Linking Essential Tremor to the Cerebellum-Neuroimaging Evidence

Serum Neurofilament Light-Chain Concentrations in Essential Tremor: a Case-Control Study

Essential tremor (ET) is a common neurological disorder, with clinical and pathophysiological links to the cerebellum. Inquiries into the etiology, pathophysiology, and nosology of ET stand to benefit from the identification of disease biomarkers. Serum neurofilament light chain (NfL) has emerged as a novel signature of conditions in which neuronal injury reflects an outcome of the ongoing disease process. We sought to investigate the concentrations of NfL in ET patients and healthy controls. In this case-control study, our powered study population of 41 ET patients and 40 age-matched healthy controls underwent clinical assessments and measurement of serum NfL concentration using Simoa technology. Serum NfL was elevated in ET patients - mean log-transformed serum NfL concentration = 1.23 ± 0.19 (95% confidence interval [CI] = 1.17-1.29) vs. 1.08 ± 0.15 (95% CI = 1.03-1.13), p = 0.0002. This difference persisted after accounting for age, sex and Montreal Cognitive Assessment score in a multiple linear regression model (p = 0.002) and in an age-matched sample subset of 35 ET cases and 35 controls (p = 0.006). There was no association between tremor severity and serum NfL levels (p = 0.73). In this sample of ET patients and controls, serum NfL concentrations were significantly higher in ET. Studies in additional cohorts of ET cases would be of value in attempting to replicate these results and assessing diagnostic utility.

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.

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.

Social cognition in hyperkinetic movement disorders: a systematic review

Numerous lines of research indicate that our social brain involves a network of cortical and subcortical brain regions that are responsible for sensing and controlling body movements. However, it remains unclear whether movement disorders have a systematic impact on social cognition. To address this question, we conducted a systematic review examining the influence of hyperkinetic movement disorders (including Huntington disease, Tourette syndrome, dystonia, and essential tremor) on social cognition. Following the PRISMA guidelines and registering the protocol in the PROSPERO database (CRD42022327459), we analyzed 50 published studies focusing on theory of mind (ToM), social perception, and empathy. The results from these studies provide evidence of impairments in ToM and social perception in all hyperkinetic movement disorders, particularly during the recognition of negative emotions. Additionally, individuals with Huntington's Disease and Tourette syndrome exhibit empathy disorders. These findings support the functional role of subcortical structures (such as the basal ganglia and cerebellum), which are primarily responsible for movement disorders, in deficits related to social cognition.

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.

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.

Visuomotor Adaptation Deficits in Patients with Essential Tremor

Essential tremor (ET) is a progressive movement disorder whose pathophysiology is not fully understood. Current evidence supports the view that the cerebellum is critically involved in the genesis of the tremor in ET. However, it is still unknown whether cerebellar dysfunction affects not only the control of current movements but also the prediction of future movements through dynamic adaptation toward a changed environment. Here, we tested the capacity of 28 patients with ET to adapt in a visuomotor adaptation task known to depend on intact cerebellar function. We found specific impairments in that task compared to age-matched healthy controls. Adaptation to the visual perturbation was disrupted in ET patients, while de-adaptation, the phase after abrupt removal of the perturbation, developed similarly to control subjects. Baseline tremor-independent motor performance was as well similar to healthy controls, indicating that adaptation deficits in ET patients were not rooted in an inability to perform goal-directed movements. There was no association between clinical severity scores of ET and early visuomotor adaptation abilities. These results provide further evidence that the cerebellum is dysfunctional in 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.

Grey matter correlates of dystonic soft signs in essential tremor

BACKGROUND

Questionable signs of dystonia are a common finding in patients with essential tremor (ET). Brain structural alterations in ET patients plus dystonic soft signs (ET + ds) in comparison to ET patients without dystonic soft signs (ET-ds) or patients with tremor associated with manifest dystonia (TAWD) have not been examined yet. Therefore, our study aims to explore alterations of brain grey matter in patients with ET + ds.

