomic profiling and Vagus nerve stimulation Outcomes Study (CONNECTiVOS): a prospective observational protocol to identify biomarkers of seizure response in children and youth

INTRODUCTION

Vagus nerve stimulation (VNS) is a neuromodulation therapy that can reduce the seizure burden of children with medically intractable epilepsy. Despite the widespread use of VNS to treat epilepsy, there are currently no means to preoperatively identify patients who will benefit from treatment. The objective of the present study is to determine clinical and neural network-based correlates of treatment outcome to better identify candidates for VNS therapy.

METHODS AND ANALYSIS

In this multi-institutional North American study, children undergoing VNS and their caregivers will be prospectively recruited. All patients will have documentation of clinical history, physical and neurological examination and video electroencephalography as part of the standard clinical workup for VNS. Neuroimaging data including resting-state functional MRI, diffusion-tensor imaging and magnetoencephalography will be collected before surgery. MR-based measures will also be repeated 12 months after implantation. Outcomes of VNS, including seizure control and health-related quality of life of both patient and primary caregiver, will be prospectively measured up to 2 years postoperatively. All data will be collected electronically using Research Electronic Data Capture.

ETHICS AND DISSEMINATION

This study was approved by the Hospital for Sick Children Research Ethics Board (REB number 1000061744). All participants, or substitute decision-makers, will provide informed consent prior to be enrolled in the study. Institutional Research Ethics Board approval will be obtained from each additional participating site prior to inclusion. This study is funded through a Canadian Institutes of Health Research grant (PJT-159561) and an investigator-initiated funding grant from LivaNova USA (Houston, TX; FF01803B IIR).

Sex-Specific Effects of Stress on Respiratory Control: Plasticity, Adaptation, and Dysfunction

As our understanding of respiratory control evolves, we appreciate how the basic neurobiological principles of plasticity discovered in other systems shape the development and function of the respiratory control system. While breathing is a robust homeostatic function, there is growing evidence that stress disrupts respiratory control in ways that predispose to disease. Neonatal stress (in the form of maternal separation) affects "classical" respiratory control structures such as the peripheral O₂ sensors (carotid bodies) and the medulla (e.g., nucleus of the solitary tract). Furthermore, early life stress disrupts the paraventricular nucleus of the hypothalamus (PVH), a structure that has emerged as a primary determinant of the intensity of the ventilatory response to hypoxia. Although underestimated, the PVH's influence on respiratory function is a logical extension of the hypothalamic control of metabolic demand and supply. In this article, we review the functional and anatomical links between the stress neuroendocrine axis and the medullary network regulating breathing. We then present the persistent and sex-specific effects of neonatal stress on respiratory control in adult rats. The similarities between the respiratory phenotype of stressed rats and clinical manifestations of respiratory control disorders such as sleep-disordered breathing and panic attacks are remarkable. These observations are in line with the scientific consensus that the origins of adult disease are often found among developmental and biological disruptions occurring during early life. These observations bring a different perspective on the structural hierarchy of respiratory homeostasis and point to new directions in our understanding of the etiology of respiratory control disorders. © 2021 American Physiological Society. Compr Physiol 11:1-38, 2021.

Biomarkers of seizure response to vagus nerve stimulation: A scoping review

Although vagus nerve stimulation (VNS) is a common procedure, seizure outcomes are heterogeneous, with few available means to preoperatively identify the ideal surgical candidate. Here, we perform a scoping review of the literature to identify biomarkers of VNS response in patients with drug-resistant epilepsy. Several databases (Ovid MEDLINE, Ovid Embase, BIOSIS Previews, and Web of Science) were searched for all relevant articles that reported at least one biomarker of VNS response following implantation for intractable epilepsy. Patient demographics, seizure data, and details related to biomarkers were abstracted from all studies. From the 288 records screened, 28 articles reporting on 16 putative biomarkers were identified. These were grouped into four categories: network/connectomic-based biomarkers, electrophysiological signatures, structural findings on neuroimaging, and systemic assays. Differences in brain network organization, connectivity, and electrophysiological synchronicity demonstrated the most robust ability to identify VNS responders. Structural findings on neuroimaging yielded inconsistent associations with VNS responsiveness. With regard to systemic biomarkers, heart rate variability was shown to be an independent marker of VNS response, whereas inflammatory markers were not useful. There is an unmet need to preoperatively identify candidates who are likely to benefit from VNS. Several biomarkers demonstrate promise in predicting seizure responsiveness to VNS, particularly measures of brain network connectivity. Further efforts are required to validate existing biomarkers to inform clinical decision-making.

Research progress of vagus nerve stimulation in the treatment of epilepsy

The International League Against Epilepsy (ILAE) defined drug‐resistant epilepsy (DRE) that epilepsy seizure symptoms cannot be controlled with two well‐tolerated and appropriately chosen antiepileptic drugs, whether they are given as monotherapy or in combination. According to the WHO reports, there is about 30%‐40% of epilepsy patients belong to DRE. These patients need some treatments other than drugs, such as epilepsy surgery, and neuromodulation treatment. Traditional surgical approaches may be limited by the patient's clinical status, pathological tissue location, or overall prognosis. Thus, neuromodulation is an alternative choice to control their symptoms. Vagus nerve stimulation (VNS) is one of the neuromodulation methods clinically, which have been approved by the Food and Drug Administration (FDA). In this review, we systematically describe the clinical application, clinical effects, possible antiepileptic mechanisms, and future research directions of VNS for epilepsy.

