Modulation of postural wrist tremors by magnetic stimulation of the motor cortex in patients with Parkinson's disease or essential tremor and in normal subjects mimicking tremor

The role of laboratory investigations in the classification of tremors

INTRODUCTION

Tremor is the most common movement disorder. Although clinical examination plays a significant role in evaluating patients with tremor, laboratory tests are useful to classify tremors according to the recent two-axis approach proposed by the International Parkinson and Movement Disorders Society.

METHODS

In the present review, we will discuss the usefulness and applicability of the various diagnostic methods in classifying and diagnosing tremors. We will evaluate a number of techniques, including laboratory and genetic tests, neurophysiology, and neuroimaging. The role of newly introduced innovative tremor assessment methods will also be discussed.

RESULTS

Neurophysiology plays a crucial role in tremor definition and classification, and it can be useful for the identification of specific tremor syndromes. Laboratory and genetic tests and neuroimaging may be of paramount importance in identifying specific etiologies. Highly promising innovative technologies are being developed for both clinical and research purposes.

CONCLUSIONS

Overall, laboratory investigations may support clinicians in the diagnostic process of tremor. Also, combining data from different techniques can help improve understanding of the pathophysiological bases underlying tremors and guide therapeutic management.

Electromagnetic radiation therapy for Parkinson's disease tremor reduction- systematic reviews and Bayesian meta-analyses for comparing the effectiveness of electric, magnetic and light stimulation methods

PURPOSE

Tremor is one of the key characteristics of Parkinson's disease (PD), leading to physical disabilities and often showing limited responses to pharmacological treatments. To suppress tremors in PD patients, several types of non-invasive and non-pharmacological methods have been proposed so far. In the current systematic review, three electromagnetic-based radiation strategies including electrical stimulation, magnetic stimulation, and light stimulation methods were reviewed and compared.

METHODS

Major databases were searched to retrieve eligible studies. For the meta-analysis, a random-effect Bayesian framework was used. Also, heterogeneity between studies was assessed using I2 statistic, prediction interval, and tau2. Publication bias was assessed using funnel plot, and the effectiveness of methods for reducing tremor was compared using network Bayesian meta-analysis.

RESULTS AND CONCLUSION

Thirty-one studies were found for qualitative analysis, and 16 studies were found for quantitative synthesis. Based on the suppression ratio, methods can be ordered as electrical stimulation, light therapy, and magnetic stimulation. Furthermore, the results showed that electrical and magnetic stimulation were more effective for tremor suppression at early stages of PD, while light therapy was found to be more effective during the later stages of PD.

Clinical Heterogeneity of Essential Tremor: Understanding Neural Substrates of Action Tremor Subtypes

Essential tremor (ET) is a common movement disorder affecting millions of people. Studies of ET patients and perturbations in animal models have provided a foundation for the neural networks involved in its pathophysiology. However, ET encompasses a wide variability of phenotypic expression, and this may be the consequence of dysfunction in distinct subcircuits in the brain. The cerebello-thalamo-cortical circuit is a common substrate for the multiple subtypes of action tremor. Within the cerebellum, three sets of cerebellar cortex-deep cerebellar nuclei connections are important for tremor. The lateral hemispheres and dentate nuclei may be involved in intention, postural and isometric tremor. The intermediate zone and interposed nuclei could be involved in intention tremor. The vermis and fastigial nuclei could be involved in head and proximal upper extremity tremor. Studying distinct cerebellar circuitry will provide important framework for understanding the clinical heterogeneity of ET.

Is essential tremor a degenerative or an electrical disorder? Electrical disorder

Essential tremor (ET) is one of the most common movement disorders, yet we do not have a complete understanding of its pathophysiology. From a phenomenology standpoint, ET is an isolated tremor syndrome of bilateral upper limb action tremor with or without tremor in other body locations. ET is a pathological tremor that arises from excessive oscillation in the central motor network. The tremor network comprises of multiple brain regions including the inferior olive, cerebellum, thalamus, and motor cortex, and there is evidence that a dynamic oscillatory disturbance within this network leads to tremor. ET is a chronic disorder, and the natural history shows a slow progression of tremor intensity with age. There are reported data suggesting that ET follows the disease model of a neurodegenerative disorder, however whether ET is a degenerative or electrical disorder has been a subject of debate. In this chapter, we will review cumulative evidence that ET as a syndrome is a fundamentally electric disorder. The etiology is likely heterogenous and may not be primarily neurodegenerative.

The Role of the Motor Cortex in Tremor Suppression in Parkinson's Disease

BACKGROUND

Patients with Parkinson's disease (PD) and rest tremor may also have tremor during posture holding, with tremor being transiently suppressed during the transition between resting and posture holding. Other PD patients show no tremor suppression between resting and posture holding. The mechanisms responsible for tremor suppression in PD are unknown. Understanding the mechanisms of tremor suppression would expand our knowledge of tremor pathophysiology in PD.

OBJECTIVE

To investigate whether tremor suppression reflects the activity of the primary motor cortex (M1) and assess whether tremor features are different in patients with and without tremor suppression.

METHODS

We compared corticomuscular coherence (CMC) at tremor frequency and transcranial magnetic stimulation tremor resetting between 10 PD patients with tremor suppression and 10 patients without suppression. We also compared tremor spectral features between the two groups.

RESULTS

Patients with tremor suppression had higher CMC at tremor frequency during both rest tremor and postural tremor, and a higher postural tremor resetting index and stability when compared with patients without tremor suppression. Rest tremor frequency was similar between the two groups, but postural tremor frequency was lower in patients with tremor suppression as compared to patients without.

CONCLUSION

M1 plays a major role in tremor suppression in PD, and the mechanisms of postural tremor may differ between patients with and without tremor suppression.