METHODS

A total of 68 elderly patients with ET-ds (n = 32), ET + ds (n = 20) or idiopathic cervical dystonia with dystonia associated action tremor of the upper limbs (TAWD, n = 16) and 42 age-matched healthy controls underwent a clinical and electrophysiological assessment and 3T MRI. For grey matter alterations T1 MRI images were analysed by voxel-based morphometry. Additionally, regression analyses with clinical parameters (tremor frequency, severity and disease duration) were performed.

RESULTS

VBM showed a significant increase of grey matter in the right lentiform nucleus in ET + ds and TAWD compared to HC and ET-ds. Further, an increase of cortical grey matter in the middle frontal gyrus in ET + ds was shown. The hypertrophy of the lentiform nucleus in ET + ds was correlated with disease severity and duration.

CONCLUSION

Patients with ET + ds showed grey matter brain structural alterations similar to TAWD. Our findings suggest an involvement of the basal ganglia-cortical loop in ET + ds which may indicate a pathophysiological similarity with TAWD rather than ET.

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.

Brain microstructural changes in essential tremor patients and correlations with clinical characteristics: a diffusion kurtosis imaging study

OBJECTIVE

Essential tremor (ET) is the second most common movement disorder; however, the pathophysiological mechanism of ET is unclear. We aimed to investigate the microstructural degeneration of gray matter (GM) and white matter (WM) and their correlations with cognition and tremor in patients with ET.

METHODS

The participants were 63 patients with ET and 63 matched healthy controls (HCs) who underwent 3D-T1 weighted and diffusion kurtosis images (DKI). Microstructural degeneration was measured using high-level diffusion parameters derived from DKI. A voxel-wise analysis of the means of the GM-based spatial statistics and tract-based spatial statistics were conducted to assess differences in diffusion parameters between the ET and HC groups. The volume differences between the two groups were also assessed, and tremor severity and multi-domain cognitive performance were evaluated. Finally, the relationship between microstructural degeneration and clinical characteristics were assessed.

RESULTS

The ET group had significantly lower mean kurtosis of the temporal, parietal, and occipital lobes and the cerebellum and lower radial kurtosis in several tracts. These microstructural changes in GM and WM were correlated with tremor and cognitive scores. However, no significant difference in volume was found between the groups.

CONCLUSION

Our findings suggest that ET entails extensive GM and WM microstructural alterations, which support the neurodegenerative hypothesis of ET. Our study contributes to a better understanding of the mechanisms underlying tremor and cognitive impairment in ET.

Locus coeruleus degeneration and cerebellar gray matter changes in essential tremor

BACKGROUND

The pathophysiology of essential tremor (ET) is not fully understood, and studies suggest pathological changes mainly occur in the cerebellum and locus coeruleus (LC).

METHODS

Fifty-three ET patients, including 30 patients with head tremor (h-ET), 23 patients without head tremor (nh-ET), 71 age and education matched healthy controls (HCs) were enrolled. All participants underwent Neuromelanin-sensitive magnetic resonance imaging (NM-MRI) and T1 scans on a 3-Tesla MR system. Next, we assessed the relationship between the contrast-to-noise ratio of LC (CNR_LC) and the score of The Essential Tremor Rating Assessment Scale (TETRAS) and cerebellum gray matter (GM) volume.

RESULTS

Significant difference of CNR_LC was found between ET and HC groups. The CNR_LC of ET groups is lower than the HC group (p = 0.031). Subgroup analysis showed that the CNR_LC in nh-ET was significantly lower than HCs (p = 0.016). Compared to HCs, h-ETs showed marked atrophy in the cerebellum: the vermis IV-V and lobule VI (GRF corrected, p < 0.05). A significant negative correlation was found between CNR_LC and the vermis lobule IV-V in h-ETs (r = - 0.651, p < 0.001). No significant correlation was found between CNR_LC and TETRAS scores.

