Transcranial direct current stimulation (tDCS) in the management of epilepsy: A systematic review

Pathogenesis, diagnosis, and treatment of epilepsy: electromagnetic stimulation-mediated neuromodulation therapy and new technologies

Epilepsy is a severe, relapsing, and multifactorial neurological disorder. Studies regarding the accurate diagnosis, prognosis, and in-depth pathogenesis are crucial for the precise and effective treatment of epilepsy. The pathogenesis of epilepsy is complex and involves alterations in variables such as gene expression, protein expression, ion channel activity, energy metabolites, and gut microbiota composition. Satisfactory results are lacking for conventional treatments for epilepsy. Surgical resection of lesions, drug therapy, and non-drug interventions are mainly used in clinical practice to treat pain associated with epilepsy. Non-pharmacological treatments, such as a ketogenic diet, gene therapy for nerve regeneration, and neural regulation, are currently areas of research focus. This review provides a comprehensive overview of the pathogenesis, diagnostic methods, and treatments of epilepsy. It also elaborates on the theoretical basis, treatment modes, and effects of invasive nerve stimulation in neurotherapy, including percutaneous vagus nerve stimulation, deep brain electrical stimulation, repetitive nerve electrical stimulation, in addition to non-invasive transcranial magnetic stimulation and transcranial direct current stimulation. Numerous studies have shown that electromagnetic stimulation-mediated neuromodulation therapy can markedly improve neurological function and reduce the frequency of epileptic seizures. Additionally, many new technologies for the diagnosis and treatment of epilepsy are being explored. However, current research is mainly focused on analyzing patients' clinical manifestations and exploring relevant diagnostic and treatment methods to study the pathogenesis at a molecular level, which has led to a lack of consensus regarding the mechanisms related to the disease.

Synaptic Plasticity in the Injured Brain Depends on the Temporal Pattern of Stimulation

Neurostimulation protocols are increasingly used as therapeutic interventions, including for brain injury. In addition to the direct activation of neurons, these stimulation protocols are also likely to have downstream effects on those neurons' synaptic outputs. It is well known that alterations in the strength of synaptic connections (long-term potentiation, LTP; long-term depression, LTD) are sensitive to the frequency of stimulation used for induction; however, little is known about the contribution of the temporal pattern of stimulation to the downstream synaptic plasticity that may be induced by neurostimulation in the injured brain. We explored interactions of the temporal pattern and frequency of neurostimulation in the normal cerebral cortex and after mild traumatic brain injury (mTBI), to inform therapies to strengthen or weaken neural circuits in injured brains, as well as to better understand the role of these factors in normal brain plasticity. Whole-cell (WC) patch-clamp recordings of evoked postsynaptic potentials in individual neurons, as well as field potential (FP) recordings, were made from layer 2/3 of visual cortex in response to stimulation of layer 4, in acute slices from control (naive), sham operated, and mTBI rats. We compared synaptic plasticity induced by different stimulation protocols, each consisting of a specific frequency (1 Hz, 10 Hz, or 100 Hz), continuity (continuous or discontinuous), and temporal pattern (perfectly regular, slightly irregular, or highly irregular). At the individual neuron level, dramatic differences in plasticity outcome occurred when the highly irregular stimulation protocol was used at 1 Hz or 10 Hz, producing an overall LTD in controls and shams, but a robust overall LTP after mTBI. Consistent with the individual neuron results, the plasticity outcomes for simultaneous FP recordings were similar, indicative of our results generalizing to a larger scale synaptic network than can be sampled by individual WC recordings alone. In addition to the differences in plasticity outcome between control (naive or sham) and injured brains, the dynamics of the changes in synaptic responses that developed during stimulation were predictive of the final plasticity outcome. Our results demonstrate that the temporal pattern of stimulation plays a role in the polarity and magnitude of synaptic plasticity induced in the cerebral cortex while highlighting differences between normal and injured brain responses. Moreover, these results may be useful for optimization of neurostimulation therapies to treat mTBI and other brain disorders, in addition to providing new insights into downstream plasticity signaling mechanisms in the normal brain.

Lack of effects of online HD-tDCS over the left or right DLPFC in an associative memory and metamemory monitoring task

Neuroimaging studies have shown that activity in the prefrontal cortex correlates with two critical aspects of normal memory functioning: retrieval of episodic memories and subjective "feelings-of-knowing" about our memory. Brain stimulation can be used to test the causal role of the prefrontal cortex in these processes, and whether the role differs for the left versus right prefrontal cortex. We compared the effects of online High-Definition transcranial Direct Current Stimulation (HD-tDCS) over the left or right dorsolateral prefrontal cortex (DLPFC) compared to sham during a proverb-name associative memory and feeling-of-knowing task. There were no significant effects of HD-tDCS on either associative recognition or feeling-of-knowing performance, with Bayesian analyses showing moderate support for the null hypotheses. Despite past work showing effects of HD-tDCS on other memory and feeling-of-knowing tasks, and neuroimaging showing effects with similar tasks, these findings add to the literature of non-significant effects with tDCS. This work highlights the need to better understand factors that determine the effectiveness of tDCS, especially if tDCS is to have a successful future as a clinical intervention.

