Vagal nerve stimulation: adjustments to reduce painful side effects

Adverse Events and Complications Associated With Vagal Nerve Stimulation: An Analysis of the Manufacturer And User Facility Device Experience Database

OBJECTIVE

Drug-resistant epilepsy (DRE) can have devastating consequences for patients and families. Vagal nerve stimulation (VNS) is used as a surgical adjunct for treating DRE not amenable to surgical resection. Although VNS is generally safe, it has its inherent complications. With the increasing number of implantations, adequate patient education with discussion of possible complications forms a critical aspect of informed consent and patient counseling. There is a lack of large-scale reviews of device malfunction, patient complaints, and surgically related complications available to date.

MATERIALS AND METHODS

Complications associated with VNS implants performed between 2011 and 2021 were identified through a search of the United States Food and Drug Administration Manufacturer And User Facility Device Experience (MAUDE) data base. We found three models on the data base, CYBERONICS, INC pulse gen Demipulse 103, AspireSR 106, and SenTiva 1000. The reports were classified into three main groups, "Device malfunction," "Patient complaints," and "Surgically managed complications."

RESULTS

A total of 5888 complications were reported over the ten-year period, of which 501 reports were inconclusive, 610 were unrelated, and 449 were deaths. In summary, there were 2272 reports for VNS 103, 1526 reports for VNS 106, and 530 reports for VNS 1000. Within VNS 103, 33% of reports were related to device malfunction, 33% to patient complaints, and 34% to surgically managed complications. For VNS 106, 35% were related to device malfunction, 24% to patient complaints, and 41% to surgically managed complications. Lastly, for VNS 1000, 8% were device malfunction, 45% patient complaints, and 47% surgically managed complications.

CONCLUSION

We present an analysis of the MAUDE data base for adverse events and complications related to VNS. It is hoped that this description of complications and literature review will help promote further improvement in its safety profile, patient education, and management of both patient and clinician expectations.

Cognitive Functions following Trigeminal Neuromodulation

Vast scientific effort in recent years have been focused on the search for effective and safe treatments for cognitive decline. In this regard, non-invasive neuromodulation has gained increasing attention for its reported effectiveness in promoting the recovery of multiple cognitive domains after central nervous system damage. In this short review, we discuss the available evidence supporting a possible cognitive effect of trigeminal nerve stimulation (TNS). In particular, we ask that, while TNS has been widely and successfully used in the treatment of various neuropsychiatric conditions, as far as research in the cognitive field is concerned, where does TNS stand? The trigeminal nerve is the largest cranial nerve, conveying the sensory information from the face to the trigeminal sensory nuclei, and from there to the thalamus and up to the somatosensory cortex. On these bases, a bottom-up mechanism has been proposed, positing that TNS-induced modulation of the brainstem noradrenergic system may affect the function of the brain networks involved in cognition. Nevertheless, despite the promising theories, to date, the use of TNS for cognitive empowering and/or cognitive decline treatment has several challenges ahead of it, mainly due to little uniformity of the stimulation protocols. However, as the field continues to grow, standardization of practice will allow for data comparisons across studies, leading to optimized protocols targeting specific brain circuitries, which may, in turn, influence cognition in a designed manner.

Additional Effect of High-output Current and/or High-duty Cycle in Vagus Nerve Stimulation for Adolescent/Adult Intractable Epilepsy

A vagus nerve stimulation (VNS) device delivers electrical pulses to the vagus nerve at a rhythm defined by the duty cycle. The standard therapeutic range is advocated for an output current of 1.5-2.25 mA and a duty cycle of 10%. As the optimal settings vary from patient to patient, some patients may benefit from additional seizure reduction when stimulated beyond the standard range. A total of 74 patients (15 children aged <12 years and 59 adolescents/adults) who underwent VNS implantation between 2011 and 2020 and who were followed up for at least 2 years were included in this retrospective study. Stimulation parameters exceeding 2.25 mA of output current, 25% of duty cycle, and 0.5625 (2.25 mA × 25%) of current × duty cycle were defined as high stimulation. The proportion achieved an additional seizure reduction of 20%, and the 50% seizure reduction rate at the last follow-up was compared between adolescents/adults and children. Approximately 40% of patients in adolescents/adults treated with high stimulation experienced an additional acute effect, resulting in a 50% or greater reduction in seizures in almost all patients. Moreover, in adolescents/adults, 22.2%-41.9% of the patients were treated with high stimulation, and the responder rate was 69.5%. Conversely, the responder rate in children was 26.7%, significantly worse than that in adolescents/adults, despite higher stimulation. VNS with high-stimulation settings is effective for adolescent and adult patients with intractable epilepsy. Even high stimulation may not be effective in extremely refractory pediatric epilepsy with a high seizure frequency.

Role of Vagus Nerve Stimulation in the Treatment of Chronic Pain

Vagus nerve stimulation (VNS) can modulate vagal activity and neuro-immune communication. Human and animal studies have provided growing evidence that VNS can produce analgesic effects in addition to alleviating refractory epilepsy and depression. The vagus nerve (VN) projects to many brain regions related to pain processing, which can be affected by VNS. In addition to neural regulation, the anti-inflammatory property of VNS may also contribute to its pain-inhibitory effects. To date, both invasive and noninvasive VNS devices have been developed, with noninvasive devices including transcutaneous stimulation of auricular VN or carotid VN that are undergoing many clinical trials for chronic pain treatment. This review aimed to provide an update on both preclinical and clinical studies of VNS in the management for chronic pain, including fibromyalgia, abdominal pain, and headaches. We further discuss potential underlying mechanisms for VNS to inhibit chronic pain.