CONCLUSION

The LC and the cerebellum might both involve in the pathophysiology of ET. LC evaluation using NM-MRI might be an effective tool for us to explore the pathophysiology of ET further.

Reproducibility of cerebellar involvement as quantified by consensus structural MRI biomarkers in advanced essential tremor

Essential tremor (ET) is the most prevalent movement disorder with poorly understood etiology. Some neuroimaging studies report cerebellar involvement whereas others do not. This discrepancy may stem from underpowered studies, differences in statistical modeling or variation in magnetic resonance imaging (MRI) acquisition and processing. To resolve this, we investigated the cerebellar structural differences using a local advanced ET dataset augmented by matched controls from PPMI and ADNI. We tested the hypothesis of cerebellar involvement using three neuroimaging biomarkers: VBM, gray/white matter volumetry and lobular volumetry. Furthermore, we assessed the impacts of statistical models and segmentation pipelines on results. Results indicate that the detected cerebellar structural changes vary with methodology. Significant reduction of right cerebellar gray matter and increase of the left cerebellar white matter were the only two biomarkers consistently identified by multiple methods. Results also show substantial volumetric overestimation from SUIT-based segmentation-partially explaining previous literature discrepancies. This study suggests that current estimation of cerebellar involvement in ET may be overemphasized in MRI studies and highlights the importance of methods sensitivity analysis on results interpretation. ET datasets with large sample size and replication studies are required to improve our understanding of regional specificity of cerebellum involvement in ET. PROTOCOL REGISTRATION: The stage 1 protocol for this Registered Report was accepted in principle on 21 March 2022. The protocol, as accepted by the journal, can be found at: https://doi.org/10.6084/m9.figshare.19697776 .

Case report: p.Glu134del SOD1 mutation in two apparently unrelated ALS patients with mirrored phenotype

With upcoming personalized approaches based on genetics, it is important to report new mutations in amyotrophic lateral sclerosis (ALS) genes in order to understand their pathogenicity and possible patient responses to specific therapies. SOD1 mutations are the second most frequent genetic cause of ALS in European populations. Here, we describe two seemingly unrelated Italian patients with ALS carrying the same SOD1 heterozygous c.400_402 deletion (p.Glu134del). Both patients had spinal onset in their lower limbs, progressive muscular weakness with respiratory involvement, and sparing bulbar function. In addition to the clinical picture, we discuss the possible pathogenic role of this unfamiliar SOD1 mutation.

Prevalence and risk factors for abnormal tandem gait in patients with essential tremor syndrome: A cross-sectional study in Southwest China

Objectives

Patients with essential tremor (ET) syndrome have more prevalent and more serious gait and balance impairments than healthy controls. In this cross-sectional study, we explored whether balance impairments are associated with falls as well as more pronounced non-motor symptoms in patients with ET syndrome.

Methods

We assessed the tandem gait (TG) test, as well as falls or near-falls that occurred over the previous year. Non-motor symptoms-including cognitive deficits, psychological and sleep disorders-were evaluated. In univariate analyses, statistical significance was corrected for multiple comparisons using the Benjamini-Hochberg method. Multiple logistic regression was utilized to evaluate the risk factors of poor TG performance in patients with ET syndrome.

Results

A total of 358 patients with ET syndrome were divided into the abnormal TG (a-TG) and normal TG (n-TG) groups based on their performances in the TG test. We revealed that 47.2% of patients with ET syndrome had a-TG. The patients with a-TG were older, were more likely female, and were more likely present with cranial tremors and falls or near-falls (all adjusted P < 0.01). The patients with a-TG had significantly lower Mini-Mental Status Examination scores, as well as significantly higher Hamilton Depression/Anxiety Rating Scale and Pittsburgh Sleep Quality Index scores. Multiple logistic regression analysis demonstrated that female sex (OR 1.913, 95% CI: 1.180-3.103), age (OR 1.050, 95% CI: 1.032-1.068), cranial tremor scores (OR 1.299, 95% CI: 1.095-1.542), a history of falls or near-falls (OR 2.952, 95% CI: 1.558-5.594), and the presence of depressive symptoms (OR 1.679, 95% CI: 1.034-2.726) were associated with the occurrence of a-TG in patients with ET syndrome.

