The effects of vagus nerve stimulation on tryptophan metabolites in children with intractable epilepsy

The Influence of Kynurenine Metabolites on Neurodegenerative Pathologies

As the kynurenine pathway's links to inflammation, the immune system, and neurological disorders became more apparent, it attracted more and more attention. It is the main pathway through which the liver breaks down Tryptophan and the initial step in the creation of nicotinamide adenine dinucleotide (NAD+) in mammals. Immune system activation and the buildup of potentially neurotoxic substances can result from the dysregulation or overactivation of this pathway. Therefore, it is not shocking that kynurenines have been linked to neurological conditions (Depression, Parkinson's, Alzheimer's, Huntington's Disease, Schizophrenia, and cognitive deficits) in relation to inflammation. Nevertheless, preclinical research has demonstrated that kynurenines are essential components of the behavioral analogs of depression and schizophrenia-like cognitive deficits in addition to mediators associated with neurological pathologies due to their neuromodulatory qualities. Neurodegenerative diseases have been extensively associated with neuroactive metabolites of the kynurenine pathway (KP) of tryptophan breakdown. In addition to being a necessary amino acid for protein synthesis, Tryptophan is also transformed into the important neurotransmitters tryptamine and serotonin in higher eukaryotes. In this article, a summary of the KP, its function in neurodegeneration, and the approaches being used currently to target the route therapeutically are discussed.

Tryptophan metabolism: Mechanism-oriented therapy for neurological and psychiatric disorders

Neurological and psychiatric disorders are a category of chronic diseases that are widespread and pose serious mental and physical health problems for patients. The substrates, products, and enzymes of Tryptophan metabolism all contribute to the development of neurological and psychiatric disorders. This paper deals with three metabolic pathways of tryptophan that produce a series of metabolites called tryptophan Catabolics (TRYCATs). These metabolites are involved in pathological processes such as excitotoxicity, neuroinflammation, oxidative stress, and mitochondrial damage and are closely associated with neurological and psychiatric disorders such as Alzheimer’s disease and depression. Here, we review the elements that affect how tryptophan metabolism is regulated, including inflammation and stress, exercise, vitamins, minerals, diet and gut microbes, glucocorticoids, and aging, as well as the downstream regulatory effects of tryptophan metabolism, including the regulation of glutamate (Glu), immunity, G-protein coupled receptor 35 (Gpr35), nicotinic acetylcholine receptor (nAChR), aryl hydrocarbon receptor (AhR), and dopamine (DA). In order to advance the general understanding of tryptophan metabolism in neurological and psychiatric disorders, this paper also summarizes the current situation and effective drugs of tryptophan metabolism in the treatment of neurological and psychiatric disorders and considers its future research prospects.

Neuromodulation in drug-resistant epilepsy: A review of current knowledge

Nearly 1% of the global population suffers from epilepsy. Drug-resistant epilepsy (DRE) affects one-third of epileptic patients who are unable to treat their condition with existing drugs. For the treatment of DRE, neuromodulation offers a lot of potential. The background, mechanism, indication, application, efficacy, and safety of each technique are briefly described in this narrative review, with an emphasis on three approved neuromodulation therapies: vagus nerve stimulation (VNS), deep brain stimulation of the anterior nucleus of the thalamus (ANT-DBS), and closed-loop responsive neurostimulation (RNS). Neuromodulatory approaches involving direct or induced electrical currents have been developed to lessen seizure frequency and duration in patients with DRE since the notion of electrical stimulation as a therapy for neurologic diseases originated in the early nineteenth century. Although few people have attained total seizure independence for more than 12 months using these treatments, more than half have benefitted from a 50% drop in seizure frequency over time. Although promising outcomes in adults and children with DRE have been achieved, challenges such as heterogeneity among epilepsy types and etiologies, optimization of stimulation parameters, a lack of biomarkers to predict response to neuromodulation therapies, high-level evidence to aid decision-making, and direct comparisons between neuromodulatory approaches remain. To solve these existing gaps, authorize new kinds of neuromodulation, and develop personalized closed-loop treatments, further research is needed. Finally, both invasive and non-invasive neuromodulation seems to be safe. Implantation-related adverse events for invasive stimulation primarily include infection and pain at the implant site. Intracranial hemorrhage is a frequent adverse event for DBS and RNS. Other stimulation-specific side-effects are mild with non-invasive stimulation.