Short bursts of transcutaneous auricular vagus nerve stimulation enhance evoked pupil dilation as a function of stimulation parameters

Transcutaneous auricular vagus nerve stimulation (taVNS) is a neurostimulatory technique hypothesised to enhance central noradrenaline. Currently, there is scarce evidence in support of a noradrenergic mechanism of taVNS and limited knowledge on its stimulation parameters (i.e., intensity and pulse width). Therefore, the present study aimed to test whether taVNS enhances pupil dilation, a noradrenergic biomarker, as a function of stimulation parameters. Forty-nine participants received sham (i.e., left ear earlobe) and taVNS (i.e., left ear cymba concha) stimulation in two separate sessions, in a counterbalanced order. We administered short bursts (5s) of seven stimulation settings varying as a function of pulse width and intensity and measured pupil size in parallel. Each stimulation setting was administered sixteen times in separate blocks. We expected short bursts of stimulation to elicit phasic noradrenergic activity as indexed by event-related pupil dilation and event-related temporal derivative. We hypothesised higher stimulation settings, quantified as the total charge per pulse (pulse width x intensity), to drive greater event-related pupil dilation and temporal derivative in the taVNS compared to sham condition. Specifically, we expected stimulation settings in the taVNS condition to be associated with a linear increase in event-related pupil dilation and temporal derivative. We found stimulation settings to linearly increase both pupil measures. In line with our hypothesis, the observed dose-dependent effect was stronger in the taVNS condition. We also found taVNS to elicit more intense and unpleasant sensations than sham stimulation. These results support the hypothesis of a noradrenergic mechanism of taVNS. However, future studies should disentangle whether stimulation elicited sensations mediate the effect of taVNS on evoked pupil dilation.

Effect of transcutaneous auricular vagus nerve stimulation on major depressive disorder with peripartum onset: A multicenter, open-label, controlled proof-of-concept clinical trial (DELOS-1)

BACKGROUND

Postpartum depression has a high prevalence in the United States (~13 %) and often goes undertreated/untreated. We conducted a multicenter, open-label, proof-of-concept trial to assess the Nēsos wearable, non-invasive, transcutaneous auricular vagus nerve stimulation (taVNS) system for the treatment of major depressive disorder with peripartum onset (PPD).

METHODS

Women (n = 25), ages 18 to 45, within 9 months postpartum, and diagnosed with PPD were enrolled at 3 sites. The study included 6 weeks open-label therapy and 2 weeks observation. Efficacy outcomes included change from baseline (CFB) in Hamilton Rating Scale for Depression (HAMD17) total scores, HAM-D17 response and remission, and patient and clinician global impression of change (PGIC, CGIC) scores. Analysis included descriptive statistics and mixed-effects models for repeated measures.

RESULTS

The most common AEs (≥5 %) were discomfort (n = 5), headache (n = 3), and dizziness (n = 2); all resolved without intervention. No serious AEs or deaths occurred. Baseline mean HAM-D17 score was 18.4. Week 6 least squares (LS) mean CFB in HAM-D17 score was -9.7; 74 % achieved response and 61 % achieved remission. At week 6, at least some improvement was reported by 21 of 22 (95 %) clinicians on CGIC and 22 of 23 (96 %) participants on PGIC.

LIMITATIONS

This was a single-arm, open-label study, and enrollment was limited to participants with mild-to-moderate peripartum depression.

CONCLUSION

Results from this proof-of-concept study suggest that the Nēsos taVNS system is well tolerated and may be an effective non-invasive, non-pharmacological treatment for major depressive disorder with peripartum onset. Further evaluation in larger sham-controlled studies is needed.

CLINICALTRIALS

govNCT03972995.

International Consensus Based Review and Recommendations for Minimum Reporting Standards in Research on Transcutaneous Vagus Nerve Stimulation (Version 2020)

Given its non-invasive nature, there is increasing interest in the use of transcutaneous vagus nerve stimulation (tVNS) across basic, translational and clinical research. Contemporaneously, tVNS can be achieved by stimulating either the auricular branch or the cervical bundle of the vagus nerve, referred to as transcutaneous auricular vagus nerve stimulation(VNS) and transcutaneous cervical VNS, respectively. In order to advance the field in a systematic manner, studies using these technologies need to adequately report sufficient methodological detail to enable comparison of results between studies, replication of studies, as well as enhancing study participant safety. We systematically reviewed the existing tVNS literature to evaluate current reporting practices. Based on this review, and consensus among participating authors, we propose a set of minimal reporting items to guide future tVNS studies. The suggested items address specific technical aspects of the device and stimulation parameters. We also cover general recommendations including inclusion and exclusion criteria for participants, outcome parameters and the detailed reporting of side effects. Furthermore, we review strategies used to identify the optimal stimulation parameters for a given research setting and summarize ongoing developments in animal research with potential implications for the application of tVNS in humans. Finally, we discuss the potential of tVNS in future research as well as the associated challenges across several disciplines in research and clinical practice.