Network-targeted transcranial direct current stimulation of the hypothalamus appetite-control network: a feasibility study

The hypothalamus is the key regulator for energy homeostasis and is functionally connected to striatal and cortical regions vital for the inhibitory control of appetite. Hence, the ability to non-invasively modulate the hypothalamus network could open new ways for the treatment of metabolic diseases. Here, we tested a novel method for network-targeted transcranial direct current stimulation (net-tDCS) to influence the excitability of brain regions involved in the control of appetite. Based on the resting-state functional connectivity map of the hypothalamus, a 12-channel net-tDCS protocol was generated (Neuroelectrics Starstim system), which included anodal, cathodal and sham stimulation. Ten participants with overweight or obesity were enrolled in a sham-controlled, crossover study. During stimulation or sham control, participants completed a stop-signal task to measure inhibitory control. Overall, stimulation was well tolerated. Anodal net-tDCS resulted in faster stop signal reaction time (SSRT) compared to sham (p = 0.039) and cathodal net-tDCS (p = 0.042). Baseline functional connectivity of the target network correlated with SSRT after anodal compared to sham stimulation (p = 0.016). These preliminary data indicate that modulating hypothalamus functional network connectivity via net-tDCS may result in improved inhibitory control. Further studies need to evaluate the effects on eating behavior and metabolism.

Transcranial electrical stimulation during functional magnetic resonance imaging in patients with genetic generalized epilepsy: a pilot and feasibility study

Objective

A third of patients with epilepsy continue to have seizures despite receiving adequate antiseizure medication. Transcranial direct current stimulation (tDCS) might be a viable adjunct treatment option, having been shown to reduce epileptic seizures in patients with focal epilepsy. Evidence for the use of tDCS in genetic generalized epilepsy (GGE) is scarce. We aimed to establish the feasibility of applying tDCS during fMRI in patients with GGE to study the acute neuromodulatory effects of tDCS, particularly on sensorimotor network activity.

Methods

Seven healthy controls and three patients with GGE received tDCS with simultaneous fMRI acquisition while watching a movie. Three tDCS conditions were applied: anodal, cathodal and sham. Periods of 60 s without stimulation were applied between each stimulation condition. Changes in sensorimotor cortex connectivity were evaluated by calculating the mean degree centrality across eight nodes of the sensorimotor cortex defined by the Automated Anatomical Labeling atlas (primary motor cortex (precentral left and right), supplementary motor area (left and right), mid-cingulum (left and right), postcentral gyrus (left and right)), across each of the conditions, for each participant.

Results

Simultaneous tDCS-fMRI was well tolerated in both healthy controls and patients without adverse effects. Anodal and cathodal stimulation reduced mean degree centrality of the sensorimotor network (Friedman's ANOVA with Dunn's multiple comparisons test; adjusted p = 0.02 and p = 0.03 respectively). Mean degree connectivity of the sensorimotor network during the sham condition was not different to the rest condition (adjusted p = 0.94).

Conclusion

Applying tDCS during fMRI was shown to be feasible and safe in a small group of patients with GGE. Anodal and cathodal stimulation caused a significant reduction in network connectivity of the sensorimotor cortex across participants. This initial research supports the feasibility of using fMRI to guide and understand network modulation by tDCS that might facilitate its clinical application in GGE in the future.

Comparing the performance of beamformer algorithms in estimating orientations of neural sources

The efficacy of transcranial electric stimulation (tES) to effectively modulate neuronal activity depends critically on the spatial orientation of the targeted neuronal population. Therefore, precise estimation of target orientation is of utmost importance. Different beamforming algorithms provide orientation estimates; however, a systematic analysis of their performance is still lacking. For fixed brain locations, EEG and MEG data from sources with randomized orientations were simulated. The orientation was then estimated (1) with an EEG and (2) with a combined EEG-MEG approach. Three commonly used beamformer algorithms were evaluated with respect to their abilities to estimate the correct orientation: Unit-Gain (UG), Unit-Noise-Gain (UNG), and Array-Gain (AG) beamformer. Performance depends on the signal-to-noise ratios for the modalities and on the chosen beamformer. Overall, the UNG and AG beamformers appear as the most reliable. With increasing noise, the UG estimate converges to a vector determined by the leadfield, thus leading to insufficient orientation estimates.

Acute effect of transcranial direct current stimulation on photoparoxysmal response

INTRODUCTION

Transcranial direct current stimulation (tDCS) is a non-invasive technique, used to modify the excitability of the central nervous system. The main mechanism of tDCS is to change the excitability by subthreshold modulation by affecting neuronal membrane potentials in the direction of depolarization or repolarization. tDCS was previously investigated as an alternative adjunctive therapy in patients with epilepsy. We aimed here to investigate the acute effect of tDCS on the photoparoxysmal response (PPR) in EEG.

METHODS

We enrolled 11 consecutive patients diagnosed with idiopathic generalized epilepsy who had PPR on at least 2 EEGs. Three different procedures, including sham, anodal, and cathodal tDCS were applied to the patients at intervals of one week by placing the active electrode over Oz, for 2 mA, 20 minutes. Spike-wave indices (SWI) were counted by two researchers independently and were compared during intermittent photic stimulation (IPS) on EEGs both before and after the application.

RESULTS

After cathodal tDCS, SWI increased compared to baseline EEG and sham EEG in 3 patients, and after anodal tDCS, SWI increased in 2 patients. Although the SWI values did not change significantly, 8 patients reported subjectively that the applications were beneficial for them and that they experienced less discomfort during photic stimulation after the sessions. There were no side effects except transient skin rash in one patient, only.

