Linking Essential Tremor to the Cerebellum: Neuropathological Evidence

Decreased Synaptic Vesicle Glycoprotein 2A Binding in the Human Postmortem Essential Tremor Cerebellum: Evidence of Reduction in Synaptic Density

OBJECTIVE

Despite being one of the most prevalent neurological diseases, the pathophysiology of essential tremor (ET) is not fully understood. Neuropathological studies have identified numerous degenerative changes in the cerebellum of ET patients, however. These data align with considerable clinical and neurophysiological data linking ET to the cerebellum. While neuroimaging studies have variably shown mild atrophy in the cerebellum, marked atrophy is not a clear feature of the cerebellum in ET and a search for a more suitable neuroimaging signature of neurodegeneration is in order. Postmortem studies in ET have examined different neuropathological alterations in the cerebellum, but as of yet have not focused on measures of generalized synaptic markers. This pilot study focuses on synaptic vesicle glycoprotein 2A (SV2A), a protein expressed in practically all synapses in the brain, as a measure of synaptic density in postmortem ET cases.

METHODS

The current study utilized autoradiography with the SV2A radioligand [18F]SDM-16 to assess synaptic density in the cerebellar cortex and dentate nucleus in three ET cases and three age-matched controls.

RESULTS

Using [18F]SDM-16, SV2A was 53% and 46% lower in the cerebellar cortex and dentate nucleus, respectively, in ET cases compared to age-matched controls.

CONCLUSION

In this pilot study, using in vitro SV2A autoradiography, we have observed significantly lower synaptic density in the cerebellar cortex and dentate nucleus of ET cases. Future research could expand on our sample size and focus on in vivo imaging in ET to explore whether SV2A imaging could serve as a much-needed disease biomarker.

Early-onset familial essential tremor is associated with nucleotide expansions of spinocerebellar ataxia in China

BACKGROUND

Essential tremor (ET) is a neurological disease characterized by action tremor in upper arms. Although its high heritability and prevalence worldwide, its etiology and association with other diseases are still unknown.

METHOD

We investigated 10 common spinocerebellar ataxias (SCAs), including SCA1, SCA2, SCA3, SCA6, SCA7, SCA8, SCA12, SCA17, SCA36, dentatorubral-pallidoluysian atrophy (DRPLA) in 92 early-onset familial ET pedigrees in China collected from 2016 to 2022.

RESULT

We found one SCA12 proband carried 51 CAG repeats within PPP2R2B gene and one SCA3 proband with intermediate CAG repeats (55) with ATXN3 gene. The other 90 ET probands all had normal repeat expansions.

CONCLUSION

Tremor can be the initial phenotype of certain SCA. For early-onset, familial ET patients, careful physical examinations are needed before genetic SCA screening.

Differences in Olivo-Cerebellar Circuit and Cerebellar Network Connectivity in Essential Tremor: a Resting State fMRI Study

The olivo-cerebellar circuit is thought to play a crucial role in the pathophysiology of essential tremor (ET). Whether olivo-cerebellar circuit dysfunction is also present at rest, in the absence of clinical tremor and linked voluntary movement, remains unclear. Assessing this network in detail with fMRI is challenging, considering the brainstem is close to major arteries and pulsatile cerebrospinal fluid-filled spaces obscuring signals of interest. Here, we used methods tailored to the analysis of infratentorial structures. We hypothesize that the olivo-cerebellar circuit shows altered intra-network connectivity at rest and decreased functional coupling with other parts of the motor network in ET. In 17 ET patients and 19 healthy controls, we investigated using resting state fMRI intracerebellar functional and effective connectivity on a dedicated cerebellar atlas. With independent component analysis, we investigated data-driven cerebellar motor network activations during rest. Finally, whole-brain connectivity of cerebellar motor structures was investigated using identified components. In ET, olivo-cerebellar pathways show decreased functional connectivity compared with healthy controls. Effective connectivity analysis showed an increased inhibitory influence of the dentate nucleus towards the inferior olive. Cerebellar independent component analyses showed motor resting state networks are less strongly connected to the cerebral cortex compared to controls. Our results indicate the olivo-cerebellar circuit to be affected at rest. Also, the cerebellum is "disconnected" from the rest of the motor network. Aberrant activity, generated within the olivo-cerebellar circuit could, during action, spread towards other parts of the motor circuit and potentially underlie the characteristic tremor of this patient group.

Mapping Essential Tremor to a Common Brain Network Using Functional Connectivity Analysis

BACKGROUND AND OBJECTIVES

The cerebello-thalamo-cortical circuit plays a critical role in essential tremor (ET). However, abnormalities have been reported in multiple brain regions outside this circuit, leading to inconsistent characterization of ET pathophysiology. Here, we test whether these mixed findings in ET localize to a common functional network and whether this network has therapeutic relevance.

METHODS

We conducted a systematic literature search to identify studies reporting structural or metabolic brain abnormalities in ET. We then used 'coordinate network mapping,' which leverages a normative connectome (n = 1,000) of resting-state fMRI data to identify regions commonly connected to findings across all studies. To assess whether these regions may be relevant for the treatment of ET, we compared our network with a therapeutic network derived from lesions that relieved ET. Finally, we investigated whether the functional connectivity of this ET symptom network is abnormal in an independent cohort of patients with ET as compared with healthy controls.

RESULTS

Structural and metabolic brain abnormalities in ET were located in heterogeneous regions throughout the brain. However, these coordinates were connected to a common functional brain network, including the cerebellum, thalamus, motor cortex, precuneus, inferior parietal lobe, and insula. The cerebellum was identified as the hub of this network because it was the only brain region that was both functionally connected to the findings of over 90% of studies and significantly different in connectivity compared with a control data set of other movement disorders. This network was strikingly similar to the therapeutic network derived from lesions improving ET, with key regions aligning in the thalamus and cerebellum. Furthermore, positive functional connectivity between the cerebellar network hub and the sensorimotor cortices was significantly reduced in patients with ET compared with healthy controls, and connectivity within this network was correlated with tremor severity and cognitive functioning.

DISCUSSION

These findings suggest that the cerebellum is the central hub of a network commonly connected to structural and metabolic abnormalities in ET. This network may have therapeutic utility in refining and informing new targets for neuromodulation of ET.

Defective cerebellar ryanodine receptor type 1 and endoplasmic reticulum calcium 'leak' in tremor pathophysiology

Essential Tremor (ET) is a prevalent neurological disease characterized by an 8-10 Hz action tremor. Molecular mechanisms of ET remain poorly understood. Clinical data suggest the importance of the cerebellum in disease pathophysiology, and pathological studies indicate Purkinje Cells (PCs) incur damage. Our recent cerebellar cortex and PC-specific transcriptome studies identified alterations in calcium (Ca2+) signaling pathways that included ryanodine receptor type 1 (RyR1) in ET. RyR1 is an intracellular Ca2+ release channel located on the Endoplasmic Reticulum (ER), and in cerebellum is predominantly expressed in PCs. Under stress conditions, RyR1 undergoes several post-translational modifications (protein kinase A [PKA] phosphorylation, oxidation, nitrosylation), coupled with depletion of the channel-stabilizing binding partner calstabin1, which collectively characterize a "leaky channel" biochemical signature. In this study, we found markedly increased PKA phosphorylation at the RyR1-S2844 site, increased RyR1 oxidation and nitrosylation, and calstabin1 depletion from the RyR1 complex in postmortem ET cerebellum. Decreased calstabin1-RyR1-binding affinity correlated with loss of PCs and climbing fiber-PC synapses in ET. This 'leaky' RyR1 signature was not seen in control or Parkinson's disease cerebellum. Microsomes from postmortem cerebellum demonstrated excessive ER Ca2+ leak in ET vs. controls, attenuated by channel stabilization. We further studied the role of RyR1 in tremor using a mouse model harboring a RyR1 point mutation that mimics constitutive site-specific PKA phosphorylation (RyR1-S2844D). RyR1-S2844D homozygous mice develop a 10 Hz action tremor and robust abnormal oscillatory activity in cerebellar physiological recordings. Intra-cerebellar microinfusion of RyR1 agonist or antagonist, respectively, increased or decreased tremor amplitude in RyR1-S2844D mice, supporting a direct role of cerebellar RyR1 leakiness for tremor generation. Treating RyR1-S2844D mice with a novel RyR1 channel-stabilizing compound, Rycal, effectively dampened cerebellar oscillatory activity, suppressed tremor, and normalized cerebellar RyR1-calstabin1 binding. These data collectively support that stress-associated ER Ca2+ leak via RyR1 may contribute to tremor pathophysiology.