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.

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.

Levodopa Changes Functional Connectivity Patterns in Subregions of the Primary Motor Cortex in Patients With Parkinson's Disease

Background

The primary motor cortex (M1) is a critical node in Parkinson’s disease (PD)-related motor circuitry; however, the functional roles of its subregions are poorly understood. In this study, we investigated changes in the functional connectivity patterns of M1 subregions and their relationships to improved clinical symptoms following levodopa administration.

Methods

Thirty-six PD patients and 37 healthy controls (HCs) were enrolled. A formal levodopa challenge test was conducted in the PD group, and the Unified Parkinson’s Disease Rating Scale motor section (UPDRS-III) was assessed before (off state) and 1 h after administration of levodopa (on state). The PD group underwent resting-state functional magnetic resonance imaging in both off and on states, whereas the HC group was scanned once. We used the Human Brainnetome Atlas template to subdivide M1 into twelve regions of interest (ROIs). Functional connectivity (FC) was compared between PD on and off states [paired t-test, voxel-level p < 0.001, cluster-level p < 0.05, Gaussian random field (GRF) correction] and between patients and HC (two-sample t-test voxel-level p < 0.001, cluster-level p < 0.05). Correlations between ΔFC (differences in FC between PD off and on states) and clinical symptom improvements were examined.

Results

There was decreased FC between the right caudal dorsolateral area 6 and the anterior cingulate gyrus (ACC), the right upper limb region and the left medial dorsal thalamus (mdTHA), as well as increased FC between the left tongue and larynx region and the left medial frontal gyrus. ΔFC between the right caudal dorsolateral area 6 and ACC was positively correlated with improvements in UPDRS-III total scores as well as the rigidity (item 22) and bradykinesia (items 23–26 and 31) subscores. ΔFC between the right upper limb region and left thalamus was positively correlated with improvements in the left upper limb tremor (items 20c and 21b) and postural tremor (item 21b) subscores.

Conclusions

Our results reveal novel information regarding the underlying mechanisms in the motor circuits in the M1 and a promising way to explore the internal function of the M1 in PD patients. Notably, M1 is a potential therapeutic target in PD, and the exploration of its subregions provides a basis and a source of new insights for clinical intervention and precise drug treatment.

Re-emergent Tremor in Parkinson's Disease: The Role of the Motor Cortex

BACKGROUND

Parkinson's disease patients may show a tremor that appears after a variable delay while the arms are kept outstretched (re-emergent tremor). The objectives of this study were to investigate re-emergent tremor pathophysiology by studying the role of the primary motor cortex in this tremor and making a comparison with rest tremor.

METHODS

We enrolled 10 Parkinson's disease patients with both re-emergent and rest tremor. Tremor was assessed by spectral analysis, corticomuscular coherence and tremor-resetting produced by transcranial magnetic stimulation over the primary motor cortex. We also recorded transcranial magnetic stimulation-evoked potentials generated by motor cortex stimulation during rest tremor, tremor suppression during wrist extension, and re-emergent tremor. Spectral analysis, corticomuscular coherence, and tremor resetting were compared between re-emergent tremor and rest tremor.

RESULTS

Re-emergent tremor showed significant corticomuscular coherence, causal relation between motor cortex activity and tremor muscle and tremor resetting. The P60 component of transcranial magnetic stimulation-evoked potentials reduced in amplitude during tremor suppression, recovered before re-emergent tremor, was facilitated at re-emergent tremor onset, and returned to values similar to those of rest tremor during re-emergent tremor. Compared with rest tremor, re-emergent tremor showed similar corticomuscular coherence and tremor resetting, but slightly higher frequency.

CONCLUSIONS

Re-emergent tremor is causally related with the activity of the primary motor cortex, which is likely a convergence node in the network that generates re-emergent tremor. Re-emergent tremor and rest tremor share common pathophysiological mechanisms in which the motor cortex plays a crucial role. © 2020 International Parkinson and Movement Disorder Society.

Tremor in Parkinson's Disease May Arise from Interactions of Central Rhythms with Spinal Reflex Loop Oscillations

It is commonly believed that tremor, one of the cardinal signs of Parkinson’s disease, is associated with cerebello-thalamo-cortical oscillations set off by the dopamine-depleted basal ganglia networks. The triggering mechanism has been, however, not entirely delineated. Several reports have pointed to the relevance of interactions with peripheral/spinal mechanisms to tremor generation. Investigations of motor unit synchronization and discharge patterns suggested that exaggerated beta-band oscillations may intermittently reach alpha-motoneurons and modulate low-amplitude membrane oscillations due to spinal loop transmission delays. As a result, the spinal reflex loop will oscillate more vigorously and at a lower frequency and, in turn, entrain larger transcortical loops. Motoneurons may thus represent the specific generator “node” in a tremor network encompassing both cerebral and peripheral/spinal recurrent circuits.

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.

Pulsatile desynchronizing delayed feedback for closed-loop deep brain stimulation

High-frequency (HF) deep brain stimulation (DBS) is the gold standard for the treatment of medically refractory movement disorders like Parkinson’s disease, essential tremor, and dystonia, with a significant potential for application to other neurological diseases. The standard setup of HF DBS utilizes an open-loop stimulation protocol, where a permanent HF electrical pulse train is administered to the brain target areas irrespectively of the ongoing neuronal dynamics. Recent experimental and clinical studies demonstrate that a closed-loop, adaptive DBS might be superior to the open-loop setup. We here combine the notion of the adaptive high-frequency stimulation approach, that aims at delivering stimulation adapted to the extent of appropriately detected biomarkers, with specifically desynchronizing stimulation protocols. To this end, we extend the delayed feedback stimulation methods, which are intrinsically closed-loop techniques and specifically designed to desynchronize abnormal neuronal synchronization, to pulsatile electrical brain stimulation. We show that permanent pulsatile high-frequency stimulation subjected to an amplitude modulation by linear or nonlinear delayed feedback methods can effectively and robustly desynchronize a STN-GPe network of model neurons and suggest this approach for desynchronizing closed-loop DBS.