CONCLUSIONS

In our sham controlled tDCS study with both cathodal and anodal stimulation, our data showed that there was no significant change in SWI during IPS, despite subjective well-being. tDCS' modulatory effect does not seem to act in the acute phase on EEG parameters after photic stimulation.

The efficacy and safety of third-generation antiseizure medications and non-invasive brain stimulation to treat refractory epilepsy: a systematic review and network meta-analysis study

Background

The new antiseizure medications (ASMs) and non-invasive brain stimulation (NIBS) are controversial in controlling seizures. So, this network meta-analysis aimed to evaluate the efficacy and safety of five third-generation ASMs and two NIBS therapies for the treatment of refractory epilepsy.

Methods

We searched PubMed, EMBASE, Cochrane Library and Web of Science databases. Brivaracetam (BRV), cenobamate (CNB), eslicarbazepine acetate (ESL), lacosamide (LCM), perampanel (PER), repetitive transcranial magnetic stimulation (rTMS), and transcranial direct current stimulation (tDCS) were selected as additional treatments for refractory epilepsy in randomized controlled studies and other cohort studies. Randomized, double-blind, placebo-controlled, add-on studies that evaluated the efficacy or safety of medication and non-invasive brain stimulation and included patients with seizures were uncontrolled by one or more concomitant ASMs were identified. A random effects model was used to incorporate possible heterogeneity. The primary outcome was the change in seizure frequency from baseline, and secondary outcomes included the proportion of patients with ≥50% reduction in seizure frequency, and the rate of treatment-emergent adverse events.

Results

Forty-five studies were analyzed. The five ASMs and two NIBS decreased seizure frequency from baseline compared with placebo. The 50% responder rates of the five antiseizure drugs were significantly higher than that of placebo, and the ASMs were associated with fewer adverse events than placebo (p < 0.05). The surface under the cumulative ranking analysis revealed that ESL was most effective in decreasing the seizure frequency from baseline, whereas CNB provided the best 50% responder rate. BRV was the best tolerated. No significant publication bias was identified for each outcome index.

Conclusion

The five third-generation ASMs were more effective in controlling seizures than placebo, among which CNB, ESL, and LCM were most effective, and BRV exhibited better safety. Although rTMS and tDCS did not reduce seizure frequency as effectively as the five drugs, their safety was confirmed.

Systematic review registration

PROSPERO, https://www.crd.york.ac.uk/prospero/ (CRD42023441097).

Intravital photoacoustic brain stimulation with high-precision

Significance

Neural regulation at high precision vitally contributes to propelling fundamental understanding in the field of neuroscience and providing innovative clinical treatment options. Recently, photoacoustic brain stimulation has emerged as a cutting-edge method for precise neuromodulation and shows great potential for clinical application.

Aim

The goal of this perspective is to outline the advancements in photoacoustic brain stimulation in recent years. And, we also provide an outlook delineating several prospective paths through which this burgeoning approach may be substantively refined for augmented capability and wider implementations.

Approach

First, the mechanisms of photoacoustic generation as well as the potential mechanisms of photoacoustic brain stimulation are provided and discussed. Then, the state-of-the-art achievements corresponding to this technology are reviewed. Finally, future directions for photoacoustic technology in neuromodulation are provided.

Results

Intensive research endeavors have prompted substantial advancements in photoacoustic brain stimulation, illuminating the unique advantages of this modality for noninvasive and high-precision neuromodulation via a nongenetic way. It is envisaged that further technology optimization and randomized prospective clinical trials will enable a wide acceptance of photoacoustic brain stimulation in clinical practice.

Conclusions

The innovative practice of photoacoustic technology serves as a multifaceted neuromodulation approach, possessing noninvasive, high-accuracy, and nongenetic characteristics. It has a great potential that could considerably enhance not only the fundamental underpinnings of neuroscience research but also its practical implementations in a clinical setting.

Cathodal HD-tDCS and attention: A study on patients with intractable left lateral frontal lobe epilepsy

OBJECTIVE

Defects in the attentional network in patients with epilepsy are influenced by factors such as the location of epileptic foci. Examining the impact of cathodal high-definition transcranial direct current stimulation (HD-tDCS) on attention components could provide insights into potential attention-related side effects of tDCS. This study aimed to investigate the effect of cathodal HD-tDCS on interictal epileptiform discharges (IEDs), auditory/visual (A/V) attention components, and reaction time (RT) in patients with intractable focal left lateral frontal lobe epilepsy (LFLE).

METHODS

To control for variations in individual epilepsy syndrome, 12 adult participants diagnosed with drug-resistant left LFLE with focal cortical IEDs on C3 underwent repeated measurements at pretest, posttest, and follow-up steps. 4 × 1 ring electrodes (cathode on C3 and four anodes on F3, P3, T3, and Cz) delivered 2 mA DC for 20 min per session for 10 consecutive days. The integrated visual and auditory continuous performance test (IVA+) assessed the A/V attention components and RT. One-way repeated-measure ANOVA was used.

RESULTS

The findings suggest a significant effect in reducing IEDs. The IVA+ results showed a significant improvement in auditory divided attention and visual selective and focused attention (p < 0.05). In the follow-up, these changes demonstrated lasting efficacy. A/V speed scales increased (p < 0.05), showing a significant decrease in reaction time.

