Low-frequency transcranial stimulation of pre-supplementary motor area alleviates levodopa-induced dyskinesia in Parkinson's disease: a randomized cross-over trial

The effect of transcranial ultrasound pulse repetition frequency on sustained inhibition in the human primary motor cortex: A double-blind, sham-controlled study

BACKGROUND

Non-invasive brain stimulation techniques such as transcranial magnetic stimulation and transcranial direct current stimulation hold promise for inducing brain plasticity. However, their limited precision may hamper certain applications. In contrast, Transcranial Ultrasound Stimulation (TUS), known for its precision and deep brain targeting capabilities, requires further investigation to establish its efficacy in producing enduring effects for treating neurological and psychiatric disorders.

OBJECTIVE

To investigate the enduring effects of different pulse repetition frequencies (PRF) of TUS on motor corticospinal excitability.

METHODS

T1-, T2-weighted, and zero echo time magnetic resonance imaging scans were acquired from 21 neurologically healthy participants for neuronavigation, skull reconstruction, and the performance of transcranial ultrasound and thermal modelling. The effects of three different TUS PRFs (10, 100, and 1000 Hz) with a constant duty cycle of 10 % on corticospinal excitability in the primary motor cortex were assessed using TMS-induced motor evoked potentials (MEPs). Each PRF and sham condition was evaluated on separate days, with measurements taken 5-, 30-, and 60-min post-TUS.

RESULTS

A significant decrease in MEP amplitude was observed with a PRF of 10 Hz (p = 0.007), which persisted for at least 30 min, and with a PRF of 100 Hz (p = 0.001), lasting over 60 min. However, no significant changes were found for the PRF of 1000 Hz and the sham conditions.

CONCLUSION

This study highlights the significance of PRF selection in TUS and underscores its potential as a non-invasive approach to reduce corticospinal excitability, offering valuable insights for future clinical applications.

Neuroplasticity in levodopa-induced dyskinesias: An overview on pathophysiology and therapeutic targets

Levodopa-induced dyskinesias (LIDs) are a common complication in patients with Parkinson's disease (PD). A complex cascade of electrophysiological and molecular events that induce aberrant plasticity in the cortico-basal ganglia system plays a key role in the pathophysiology of LIDs. In the striatum, multiple neurotransmitters regulate the different forms of physiological synaptic plasticity to provide it in a bidirectional and Hebbian manner. In PD, impairment of both long-term potentiation (LTP) and long-term depression (LTD) progresses with disease and dopaminergic denervation of striatum. The altered balance between LTP and LTD processes leads to unidirectional changes in plasticity that cause network dysregulation and the development of involuntary movements. These alterations have been documented, in both experimental models and PD patients, not only in deep brain structures but also at motor cortex. Invasive and non-invasive neuromodulation treatments, as deep brain stimulation, transcranial magnetic stimulation, or transcranial direct current stimulation, may provide strategies to modulate the aberrant plasticity in the cortico-basal ganglia network of patients affected by LIDs, thus restoring normal neurophysiological functioning and treating dyskinesias. In this review, we discuss the evidence for neuroplasticity impairment in experimental PD models and in patients affected by LIDs, and potential neuromodulation strategies that may modulate aberrant plasticity.

Outcome measures for electric field modeling in tES and TMS: A systematic review and large-scale modeling study

BACKGROUND

Electric field (E-field) modeling is a potent tool to estimate the amount of transcranial magnetic and electrical stimulation (TMS and tES, respectively) that reaches the cortex and to address the variable behavioral effects observed in the field. However, outcome measures used to quantify E-fields vary considerably and a thorough comparison is missing.

OBJECTIVES

This two-part study aimed to examine the different outcome measures used to report on tES and TMS induced E-fields, including volume- and surface-level gray matter, region of interest (ROI), whole brain, geometrical, structural, and percentile-based approaches. The study aimed to guide future research in informed selection of appropriate outcome measures.

METHODS

Three electronic databases were searched for tES and/or TMS studies quantifying E-fields. The identified outcome measures were compared across volume- and surface-level E-field data in ten tES and TMS modalities targeting two common targets in 100 healthy individuals.

RESULTS

In the systematic review, we extracted 308 outcome measures from 202 studies that adopted either a gray matter volume-level (n = 197) or surface-level (n = 111) approach. Volume-level results focused on E-field magnitude, while surface-level data encompassed E-field magnitude (n = 64) and normal/tangential E-field components (n = 47). E-fields were extracted in ROIs, such as brain structures and shapes (spheres, hexahedra and cylinders), or the whole brain. Percentiles or mean values were mostly used to quantify E-fields. Our modeling study, which involved 1,000 E-field models and > 1,000,000 extracted E-field values, revealed that different outcome measures yielded distinct E-field values, analyzed different brain regions, and did not always exhibit strong correlations in the same within-subject E-field model.