A proposed guideline for vagus nerve stimulator handling in palliative care and after death

Vagus nerve stimulation (VNS) is often used for patients with drug-resistant epilepsy. Although this intervention may improve seizure control and mood, a number of factors must be considered when patients with VNS near end of life. We reviewed relevant literature to create a proposed guideline for management of patients with VNS in palliative care and after death. VNS has multiple possible side effects, including cough and swallowing difficulties. For patients with neurologic disease in palliative care, such adverse effects can severely affect quality of life and increase the risk for complications such as aspiration pneumonia. Patients with VNS should be screened regularly for such side effects, and VNS parameters should be adjusted if they are identified. If a patient requires urgent cardiac resuscitation involving external defibrillation, the VNS should be interrogated immediately afterwards to evaluate its function. During defibrillation, paddles should be placed perpendicular to the VNS, and as far as possible away from it. The VNS can be acutely turned off by taping the magnet to the patient's chest, thereby preventing any possible interference with restoration of a normal heart rhythm. After death, any staff involved with handling the body should be notified that a VNS is in place. The device must be removed prior to cremation, as it can explode with high heat. If the cause of death is unclear, a full postmortem examination should be undertaken, per sudden unexpected death in epilepsy guidelines. If there is concern about device malfunction, the device should be returned to the manufacturer for evaluation.

Therapeutic difficulties in recurrent, multidrug-resistant epilepsy and vagal nerve stimulation, with recent traumatic brain complications needing iterative neurosurgical interventions

Introduction: Focal epilepsy (possible with secondary generalization) may be a secondary complication to any brain damage (traumatic, vascular, infectious), and is characterized by abnormal excessive neuronal activity with motor, cognitive and psychosocial manifestations. About one third of the patients who suffer from epilepsy have a refractory, multidrug clinical form. Falls are one of the most common medical complications in neurologic patients, occurring during paroxysmal epileptic attacks, or due to sequelary gait limitations. Physical injuries are common, and about 47% patients with epilepsy report at least one injury in the past 12 months. Case presentation: We present a 36-years-old male patient with a medical history of right frontal congenital arteriovenous malformation, ruptured and operated at the age of eight, complicated with left spastic hemiplegia and refractory epilepsy, needing three antiepileptic drugs (AED) and vagus nerve-stimulation (VNS).This presentation was approved by THEBA Bioethics Committee (No.17464/14.06.2019). During a recent epileptic seizure he suffered a severe traumatic brain injury with coma (GCS 5), needing iterative neurosurgical interventions, intensive care supervision, and orotracheal intubation. CT cerebral scan revealed right hemispheric subdural hematoma, operated (on the 2nd May 2019). Rebleeding occurred seven days later, due to an extradural hematoma, and neurosurgical intervention was performed again. The patient was transferred in our neurorehabilitation clinic with left spastic hemiplegia (global motor score was 65/100, and functional independence measure (FIM) 24/91. Neuropsychological assessment revealed an obtunded level of consciousness, depression and dysmnesia for recent events, MMSE 9/30. During hospitalization the patient has continued his previous daily AED treatment with: levetiracetamum 2000 mg + clonazepamum 1mg + carbamazepinum retard 600 mg, associated with VNS. During hospitalization emerged three new short jacksonian seizures, who gave up spontaneously. The overall evolution was favorable with rehabilitation program and psychological support, with improvement of the global motor score, FIM (44/91), and partial restoration of walking ability, but still needing human help. Discussion: The pathophysiological mechanism of relapsed seizures has complex, multiple causes: imbalance of the local brain metabolism and /or a dysfunctional VNS procedure (a possible technical issue due to an impaired electronic device or a bioelectrical one, due to local fibrosis and increased impedance at the contact level between the electrode and the vagal nerve). Specialized technical control disclosed normal electric parameters provided by the electronic device. Other neuromodulatory devices and related technologies, such as deep brain stimulation (DBS) immediately demonstrate their effect control (motor correction) of Parkinson's or dystonic movements. Unfortunately VNS has not the possibility of immediate clinical feed-back control. The AED schedule was modified, by increasing clonazepamum to 2 mg daily. Video-EEG monitoring was recommended. The quod ad vitam prognosis might be unfavorable, because seizures can relapse anytime and evolution is uncontrolled. Furthermore, new brain injures may exacerbate the severity of the epilepsy, any new seizure may worsen the neurologic evolution. The quod ad functionem rehabilitation outcome might be precarious. The family support is essential in the therapeutic efforts. This clinical case underlines the necessity to implement a fall prevention program in patients with epilepsy, the importance of therapeutically tailoring AED for different pathophysiological stages of the disease, and emphasizes the limits of the modern techniques for seizures control. A multi-/ interdisciplinary team management of a such complex clinical case is mandatory. Key words: cerebral arteriovenous malformation, multidrug-resistant (refractory) epilepsy, vagal nerve electrostimulation (VNS), falls, traumatic brain injury,

A Review of Neurostimulation for Epilepsy in Pediatrics

Neurostimulation for epilepsy refers to the application of electricity to affect the central nervous system, with the goal of reducing seizure frequency and severity. We review the available evidence for the use of neurostimulation to treat pediatric epilepsy, including vagus nerve stimulation (VNS), responsive neurostimulation (RNS), deep brain stimulation (DBS), chronic subthreshold cortical stimulation (CSCS), transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS). We consider possible mechanisms of action and safety concerns, and we propose a methodology for selecting between available options. In general, we find neurostimulation is safe and effective, although any high quality evidence applying neurostimulation to pediatrics is lacking. Further research is needed to understand neuromodulatory systems, and to identify biomarkers of response in order to establish optimal stimulation paradigms.