CONCLUSIONS

Cathodal HD-tDCS significantly reduced IEDs and improved the components of auditory divided attention, visual focused attention, and visual selective attention, with a reduction in patient reaction time. A significant lasting, side-effect-free positive effect was observed for up to one month after the intervention.

Effect of intracranial electrical stimulation on dynamic functional connectivity in medically refractory epilepsy

Objective

The objective of this study was to explore the distributed network effects of intracranial electrical stimulation in patients with medically refractory epilepsy using dynamic functional connectivity (dFC) and graph indicators.

Methods

The time-varying connectivity patterns of dFC (state-based metrics) as well as topological properties of static functional connectivity (sFC) and dFC (graph indicators) were assessed before and after the intracranial electrical stimulation. The sliding window method and k-means clustering were used for the analysis of dFC states, which were characterized by connectivity strength, occupancy rate, dwell time, and transition. Graph indicators for sFC and dFC were obtained using group statistical tests.

Results

DFCs were clustered into two connectivity configurations: a strongly connected state (state 1) and a sparsely connected state (state 2). After electrical stimulation, the dwell time and occupancy rate of state 1 decreased, while that of state 2 increased. Connectivity strengths of both state 1 and state 2 decreased. For graph indicators, the clustering coefficient, k-core, global efficiency, and local efficiency of patients showed a significant decrease, but the brain networks of patients exhibited higher modularity after electrical stimulation. Especially, for state 1, there was a significant decrease in functional connectivity strength after stimulation within and between the frontal lobe and temporary lobe, both of which are associated with the seizure onset.

Conclusion

Our findings demonstrated that intracranial electrical stimulation significantly changed the time-varying connectivity patterns and graph indicators of the brain in patients with medically refractory epilepsy. Specifically, the electrical stimulation decreased functional connectivity strength in both local-level and global-level networks. This might provide a mechanism of understanding for the distributed network effects of intracranial electrical stimulation and extend the knowledge of the pathophysiological network of medically refractory epilepsy.

A voltage-controlled current source for temporal interference stimulation: Analysis, design, and study

The temporal interference stimulation is a new technique to modulate brain activity by applying multiple channels of voltage-controlled current source (VCCS) simultaneously to the receptor surface. In this paper, the envelopes of the overlapped current waves at several areas of the receptor were analyzed and discussed with different circuit structures of VCCS. A complementary differential current source (CDCS) was designed to fit the best circuit topology based on the analysis of the enhanced Howland current source structure. Experiments were conducted by injecting current to a swine tissue using the CDCS and conventional VCCS and acquiring the voltage waveform data from different parts of the tissue. The waveforms were compared and analyzed, revealing that the conventional VCCS may generate an interference envelope in unexpected regions, while the CDCS did not. The CDCS design approach in this paper provides a new solution for temporal interference stimulation VCCS.

Tackling seizures in patients with Alzheimer's disease

INTRODUCTION

In past years, a possible bidirectional link between epilepsy and Alzheimer's disease (AD) has been proposed: if AD patients are more likely to develop epilepsy, people with late-onset epilepsy evidence an increased risk of dementia. Furthermore, current research suggested that subclinical epileptiform discharges may be more frequent in patients with AD and network hyperexcitability may hasten cognitive impairment.

AREAS COVERED

In this narrative review, the authors discuss the recent evidence linking AD and epilepsy as well as seizures semeiology and epileptiform activity observed in patients with AD. Finally, anti-seizure medications (ASMs) and therapeutic trials to tackle seizures and network hyperexcitability in this clinical scenario have been summarized.

EXPERT OPINION

There is growing experimental evidence demonstrating a strong connection between seizures, neuronal hyperexcitability, and AD. Epilepsy in AD has shown a good response to ASMs both at the late and prodromal stages. The new generation ASMs with fewer cognitive adverse effects seem to be a preferable option. Data on the possible effects of network hyperexcitability and ASMs on AD progression are still inconclusive. Further clinical trials are mandatory to identify clear guidelines about treatment of subclinical epileptiform discharges in patients with AD without seizures.

An Operational Approach for Optimizing Transcranial Direct Current Stimulation

Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique that has been utilized for treating brain disorders and improving cognitive function. In order to achieve targeted tDCS, many optimization methods of montages and electric currents have been proposed. However, these methods have some limitations. Most of them were proposed for single-objective optimization (focality or intensity) and have no constrain with the number of electrodes (Most devices only have less than 8 electrodes currently). In this study, we proposed an operational optimization approach for well-targeted tDCS, which aims to optimize for two objectives of electric field (EF) intensity and focality with constraints on the number of electrodes. Compared with traditional tDCS in our cohort (10 subjects), our method significantly improves the EF focality. When compared to commonly used 4×1 high-definition tDCS (HD-tDCS), our method can achieve higher EF intensity in the target region with less than 8 electrodes. Our method can balance the two objectives of EF and shorten optimization time, which is convenient for practical application.

Focal Cortex Stimulation With a Novel Implantable Device and Antiseizure Outcomes in 2 Prospective Multicenter Single-Arm Trials

Importance

For the large population of people with drug-refractory epilepsy, alternative treatment approaches are needed. Clinical trial outcomes of a novel stimulation device, which is newly available in Europe for the treatment of patients with a predominant seizure focus, are reported for the first time.