Conclusion

TG abnormalities may be a predictor of fall risk in patients with ET syndrome and are associated with non-motor symptoms, especially depression.

Essential tremor

Essential tremor (ET) is a chronic and progressive neurologic disease. Its central and defining clinical feature is a 4-12Hz kinetic tremor, that is, tremor that occurs during voluntary movements such as drinking from a cup or writing. Patients may also exhibit a range of other tremors-postural, rest, intention, additional motor features (e.g., mild gait ataxia, mild dystonia), as well as nonmotor features. The disease itself seems to be a risk factor for other degenerative diseases such as Alzheimer's disease and Parkinson's disease. Both genetic and toxic environmental factors have been explored as etiologic factors. In addition to a growing appreciation of the presence of clinical, etiologic, and pathologic heterogeneity, there is some support for the notion that ET itself may not be a single disease, but may be a family of diseases whose central defining feature is kinetic tremor of the arms, and which might more accurately be referred to as "the essential tremors." Recent research has increasingly placed the seat of the disease in the cerebellum and cerebellar system and identified a host of neurodegenerative changes within the cerebellum, indicating that this progressive disorder is likely degenerative.

Voice Tremor and Botulinum Neurotoxin Therapy: A Contemporary Review

Voice tremor is a common, yet debilitating symptom for patients suffering from a number of tremor-associated disorders. The key to targeting effective treatments for voice tremor requires a fundamental understanding of the pathophysiology that underpins the tremor mechanism and accurate identification of the disease in affected patients. An updated review of the literature detailing the current understanding of voice tremor (with or without essential tremor), its accurate diagnosis and targeted treatment options was conducted, with a specific focus on the role of botulinum neurotoxin. Judicious patient selection, following detailed characterisation of voice tremor qualities, is essential to optimising treatment outcomes for botulinum neurotoxin therapy, as well as other targeted therapies. Further focused investigation is required to characterise the response to targeted treatment in voice tremor patients and to guide the development of innovative treatment options.

Multimodal imaging of essential tremor and dystonic tremor

Despite recent advances in tremor and dystonia classification, it remains difficult to discriminate essential tremor from dystonic tremor as they are similar in appearance and no biomarker exists. Further, tremor can appear in the same or a different body part than the dystonia. The aim of the current study was to better understand the differential pathophysiology of these tremors. We designed a cross-sectional case-control study and recruited 16 patients with essential tremor, 16 patients with dystonic tremor, and 17 age-matched healthy volunteers. We used multi-modal imaging combining resting-state functional MRI, diffusion tensor imaging, and magnetic resonance spectroscopy. We measured functional connectivity of resting-state fMRI to assess connectivity in the tremor network, fractional anisotropy and mean diffusivity with diffusion tensor imaging, and GABA+, Glutamate/Glutamine, Choline, and N-Acetylaspartate with spectroscopy (adjusted to Creatine). Our results showed reduced functional connectivity of resting-state fMRI between the cerebellum and dentate nucleus bilaterally for the essential tremor group, but not the dystonic tremor group, compared to healthy volunteers. There was higher fractional anisotropy in the middle cerebellar peduncle bilaterally for the dystonic tremor group compared to the essential tremor group as well as for essential tremor group compared to healthy volunteers. There was also higher fractional anisotropy in the red nucleus and corticospinal tract for essential tremor and dystonic tremor groups compared to healthy volunteers. We also showed reduced mean diffusivity in the cerebellum of both essential tremor and dystonic tremor groups compared to healthy volunteers. Finally, we found elevated GABA+/Cr in the cerebellum of the essential tremor and dystonic tremor groups compared to healthy volunteers, but no difference emerged between essential tremor and dystonic tremor groups. We did not find group differences in the other metabolites. Our results indicate cerebellar alterations in essential tremor and dystonic tremor patients compared to healthy volunteers, and further changes in the cerebellum network for the dystonic tremor patients. suggesting that the cerebellum is affected differently in both tremors.