Functional correlates of the therapeutic and adverse effects evoked by thalamic stimulation for essential tremor

Thalamic deep brain stimulation (DBS) is an effective therapy for essential tremor. Gibson et al. use functional MRI to reveal patterns of activation that correlate with stimulation-induced therapeutic and adverse effects. Their results suggest that thalamic DBS controls tremor, and induces paraesthesias, through distal modulation of tremor-related network nodes.

Deep brain stimulation is an established neurosurgical therapy for movement disorders including essential tremor and Parkinson’s disease. While typically highly effective, deep brain stimulation can sometimes yield suboptimal therapeutic benefit and can cause adverse effects. In this study, we tested the hypothesis that intraoperative functional magnetic resonance imaging could be used to detect deep brain stimulation-evoked changes in functional and effective connectivity that would correlate with the therapeutic and adverse effects of stimulation. Ten patients receiving deep brain stimulation of the ventralis intermedius thalamic nucleus for essential tremor underwent functional magnetic resonance imaging during stimulation applied at a series of stimulation localizations, followed by evaluation of deep brain stimulation-evoked therapeutic and adverse effects. Correlations between the therapeutic effectiveness of deep brain stimulation (3 months postoperatively) and deep brain stimulation-evoked changes in functional and effective connectivity were assessed using region of interest-based correlation analysis and dynamic causal modelling, respectively. Further, we investigated whether brain regions might exist in which activation resulting from deep brain stimulation might correlate with the presence of paraesthesias, the most common deep brain stimulation-evoked adverse effect. Thalamic deep brain stimulation resulted in activation within established nodes of the tremor circuit: sensorimotor cortex, thalamus, contralateral cerebellar cortex and deep cerebellar nuclei (FDR q < 0.05). Stimulation-evoked activation in all these regions of interest, as well as activation within the supplementary motor area, brainstem, and inferior frontal gyrus, exhibited significant correlations with the long-term therapeutic effectiveness of deep brain stimulation (P < 0.05), with the strongest correlation (P < 0.001) observed within the contralateral cerebellum. Dynamic causal modelling revealed a correlation between therapeutic effectiveness and attenuated within-region inhibitory connectivity in cerebellum. Finally, specific subregions of sensorimotor cortex were identified in which deep brain stimulation-evoked activation correlated with the presence of unwanted paraesthesias. These results suggest that thalamic deep brain stimulation in tremor likely exerts its effects through modulation of both olivocerebellar and thalamocortical circuits. In addition, our findings indicate that deep brain stimulation-evoked functional activation maps obtained intraoperatively may contain predictive information pertaining to the therapeutic and adverse effects induced by deep brain stimulation.

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.

Exploring the effect of electrical muscle stimulation as a novel treatment of intractable tremor in Parkinson's disease

BACKGROUND

As the pathophysiology of tremor in Parkinson disease (PD) involves a complex interaction between central and peripheral mechanisms, we propose that modulation of peripheral reflex mechanism by electrical muscle stimulation (EMS) may improve tremor temporarily.

OBJECTIVES

To determine the efficacy of EMS as a treatment for drug resistant tremor in PD patients.

METHODS

This study was a single-blinded, quasi-experimental study involving 34 PD patients with classic resting tremor as confirmed by tremor analysis. The EMS was given at 50Hz over the abductor pollicis brevis and interrosseus muscles for 10s with identified tremor parameters before and during stimulation as primary outcomes.

RESULTS

Compared to before stimulation, we observed a significant reduction in the root mean square (RMS) of the angular velocity (p<0.001) and peak magnitude (p<0.001) of resting tremor while tremor frequency (p=0.126) and dispersion (p=0.284) remained unchanged during stimulation. The UPDRS tremor score decreased from 10.59 (SD=1.74) before stimulation to 8.85 (SD=2.19) during stimulation (p<0.001). The average percentage of improvement of the peak magnitude and RMS angular velocity was 49.57% (SD=38.89) and 43.81% (SD=33.15) respectively. 70.6% and 61.8% of patients experienced at least 30% tremor attenuation as calculated from the peak magnitude and RMS angular velocity respectively.

CONCLUSIONS

Our study demonstrated the efficacy of EMS in temporarily improving resting tremor in medically intractable PD patients. Although tremor severity decreased, they were not completely eliminated and continued with a similar frequency, thus demonstrating the role of peripheral reflex mechanism in the modulation of tremor, but not as a generator. EMS should be further explored as a possible therapeutic intervention for tremor in PD.

Resetting tremor by single and paired transcranial magnetic stimulation in Parkinson's disease and essential tremor

OBJECTIVE

The pathogenesis of tremor in Parkinson's disease (PD) and essential tremor (ET) is not fully understood. This study tested the role of primary motor cortex (M1), supplementary motor area (SMA) and cerebellar cortex on PD and ET tremor by single- and paired-pulse transcranial magnetic stimulation (TMS).

METHODS

Ten PD patients with resting tremor, six of them also with postural tremor, and ten ET patients with postural tremor were studied. Randomized single- and paired-pulse TMS with an interstimulus interval of 100 ms were delivered over M1, SMA and cerebellum. TMS effects were evaluated by calculating a tremor-resetting index (RI).