Objective

To perform a pooled analysis of the results of 2 prospective, multicenter, single-arm trials, A Pilot Study to Assess the Feasibility of Neurostimulation With the EASEE System to Treat Medically Refractory Focal Epilepsy (EASEE II) and A Pilot Study to Assess the Feasibility of Patient-Controlled Neurostimulation With the EASEE System to Treat Medically Refractory Focal Epilepsy (PIMIDES I), assessing the safety and efficacy of epicranial focal cortex stimulation (FCS) with a novel implantable device (EASEE [Precisis]) as adjunctive treatment for adult patients with drug-refractory focal epilepsy.

Design, Setting, and Participants

This study was a pooled analysis of 2 nonrandomized uncontrolled trials, EASEE II and PIMIDES I, which began on January 15, 2019, and January 14, 2020, respectively, and ended on July 28, 2021. EASEE II and PIMIDES I were the first in-human, prospective, single-arm trials with an 8-month evaluation period. Patients were recruited at 7 European epilepsy centers. Consecutive participants with drug-refractory focal epilepsy were enrolled. Study data were analyzed from September 29, 2021, to February 2, 2022.

Interventions

After a 1-month prospective baseline period, patients were implanted with the neurostimulation device. After a 1-month postimplantation recovery period, unblinded FCS was activated using both high-frequency and direct current (DC)-like components performed via electrode arrays placed epicranially above the individual epileptic focus region.

Main Outcomes and Measures

Efficacy was prospectively assessed by the responder rate in the sixth month of stimulation compared with baseline; safety and additional end points were assessed after device implantation and during the stimulation period.

Results

Of the 34 adult patients enrolled at 6 German and 1 Belgian investigational site, 33 (mean [SD] age, 34.6 [13.5] years; 18 male patients [54.5%]) received the neurostimulation device implant. A total of 32 patients underwent combined high-frequency direct current-like stimulation at least until the 8-month postimplant follow-up visit. After 6 months of stimulation, 17 of 32 patients (53.1%) were responders to treatment with at least a 50% reduction in seizure frequency compared with baseline, corresponding to a significant median seizure reduction by 52% (95% CI, 0.37%-0.76%; P < .001). No device- or procedure-related serious adverse events were reported (0; 95% CI, 0%-10.58%). There were no significant alterations in cognition, mood, or overall quality of life.

Conclusions and Relevance

Results of this pooled analysis of 2 nonrandomized uncontrolled trials suggest that FCS with a novel neurostimulation device was associated with an effective reduction in seizure frequency in patients with drug-refractory focal epilepsy and may offer a promising treatment option for patients with a predominant epileptic focus.

Trial Registration

German Clinical Trials Register: DRKS00015918 and DRKS00017833, respectively, and jointly under PROSPERO: CRD42021266440.

Tolerability and Effectiveness of Cathodal Transcranial Direct Current Stimulation in Children with Refractory Epilepsy: A Case Series

There are limited treatment options for drug-resistant epilepsy (DRE) in children. We performed a pilot study to investigate the tolerability and effectiveness of cathodal transcranial direct current stimulation (tDCS) in DRE. Twelve children with DRE of varied etiology underwent three to four daily sessions of cathodal tDCS. The seizure frequency at 2 weeks before and after tDCS was obtained from seizure diaries; clinic reviews at 3 and 6 months assessed any longer-term benefits or adverse effects. The spike wave index (SWI) was analyzed in the EEGs done immediately before and after tDCS on the first and last day of tDCS. One child remained seizure free for a year after tDCS. One child had reduced frequency of ICU admissions for status epilepticus for 2 weeks, likely due to reduced severity of seizures. In four children, an improvement in alertness and mood was reported for 2-4 weeks after tDCS. There was no benefit following tDCS in the other children. There were no unexpected or serious adverse effects in any child. Benefit was seen in two children, and the reasons for the lack of benefit in the other children need further study. It is likely that tDCS stimulus parameters will need to be tailored for different epilepsy syndromes and etiologies.

Transferability of cathodal tDCS effects from the primary motor to the prefrontal cortex: A multimodal TMS-EEG study

Neurophysiological effects of transcranial direct current stimulation (tDCS) have been extensively studied over the primary motor cortex (M1). Much less is however known about its effects over non-motor areas, such as the prefrontal cortex (PFC), which is the neuronal foundation for many high-level cognitive functions and involved in neuropsychiatric disorders. In this study, we, therefore, explored the transferability of cathodal tDCS effects over M1 to the PFC. Eighteen healthy human participants (11 males and 8 females) were involved in eight randomized sessions per participant, in which four cathodal tDCS dosages, low, medium, and high, as well as sham stimulation, were applied over the left M1 and left PFC. After-effects of tDCS were evaluated via transcranial magnetic stimulation (TMS)-electroencephalography (EEG), and TMS-elicited motor evoked potentials (MEP), for the outcome parameters TMS-evoked potentials (TEP), TMS-evoked oscillations, and MEP amplitude alterations. TEPs were studied both at the regional and global scalp levels. The results indicate a regional dosage-dependent nonlinear neurophysiological effect of M1 tDCS, which is not one-to-one transferable to PFC tDCS. Low and high dosages of M1 tDCS reduced early positive TEP peaks (P30, P60), and MEP amplitudes, while an enhancement was observed for medium dosage M1 tDCS (P30). In contrast, prefrontal low, medium and high dosage tDCS uniformly reduced the early positive TEP peak amplitudes. Furthermore, for both cortical areas, regional tDCS-induced modulatory effects were not observed for late TEP peaks, nor TMS-evoked oscillations. However, at the global scalp level, widespread effects of tDCS were observed for both, TMS-evoked potentials and oscillations. This study provides the first direct physiological comparison of tDCS effects applied over different brain areas and therefore delivers crucial information for future tDCS applications.