Functional connectivity of the cortico-subcortical sensorimotor loop is modulated by the severity of nigrostriatal dopaminergic denervation in Parkinson’s Disease

To assess if the severity of nigrostriatal innervation loss affects the functional connectivity (FC) of the sensorimotor cortico-striato-thalamic-cortical loop (CSTCL) in Parkinson’s Disease (PD), Resting-State functional MRI and ¹⁸F-DOPA PET data, simultaneously acquired on a hybrid PET/MRI scanner, were retrospectively analyzed in 39 PD and 16 essential tremor patients. Correlations between posterior Putamen DOPA Uptake (pPDU) and the FC of the main CSTCL hubs were assessed separately in the two groups, analyzing the differences between the two groups by a group-by-pPDU interaction analysis of the resulting clusters’ FC. Unlike in essential tremor, in PD patients pPDU correlated inversely with the FC of the thalamus with the sensorimotor cortices, and of the postcentral gyrus with the dorsal cerebellum, and directly with the FC of pre- and post-central gyri with both the superior and middle temporal gyri and the paracentral lobule, and of the caudate with the superior parietal cortex. The interaction analysis confirmed the significance of the difference between the two groups in these correlations. In PD patients, the post-central cortex FC, in the clusters correlating directly with pPDU, negatively correlated with both UPDRS motor examination score and Hoehn and Yahr stage, independent of the pPDU, suggesting that these FC changes contribute to motor impairment. In PD, nigrostriatal innervation loss correlates with a decrease in the FC within the sensorimotor network and between the sensorimotor network and the superior temporal cortices, possibly contributing to motor impairment, and with a strengthening of the thalamo-cortical FC, that may represent ineffective compensatory phenomena.

Essential tremor impairs the ability to suppress involuntary action impulses

Essential tremor (ET) is a movement disorder characterized primarily by action tremor which affects the regulation of movements. Disruptions in cerebello-thalamocortical networks could interfere with cognitive control over actions in ET, for example, the ability to suppress a strong automatic impulse over a more appropriate action (conflict control). The current study investigated whether ET impacts conflict control proficiency. Forty-one ET patients and 29 age-matched healthy controls (HCs) performed a conflict control task (Simon task). Participants were instructed to give a left or right response to a spatially lateralized arrow (direction of the arrow). When the action signaled by the spatial location and direction of the arrow were non-corresponding (induced conflict), the inappropriate action impulse required suppression. Overall, ET patients responded slower and less accurately compared to HCs. ET patients were especially less accurate on non-corresponding conflict (Nc) versus corresponding (Cs) trials. A focused analysis on fast impulsive response rates (based on the accuracy rate at the fastest reaction times on Nc trials) showed that ET patients made more fast errors compared to HCs. Results suggest impaired conflict control in ET compared to HCs. The increased impulsive errors seen in the ET population may be a symptom of deficiencies in the cerebello-thalamocortical networks, or, be caused by indirect effects on the cortico-striatal pathways. Future studies into the functional networks impacted by ET (cortico-striatal and cerebello-thalamocortical pathways) could advance our understanding of inhibitory control in general and the cognitive deficits in ET.

Cerebellar Oscillations in Familial and Sporadic Essential Tremor

Enhanced cerebellar oscillations have recently been identified in essential tremor (ET) patients as a key pathophysiological change. Since ET is considered a heterogeneous group of diseases, we investigated whether cerebellar oscillations differ in ET subtypes (familial vs. sporadic). This study aims to determine cerebellar physiology in familial and sporadic ET. Using surface electroencephalogram, we studied cerebellar physiology in 40 ET cases (n = 22 familial and n = 18 sporadic) and 20 age-matched controls. Both familial and sporadic ET cases had an increase in the intensity of cerebellar oscillations when compared to controls. Interestingly, cerebellar oscillations correlated with tremor severity in familial ET but not in sporadic ET. Our study demonstrated that ET cases have enhanced cerebellar oscillations, and the different relationships between cerebellar oscillations and tremor severity in familial and sporadic ET suggest diverse cerebellar pathophysiology.