Decreased Synaptic Vesicle Glycoprotein 2A Binding in the Human Postmortem Essential Tremor Cerebellum: Evidence of Reduction in Synaptic Density

Objective Despite being one of the most prevalent neurological diseases, the pathophysiology of essential tremor (ET) is not fully understood. Neuropathological studies have identified numerous degenerative changes in the cerebellum of ET patients, however. These data align with considerable clinical and neurophysiological data linking ET to the cerebellum. While neuroimaging studies have variably shown mild atrophy in the cerebellum, marked atrophy is not a clear feature of the cerebellum in ET and that a search for a more suitable neuroimaging signature of neurodegeneration is in order. Postmortem studies in ET have examined different neuropathological alterations in the cerebellum, but as of yet have not focused on measures of generalized synaptic markers. This pilot study focuses on synaptic vesicle glycoprotein 2A (SV2A), a protein expressed in practically all synapses in the brain, as a measure of synaptic density in postmortem ET cases. Methods The current study utilized autoradiography with the SV2A radioligand [ 18 F]SDM-16 to assess synaptic density in the cerebellar cortex and dentate nucleus in three ET cases and three age-matched controls. Results Using [ 18 F]SDM-16, SV2A was 53% and 46% lower in the cerebellar cortex and dentate nucleus, respectively, in ET cases compared to age-matched controls. Conclusion For the first time, using in vitro SV2A autoradiography, we have observed significantly lower synaptic density in the cerebellar cortex and dentate nucleus of ET cases. Future research could focus on in vivo imaging in ET to explore whether SV2A imaging could serve as a much-needed disease biomarker.

Eye movement study in essential tremor patients and its clinical correlates

Essential tremor (ET) encompasses a wide spectrum of motor and non-motor features. Eye movement abnormalities were first reported two decades ago as an atypical finding in ET. Today, a growing number of publications about eye movement abnormalities in neurodegenerative diseases have helped understand their pathophysiology and the basis of their phenotypic variability. Thus, addressing such aspect in ET may disentangle, based on the oculomotor network abnormalities, the dysfunctional brain pathways in ET. In this study, we aimed to describe neurophysiological eye movement abnormalities in ET and their clinical correlates in terms of cognition and other associated clinical signs. We conducted a cross-sectional study in a tertiary neurology referral center including consecutive ET patients and cognitively normal healthy controls (HC) matched for age and sex. The study protocol included the assessment of voluntary horizontal saccades, smooth pursuit, anti-saccades and saccadic intrusions. We assessed the associated motor signs, cognitive functions and the presence of rapid eye movement disorder (RBD). Sixty-two ET patients and 66 HC were enrolled in the study. Eye movement examination showed significant abnormalities in comparison with HC (46.7% vs 20%, p = 0.002). Prolonged saccadic latency (38.7%, p = 0.033) and altered smooth pursuit (38.7%, p = 0.033) were the most common abnormalities in ET patients. Anti-saccadic errors (16% vs 0% in HC, p = 0.034) correlated with the presence of rigidity (p = 0.046), bradykinesia (p = 0.001), cognitive dysfunction (p = 0.006), executive dysfunction (p = 0.0002), apraxia (p = 0.0001), altered verbal fluency (p = 0.013) and altered backward digit span (p = 0.045) along with the presence of RBD (p = 0.035). Square-wave jerks (11.5% vs 0% in HC, p = 0.0024) correlated with rest tremor. A distinctive phenotype of ET could emerge out of this study characterized by anti-saccadic errors and a sub-cortical cognitive profile, consecutive to the disruption of the cerebello-thalamo-cortical loop. Patients with anti-saccadic errors could be cognitively vulnerable and in need of a close monitoring of their cognitive efficiency during the disease's progression. They may as well convert to Parkinson disease if they present with parkinsonism, RBD and square-wave jerks and require, consequently, a close observation of their motor progression.

Differences in intrinsic functional networks in patients with essential tremor who had good and poor long-term responses after thalamotomy performed using MR-guided ultrasound

OBJECTIVE

Thalamotomy at the nucleus ventralis intermedius using MR-guided focused ultrasound has been an effective treatment method for essential tremor (ET). However, this is not true for all cases, even for successful ablation. How the brain differs in patients with ET between those with long-term good and poor outcomes is not clear. To analyze the functional connectivity difference between patients in whom thalamotomy was effective and those in whom thalamotomy was ineffective and its prognostic role in ET treatment, the authors evaluated preoperative resting-state functional MRI in thalamotomy-treated patients.

METHODS

Preoperative resting-state functional MRI data in 85 patients with ET, who were experiencing tremor relief at the time of treatment and were followed up for a minimum of 6 months after the procedure, were collected for the study. The authors conducted a graph independent component analysis of the functional connectivity matrices of tremor-related networks. The patients were divided into thalamotomy-effective and thalamotomy-ineffective groups (thalamotomy-effective group, ≥ 50% motor symptom reduction; thalamotomy-ineffective group, < 50% motor symptom reduction at 6 months after treatment) and the authors compared network components between groups.

RESULTS

Seventy-two (84.7%) of the 85 patients showed ≥ 50% tremor reduction from baseline at 6 months after thalamotomy. The network analysis shows significant suppression of functional network components with connections between the areas of the cerebellum and the basal ganglia and thalamus, but enhancement of those between the premotor cortex and supplementary motor area in the noneffective group compared to the effective group.

CONCLUSIONS

The present study demonstrates that patients in the noneffective group have suppressed functional subnetworks in the cerebellum and subcortex regions and have enhanced functional subnetworks among motor-sensory cortical networks compared to the thalamotomy-effective group. Therefore, the authors suggest that the functional connectivity pattern might be a possible predictive factor for outcomes of MR-guided focused ultrasound thalamotomy.

Propranolol Modulates Cerebellar Circuit Activity and Reduces Tremor

Tremor is the most common movement disorder. Several drugs reduce tremor severity, but no cures are available. Propranolol, a β-adrenergic receptor blocker, is the leading treatment for tremor. However, the in vivo circuit mechanisms by which propranolol decreases tremor remain unclear. Here, we test whether propranolol modulates activity in the cerebellum, a key node in the tremor network. We investigated the effects of propranolol in healthy control mice and Car8^wdl/wdl mice, which exhibit pathophysiological tremor and ataxia due to cerebellar dysfunction. Propranolol reduced physiological tremor in control mice and reduced pathophysiological tremor in Car8^wdl/wdl mice to control levels. Open field and footprinting assays showed that propranolol did not correct ataxia in Car8^wdl/wdl mice. In vivo recordings in awake mice revealed that propranolol modulates the spiking activity of control and Car8^wdl/wdl Purkinje cells. Recordings in cerebellar nuclei neurons, the targets of Purkinje cells, also revealed altered activity in propranolol-treated control and Car8^wdl/wdl mice. Next, we tested whether propranolol reduces tremor through β₁ and β₂ adrenergic receptors. Propranolol did not change tremor amplitude or cerebellar nuclei activity in β₁ and β₂ null mice or Car8^wdl/wdl mice lacking β₁ and β₂ receptor function. These data show that propranolol can modulate cerebellar circuit activity through β-adrenergic receptors and may contribute to tremor therapeutics.

Cognitive subtypes in individuals with essential tremor seeking deep brain stimulation

Objective: Essential tremor (ET) is a common neurological disorder that has been associated with 60% increased risk of developing dementia. The goals of the present study were to: (a) learn whether individuals with advanced ET symptoms seeking deep brain stimulation (DBS) surgery would fall into distinct cognitive subgroups, and (b) learn how empirically derived subgroups map onto criteria for mild cognitive impairment (MCI). Method: Patients with ET (N = 201; mean age = 68.9 ± 8.9 years) undergoing pre-surgical evaluation for DBS completed a multi-domain neurocognitive assessment consisting of memory, executive function, visuospatial skill, language, and processing speed. Two cluster analytic approaches (K-means, hierarchical) were independently conducted to classify cognitive patterns using domain composites. Demographics, clinical characteristics, and proportion of cases meeting neuropsychologically defined criteria for MCI were examined among clusters. Results: A three-cluster solution reflected a Low Executive group (N = 64), Low Memory Multi-Domain group (N = 41), and Cognitively Normal group (N = 96). The Cognitively Normal group was older and more educated, with a higher Dementia Rating Scale-2 score. In total, 27.4% of participants met criteria for MCI. Of the MCI cases, most were in the Low Executive (41.8%) or Low Memory Multi-Domain groups (49.1%). In the latter, 65.9% of its members were classified as MCI versus 35.9% in the Low Executive group. Conclusions: Our study identified three cognitive subtypes of ET patients presenting for DBS. Future work should examine the subgroups for progression to dementia, particularly the Low Memory Multi-Domain subgroup which may be at highest risk.