Practical considerations in epilepsy neurostimulation

Neuromodulation is a key therapeutic tool for clinicians managing patients with drug-resistant epilepsy. Multiple devices are available with long-term follow-up and real-world experience. The aim of this review is to give a practical summary of available neuromodulation techniques to guide the selection of modalities, focusing on patient selection for devices, common approaches and techniques for initiation of programming, and outpatient management issues. Vagus nerve stimulation (VNS), deep brain stimulation of the anterior nucleus of the thalamus (DBS-ANT), and responsive neurostimulation (RNS) are all supported by randomized controlled trials that show safety and a significant impact on seizure reduction, as well as a suggestion of reduction in the risk of sudden unexplained death in epilepsy (SUDEP). Significant seizure reductions are observed after 3 months for DBS, RNS, and VNS in randomized controlled trials, and efficacy appears to improve with time out to 7 to 10 years of follow-up for all modalities, albeit in uncontrolled follow-up or retrospective studies. A significant number of patients experience seizure-free intervals of 6 months or more with all three modalities. Number and location of epileptogenic foci are important factors affecting efficacy, and together with comorbidities such as severe mood or sleep disorders, may influence the choice of modality. Programming has evolved-DBS is typically initiated at lower current/voltage than used in the pivotal trial, whereas target charge density is lower with RNS, however generalizable optimal parameters are yet to be defined. Noninvasive brain stimulation is an emerging stimulation modality, although it is currently not used widely. In summary, clinical practice has evolved from those established in pivotal trials. Guidance is now available for clinicians who wish to expand their approach, and choice of neuromodulation technique may be tailored to individual patients based on their epilepsy characteristics, risk tolerance, and preferences.

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.

Functional Neurological Disorders: Clinical Spectrum, Diagnosis, and Treatment

BACKGROUND

Functional neurological disorders (FNDs) are common but often misdiagnosed.

REVIEW SUMMARY

The incidence of FNDs is between 4 and 12 per 100,000, comparable to multiple sclerosis and amyotrophic lateral sclerosis, and it is the second most common diagnosis in neurology clinics. Some clues in the history are sudden onset, intermittent time course, variability of manifestation over time, childhood trauma, and history of other somatic symptoms. Anxiety and depression are common, but not necessarily more than in the general population. Although there are no tests currently capable of demonstrating whether symptoms are willfully produced, there may not be a clear categorical difference between voluntary and involuntary symptoms. The prognosis of an FND is linked to early diagnosis and symptom duration, but unfortunately, the majority of the patients are diagnosed after considerable delays.

CONCLUSIONS

A positive diagnosis of FNDs can be made on the basis of history and neurological signs without reliance on psychological stressors. Past sensitizing events and neurobiological abnormalities contribute to the pathogenesis of FNDs. Physical rehabilitation and psychological interventions alone or in combination are helpful in the treatment.

Focal non-invasive deep-brain stimulation with temporal interference for the suppression of epileptic biomarkers

Introduction

Neurostimulation applied from deep brain stimulation (DBS) electrodes is an effective therapeutic intervention in patients suffering from intractable drug-resistant epilepsy when resective surgery is contraindicated or failed. Inhibitory DBS to suppress seizures and associated epileptogenic biomarkers could be performed with high-frequency stimulation (HFS), typically between 100 and 165 Hz, to various deep-seated targets, such as the Mesio-temporal lobe (MTL), which leads to changes in brain rhythms, specifically in the hippocampus. The most prominent alterations concern high-frequency oscillations (HFOs), namely an increase in ripples, a reduction in pathological Fast Ripples (FRs), and a decrease in pathological interictal epileptiform discharges (IEDs).

Materials and methods

In the current study, we use Temporal Interference (TI) stimulation to provide a non-invasive DBS (130 Hz) of the MTL, specifically the hippocampus, in both mouse models of epilepsy, and scale the method using human cadavers to demonstrate the potential efficacy in human patients. Simulations for both mice and human heads were performed to calculate the best coordinates to reach the hippocampus.

Results

This non-invasive DBS increases physiological ripples, and decreases the number of FRs and IEDs in a mouse model of epilepsy. Similarly, we show the inability of 130 Hz transcranial current stimulation (TCS) to achieve similar results. We therefore further demonstrate the translatability to human subjects via measurements of the TI stimulation vs. TCS in human cadavers. Results show a better penetration of TI fields into the human hippocampus as compared with TCS.

Significance

These results constitute the first proof of the feasibility and efficiency of TI to stimulate at depth an area without impacting the surrounding tissue. The data tend to show the sufficiently focal character of the induced effects and suggest promising therapeutic applications in epilepsy.