CONCLUSIONS

Outcome measure selection significantly impacts the locations and intensities of extracted E-field data in both tES and TMS E-field models. The suitability of different outcome measures depends on the target region, TMS/tES modality, individual anatomy, the analyzed E-field component and the research question. To enhance the quality, rigor, and reproducibility in the E-field modeling domain, we suggest standard reporting practices across studies and provide four recommendations.

Design issues in crossover trials involving patients with Parkinson's disease

Background and objectives

Crossover designs are frequently used to assess treatments for patients with Parkinson's disease. Typically, two-period two-treatment trials include a washout period between the 2 periods and assume that the washout period is sufficiently long to eliminate carryover effects. A complementary strategy might be to jointly model carryover and treatment effects, though this has rarely been done in Parkinson's disease crossover studies. The primary objective of this research is to demonstrate a modeling approach that assesses treatment and carryover effects in one unified mixed model analysis and to examine how it performs in a simulation study and a real data analysis example, as compared to other data analytic approaches used in Parkinson's disease crossover studies.

Methods

We examined how three different methods of analysis (standard crossover t-test, mixed model with a carryover term included in model statement, and mixed model with no carryover term) performed in a simulation study and illustrated the methods in a real data example in Parkinson's disease.

Results

The simulation study based on the presence of a carryover effect indicated that mixed models with a carryover term and an unstructured correlation matrix provided unbiased estimates of treatment effect and appropriate type I error. The methods are illustrated in a real data example involving Parkinson's disease. Our literature review revealed that a majority of crossover studies included a washout period but did not assess whether the washout was sufficiently long to eliminate the possibility of carryover.

Discussion

We recommend using a mixed model with a carryover term and an unstructured correlation matrix to obtain unbiased estimates of treatment effect.

The Role of the Left Inferior Parietal Cortex in Gilles de la Tourette Syndrome—An rTMS Study

Increased activity in the left inferior parietal cortex (BA40) plays a role in the generation of tics in the Gilles de la Tourette syndrome (GTS). Thus, inhibitory repetitive transcranial magnetic stimulation (rTMS) applied to BA40 was hypothesized to alleviate symptoms in GTS. We investigated the immediate effects of single-session 1 Hz rTMS and sham stimulation delivered to the left BA40 on tics assessed with the Rush video protocol in 29 adults with GTS. There were no significant effects on tic symptoms following rTMS or sham stimulation. Moreover, there was no difference when comparing the effects of both stimulation conditions. Bayesian statistics indicated substantial evidence against an intervention effect. The left BA40 appears not to be a useful target for 1 Hz rTMS to modulate tic symptoms in GTS patients.

A Systematic Review and Large-Scale tES and TMS Electric Field Modeling Study Reveals How Outcome Measure Selection Alters Results in a Person- and Montage-Specific Manner

Background

Electric field (E-field) modeling is a potent tool to examine the cortical effects of transcranial magnetic and electrical stimulation (TMS and tES, respectively) and to address the high variability in efficacy observed in the literature. However, outcome measures used to report E-field magnitude vary considerably and have not yet been compared in detail.

Objectives

The goal of this two-part study, encompassing a systematic review and modeling experiment, was to provide an overview of the different outcome measures used to report the magnitude of tES and TMS E-fields, and to conduct a direct comparison of these measures across different stimulation montages.

Methods

Three electronic databases were searched for tES and/or TMS studies reporting E-field magnitude. We extracted and discussed outcome measures in studies meeting the inclusion criteria. Additionally, outcome measures were compared via models of four common tES and two TMS modalities in 100 healthy younger adults.

Results

In the systematic review, we included 118 studies using 151 outcome measures related to E-field magnitude. Structural and spherical regions of interest (ROI) analyses and percentile-based whole-brain analyses were used most often. In the modeling analyses, we found that there was an average of only 6% overlap between ROI and percentile-based whole-brain analyses in the investigated volumes within the same person. The overlap between ROI and whole-brain percentiles was montage- and person-specific, with more focal montages such as 4Ã-1 and APPS-tES, and figure-of-eight TMS showing up to 73%, 60%, and 52% overlap between ROI and percentile approaches respectively. However, even in these cases, 27% or more of the analyzed volume still differed between outcome measures in every analyses.