Is essential tremor a degenerative disorder or an electric disorder? Degenerative disorder

Essential tremor (ET) is a highly prevalent neurologic disease and is the most common of the many tremor disorders. ET is a progressive condition with marked clinical heterogeneity, associated with a spectrum of both motor and non-motor features. However, its disease mechanisms remain poorly understood. Much debate has centered on whether ET should be considered a degenerative disorder, with underlying pathological changes in brain causing progressive disease manifestations, or an electric disorder, with overactivity of intrinsically oscillatory motor networks that occur without underlying structural brain abnormalities. Converging data from clinical, neuroimaging and pathological studies in ET now provide considerable evidence for the neurodegenerative hypothesis. A major turning point in this debate is that rigorous tissue-based studies have recently identified a series of structural changes in the ET cerebellum. Most of these pathological changes are centered on the Purkinje cell and connected neuronal populations, which can result in partial loss of Purkinje cells and circuitry reorganizations that would disturb cerebellar function. There is significant overlap in clinical and pathological features of ET with other disorders of cerebellar degeneration, and an increased risk of developing other degenerative diseases in ET. The combined implication of these studies is that ET could be degenerative. The evidence in support of the degenerative hypothesis is presented.

Tremor associated with similar structural networks in Parkinson's disease and essential tremor

INTRODUCTION

Despite substantial clinical and pathophysiological differences, the characteristics of tremor in Parkinson's disease (PD) and essential tremor (ET) patients bear certain similarities. The presented study delineates tremor-related structural networks in these two disorders.

METHODS

42 non-advanced PD patients (18 tremor-dominant, 24 without substantial tremor), 17 ET, and 45 healthy controls underwent high-angular resolution diffusion-weighted imaging acquisition to reconstruct their structural motor connectomes as a proxy of the anatomical interconnections between motor network regions, implementing state-of-the-art globally optimised probabilistic tractography.

RESULTS

When compared to healthy controls, ET patients exhibited higher structural connectivity in the cerebello-thalamo-cortical network. Interestingly, the comparison of tremor-dominant PD patients and PD patients without tremor yielded very similar results - higher structural connectivity in tremor-dominant PD sharing multiple nodes with the tremor network detected in ET, despite the generally lower structural connectivity between basal ganglia and frontal cortex in the whole PD group when compared to healthy controls.

CONCLUSION

The higher structural connectivity of the cerebello-thalamo-cortical network seems to be the dominant tremor driver in both PD and ET. While it appears to be the only tremor-related network in ET, its combination with large scale hypoconnectivity in the frontal cortico-subcortical network in PD may explain different clinical features of tremor in these two disorders.

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.

Pathophysiology of Cerebellar Tremor: The Forward Model-Related Tremor and the Inferior Olive Oscillation-Related Tremor