Connecting tremors - a circuits perspective

PURPOSE OF REVIEW

Tremor is one of the most prevalent movement disorders in clinical practice. Here, we review new insights in the pathophysiology of tremor. We focus on the three most common tremor disorders: essential tremor (ET), dystonic tremor syndrome (DTS), and Parkinson's disease (PD) tremor.

RECENT FINDINGS

Converging evidence suggests that ET, DTS, and PD tremor are all associated with (partly) overlapping cerebral networks involving the basal ganglia and cerebello-thalamo-cortical circuit. Recent studies have assessed the role of these networks in tremor by measuring tremor-related activity and connectivity with electrophysiology and neuroimaging, and by perturbing network components using invasive and noninvasive brain stimulation. The cerebellum plays a more dominant and causal role in action tremors than in rest tremor, as exemplified by recent findings in ET, DTS, and re-emergent tremor in PD. Furthermore, the role of the cerebellum in DTS is related to clinical differences between patients, for example, whether or not the tremor occurs in a dystonic limb, and whether the tremor is jerky or sinusoidal.

SUMMARY

Insight into the pathophysiological mechanisms of tremor may provide a more direct window into mechanism-based treatment options than either the etiology or the clinical phenotype of a tremor syndrome.

Targeting the vim by direct visualization of the cerebello-thalamo-cortical pathway in 3 T proton density MRI: correlation with focused ultrasound lesioning

Surgical targeting of the ventral intermediate nucleus of the thalamus (VIM) has been historically done using indirect strategies. Here we depict the cerebello-thalamo-cortical tract (CTCT) through 3 T proton density (PD) in a cohort of patients who underwent high-intensity focus ultrasound (HIFUS) thalamotomy. Forty-seven patients treated in our institution with MR-guided HIFUS VIM thalamotomy were included in this study. PD weighted 3 T MRI used for presurgical planning was compared with postoperative MRI obtained 1 month after surgery. Images were processed with ISTX software (Brain lab, Munich, Germany). The coordinates of the VIM lesion concerning the inter-commissural line (ICL) were annotated. Deterministic tractographies using three ROIs were used to verify the different tracts. The triangle seen in the 3 T PD sequence at the level of the mesencephalic-diencephalic junction was systematically recognized. The posterior angle of this triangle at the junction of the CTCT and the ZI was denominated as "point P." The area of this triangle corresponds to the posterior subthalamic area (PSA) harboring the Raprl fibers. The CTCT was visible from 1 to 2.5 mm below the ICL. The average center of the final HIFUS lesion (point F) was 11 mm from the medial thalamic border of the thalamus (14.9 mm from the midline), 6.4 mm anterior to PC, and 0.6 mm above the ICL. The FUS point was consistently 1-2 mm directly above point P. The anterior border of the external angle of this triangle (point P) can be used as an intraparenchymal point for targeting the ventral border of the VIM. Three ROIs placed in a single slice around this triangle are a fast way to originate tractography of the CTCT, lemniscus medialis, and pyramidal tract.

Cerebello-Thalamo-Cortical Network Dynamics in the Harmaline Rodent Model of Essential Tremor

Essential Tremor (ET) is a common movement disorder, characterised by a posture or movement-related tremor of the upper limbs. Abnormalities within cerebellar circuits are thought to underlie the pathogenesis of ET, resulting in aberrant synchronous oscillatory activity within the thalamo-cortical network leading to tremors. Harmaline produces pathological oscillations within the cerebellum, and a tremor that phenotypically resembles ET. However, the neural network dynamics in cerebellar-thalamo-cortical circuits in harmaline-induced tremor remains unclear, including the way circuit interactions may be influenced by behavioural state. Here, we examined the effect of harmaline on cerebello-thalamo-cortical oscillations during rest and movement. EEG recordings from the sensorimotor cortex and local field potentials (LFP) from thalamic and medial cerebellar nuclei were simultaneously recorded in awake behaving rats, alongside measures of tremor using EMG and accelerometery. Analyses compared neural oscillations before and after systemic administration of harmaline (10 mg/kg, I.P), and coherence across periods when rats were resting vs. moving. During movement, harmaline increased the 9-15 Hz behavioural tremor amplitude and increased thalamic LFP coherence with tremor. Medial cerebellar nuclei and cerebellar vermis LFP coherence with tremor however remained unchanged from rest. These findings suggest harmaline-induced cerebellar oscillations are independent of behavioural state and associated changes in tremor amplitude. By contrast, thalamic oscillations are dependent on behavioural state and related changes in tremor amplitude. This study provides new insights into the role of cerebello-thalamo-cortical network interactions in tremor, whereby neural oscillations in thalamocortical, but not cerebellar circuits can be influenced by movement and/or behavioural tremor amplitude in the harmaline model.

Essential Tremor Within the Broader Context of Other Forms of Cerebellar Degeneration

Essential tremor (ET) has recently been reconceptualized by many as a degenerative disease of the cerebellum. Until now, though, there has been no attempt to frame it within the context of these diseases. Here, we compare the clinical and postmortem features of ET with other cerebellar degenerations, thereby placing it within the broader context of these diseases. Action tremor is the hallmark feature of ET. Although often underreported in the spinocerebellar ataxias (SCAs), action tremors occur, and it is noteworthy that in SCA12 and 15, they are highly prevalent, often severe, and can be the earliest disease manifestation, resulting in an initial diagnosis of ET in many cases. Intention tremor, sometimes referred to as "cerebellar tremor," is a common feature of ET and many SCAs. Other features of cerebellar dysfunction, gait ataxia and eye motion abnormalities, are seen to a mild degree in ET and more markedly in SCAs. Several SCAs (e.g., SCA5, 6, 14, and 15), like ET, follow a milder and more protracted disease course. In ET, numerous postmortem changes have been localized to the cerebellum and are largely confined to the cerebellar cortex, preserving the cerebellar nuclei. Purkinje cell loss is modest. Similarly, in SCA3, 12, and 15, Purkinje cell loss is limited, and in SCA12 and 15, there is preservation of cerebellar nuclei and relative sparing of other central nervous system regions. Both clinically and pathologically, there are numerous similarities and intersection points between ET and other disorders of cerebellar degeneration.

White Matter Hyperintensities Are Associated With Severity of Essential Tremor in the Elderly

Background: Essential tremor (ET) occurs with steeply increasing prevalence in the elderly, and apart from disease duration, age is independently associated with an increase of tremor amplitude and a decrease of frequency. White matter hyperintensities (WMHs) are a common finding in the elderly, and their role in the pathophysiology of ET is unknown. The aims of this study were to examine whether ET patients differ in their total or region-specific WMH volumes from healthy controls and to determine the impact of WMH on tremor characteristics. Methods: A total of 47 elderly ET patients with a mean age of 72 years and 39 age-matched healthy controls underwent a thorough clinical assessment and 3T MRI. Total WMH volumes were derived from T2-weighted fluid-attenuated inversion recovery (FLAIR) MR images. Additionally, region of interest-based WMH volumes for the Johns Hopkins University (JHU) white matter tracts and labels were calculated, and WMHs were assessed semiquantitatively using the Fazekas scale. Results: Essential tremor patients and healthy controls did not differ in their total or tract-specific WMH volumes or Fazekas scores. However, WMH volume was significantly positively correlated with tremor severity on the TETRAS scale, and there was a significant negative correlation with the mean accelerometric tremor frequency. In a multiple linear regression model including disease duration, age, and age-adjusted total WMH volume, only the WMH volume significantly predicted tremor severity, while age and disease duration were not significant. Conclusion: We found evidence for a direct association between WMH volume and tremor severity. If confirmed by larger studies, our findings could explain the well-known relation between age and tremor severity.

GABA, Glutamate, and NAA Levels in the Deep Cerebellar Nuclei of Essential Tremor Patients

Background: Essential tremor is among the commonly observed movement disorders in clinical practice, however the exact pathophysiological mechanisms underlying tremor are unknown. It has been suggested that Purkinje cell alterations play a causal factor in tremorgenesis. Altered levels of inhibitory (GABA) and excitatory (glutamate+glutamine, Glx) neurotransmitters could be markers for Purkinje cell alterations. We hypothesize that GABA and Glx levels in the dentate nuclei could be differentially altered in patients responsive to either anticonvulsants or β-adrenergic blockers. Methods: In this explorative study in patients with essential tremor, we measured gamma-aminobutyric acid (GABA) and glutamate+glutamine (Glx) levels in the dentate nucleus region using 1H-magnetic resonance spectroscopy (MRS) in seven patients using propranolol, five patients using anticonvulsants, and eight healthy controls. Results: There were no group differences with respect to GABA+/Cr, Glx/Cr, NAA/Cr, and GABA+/Glx ratios. There was no correlation with tremor severity. Discussion: Our results are in line with previously published studies; however, additional studies on a larger number of patients are warranted to confirm these findings. Furthermore medication-subgroups did not exhibit differences with respect to GABA+/Cr, Glx/Cr, NAA/Cr, and GABA+/Glx ratios. A recent study, of similar size, found an inverse association between tremor severity and the GABA+/Glx ratio in the cerebellum of essential tremor patients. We were unable to replicate these findings. The field of tremor research is plagued by heterogeneous results, and we would caution against drawing firm conclusions based on pilot studies.