The Present and Future of Vagus Nerve Stimulation

Epilepsy is one of the major chronic neurological diseases affecting many patients. Resection surgery is the most effective therapy for medically intractable epilepsy, but it is not feasible in all patients. Vagus nerve stimulation (VNS) is an adjunctive neuromodulation therapy that was approved in 1997 for the alleviation of seizures; however, efforts to control epilepsy by stimulating the vagus nerve have been studied for over 100 years. Although its exact mechanism is still under investigation, VNS is thought to affect various brain areas. Hence, VNS has a wide indication for various intractable epileptic syndromes and epilepsyrelated comorbidities. Moreover, recent studies have shown anti-inflammatory effects of VNS, and the indication is expanding beyond epilepsy to rheumatoid arthritis, chronic headaches, and depression. VNS yields a more than 50% reduction in seizures in approximately 60% of recipients, with an increase in reduction rates as the follow-up duration increases. The complication rate of VNS is 3–6%, and infection is the most important complication to consider. However, revision surgery was reported to be feasible and safe with appropriate measures. Recently, noninvasive VNS (nVNS) has been introduced, which can be performed transcutaneously without implantation surgery. Although more clinical trials are being conducted, nVNS can reduce the risk of infection and subsequent device failure. In conclusion, VNS has been demonstrated to be beneficial and effective in the treatment of epilepsy and various diseases, and more development is expected in the future.

Invasive Neuromodulation for the Treatment of Pediatric Epilepsy

Neuromodulatory strategies are increasingly adopted for the treatment of intractable epilepsy in children. These encompass a wide range of treatments aimed at externally stimulating neural circuitry in order to decrease seizure frequency. In the current review, the authors discuss the evidence for invasive neuromodulation, namely vagus nerve and deep brain stimulation in affected children. Putative mechanisms of action and biomarkers of treatment success are explored and evidence of the efficacy of invasive neuromodulation is highlighted.

The vagus afferent network: emerging role in translational connectomics

Vagus nerve stimulation (VNS) is increasingly considered for the treatment of intractable epilepsy and holds potential for the management of a variety of neuropsychiatric conditions. The emergence of the field of connectomics and the introduction of large-scale modeling of neural networks has helped elucidate the underlying neurobiology of VNS, which may be variably expressed in patient populations and related to responsiveness to stimulation. In this report, the authors outline current data on the underlying neural circuitry believed to be implicated in VNS responsiveness in what the authors term the “vagus afferent network.” The emerging role of biomarkers to predict treatment effect is further discussed and important avenues for future work are highlighted.

Presurgical thalamocortical connectivity is associated with response to vagus nerve stimulation in children with intractable epilepsy

Although chronic vagus nerve stimulation (VNS) is an established treatment for medically-intractable childhood epilepsy, there is considerable heterogeneity in seizure response and little data are available to pre-operatively identify patients who may benefit from treatment. Since the therapeutic effect of VNS may be mediated by afferent projections to the thalamus, we tested the hypothesis that intrinsic thalamocortical connectivity is associated with seizure response following chronic VNS in children with epilepsy. Twenty-one children (ages 5-21 years) with medically-intractable epilepsy underwent resting-state fMRI prior to implantation of VNS. Ten received sedation, while 11 did not. Whole brain connectivity to thalamic regions of interest was performed. Multivariate generalized linear models were used to correlate resting-state data with seizure outcomes, while adjusting for age and sedation status. A supervised support vector machine (SVM) algorithm was used to classify response to chronic VNS on the basis of intrinsic connectivity. Of the 21 subjects, 11 (52%) had 50% or greater improvement in seizure control after VNS. Enhanced connectivity of the thalami to the anterior cingulate cortex (ACC) and left insula was associated with greater VNS efficacy. Within our test cohort, SVM correctly classified response to chronic VNS with 86% accuracy. In an external cohort of 8 children, the predictive model correctly classified the seizure response with 88% accuracy. We find that enhanced intrinsic connectivity within thalamocortical circuitry is associated with seizure response following VNS. These results encourage the study of intrinsic connectivity to inform neural network-based, personalized treatment decisions for children with intractable epilepsy.

Vagus nerve stimulation: a new promising therapeutic tool in inflammatory bowel disease

Inflammatory bowel disease (IBD), that is Crohn's disease (CD) and ulcerative colitis, affects about 1.5 million persons in the USA and 2.2 million in Europe. The pathophysiology of IBD involves immunological, genetic and environmental factors. The treatment is medico-surgical but suspensive. Anti-TNFα agents have revolutionized the treatment of IBD but have side effects. In addition, a non-negligible percentage of patients with IBD stop or take episodically their treatment. Consequently, a nondrug therapy targeting TNFα through a physiological pathway, devoid of major side effects and with a good cost-effectiveness ratio, would be of interest. The vagus nerve has dual anti-inflammatory properties through its afferent (i.e. hypothalamic-pituitary-adrenal axis) and efferent (i.e. the anti-TNFα effect of the cholinergic anti-inflammatory pathway) fibres. We have shown that there is an inverse relationship between vagal tone and plasma TNFα level in patients with CD, and have reported, for the first time, that chronic vagus nerve stimulation has anti-inflammatory properties in a rat model of colitis and in a pilot study performed in seven patients with moderate CD. Two of these patients failed to improve after 3 months of vagus nerve stimulation but five were in deep remission (clinical, biological and endoscopic) at 6 months of follow-up and vagal tone was restored. No major side effects were observed. Thus, vagus nerve stimulation provides a new therapeutic option in the treatment of CD.