Lesions in the Guillain-Mollaret (G-M) triangle frequently cause various types of tremors or tremor-like movements. Nevertheless, we know relatively little about their generation mechanisms. The deep cerebellar nuclei (DCN), which is a primary node of the triangle, has two main output paths: the primary excitatory path to the thalamus, the red nucleus (RN), and other brain stem nuclei, and the secondary inhibitory path to the inferior olive (IO). The inhibitory path contributes to the dentato-olivo-cerebellar loop (the short loop), while the excitatory path contributes to the cerebrocerebellar loop (the long loop). We propose a novel hypothesis: each loop contributes to physiologically distinct type of tremors or tremor-like movements. One type of irregular tremor-like movement is caused by a lesion in the cerebrocerebellar loop, which includes the primary path. A lesion in this loop affects the cerebellar forward model and deteriorates its accuracy of prediction and compensation of the feedback delay, resulting in irregular instability of voluntary motor control, i.e., cerebellar ataxia (CA). Therefore, this type of tremor, such as kinetic tremor, is usually associated with other symptoms of CA such as dysmetria. We call this type of tremor forward model-related tremor. The second type of regular tremor appears to be correlated with synchronized oscillation of IO neurons due, at least in animal models, to reduced degrees of freedom in IO activities. The regular burst activity of IO neurons is precisely transmitted along the cerebellocerebral path to the motor cortex before inducing rhythmical reciprocal activities of agonists and antagonists, i.e., tremor. We call this type of tremor IO-oscillation-related tremor. Although this type of regular tremor does not necessarily accompany ataxia, the aberrant IO activities (i.e., aberrant CS activities) may induce secondary maladaptation of cerebellar forward models through aberrant patterns of long-term depression (LTD) and/or long-term potentiation (LTP) of the cerebellar circuitry. Although our hypothesis does not cover all tremors or tremor-like movement disorders, our approach integrates the latest theories of cerebellar physiology and provides explanations how various lesions in or around the G-M triangle results in tremors or tremor-like movements. We propose that tremor results from errors in predictions carried out by the cerebellar circuitry.

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.

Wearable Peripheral Electrical Stimulation Devices for the Reduction of Essential Tremor: A Review

Essential tremor is the most common pathological tremor, with a prevalence of 6.3% in people over 65 years of age. This disorder interferes with a patient's ability to carry out activities of daily living independently, and treatment with medical and surgical interventions is often insufficient or contraindicated. Mechanical orthoses have not been widely adopted by patients due to discomfort and lack of discretion. Over the past 30 years, peripheral electrical stimulation has been investigated as a possible treatment for patients who have not found other treatment options to be satisfactory, with wearable devices revolutionizing this emerging approach in recent years. In this paper, an overview of essential tremor and its current medical and surgical treatment options are presented. Following this, tremor detection, measurement and characterization methods are explored with a focus on the measurement options that can be incorporated into wearable devices. Then, novel interventions for essential tremor are described, with a detailed review of open and closed-loop peripheral electrical stimulation methods. Finally, discussion of the need for wearable closed-loop peripheral electrical stimulation devices for essential tremor, approaches in their implementation, and gaps in the literature for further research are presented.

Abnormalities of intrinsic brain activity in essential tremor: A meta-analysis of resting-state functional imaging

Neuroimaging studies using a variety of techniques have demonstrated abnormal patterns of spontaneous brain activity in patients with essential tremor (ET). However, the findings are variable and inconsistent, hindering understanding of underlying neuropathology. We conducted a meta‐analysis of whole‐brain resting‐state functional neuroimaging studies in ET compared to healthy controls (HC), using anisotropic effect‐size seed‐based d mapping, to identify the most consistent brain activity alterations and their relation to clinical features. After systematic literature search, we included 13 studies reporting 14 comparisons, describing 286 ET patients and 254 HC. Subgroup analyses were conducted considering medication status, head tremor status, and methodological factors. Brain activity in ET is altered not only in the cerebellum and cerebral motor cortex, but also in nonmotor cortical regions including prefrontal cortex and insula. Most of the results remained unchanged in subgroup analyses of patients with head tremor, medication‐naive patients, studies with statistical threshold correction, and the large subgroup of studies using functional magnetic resonance imaging. These findings not only show consistent and robust abnormalities in specific brain regions but also provide new information on the biology of patient heterogeneity, and thus help to elucidate the pathophysiology of ET.

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.

