Cerebellar Continuous Theta Burst Stimulation in Essential Tremor

Feasibility of phase-locked transcranial magnetic stimulation of cerebellum for the treatment of essential tremor

Objective.Cerebellar transcranial magnetic stimulation (TMS) has been proposed to suppress limb tremors in essential tremor (ET), but mixed results have been reported so far, both when pulses are applied repetitively TMS (rTMS) and in bursts. We aim to investigate the cellular effects of TMS on the cerebellum under ET through numerical simulations.Approach.A computational model of the olivo-cerebello-thalamocortical pathways exhibiting the main neural biomarkers of ET (i.e. circuit-wide tremor-locked neural oscillations) was expanded to incorporate the effects of TMS-induced electric field (E-field) on Purkinje cells. TMS pulse amplitude, frequency, and temporal pattern were varied, and the resultant effects on ET biomarkers were assessed. Four levels of cellular response to TMS were considered, ranging from low to high cell recruitment underneath the coil, and three stimulation patterns were tested, i.e. rTMS, irregular TMS (ir-TMS, pulses were arranged according to Sobol sequences with average frequency matching rTMS), and phase-locked TMS (PL-TMS).Main results.rTMS can suppress ET oscillations, but its efficacy depends on tremor frequency and recruitment level, with these factors shaping a narrow range of effective settings. The ratio between tremor and rTMS frequencies also affects the neural response and further narrows the span of viable settings, while ir-TMS is ineffective. PL-TMS is highly effective and robust against changes to cell recruitment level and tremor frequency. Across all scenarios, PL-TMS provides a rapid (i.e. within seconds) suppression of tremor oscillations and, when both PL-TMS and rTMS are effective, the time to tremor suppression decreases by 50% or more in PL-TMS versus rTMS. At the cellular level, PL-TMS operates by disrupting the synchronization along the olivo-cerebellar loop, and the preferred phases map onto the mid-region of the silent period between complex spikes of the Purkinje cells.Significance.Cerebellar PL-TMS can provide robust suppression of ET oscillations while operating within safety boundaries.

Pathophysiological Role of Primary Motor Cortex in Essential Tremor

BACKGROUND

Essential tremor (ET) is one of the most prevalent movement disorders. However, the complete understanding of ET pathophysiology remains elusive.

OBJECTIVE

To explore the pathophysiological role of primary motor cortex (M1) in ET, specifically exploring its neurophysiological changes and their correlation with voluntary motor abnormalities.

METHODS

We recruited 30 ET patients and 18 healthy controls (HC). Evaluations were conducted on patients using clinical scales. Transcranial magnetic stimulation (TMS) was used to assess M1 excitability, including motor thresholds and motor evoked potentials (MEPs) input/output curve, together with intracortical excitability measures. Long-term potentiation (LTP)-like plasticity of M1 was tested using intermittent theta-burst stimulation (iTBS). Objective assessments of tremor and voluntary movement execution during finger-tapping were conducted through kinematic analysis. Finally, we explored the potential relationship between TMS, clinical, and kinematic data.

RESULTS

Compared with HC, ET patients had lower excitability, intracortical inhibition, and lower LTP-like plasticity of M1. ET patients also exhibited slower finger-tapping performance compared with HC. Among ET patients, the degree of movement slowing during finger-tapping correlated with alterations in corticospinal excitability. Specifically, reduced M1 excitability was associated with lower finger-tapping velocity. No other correlations were found.

CONCLUSIONS

The study findings reveal neurophysiological alterations of M1 in ET and demonstrate correlations between excitability measures and voluntary motor performance. These results provide novel insight into the pathophysiology of ET, emphasizing the role of M1 changes in this condition. © 2025 The Author(s). Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Essential tremor disrupts rhythmic brain networks during naturalistic movement

Essential Tremor (ET) is a very common neurological disorder characterised by involuntary rhythmic movements attributable to pathological synchronization within corticothalamic circuits. Previous work has focused on tremor in isolation, overlooking broader disturbances to motor control during naturalistic movements such as reaching. We hypothesised that ET disrupts the sequential engagement of large-scale rhythmic brain networks, leading to both tremor and deficits in motor planning and execution. To test this, we performed whole-head neuroimaging during an upper-limb reaching task using high-density electroencephalography in ET patients and healthy controls, alongside optically pumped magnetoencephalography in a smaller cohort. Key motor regions-including the supplementary motor area, premotor cortex, posterior parietal cortex, and motor cerebellum-were synchronized to tremor rhythms. Patients exhibited a 15 % increase in low beta (14-21 Hz) desynchronization over the supplementary motor area during movement, which strongly correlated with tremor severity (R2 = 0.85). A novel dimensionality reduction technique revealed four distinct networks accounting for 97 % of the variance in motor-related brain-wide oscillations, with ET altering their sequential engagement. Consistent with our hypothesis, the frontoparietal beta network- normally involved in motor planning-exhibited additional desynchronization during movement execution in ET patients. This altered engagement correlated with slower movement velocities, suggesting an adaptation towards feedback-driven motor control. These findings reveal fundamental disruptions in distributed motor control networks in ET and identify novel biomarkers as targets for next-generation brain stimulation therapies.

Effect of a single session of transcranial pulse stimulation (TPS) on resting tremor in patients with Parkinson's disease

INTRODUCTION

Tremor is a common symptom in movement disorders and is evident at rest in Parkinson's Disease (PD). In PD, tremor may be responsive to brain stimulation, ranging from Deep Brain Stimulation to Transcranial Magnetic Stimulation. Transcranial Pulse Stimulation (TPS) is a novel/painless/non-invasive technique which appears to induce biomolecular changes through shock waves. Here, as one of the first studies in the field of PD, we exploratively investigate the possibility to observe changes in tremor, induced by single-session TPS delivered on the motor cortex of PD patients.

METHODS

TPS was delivered in 16 patients. Of these, 9 were admitted to sham (placebo). Resting tremor was measured at baseline (T0), after TPS (T1), and after 24 ​h from intervention (T2).

RESULTS

At baseline, tremor was always present. After TPS, tremor reduction was noted at T1 and T2 (compared to T0 and placebo). We noted a decrease in the amplitude of resting tremor (not its frequency).

DISCUSSION

TPS is a non-invasive technique that may be a novel solution for reducing tremor in PD, lasting at least 24 ​h after single-sessions. No side effects were reported. We discuss evidence suggesting potential physiological changes in mechanisms of neural circuits that are affected in PD.

Protocol for a Phase 2 randomized controlled patient-assessor blinded study: efficacy and safety of combined cortical and cerebellar dual-target transcranial magnetic stimulation for the treatment of essential tremor

Background

Essential tremor (ET) is the most common neurological movement disorder with few treatments and limited therapeutic efficacy, research into noninvasive and effective treatments is critical. Abnormal cerebello-thalamo-cortical (CTC) loop function are thought to be significant pathogenic causes of ET, with the cerebellum and cortex are common targets for ET treatment. In recent years, transcranial magnetic stimulation (TMS) has been recognized as a promising brain research technique owing to its noninvasive nature and safety. In this study, we will use left M1 cortex continuous theta-burst stimulation (cTBS) combined with right cerebellar hemisphere 1 Hz repetitive transcranial magnetic stimulation (rTMS) dual-target stimulation to explore the Safety, feasibility and efficiency of this dual-target stimulation mode, and the mechanism of its therapeutic effect.

