Intermittent cortical involvement in the preservation of tremor in essential tremor

Deep Brain Stimulation for Orthostatic Tremor: An Observational Study

BACKGROUND

Primary orthostatic tremor (OT) can affect patients' life. Treatment of OT with deep brain stimulation (DBS) of the thalamic ventral intermediate nucleus (Vim) is described in a limited number of patients. The Vim and posterior subthalamic area (PSA) can be targeted in a single trajectory, allowing both stimulation of the Vim and/or dentatorubrothalamic tract (DRT). In essential tremor this is currently often used with positive effects.

OBJECTIVE

To evaluate the efficacy of Vim/DRT-DBS in OT-patients, based on standing time and Quality of Life (QoL), also on the long-term. Furthermore, to relate stimulation of the Vim and DRT, medial lemniscus (ML) and pyramidal tract (PT) to beneficial clinical and side-effects.

METHODS

Nine severely affected OT-patients received bilateral Vim/DRT-DBS. Primary outcome measure was standing time; secondary measures included self-reported measures, neurophysiological measures, structural analyses, surgical complications, stimulation-induced side-effects, and QoL up to 56 months. Stimulation of volume of tissue activated (VTA) were related to outcome measures.

RESULTS

Average maximum standing time increased from 41.0 s ± 51.0 s to 109.3 s ± 65.0 s after 18 months, with improvements measured in seven of nine patients. VTA (n = 7) overlapped with the DRT in six patients and with the ML and/or PT in six patients. All patients experienced side-effects and QoL worsened during the first year after surgery, which improved again during long-term follow-up, although remaining below age-related normal values. Most patients reported a positive effect of DBS.

CONCLUSION

Vim/DRT-DBS improved standing time in patients with severe OT. Observed side-effects are possibly related to stimulation of the ML and PT.

Dynamic functional changes upon thalamotomy in essential tremor depend on baseline brain morphometry

Patients with drug-resistant essential tremor (ET) may undergo Gamma Knife stereotactic radiosurgical thalamotomy (SRS-T), where the ventro-intermediate nucleus of the thalamus (Vim) is lesioned by focused beams of gamma radiations to induce clinical improvement. Here, we studied SRS-T impacts on left Vim dynamic functional connectivity (dFC, n = 23 ET patients scanned before and 1 year after intervention), and on surface-based morphometric brain features (n = 34 patients, including those from dFC analysis). In matched healthy controls (HCs), three dFC states were extracted from resting-state functional MRI data. In ET patients, state 1 spatial stability increased upon SRS-T (F1,22 = 19.13, p = 0.004). More frequent expression of state 3 over state 1 before SRS-T correlated with greater clinical recovery in a way that depended on the MR signature volume (t6 = 4.6, p = 0.004). Lower pre-intervention spatial variability in state 3 expression also did (t6 = - 4.24, p = 0.005) and interacted with the presence of familial ET so that these patients improved less (t6 = 4.14, p = 0.006). ET morphometric profiles showed significantly lower similarity to HCs in 13 regions upon SRS-T (z ≤ - 3.66, p ≤ 0.022), and a joint analysis revealed that before thalamotomy, morphometric similarity and states 2/3 mean spatial similarity to HCs were anticorrelated, a relationship that disappeared upon SRS-T (z ≥ 4.39, p < 0.001). Our results show that left Vim functional dynamics directly relates to upper limb tremor lowering upon intervention, while morphometry instead has a supporting role in reshaping such dynamics.

Physiological Recordings of the Cerebellum in Movement Disorders

The cerebellum plays an important role in movement disorders, specifically in symptoms of ataxia, tremor, and dystonia. Understanding the physiological signals of the cerebellum contributes to insights into the pathophysiology of these movement disorders and holds promise in advancing therapeutic development. Non-invasive techniques such as electroencephalogram and magnetoencephalogram can record neural signals with high temporal resolution at the millisecond level, which is uniquely suitable to interrogate cerebellar physiology. These techniques have recently been implemented to study cerebellar physiology in healthy subjects as well as individuals with movement disorders. In the present review, we focus on the current understanding of cerebellar physiology using these techniques to study movement disorders.

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.

Disrupted cortico-peripheral interactions in motor disorders

Motor disorders may arise from neurological damage or diseases at different levels of the hierarchical motor control system and side-loops. Altered cortico-peripheral interactions might be essential characteristics indicating motor dysfunctions. By integrating cortical and peripheral responses, top-down and bottom-up cortico-peripheral coupling measures could provide new insights into the motor control and recovery process. This review first discusses the neural bases of cortico-peripheral interactions, and corticomuscular coupling and corticokinematic coupling measures are addressed. Subsequently, methodological efforts are summarized to enhance the modeling reliability of neural coupling measures, both linear and nonlinear approaches are introduced. The latest progress, limitations, and future directions are discussed. Finally, we emphasize clinical applications of cortico-peripheral interactions in different motor disorders, including stroke, neurodegenerative diseases, tremor, and other motor-related disorders. The modified interaction patterns and potential changes following rehabilitation interventions are illustrated. Altered coupling strength, modified coupling directionality, and reorganized cortico-peripheral activation patterns are pivotal attributes after motor dysfunction. More robust coupling estimation methodologies and combination with other neurophysiological modalities might more efficiently shed light on motor control and recovery mechanisms. Future studies with large sample sizes might be necessary to determine the reliabilities of cortico-peripheral interaction measures in clinical practice.