A Study on the Essential and Parkinson’s Arm Tremor Classification

In this article, the challenge of discriminating between essential and Parkinson’s tremor is addressed. Although a variety of methods have been proposed for diagnosing the severity of these highly occurring tremor types, their rapid and effective identification, especially in their early stages, proves particularly difficult and complicated due to their wide range of causes and similarity of symptoms. To this goal, a clinical analysis was performed, where a number of volunteers including essential and Parkinson’s tremor-diagnosed patients underwent a series of pre-defined motion patterns, during which a wearable sensing setup was used to measure their lower arm tremor characteristics from multiple selected points. Extracted features from the acquired accelerometer signals were used to train classification algorithms, including decision trees, discriminant analysis, support vector machine (SVM), K-nearest neighbor (KNN) and ensemble learning algorithms, for providing a comparative study and evaluating the potential of utilizing machine learning to accurately discriminate among different tremor types. Overall, SVM related classifiers proved to be the most successful in terms of classifying between Parkinson’s, essential and no tremor diagnosed with percentages reaching up to 100% for a single accelerometer measurement at the metacarpal area. In general and in motion while holding an object position, Coarse Gaussian SVM classifier reached 82.62% accuracy.

Voxel-based morphometry and task functional magnetic resonance imaging in essential tremor: evidence for a disrupted brain network

The pathophysiology of essential tremor (ET) is controversial and might be further elucidated by advanced neuroimaging. Focusing on homogenous ET patients diagnosed according to the 2018 consensus criteria, this study aimed to: (1) investigate whether task functional MRI (fMRI) can identify networks of activated and deactivated brain areas, (2) characterize morphometric and functional modulations, relative to healthy controls (HC). Ten ET patients and ten HC underwent fMRI while performing two motor tasks with their upper limb: (1) maintaining a posture (both groups); (2) simulating tremor (HC only). Activations/deactivations were obtained from General Linear Model and compared across groups/tasks. Voxel-based morphometry and linear regressions between clinical and fMRI data were also performed. Few cerebellar clusters of gray matter loss were found in ET. Conversely, widespread fMRI alterations were shown. Tremor in ET (task 1) was associated with extensive deactivations mainly involving the cerebellum, sensory-motor cortex, and basal ganglia compared to both tasks in HC, and was negatively correlated with clinical tremor scales. Homogeneous ET patients demonstrated deactivation patterns during tasks triggering tremor, encompassing a network of cortical and subcortical regions. Our results point towards a marked cerebellar involvement in ET pathophysiology and the presence of an impaired cerebello-thalamo-cortical tremor network.

Essential tremor: the most common form of cerebellar degeneration?

Background

The degenerative cerebellar ataxias comprise a large and heterogeneous group of neurological diseases whose hallmark clinical feature is ataxia, and which are accompanied, to variable degrees, by other features that are attributable to cerebellar dysfunction. Essential tremor (ET) is an exceptionally common neurological disease whose primary motor feature is action tremor, although patients often manifest intention tremor, mild gait ataxia and several other features of cerebellar dysfunction.

Main Body

In this paper, we review the abundant evidence derived from clinical, neuroimaging and postmortem studies, linking ET to cerebellar dysfunction. Furthermore, we review the combination of clinical, natural history and postmortem features suggesting that ET is neurodegenerative. We then compare the prevalence of ET (400 – 900 cases per 100,000) to that of the other cerebellar degenerations (ranging from <0.5 – 9 cases per 100,000, and in composite likely to be on the order of 20 cases per 100,000) and conclude that ET is 20 to 45 times more prevalent than all other forms of cerebellar degeneration combined.

Conclusion

Given the data we present, it is logical to conclude that ET is, by far, the most common form of cerebellar degeneration.

Brain microstructural changes and cognitive function in non-demented essential tremor patients: a diffusion tensor imaging study

BACKGROUND

Essential tremor (ET) is disease with both motor and non-motor features. Notable among the non-motor features is cognitive impairment. While this impairment has been attributed to cortico-thalamo-cerebellar pathway pathology, it is likely that a more complicated involvement of brain structures underlies cognitive function in ET.

OBJECTIVE

To evaluate the brain microstructural changes of both white matter and grey matter in ET using region of interest based diffusion tensor imaging (DTI), and to correlate these changes with cognitive function assessed during detailed neuropsychological testing.

METHOD

Thirty-five non-demented ET patients with a range of cognitive function (Clinical Dementia Rating = 0-0.5, mean age = 57.5 ± 16.7 years, age range = 23-76 years) underwent a comprehensive neuropsychological evaluation and brain magnetic resonance imaging, including DTI. DTI findings were reported as fractional anisotropy, average diffusion coefficient, these values were evaluated for 32 ROIs. Cognitive domains included attention, visuospatial functions, executive function, verbal memory, visual memory, and language. Domain Z-scores were calculated each cognitive domain and compared for each brain region.

RESULTS

Microstructural changes in prefrontal cortical areas (dorsolateral, ventrolateral), paralimbic and limbic structures (posterior cingulate cortex, precuneus, hippocampus), basal ganglia (substantia nigra, putamen, caudate nucleus) and white matter bundles (corpus callosum, anterior thalamic radiation, longitudinal fasciculus, frontooccipital fasciculus, etc.) correlated with specific domains of cognitive function in ET patients.

CONCLUSION

These data suggest that not only the cerebello thalamocortical pathway, but numerous other brain structures are related to level of cognitive performance and possibly underlie cognitive dysfunction in ET.

Evidence of central involvement in essential tremor: a detailed study of auditory pathway physiology

Essential tremor (ET) is a common tremor disorder that is likely neurodegenerative. The pathophysiology of ET involves the cerebellum and its connections in the brainstem and thalamus. Hearing dysfunction has been shown to be a non-motor finding in ET patients. A limited number of studies have suggested that cochlear pathology is the cause, but studies have not evaluated the integrity of the primary auditory pathway in ET. The main aim of this study is to investigate the integrity of the auditory pathway via auditory brainstem response (ABR) and auditory middle latency response (AMLR), thereby allowing us to evaluate the auditory pathway from the 8th cranial nerve to the cerebral cortex. Sixteen ET patients and sixteen age- and gender-matched controls (64 ears) were evaluated. In the ABR study, we detected prolongation of wave V peak latencies (ms) in ET (p = 0.02). In the AMLR study, P0 (p = 0.03), Pa (p = 0.008), Na (p = 0.03), and Nb (p = 0.01) waves differed between the two groups. Eleven ET patients and four control subjects had abnormal electrophysiological findings (ABR or AMLR or both) (68.8% vs. 25%, p = 0.01). Tremor duration was greater in ET patients with abnormal electrophysiological findings (p = 0.01). Finally, we observed prolongation of latencies after the ABR III wave, indicating that abnormalities exist within the superior olivary complex. For the first time, our research provides evidence that ET-related pathology is present at the subcortical and cortical levels of the auditory pathway.

Structural Correlates of the Sensorimotor Cerebellum in Parkinson's Disease and Essential Tremor

BACKGROUND

Parkinson's disease (PD) and essential tremor (ET) are commonly encountered movement disorders. Pathophysiologic processes that localize to the cerebellum are described in both. There are limited studies investigating cerebellar structural changes in these conditions, largely because of inherent challenges in the efficiency of segmentation.

METHODS

We applied a novel multiatlas cerebellar segmentation method to T1-weighted images in 282 PD and 111 essential tremor patients to define 26 cerebellar lobule volumes. The severity of postural and resting tremor in both populations and gait and postural instability in PD patients were defined using subscores of the UPDRS and Washington Heights-Inwood Genetic Study motor scales. These clinical measurements were related to lobule volume size. Multiple comparisons were controlled using a false discovery rate method.