RESULTS

Single- vs. paired-pulse TMS showed no difference. M1-TMS and SMA-TMS but not by cerebellar TMS induced a significant RI in PD and ET. M1-TMS resulted in a significantly higher RI in PD than ET. Furthermore, M1-TMS in PD but not in ET resulted in a significantly higher RI than SMA-TMS.

CONCLUSIONS

Findings suggest a stronger involvement of M1 in resting and postural tremor in PD than postural tremor in ET.

SIGNIFICANCE

RI provides a useful marker to explore the differential functional role of M1, SMA and cerebellum in PD vs. ET tremor.

Closing the loop of deep brain stimulation

High-frequency deep brain stimulation is used to treat a wide range of brain disorders, like Parkinson's disease. The stimulated networks usually share common electrophysiological signatures, including hyperactivity and/or dysrhythmia. From a clinical perspective, HFS is expected to alleviate clinical signs without generating adverse effects. Here, we consider whether the classical open-loop HFS fulfills these criteria and outline current experimental or theoretical research on the different types of closed-loop DBS that could provide better clinical outcomes. In the first part of the review, the two routes followed by HFS-evoked axonal spikes are explored. In one direction, orthodromic spikes functionally de-afferent the stimulated nucleus from its downstream target networks. In the opposite direction, antidromic spikes prevent this nucleus from being influenced by its afferent networks. As a result, the pathological synchronized activity no longer propagates from the cortical networks to the stimulated nucleus. The overall result can be described as a reversible functional de-afferentation of the stimulated nucleus from its upstream and downstream nuclei. In the second part of the review, the latest advances in closed-loop DBS are considered. Some of the proposed approaches are based on mathematical models, which emphasize different aspects of the parkinsonian basal ganglia: excessive synchronization, abnormal firing-rate rhythms, and a deficient thalamo-cortical relay. The stimulation strategies are classified depending on the control-theory techniques on which they are based: adaptive and on-demand stimulation schemes, delayed and multi-site approaches, stimulations based on proportional and/or derivative control actions, optimal control strategies. Some of these strategies have been validated experimentally, but there is still a large reservoir of theoretical work that may point to ways of improving practical treatment.

Sensory reweighting in controls and stroke patients

OBJECTIVE

To test sensitivity to proprioceptive, vestibular and visual stimulations of stroke patients with regard to balance.

METHOD

The postural control of 20 hemiparetic patients after a single hemispheric stroke that had occurred at least 6 months before the study along with 20 controls was probed with vibration, optokinetic, and vestibular galvanic stimulations. Balance was assessed using a force platform (PF) with two miniature inertial sensors placed on the head (C1) and the trunk (C2) under each sensory condition and measured by three composite scores as the mean displacement of the body (PF, C1, C2) during the stimulation. A subject with a composite score greater than the 75th percentile of the composite scores found in the control subjects was arbitrarily considered to be sensitive to that stimulation.

RESULTS

Both control and stroke patients showed large inter-individual variations in response to the three types of sensory stimulation. Among the hemiparetic patients, nearly 65% were sensitive to the optokinetic stimulation, 60% to the galvanic stimulation and 65% to the vibration stimulation. In contrast to the control group, all the hemiparetic subjects were sensitive to at least one type of stimulation.

CONCLUSION

Stroke patients are highly dependent on visual, proprioceptive and vestibular information in order to control their standing posture and individually differ in their relative sensitivity to each type of sensory stimulation.

SIGNIFICANCE

Contrarily to what one might suppose, the increased visual dependence manifested by stroke patients does not necessarily entail any neglect of proprioceptive and vestibular information.

Short latency activation of cortex by clinically effective thalamic brain stimulation for tremor

Deep brain stimulation (DBS) relieves disabling symptoms of neurologic and psychiatric diseases when medical treatments fail, yet its therapeutic mechanism is unknown. We hypothesized that ventral intermediate (VIM) nucleus stimulation for essential tremor activates the cortex at short latencies, and that this potential is related to the suppression of tremor in the contralateral arm. We measured cortical activity with electroencephalography in 5 subjects (seven brain hemispheres) across a range of stimulator settings, and reversal of the anode and cathode electrode contacts minimized the stimulus artifact, allowing visualization of brain activity. Regression quantified the relationship between stimulation parameters and both the peak of the short latency potential and tremor suppression. Stimulation generated a polyphasic event-related potential in the ipsilateral sensorimotor cortex, with peaks at discrete latencies beginning less than 1 ms after stimulus onset (mean latencies 0.9 ± 0.2, 5.6 ± 0.7, and 13.9 ± 1.4 ms, denoted R1, R2, and R3, respectively). R1 showed more fixed timing than the subsequent peaks in the response (P < 0.0001, Levene's test), and R1 amplitude and frequency were both closely associated with tremor suppression (P < 0.0001, respectively). These findings demonstrate that effective VIM thalamic stimulation for essential tremor activates the cerebral cortex at approximately 1 ms after the stimulus pulse. The association between this short latency potential and tremor suppression suggests that DBS may improve tremor by synchronizing the precise timing of discharges in nearby axons and, by extension, the distributed motor network to the stimulation frequency or one of its subharmonics.

The oscillating central network of Essential tremor

Essential tremor (ET) is a centrally driven tremor. It is meanwhile well established that it does not emerge from one single oscillator but an oscillatory network comprising most parts of the physiological central motor network. Several lines of evidence hint at the olivocerebellar system and the thalamus as key structures within this network whereas the cortical motor regions are only intermittently entrained in the tremor rhythm in thalamocortical loops. Dynamic changes in network composition and the interaction in symmetric loops seem to be specific to the generation of tremor. The same network in voluntary motor control is more fixed and subcortico-cortical interactions are preferentially via thalamocortical relays. Thus it is not primarily the network topography but the dynamics and interaction within the network that determines whether involuntary tremor or voluntary movements emerge. And this may be the basis for the selective effect of deep brain stimulation on tremor.