Methods

Twenty-four patients with ET will be randomly assigned to three groups: dual-target stimulation, single-target stimulation, or sham stimulation. The single-target stimulation group will receive stimulation of the right cerebellar hemisphere for 10 days, whereas the dual-target stimulation group will be given stimulation of both the left M1 cortex and the right cerebellar hemisphere. The sham stimulation group will be given sham stimulation for 10 days. Tremor will be assessed using both the subjective The Essential Tremor Rating Assessment Scale (TETRAS) and objective accelerometer-based tremor analysis. at baseline (before stimulation), after the first, fifth, tenth days of treatment (D1, 5, 10), 24 h after 10 days of treatment (D10-24 h), and 1, 2, 3, and 4 weeks after stimulation (W1, 2, 3, 4).

Discussion

This is a Phase 2 randomized, controlled, patient-assessor blinded clinical trial. The goal of this study is to investigate the Safety, feasibility and efficiency of TMS for the treatment of ET.

Functional neural substrates of Parkinson's disease and potential underpinnings of acute responses to acupuncture stimulation

Parkinson's disease is a heterogenous neurodegenerative disorder with a wide variety of motor and non-motor symptoms. This study used resting-state fMRI to identify the neural substrates of PD and explore the acute neural response to acupuncture stimulation in 74 participants (50 patients with PD and 24 healthy controls). All participants with PD were evaluated for the severity of symptoms using the Unified Parkinson's Disease Rating Scale and Balance Master. The z-transformed fractional amplitude of low-frequency fluctuation analysis showed significant differences between the PD and healthy controls in the cerebellar regions, which are thought to play a crucial role in PD pathology. Subsequently, seed-based functional connectivity of the cerebellum with the frontal, parietal, and limbic regions was identified as a potential diagnostic marker for PD. In addition, spontaneous neural activity in the precentral gyrus and thalamus was significantly associated with the severity of PD symptoms. Neural activity in the precentral gyrus, precuneus, and superior temporal gyrus showed a significant correlation with Balance Master indicators. Finally, acupuncture stimulation at GB34 significantly reduced the activity of the occipital regions in patients with PD, but this effect was not observed in healthy controls. The mixed-effects analysis revealed an interaction effects between group and acupuncture stimulation, suggesting that the modulatory effects of acupuncture could differ depending on disease status. Therefore, this study suggests the neural substrates of PD and potential underpinnings of acute neural response to acupuncture stimulation.

Clinical neurophysiology in the treatment of movement disorders: IFCN handbook chapter

In this review, different aspects of the use of clinical neurophysiology techniques for the treatment of movement disorders are addressed. First of all, these techniques can be used to guide neuromodulation techniques or to perform therapeutic neuromodulation as such. Neuromodulation includes invasive techniques based on the surgical implantation of electrodes and a pulse generator, such as deep brain stimulation (DBS) or spinal cord stimulation (SCS) on the one hand, and non-invasive techniques aimed at modulating or even lesioning neural structures by transcranial application. Movement disorders are one of the main areas of indication for the various neuromodulation techniques. This review focuses on the following techniques: DBS, repetitive transcranial magnetic stimulation (rTMS), low-intensity transcranial electrical stimulation, including transcranial direct current stimulation (tDCS) and transcranial alternating current stimulation (tACS), and focused ultrasound (FUS), including high-intensity magnetic resonance-guided FUS (MRgFUS), and pulsed mode low-intensity transcranial FUS stimulation (TUS). The main clinical conditions in which neuromodulation has proven its efficacy are Parkinson's disease, dystonia, and essential tremor, mainly using DBS or MRgFUS. There is also some evidence for Tourette syndrome (DBS), Huntington's disease (DBS), cerebellar ataxia (tDCS), and axial signs (SCS) and depression (rTMS) in PD. The development of non-invasive transcranial neuromodulation techniques is limited by the short-term clinical impact of these techniques, especially rTMS, in the context of very chronic diseases. However, at-home use (tDCS) or current advances in the design of closed-loop stimulation (tACS) may open new perspectives for the application of these techniques in patients, favored by their easier use and lower rate of adverse effects compared to invasive or lesioning methods. Finally, this review summarizes the evidence for keeping the use of electromyography to optimize the identification of muscles to be treated with botulinum toxin injection, which is indicated and widely performed for the treatment of various movement disorders.

Understanding the role of cerebellum in early Parkinson's disease: a structural and functional MRI study

Increasing evidence suggests that the cerebellum may have a role in the pathophysiology of Parkinson's disease (PD). Hence, the scope of this study was to investigate whether there are structural and functional alterations of the cerebellum and whether they correlate with motor and non-motor symptoms in early PD patients. Seventy-six patients with early PD and thirty-one age and sex-matched healthy subjects (HS) were enrolled and underwent a 3 T magnetic resonance imaging (MRI) protocol. The following MRI analyses were performed: (1) volumes of 5 cerebellar regions of interest (sensorimotor and cognitive cerebellum, dentate, interposed, and fastigial nuclei); (2) microstructural integrity of the cerebellar white matter connections (inferior, middle, and superior cerebellar peduncles); (3) functional connectivity at rest of the 5 regions of interest already described in point 1 with the rest of brain. Compared to controls, early PD patients showed a significant decrease in gray matter volume of the dentate, interposed and fastigial nuclei, bilaterally. They also showed abnormal, bilateral white matter microstructural integrity in all 3 cerebellar peduncles. Functional connectivity of the 5 cerebellar regions of interest with several areas in the midbrain, basal ganglia and cerebral cortex was altered. Finally, there was a positive correlation between abnormal functional connectivity of the fastigial nucleus with the volume of the nucleus itself and a negative correlation with axial symptoms severity. Our results showed that structural and functional alterations of the cerebellum are present in PD patients and these changes contribute to the pathophysiology of PD in the early phase.

Theta-burst stimulation as a therapeutic tool in neurological pathology: a systematic review

TBS (theta-burst stimulation) is a novel therapeutic approach in a wide range of neurological diseases. The present systematic review aims to identify the various protocols used in the last years, to assess study quality and to offer a general overview of the current state of the literature. The systematic review was conducted according to the Preferred Reporting Item for Systematic Review and Meta-Analyses (PRISMA) guidelines. We applied the following inclusion criteria: (1) population over 18 years old with diagnosed neurological disorders, (2) patients treated with sessions of theta-burst stimulation, (3) randomized-controlled clinical trials, (4) articles in the English language, and (5) studies that report response and score reduction on a validated scale of the investigated disorder or remission rates. We included in the final analysis 56 randomized controlled trials focusing on different neurological pathologies (stroke, Parkinson`s disease, multiple sclerosis, tinnitus, dystonia, chronic pain, essential tremor and tic disorder), and we extracted data regarding study design, groups and comparators, sample sizes, type of coil, stimulation parameters (frequency, number of pulses, intensity, stimulation site etc.), number of sessions, follow-up, assessment through functional connectivity and neurological scales used. We observed a great interstudy heterogenicity that leads to a difficulty in drawing plain conclusions. TBS protocols have shown promising results in improving various symptoms in patients with neurological disorders, but larger and more coherent studies, using similar stimulation protocols and evaluation scales, are needed to establish guideline recommendations.