RESULTS

Group differences were identified between ET and PD patients, with reductions in deep cerebellar nucleus volume in ET versus reduced lobule VI volume in PD. In ET patients, lobule VIII was negatively correlated with the severity of postural tremor. In PD patients, lobule IV was positively correlated with resting tremor and total tremor severity. We observed differences in cerebellar structure that localized to sensorimotor lobules of the cerebellum. Lobule volumes appeared to differentially relate to clinical symptoms, suggesting important clinicopathologic distinctions between these conditions. These results emphasize the role of the cerebellum in tremor symptoms and should foster future clinical and pathologic investigations of the sensorimotor lobules of the cerebellum. © 2020 International Parkinson and Movement Disorder Society.

Clinical characteristics of patients with essential tremor or essential tremor plus

BACKGROUND

The International Parkinson and Movement Disorder Society introduced the category of essential tremor (ET)-plus in its new ET classification scheme, but how the clinical correlates of ET-plus differ from those of "pure" ET is unclear. By comparing the clinical characteristics of ET and ET-plus patients, we expect to better understand the impact and invoked questions of the new classification on clinical practice.

METHODS

We reviewed the medical records of 280 ET syndrome patients in an ongoing cross-sectional study in a Chinese population and reclassified them according to the new criteria. Clinico-demographic characteristics were compared between ET and ET-plus patients. Risk factors of diagnosis of ET-plus were explored using logistic regression.

RESULTS

A total of 121 patients (50.8%) were reclassified as having ET and 117 as having ET-plus. ET-plus group was significantly older at tremor onset, less educated, and more likely to have cranial tremor, depression symptom, anxiety symptom, and probable REM sleep behavior disorder. ET-plus group also showed more severe upper limb tremor and cognition impairment. Regression analysis identified four independent risk factors associated with ET-plus: late tremor onset (OR 3.04, 95%CI 1.60-5.79), less educated (OR 0.91, 95%CI 0.85-0.97), severe upper limb tremor (OR 2.46, 95%CI 1.30-4.62), and presence of cranial tremor (OR 2.30, 95%CI 1.20-4.41).

CONCLUSIONS

The new classification scheme emphasized that ET syndrome is heterogeneous. ET-plus cannot be seen as a subtype or a diagnosis of ET syndrome, but rather as a placeholder, representing an area of current scientific uncertainty.

Purkinje cell misfiring generates high-amplitude action tremors that are corrected by cerebellar deep brain stimulation

Tremor is currently ranked as the most common movement disorder. The brain regions and neural signals that initiate the debilitating shakiness of different body parts remain unclear. Here, we found that genetically silencing cerebellar Purkinje cell output blocked tremor in mice that were given the tremorgenic drug harmaline. We show in awake behaving mice that the onset of tremor is coincident with rhythmic Purkinje cell firing, which alters the activity of their target cerebellar nuclei cells. We mimic the tremorgenic action of the drug with optogenetics and present evidence that highly patterned Purkinje cell activity drives a powerful tremor in otherwise normal mice. Modulating the altered activity with deep brain stimulation directed to the Purkinje cell output in the cerebellar nuclei reduced tremor in freely moving mice. Together, the data implicate Purkinje cell connectivity as a neural substrate for tremor and a gateway for signals that mediate the disease.

Essential tremor-plus: a controversial new concept

In addition to redefining essential tremor (ET), the 2018 consensus statement of the Movement Disorder Society on tremor coined a new term: essential tremor-plus (ET-plus). This term is uncertainly defined as tremor with the characteristics of ET, with additional neurological signs of uncertain clinical significance. If ET-plus had been defined on the basis of a difference in underlying pathology or an appreciable difference in prognosis, it would have a valid, scientific rationale, as does the term Parkinson-plus. However, there is no such evidence, so the basis for the term is questionable. In fact, ET-plus might only represent a state condition (ie, patients with ET might develop these additional clinical features when the disease is at a more advanced stage). We caution against coining new terms that are not supported by a firm scientific basis and encourage research into the creation of essential tremor subsets that are defined with respect to differences in underlying causes or pathophysiology.

Clofibrate, a PPAR-α agonist, abrogates sodium fluoride-induced neuroinflammation, oxidative stress, and motor incoordination via modulation of GFAP/Iba-1/anti-calbindin signaling pathways

Fluoride is an environmental contaminant that is ubiquitously present in air, water, and soil. It is commonly added in minute quantity to drinking water, toothpaste, and mouth rinses to prevent tooth decay. Epidemiological findings have demonstrated that exposure to fluoride induced neurodevelopmental toxicity, developmental neurotoxicity, and motor disorders. The neuroprotective effect of clofibrate, a peroxisome proliferator-activated receptor alpha agonist, was investigated in the present study. Forty male Wistar rats were used for this study and randomly grouped into 10 rats per group as control, sodium fluoride (NaF) alone (300 ppm), NaF plus clofibrate (250 mg/kg), and NaF plus lisinopril (10 mg/kg), respectively, for 7 days. NaF was administered in drinking water while clofibrate and lisinopril were administered by oral gavage. Markers of neuronal inflammation and oxidative stress, acetylcholinesterase activity, and neurobehavioral (hanging wire and open field) tests were performed. Immunohistochemistry was performed on brain tissues, and they were probed with glial fibrillary acidic protein, ionized calcium-binding adaptor molecule 1, and cerebellar Ca²⁺ -binding protein calbindin-D28k. The results showed that NaF significantly increased of oxidative stress and neuroinflammation and inhibited AChE activity. Immunostaining showed reactive astrocytes, microgliosis, loss of dendritic spines, and arborization in Purkinje cells in rats administered only NaF. Neurobehavioral results showed that cotreatment of NaF with clofibrate improved muscular strength and locomotion, reduced anxiety, and significantly reduced astrocytic count. Overall, cotreatment of NaF with either clofibrate or lisinopril showed neuroprotective effects by mitigating neuronal inflammation and oxidative and motor incoordination. Hence, clofibrate could be seen as a novel drug candidate against neurodegeneration and motor disorders.

Profiles of Normal Cognition in Essential Tremor

OBJECTIVES

Patients with essential tremor exhibit heterogeneous cognitive functioning. Although the majority of patients fall under the broad classification of cognitively "normal," essential tremor is associated with increased risk for mild cognitive impairment and dementia. It is possible that patterns of cognitive performance within the wide range of normal functioning have predictive utility for mild cognitive impairment or dementia. These cross-sectional analyses sought to determine whether cognitive patterns, or "clusters," could be identified among individuals with essential tremor diagnosed as cognitively normal. We also determined whether such clusters, if identified, were associated with demographic or clinical characteristics of patients.

METHODS

Elderly subjects with essential tremor (age >55 years) underwent comprehensive neuropsychological testing. Domain means (memory, executive function, attention, visuospatial abilities, and language) from 148 individuals diagnosed as cognitively normal were partitioned using k-means cluster analysis. Individuals in each cluster were compared according to cognitive functioning (domain means and test scores), demographic factors, and clinical variables.

RESULTS

There were three clusters. Cluster 1 (n = 64) was characterized by comparatively low memory scores (p < .001), Cluster 2 (n = 39) had relatively low attention and visuospatial scores (p < .001), and Cluster 3 (n = 45) exhibited consistently high performance across all domains. Cluster 1 had lower Montreal Cognitive Assessment scores and reported more prescription medication use and lower balance confidence.

CONCLUSIONS

Three patterns of cognitive functioning within the normal range were evident and tracked with certain clinical features. Future work will examine the extent to which such patterns predict conversion to mild cognitive impairment and/or dementia.

Essential tremor pathology: neurodegeneration and reorganization of neuronal connections

Essential tremor (ET) is the most common tremor disorder globally and is characterized by kinetic tremor of the upper limbs, although other clinical features can also occur. Postmortem studies are a particularly important avenue for advancing our understanding of the pathogenesis of ET; however, until recently, the number of such studies has been limited. Several recent postmortem studies have made important contributions to our understanding of the pathological changes that take place in ET. These studies identified abnormalities in the cerebellum, which primarily affected Purkinje cells (PCs), basket cells and climbing fibres, in individuals with ET. We suggest that some of these pathological changes (for example, focal PC axonal swellings, swellings in and regression of the PC dendritic arbor and PC death) are likely to be primary and degenerative. By contrast, other changes, such as an increase in PC recurrent axonal collateral formation and hypertrophy of GABAergic basket cell axonal processes, could be compensatory responses to restore cerebellar GABAergic tone and cerebellar cortical inhibitory efficacy. Such compensatory responses are likely to be insufficient, enabling the disease to progress. Here, we review the results of recent postmortem studies of ET and attempt to place these findings into an anatomical-physiological disease model.