Involvement of the cerebellothalamocortical pathway in Parkinson disease

OBJECTIVE

Lesioning or stimulation of the cerebellar thalamus is an established treatment for rest and postural tremors in Parkinson disease (PD). The cerebellothalamocortical (CTC) pathway can be assessed by transcranial magnetic stimulation (TMS) of the cerebellum, which suppresses the contralateral primary motor cortex (M1), a phenomenon termed cerebellar inhibition (CBI). Tremor reset can be used to assess whether the stimulated brain area is involved in the generation or transmission of tremor. We tested whether M1 or cerebellar stimulation can reset PD tremor, and investigated the excitability of the CTC pathway in PD.

METHODS

Ten mild to moderate PD patients in the OFF medication state and 10 healthy controls were studied. Tremor reset was tested with TMS delivered to the cerebellum or M1. CBI was assessed by cerebellar stimulation followed by M1 stimulation at interstimulus intervals of 3 to 8 milliseconds. Subjects were tested both at rest and during arm extension.

RESULTS

Rest tremor in PD was reset by M1 stimulation but not by cerebellar stimulation. Postural tremor was reset by both types of stimulation. At rest, CBI was reduced in PD patients compared to controls. Arm extension decreased CBI in controls and turned the inhibition into facilitation in patients. CBI correlated with the degree of tremor reset caused by the cerebellar stimulation.

INTERPRETATION

The excitability of CTC pathway is decreased in PD. Rest and postural tremors in PD are mediated by different neuronal pathways, and the CTC pathway is involved in the generation or transmission of postural tremor.

The Bereitschaftspotential in essential tremor

OBJECTIVE

Essential tremor (ET) is an involuntary postural oscillation. It is unclear to which extent motor cortical activity in preparation of volitional movement is abnormal in ET. We measured the Bereitschaftspotential (BP) to address this question.

METHODS

Given the known influence of the cerebello-dentato-thalamo-cortical projection in the generation of the BP, patients were divided into two groups, defined by purely postural tremor (ET(PT)) or additional presence of intention tremor (ET(IT)) and compared to healthy controls. BP was recorded during self-paced rapid wrist extension movements.

RESULTS

The late BP (500-0 ms before movement onset) was increased over the mid-frontal area in ET(PT), whereas it was reduced over the mid-parietal area in ET(IT) when compared to healthy controls. In addition, the late BP was reduced over a widespread centro-parietal area in ET(IT) compared to ET(PT).

CONCLUSIONS

Findings suggest that presence vs. absence of cerebellar signs (intention tremor) in ET results in differential affection of volitional preparatory motor cortical activity. The BP increase in ET(PT) may indicate compensatory activity, whereas the widespread centro-parietal BP reduction in ET(IT) suggests dysfunction of the cerebello-dentato-thalamo-cortical projection.

SIGNIFICANCE

Reduction of the late BP amplitude may serve as a surrogate marker for dysfunction of the cerebello-dentato-thalamo-cortical projection in ET.

Observations on the cortical silent period in Parkinson's disease

Transcranial magnetic stimulation is a tool in the neurosciences to study motor functions and nervous disorders, amongst others. Single pulses of TMS applied over the primary motor cortex lead to a so-called cortical silent period in the recording from the corresponding muscle, i.e. a period of approximately 100ms with no muscle activity. We here show that in Parkinson's disease (PD), this cortical silent period in some cases is interrupted by short bursts of EMG activity. We describe in detail these interruptions in two patients with PD. These interruptions may number up to 3 per cortical silent period and show a consistent frequency across trials and hemispheres within a given patient; the two patients described here do differ, however, in the time-delay of the interruptions and hence the induced frequency. For one patient, the frequency of the interruptions proved to be around 13 Hz, the other patient showed a frequency of around 17 Hz. The results corroborate earlier findings of cortical oscillations elicited by pulses of TMS and may be related to abnormal oscillatory activity found in the cortical-subcortical motor system in PD.

Using wavelet analysis to detect the influence of low frequency magnetic fields on human physiological tremor

The influence of extremely low frequency magnetic fields (ELF-MFs) on human physiological processes and, in particular, on motor activity is still not established with certainty. Using the wavelet-transform approach, changes in the characteristics of human finger micromovement are studied in the presence of a low intensity MF centred at the level of the head. Different approaches to nonstationary signal analysis involving real as well as complex wavelet functions are considered. We find evidence that ELF-MFs lead to more regular postural tremor and more homogeneous energy distribution.

The neural response to transcranial magnetic stimulation of the human motor cortex. I. Intracortical and cortico-cortical contributions

We investigated the properties of the neural response to transcranial magnetic stimulation (TMS) applied over the human primary motor cortex. Consistent with our previous findings, single pulses of TMS induce a characteristic negative deflection at 45 ms (N45) and a transient oscillation in the beta frequency-range (15-30 Hz), as measured using electroencephalograpy (EEG). Here we show the relative specificity of the beta oscillation and the N45; both are stronger when elicited by stimulation applied over the primary motor cortex, as compared with stimulation over the dorsal premotor cortex. We also provide a quantitative analysis of the beta responses to single pulses of TMS and show that the responses are highly phaselocked to the TMS pulses within single subjects; this phaselocking is similar from subject to subject. A single pulse of TMS applied over the primary motor cortex thus appears to reset the ongoing oscillations of the neurons, bringing them transiently into synchrony. Finally, we examine the effect of local or distal modulation of the excitability of the primary motor cortex on the beta oscillation and the N45 in response to single-pulse TMS. We applied low-frequency subthreshold repetitive TMS either over the primary motor cortex (local modulation) or, on a separate day, over the dorsal premotor cortex (distal modulation). The modulation was evaluated with single suprathreshold test pulses of TMS applied over the primary motor cortex before and after the subthreshold low-frequency rTMS. We recorded the EEG response throughout the testing session, i.e. to both the subthreshold and the suprathreshold pulses. After repetitive TMS applied over the primary motor cortex, but not the dorsal premotor cortex, the amplitude of the N45 in response to suprathreshold pulses tended to decrease (not significant), and subsequently increased (significant); neither type of repetitive TMS affected the amplitude of the beta oscillation. We conclude that (1) the N45 depends on circuits intrinsic to the primary motor cortex; (2) the beta oscillation is specific to stimulation of the primary motor cortex, but is not affected by modulation of either cortical area and; (3) the beta oscillatory response to pulses of TMS arises from resetting of ongoing oscillations rather than their induction.