An affordable implantable vagus nerve stimulator system for use in animal research

In this research, a vagus nerve stimulator has been developed and miniaturized for use in epilepsy research. The board contains all the components necessary for its operation during the standard duration of the experiments, being possible to control it once implanted and even being able to reuse it. The VNS system has been designed for rodents since the VNS devices available for human are not only too large for laboratory animals, but also too expensive. With this solution the expenditure on materials made by laboratories is greatly reduced and bioethical considerations were kept in mind. The system was validated in hamsters. This article is part of the theme issue 'Advanced neurotechnologies: translating innovation for health and well-being'.

A Miniaturized Closed-Loop Optogenetic Brain Stimulation Device

This paper presents a tetherless and miniaturized closed-loop optogenetic brain stimulation device, designed as a back mountable device for laboratory mice. The device has the ability to sense the biomarkers corresponding to major depressive disorder (MDD) from local field potential (LFP), and produces a feedback signal to control the closed-loop operation after on-device processing of the sensed signals. MDD is a chronic neurological disorder and there are still many unanswered questions about the underlying neurological mechanisms behind its occurrence. Along with other brain stimulation paradigms, optogenetics has recently proved effective in the study of MDD. Most of these experiments have used tethered and connected devices. However, the use of tethered devices in optogenetic brain stimulation experiments has the drawback of hindering the free movement of the laboratory animal subjects undergoing stimulation. To address this issue, the proposed device is small, light-weight, untethered, and back-mountable. The device consists of: (i) an optrode which houses an electrode for collecting neural signals, an optical source for delivering light stimulations, and a temperature sensor for monitoring the temperature increase at the stimulation site, (ii) a neural sensor for acquisition and pre-processing of the neural signals to obtain LFP signals in the frequency range of 4 to 200 Hz, as electrophysiological biomarkers of MDD (iii) a classifier for classification of the signal into four classes: normal, abnormal alpha, abnormal theta, and abnormal gamma oscillations, (iv) a control algorithm to select stimulation parameters based on the input class, and (v) a stimulator for generating light stimulations. The design, implementation, and evaluation of the device are presented, and the results are discussed. The neural sensor and the stimulator are circular in shape with a radius of 8 mm. Pre-recorded neural signals from the mouse hippocampus are used for the evaluation of the device.

A pilot randomized controlled clinical trial of Transcranial Alternating Current Stimulation in patients with multifocal pharmaco-resistant epilepsy

Transcranial Alternating Current Stimulation (tACS) is a promising noninvasive electrical stimulation therapy for neuropsychiatric diseases. Invasive neuromodulation using alternating current has been efficacious for drug-resistant epilepsy, but it is associated with surgical and medical complications. We aimed to explore the safeness and effectivity on seizure frequency reduction of two tACS protocols against placebo in patients with multifocal refractory epilepsy. This was a randomized, double-blinded, placebo-controlled clinical trial with 3-arm parallel-group (placebo, 30 min/2 mA daily sessions for 3 days [tACS-30], and 60 min/2 mA weekday sessions [tACS-60]). The main outcome was considered a change in reducing seizure frequency at 2 months after the intervention. Secondary outcomes were the apparition of any adverse effects during follow-up. At the second month, we observed a nonsignificant reduction in the seizure frequency in the placebo (7.3 ± 40.4%, p > 0.05) and the tACS-60 (26 ± 37.7%, p > 0.05). While the tACS-30 group showed a nonsignificant increase in seizure frequency (63.6 ± 155.3%, p > 0.05). No changes were statistically different from the placebo group. Otherwise, participants experienced only minor adverse events - the most common being an initial local transient tingling sensation (21%). This pilot study of tACS raises no severe safety issues, but provides negligible evidence for efficacy using this brief treatment protocol. Therefore, more studies are warranted testing different parameters to further verify the safety and effectivity of tACS in multifocal epilepsy.

High Estrogen Levels Cause Greater Leg Muscle Fatigability in Eumenorrheic Young Women after 4 mA Transcranial Direct Current Stimulation

Transcranial direct current stimulation (tDCS) research has shown great outcome variability in motor performance tasks, with one possible source being sex differences. The goal of this study was to evaluate the effects of estrogen levels on leg muscle fatigability during a fatigue task (FT) after 4 mA tDCS over the left motor cortex (M1). Ten young, healthy eumenorrheic women received 4 mA anodal active or sham stimulation over the left M1 during periods of high and low estrogen levels. A fatigue index (FI) was calculated to quantify fatigability, and the electromyography (EMG) of the knee extensors and flexors was recorded during the FT. The findings showed that tDCS applied during high estrogen levels resulted in greater leg muscle fatigability. Furthermore, a significant increase in EMG activity of the right knee extensors was observed during periods of active stimulation, independent of estrogen level. These results suggest that estrogen levels should be considered in tDCS studies with young healthy women.

Pathological Delta Oscillations in Hallucinogen Persisting Perception Disorder: A Case Report

BACKGROUND

Hallucinogen persisting perception disorder (HPPD) is characterized by spontaneous recurrence of visual hallucinations or disturbances after previous consumption of hallucinogens, such as lysergic acid diethylamide (LSD). The underlying physiological mechanisms are unknown and there is no standardized treatment strategy available.