Carbon fiber on polyimide ultra-microelectrodes

OBJECTIVE

Most preparations for making neural recordings degrade over time and eventually fail due to insertion trauma and reactive tissue response. The magnitudes of these responses are thought to be related to the electrode size (specifically, the cross-sectional area), the relative stiffness of the electrode, and the degree of tissue tolerance for the material. Flexible carbon fiber ultra-microelectrodes have a much smaller cross-section than traditional electrodes and low tissue reactivity, and thus may enable improved longevity of neural recordings in the central and peripheral nervous systems. Only two carbon fiber array designs have been described previously, each with limited channel densities due to limitations of the fabrication processes or interconnect strategies. Here, we describe a method for assembling carbon fiber electrodes on a flexible polyimide substrate that is expected to facilitate the construction of high-density recording and stimulating arrays.

APPROACH

Individual carbon fibers were aligned using an alignment tool that was 3D-printed with sub-micron resolution using direct laser writing. Indium deposition on the carbon fibers, followed by low-temperature microsoldering, provided a robust and reliable method of electrical connection to the polyimide interconnect.

MAIN RESULTS

Spontaneous multiunit activity and stimulation-evoked compound responses with SNR  >10 and  >120, respectively, were recorded from a small (125 µm) peripheral nerve. We also improved the typically poor charge injection capacity of small diameter carbon fibers by electrodepositing 100 nm-thick iridium oxide films, making the carbon fiber arrays usable for electrical stimulation as well as recording.

SIGNIFICANCE

Our innovations in fabrication technique pave the way for further miniaturization of carbon fiber ultra-microelectrode arrays. We believe these advances to be key steps to enable a shift from labor intensive, manual assembly to a more automated manufacturing process.

Current Directions in the Auricular Vagus Nerve Stimulation I - A Physiological Perspective

Electrical stimulation of the auricular vagus nerve (aVNS) is an emerging technology in the field of bioelectronic medicine with applications in therapy. Modulation of the afferent vagus nerve affects a large number of physiological processes and bodily states associated with information transfer between the brain and body. These include disease mitigating effects and sustainable therapeutic applications ranging from chronic pain diseases, neurodegenerative and metabolic ailments to inflammatory and cardiovascular diseases. Given the current evidence from experimental research in animal and clinical studies we discuss basic aVNS mechanisms and their potential clinical effects. Collectively, we provide a focused review on the physiological role of the vagus nerve and formulate a biology-driven rationale for aVNS. For the first time, two international workshops on aVNS have been held in Warsaw and Vienna in 2017 within the framework of EU COST Action "European network for innovative uses of EMFs in biomedical applications (BM1309)." Both workshops focused critically on the driving physiological mechanisms of aVNS, its experimental and clinical studies in animals and humans, in silico aVNS studies, technological advancements, and regulatory barriers. The results of the workshops are covered in two reviews, covering physiological and engineering aspects. The present review summarizes on physiological aspects - a discussion of engineering aspects is provided by our accompanying article (Kaniusas et al., 2019). Both reviews build a reasonable bridge from the rationale of aVNS as a therapeutic tool to current research lines, all of them being highly relevant for the promising aVNS technology to reach the patient.

Current Directions in the Auricular Vagus Nerve Stimulation II - An Engineering Perspective

Electrical stimulation of the auricular vagus nerve (aVNS) is an emerging electroceutical technology in the field of bioelectronic medicine with applications in therapy. Artificial modulation of the afferent vagus nerve - a powerful entrance to the brain - affects a large number of physiological processes implicating interactions between the brain and body. Engineering aspects of aVNS determine its efficiency in application. The relevant safety and regulatory issues need to be appropriately addressed. In particular, in silico modeling acts as a tool for aVNS optimization. The evolution of personalized electroceuticals using novel architectures of the closed-loop aVNS paradigms with biofeedback can be expected to optimally meet therapy needs. For the first time, two international workshops on aVNS have been held in Warsaw and Vienna in 2017 within the scope of EU COST Action "European network for innovative uses of EMFs in biomedical applications (BM1309)." Both workshops focused critically on the driving physiological mechanisms of aVNS, its experimental and clinical studies in animals and humans, in silico aVNS studies, technological advancements, and regulatory barriers. The results of the workshops are covered in two reviews, covering physiological and engineering aspects. The present review summarizes on engineering aspects - a discussion of physiological aspects is provided by our accompanying article (Kaniusas et al., 2019). Both reviews build a reasonable bridge from the rationale of aVNS as a therapeutic tool to current research lines, all of them being highly relevant for the promising aVNS technology to reach the patient.

Vagus nerve stimulation (VNS) therapy update

Epilepsy affects millions of people worldwide. Approximately one-third have pharmacoresistant epilepsy, and of these, the majority are not candidates for epilepsy surgery. Vagus nerve stimulation (VNS) therapy has been an option to treat pharmacoresistant seizures for 30 years. In this update, we will review the clinical data that support the device's efficacy in children, adolescents, and adults. We will also review its side-effect profile, quality of life and cost benefits, and the impact the device has on sudden unexpected death in epilepsy (SUDEP). We will then discuss candidate selection and provide guidance on dosing and future models. Vagus nerve stimulation therapy is an effective treatment for many seizure types and epilepsy syndromes with a predictable and benign side-effect profile that supports its role as the most commonly prescribed device to treat pharmacoresistant epilepsy. "This article is part of the Supplement issue Neurostimulation for Epilepsy."