Latest Views on the Mechanisms of Action of Surgically Implanted Cervical Vagal Nerve Stimulation in Epilepsy

BACKGROUND

Vagus nerve stimulation (VNS) is approved as an adjunctive treatment for drug-resistant epilepsy. Although there is a substantial amount of literature aiming at unraveling the mechanisms of action of VNS in epilepsy, it is still unclear how the cascade of events triggered by VNS leads to its antiepileptic effect.

OBJECTIVE

In this review, we integrated available peer-reviewed data on the effects of VNS in clinical and experimental research to identify those that are putatively responsible for its therapeutic effect. The topic of transcutaneous VNS will not be covered owing to the current lack of data supporting the differences and commonalities of its mechanisms of action in relation to invasive VNS.

SUMMARY OF THE MAIN FINDINGS

There is compelling evidence that the effect is obtained through the stimulation of large-diameter afferent myelinated fibers that project to the solitary tract nucleus, then to the parabrachial nucleus, which in turn alters the activity of the limbic system, thalamus, and cortex. VNS-induced catecholamine release from the locus coeruleus in the brainstem plays a pivotal role. Functional imaging studies tend to point toward a common vagal network that comes into play, made up of the amygdalo-hippocampal regions, left thalamus, and insular cortex.

CONCLUSIONS

Even though some crucial pieces are missing, neurochemical, molecular, cellular, and electrophysiological changes occur within the vagal afferent network at three main levels (the brainstem, the limbic system [amygdala and hippocampus], and the cortex). At this final level, VNS notably alters functional connectivity, which is known to be abnormally high within the epileptic zone and was shown to be significantly decreased by VNS in responders. The effect of crucial VNS parameters such as frequency or current amplitude on functional connectivity metrics is of utmost importance and requires further investigation.

Vagus nerve stimulation for focal seizures

BACKGROUND

This is an updated version of the Cochrane Review published in 2015. Epilepsy is a chronic neurological disorder, characterised by recurring, unprovoked seizures. Vagus nerve stimulation (VNS) is a neuromodulatory treatment that is used as an adjunctive therapy for treating people with drug-resistant epilepsy. VNS consists of chronic, intermittent electrical stimulation of the vagus nerve, delivered by a programmable pulse generator.

OBJECTIVES

To evaluate the efficacy and tolerability of VNS when used as add-on treatment for people with drug-resistant focal epilepsy.

SEARCH METHODS

For this update, we searched the Cochrane Register of Studies (CRS), and MEDLINE Ovid on 3 March 2022. We imposed no language restrictions. CRS Web includes randomised or quasi-randomised controlled trials from the Specialised Registers of Cochrane Review Groups, including Epilepsy, CENTRAL, PubMed, Embase, ClinicalTrials.gov, and the World Health Organization International Clinical Trials Registry Platform.

SELECTION CRITERIA

We considered parallel or cross-over, randomised, double-blind, controlled trials of VNS as add-on treatment, which compared high- and low-level stimulation (including three different stimulation paradigms: rapid, mild, and slow duty-cycle), and VNS stimulation versus no stimulation, or a different intervention. We considered adults or children with drug-resistant focal seizures who were either not eligible for surgery, or who had failed surgery.

DATA COLLECTION AND ANALYSIS

We followed standard Cochrane methods, assessing the following outcomes: 1. 50% or greater reduction in seizure frequency 2. Treatment withdrawal (any reason) 3. Adverse effects 4. Quality of life (QoL) 5. Cognition 6. Mood