Pancreatic polypeptide and its central Y4 receptors are essential for cued fear extinction and permanent suppression of fear

Background and purpose

Avoiding danger and finding food are closely related behaviours that are essential for surviving in a natural environment. Growing evidence supports an important role of gut‐brain peptides in modulating energy homeostasis and emotional‐affective behaviour. For instance, postprandial release of pancreatic polypeptide (PP) reduced food intake and altered stress‐induced motor activity and anxiety by activating central Y₄ receptors.

Experimental approach

We characterized [K³⁰(PEG2)]hPP_2‐36 as long‐acting Y₄ receptor agonist and injected it peripherally into wildtype and Y₄ receptor knockout (Y4KO) C57Bl/6NCrl mice to investigate the role of Y₄ receptors in fear conditioning. Extinction and relapse after extinction was measured by spontaneous recovery and renewal.

Key results

The Y4KO mice showed impaired cued and context fear extinction without affecting acquisition, consolidation or recall of fear. Correspondingly, peripheral injection of [K³⁰(PEG2)]hPP_2‐36 facilitated extinction learning upon fasting, an effect that was long‐lasting and generalized. Furthermore, peripherally applied [K³⁰(PEG2)]hPP_2‐36 before extinction inhibited the activation of orexin‐expressing neurons in the lateral hypothalamus in WT, but not in Y4KO mice.

Conclusions and implications

Our findings suggests suppression of excessive arousal as a possible mechanism for the extinction‐promoting effect of central Y₄ receptors and provide strong evidence that fear extinction requires integration of vegetative stimuli with cortical and subcortical information, a process crucially depending on Y₄ receptors. Importantly, in the lateral hypothalamus two peptide systems, PP and orexin, interact to generate an emotional response adapted to the current homeostatic state. Detailed investigations of feeding‐relevant genes may thus deliver multiple intervention points for treating anxiety‐related disorders.

Central nervous system control of gastrointestinal motility and secretion and modulation of gastrointestinal functions

Although the gastrointestinal (GI) tract possesses intrinsic neural plexuses that allow a significant degree of autonomy over GI functions, the central nervous system (CNS) provides extrinsic neural inputs that regulate, modulate, and control these functions. While the intestines are capable of functioning in the absence of extrinsic inputs, the stomach and esophagus are much more dependent upon extrinsic neural inputs, particularly from parasympathetic and sympathetic pathways. The sympathetic nervous system exerts a predominantly inhibitory effect upon GI muscle and provides a tonic inhibitory influence over mucosal secretion while, at the same time, regulates GI blood flow via neurally mediated vasoconstriction. The parasympathetic nervous system, in contrast, exerts both excitatory and inhibitory control over gastric and intestinal tone and motility. Although GI functions are controlled by the autonomic nervous system and occur, by and large, independently of conscious perception, it is clear that the higher CNS centers influence homeostatic control as well as cognitive and behavioral functions. This review will describe the basic neural circuitry of extrinsic inputs to the GI tract as well as the major CNS nuclei that innervate and modulate the activity of these pathways. The role of CNS-centered reflexes in the regulation of GI functions will be discussed as will modulation of these reflexes under both physiological and pathophysiological conditions. Finally, future directions within the field will be discussed in terms of important questions that remain to be resolved and advances in technology that may help provide these answers.

Neuronal control of breathing: sex and stress hormones

There is a growing public awareness that hormones can have a significant impact on most biological systems, including the control of breathing. This review will focus on the actions of two broad classes of hormones on the neuronal control of breathing: sex hormones and stress hormones. The majority of these hormones are steroids; a striking feature is that both groups are derived from cholesterol. Stress hormones also include many peptides which are produced primarily within the paraventricular nucleus of the hypothalamus (PVN) and secreted into the brain or into the circulatory system. In this article we will first review and discuss the role of sex hormones in respiratory control throughout life, emphasizing how natural fluctuations in hormones are reflected in ventilatory metrics and how disruption of their endogenous cycle can predispose to respiratory disease. These effects may be mediated directly by sex hormone receptors or indirectly by neurotransmitter systems. Next, we will discuss the origins of hypothalamic stress hormones and their relationship with the respiratory control system. This relationship is 2-fold: (i) via direct anatomical connections to brainstem respiratory control centers, and (ii) via steroid hormones released from the adrenal gland in response to signals from the pituitary gland. Finally, the impact of stress on the development of neural circuits involved in breathing is evaluated in animal models, and the consequences of early stress on respiratory health and disease is discussed.

Intensity-dependent modulatory effects of vagus nerve stimulation on cortical excitability

OBJECTIVES

Vagus nerve stimulation (VNS) is an effective treatment for refractory epilepsy. It remains unknown whether VNS efficacy is dependent on output current intensity. The present study investigated the effect of various VNS output current intensities on cortical excitability in the motor cortex stimulation rat model. The hypothesis was that output current intensities in the lower range are sufficient to significantly affect cortical excitability.

MATERIAL AND METHODS

VNS at four output current intensities (0 mA, 0.25 mA, 0.5 mA and 1 mA) was randomly administered in rats (n = 15) on four consecutive days. Per output current intensity, the animals underwent five-one-hour periods: (i) baseline, (ii) VNS1, (iii) wash-out1, (iv) VNS2 and (v) wash-out2. After each one-hour period, the motor seizure threshold (MST) was measured and compared to baseline (i.e. ∆MSTbaseline , ∆MSTVNS 1 , ∆MSTwash-out1 , ∆MSTVNS 2 and ∆MSTwash-out2 ). Finally, the mean ∆MSTbaseline , mean ∆MSTwash-out1 , mean ∆MSTwash-out2 and mean ∆MSTVNS per VNS output current intensity were calculated.