The interaction of pulse width and current intensity on the extent of cortical plasticity evoked by vagus nerve stimulation

BACKGROUND

Repeatedly pairing a tone with a brief burst of vagus nerve stimulation (VNS) results in a reorganization of primary auditory cortex (A1). The plasticity-enhancing and memory-enhancing effects of VNS follow an inverted-U response to stimulation intensity, in which moderate intensity currents yield greater effects than low or high intensity currents. It is not known how other stimulation parameters effect the plasticity-enhancing effects of VNS.

OBJECTIVE

We sought to investigate the effect of pulse-width and intensity on VNS efficacy. Here, we used the extent of plasticity induced by VNS-tone pairing to assess VNS efficacy.

METHODS

Rats were exposed to a 9 kHz tone paired to VNS with varying current intensities and pulse widths. Cortical plasticity was measured as changes in the percent of area of primary auditory cortex responding to a range of sounds in VNS-treated rats relative to naïve rats.

RESULTS

We find that a combination of low current intensity (200 μA) and short pulse duration (100 μs) is insufficient to drive cortical plasticity. Increasing the pulse duration to 500 μs results in a reorganization of receptive fields in A1 auditory cortex. The extent of plasticity engaged under these conditions is less than that driven by conditions previously reported to drive robust plasticity (800 μA with 100 μs wide pulses).

CONCLUSION

These results suggest that the plasticity-enhancing and memory-enhancing effects of VNS follow an inverted-U response of stimulation current that is influenced by pulse width. Furthermore, shorter pulse widths may offer a clinical advantage when determining optimal stimulation current. These findings may facilitate determination of optimal VNS parameters for clinical application.

The role of vagus nerve stimulation in refractory epilepsy

ABSTRACT Vagus nerve stimulation is an adjunctive therapy used to treat patients with refractory epilepsy who are not candidates for resective surgery or had poor results after surgical procedures. Its mechanism of action is not yet fully comprehended but it possibly involves modulation of the locus coeruleus, thalamus and limbic circuit through noradrenergic and serotonergic projections. There is sufficient evidence to support its use in patients with focal epilepsy and other seizure types. However, it should be recognized that improvement is not immediate and increases over time. The majority of adverse events is stimulation-related, temporary and decreases after adjustment of settings. Future perspectives to improve efficacy and reduce side effects, such as different approaches to increase battery life, transcutaneous stimulation and identification of prognostic factors, should be further investigated.

Late-onset jaw and teeth pain mimicking trigeminal neuralgia associated with chronic vagal nerve stimulation: case series and review of the literature

BACKGROUND

Vagal nerve stimulation (VNS) for refractory epilepsy is well established. Trigeminal neuralgia itself is a common disease in adults, and thus, late-onset pain in the trigeminal region under VNS, which is extremely rare, may not be recognized as caused by VNS.

CASE PRESENTATION

Two patients with drug-resistant symptomatic epilepsy treated with chronic VNS experienced stimulation-related pain in the lower and upper jaw and teeth on the side of stimulation. No evidence of local spread of the stimulation current was present. The pain started with a delay of years after device implantation and weeks after the last increase in the pacing parameters. At the time of onset, the pain was not recognized as VNS-related, leading to extensive examinations. The trigeminal neuralgia-like pain resolved after adjustment of the stimulation current intensity. In one of the patients, the pain disappeared within one to two days following every epileptic seizure. To our knowledge, this is the first case report of late-onset trigeminal pain under VNS revealing a direct link between epileptogenic and pain processes.

CONCLUSION

A painless interval between the last change of the pacing parameters and trigeminal pain can lead to the erroneous interpretation that this is a typical trigeminal neuralgia. The lack of its recognition as a side effect of VNS can lead to unnecessary examinations and delayed adjustment of stimulation parameters. In patients with signs of late-onset trigeminal pain under VNS with normal electrode impedance and no evidence of local current spread, the replacement of the VNS lead does not seem to be beneficial. A review of the literature on VNS side effects including pain and device malfunctions was undertaken.

Parametric characterization of neural activity in the locus coeruleus in response to vagus nerve stimulation

Vagus nerve stimulation (VNS) has emerged as a therapy to treat a wide range of neurological disorders, including epilepsy, depression, stroke, and tinnitus. Activation of neurons in the locus coeruleus (LC) is believed to mediate many of the effects of VNS in the central nervous system. Despite the importance of the LC, there is a dearth of direct evidence characterizing neural activity in response to VNS. A detailed understanding of the brain activity evoked by VNS across a range of stimulation parameters may guide selection of stimulation regimens for therapeutic use. In this study, we recorded neural activity in the LC and the mesencephalic trigeminal nucleus (Me5) in response to VNS over a broad range of current amplitudes, pulse frequencies, train durations, inter-train intervals, and pulse widths. Brief 0.5s trains of VNS drive rapid, phasic firing of LC neurons at 0.1mA. Higher current intensities and longer pulse widths drive greater increases in LC firing rate. Varying the pulse frequency substantially affects the timing, but not the total amount, of phasic LC activity. VNS drives pulse-locked neural activity in the Me5 at current levels above 1.2mA. These results provide insight into VNS-evoked phasic neural activity in multiple neural structures and may be useful in guiding the selection of VNS parameters to enhance clinical efficacy.