Effect of a low intensity magnetic field on human motor behavior

Extremely low frequency (ELF) magnetic fields (MF) are omnipresent in our modern daily environment, but their effects on humans are still not clearly established. The aim of this study was to determine the effect of a 50 Hz, 1,000 microT MF centered at the level of the head on human index finger micro-displacements. Twenty-four men recruited among the personnel of the French company, Electricité de France (EDF), completed the experiment. Their postural and kinetic tremors were recorded under four "field-on" and four "field-off" conditions, each tested during a real and a sham sequence. Eight postural and four kinetic tremor characteristics were calculated on recorded time series and were used for statistical analysis. No effect of the MF was found for kinetic tremor. Concerning postural tremor, the proportion of oscillations at low frequencies (between 2 and 4 Hz) was higher during the real than during the sham exposure sequence (P<.05). It suggests that MF could have a subtle delayed effect on human behavior, which is clearly not pathological. These results should be taken into account for the establishment of new exposure limits.

Effect of Transcranial Magnetic Stimulation on Bimanual Movements

Transcranial magnetic stimulation (TMS) of the motor cortex can interrupt voluntary contralateral rhythmic limb movements. Using the method of “resetting index” (RI), our study investigated the TMS effect on different types of bimanual movements. Six normal subjects participated. For unimanual movement, each subject tapped either the right or left index finger at a comfortable rate. For bimanual movement, index fingers of both hands tapped in the same (in-phase) direction or in the opposite (antiphase) direction. TMS was applied to each hemisphere separately at various intensities from 0.5 to 1.5 times motor threshold (MT). TMS interruption of rhythm was quantified by RI. For the unimanual movements, TMS disrupted both contralateral and ipsilateral rhythmic hand movements, although the effect was much less in the ipsilateral hand. For the bimanual in-phase task, TMS could simultaneously reset the rhythmic movements of both hands, but the effect on the contralateral hand was less and the effect on the ipsilateral hand was more compared with the unimanual tasks. Similar effects were seen from right and left hemisphere stimulation. TMS had little effect on the bimanual antiphase task. The equal effect of right and left hemisphere stimulation indicates that neither motor cortex is dominant for simple bimanual in-phase movement. The smaller influence of contralateral stimulation and the greater effect of ipsilateral stimulation during bimanual in-phase movement compared with unimanual movement suggest hemispheric coupling. The antiphase movements were resistant to TMS disruption, and this suggests that control of rhythm differs in the 2 tasks. TMS produced a transient asynchrony of movements on the 2 sides, indicating that both motor cortices might be downstream of the clocking command or that the clocking is a consequence of the 2 hemispheres communicating equally with each other.

Impact of different stimulation types on orthostatic tremor

OBJECTIVE

Primary orthostatic tremor (OT) is thought to be generated by a unique supraspinal tremor generator. Here we studied the effect of ipsi- and contralateral stimulation of the central and peripheral nervous system on OT.

METHODS

In 7 patients with primary OT, surface EMG was recorded from both tibialis anterior muscles. We performed transcranial magnetic stimulation (TMS) over the vertex, and lumbar magnetic stimulation (LMS) over the lumbar spine. Supramaximal electrical nerve stimuli were applied to the tibial or peroneal nerve at the knee. Proprioceptive input was evoked by rhythmical submaximal stimulation of the tibial, peroneal or sural nerve at the ankle.

RESULTS

TMS reset OT significantly in the contralateral as well as the ipsilateral tibialis anterior muscle. The resetting in both muscles was identical. In contrast, peripheral input by means of LMS, supra- or submaximal nerve stimulation had no impact on OT.

CONCLUSIONS

Transcranial magnetic stimulation of one cortical leg area resets OT in both legs whereas OT is not modified by any peripheral stimuli applied in this study.

SIGNIFICANCE

Our results support the hypothesis of n unique supraspinal OT generator. This generator receives a modulating input from the motor cortex.

Acute effect of transcutaneous electrical nerve stimulation on tremor

Based on the claims that transcutaneous electrical nerve stimulation is effective in myoclonic dystonia and essential tremor, we evaluated its acute effects in 5 patients with essential tremor and 2 patients with tremor attributed to peripheral neuropathy using as parameters the Washington Heights-Inwood Genetic Study of Essential Tremor rating scale, self-reported impression, and recording of electromyographic activity. We found no significant improvement in any of the parameters tested.

Abnormal gating of somatosensory inputs in essential tremor

OBJECTIVE

To study whether sensorimotor cortical areas are involved in Essential Tremor (ET) generation.

BACKGROUND

It has been suggested that sensorimotor cortical areas can play a role in ET generation. Therefore, we studied median nerve somatosensory evoked potentials (SEPs) in 10 patients with definite ET.