Directionality of corticomuscular coupling in essential tremor and cortical myoclonic tremor

OBJECTIVE

A role of the motor cortex in tremor generation in essential tremor (ET) is assumed, yet the directionality of corticomuscular coupling is unknown. Our aim is to clarify the role of the motor cortex. To this end we also study 'familial cortical myoclonic tremor with epilepsy' (FCMTE) and slow repetitive voluntary movements with a known cortical drive.

METHODS

Directionality of corticomuscular coupling (EEG-EMG) was studied with renormalized partial directed coherence (rPDC) during tremor in 25 ET patients, 25 healthy controls (mimicked) and in seven FCMTE patients; and during a self-paced 2 Hz task in eight ET patients and seven healthy controls.

RESULTS

Efferent coupling around tremor frequency was seen in 33% of ET patients, 45.5% of healthy controls, all FCMTE patients, and, around 2 Hz, in all ET patients and all healthy controls. Ascending coupling, seen in the majority of all participants, was weaker in ET than in healthy controls around 5-6 Hz.

CONCLUSIONS

Possible explanations are that tremor in ET results from faulty subcortical output bypassing the motor cortex; rate-dependent transmission similar to generation of rhythmic movements; and/or faulty feedforward mechanism resulting from decreased afferent (sensory) coupling.

SIGNIFICANCE

A linear cortical drive is lacking in the majority of ET patients.

CMC is more than a measure of corticospinal tract integrity in acute stroke patients

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CMC is weaker and occurs at lower frequencies in acute stroke patients.

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Both afferent and efferent input signals contribute to CMC.

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CMC should not be used as a direct measure of corticospinal tract integrity.

In healthy subjects, motor cortex activity and electromyographic (EMG) signals from contracting contralateral muscle show coherence in the beta (15–30 Hz) range. Corticomuscular coherence (CMC) is considered a sign of functional coupling between muscle and brain. Based on prior studies, CMC is altered in stroke, but functional significance of this finding has remained unclear. Here, we examined CMC in acute stroke patients and correlated the results with clinical outcome measures and corticospinal tract (CST) integrity estimated with diffusion tensor imaging (DTI). During isometric contraction of the extensor carpi radialis muscle, EMG and magnetoencephalographic oscillatory signals were recorded from 29 patients with paresis of the upper extremity due to ischemic stroke and 22 control subjects. CMC amplitudes and peak frequencies at 13–30 Hz were compared between the two groups. In the patients, the peak frequency in both the affected and the unaffected hemisphere was significantly (p < 0.01) lower and the strength of CMC was significantly (p < 0.05) weaker in the affected hemisphere compared to the control subjects. The strength of CMC in the patients correlated with the level of tactile sensitivity and clinical test results of hand function. In contrast, no correlation between measures of CST integrity and CMC was found. The results confirm the earlier findings that CMC is altered in acute stroke and demonstrate that CMC is bidirectional and not solely a measure of integrity of the efferent corticospinal tract.

Tremor habituation to deep brain stimulation: Underlying mechanisms and solutions

DBS of the ventral intermediate nucleus is an extremely effective treatment for essential tremor, although a waning benefit is observed after a variable time in a variable proportion of patients (ranging from 0% to 73%), a concept historically defined as "tolerance." Tolerance is currently an established concept in the medical community, although there is debate on its real existence. In fact, very few publications have actually addressed the problem, thus making tolerance a typical example of science based on "eminence rather than evidence." The underpinnings of the phenomena associated with the progressive loss of DBS benefit are not fully elucidated, although the interplay of different-not mutually exclusive-factors has been advocated. In this viewpoint, we gathered the evidence explaining the progressive loss of benefit observed after DBS. We grouped these factors in three categories: disease-related factors (tremor etiology and progression); surgery-related factors (electrode location, microlesional effect and placebo); and stimulation-related factors (not optimized stimulation, stimulation-induced side effects, habituation, and tremor rebound). We also propose possible pathophysiological explanations for the phenomenon and define a nomenclature of the associated features: early versus late DBS failure; tremor rebound versus habituation (to be preferred over tolerance). Finally, we provide a practical approach for preventing and treating this loss of DBS benefit, and we draft a possible roadmap for the research to come. © 2019 International Parkinson and Movement Disorder Society.