Conclusions

The choice of outcome measures meaningfully alters the interpretation of tES and TMS E-field models. Well-considered outcome measure selection is imperative for accurate interpretation of results, valid between-study comparisons, and depends on stimulation focality and study goals. We formulated four recommendations to increase the quality and rigor of E-field modeling outcome measures. With these data and recommendations, we hope to guide future studies towards informed outcome measure selection, and improve the comparability of studies.

Functional Connectivity of the Supplementary and Presupplementary Motor Areas in Postoperative Transition Between Stages of Frailty

BACKGROUND

Frailty is a multietiological geriatric syndrome of run-down physical reserves with high vulnerability to stressors. Transitions between physical robustness and frailty often occur in the context of medical interventions. Studies suggest that neurological disorders contribute to faster progression of frailty. In a previous cross-sectional study we found altered functional connectivity of supplementary motor area (SMA) in (pre)frail compared to robust patients. We analyzed functional connectivity of the SMA and presupplementary motor area (pre-SMA) in patients with postoperative transitions between physical robustness and stages of frailty.

METHODS

We investigated 120 cognitively healthy patients (49.2% robust, 47.5% prefrail, 3.3% frail, 37.5% female, median age 71 [65-87] years) undergoing elective surgery from the BioCog project, a multicentric prospective cohort study on postoperative delirium and cognitive dysfunction. Assessments took place 14 days before and 3 months after surgery, comprising assessments of a modified frailty phenotype according to Fried and resting-state functional magnetic resonance imaging at 3 T. The associations between functional connectivity of the SMA and pre-SMA networks, preoperative frailty stages, and postoperative transitions were examined using mixed linear effects models.

RESULTS

Nineteen patients showed physical improvement after surgery, 24 patients progressed to (pre)frailty and in 77 patients no transition was observed. At follow-up, 57 (47.5%) patients were robust, 52 (43.3%) prefrail, and 11 (9.2%) frail. Lower functional connectivity in the pre-SMA network was associated with more unfavorable postoperative transition types. An exploratory analysis suggested that the association was restricted to patients who were prefrail at baseline. There was no association of transition type with SMA functional connectivity in the primary analysis. In an exploratory analysis, transition from prefrailty to robustness was associated with higher functional connectivity and progression in robust patients was associated with higher SMA network segregation.

CONCLUSIONS

Our findings implicate that dysfunctions of cortical networks involved in higher cognitive control of motion are associated with postoperative transitions between frailty stages. The pre-SMA may be a target for neurofeedback or brain stimulation in approaches to prevent frailty. Clinical Trials Registration Number: NCT02265263.

Novel Non-invasive Transcranial Electrical Stimulation for Parkinson's Disease

Conventional transcranial electrical stimulation (tES) is a non-invasive method to modulate brain activity and has been extensively used in the treatment of Parkinson's disease (PD). Despite promising prospects, the efficacy of conventional tES in PD treatment is highly variable across different studies. Therefore, many have tried to optimize tES for an improved therapeutic efficacy by developing novel tES intervention strategies. Until now, these novel clinical interventions have not been discussed or reviewed in the context of PD therapy. In this review, we focused on the efficacy of these novel strategies in PD mitigation, classified them into three categories based on their distinct technical approach to circumvent conventional tES problems. The first category has novel stimulation modes to target different modulating mechanisms, expanding the rang of stimulation choices hence enabling the ability to modulate complex brain circuit or functional networks. The second category applies tES as a supplementary intervention for PD hence amplifies neurological or behavioral improvements. Lastly, the closed loop tES stimulation can provide self-adaptive individualized stimulation, which enables a more specialized intervention. In summary, these novel tES have validated potential in both alleviating PD symptoms and improving understanding of the pathophysiological mechanisms of PD. However, to assure wide clinical used of tES therapy for PD patients, further large-scale trials are required.

Molecular Mechanisms and Therapeutic Strategies for Levodopa-Induced Dyskinesia in Parkinson’s Disease: A Perspective Through Preclinical and Clinical Evidence

Parkinson’s disease (PD) is the second leading neurodegenerative disease that is characterized by severe locomotor abnormalities. Levodopa (L-DOPA) treatment has been considered a mainstay for the management of PD; however, its prolonged treatment is often associated with abnormal involuntary movements and results in L-DOPA-induced dyskinesia (LID). Although LID is encountered after chronic administration of L-DOPA, the appearance of dyskinesia after weeks or months of the L-DOPA treatment has complicated our understanding of its pathogenesis. Pathophysiology of LID is mainly associated with alteration of direct and indirect pathways of the cortico-basal ganglia-thalamic loop, which regulates normal fine motor movements. Hypersensitivity of dopamine receptors has been involved in the development of LID; moreover, these symptoms are worsened by concurrent non-dopaminergic innervations including glutamatergic, serotonergic, and peptidergic neurotransmission. The present study is focused on discussing the recent updates in molecular mechanisms and therapeutic approaches for the effective management of LID in PD patients.