[Neurochemical mechanisms of tremor in Parkinson's disease]

Tremor is one of the main motor symptoms of Parkinson's disease, and its pathophysiology remains largely unknown. The clinical and pathomorphological heterogeneity of tremor and the not always response to therapy complicate the task of researchers and clinicians. This review discusses the specific degeneration of neurotransmitter systems driving the development of tremor, and the influence of neurotransmitters on specific anatomical entities according to current models explaining tremor. It is discusses how changes in neurotransmitter systems may influence the clinical diversity of tremor and differences in response to therapy. Data from clinical trials demonstrating the effect of the dopamine receptor agonist piribedil on tremor are presented.

Cerebellar Modulation of Sensorimotor Associative Plasticity Is Impaired in Cervical Dystonia

BACKGROUND

In recent years, cervical dystonia (CD) has been recognized as a network disorder that involves not only the basal ganglia but other brain regions, such as the primary motor and somatosensory cortex, brainstem, and cerebellum. So far, the role of the cerebellum in the pathophysiology of dystonia is only poorly understood.

OBJECTIVE

The objective of this study was to investigate the role of the cerebellum on sensorimotor associative plasticity in patients with CD.

METHODS

Sixteen patients with CD and 13 healthy subjects received cerebellar transcranial direct current stimulation (ctDCS) followed by a paired associative stimulation (PAS) protocol based on transcranial magnetic stimulation that induces sensorimotor associative plasticity. Across three sessions the participants received excitatory anodal, inhibitory cathodal, and sham ctDCS in a double-blind crossover design. Before and after the intervention, motor cortical excitability and motor symptom severity were assessed.

RESULTS

PAS induced an increase in motor cortical excitability in both healthy control subjects and patients with CD. In healthy subjects this effect was attenuated by both anodal and cathodal ctDCS with a stronger effect of cathodal stimulation. In patients with CD, anodal stimulation suppressed the PAS effect, whereas cathodal stimulation had no influence on PAS. Motor symptom severity was unchanged after the intervention.

CONCLUSIONS

Cerebellar modulation with cathodal ctDCS had no effect on sensorimotor associative plasticity in patients with CD, in contrast with the net inhibitory effect in healthy subjects. This is further evidence that the cerebello-thalamo-cortical network plays a role in the pathophysiology of dystonia. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Subtle changes in central dopaminergic tone underlie bradykinesia in essential tremor

INTRODUCTION

In this research, our primary objective was to explore the correlation between basal ganglia dopaminergic neurotransmission, assessed using 123I-FP-CIT (DAT-SPECT), and finger movements abnormalities in patients with essential tremor (ET) and Parkinson's disease (PD).

METHODS

We enrolled 16 patients with ET, 17 with PD, and 18 healthy controls (HC). Each participant underwent comprehensive clinical evaluations, kinematic assessments of finger tapping. ET and PD patients underwent DAT-SPECT imaging. The DAT-SPECT scans were subjected to both visual and semi-quantitative analysis using DaTQUANT®. We then investigated the correlations between the clinical, kinematic, and DAT-SPECT data, in patients.

RESULTS

Our findings confirm that individuals with ET exhibited slower finger tapping than HC. Visual evaluation of radiotracer uptake in both striata demonstrated normal levels within the ET patient cohort, while PD patients displayed reduced uptake. However, there was notable heterogeneity in the quantification of uptake within the striata among ET patients. Additionally, we found a correlation between the amount of radiotracer uptake in the striatum and movement velocity during finger tapping in patients. Specifically, lower radioligand uptake corresponded to decreased movement velocity (ET: coef. = 0.53, p-adj = 0.03; PD: coef. = 0.59, p-adj = 0.01).

CONCLUSION

The study's findings suggest a potential link between subtle changes in central dopaminergic tone and altered voluntary movement execution, in ET. These results provide further insights into the pathophysiology of ET. However, longitudinal studies are essential to determine whether the slight reduction in dopaminergic tone observed in ET patients represents a distinct subtype of the disease or could serve as a predictor for the clinical progression into PD.

Cerebellar Transcranial Alternating Current Stimulation in Essential Tremor Patients with Thalamic Stimulation: A Proof-of-Concept Study

Essential tremor (ET) is a disabling condition resulting from a dysfunction of cerebello-thalamo-cortical circuitry. Deep brain stimulation (DBS) or lesion of the ventral-intermediate thalamic nucleus (VIM) is an effective treatment for severe ET. Transcranial cerebellar brain stimulation has recently emerged as a non-invasive potential therapeutic option. Here, we aim to investigate the effects of high-frequency non-invasive cerebellar transcranial alternating current stimulation (tACS) in severe ET patients already operated for VIM-DBS. Eleven ET patients with VIM-DBS, and 10 ET patients without VIM-DBS and matched for tremor severity, were included in this double-blind proof-of-concept controlled study. All patients received unilateral cerebellar sham-tACS and active-tACS for 10 min. Tremor severity was blindly assessed at baseline, without VIM-DBS, during sham-tACS, during and at 0, 20, 40 min after active-tACS, using kinetic recordings during holding posture and action ('nose-to-target') task and videorecorded Fahn-Tolosa-Marin (FTM) clinical scales. In the VIM-DBS group, active-tACS significantly improved both postural and action tremor amplitude and clinical (FTM scales) severity, relative to baseline, whereas sham-tACS did not, with a predominant effect for the ipsilateral arm. Tremor amplitude and clinical severity were also not significantly different between ON VIM-DBS and active-tACS conditions. In the non-VIM-DBS group, we also observed significant improvements in ipsilateral action tremor amplitude, and clinical severity after cerebellar active-tACS, with a trend for improved postural tremor amplitude. In non-VIM-DBS group, sham- active-tACS also decreased clinical scores. These data support the safety and potential efficacy of high-frequency cerebellar-tACS to reduce ET amplitude and severity.

Transcranial Magnetic Stimulation in the Treatment of Neurological Diseases

Transcranial Magnetic Stimulation (TMS) has widespread use in research and clinical application. For psychiatric applications, such as depression or OCD, repetitive TMS protocols (rTMS) are an established and globally applied treatment option. While promising, rTMS is not yet as common in treating neurological diseases, except for neurorehabilitation after (motor) stroke and neuropathic pain treatment. This may soon change. New clinical studies testing the potential of rTMS in various other neurological conditions appear at a rapid pace. This can prove challenging for both practitioners and clinical researchers. Although most of these neurological applications have not yet received the same level of scientific/empirical scrutiny as motor stroke and neuropathic pain, the results are encouraging, opening new doors for TMS in neurology. We here review the latest clinical evidence for rTMS in pioneering neurological applications including movement disorders, Alzheimer's disease/mild cognitive impairment, epilepsy, multiple sclerosis, and disorders of consciousness.

Cerebellar noninvasive neuromodulation influences the reactivity of the contralateral primary motor cortex and surrounding areas: a TMS-EMG-EEG study

Understanding cerebellar-cortical physiological interactions is of fundamental importance to advance the efficacy of neurorehabilitation strategies for patients with cerebellar damage. Previous works have aimed to modulate this pathway by applying transcranial electrical or magnetic stimulation (TMS) over the cerebellum and probing the resulting changes in the primary motor cortex (M1) excitability with motor-evoked potentials (MEPs). While these protocols produce changes in cerebellar excitability, their ability to modulate MEPs has produced inconsistent results, mainly due to the MEP being a highly variable outcome measure that is susceptible to fluctuations in the excitability of M1 neurons and spinal interneurons. To overcome this limitation, we combined TMS with electroencephalography (EEG) to directly record TMS-evoked potentials (TEPs) and oscillations from the scalp. In three sessions, we applied intermittent theta-burst stimulation (iTBS), cathodal direct current stimulation (c-DC) or sham stimulation to modulate cerebellar activity. To assess the effects on M1 and nearby cortex, we recorded TMS-EEG and MEPs before, immediately after (T1) and 15 min (T2) following cerebellar neuromodulation. We found that cerebellar iTBS immediately increased TMS-induced alpha oscillations and produced lasting facilitatory effects on TEPs, whereas c-DC immediately decreased TMS-induced alpha oscillations and reduced TEPs. We also found increased MEP following iTBS but not after c-DC. All of the TMS-EEG measures showed high test-retest repeatability. Overall, this work importantly shows that cerebellar neuromodulation influences both cortical and corticospinal physiological measures; however, they are more pronounced and detailed when utilizing TMS-EEG outcome measures. These findings highlight the advantage of using TMS-EEG over MEPs when assessing the effects of neuromodulation.