MAIN RESULTS

We did not identify any new studies for this update, therefore, the conclusions are unchanged. We included the five randomised controlled trials (RCT) from the last update, with a total of 439 participants. The baseline phase ranged from 4 to 12 weeks, and double-blind treatment phases from 12 to 20 weeks. We rated two studies at an overall low risk of bias, and three at an overall unclear risk of bias, due to lack of reported information about study design. Effective blinding of studies of VNS is difficult, due to the frequency of stimulation-related side effects, such as voice alteration. The risk ratio (RR) for 50% or greater reduction in seizure frequency was 1.73 (95% confidence interval (CI) 1.13 to 2.64; 4 RCTs, 373 participants; moderate-certainty evidence), showing that high frequency VNS was over one and a half times more effective than low frequency VNS. The RR for treatment withdrawal was 2.56 (95% CI 0.51 to 12.71; 4 RCTs, 375 participants; low-certainty evidence). Results for the top five reported adverse events were: hoarseness RR 2.17 (99% CI 1.49 to 3.17; 3 RCTs, 330 participants; moderate-certainty evidence); cough RR 1.09 (99% CI 0.74 to 1.62; 3 RCTs, 334 participants; moderate-certainty evidence); dyspnoea RR 2.45 (99% CI 1.07 to 5.60; 3 RCTs, 312 participants; low-certainty evidence); pain RR 1.01 (99% CI 0.60 to 1.68; 2 RCTs; 312 participants; moderate-certainty evidence); paraesthesia 0.78 (99% CI 0.39 to 1.53; 2 RCTs, 312 participants; moderate-certainty evidence). Results from two studies (312 participants) showed that a small number of favourable QOL effects were associated with VNS stimulation, but results were inconclusive between high- and low-level stimulation groups. One study (198 participants) found inconclusive results between high- and low-level stimulation for cognition on all measures used. One study (114 participants) found the majority of participants showed an improvement in mood on the Montgomery-Åsberg Depression Rating Scale compared to baseline, but results between high- and low-level stimulation were inconclusive. We found no important heterogeneity between studies for any of the outcomes.

AUTHORS' CONCLUSIONS

VNS for focal seizures appears to be an effective and well-tolerated treatment. Results of the overall efficacy analysis show that high-level stimulation reduced the frequency of seizures better than low-level stimulation. There were very few withdrawals, which suggests that VNS is well tolerated. Adverse effects associated with implantation and stimulation were primarily hoarseness, cough, dyspnoea, pain, paraesthesia, nausea, and headache, with hoarseness and dyspnoea more likely to occur with high-level stimulation than low-level stimulation. However, the evidence for these outcomes is limited, and of moderate to low certainty. Further high-quality research is needed to fully evaluate the efficacy and tolerability of VNS for drug-resistant focal seizures.

Age-related differences in corneal nerve regeneration after SMILE and the mechanism revealed by metabolomics

PURPOSE

To investigate the effect of age on wound healing after small incision lenticule extraction (SMILE) and the underlying metabolomic mechanisms.

METHODS

This prospective study was conducted on 216 patients in four groups: the 18-20 (n = 38, Group I), 21-30 (n = 84, Group Ⅱ), 31-40 (n = 58, Group Ⅲ), and 41-50 (n = 36, Group IV) age groups. The density of corneal epithelial wing cells, basal cells, corneal stromal cells, endothelial cells and corneal nerves were examined with a laser confocal microscope (HRT III-RCM) before and 1 month, 3 month, 6 month and 1 year after SMILE. The central nerve fiber length (CNFL), the central corneal nerve fibre density (CNFD), and the central corneal nerve branch density (CNBD) were analyzed by Nero J. The corneal stroma lenticules were obtained from SMILE to analyze metabolites by high-performance liquid chromatography coupled with quadrupole time-of-flight tandem mass spectrometry (HPLC-QTOF-MS).

RESULTS

The density of corneal wing epithelial cells and basal epithelial cells have no significant difference among the four groups. The CNFL was 21.90 ± 1.68 mm/mm² in Group Ⅰ and 21.63 ± 2.09 mm/mm² in Group Ⅱ after 1 year of SMILE, which represented a return to the preoperative level, whereas the CNFL of Group Ⅲ (19.40 ± 0.98 mm/mm²) and Group Ⅳ (18.94 ± 0.72 mm/mm²) were lower than that preoperation (P ˂0.01). CNFL repair had a negative correlation with age after surgery (Pearson's R = -0.572, P ˂0.01). The CNFD and the CNBD showed the same trend with the CNFL (Pearson's R = -0.602 and -0.531, P ˂0.05). Through screening the significantly different metabolites between the 18-30 age group (including Group I and Group Ⅱ) and other two groups, 6 common remarkably different metabolites were identified. Meanwhile, 5 unique different metabolites were identified only between the 18-30 age group and the 31-40 age group. Six unique different metabolites were identified only between the 18-30 age group and the 41-50 age group.