RESULTS

No differences were found between the mean ∆MSTbaseline , mean ∆MSTwash-out1 and mean ∆MSTwash-out2 within each VNS output current intensity. The mean ∆MSTVNS at 0 mA, 0.25 mA, 0.5 mA and 1 mA was 15.3 ± 14.6 μA, 101.8 ± 23.5 μA, 108.1 ± 24.4 μA and 85.7 ± 18.1 μA respectively. The mean ∆MSTVNS at 0.25 mA, 0.5 mA and 1 mA were significantly larger compared to the mean ∆MSTVNS at 0 mA (P = 0.002 for 0.25 mA; P = 0.001 for 0.5 mA; P = 0.011 for 1 mA).

CONCLUSIONS

This study confirms efficacy of VNS in the motor cortex stimulation rat model and indicates that, of the output current intensities tested, 0.25 mA is sufficient to decrease cortical excitability and higher output current intensities may not be required.

Pontine mechanisms of respiratory control

Pontine respiratory nuclei provide synaptic input to medullary rhythmogenic circuits to shape and adapt the breathing pattern. An understanding of this statement depends on appreciating breathing as a behavior, rather than a stereotypic rhythm. In this review, we focus on the pontine-mediated inspiratory off-switch (IOS) associated with postinspiratory glottal constriction. Further, IOS is examined in the context of pontine regulation of glottal resistance in response to multimodal sensory inputs and higher commands, which in turn rules timing, duration, and patterning of respiratory airflow. In addition, network plasticity in respiratory control emerges during the development of the pons. Synaptic plasticity is required for dynamic and efficient modulation of the expiratory breathing pattern to cope with rapid changes from eupneic to adaptive breathing linked to exploratory (foraging and sniffing) and expulsive (vocalizing, coughing, sneezing, and retching) behaviors, as well as conveyance of basic emotions. The speed and complexity of changes in the breathing pattern of behaving animals implies that "learning to breathe" is necessary to adjust to changing internal and external states to maintain homeostasis and survival.

A novel implantable vagus nerve stimulation system (ADNS-300) for combined stimulation and recording of the vagus nerve: pilot trial at Ghent University Hospital

PURPOSE

Vagus nerve stimulation (VNS) is an established treatment for refractory epilepsy. The ADNS-300 is a new system for VNS that includes a rechargeable stimulus generator and an electrode for combined stimulation and recording. In this feasibility study, three patients were implanted with ADNS-300 for therapeutic VNS. In addition, compound action potentials (CAPs) were recorded to evaluate activation of the vagus nerve in response to VNS.

METHODS

Three patients were implanted with a cuff-electrode around the left vagus nerve, that was connected to a rechargeable pulse generator under the left clavicula. Two weeks after surgery, therapeutic VNS (0.25-1.25 mA, 500 μs, 30s on, 10 min off and 30Hz) was initiated and stimulus-induced CAPs were recorded.

RESULTS

The ADNS-300 system was successfully implanted in all three patients and patients were appropriately stimulated during six months of follow-up. A reduction in seizure frequency was demonstrated in two patients (43% and 40% in patients 1 and 3, respectively), while in patient 2 seizure frequency remained unchanged. CAPs could be recorded in patients 1 and 2, proving stimulation-induced activation of the vagus nerve.

CONCLUSION

This feasibility study demonstrates that the ADNS-300 system can be used for combined therapeutic stimulation (in 3/3 patients) and recording of CAPs in response to VNS (in 2/3 patients) up to three weeks after surgery. Implantation in a larger number of patients will lead to a better understanding of the electrophysiology of the vagus nerve, which in turn could result in more adequate and individualized VNS parameter choice.

Modulation of seizure threshold by vagus nerve stimulation in an animal model for motor seizures

OBJECTIVE

The precise mechanism of action of vagus nerve stimulation (VNS) in suppressing epileptic seizures remains to be elucidated. This study investigates whether VNS modulates cortical excitability by determining the threshold for provoking focal motor seizures by cortical electrical stimulation before and after VNS.

MATERIAL AND METHODS

Male Wistar rats (n = 8) were implanted with a cuff-electrode around the left vagus nerve and with stimulation electrodes placed bilaterally on the rat motor cortex. Motor seizure threshold (MST) was assessed for each rat before and immediately after 1 h of VNS with standard stimulation parameters, during two to three sessions on different days.

RESULTS

An overall significant increase of the MST was observed following 1 h of VNS compared to the baseline value (1420 microA and 1072 microA, respectively; P < 0.01). The effect was reproducible over time with an increase in MST in each experimental session.

CONCLUSIONS

VNS significantly increases the MST in a cortical stimulation model for motor seizures. These data indicate that VNS is capable of modulating cortical excitability.