Deep brain stimulation in the caudal zona incerta modulates the sensorimotor cerebello-cerebral circuit in essential tremor

Essential tremor is effectively treated with deep brain stimulation (DBS), but the neural mechanisms underlying the treatment effect are poorly understood. Essential tremor is driven by a dysfunctional cerebello-thalamo-cerebral circuit resulting in pathological tremor oscillations. DBS is hypothesised to interfere with these oscillations at the stimulated target level, but it is unknown whether the stimulation modulates the activity of the cerebello-thalamo-cerebral circuit during different task states (with and without tremor) in awake essential tremor patients. To address this issue, we used functional MRI in 16 essential tremor patients chronically implanted with DBS in the caudal zona incerta. During scanning, the patients performed unilateral tremor-inducing postural holding and pointing tasks as well as rest, with contralateral stimulation turned On and Off. We show that DBS exerts both task-dependent as well as task-independent modulation of the sensorimotor cerebello-cerebral regions (p ​≤ ​0.05, FWE cluster-corrected for multiple comparisons). Task-dependent modulation (DBS ​× ​task interaction) resulted in two patterns of stimulation effects. Firstly, activity decreases (blood oxygen level-dependent signal) during tremor-inducing postural holding in the primary sensorimotor cortex and cerebellar lobule VIII, and activity increases in the supplementary motor area and cerebellar lobule V during rest (p ​≤ ​0.05, post hoc two-tailed t-test). These effects represent differences at the effector level and may reflect DBS-induced tremor reduction since the primary sensorimotor cortex, cerebellum and supplementary motor area exhibit less motor task-activity as compared to the resting condition during On stimulation. Secondly, task-independent modulation (main effect of DBS) was observed as activity increase in the lateral premotor cortex during all motor tasks, and also during rest (p ​≤ ​0.05, post hoc two-tailed t-test). This task-independent effect may mediate the therapeutic effects of DBS through the facilitation of the premotor control over the sensorimotor circuit, making it less susceptible to tremor entrainment. Our findings support the notion that DBS in essential tremor is modulating the sensorimotor cerebello-cerebral circuit, distant to the stimulated target, and illustrate the complexity of stimulation mechanisms by demonstrating task-dependent as well as task-independent actions in cerebello-cerebral regions.

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.

Expansion of GGC repeat in the human-specific NOTCH2NLC gene is associated with essential tremor

Essential tremor is one of the most common movement disorders. Despite its high prevalence and heritability, the genetic aetiology of essential tremor remains elusive. Up to now, only a few genes/loci have been identified, but these genes have not been replicated in other essential tremor families or cohorts. Here we report a genetic study in a cohort of 197 Chinese pedigrees clinically diagnosed with essential tremor. Using a comprehensive strategy combining linkage analysis, whole-exome sequencing, long-read whole-genome sequencing, repeat-primed polymerase chain reaction and GC-rich polymerase chain reaction, we identified an abnormal GGC repeat expansion in the 5′ region of the NOTCH2NLC gene that co-segregated with disease in 11 essential tremor families (5.58%) from our cohort. Clinically, probands that had an abnormal GGC repeat expansion were found to have more severe tremor phenotypes, lower activities of daily living ability. Obvious genetic anticipation was also detected in these 11 essential tremor-positive families. These results indicate that abnormal GGC repeat expansion in the 5′ region of NOTCH2NLC gene is associated with essential tremor, and provide strong evidence that essential tremor is a family of diseases with high clinical and genetic heterogeneities.

Revisiting the Clinical Phenomenology of "Cerebellar Tremor": Beyond the Intention Tremor

Tremor is an involuntary, rhythmic, oscillatory movement of a body part. It is a central feature of a range of diseases resulting from pathological changes in the cerebellum. Interestingly, in modern times, the terms "cerebellar tremor" and "intention tremor" are often used synonymously and interchangeably. However, "cerebellar tremor" (i.e., tremors of cerebellar origin) do not always present exclusively as intention tremor. In this article, we comprehensively revisit the clinical phenomenology of tremors observed in various diseases that are based in the cerebellum. By this, we mean diseases for which the cerebellum and its various connections are often seen as playing a central and defining role. These include spinocerebellar ataxias, essential tremor, orthostatic tremor, dystonia, acute cerebellitis, cerebellar tumors, paraneoplastic cerebellar degeneration, and cerebellar strokes. The theme of this article is to highlight, through published data available in the current literature, that the clinical phenomenology of tremor of cerebellar origin is heterogeneous, and it extends beyond that of intention tremor to include postural tremors, kinetic tremor, rest tremor, and orthostatic tremor. This heterogeneity is consistent with the seminal work of Gordon Holmes, in which he described a variety of tremors aside from intention tremor in the setting of cerebellar lesions. In the end, it would seem that the notion that intention tremor is the sole signature of cerebellar lesions is an over-simplification and is not correct. Future studies are warranted to identify and further characterize the heterogeneity of tremors arising from the various cerebellar etiologies.

Current Opinions and Consensus for Studying Tremor in Animal Models

Tremor is the most common movement disorder; however, we are just beginning to understand the brain circuitry that generates tremor. Various neuroimaging, neuropathological, and physiological studies in human tremor disorders have been performed to further our knowledge of tremor. But, the causal relationship between these observations and tremor is usually difficult to establish and detailed mechanisms are not sufficiently studied. To overcome these obstacles, animal models can provide an important means to look into human tremor disorders. In this manuscript, we will discuss the use of different species of animals (mice, rats, fruit flies, pigs, and monkeys) to model human tremor disorders. Several ways to manipulate the brain circuitry and physiology in these animal models (pharmacology, genetics, and lesioning) will also be discussed. Finally, we will discuss how these animal models can help us to gain knowledge of the pathophysiology of human tremor disorders, which could serve as a platform towards developing novel therapies for tremor.

Outcomes of Onabotulinum Toxin A Treatment for Adductor Spasmodic Dysphonia and Laryngeal Tremor

Importance

The relative outcomes of onabotulinum toxin A injections for treatment of adductor spasmodic dysphonia (ADSD), ADSD with lateral laryngeal tremor (ADSD+LT), and lateral LT without ADSD are unclear.

Objective

To compare the outcomes of onabotulinum toxin A treatment on ADSD, ADSD+LT, and lateral LT without ADSD.

Design, Setting, and Participants

A retrospective cohort study was conducted from January 1, 1990, to September 30, 2016, at a tertiary referral voice center. Participants included 817 patients treated with onabotulinum toxin A injections for diagnosis of ADSD, ADSD+LT, and lateral LT without ADSD.

Exposure

Injection of onabotulinum toxin A into the thyroarytenoid/lateral cricoarytenoid muscle complex.

Main Outcomes and Measures

Data from patient diaries were used to evaluate patient-perceived effectiveness of onabotulinum toxin A injection. Primary outcomes were (1) patient-reported good voice days (voice breaks or tremor minimized to patient satisfaction) and (2) percentage of injections in which maximal voice quality was reached (significant or complete reduction in vocal tremor or spasms during a treatment cycle). Multivariate analysis of variance tests compared differences in outcomes between groups. Subanalysis was performed to compare outcomes in patients with isolated LT with those who had mixed tremor (lateral with concomitant anterior-posterior and/or vertical components).

Results

Of 817 patients treated with onabotulinum toxin A injections for laryngeal movement disorders, 548 patients (12 771 injection sessions) met inclusion criteria (ADSD: n = 328, ADSD+LT: n = 77, lateral LT without ADSD: n = 143). Of these, 408 (80.8%) were women; mean (SD) age was 57.2 (13.7) years. Among patients with tremor, those with isolated LT had better outcomes than those with mixed tremor. In adjusted analysis, good voice days in patients with ADSD, ADSD+LT, and lateral LT without ADSD were 81.1, 75.4, and 71.3 days, respectively (partial η2, 0.05; 95% CI, 0.01-0.09). The percentage of maximally beneficial injections was 88.1% for ADSD, 83.4% for ADSD+LT, and 70.4% for LT without ADSD (partial η2, 0.12; 95% CI, 0.06-0.17).

Conclusions and Relevance

Onabotulinum toxin A injections into the thyroarytenoid/lateral cricoarytenoid muscle complex are an effective treatment for ADSD, ADSD+LT, and LT without ADSD; however, the greatest effectiveness was observed among patients with ADSD. Defining tremor directionality may help to prognosticate the effectiveness of onabotulinum toxin A injection among patients presenting with tremor components.