Vagus Nerve Stimulation Applied with a Rapid Cycle Has More Profound Influence on Hippocampal Electrophysiology Than a Standard Cycle

Although vagus nerve stimulation (VNS) is widely used, therapeutic mechanisms and optimal stimulation parameters remain elusive. In the present study, we investigated the effect of VNS on hippocampal field activity and compared the efficiency of different VNS paradigms. Hippocampal electroencephalography (EEG) and perforant path dentate field-evoked potentials were acquired before and during VNS in freely moving rats, using 2 VNS duty cycles: a rapid cycle (7 s on, 18 s off) and standard cycle (30 s on, 300 s off) and various output currents. VNS modulated the evoked potentials, reduced total power of the hippocampal EEG, and slowed the theta rhythm. In the hippocampal EEG, theta (4-8 Hz) and high gamma (75-150 Hz) activity displayed strong phase amplitude coupling that was reduced by VNS. Rapid-cycle VNS had a greater effect than standard-cycle VNS on all outcome measures. Using rapid cycle VNS, a maximal effect on EEG parameters was found at 300 μA, beyond which effects saturated. The findings suggest that rapid-cycle VNS produces a more robust outcome than standard cycle VNS and support already existing preclinical evidence that relatively low output currents are sufficient to produce changes in brain physiology and thus likely also therapeutic efficacy.

The antidepressant-like effect of vagus nerve stimulation is mediated through the locus coeruleus

It has been shown that vagus nerve stimulation (VNS) has an antidepressant-like effect in the forced swim test. The mechanism of action underlying this effect is incompletely understood, but there is evidence suggesting that the locus coeruleus (LC) may play an important role. In this study, noradrenergic LC neurons were selectively lesioned to test their involvement in the antidepressant-like effect of VNS in the forced swim test. Forced swim test behavior was assessed in rats that were subjected to VNS or sham treatment. In half of the VNS-treated animals, the noradrenergic neurons from the LC were lesioned using the selective neurotoxin DSP-4 [N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride], yielding three experimental arms: sham, VNS and DSP-4-VNS (n = 8 per group). Furthermore, the open field test was performed to evaluate locomotor activity. A dopamine-β-hydroxylase immunostaining was performed to confirm lesioning of noradrenergic LC neurons. VNS significantly reduced the percentage of immobility time in the forced swim test compared to sham treatment (median: 56%, interquartile range: 41% vs. median: 75%, interquartile range: 12%). This antidepressant-like effect of VNS could not be demonstrated in the DSP-4-VNS group (median: 79%, interquartile range: 33%). Locomotor activity in the open field test was not different between the three treatment arms. The absence of hippocampal dopamine-β-hydroxylase immunostaining in the DSP-4-treated rats confirmed the lesioning of noradrenergic neurons originating from the brainstem LC. The results of this study demonstrate that the noradrenergic neurons from the LC play an important role in the antidepressant-like effect of VNS.

Future of epilepsy treatment: integration of devices

ABSTRACT 

The use of devices in the treatment of epilepsy is an emerging therapy for those patients whose seizures are not controlled by medications. This article will discuss current treatment options with devices for vagus nerve stimulation, deep brain stimulation and responsive neurostimulation. Emerging therapies in noninvasive neurostimulation such as with trigeminal nerve stimulation, transcranial magnetic stimulation and transcranial direct current stimulation may prove to be promising solutions. Finally, new and enhanced techniques of drug delivery are discussed as well as other devices with potential use in the study and treatment of epilepsy.

Should I offer vagus nerve stimulation as part of my neurology practice?

Vagus nerve stimulation (VNS) is a safe and effective adjunctive therapy approved for patients with partial-onset seizures. A pulse generator, which is implanted in the chest wall, delivers programmed electrical pulses through an electrode that is attached to the left vagus nerve. VNS plays an important role in the treatment of patients with drug-resistant epilepsy. It is currently offered in academic as well as private practice settings. After a comprehensive workup is performed, VNS should be offered to patients with drug-resistant epilepsy who are not candidates for surgery.

Interventional psychiatry: how should psychiatric educators incorporate neuromodulation into training?

OBJECTIVE

Interventional psychiatry is an emerging subspecialty that uses a variety of procedural neuromodulation techniques in the context of an electrocircuit-based view of mental dysfunction as proximal causes for psychiatric diseases.

METHODS

The authors propose the development of an interventional psychiatry-training paradigm analogous to those found in cardiology and neurology.

RESULTS

The proposed comprehensive training in interventional psychiatry would include didactics in the theory, proposed mechanisms, and delivery of invasive and noninvasive brain stimulation.

CONCLUSIONS

The development and refinement of this subspecialty would facilitate safe, effective growth in the field of brain stimulation by certified and credentialed practitioners within the field of psychiatry while also potentially improving the efficacy of current treatments.

High-resolution measurement of electrically-evoked vagus nerve activity in the anesthetized dog

OBJECTIVE

Not fully understanding the type of axons activated during vagus nerve stimulation (VNS) is one of several factors that limit the clinical efficacy of VNS therapies. The main goal of this study was to characterize the electrical recruitment of both myelinated and unmyelinated fibers within the cervical vagus nerve.

APPROACH

In anesthetized dogs, recording nerve cuff electrodes were implanted on the vagus nerve following surgical excision of the epineurium. Both the vagal electroneurogram (ENG) and laryngeal muscle activity were recorded in response to stimulation of the right vagus nerve.