Modulatory effect of continuous theta burst stimulation in patients with essential tremor

INTRODUCTION

We aimed to study the cortical and intracortical functions in patients of ET using transcranial magnetic stimulation (TMS) and to evaluate the effect of continuous theta burst stimulation (cTBS) on the tremor characteristics.

METHODS

Ten ET and 20 healthy controls were included in the study. All the participants were evaluated with TMS with recording of resting motor threshold (RMT), central motor conduction time, contralateral silent period (cSP), short interval intracortical inhibition (SICI) and intracortical facilitation (ICF). Subsequently only ET patients underwent cTBS of the motor cortex (M1) followed by repeat TMS.

RESULTS

The mean age of the patients (46.5 ± 17.2 years) was comparable to healthy controls (55.4 ± 9.2 years; p = 0.16). There was a non-significant increase in RMT in ET patients (44 ± 12.5%) when compared to healthy controls (40.9 ± 6.9%; p = 0.48). There was a significant reduction of cSP in the ET group (102.03 ± 15.26 msec) compared to healthy controls (116.1 ± 15.2, p = 0.03). In addition, a significant reduction in ICF was observed in ET patients (0.9 ± 0.7) compared to healthy controls (1.8 ± 0.8, p = 0.01). Following cTBS there was a significant reduction in the tremor scores [FTMRS (Pre-cTBS: 29.3 ± 18.7, Post-cTBS: 25.3 ± 16.8; p < 0.001) and TETRAS (pre-cTBS: 34.4 ± 16.2, post-cTBS: 29.8 ± 12.1; p = 0.01)] and improvement (increase) of the duration of cSP (pre-cTBS: 102.03 ± 15.3 msec., post-cTBS: 119.4 ± 12.03 msec; p = 0.05).

CONCLUSIONS

Patients with ET have GABAergic and glutaminergic dysfunction as demonstrated by reduced cSP and ICF. However, only the cSP improved following cTBS of M1 region, with a corresponding improvement of tremor severity suggesting the effect of cTBS on the cerebello-thalamo-cortical network.

The Contribution of Neuroimaging to the Understanding of Essential Tremor Pathophysiology: a Systematic Review

Essential tremor (ET) is one of the most common movement disorders. Over the last 10 years, magnetic resonance imaging (MRI) has shed light on the structural and functional abnormalities possibly involved in ET pathophysiology. In this systematic review, we aimed to identify the cortical and subcortical structures involved and the role that different brain areas play in the pathophysiology of motor and non-motor ET features. We found that structural (grey and white matter) cerebellar damage and connectivity alterations between the cerebellum and various cortical areas play a role in both motor and non-motor symptoms of ET. In particular, many studies found an association between MRI findings and non-motor symptoms.

Parietal Cortex Connectivity as a Marker of Shift in Spatial Attention Following Continuous Theta Burst Stimulation

Non-invasive brain stimulation is a useful tool to probe brain function and provide therapeutic treatments in disease. When applied to the right posterior parietal cortex (PPC) of healthy participants, it is possible to temporarily shift spatial attention and mimic symptoms of spatial neglect. However, the field of brain stimulation is plagued by issues of high response variability. The aim of this study was to investigate baseline functional connectivity as a predictor of response to an inhibitory brain stimulation paradigm applied to the right PPC. In fourteen healthy adults (9 female, aged 24.8 ± 4.0 years) we applied continuous theta burst stimulation (cTBS) to suppress activity in the right PPC. Resting state functional connectivity was quantified by recording electroencephalography and assessing phase consistency. Spatial attention was assessed before and after cTBS with the Landmark Task. Finally, known determinants of response to brain stimulation were controlled for to enable robust investigation of the influence of resting state connectivity on cTBS response. We observed significant inter-individual variability in the behavioral response to cTBS with 53.8% of participants demonstrating the expected rightward shift in spatial attention. Baseline high beta connectivity between the right PPC, dorsomedial pre-motor region and left temporal-parietal region was strongly associated with cTBS response (R² = 0.51). Regression analysis combining known cTBS determinants (age, sex, motor threshold, physical activity, stress) found connectivity between the right PPC and left temporal-parietal region was the only significant variable (p = 0.011). These results suggest baseline resting state functional connectivity is a strong predictor of a shift in spatial attention following cTBS. Findings from this study help further understand the mechanism by which cTBS modifies cortical function and could be used to improve the reliability of brain stimulation protocols.

Effects of low-frequency repetitive transcranial magnetic stimulation combined with cerebellar continuous theta burst stimulation on spasticity and limb dyskinesia in patients with stroke

Background

Repetitive transcranial magnetic stimulation (rTMS) has been reported to treat muscle spasticity in post-stroke patients. The purpose of this study was to explore whether combined low-frequency rTMS (LF-rTMS) and cerebellar continuous theta burst stimulation (cTBS) could provide better relief than different modalities alone for muscle spasticity and limb dyskinesia in stroke patients.

Methods

This study recruited ninety stroke patients with hemiplegia, who were divided into LF-rTMS+cTBS group (n=30), LF-rTMS group (n=30) and cTBS group (three pulse bursts at 50 Hz, n=30). The LF-rTMS group received 1 Hz rTMS stimulation of the motor cortical (M1) region on the unaffected side of the brain, the cTBS group received cTBS stimulation to the cerebellar region, and the LF-rTMS+cTBS group received 2 stimuli as described above. Each group received 4 weeks of stimulation followed by rehabilitation. Muscle spasticity, motor function of limb and activity of daily living (ADL) were evaluated by modified Ashworth Scale (MAS), Fugl-Meyer Assessment (FMA) and Modified Barthel Index (MBI) scores, respectively.

Results

The MAS score was markedly decreased, FMA and MBI scores were markedly increased in the three groups after therapy than before therapy. In addition, after therapy, LF-rTMS+cTBS group showed lower MAS score, higher FMA and MBI scores than the LF-rTMS group and cTBS group.

Conclusion

Muscle spasticity and limb dyskinesia of the three groups are all significantly improved after therapy. Combined LF-rTMS and cTBS treatment is more effective in improving muscle spasticity and limb dyskinesia of patients after stroke than LF-rTMS and cTBS treatment alone.