Vagus nerve stimulation for intractable epilepsy: outcome in two series combining 90 patients

Vagus nerve stimulation (VNS) is the most widely used non-pharmacological treatment for medically intractable epilepsy and has been in clinical use for over a decade. It is indicated in patients who are refractory to medical treatment or who experience intolerable side effects, and who are not candidates for resective surgery. VNS used in the acute setting can both abort seizures and have an acute prophylactic effect. This effect increases over time in chronic treatment to a maximum at around 18 months. The evidence base supporting the efficacy of VNS is strong, but its exact mechanism of action remains unknown. A vagus nerve stimulator consists of two electrodes embedded in a silastic helix that is wrapped around the cervical vagus nerve. The stimulator is always implanted on the left vagus nerve in order to reduce the likelihood of adverse cardiac effects. The electrodes are connected to an implantable pulse generator (IPG) which is positioned subcutaneously either below the clavicle or in the axilla. The IPG is programmed by computer via a wand placed on the skin over it. In addition, extra pulses of stimulation triggered by a hand-held magnet may help to prevent or abort seizures. VNS is essentially a palliative treatment and the number of patients who become seizure free is very small. A significant reduction in the frequency and severity of seizures can be expected in about one third of patients and efficacy tends to improve with time. Vagus nerve stimulation is well tolerated and has few significant side effects. We describe our experience on the use of VNS on drug-resistant epilepsy in 90 patients treated in two departments (in Athens, Greece and Newcastle, England).

Vagus nerve stimulation for induced spinal cord seizures: insights into seizure cessation

OBJECT

Vagus nerve stimulation is known to decrease the frequency, duration, and intensity of some types of intracranial seizures in both humans and animals. Although many theories abound concerning the mechanism for this action, the true cause remains speculative. To potentially elucidate a pathway in which vagus nerve stimulation aborts seizure activity, seizures were initiated not in the cerebral cortex but in the spinal cord and then vagus nerve stimulation was performed.

METHODS

Ten pigs were anesthetized and placed in the lateral position, and a small laminectomy was performed in the lumbar region. Topical penicillin, a known epileptogenic drug to the cerebral cortex and spinal cord, was applied to the dorsal surface of the exposed cord. With the exception of two animals that were used as controls, once seizure activity was discernible via motor convulsion or increased electrical activity the left vagus nerve, which had been previously isolated in the neck, was stimulated. Following multiple stimulations of the vagus nerve and with seizure activity confirmed, the cord was transected in the midthoracic region and vagus nerve stimulation was performed. Vagus nerve stimulation resulted in cessation of spinal cord seizure activity in all (87.5%) but one experimented animal. Transection of the spinal cord superior to the site of seizure induction resulted in the ineffectiveness of vagus nerve stimulation to cause cessation of seizure activity in all study animals.

CONCLUSIONS

The effects of vagus nerve stimulation on induced spinal cord seizures involve descending spinal pathways. The authors believe that this experiment is the first to demonstrate that spinal cord neuronal hyperactivity can be suppressed by stimulation of a cranial nerve. These data may aid in the development of alternative mechanisms for electrical stimulation in patients with medically intractable seizures. Further studies are now necessary to isolate which specific tracts, nuclei, and neurotransmitters are involved in this process.

Chemosensory Function in Patients With Vagal Nerve Stimulators

Chemosensory function is determined by the interplay of numerous sensory modalities. The present study aimed to evaluate the possible influence of electrical stimulation of the left-sided vagal nerve on gustatory and olfactory function in patients with vagal nerve stimulation (VNS). Gustation and olfaction were tested using psychophysical techniques; olfactory function was additionally evaluated using event-related potentials. A total of 11 subjects participated (six men and five women, aged 21 to 56 years). The vagal stimulator was run in "rapid cycle mode" in 10 patients, whereas one patient was treated with "normal mode" VNS. Subjects participated in two sessions, with the vagal stimulator switched on and off, respectively. The sequence of the two sessions was randomized across all participants. Using air-dilution, olfactometry event-related potentials to the specific olfactory stimulant H2S were recorded. Psychophysical tests were performed using the "Sniffin' Sticks" test kit, a test for retronasal olfactory function, and a gustatory test based on impregnated filter paper. The study yielded the following major results: (1) VNS produced a prolongation of P2 latencies of olfactory ERP, and (2) patients with therapeutic benefit from VNS in terms of seizure control had larger amplitudes during the on period than during the off period. In conclusion, using electrophysiological measures of olfactory function, the present study indicated a significant role of VNS in the processing of olfactory information.

Left-sided vagus nerve stimulation decreases intracranial pressure without resultant bradycardia in the pig: a potential therapeutic modality for humans

INTRODUCTION

The medical literature is lacking in data regarding intracranial pressure in an animal model with concomitant vagus nerve and spinal cord stimulation.

METHODS

Ten pigs were anesthetized and placed in the supine position and an intracranial pressure monitor was inserted. Each study animal had a small laminectomy performed in the lumbar region and the left vagus nerve exposed within the carotid sheath. Intracranial pressure was monitored for 3 h in all animals. Eight animals at the end of 3 h of monitoring underwent vagus nerve stimulation and or spinal cord stimulation. Study and control animals had their intracranial pressure monitored for an additional 1 and 2 h respectively.

RESULTS

Following vagus nerve stimulation, all animals had significant (p<0.05) decreases in their intracranial pressure with a lasting effect of 15-35 min (mean 18.5 min). No significant change in blood pressure was noted during stimulation of the vagus nerve. After spinal cord stimulation all animals responded with acute increases in their intracranial pressure (p<0.05).