MAIN RESULTS

Desheathing the nerve significantly increased the signal-to-noise ratio of the ENG by 1.2 to 9.9 dB, depending on the nerve fiber type. Repeated VNS following nerve transection or neuromuscular block (1) enabled the characterization of A-fibers, two sub-types of B-fibers, and unmyelinated C-fibers, (2) confirmed the absence of stimulation-evoked reflex compound nerve action potentials in both the ipsilateral and contralateral vagus nerves, and (3) provided evidence of stimulus spillover into muscle tissue surrounding the stimulating electrode.

SIGNIFICANCE

Given the anatomical similarities between the canine and human vagus nerves, the results of this study provide a template for better understanding the nerve fiber recruitment patterns associated with VNS therapies.

Optimization of vagus nerve stimulation parameters using the firing activity of serotonin neurons in the rat dorsal raphe

Vagus nerve stimulation (VNS) is a recently approved adjunctive intervention for treatment-resistant depression. This therapy enhances the firing rate of rat norepinephrine neurons after 1 h and that of serotonin (5-HT) neurons only after 14 days. Various stimulation parameters were thus tested on their capacity to enhance 5-HT neuronal firing because of the delayed action of VNS on the 5-HT system and its important role in the antidepressant response. Rats were implanted with a stimulator and treated for 14 days, each group of rats having only one stimulation parameter modified from the standard ones (0.25 mA, 20 Hz, 500 micros, 30 s ON/5 min OFF). Electrophysiological recordings showed that the usual parameters utilized in depressed patients, with the exception of current intensity, produced an optimal activation of 5-HT neurons. Excessive enhancement of the charge delivered to the nerve can lead to a loss of VNS effect on 5-HT neuronal firing.

Brain stimulation for epilepsy

Brain stimulation has been receiving increasing attention as an alternative therapy for epilepsy that cannot be treated by either antiepileptic medication or surgical resection of the epileptogenic focus. The stimulation methods include transcranial magnetic stimulation (TMS) or electrical stimulation by implanted devices of the vagus nerve (VNS), deep brain structures (DBS) (thalamic or hippocampal), cerebellar or cortical areas. TMS is the simplest and least invasive approach. However, the most common epileptogenic areas (mesial temporal structures) probably lie too deep beneath the surface of the skull for effective TMS. The efficacy of VNS in reducing the frequency or severity of seizures is quite variable and depends on many factors which are currently investigated. VNS is well-tolerated and approved in many countries. DBS is much more invasive than either TMS or VNS. Currently, a number of targets for DBS are investigated including caudate, centromedian or anterior thalamic nuclei, and subthalamic nucleus. Direct stimulation of the epileptic cortical focus is another approach to the neuromodulation in epilepsy. Finally, another line of research investigates the usefulness of implantable seizure detection devices. The current chapter presents the most important evidence on the above methods. Furthermore, other important issues are reviewed such as the selection criteria of patients for brain stimulation and the potential role of brain stimulation in the treatment of depression in epileptic patients.

Vagus nerve stimulation therapy in depression and epilepsy: therapeutic parameter settings

Vagus nerve stimulation (VNS) therapy is an effective adjunctive treatment for chronic or recurrent treatment-resistant depression in adults, and for pharmacoresistant epilepsy in adults and adolescents. VNS therapy is administered through an implanted pulse generator that delivers programmed electrical pulses through an implanted lead to the left vagus nerve. Programmable pulse parameters include output current, frequency, pulse width, and ON/OFF times. Within a range of typical values, individual patients respond best to different combinations of parameter settings. The physician must identify the optimum settings for each patient while balancing the goals of maximizing efficacy, minimizing side effects, and preserving battery life. Output current is gradually increased from 0.25 mA to the maximum tolerable level (maximum, 3.5 mA); typical therapeutic settings range from 1.0 to 1.5 mA. Greater output current is associated with increased side effects, including voice alteration, cough, a feeling of throat tightening, and dyspnea. Frequency is typically programmed at 20 Hz in depression and 30 Hz in epilepsy. Pulse width is typically 250 or 500 micros. The recommended initial ON time is 30 s, followed by 5 min OFF; OFF time > ON time is recommended. As with pharmacotherapy, VNS therapy must be adjusted in a gradual, systematic fashion to individualize therapy for each patient.

Vagus nerve stimulation use and effect in epilepsy: what have we learned?

Vagus nerve stimulation (VNS) for epilepsy has been available in the United States for 8 years. Pivotal randomized, blinded clinical trials leading to FDA approval in patients age 12 and older with refractory partial seizures have not been performed for other age groups or epilepsy syndromes. This practical review takes stock of the current information about VNS use and efficacy in various types of epilepsy. We review the evidence for commonly used stimulation parameters, end of battery life, predictors of response including duration of epilepsy, seizure type/epilepsy syndrome, bihemispheric seizures, age at implant, and prior cranial surgery. We review adverse events and VNS effects on respiratory patterns, cardiac function, and mood and behavior. With the recent U.S. approval of VNS for treatment-resistant depression, we anticipate that lessons learned from treating patients with epilepsy will be useful to physicians using VNS to treat patients with depression and possibly other conditions.