Transcranial Magnetic Stimulation in Tremor Syndromes: Pathophysiologic Insights and Therapeutic Role

Transcranial magnetic stimulation (TMS) is a painless, non-invasive, and established brain stimulation technique to investigate human brain function. Over the last three decades, TMS has shed insight into the pathophysiology of many neurological disorders. Tremor is an involuntary, rhythmic oscillatory movement disorder commonly related to pathological oscillations propagated via the cerebello-thalamo-cortical pathway. Although tremor is the most common movement disorder and recent imaging studies have enhanced our understanding of the critical pathogenic networks, the underlying pathophysiology of different tremor syndromes is complex and still not fully understood. TMS has been used as a tool to further our understanding of tremor pathophysiology. In addition, repetitive TMS (rTMS) that can modulate brain functions through plasticity effects has been targeted to the tremor network to gain potential therapeutic benefits. However, evidence is available for only a few studies that included small patient samples with limited clinical follow-up. This review aims to discuss the role of TMS in advancing the pathophysiological understanding as well as emerging applications of rTMS for treating individual tremor syndromes. The review will focus on essential tremor, Parkinson's disease tremor, dystonic tremor syndrome, orthostatic tremor, and functional tremor.

Use of non-invasive stimulation in movement disorders: a critical review

BACKGROUND

Noninvasive stimulation has been widely used in the past 30 years to study and treat a large number of neurological diseases, including movement disorders.

OBJECTIVE

In this critical review, we illustrate the rationale for use of these techniques in movement disorders and summarize the best medical evidence based on the main clinical trials performed to date.

METHODS

A nationally representative group of experts performed a comprehensive review of the literature in order to analyze the key clinical decision-making factors driving transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) in movement disorders. Classes of evidence and recommendations were described for each disease.

RESULTS

Despite unavoidable heterogeneities and low effect size, TMS is likely to be effective for treating motor symptoms and depression in Parkinson's disease (PD). The efficacy in other movement disorders is unclear. TMS is possibly effective for focal hand dystonia, essential tremor and cerebellar ataxia. Additionally, it is likely to be ineffective in reducing tics in Tourette syndrome. Lastly, tDCS is likely to be effective in improving gait in PD.

CONCLUSIONS

There is encouraging evidence for the use of noninvasive stimulation on a subset of symptoms in selected movement disorders, although the means to optimize protocols for improving positive outcomes in routine clinical practice remain undetermined. Similarly, the best stimulation paradigms and responder profile need to be investigated in large clinical trials with established therapeutic and assessment paradigms that could also allow genuine long-term benefits to be determined.

A narrative review on non-invasive stimulation of the cerebellum in neurological diseases

IMPORTANCE

The cerebellum plays an important role in motor, cognitive, and affective functions owing to its dense interconnections with basal ganglia and cerebral cortex. This review aimed at summarizing the non-invasive cerebellar stimulation (NICS) approaches used to modulate cerebellar output and treat cerebellar dysfunction in the motor domain.

OBSERVATION

The utility of NICS in the treatment of cerebellar and non-cerebellar neurological diseases (including Parkinson's disease, dementia, cerebellar ataxia, and stroke) is discussed. NICS induces meaningful clinical effects from repeated sessions alone in both cerebellar and non-cerebellar diseases. However, there are no conclusive data on this issue and several concerns need to be still addressed before NICS could be considered a valuable, standard therapeutic tool.

CONCLUSIONS AND RELEVANCE

Even though some challenges must be overcome to adopt NICS in a wider clinical setting, this tool might become a useful strategy to help patients with lesions in the cerebellum and cerebral areas that are connected with the cerebellum whether one could enhance cerebellar activity with the intention of facilitating the cerebellum and the entire, related network, rather than attempting to facilitate a partially damaged cortical region or inhibiting the homologs' contralateral area. The different outcome of each approach would depend on the residual functional reserve of the cerebellum, which is confirmed as a critical element to be probed preliminary in order to define the best patient-tailored NICS.

Little Brain, Big Expectations

The cerebellum has been implicated in the mechanisms of several movement disorders. With the recent reports of successful modulation of its functioning, this highly connected structure has emerged as a promising way to provide symptomatic relief not yet obtained by usual treatments. Here we review the most relevant papers published to date, the limitations and gaps in literature, discuss why several papers have failed in showing efficacy, and present a new way of stimulating the cerebellum. References for this critique review were identified by searches on PubMed for the terms “Parkinson’s disease”, “ataxia”, “dystonia”, “tremor”, and “dyskinesias” in combination with the type of stimulation and the stimulation site. Studies conducted thus far have shed light on the potential of cerebellar neuromodulation for attenuating symptoms in patients with some forms of isolated and combined dystonia, dyskinesia in Parkinson’s disease, and neurodegenerative ataxia. However, there is still a high heterogeneity of results and uncertainty about the possibility of maintaining long-term benefits. Because of the complicated architecture of the cerebellum, the modulation techniques employed may have to focus on targeting the activity of the cerebellar nuclei rather than the cerebellar cortex. Measures of cerebellar activity may reduce the variability in outcomes.

Safety Considerations for Cerebellar Theta Burst Stimulation

PURPOSE

The cerebellum is an intricate neural structure that orchestrates various cognitive and behavioral functions. In recent years, there has been an increasing interest in neuromodulation of the cerebellum with transcranial magnetic stimulation (TMS) for therapeutic and basic science applications. Theta burst stimulation (TBS) is an efficient and powerful TMS protocol that is able to induce longer-lasting effects with shorter stimulation times compared with traditional TMS. Parameters for cerebellar TBS are traditionally framed in the bounds of TBS to the cerebral cortex, even when the 2 have distinct histologic, anatomical, and functional characteristics. Tolerability limits have not been systematically explored in the literature for this specific application. Therefore, we aimed to determine the stimulation parameters that have been used for cerebellar. TBS to date and evaluate adverse events and adverse effects related to stimulation parameters.

METHODS

We used PubMed to perform a critical review of the literature based on a systematic review of original research studies published between September 2008 and November 2019 that reported on cerebellar TBS. We recovered information from these publications and communication with authors about the stimulation parameters used and the occurrence of adverse events.

FINDINGS

We identified 61 research articles on interventions of TBS to the cerebellum. These articles described 3176 active sessions of cerebellar TBS in 1203 individuals, including healthy participants and patients with various neurologic conditions, including brain injuries. Some studies used substantial doses (eg, pulse intensity and number of pulses) in short periods. No serious adverse events were reported. The specific number of patients who experienced adverse events was established for 48 studies. The risk of an adverse event in this population (n = 885) was 4.1%. Adverse events consisted mostly of discomfort attributable to involuntary muscle contractions. Authors used a variety of methods for calculating stimulation dosages, ranging from the long-established reference of electromyography of a hand muscle to techniques that atone for some of the differences between cerebrum and cerebellum.

IMPLICATIONS

No serious adverse events have been reported for cerebellar TBS. There is no substantial evidence of a tolerable maximal-efficacy stimulation dose in humans. There is no assurance of equivalence in the translation of cortical excitability and stimulation intensities from the cerebral cortex to cerebellar regions. Further research for the stimulation dose in cerebellar TBS is warranted, along with consistent report of adverse events. © 2020 Elsevier HS Journals, Inc.

Tremor Distribution and the Variable Clinical Presentation of Essential Tremor

In addition to having postural and kinetic tremor of the upper limbs, some patients with essential tremor (ET) may have head tremor as well as cognitive and psychiatric disorders. We aimed to investigate whether the variable clinical presentation in ET patients, including motor and non-motor symptoms, differs in patients with and without head tremor. We consecutively enrolled 70 patients with a diagnosis of ET. Tremor severity was assessed by means of clinical rating scales. Patients also underwent kinematic recordings of postural and kinetic tremor of the upper limbs based on an optoelectronic system. Several neuropsychological tests were also administered. Finally, we adopted the structured interviews for DSM-IV, SCID-I, and SCID-II to investigate psychiatric and personality disorders. ET patients with upper limb tremor plus head tremor exhibited more severe kinetic tremor of the upper limbs and a higher occurrence of axis I psychiatric disorders than ET patients with upper limb tremor only. Cognitive and other motor and psychiatric features did not differ significantly with respect to tremor distribution. The study findings support the hypothesis that body tremor distribution, i.e., the presence of head tremor, influences the variable clinical presentation of ET. The study results support the notion that cases with head tremor may represent a distinct ET subtype, characterized by a prominent cerebellar involvement, and that psychiatric disorders should be considered as a specific manifestation of ET.