CASE PRESENTATION

A 33-year-old male patient presented with persistent visual distortions (halos around objects, intensified colors, positive after images, and trails following moving objects) that developed after repeated use of hallucinogenic drugs at the age of 18. Symptoms developed gradually and worsened several months later, resulting in various pharmacological and psychosocial treatment attempts that remained unsuccessful, however. At presentation, 32-channel electroencephalography (EEG) showed increased delta activity over the occipital brain regions, reminiscent of occipital intermittent rhythmic delta activity (OIRDA) usually seen in children. Two sessions of cathodal (inhibitory) transcranial direct current stimulation (tDCS) over 30 min attenuated visual hallucinations and occipital delta activity by approximately 60%. The response persisted for over four weeks.

CONCLUSION

Pathological delta activity over occipital brain regions may play an important role in the development and perpetuation of HPPD and can be attenuated by non-invasive brain stimulation.

Brain stimulation: a therapeutic approach for the treatment of neurological disorders

Brain stimulation has become one of the most acceptable therapeutic approaches in recent years and a powerful tool in the remedy against neurological diseases. Brain stimulation is achieved through the application of electric currents using non-invasive as well as invasive techniques. Recent technological advancements have evolved into the development of precise devices with capacity to produce well-controlled and effective brain stimulation. Currently, most used non-invasive techniques are repetitive transcranial magnetic stimulation (rTMS) and transcranial direct current stimulation (tDCS), whereas the most common invasive technique is deep brain stimulation (DBS). In last decade, application of these brain stimulation techniques has not only exploded but also expanded to wide variety of neurological disorders. Therefore, in the current review, we will provide an overview of the potential of both non-invasive (rTMS and tDCS) and invasive (DBS) brain stimulation techniques in the treatment of such brain diseases.

Cerebellar Transcranial Direct Current Stimulation in Children with Autism Spectrum Disorder: A Pilot Study on Efficacy, Feasibility, Safety, and Unexpected Outcomes in Tic Disorder and Epilepsy

Patients with autism spectrum disorder (ASD) display distinctive neurophysiological characteristics associated with significant cognitive, emotional, and behavioral symptoms. Transcranial direct current stimulation (tDCS) applied to the frontal or temporoparietal lobes has demonstrated potential to reduce the severity of ASD-related symptoms. Recently, the cerebellum has been identified as a brain area involved in ASD pathophysiology. In this open-label pilot study, seven ASD patients aged between 9 and 13 years underwent 20 daily sessions of 20 min cathodal stimulation of the right cerebellar lobe. At the end of the treatment, the Aberrant Behavior Checklist (ABC) scores showed a 25% mean reduction in global severity of symptoms, with a more pronounced reduction in the “social withdrawal and lethargy” (−35%), “hyperactivity and noncompliance” (−26%), and “irritability, agitation, and crying” (−25%) subscales. Minor and no improvement were observed in the “stereotypic behavior” (−18%) and “inappropriate speech” (−0%) subscales, respectively. Improvements were not detected in the two patients who were taking psychotropic drugs during the study. Clinical response showed a symptom-specific time course. Quality of sleep and mood improved earlier than hyperactivity and social withdrawal. The treatment was generally accepted by patients and well tolerated. No serious adverse events were reported. Stimulation also appeared to markedly reduce the severity of tics in a patient with comorbid tic disorder and led to the disappearance of a frontal epileptogenic focus in another patient with a history of seizures. In conclusion, cerebellar tDCS is safe, feasible, and potentially effective in the treatment of ASD symptoms among children. Strategies to improve recruitment and retention are discussed.

Electrical stimulation in animal models of epilepsy: A review on cellular and electrophysiological aspects

Epilepsy is a debilitating condition, primarily refractory individuals, leading to the search for new efficient therapies. Electrical stimulation is an important method used for years to treat several neurological disorders. Currently, electrical stimulation is used to reduce epileptic crisis in patients and shows promising results. Even though the use of electricity to treat neurological disorders has grown worldwide, there are still many caveats that must be clarified, such as action mechanisms and more efficient stimulation treatment parameters. Thus, this review aimed to explore the comprehension of the main stimulation methods in animal models of epilepsy using rodents to develop new experimental protocols and therapeutic approaches.

Non-invasive cortical stimulation: Transcranial direct current stimulation (tDCS)

Transcranial direct current stimulation (tDCS) is a re-emerging non-invasive brain stimulation technique that has been used in animal models and human trials aimed to elucidate neurophysiology and behavior interactions. It delivers subthreshold electrical currents to neuronal populations that shift resting membrane potential either toward depolarization or hyperpolarization, depending on stimulation parameters and neuronal orientation in relation to the induced electric field (EF). Although the resulting cerebral EFs are not strong enough to induce action potentials, spontaneous neuronal firing in response to inputs from other brain areas is influenced by tDCS. Additionally, tDCS induces plastic synaptic changes resembling long-term potentiation (LTP) or long-term depression (LTD) that outlast the period of stimulation. Such properties place tDCS as an appealing intervention for the treatment of diverse neuropsychiatric disorders. Although findings of clinical trials are preliminary for most studied conditions, there is already convincing evidence regarding its efficacy for unipolar depression. The main advantages of tDCS are the absence of serious or intolerable side effects and the portability of the devices, which might lead in the future to home-use applications and improved patient care. This chapter provides an up-to-date overview of a number tDCS relevant topics such as mechanisms of action, contemporary applications and safety. Furthermore, we propose ways to further develop tDCS research.