CONCLUSIONS

Left vagus nerve stimulation reliably decreases intracranial pressure in the pig. The mechanism of this action remains unclear and does not appear to be due to resultant bradycardia. Conversely, stimulation of the upper lumbar spinal cord increases intracranial pressure with simultaneous increases in heart rate. Following additional studies and with close observation of cerebral perfusion pressure, we believe that left vagus nerve stimulation may represent a novel adjunctive therapy for decreasing elevated intracranial pressure in posttraumatic human patients with head injuries. Additionally, according to this animal data, minimizing spinal cord stimulation should be considered in the acute setting following head injury so as to minimize iatrogenic elevation of intracranial pressure.

The central nucleus of the amygdala modulates gut-related neurons in the dorsal vagal complex in rats

Using retrograde tract-tracing and electrophysiological methods, we characterized the anatomical and functional relationship between the central nucleus of the amygdala and the dorsal vagal complex. Retrograde tract-tracing techniques revealed that the central nucleus of the amygdala projects to the dorsal vagal complex with a topographic distribution. Following injection of retrograde tracer into the vagal complex, retrogradely labelled neurons in the central nucleus of the amygdala were clustered in the central portion at the rostral level and in the medial part at the middle level of the nucleus. Few labelled neurons were seen at the caudal level. Electrical stimulation of the central nucleus of the amygdala altered the basal firing rates of 65 % of gut-related neurons in the nucleus of the solitary tract and in the dorsal motor nucleus of the vagus. Eighty-one percent of the neurons in the nucleus of the solitary tract and 47 % of the neurons in the dorsal motor nucleus were inhibited. Electrical stimulation of the central nucleus of the amygdala also modulated the response of neurons in the dorsal vagal complex to gastrointestinal stimuli. The predominant effect on the neurons of the nucleus of the solitary tract was inhibition. These results suggest that the central nucleus of the amygdala influences gut-related neurons in the dorsal vagal complex and provides a neuronal circuitry that explains the regulation of gastrointestinal activity by the amygdala.

Complications of chronic vagus nerve stimulation for epilepsy in children

OBJECT

The aim of this study was to define better the incidence of surgical complications and untoward side effects of chronic vagus nerve stimulation (VNS) in a population of children with medically refractory epilepsy.

METHODS

The authors retrospectively reviewed the cases of 74 consecutive patients (41 male and 33 female) 18 years of age or younger (mean age 8.8 years, range 11 months-18 years) who had undergone implantation of a vagal stimulator between 1998 and 2001 with a minimum follow up of 1 year (mean 2.2 years). Of the 74 patients treated, seven (9.4%) had a complication ultimately resulting in removal of the stimulator. The rate of deep infections necessitating device removal was 3.5% (three of 74 patients who had undergone 85 implantation and/or revision procedures). An additional three superficial infections occurred in patients in whom the stimulators were not removed: one was treated with superficial operative debridement and antibiotic agents and the other two with oral antibiotics only. Another four stimulators (5.4%) were removed because of the absence of clinical benefit and device intolerance. Two devices were revised because of lead fracture (2.7%). Among the cohort, 11 battery changes have been performed thus far, although none less than 33 months after initial implantation. Several patients experienced stimulation-induced symptoms (hoarseness, cough, drooling, outbursts of laughter, shoulder abduction, dysphagia, or urinary retention) that did not require device removal. Ipsilateral vocal cord paralysis was identified in one patient. One patient died of aspiration pneumonia more than 30 days after device implantation.

CONCLUSIONS

Vagus nerve stimulation remains a viable option for improving seizure control in difficult to treat pediatric patients with epilepsy. Surgical complications such as hardware failure (2.7%) or deep infection (3.5%) occurred, resulting in device removal or revision. Occasional stimulation-induced symptoms such as hoarseness, dysphagia, or torticollis may be expected (5.4%).

Cerebral activation during vagus nerve stimulation: a functional MR study

PURPOSE

To study the short-term effects of vagus nerve stimulation (VNS) on brain activation and cerebral blood flow by using functional magnetic resonance imaging (fMRI).

METHODS

Five patients (three women, two men; mean age, 35.4 years) who were treated for medically refractory epilepsy with VNS, underwent fMRI. All patients had a nonfocal brain MRI. The VNS was set at 30 Hz, 0.5-2.0 mA for intervals of activation of 30 s on and 30 s off, during which the fMRI was performed. Statistical parametric mapping (SPM) was used to determine significant areas of activation or inhibition during vagal nerve stimulation (p < 0.05).

RESULTS

VNS-induced activation was detected in the thalami bilaterally (left more than right), insular cortices bilaterally, ipsilateral basal ganglia and postcentral gyri, right posterior superior temporal gyrus, and inferomedial occipital gyri (left more than right). The most robust activation was seen in the thalami (left more than right) and insular cortices.

CONCLUSIONS

VNS-induced thalamic and insular cortical activation during fMRI suggests that these areas may play a role in modulating cerebral cortical activity, and the observed decrease in seizure frequency in patients who are given VNS may be a consequence of this increased activation.

Vagus nerve stimulation in pediatric epilepsy: a review

Therapeutic options for intractable epilepsy include new and investigational antiepileptic drugs, ketogenic diet, epilepsy surgery, and, now, vagus nerve stimulation, which is approved by the U.S. Food and Drug Administration for the treatment of refractory partial seizures in adolescents and adults. The exact mechanisms of action are unknown. Although the use of vagus nerve stimulation in children has increased, including those younger than 12 years of age or those with generalized epilepsy, there has been no large controlled pediatric study to date. The identification of favorable prognostic indicators, especially in children, would be useful. Preliminary results suggest that children with Lennox-Gastaut syndrome may have a favorable response, with improvement in both seizure control and global evaluation scores. Improved global evaluation scores have occurred even without an associated improvement in seizure control.