Lamotrigine Induces Tremor among Epilepsy Patients Probably via Cerebellar Pathways

Lamotrigine, a frequently used antiepileptic drug, inhibits voltage-gated sodium-channels. By suppressing the release of glutamate and aspartate, lamotrigine acts as a membrane stabilizer, and it is also effective in bipolar disorder and migraine. However, lamotrigine is known to induce tremor among 4-10% of patients. We examined the lamotrigine-induced tremor in 28 epilepsy patients (age: 38.06 ± 13.56 years; 24 females and 4 males) receiving lamotrigine monotherapy and compared the data to 30 age- and sex-matched controls (age: 33.06 ± 10.71 years; 25 females and 5 males). Tremor was visually assessed by clinical tremor rating scales. Quantitative characteristics (intensity, center frequency and frequency dispersion) which are regularly used to differentiate various tremor syndromes were measured by validated, sensitive biaxial accelerometry in resting, postural and intentional positions. Regularity of repetitive finger and hand movements and reaction time were also determined. Data were statistically analyzed. Clinical tremor rating scales detected pathological tremor in three patients (10%), while accelerometry revealed tremor in seven patients (25%). Center frequency of patients with pathological tremor was similar to controls, but the frequency dispersion was significantly lower and tremor intensity was significantly higher in both postural and intentional positions. Rhythmic movements and reaction time were normal. Our results show that objective measurements detect pathological intention tremor in 25% of epilepsy patients receiving lamotrigine monotherapy. Quantitative characteristics suggest the involvement of the cerebellum in the pathomechanism of lamotrigine-induced tremor. Determining the parameters of drug-induced tremor syndromes might help to understand the complex action of tremor generator networks.

Identifying therapeutic targets from spontaneous beneficial brain lesions

Brain damage can occasionally result in paradoxical functional benefit, which could help identify therapeutic targets for neuromodulation. However, these beneficial lesions are rare and lesions in multiple different brain locations can improve the same symptom. Using a technique called lesion network mapping, we show that heterogeneous lesion locations resulting in tremor relief are all connected to common nodes in the cerebellum and thalamus, the latter of which is a proven deep brain stimulation target for tremor. These results suggest that lesion network mapping can identify the common substrate underlying therapeutic lesions and effective therapeutic targets. Ann Neurol 2018;83:153-157.

Essential tremor: a nuanced approach to the clinical features

Click here to listen to the Podcast Essential tremor is the most common form of tremor in humans. Given neurologists' high exposure to this condition, and its seemingly straightforward phenotype, it might seem easy to diagnose. However, 30%-50 % of patients labelled as having 'essential tremor' have other diagnoses, mostly Parkinson's disease and dystonia. The tremor of essential tremor is neither non-descript nor featureless but is multifaceted and highly patterned. This review focuses on its clinical features, beginning with a discussion of tremors and then briefly discussing its additional motor features, and presents several aids to help distinguish essential tremor from Parkinson's disease and dystonia. Careful attention to certain clinical nuances will aid the diagnosis and care of patients with essential tremor.

Ataxic Gait in Essential Tremor: A Disease-Associated Feature?

Background

While accumulating evidence suggests that balance and gait impairments are commonly seen in patients with essential tremor (ET), questions remain regarding their prevalence, their relationship with normal aging, whether they are similar to the impairments seen in spinocerebellar ataxias, their functional consequences, and whether some ET patients carry greater susceptibility.

Methods

We conducted a literature search (until December 2018) on this topic.

Results

We identified 23 articles on gait or balance impairments in ET. The prevalence of balance impairment (missteps on tandem walk test) was seven times higher in ET patients than controls. Gait impairments in ET included reduced speed, increased asymmetry, and impaired dynamic balance. While balance and gait problems worsened with age, ET patients were more impaired than controls, independent of age. The pattern of impairments seen in ET was qualitatively similar to that seen in spinocerebellar ataxias. Balance and gait impairments resulted in greater number of near falls in ET patients. Factors associated with balance and gait impairments in ET included age, presence of tremor in midline structures, and cognitive dysfunction.

Discussion

Accumulating evidence suggests that balance and gait impairments are common in ET patients and occur to a greater extent in controls. Thus, they represent a disease-associated feature. These impairments, which are qualitatively similar to those seen in spinocerebellar ataxias, are not merely subclinical but result in difficulty performing functional tasks and increase falls risk. A subset of patients is more susceptible to balance and gait impairments. The full spectrum of impairments remains to be characterized.

Atrophy of cerebellar peduncles in essential tremor: a machine learning-based volumetric analysis

BACKGROUND

Subtle cerebellar signs are frequently observed in essential tremor (ET) and may be associated with cerebellar dysfunction. This study aims to evaluate the macrostructural integrity of the superior, middle, and inferior cerebellar peduncles (SCP, MCP, ICP) and cerebellar gray and white matter (GM, WM) volumes in patients with ET, and compare these volumes between patients with and without cerebellar signs (ETc and ETnc).

METHODS

Forty patients with ET and 37 age- and gender-matched healthy controls were recruited. Atlas-based region-of-interest analysis of the SCP, MCP, and ICP and automated analysis of cerebellar GM and WM volumes were performed. Peduncular volumes were employed in a multi-variate classification framework to attempt discrimination of ET from controls.

RESULTS

Significant atrophy of bilateral MCP and ICP and bilateral cerebellar GM was observed in ET. Cerebellar signs were present in 20% of subjects with ET. Comparison of peduncular and cerebellar volumes between ETnc and ETc revealed atrophy of right SCP, bilateral MCP and ICP, and left cerebellar WM in ETc. The multi-variate classifier could discriminate between ET and controls with a test accuracy of 86.66%.

CONCLUSIONS

Patients with ET have significant atrophy of cerebellar peduncles, particularly the MCP and ICP. Additional atrophy of the SCP is observed in the ETc group. These abnormalities may contribute to the pathogenesis of cerebellar signs in ET.

KEY POINTS

• Patients with ET have significant atrophy of bilateral middle and inferior cerebellar peduncles and cerebellar gray matter in comparison with healthy controls. • Patients of ET with cerebellar signs have significant atrophy of right superior cerebellar peduncle, bilateral middle and inferior cerebellar peduncle, and left cerebellar white matter in comparison with ET without cerebellar signs. • A multi-variate classifier employing peduncular volumes could discriminate between ET and controls with a test accuracy of 86.66%.

Effects of cerebellar transcranial direct current stimulation on cerebellar-brain inhibition in humans: A systematic evaluation

BACKGROUND

Cerebellar transcranial direct current stimulation (ctDCS) is increasingly used to modulate cerebellar excitability and plasticity in healthy subjects and various patient populations. ctDCS parameters are poorly standardized, and its physiology remains little understood. Our aim was to compare the physiological effects of three different non-target electrode positions (buccinator muscle, supraorbital region, deltoid muscle).

METHODS

In the first experiment, physiological after-effects of ctDCS were compared based on cerebellar-brain inhibition (CBI) in a group of 15 healthy right-handed participants. In the second experiment, CBI after-effects of ctDCS were assessed using different transcranial magnetic stimulation (TMS) intensities in 14 participants (CBI recruitment curve). The electric field distribution was calculated for each of the electrode montages based on a single anatomically accurate head model.

RESULTS

Anodal and cathodal ctDCS polarities significantly decreased cerebellar-brain inhibition (CBI) with no substantial differences between the montages. Lower cerebellar TMS intensities resulted in decreased CBI following cathodal and increased CBI after anodal ctDCS. Computational modeling revealed minor differences in the electric field distribution between non-target electrode positions based on the effect size.

CONCLUSION

Our results show that the non-target electrode position has no significant impact on modeling results and physiological ctDCS after-effects. The recruitment of the cerebellar-M1 connection, however, varied depending on ctDCS polarity and cerebellar transcranial magnetic stimulation intensity, possibly due to diverse effects on different cell populations in the cerebellar cortex. This may be one of the reasons why ctDCS effects on functional measures are difficult to predict.

Consensus paper: Decoding the Contributions of the Cerebellum as a Time Machine. From Neurons to Clinical Applications

Time perception is an essential element of conscious and subconscious experience, coordinating our perception and interaction with the surrounding environment. In recent years, major technological advances in the field of neuroscience have helped foster new insights into the processing of temporal information, including extending our knowledge of the role of the cerebellum as one of the key nodes in the brain for this function. This consensus paper provides a state-of-the-art picture from the experts in the field of the cerebellar research on a variety of crucial issues related to temporal processing, drawing on recent anatomical, neurophysiological, behavioral, and clinical research.The cerebellar granular layer appears especially well-suited for timing operations required to confer millisecond precision for cerebellar computations. This may be most evident in the manner the cerebellum controls the duration of the timing of agonist-antagonist EMG bursts associated with fast goal-directed voluntary movements. In concert with adaptive processes, interactions within the cerebellar cortex are sufficient to support sub-second timing. However, supra-second timing seems to require cortical and basal ganglia networks, perhaps operating in concert with cerebellum. Additionally, sensory information such as an unexpected stimulus can be forwarded to the cerebellum via the climbing fiber system, providing a temporally constrained mechanism to adjust ongoing behavior and modify future processing. Patients with cerebellar disorders exhibit impairments on a range of tasks that require precise timing, and recent evidence suggest that timing problems observed in other neurological conditions such as Parkinson's disease, essential tremor, and dystonia may reflect disrupted interactions between the basal ganglia and cerebellum.The complex concepts emerging from this consensus paper should provide a foundation for further discussion, helping identify basic research questions required to understand how the brain represents and utilizes time, as well as delineating ways in which this knowledge can help improve the lives of those with neurological conditions that disrupt this most elemental sense. The panel of experts agrees that timing control in the brain is a complex concept in whom cerebellar circuitry is deeply involved. The concept of a timing machine has now expanded to clinical disorders.