Tools to explore neuroplasticity in humans: Combining interventional neurophysiology with functional and structural magnetic resonance imaging and spectroscopy

This chapter summarizes how brain imaging can be used in combination with non-invasive transcranial stimulation to probe and induce neuroplasticity in the human brain. We aim to give a conceptual account and highlight exemplary studies. We showcase the scientific and clinical potentials of studies focusing on the combination of transcranial magnetic stimulation (TMS) with Magnetic Resonance Imaging (MRI) or Magnetic Resonance Spectroscopy (MRS). MRI and MRS can be used before brain stimulation to identify target networks and loci but also to inform individual dosing. After a brain stimulation session, MRI and MRS can be used to pinpoint how the stimulation protocol alters brain function, structure, or metabolism and relate these after-effects to behavioral and clinical outcomes. Complementing these "offline" approaches, TMS can also be applied "online" during MRI or MRS to delineate how stimulation acutely engages the stimulated brain regions and networks. In this case, it is critical to account for confounds introduced by off-target stimulation of peripheral structures of the nervous system that may not only confound MR-based readouts but also induce neuroplastic phenomena.

Transcranial Magnetic Stimulation Alleviates Levodopa-Induced Dyskinesia in Parkinson's Disease and the Related Mechanisms: A Mini-Review

After long-term use of levodopa, Parkinson's patients almost inevitably develop dyskinesia, a kind of drug side effect manifesting as uncontrollable choreic movements and dystonia, which could be crippling yet have limited therapeutic options. Transcranial magnetic stimulation is the most widely studied non-invasive neuromodulation technology to treat levodopa-induced dyskinesia. Many studies have shown that transcranial magnetic stimulation has beneficial effects on levodopa-induced dyskinesia and is patient-tolerable, barely with reported adverse effects. Changes in brain connectivity, neuroplasticity, neurotransmitter, neurorestoration, and blood flow modulation could play crucial roles in the efficacy of transcranial magnetic stimulation for levodopa-induced dyskinesia. The appearance of new modes and application for emerging targets are possible solutions for transcranial magnetic stimulation to achieve sustained efficacy. Since the sample size in all available studies is small, more randomized double-blind controlled studies are needed to elucidate the specific treatment mechanisms and optimize treatment parameters.

Therapeutic application of rTMS in neurodegenerative and movement disorders: A review

Transcranial magnetic stimulation (TMS) is a non-invasive form of brain stimulation that makes use of the magnetic field generated when an electric current passes through a magnetic coil placed over the scalp. It can be applied as a single stimulus at a time, in pairs of stimuli, or repetitively in trains of stimuli (repetitive TMS, rTMS). RTMS can induce changes in brain activity, whose after-effects reflect the processes of long-term potentiation and long-term depression, as certain protocols, namely those using low frequencies (≤1 Hz) seem to suppress cortical excitability, while those using high frequencies (>1 Hz) seem to enhance it. It is a technique with very few and mostly mild side-effects, whose effects can persist for long time periods, and as such, it has been studied as a potential treatment option in a multitude of neurodegenerative diseases, including those affecting movement. Although rTMS has received approval as a treatment strategy of only a few aspects in movement disorders in the latest guidelines, its further use seems to also be promising in their context. In this review, we gathered the available literature on the therapeutic application of rTMS in movement disorders, namely Parkinson's disease, Amyotrophic Lateral Sclerosis, Huntington's disease, Dystonia, Tic disorders and Essential Tremor.

Current Knowledge on the Background, Pathophysiology and Treatment of Levodopa-Induced Dyskinesia-Literature Review

Levodopa remains the primary drug for controlling motor symptoms in Parkinson’s disease through the whole course, but over time, complications develop in the form of dyskinesias, which gradually become more frequent and severe. These abnormal, involuntary, hyperkinetic movements are mainly characteristic of the ON phase and are triggered by excess exogenous levodopa. They may also occur during the OFF phase, or in both phases. Over the past 10 years, the issue of levodopa-induced dyskinesia has been the subject of research into both the substrate of this pathology and potential remedial strategies. The purpose of the present study was to review the results of recent research on the background and treatment of dyskinesia. To this end, databases were reviewed using a search strategy that included both relevant keywords related to the topic and appropriate filters to limit results to English language literature published since 2010. Based on the selected papers, the current state of knowledge on the morphological, functional, genetic and clinical features of levodopa-induced dyskinesia, as well as pharmacological, genetic treatment and other therapies such as deep brain stimulation, are described.