Vagus nerve stimulation for refractory epilepsy

Vagus nerve stimulation (VNS) is a neurophysiological treatment for patients with medically or surgically refractory epilepsy. Since the first human implant in 1989, more than 10 000 patients have been treated with VNS. Two randomized controlled studies have shown a statistically significant decrease in seizure frequency during a 12-week treatment period versus a baseline period when 'high stimulation' mode was compared with 'low stimulation' mode. The efficacy appears to increase over time. In general, one third of the patients show a >50% reduction of seizure frequency; one third show a 30-50% seizure reduction, and one third of patients show no response. Few patients become seizure-free. Side effects during stimulation are mainly voice alteration, coughing, throat paraesthesia and discomfort. When studied on a long-term basis, VNS is an efficacious, safe and cost-effective treatment not only in adults but also in children and the elderly. The precise mechanism of action remains to be elucidated. In recent years much progress has been made through neurophysiological, neuroanatomical, neurochemical and cerebral blood flow studies in animals and patients treated with VNS. Further elucidation of the mechanism of action of VNS may increase its clinical efficacy and our general understanding of some physiopathological aspects of epilepsy. Finally, VNS may become an alternative treatment for other conditions such as depression and pain.

The mechanism of action of vagus nerve stimulation for refractory epilepsy: the current status

Vagus nerve stimulation (VNS) is a neurophysiologic treatment for patients with medically or surgically refractory epilepsy. Since the first human implant in 1989, more than 10,000 patients have been treated with VNS. The precise mechanism of action remains to be elucidated. Animal experiments with VNS were initially performed to demonstrate efficacy and safety preceding the clinical trials in human patients. Mechanism of action research involving animal experiments can provide essential clues. Animal experiments are often labor-intensive even in the hands of experienced researchers, however, and the results remain only a reflection of the complicated pathophysiologic systems of the human brain. Mechanism of action research in human patients treated with VNS is particularly challenging because of safety concerns, the large number of patients required, and the heterogeneous nature of various small patient series. This study provides an overview of the progress that has been made in the past 10 years through neurophysiologic, neuroanatomic, neurochemical, and cerebral blood flow studies in animals and patients treated with VNS. Further elucidation of the mechanism of action of VNS may increase its clinical efficacy. It may also provide inspiration for the development of new therapeutic modalities for refractory epilepsy.

Acute single photon emission computed tomographic study of vagus nerve stimulation in refractory epilepsy

PURPOSE

Left-sided vagus nerve stimulation (VNS) is an efficacious treatment for patients with refractory epilepsy. The precise mechanism of action remains to be elucidated. Only limited data on VNS-induced changes in regional cerebral blood flow (rCBF) are available. The aim of this study was to investigate rCBF changes during initial VNS with single-photon emission computed tomography (SPECT).

METHODS

In 12 patients (8 women, 4 men) with mean age of 32 years and mean duration of epilepsy of 19 years, VNS-induced rCBF changes were studied by means of a 99mTc-ethyl cysteinate dimer activation study with a single-day split-dose protocol before and immediately after initial stimulation. Images were acquired on a triple-head camera with fan-beam collimators and were reconstructed with scatter and attenuation correction. After coregistration to a standardized template, both a semiquantitative analysis using predefined volumes-of-interest (VOIs) as well as voxel-by-voxel analysis of the intrasubject activation were performed. During follow-up, efficacy of VNS in terms of seizure-frequency reduction was studied.

RESULTS

The semiquantitative analysis, with reference to the total counts in all VOIs, revealed a significant decrease of activity in the left thalamus immediately after the initial stimulation train. These results agreed with voxel-by-voxel analysis. In our study ipsilateral thalamic hypoperfusion was the most significant finding. Mean frequency of complex partial seizures was reduced from 30 per month before implantation to six per month after implantation.

CONCLUSIONS

VNS induces rCBF changes immediately after initial stimulation that can be studied with SPECT. VNS-induced changes in the thalamus may play an important role in suppression of seizures. However, no significant relation between the level of hypoperfusion and subsequent clinical efficacy was found.

Vagus nerve stimulation for epilepsy: a review

Vagus nerve stimulation is an empirically based method for treatment of epilepsy by repeated stimulation of the left vagus nerve through implanted electrodes. Despite studies in animals and man, which show changes in brain electrophysiology, metabolism and neurochemistry, the mode of action remains unknown. Clinical testing has presented methodological challenges, as it is difficult to assess under double blind conditions a treatment which requires surgery and produces a sensation every time the stimulator comes on. This has nevertheless been successfully addressed in parallel design, controlled trials comparing high and low stimulation schedules. These have been performed in adults with medically intractable partial seizures, and demonstrated efficacy, safety and good tolerability. Efficacy, both in the controlled trials and in numerous reports arising from the considerable post-marketing experience is modest. Some 30% of patients achieve a 50% seizure reduction after 3 months of treatment, but this proportion progressively increases to about 50% after 18 months. Side-effects comprise: discomfort in the face or neck when the stimulator is activated, coughing, breathlessness on exertion and hoarseness of voice. All are related to intensity of stimulation and rapidly habituate in most subjects. In those patients who respond, a stimulus level can therefore generally be found which is acceptable to the subject. No indication other than refractory partial seizures in adults has been the subject of controlled trials, but post-marketing experience and uncontrolled reports indicate comparable efficacy and safety in a wide range of epilepsies, partial and generalized, idiopathic, cryptogenic, or symptomatic, in patients of all ages.