METHODS

To distinguish SEP changes due to hand movements from those specifically related to central mechanisms of tremor, SEPs were recorded at rest, during postural tremor and during active and passive movement of the hand. Moreover, we recorded SEPs from 5 volunteers who mimicked hand tremor. The traces were further submitted to dipolar source analysis.

RESULTS

Mimicked tremor in controls as well as active and passive hand movements in ET patients caused a marked attenuation of all scalp SEP components. These SEP changes can be explained by the interference between movement and somatosensory input ('gating' phenomenon). By contrast, SEPs during postural tremor in ET patients showed a reduction of N20, P22, N24 and P24 cortical SEP components, whereas the fronto-central N30 wave remained unaffected.

CONCLUSIONS

Our findings suggest that in ET patients the physiological interference between movement and somatosensory input to the cortex is not effective on the N30 response. This finding thus indicates that a dysfunction of the cortical generator of the N30 response may play a role in the pathogenesis of ET.

Control of post-stroke movement disorders using chronic motor cortex stimulation

The effects of motor cortex (MC) stimulation on post-stroke movement disorders were analyzed in 50 patients. These individuals either underwent MC stimulation primarily for the purpose of controlling their post-stroke involuntary movements (n = 8) or underwent MC stimulation for the purpose of controlling their post-stroke central pain (n = 42). In the latter patients, the effects of MC stimulation on co-existent involuntary or voluntary movement disorders were analyzed retrospectively. Good control of involuntary movements was observed in 2 of 3 patients with hemichoreo-athetosis, 2 of 2 patients with distal resting or action tremor, and 1 of 3 patients with proximal postural tremor. Subjective improvements in motor performance were reported by 8 patients who had mild motor weakness, and the effects appeared to be attributable to attenuation of rigidity. We consider that these findings justify further clinical studies on MC stimulation for the control of post-stroke movement disorders.

Applications of transcranial magnetic stimulation in movement disorders

The author reviews the applications of transcranial magnetic stimulation (TMS) in a series of movement disorders--namely, Parkinson's disease, corticobasal degeneration, multiple system atrophy, progressive supranuclear palsy, essential tremor, dystonia, Huntington's chorea, myoclonus, the ataxias, Tourette's syndrome, restless legs syndrome, Wilson's disease, Rett syndrome, and stiff-person syndrome. Single- and paired-pulse TMS studies have been done mainly for pathophysiologic purposes. Repetitive TMS has been used largely for therapy. Many TMS abnormalities are seen in the different diseases. They concur to show that motor cortical areas and their projections are the main target of the basal ganglia dysfunction typical of movement disorders. Interpretation has not always been clear, and sometimes there were discrepancies and contradictions. Largely, this may be the result of the extreme heterogeneity of the methods used and of the patients studied. It is premature to give repetitive TMS a role in treatment. Overall, however, TMS gives rise to a new, outstanding enthusiasm in the neurophysiology of movement disorders. There is reason to predict that TMS, with its continuous technical refinement, will prove even more helpful in the near future. Then, research achievements are reasonably expected to spill over into clinical practice.

Peripheral silent periods in essential tremor

BACKGROUND

Pathophysiology of essential tremor (ET) is controversial. In the present study, peripherally induced silent period (SP) in ET patients is studied.

AIMS AND OBJECTIVES

To study if the peripherally induced SP was different in ET patients as compared to age matched healthy controls.

MATERIAL AND METHODS

24 patients of ET diagnosed according to diagnostic criteria of Louis et al. [Neurology 50 (1998) 1351] (mean age 45.37+/-14.86 years) and an equal number of healthy controls (mean age 36.21+/-15.72 years) were recruited for the study. Peripherally induced SP was recorded according to the methods already described. Student's t-test and Wilcoxon sign rank test were used for statistical analysis.

RESULTS

The peripheral SP was 50.29+/-50.15 and 93.04+/-35.93 ms (p=0.0014) in ET patients and controls respectively.

CONCLUSION

Our study shows that peripheral silent period is shorter in patients of ET as compared to normal individuals.

Rhythm generation in monkey motor cortex explored using pyramidal tract stimulation

We investigated whether stimulation of the pyramidal tract (PT) could reset the phase of 15-30 Hz beta oscillations observed in the macaque motor cortex. We recorded local field potentials (LFPs) and multiple single-unit activity from two conscious macaque monkeys performing a precision grip task. EMG activity was also recorded from the second animal. Single PT stimuli were delivered during the hold period of the task, when oscillations in the LFP were most prominent. Stimulus-triggered averaging of the LFP showed a phase-locked oscillatory response to PT stimulation. Frequency domain analysis revealed two components within the response: a 15-30 Hz component, which represented resetting of on-going beta rhythms, and a lower frequency 10 Hz response. Only the higher frequency could be observed in the EMG activity, at stronger stimulus intensities than were required for resetting the cortical rhythm. Stimulation of the PT during movement elicited a greatly reduced oscillatory response. Analysis of single-unit discharge confirmed that PT stimulation was capable of resetting periodic activity in motor cortex. The firing patterns of pyramidal tract neurones (PTNs) and unidentified neurones exhibited successive cycles of suppression and facilitation, time locked to the stimulus. We conclude that PTN activity directly influences the generation of the 15-30 Hz rhythm. These PTNs facilitate EMG activity in upper limb muscles, contributing to corticomuscular coherence at this same frequency. Since the earliest oscillatory effect observed following stimulation was a suppression of firing, we speculate that inhibitory feedback may be the key mechanism generating such oscillations in the motor cortex.