CONCLUSION

Corneal nerve repair after SMILE was significantly affected by age. The identified age-associated differences in metabolites were mainly related to inflammation, oxidation, nerve protection and regeneration.

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.

Vagus nerve stimulation for drug-resistant epilepsy

Vagus nerve stimulation (VNS) is a neuromodulatory therapeutic option for drug-resistant epilepsy. In randomised controlled trials, VNS implantation has resulted in over 50% reduction in seizure frequency in 26%-40% of patients within 1 year. Long-term uncontrolled studies suggest better responses to VNS over time; however, the assessment of other potential predictive factors has led to contradictory results. Although initially designed for managing focal seizures, its use has been extended to other forms of drug-resistant epilepsy. In this review, we discuss the evidence supporting the use of VNS, its impact on seizure frequency and quality of life, and common adverse effects of this therapy. We also include practical guidance for the approach to and the management of patients with VNS in situ.

A retrospective study of transcutaneous vagus nerve stimulation for poststroke epilepsy

BACKGROUND

This study assessed the effect transcutaneous vagus nerve stimulation (TVNS) for poststroke epilepsy (PSE).

METHODS

Fifty-two patients with PSE were included in this study. Twenty-seven patients received TVNS, 30 minutes each session, once daily, twice weekly for a total of 4 weeks; and were assigned to the treatment group. Twenty-five patients were at waiting list and were assigned to the control group. The primary outcome included weekly seizure frequency. The secondary outcomes consisted of each seizure episode, and quality of life, measured by the Quality of Life in Epilepsy Inventory-31 (QOLIE-31), as well as the adverse events. All outcomes were measured before and after 4-week treatment.

RESULTS

After treatment, TVNS failed to show better outcomes in weekly seizure frequency (treatment group, P = .12; control group, P = .56), seizure episode (treatment group, P = .65; control group, P = .92), and QOLIE-31 (treatment group, P = .73; control group, P = .84) compared with these before the treatment. Furthermore, TVNS also did not elaborate the promising effect in seizure frequency (P = .81), seizure episode (P = .75), and QOLIE-31 (P = .33), compared with these in the control group. In addition, minor and acceptable adverse events were recorded in this study.

CONCLUSION

The results of this study showed that TVNS may be not effective for Chinese patients PSE after 4-week treatment.

Surgical strategies for pediatric epilepsy

Pediatric epilepsy is a debilitating condition that impacts millions of patients throughout the world. Approximately 20-30% of children with recurrent seizures have drug-resistant epilepsy (DRE). For these patients, surgery offers the possibility of not just seizure freedom but significantly improved neurocognitive and behavioral outcomes. The spectrum of surgical options is vast, ranging from outpatient procedures such as vagus nerve stimulation to radical interventions including hemispherectomy. The thread connecting all of these interventions is a common goal-seizure freedom, an outcome that can be achieved safely and durably in a large proportion of patients. In this review, we discuss many of the most commonly performed surgical interventions and describe the indications, complications, and outcomes specific to each.

Vagus nerve stimulation for partial seizures

BACKGROUND

Vagus nerve stimulation (VNS) is a neuromodulatory treatment that is used as an adjunctive therapy for treating people with medically refractory epilepsy. VNS consists of chronic intermittent electrical stimulation of the vagus nerve, delivered by a programmable pulse generator. The majority of people given a diagnosis of epilepsy have a good prognosis, and their seizures will be controlled by treatment with a single antiepileptic drug (AED), but up to 20%-30% of patients will develop drug-resistant epilepsy, often requiring treatment with combinations of AEDs. The aim of this systematic review was to overview the current evidence for the efficacy and tolerability of vagus nerve stimulation when used as an adjunctive treatment for people with drug-resistant partial epilepsy. This is an updated version of a Cochrane review published in Issue 7, 2010.

OBJECTIVES

To determine:(1) The effects on seizures of VNS compared to controls e.g. high-level stimulation compared to low-level stimulation (presumed sub-therapeutic dose); and(2) The adverse effect profile of VNS compared to controls e.g. high-level stimulation compared to low-level stimulation.