Heterotopic Purkinje Cells: a Comparative Postmortem Study of Essential Tremor and Spinocerebellar Ataxias 1, 2, 3, and 6

Essential tremor (ET) is among the most common neurological diseases. Postmortem studies have noted a series of pathological changes in the ET cerebellum. Heterotopic Purkinje cells (PCs) are those whose cell body is mis-localized in the molecular layer. In neurodegenerative settings, these are viewed as a marker of the progression of neuronal degeneration. We (1) quantify heterotopias in ET cases vs. controls, (2) compare ET cases to other cerebellar degenerative conditions (spinocerebellar ataxias (SCAs) 1, 2, 3, and 6), (3) compare these SCAs to one another, and (4) assess heterotopia within the context of associated PC loss in each disease. Heterotopic PCs were quantified using a standard LH&E-stained section of the neocerebellum. Counts were normalized to PC layer length (n-heterotopia count). It is also valuable to consider PC counts when assessing heterotopia, as loss of PCs extends both to normally located as well as heterotopic PCs. Therefore, we divided n-heterotopias by PC counts. There were 96 brains (43 ET, 31 SCA [12 SCA1, 7 SCA2, 7 SCA3, 5 SCA6], and 22 controls). The median number of n-heterotopias in ET cases was two times higher than that of the controls (2.6 vs. 1.2, p < 0.05). The median number of n-heterotopias in the various SCAs formed a spectrum, with counts being highest in SCA3 and SCA1. In analyses that factored in PC counts, ET had a median n-heterotopia/Purkinje cell count that was three times higher than the controls (0.35 vs. 0.13, p < 0.01), and SCA1 and SCA2 had counts that were 5.5 and 11 times higher than the controls (respective p < 0.001). The median n-heterotopia/PC count in ET was between that of the controls and the SCAs. Similarly, the median PC count in ET was between that of the controls and the SCAs; the one exception was SCA3, in which the PC population is well known to be preserved. Heterotopia is a disease-associated feature of ET. In comparison, several of the SCAs evidenced even more marked heterotopia, although a spectrum existed across the SCAs. The median n-heterotopia/PC count and median PC in ET was between that of the controls and the SCAs; hence, in this regard, ET could represent an intermediate state or a less advanced state of spinocerebellar atrophy.

A Quantitative Study of Empty Baskets in Essential Tremor and Other Motor Neurodegenerative Diseases

The underlying biology of essential tremor (ET) is poorly understood. Purkinje cell (PC) loss has been observed in some studies, although this finding remains somewhat controversial. Basket cells are interneurons whose axonal collaterals form a plexus around PC soma. When there is PC loss, this basket plexus appears empty. We used dual immunohistochemical staining for calbindin D28k and glutamic acid decarboxylase to quantify "empty baskets" as an indirect and alternative method of detecting PC loss. Microscopic analyses on 127 brains included ET and a spectrum of motor neurodegenerative diseases (50 ET, 27 spinocerebellar ataxias [SCAs], 25 Parkinson disease, 25 controls). The median percentage of empty baskets in ET patients was 1.5 times higher than controls (48.8% vs 33.5%, p < 0.001) but lower in ET than in SCA1 (59.7%, p = 0.011), SCA2 (77.5%, p = 0.003), and SCA6 (87.0%, p < 0.001). PC loss is not a feature of SCA3, and the median percentage of empty baskets (30.1%) was similar to controls (p = 0.303). These data provide support for PC loss in ET and are consistent with the notion that ET could represent a mild form of cerebellar degeneration with an intermediate degree of PC loss.

Defining differences in patient characteristics between spasmodic dysphonia and laryngeal tremor

OBJECTIVE

To compare presenting characteristics of patients with adductor spasmodic dysphonia (ADSD), ADSD with laryngeal tremor (ADSD + LT), and laryngeal tremor without ADSD (LT).

DESIGN

Cross-sectional analysis.

METHODS

Patients treated for laryngeal movement disorders (1990-2016) were included. Analysis of variance and chi square tests measured differences in patient characteristics across the three disease groups. Using ADSD as the referent, multivariable logistic regression models were used to determine whether potential risk factors including patient demographics, family history, presence of potential inciting events prior to disease onset, and coprevalent movement disorders were associated with ADSD + LT or LT.

RESULTS

In all, 652 patients with ADSD (n = 377), ADSD + LT (n = 98), and LT (n = 177) were included. ADSD patients were significantly younger than those with ADSD + LT and LT (52.5 ± 13.4, 63.9 ± 11.3, and 69.3 ± 10.5 years, respectively; P < 0.001). Coprevalent movement disorders were more common in ADSD + LT (38.7%) and LT (57.1%) groups than in the ADSD group (11.5%; P < 0.001). Compared to ADSD, patients with ADSD + LT and LT were more likely to develop an additional movement disorder during follow-up. In multivariable analyses, increasing age, female gender, and having a movement disorder at presentation were associated with significantly greater odds of having ADSD + LT or LT when compared to ADSD.

CONCLUSION

ADSD + LT patients demonstrate intermediate gender composition and age distributions between those with ADSD and LT. These findings suggest that ADSD + LT may be a distinct phenotype in the spectrum of laryngeal movement disorders.

LEVEL OF EVIDENCE

4 Laryngoscope, 129:170-176, 2019.

Distinct cortical responses evoked by electrical stimulation of the thalamic ventral intermediate nucleus and of the subthalamic nucleus

Objective

To investigate the spatial and temporal pattern of cortical responses evoked by deep brain stimulation (DBS) of the subthalamic nucleus (STN) and ventral intermediate nucleus of the thalamus (VIM).

Methods

We investigated 7 patients suffering from Essential tremor (ET) and 7 patients with Parkinson's Disease (PD) following the implantation of DBS electrodes (VIM for ET patients, STN for PD patients). Magnetoencephalography (MEG) was used to record cortical responses evoked by electric stimuli that were applied via the DBS electrode in trains of 5 Hz. Dipole fitting was applied to reconstruct the origin of evoked responses.

Results

Both VIM and STN DBS led to short latency cortical responses at about 1 ms. The pattern of medium and long latency cortical responses following VIM DBS consisted of peaks at 13, 40, 77, and 116 ms. The associated equivalent dipoles were localized within the central sulcus, 3 patients showed an additional response in the cerebellum at 56 ms. STN DBS evoked cortical responses peaking at 4 ms, 11 ms, and 27 ms, respectively. While most dipoles were localized in the pre- or postcentral gyrus, the distribution was less homogenous compared to VIM stimulation and partially included prefrontal brain areas.

Conclusion

MEG enables localization of cortical responses evoked by DBS of the VIM and the STN, especially in the sensorimotor cortex. Short latency responses of 1 ms suggest cortical modulation which bypasses synaptic transmission, i.e. antidromic activation of corticofugal fiber pathways.

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Cortical responses evoked by VIM or STN DBS can be precisely described using MEG.

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Both STN and VIM DBS primarily evoke cortical responses within the sensorimotor region.

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Short latency responses of 1 ms both observed in VIM and STN DBS suggest antidromic activation of corticofugal fibers.

Absence of Mutation Enrichment for Genes Phylogenetically Conserved in the Olivocerebellar Motor Circuitry in a Cohort of Canadian Essential Tremor Cases

Essential Tremor is a prevalent neurological disorder of unknown etiology. Studies suggest that genetic factors contribute to this pathology. To date, no causative mutations in a gene have been reproducibly reported. All three structures of the olivocerebellar motor circuitry have been linked to Essential Tremor. We postulated that genes enriched for their expression in the olivocerebellar circuitry would be more susceptible to harbor mutations in Essential Tremor patients. A list of 11 candidate genes, enriched for their expression in the olivocerebellar circuitry, was assessed for their variation spectrum and frequency in a cohort of Canadian Essential Tremor cases. Our results from this list of 11 candidate genes do not support an association for Essential Tremor in our cohort of Canadian cases. The heterogenic nature of ET and modest size of the cohort used in this study are two confounding factors that could explain these results.