Vibratory proprioceptive stimulation affects Parkinsonian tremor

Previous research on tremor pathophysiology showed that tremor can be affected, e.g. by electrical stimulation of the peripheral nerve, mechanical perturbation of the limb and by transcranial magnetic stimulation of the motor cortex. This report is focused on possible effects of muscle vibration (MV) on resting tremor in Parkinson's Disease (PD). Vibratory stimulation was applied to the tendons of M. extensor carpi radialis longus and M. flexor ulnaris in 27 subjects with moderate PD resting tremor. The following effects were observed: (1) tremor stopped or started time-locked to MV onset and offset, (2) tremor persisted during MV but its frequency pattern changed. These results are discussed with specific emphasis to effects of MV on spinal and supraspinal levels.

Modulation of symptomatic palatal tremor by magnetic stimulation of the motor cortex

OBJECTIVES

Magnetic stimulation of the motor cortex can be used to determine the involvement of the cortex in rhythmic movement disorders. Symptomatic palatal tremor (SPT) is thought to come from a pacemaker that is relatively resistant to internal and external stimulation. In this study, we investigated the effect of magnetic stimulation of motor cortex on SPT.

METHODS

Five male patients, aged 67-79 years, with SPT after brain stem infarction or hemorrhage, all had a synchronous mouth angle twitch with the palatal movement. Electromyographic activity was recorded with a monopolar needle electrode from orbicularis oris. In experiment 1, transcranial magnetic stimulation (TMS) was delivered at 200% motor threshold (MT) to reset SPT. In experiment 2, the effect of TMS intensities was studied at 80-240% MT in two SPT patients. To determine the influence of the TMS, we used the resetting index (RI).

RESULTS

TMS reset the tremor in all 5 SPT patients at 200% MT with RIs of 0.86-0.96. The latency of the tremor reappearance after TMS was longer than the pre-stimulus tremor interval, and the intervals between the subsequent tremor bursts were also prolonged. The degree of tremor resetting was closely correlated with the magnetic stimulus intensity and the latency of the tremor reappearance after TMS.

CONCLUSIONS

Stimulation of the motor cortex may modulate the generator of SPT.

Overview of human tremor physiology

The physiology differs in the many forms of human tremor. Tremors may derive from mechanical oscillations, mechanical reflex oscillations, normal central oscillators, and pathologic central oscillators. Methods of studying tremor include accelerometry and electromyography (EMG). An excellent method consists of accelerometry and EMG combined with spectral analysis and weighting of the body part, which allows separation of tremors coming from mechanical reflex and central oscillators. Physiologic tremor is a mechanical tremor with a possible contribution of the normal 8-12 Hz central oscillator; exaggerated physiologic tremor is a mechanical reflex tremor. Essential tremor (ET) comes from a central oscillator that can be easily influenced with sensory input. The classic rest tremor of Parkinson's disease (PD) comes from a central oscillator that seems less easily influenced with sensory input but can be affected by transcranial magnetic stimulation. Other tremors with central oscillators are palatal tremor and orthostatic tremor. Other tremors whose physiology involves central loops includes cerebellar tremor and cortical tremor. Neuropathic tremors may be a result of delays in peripheral loops, but central oscillators play a role in some.

Cortical excitability in patients with essential tremor

We used transcranial magnetic stimulation in 10 patients with essential tremor and 8 matched healthy subjects. A round stimulating coil was placed over the vertex and electromyographic activity was recorded from the first dorsal interosseous muscle. Paired transcranial stimuli were delivered at interstimulus intervals of 3, 5, 20, 100, 150, and 200 ms. The intensity of the conditioning stimulus was 80% of motor threshold at short and 150% at long interstimulus intervals (ISIs). We also measured the silent period obtained after a single magnetic pulse delivered at 150% of motor threshold during a submaximal muscle contraction. Patients and controls had similar motor threshold and similar latencies. Paired magnetic stimuli given at short and long ISIs at rest, and during a voluntary muscle contraction, elicited similar responses in both groups. The silent period evoked by transcranial magnetic stimulation had a similar duration in patients with ET and controls. In conclusion, these findings suggest that patients with essential tremor have normal cortical motor area excitability.

Reciprocal inhibition in forearm muscles in patients with essential tremor

Reciprocal inhibition of the H-reflex in the forearm flexor muscles was studied in 11 patients with essential tremor and in 10 normal controls. Whereas patients and controls had a similar first, disynaptic phase of reciprocal inhibition, patients had a significantly reduced second phase. Patients with more severe functional impairment had more pronounced abnormalities of reciprocal inhibition. Abnormalities of reciprocal inhibition may play a role in the pathophysiology of essential tremor and probably arise from defective suprasegmental control.

Central mechanisms in human enhanced physiological tremor

The sites of the central nervous structures involved in enhanced physiological tremor (EPT) are still unclear. The syndrome of persistent mirror movements (PMM) is characterized by abnormal bilateral corticospinal projections. If a supraspinal mechanism is involved in EPT, the activity of EPT should be coherent between both sides in subjects with this abnormality. We investigated three PMM subjects and three normal controls. Focal transcranial magnetic stimulation (TMS) resulted in contralateral hand muscle responses in the controls. The PMM subjects, in contrast, had bilateral responses. Similarly, long-latency reflexes (LLR) in PMM could be recorded bilaterally, while the control subjects showed responses only on the stimulated side. EPT was evoked by intravenous salbutamol. EMG time series were recorded bilaterally from the wrist extensor muscles and cross spectra were calculated. If there was a significant right-left-coherence, phase analysis was performed. No control subject showed a significant right-left-coherence of tremor activity. In contrast, a significant coherence was found in PMM between 8 and 12 Hz. When the mechanical tremor frequency of one hand was reduced by loading, coherences and phase spectra of the EMGs remained unchanged. By comparing the results from TMS, LLR and cross spectral analysis we come to the conclusion, that the 8 to 12 Hz component of EPT is transmitted transcortically, most likely originating from two separate generators for both sides.