SEARCH METHODS

We searched the Cochrane Epilepsy Group's Specialised Register (23 February 2015), the Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library 23 February 2015), MEDLINE (1946 to 23 February 2015), SCOPUS (1823 to 23 February 2015), ClinicalTrials.gov (23 February 2015) and ICTRP (23 February 2015). No language restrictions were imposed.

SELECTION CRITERIA

The following study designs were eligible for inclusion: randomised, double-blind, parallel or crossover studies, controlled trials of VNS as add-on treatment comparing high and low stimulation paradigms (including three different stimulation paradigms - duty cycle: rapid, mid and slow) and VNS stimulation versus no stimulation or a different intervention. Eligible participants were adults or children with drug-resistant partial seizures not eligible for surgery or who failed surgery.

DATA COLLECTION AND ANALYSIS

Two review authors independently selected trials for inclusion and extracted data. The following outcomes were assessed: (a) 50% or greater reduction in total seizure frequency; (b) treatment withdrawal (any reason); (c) adverse effects; (d) quality of life; (e) cognition; (f) mood. Primary analyses were intention-to-treat. Sensitivity best and worst case analyses were also undertaken to account for missing outcome data. Pooled Risk Ratios (RR) with 95% confidence intervals (95% Cl) were estimated for the primary outcomes of seizure frequency and treatment withdrawal. For adverse effects, pooled RRs and 99% CI's were calculated.

MAIN RESULTS

Five trials recruited a total of 439 participants and between them compared different types of VNS stimulation therapy. Baseline phase ranged from 4 to 12 weeks and double-blind treatment phases from 12 to 20 weeks in the five trials. Overall, two studies were rated as having a low risk of bias and three had an unclear risk of bias due to lack of reported information around study design. Effective blinding of studies of VNS is difficult due to the frequency of stimulation-related side effects such as voice alteration; this may limit the validity of the observed treatment effects. Four trials compared high frequency stimulation to low frequency stimulation and were included in quantitative syntheses (meta-analyses).The overall risk ratio (95% CI) for 50% or greater reduction in seizure frequency across all studies was 1.73 (1.13 to 2.64) showing that high frequency VNS was over one and a half times more effective than low frequency VNS. For this outcome, we rated the evidence as being moderate in quality due to incomplete outcome data in one included study; however results did not vary substantially and remained statistically significant for both the best and worst case scenarios. The risk ratio (RR) for treatment withdrawal was 2.56 (0.51 to 12.71), however evidence for this outcome was rated as low quality due to imprecision of the result and incomplete outcome data in one included study. The RR of adverse effects were as follows: (a) voice alteration and hoarseness 2.17 (99% CI 1.49 to 3.17); (b) cough 1.09 (99% CI 0.74 to 1.62); (c) dyspnea 2.45 (99% CI 1.07 to 5.60); (d) pain 1.01 (99% CI 0.60 to 1.68); (e) paresthesia 0.78 (99% CI 0.39 to 1.53); (f) nausea 0.89 (99% CI 0.42 to 1.90); (g) headache 0.90 (99% CI 0.48 to 1.69); evidence of adverse effects was rated as moderate to low quality due to imprecision of the result and/or incomplete outcome data in one included study. No important heterogeneity between studies was found for any of the outcomes.

AUTHORS' CONCLUSIONS

VNS for partial seizures appears to be an effective and well tolerated treatment in 439 included participants from five trials. Results of the overall efficacy analysis show that VNS stimulation using the high stimulation paradigm was significantly better than low stimulation in reducing frequency of seizures. Results for the outcome "withdrawal of allocated treatment" suggest that VNS is well tolerated as withdrawals were rare. No significant difference was found in withdrawal rates between the high and low stimulation groups, however limited information was available from the evidence included in this review so important differences between high and low stimulation cannot be excluded . Adverse effects associated with implantation and stimulation were primarily hoarseness, cough, dyspnea, pain, paresthesia, nausea and headache, with hoarseness and dyspnea more likely to occur on high stimulation than low stimulation. However, the evidence on these outcomes is limited and of moderate to low quality. Further high quality research is needed to fully evaluate the efficacy and tolerability of VNS for drug resistant partial seizures.