Vagus Nerve Stimulation for Refractory Epilepsy: A Transatlantic Experience

Vagus nerve stimulation (VNS) is an alternative treatment for medically or surgically refractory epilepsy. The long-term efficacy and safety of VNS were evaluated in a large patient series at Ghent University Hospital and Dartmouth-Hitchcock Medical Center. Between March 1995 and February 2003, seizure frequency and type as well as prescribed antiepileptic drugs and side effects were prospectively assessed in 131 patients treated with VNS in either center. Patients with a minimum follow-up duration of 6 months were included in the efficacy and safety analysis. A total of 118 of 131 implanted patients had a minimum postimplantation follow-up period of 6 months (mean, 33 months). The mean age of these patients was 32 years and the mean duration of refractory epilepsy was 22 years. The mean reduction in monthly seizure frequency in all patients was 55% (range, 0-100; SD = 31.6). Seven percent of patients were free of seizures with impaired consciousness, 50% of patients had a seizure frequency reduction of more than 50%, and 21% of patients were nonresponders. Fifteen patients reported stimulation-related side effects such as hoarseness or gagging. In a large patient series from two geographically distinct epilepsy centers located in two different continents, VNS proved to be efficacious and safe during long-term follow-up.

Brain stimulation for epilepsy

Neural stimulation is a promising new technology for the treatment of medically-intractable seizures. Vagus-nerve stimulation (VNS) is licensed in several countries as an adjunctive therapy. VNS is as effective as antiepileptic drug therapy, and serious complications are rare. Transcranial magnetic stimulation is simple, non-invasive, and widely used in neurophysiology. Therapeutic results in a few studies are equivocal at best. Deep brain stimulation, although experimental, has been applied to the cerebellum, caudate nucleus, centromedian thalamus, anterior thalamus, subthalamus, hippocampus, and neocortical seizure foci. Preliminary results are encouraging, but not conclusive. Electrode implantation in the brain for indications other than seizures has been associated with a 5% risk for intracranial haemorrhage and 5% for infection. A controlled study of anterior thalamic stimulation in patients with intractable partial and secondarily generalised seizures has been started. Future investigations are likely to study extrathalamic sites of stimulation, and effects of stimulation contingent upon detection of or prediction of EEG patterns of epileptiform activity.

Vagus nerve stimulation in awake rats reduces formalin-induced nociceptive behaviour and fos-immunoreactivity in trigeminal nucleus caudalis

Besides its well-established efficacy in epilepsy, vagus nerve stimulation (VNS) may be of potential interest in pain treatment. It has, however, not yet been assessed in animal pain models with the devices and stimulation protocols used in humans. We have therefore studied in awake rats the effects of left cervical VNS on trigeminal nociception using an implantable electrode and stimulator (NCP-Cyberonics). VNS was applied for 24h at 2 mA intensity, 20 Hz frequency, 0.5 ms pulse width and a duty cycle of 20s ON/18s OFF. As a nociceptive stimulus, we injected formalin into the left mystacial vibrissae, assessed behaviour for 45 min and sacrificed the animals 45 min later. Fos-immunoreactive (Fos-Ir) neurons were counted in laminae I-II of trigeminal nucleus caudalis (TNC) on both sides. We used three groups of control animals: VNS without formalin, formalin without VNS and sham VNS (implanted without stimulation or formalin). Whereas sham VNS had no significant effect, VNS alone increased Fos expression in ipsilateral TNC in addition to the expected increase in nucleus tractus solitarius. It also significantly attenuated the increase of Fos-Ir neurons observed in ipsilateral TNC laminae I-II after formalin injection. If the proper VNS effect on Fos-expression was subtracted, the reduction of formalin-induced nociceptor activation was 55%. VNS also reduced nociceptive behaviour on average by 96.1% during the early phase (0-6 min) and by 60.7% during the late phase (6-45 min) after the formalin injection. These results suggest that VNS applied with a device used in human therapy may have in awake rats a significant antinociceptive effect in a model of trigeminal pain.

Sensory neurone responses to mucosal noxae in the upper gut: relevance to mucosal integrity and gastrointestinal pain

The digestive tract is supplied by extrinsic and intrinsic sensory neurones that, together with endocrine and immune cells, form a surveillance network that is essential to gut function. This article focuses on the responses of extrinsic afferent neurones to chemical insults of the gastrointestinal mucosa and their pathophysiological relevance to mucosal integrity and abdominal pain. Within the gastroduodenal region, spinal afferents subserve an emergency function because, in case of alarm by influxing acid, they stimulate mechanisms of mucosal protection via an efferent-like release of transmitters. Other sensory neurones signal chemical noxae to the brain, a task that is not confined to spinal afferents because vagal afferents communicate gastric acid and peripheral immune challenges to the brainstem and in this way elicit autonomic, endocrine, affective and behavioural reactions. Emerging evidence indicates that hypersensitivity of extrinsic afferent pathways to mechanical and chemical stimuli makes an important contribution to the abdominal hyperalgesia seen in functional dyspepsia and irritable bowel syndrome. Sensitization may be brought about by inflammatory processes that lead to up-regulation and functional alterations of receptors and ion channels on sensory neurones. Such sensory neurone-specific molecules, which include vanilloid (capsaicin) receptors, may represent important targets for novel drugs to treat abdominal pain.

Vagus nerve stimulation: where are we?

Now nearly 5 years post-approval, vagus nerve stimulation has emerged as a major non-pharmacological treatment for epilepsy. The place of vagus nerve stimulation among antiepileptic drugs and other surgical therapies is still evolving. This review evaluates the role of vagus nerve stimulation in light of recently published research of its mechanism(s) of action, long-term efficacy, safety and tolerability, and application to other disorders besides epilepsy.