Corticomuscular Coherence and Its Applications: A Review

Corticomuscular coherence (CMC) is an index utilized to indicate coherence between brain motor cortex and associated body muscles, conventionally. As an index of functional connections between the cortex and muscles, CMC research is the focus of neurophysiology in recent years. Although CMC has been extensively studied in healthy subjects and sports disorders, the purpose of its applications is still ambiguous, and the magnitude of CMC varies among individuals. Here, we aim to investigate factors that modulate the variation of CMC amplitude and compare significant CMC between these factors to find a well-developed research prospect. In the present review, we discuss the mechanism of CMC and propose a general definition of CMC. Factors affecting CMC are also summarized as follows: experimental design, band frequencies and force levels, age correlation, and difference between healthy controls and patients. In addition, we provide a detailed overview of the current CMC applications for various motor disorders. Further recognition of the factors affecting CMC amplitude can clarify the physiological mechanism and is beneficial to the implementation of CMC clinical methods.

Unilateral Thalamic Deep Brain Stimulation for Voice Tremor

BACKGROUND

Voice tremor (VT) is the involuntary and rhythmical phonatory instability of the voice. Recent findings suggest that unilateral deep brain stimulation of the ventral intermediate nucleus (Vim-DBS) can sometimes be effective for VT. In this exploratory analysis, we investigated the effect of Vim-DBS on VT and tested the hypothesis that unilateral thalamic stimulation is effective for patients with VT.

METHODS

Seven patients with VT and previously implanted bilateral Vim-DBS were enrolled in the study. Each patient was randomized and recorded performing sustained phonation during the following conditions: left thalamic stimulation, right thalamic stimulation, bilateral thalamic stimulation (Bil-ON), and no stimulation (Bil-OFF). Perceptual VT ratings and an acoustic analysis to find the rate of variation of the fundamental frequency measured by the standard deviation of the pitch (f0SD) were performed in a blinded manner. For the purposes of this study, a "dominant" side was defined as one with more than twice as much reduction in VT following Vim-DBS compared to the contralateral side. The Wilcoxon signed-rank test was performed to compare the effect of the dominant side stimulation in the reduction of VT scores and f0SD. The volume of activated tissue (VAT) of the dominant stimulation side was modelled against the degree of improvement in VT to correlate the significant stimulation cluster with thalamic anatomy. Finally, tractography analysis was performed to analyze the connectivity of the significant stimulation cluster.

RESULTS

Unilateral stimulation was beneficial in all 7 patients. Five patients clearly had a "dominant" side with either benefit only seen following stimulation of one side or more than twice as much benefit from one side compared to the other. Two patients had similar benefit with unilateral stimulation from either side. The Wilcoxon paired test showed significant differences between unilateral dominant and unilateral nondominant stimulation for VT scores (p = 0.04), between unilateral dominant and Bil-OFF (p = 0.04), and between Bil-ON and unilateral nondominant stimulation (p = 0.04). No significant differences were found between Bil-ON and unilateral dominant condition (p = 0.27), or between Bil-OFF and unilateral nondominant (p = 0.23). The dominant VAT showed that the significant voxels associated with the best VT control were located in the most ventral and medial part of the Vim nucleus and the ventralis caudalis anterior internus nucleus. The connectivity analysis showed significant connectivity with the cortical areas of the speech circuit.

CONCLUSIONS

Unilateral dominant-side thalamic stimulation and bilateral thalamic stimulation were equally effective in reducing VT. Nondominant unilateral stimulation alone did not significantly improve VT.

Essential tremor severity and anatomical changes in brain areas controlling movement sequencing

Objective

Although the cerebello‐thalamo‐cortical network has often been suggested to be of importance in the pathogenesis of essential tremor (ET), the origins of tremorgenic activity in this disease are not fully understood. We used a combination of cortical thickness imaging and neurophysiological studies to analyze whether the severity of tremor was associated with anatomical changes in the brain in ET patients.

Methods

Magnetic resonance imaging (MRI) and a neurophysiological assessment were performed in 13 nondemented ET patients. High field structural brain MRI images acquired in a 3T scanner and analyses of cortical thickness and surface were carried out. Cortical reconstruction and volumetric segmentation was performed with the FreeSurfer image analysis software. We used high‐density surface electromyography (hdEMG) and inertial measurement units (IMUs) to quantify the tremor severity in upper extrimities of patients. In particular, advanced computer tool was used to reliably identify discharge patterns of individual motor units from surface hdEMG and quantify motor unit synchronization.

Results

We found significant association between increased motor unit synchronization (i.e., more severe tremor) and cortical changes (i.e., atrophy) in widespread cerebral cortical areas, including the left medial orbitofrontal cortex, left isthmus of the cingulate gyrus, right paracentral lobule, right lingual gyrus, as well as reduced left supramarginal gyrus (inferior parietal cortex), right isthmus of the cingulate gyrus, left thalamus, and left amygdala volumes.

Interpretation

Given that most of these brain areas are involved in controlling movement sequencing, ET tremor could be the result of an involuntary activation of a program of motor behavior used in the genesis of voluntary repetitive movements.