Evidence-based guidelines on the therapeutic use of repetitive transcranial magnetic stimulation (rTMS): An update (2014-2018)

A group of European experts reappraised the guidelines on the therapeutic efficacy of repetitive transcranial magnetic stimulation (rTMS) previously published in 2014 [Lefaucheur et al., Clin Neurophysiol 2014;125:2150-206]. These updated recommendations take into account all rTMS publications, including data prior to 2014, as well as currently reviewed literature until the end of 2018. Level A evidence (definite efficacy) was reached for: high-frequency (HF) rTMS of the primary motor cortex (M1) contralateral to the painful side for neuropathic pain; HF-rTMS of the left dorsolateral prefrontal cortex (DLPFC) using a figure-of-8 or a H1-coil for depression; low-frequency (LF) rTMS of contralesional M1 for hand motor recovery in the post-acute stage of stroke. Level B evidence (probable efficacy) was reached for: HF-rTMS of the left M1 or DLPFC for improving quality of life or pain, respectively, in fibromyalgia; HF-rTMS of bilateral M1 regions or the left DLPFC for improving motor impairment or depression, respectively, in Parkinson's disease; HF-rTMS of ipsilesional M1 for promoting motor recovery at the post-acute stage of stroke; intermittent theta burst stimulation targeted to the leg motor cortex for lower limb spasticity in multiple sclerosis; HF-rTMS of the right DLPFC in posttraumatic stress disorder; LF-rTMS of the right inferior frontal gyrus in chronic post-stroke non-fluent aphasia; LF-rTMS of the right DLPFC in depression; and bihemispheric stimulation of the DLPFC combining right-sided LF-rTMS (or continuous theta burst stimulation) and left-sided HF-rTMS (or intermittent theta burst stimulation) in depression. Level A/B evidence is not reached concerning efficacy of rTMS in any other condition. The current recommendations are based on the differences reached in therapeutic efficacy of real vs. sham rTMS protocols, replicated in a sufficient number of independent studies. This does not mean that the benefit produced by rTMS inevitably reaches a level of clinical relevance.

The role of the cerebellum in degenerative ataxias and essential tremor: Insights from noninvasive modulation of cerebellar activity

Over the last three decades, measuring and modulating cerebellar activity and its connectivity with other brain regions has become an emerging research topic in clinical neuroscience. The most important connection is the cerebellothalamocortical pathway, which can be functionally interrogated using a paired‐pulse transcranial magnetic stimulation paradigm. Cerebellar brain inhibition reflects the magnitude of suppression of motor cortex excitability after stimulating the contralateral cerebellar hemisphere and therefore represents a neurophysiological marker of the integrity of the efferent cerebellar tract. Observations that cerebellar noninvasive stimulation techniques enhanced performance of certain motor and cognitive tasks in healthy individuals have inspired attempts to modulate cerebellar activity and connectivity in patients with cerebellar diseases in order to achieve clinical benefit. We here comprehensively explore the therapeutic potential of these techniques in two movement disorders characterized by prominent cerebellar involvement, namely the degenerative ataxias and essential tremor. The article aims to illustrate the (patho)physiological insights obtained from these studies and how these translate into clinical practice, where possible by addressing the association with cerebellar brain inhibition. Finally, possible explanations for some discordant interstudy findings, shortcomings in our current understanding, and recommendations for future research will be provided. © 2019 The Authors. Movement Disorders published by Wiley Periodicals, Inc. on behalf of International Parkinson and Movement Disorder Society.

Does cerebellar non-invasive brain stimulation affect corticospinal excitability in healthy individuals? A systematic review of literature and meta-analysis

Numerous studies have indicated that non-invasive brain stimulation (NIBS) of the cerebellum could modulate corticospinal excitability (CSE) in young healthy individuals. However, there is no systematic review and meta-analysis that clarifies the effects of cerebellar NIBS on CSE. The aim of this study was to provide a meta-analytic summary of the effects of cerebellar NIBS on CSE. Seven search engines were used to identify any trial evaluating CSE before and after one session of cerebellar NIBS in healthy individuals up to June 2018. Twenty-six studies investigating the corticospinal responses following cerebellar NIBS were included. Meta-analysis was used to pool the findings from included studies. Effects were expressed as mean differences (MD) and the standard deviation (SD). Risk of bias was assessed with the Cochrane tool. Meta-analysis found that paired associative stimulation (PAS) with 2 ms interval, a combination of PAS with 21.5 ms interval and anodal transcranial direct current stimulation, and repetitive transcranial magnetic stimulation with a frequency of < 5 Hz increase CSE (P _PAS2 < 0.00001, P _{PAS21.5 +a-tDCS} = 0.02, P _rTMS = 0.04). However, continuous theta burst stimulation, a combination of PAS with 25 ms interval and anodal transcranial direct current stimulation, and PAS with a 6 ms interval decreased CSE (P _PAS6 < 0.00001, P _cTBS < 0.00001, P _{PAS25 +a-tDCS} = 0.003). The results of this review show that cerebellar NIBS techniques are a promising tool for increasing CSE.

The use of transcranial magnetic stimulation as a treatment for movement disorders: A critical review

BACKGROUND

Transcranial magnetic stimulation is a safe and painless non-invasive brain stimulation technique that has been largely used in the past 30 years to explore cortical function in healthy participants and, inter alia, the pathophysiology of movement disorders. During the years, its use has evolved from primarily research purposes to treatment of a large variety of neurological and psychiatric diseases. In this article, we illustrate the basic principles on which the therapeutic use of transcranial magnetic stimulation is based and review the clinical trials that have been performed in patients with movement disorders.

METHODS

A search of the PubMed database for research and review articles was performed on therapeutic applications of transcranial magnetic stimulation in movement disorders. The search included the following conditions: Parkinson's disease, dystonia, Tourette syndrome and other chronic tic disorders, Huntington's disease and choreas, and essential tremor. The results of the studies and possible mechanistic explanations for the relatively minor effects of transcranial magnetic stimulation are discussed. Possible ways to improve the methodology and achieve greater therapeutic efficacy are discussed.

CONCLUSION

Despite the promising and robust rationales for the use of transcranial magnetic stimulations as a treatment tool in movement disorders, the results taken as a whole are not as successful as were initially expected. There is encouraging evidence that transcranial magnetic stimulation may improve motor symptoms and depression in Parkinson's disease, but the efficacy in other movement disorders is unclear. Possible improvements in methodology are on the horizon but have yet to be implemented in large clinical studies. © 2019 International Parkinson and Movement Disorder Society © 2019 International Parkinson and Movement Disorder Society.