Vagus nerve prolonged stimulation in cats: effects on epileptogenesis (amygdala electrical kindling): behavioral and electrographic changes

PURPOSE

To analyze the effect of prolonged (daily) electrical vagus nerve stimulation (VNS) on daily amygdaloid kindling (AK) in freely moving cats.

METHODS

Fifteen adult male cats were implanted in both temporal lobe amygdalae, both lateral geniculate bodies, and prefrontal cortices. A bipolar hook (5-mm separation) stainless steel electrode also was implanted in the unsectioned left vagus nerve. AK only was performed on five of the cats as a control. The remaining 10 cats were recorded under the following experimental conditions: VNS (1.2-2.0 mA, 0.5-ms pulses, 30 Hz) for 1 min along with AK (1-s train, 1-ms pulses, 60 Hz, 300-600 microA), followed by VNS alone for 1 min, four times between 11:00 a.m. and 2 p.m. At different times, VNS was arrested, and AK was continued until stage VI kindling was reached.

RESULTS

The behavioral changes evoked by VNS were as follows: left miosis, blinking, licking, abdominal contractions, swallowing, and eventually yawning, meowing, upward gaze, and short head movements. Compulsive eating also was present with a variable latency. Outstanding polygraphic changes consisted of augmentation of eye movements and visual evoked potentials while the animal was awake and quiet, with immobility and upward gaze. An increase of the pontogeniculooccipital (PGO) wave density in rapid eye movement (REM) sleep also was noticeable. AK was completed (to stage VI) in the control animals without a vagus nerve implantation in 23.4+/-3.7 trials. In animals with VNS, the AK was significantly delayed, remaining for a long time in the behavioral stages I-III and showing a reduction of afterdischarge duration and frequency. Stage VI was never reached despite 50 AK trials, except when the vagus nerve electrodes were accidentally broken or vagal stimulation was intentionally arrested. Under these circumstances, 24.4+/-8.16 AK trials alone were necessary to reach stage VI of kindling.

CONCLUSIONS

Our results indicate that left, electrical VNS interferes with AK epileptogenesis. This anticonvulsant effect could be related to the increase of REM sleep.

An institutional experience with cervical vagus nerve trunk stimulation for medically refractory epilepsy: rationale, technique, and outcome

OBJECTIVE

Intermittent stimulation of the left cervical vagus nerve trunk is emerging as a novel adjunct in the treatment of medically refractory seizures. We sought to evaluate theoretical and practical issues attendant to this concept. We review the anatomic and physiological background arguing for clinical application of vagus nerve stimulation, discuss salient aspects of patient selection and the nuances of surgical technique, and present our observations of and results from application of the method.

METHODS

Each of 18 patients with medically refractory epilepsy and at least six complex partial or secondarily generalized seizures per month underwent placement of a NeuroCybernetic Prosthesis pulse generator (Cyberonics, Webster, TX) in the chest, connected to helical platinum leads applied to the left cervical vagus nerve trunk. The patients were then randomized in a double-blinded fashion to receive either high (presumably therapeutic) or low (presumably less therapeutic) levels of vagus nerve stimulation. Reduction in seizure frequency, global assessments of quality of life, physiological measurements, and adverse events were recorded during a 3-month period. Patients in the low group were then crossed over to high-stimulation paradigms during a 15-month extension trial.

RESULTS

All operations were successful, uneventful, and without adverse postoperative sequelae. One patient was excluded from analysis because of inadequate seizure calendars. Of the seven patients initially assigned to high stimulation, the mean reduction in seizure frequency was 71% at 3 months and 81% at 18 months. Five (72%) of these patients had a greater than 75% reduction in seizure frequency, and one (14%) remained seizure-free after more than 1.5 years of follow-up. The mean reduction in seizure frequency among the low-stimulation group was only 6% at 3 months. No serious complications, device failures, or physiological perturbations occurred.

CONCLUSION

In our experience, vagus nerve stimulation has proven to be a safe, feasible, and potentially effective method of reducing seizures in select patient populations. However, the elements of strict definition for the application of the method require further study.

Regional induction of fos immunoreactivity in the brain by anticonvulsant stimulation of the vagus nerve

Electrical stimulation of the vagus nerve exerts an antiepileptic effect on human partial-onset epilepsy, but little is known about the brain structures that mediate this phenomenon. Fos is a nuclear protein that is expressed under conditions of high neuronal activity. We utilized fos immunolabeling techniques on Sprague-Dawley rat brains to identify regions that are activated by antiepileptic stimulation of the left vagus nerve. Vagus nerve stimulation (VNS) induced specific nuclear fos immunolabeling in several forebrain structures, including the posterior cortical amygdaloid nucleus, cingulate and retrosplenial cortex, ventromedial and arcuate hypothalamic nuclei. In the brainstem, there was specific immunolabeling in vagus nerve nuclei, in the A5 and locus ceruleus noradrenergic nuclei, and in the cochlear nucleus. No labeling of these structures occurred in sham-operated, unstimulated control animals. Intense labeling also occurred in habenular nucleus of thalamus after vagus nerve stimulation, whereas only mild staining occurred in unstimulated animals. Several of the brain structures activated by VNS are important for genesis or regulation of seizures in the forebrain. These structures may mediate the antiepileptic effect of VNS.