Cerebellar Theta-Burst Stimulation Impairs Memory Consolidation in Eyeblink Classical Conditioning

Associative learning of sensorimotor contingences, as it occurs in eyeblink classical conditioning (EBCC), is known to involve the cerebellum, but its mechanism remains controversial. EBCC involves a sequence of learning processes which are thought to occur in the cerebellar cortex and deep cerebellar nuclei. Recently, the extinction phase of EBCC has been shown to be modulated after one week by cerebellar continuous theta-burst stimulation (cTBS). Here, we asked whether cerebellar cTBS could affect retention and reacquisition of conditioned responses (CRs) tested immediately after conditioning. We also investigated a possible lateralized cerebellar control of EBCC by applying cTBS on both the right and left cerebellar hemispheres. Both right and left cerebellar cTBSs induced a statistically significant impairment in retention and new acquisition of conditioned responses (CRs), the disruption effect being marginally more effective when the left cerebellar hemisphere was stimulated. These data support a model in which cTBS impairs retention and reacquisition of CR in the cerebellum, possibly by interfering with the transfer of memory to the deep cerebellar nuclei.

Differential effects of propranolol on head and upper limb tremor in patients with essential tremor and dystonia

Propranolol is used as the first-line treatment in essential tremor and it has also been proposed as a treatment for tremor in dystonia. However, several issues remain uncertain. For example, it is still not clear whether propranolol exerts a beneficial effect on head tremor. Moreover, no studies have investigated whether the effect of propranolol on head and upper limb tremor in essential tremor differs from that in dystonia. We aimed to assess the effects of propranolol on tremor in different body parts in essential tremor and in patients with tremor and dystonia. Twenty-nine patients with head and upper limb tremor were enrolled in the study, 14 with essential tremor, and 15 with dystonia. Participants underwent a clinical and kinematic analysis of tremor in two sessions, i.e., without (baseline) and 'on therapy' with propranolol. We found that head tremor was more severe in patients with dystonia, while upper limb tremor was more evident in patients with essential tremor (P < 0.05). Propranolol had no effect on head tremor in either group (all Ps > 0.05), but it did reduce upper limb tremor in patients with essential tremor. The present study demonstrates differential effects of propranolol on head and upper limb tremor in patients with essential tremor. The lack of effect on head and upper limb tremor in patients with dystonia suggests that the pathophysiological mechanisms underlying tremor in these two conditions and in different body parts may be distinct.

Variability and Predictors of Response to Continuous Theta Burst Stimulation: A TMS-EEG Study

Continuous theta-burst stimulation (cTBS) is a repetitive transcranial magnetic stimulation paradigm reported to decrease the excitability of the stimulated cortical area and which is thought to reflect a form of inhibitory synaptic plasticity. However, since its introduction, the effect of cTBS has shown a remarkable variability in its effects, which are often quantified by measuring the amplitude of motor evoked potentials (MEPs). Part of this inconsistency in experimental results might be due to an intrinsic variability of TMS effects caused by genetic or neurophysiologic factors. However, it is also possible that MEP only reflect the excitability of a sub-population of output neurons; resting EEG power and measures combining TMS and electroencephalography (TMS-EEG) might represent a more thorough reflection of cortical excitability. The aim of the present study was to verify the robustness of several predictors of cTBS response, such as I wave recruitment and baseline MEP amplitude, and to test cTBS after-effects on multiple neurophysiologic measurements such as MEP, resting EEG power, local mean field power (LMFP), TMS-related spectral perturbation (TRSP), and inter-trial phase clustering (ITPC). As a result, we were not able to confirm either the expected decrease of MEP amplitude after cTBS or the ability of I wave recruitment and MEP amplitude to predict the response to cTBS. Resting EEG power, LMFP, TRSP, and ITPC showed a more consistent trend toward a decrease after cTBS. Overall, our data suggest that the effect of cTBS on corticospinal excitability is variable and difficult to predict with common electrophysiologic markers, while its effect might be clearer when probed with combined TMS and EEG.

Effects of cerebellar neuromodulation in movement disorders: A systematic review

BACKGROUND

The cerebellum is involved in the pathophysiology of many movement disorders and its importance in the field of neuromodulation is growing.

OBJECTIVES

To review the current evidence for cerebellar modulation in movement disorders and its safety profile.

METHODS

Eligible studies were identified after a systematic literature review of the effects of cerebellar modulation in cerebellar ataxia, Parkinson's disease (PD), essential tremor (ET), dystonia and progressive supranuclear palsy (PSP). Neuromodulation techniques included transcranial magnetic stimulation (TMS), transcranial direct current stimulation (tDCS) and deep brain stimulation (DBS). The changes in motor scores and the incidence of adverse events after the stimulation were reviewed.

RESULTS

Thirty-four studies were included in the systematic review, comprising 431 patients. The evaluation after stimulation ranged from immediately after to 12 months after. Neuromodulation techniques improved cerebellar ataxia due to vascular or degenerative etiologies (TMS, tDCS and DBS), dyskinesias in PD patients (TMS), gross upper limb movement in PD patients (tDCS), tremor in ET (TMS and tDCS), cervical dystonia (TMS and tDCS) and dysarthria in PSP patients (TMS). All the neuromodulation techniques were safe, since only three studies reported the existence of side effects (slight headache after TMS, local skin erythema after tDCS and infectious complication after DBS). Eleven studies did not mention if adverse events occurred.

CONCLUSIONS

Cerebellar modulation can improve specific symptoms in some movement disorders and is a safe and well-tolerated procedure. Further studies are needed to lay the groundwork for new researches in this promising target.

Does non-invasive brain stimulation reduce essential tremor? A systematic review and meta-analysis

Essential tremor (ET) is the most common age-related disease leading to abnormal tremulous behaviors in the upper and lower extremities. Non-invasive brain stimulation (NIBS) may be an effective ET therapy by modulating the oscillating network of the brain. The current systematic review and meta-analysis examined the effects of NIBS interventions on tremor symptoms in ET patients. Our comprehensive search identified eight studies that used 1 Hz of rTMS, cTBS, or ctDCS protocols. Twenty total comparisons from the eight qualified studies were statistically synthesized, and the meta-analytic findings revealed that NIBS techniques reduced tremulous behaviors in individuals with ET. Moreover, the four moderator variable analyses demonstrated that the positive therapeutic effects of NIBS appeared across the following subgroups: (a) tremor assessment (clinical test vs. quantitative tremor assessment), (b) stimulation site (cerebellum vs. motor cortex), (c) session number (single session vs. multiple sessions), and (d) sustained positive treatment effect (posttest vs. retention test). This comprehensive systematic review and meta-analysis provided evidence that support positive treatment effects of NIBS techniques on ET motor therapy.

Body Sway Increases After Functional Inactivation of the Cerebellar Vermis by cTBS

Balance stability correlates with cerebellar vermis volume. Furthermore, the cerebellum is involved in precise timing of motor processes by fine-tuning the sensorimotor integration. We tested the hypothesis that any cerebellar action in stance control and in timing of visuomotor integration for balance is impaired by continuous theta-burst stimulation (cTBS) of the vermis. Ten subjects stood quietly and underwent six sequences of 10-min acquisition of center of foot pressure (CoP) data after cTBS, sham stimulation, and no stimulation. Visual shifts from eyes closed (EC) to eyes open (EO) and vice versa were presented via electronic goggles. Mean anteroposterior and mediolateral CoP position and oscillation, and the time delay at which body sway changed after visual shift were calculated. CoP position under both EC and EO condition was not modified after cTBS. Sway path length was greater with EC than EO and increased in both visual conditions after cTBS. CoP oscillation was also larger with EC and increased under both visual conditions after cTBS. The delay at which body oscillation changed after visual shift was longer after EC to EO than EO to EC, but unaffected by cTBS. The time constant of decrease or increase of oscillation was longer in EC to EO shifts, but unaffected by cTBS. Functional inactivation of the cerebellar vermis is associated with increased sway. Despite this, cTBS does not detectably modify onset and time course of the sensorimotor integration process of adaptation to visual shifts. Cerebellar vermis normally controls oscillation, but not timing of adaptation to abrupt changes in stabilizing information.

Non-invasive Brain Stimulation for Essential Tremor

Background

There is increasing interest in the use of non-invasive brain stimulation to characterize and potentially treat essential tremor (ET). Studies have used a variety of stimulation coils, paradigms, and target locations to make these observations. We reviewed the literature to compare prior studies and to evaluate the rationale and the methods used in these studies.

Methods

We performed a systematic literature search of the PubMed database using the terms “transcranial,” “noninvasive,” “brain stimulation,” “transcranial magnetic stimulation (TMS),” “transcranial direct current stimulation (tDCS),” “transcranial alternating current stimulation (tACS),” and “essential tremor.”

Results

Single pulses of TMS to the primary motor cortex have long been known to reset tremor. Although there are relatively few studies showing alterations in motor cortical physiology, such as motor threshold, short and long intracortical inhibition, and cortical silent period, there may be some evidence of altered intracortical facilitation and cerebello-brain inhibition in ET. Repetitive TMS, theta burst stimulation, tDCS, and tACS have been applied to human subjects with tremor with some preliminary signs of tremor reduction, particularly in those studies that employed consecutive daily sessions.

Discussion

A variety of stimulation paradigms and targets have been explored, with the increasing rationale an interest in targeting the cerebellum. Rigorous assessment of coil geometry, stimulation paradigm, rationale for selection of the specific anatomic target, and careful phenotypic and physiologic characterization of the subjects with ET undergoing these interventions may be critical in extending these preliminary findings into effective stimulation therapies.

Cerebellar Intermittent Theta-Burst Stimulation and Motor Control Training in Individuals with Cervical Dystonia

Background: There is emerging evidence that cervical dystonia is a neural network disorder with the cerebellum as a key node. The cerebellum may provide a target for neuromodulation as a therapeutic intervention in cervical dystonia. Objective: This study aimed to assess effects of intermittent theta-burst stimulation of the cerebellum on dystonia symptoms, quality of life, hand motor dexterity and cortical neurophysiology using transcranial magnetic stimulation. Methods: Sixteen participants with cervical dystonia were randomised into real or sham stimulation groups. Cerebellar neuromodulation was combined with motor training for the neck and an implicit learning task. The intervention was delivered over 10 working days. Outcome measures included dystonia severity and pain, quality of life, hand dexterity, and motor-evoked potentials and cortical silent periods recorded from upper trapezius muscles. Assessments were taken at baseline and after 5 and 10 days, with quality of life also measured 4 and 12 weeks later. Results: Intermittent theta-burst stimulation improved dystonia severity (Day 5, −5.44 points; p = 0.012; Day 10, −4.6 points; p = 0.025), however, effect sizes were small. Quality of life also improved (Day 5, −10.6 points, p = 0.012; Day 10, −8.6 points, p = 0.036; Week 4, −12.5 points, p = 0.036; Week 12, −12.4 points, p = 0.025), with medium or large effect sizes. There was a reduction in time to complete the pegboard task pre to post intervention (both p < 0.008). Cortical neurophysiology was unchanged by cerebellar neuromodulation. Conclusion: Intermittent theta-burst stimulation of the cerebellum may improve cervical dystonia symptoms, upper limb motor control and quality of life. The mechanism likely involves promoting neuroplasticity in the cerebellum although the neurophysiology remains to be elucidated. Cerebellar neuromodulation may have potential as a novel treatment intervention for cervical dystonia, although larger confirmatory studies are required.

Non-invasive brain stimulation as a tool to study cerebellar-M1 interactions in humans

The recent development of non-invasive brain stimulation techniques such as transcranial magnetic stimulation (TMS) has allowed the non-invasive assessment of cerebellar function in humans. Early studies showed that cerebellar activity, as reflected in the excitability of the dentate-thalamo-cortical pathway, can be assessed with paired stimulation of the cerebellum and the primary motor cortex (M1) (cerebellar inhibition of motor cortex, CBI). Following this, many attempts have been made, using techniques such as repetitive TMS and transcranial electrical stimulation (TES), to modulate the activity of the cerebellum and the dentate-thalamo-cortical output, and measure their impact on M1 activity. The present article reviews literature concerned with the impact of non-invasive stimulation of cerebellum on M1 measures of excitability and "plasticity" in both healthy and clinical populations. The main conclusion from the 27 reviewed articles is that the effects of cerebellar "plasticity" protocols on M1 activity are generally inconsistent. Nevertheless, two measurements showed relatively reproducible effects in healthy individuals: reduced response of M1 to sensorimotor "plasticity" (paired-associative stimulation, PAS) and reduced CBI following repetitive TMS and TES. We discuss current challenges, such as the low power of reviewed studies, variability in stimulation parameters employed and lack of understanding of physiological mechanisms underlying CBI.

Effects of cerebellar theta-burst stimulation on arm and neck movement kinematics in patients with focal dystonia

OBJECTIVE

To investigate the cerebellar inhibitory influence on the primary motor cortex in patients with focal dystonia using a cerebellar continuous theta-burst stimulation protocol (cTBS) and to evaluate any relationship with movement abnormalities.

METHODS

Thirteen patients with focal hand dystonia, 13 patients with cervical dystonia and 13 healthy subjects underwent two sessions: (i) cTBS over the cerebellar hemisphere (real cTBS) and (ii) cTBS over the neck muscles (sham cTBS). The effects of cerebellar cTBS were quantified as excitability changes in the contralateral primary motor cortex, as well as possible changes in arm and neck movements in patients.

RESULTS

Real cerebellar cTBS reduced the excitability in the contralateral primary motor cortex in healthy subjects and in patients with cervical dystonia, though not in patients with focal hand dystonia. There was no correlation between changes in primary motor cortex excitability and arm and neck movement kinematics in patients. There were no changes in clinical scores or in kinematic measures, after either real or sham cerebellar cTBS in patients.

CONCLUSIONS

The reduced cerebellar inhibitory modulation of primary motor cortex excitability in focal dystonia may be related to the body areas affected by dystonia as opposed to being a widespread pathophysiological abnormality.

SIGNIFICANCE

The present study yields information on the differential role played by the cerebellum in the pathophysiology of different focal dystonias.

Non-invasive Central and Peripheral Stimulation: New Hope for Essential Tremor?

Essential tremor (ET) is among the most frequent movement disorders. It usually manifests as a postural and kinematic tremor of the arms, but may also involve the head, voice, lower limbs, and trunk. An oscillatory network has been proposed as a neural correlate of ET, and is mainly composed of the olivocerebellar system, thalamus, and motor cortex. Since pharmacological agents have limited benefits, surgical interventions like deep brain stimulation are the last-line treatment options for the most severe cases. Non-invasive brain stimulation techniques, particularly transcranial magnetic or direct current stimulation, are used to ameliorate ET. Their non-invasiveness, along with their side effects profile, makes them an appealing treatment option. In addition, peripheral stimulation has been applied in the same perspective. Hence, the aim of the present review is to shed light on the emergent use of non-invasive central and peripheral stimulation techniques in this interesting context.