Relative increase in cerebellar gray matter in young onset essential tremor: Evidence from voxel-based morphometry analysis

This study is to investigate the presence of brain gray matter abnormalities in young onset essential tremor (ET) patients with arm tremor. Thirty ET patients together with 30 healthy volunteers were taken as candidates. Magnetic resonance imaging (MRI) was performed and voxel-based morphometry was used to compare gray matter density between the patients and volunteers. Washington Heights-Inwood Genetic Study of Essential Tremor (WHIGET) rating scale was applied to assess tremor severity in the patients. Eventually Twenty-Seven ET patients and Twenty-Seven volunteers were enrolled in the study. Voxel-based morphometry showed significant expansion of the bilateral cerebellum, occipital fusiform cortices, right inferior temporal gyrus, and precentral lobes (P < 0.05, TFCE corrected). Decrease in gray matter was detected only in the left parietal lobe. Region of interest analysis showed volume enlargement in thalamus, midbrain, and precuneus (P < 0.05, TFCE corrected). Importantly, significant negative correlation was found between the lateralized index of cerebellum and the tremor score which might implicate that the altered rightward lateralization in the cerebellum is possibly a response of the tremor effects in ET patients. Cerebellar gray matter expansion in young onset ET patients with arm tremor might be the result of compensation towards decline of cerebellar function.

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.

Subtle Cerebellar Features in Relatives of Essential Tremor Cases

Background: Essential tremor (ET) cases often exhibit a range of mild cerebellar signs. Their unaffected relatives have been shown in prior studies to exhibit subtle (i.e., preclinical) disease features.

Objective: To quantify subtle cerebellar signs in unaffected first-degree relatives of ET cases stratified based on their tremor severity.

Methods: Two hundred sixty-nine first-degree relatives of ET cases, none of whom reported tremor or a diagnosis of ET, or were diagnosed with ET based on detailed neurological examination, were stratified based on total tremor score (TTS) into two groups (lower TTS vs. higher TTS) and quartiles. Changes in gait, balance, and intention tremor were quantified on neurological examination.

Results: Higher TTS performed worse on the tandem stance task (p = 0.011). When stratified into TTS quartiles, higher quartile was associated with worse performance in tandem stance (p = 0.011) and stance with feet together (p = 0.028). Similarly, intention tremor in the arms (p = 0.0002) and legs (p = 0.047) were higher in the groups with more tremor.

Discussion: The links between ET and the cerebellum are multiple. These data provide intriguing evidence that subtle cerebellar signs (i.e., changes in balance and intention tremor) are more prevalent among first-degree relatives of ET cases with more tremor (i.e., those who may be themselves on the pathway to developing ET). These data contribute to a better characterization of what may be an early subclinical stage of the disease.

Quantitative Separation of Tremor and Ataxia in Essential Tremor

OBJECTIVE

This study addresses an important problem in neurology, distinguishing tremor and ataxia using quantitative methods. Specifically, we aimed to quantitatively separate dysmetria, a cardinal sign of ataxia, from tremor in essential tremor (ET).

METHODS

In Experiment 1, we compared 19 participants diagnosed with ET undergoing thalamic deep brain stimulation (DBS; ET_DBS ) to 19 healthy controls (HC). We quantified tremor during postural tasks using accelerometry and dysmetria with fast, reverse-at-target goal-directed movements. To ensure that endpoint accuracy was unaffected by tremor, we quantified dysmetria in selected trials manifesting a smooth trajectory to the endpoint. Finally, we manipulated tremor amplitude by switching DBS ON and OFF to examine its effect on dysmetria. In Experiment 2, we compared 10 ET participants with 10 HC to determine whether we could identify and distinguish dysmetria from tremor in non-DBS ET.

RESULTS

Three findings suggest that we can quantify dysmetria independently of tremor in ET. First, ET_DBS and ET exhibited greater dysmetria than HC and dysmetria did not correlate with tremor (R²  < 0.01). Second, even for trials with tremor-free trajectories to the target, ET exhibited greater dysmetria than HC (p < 0.01). Third, activating DBS reduced tremor (p < 0.01) but had no effect on dysmetria (p > 0.2).

INTERPRETATION

We demonstrate that dysmetria can be quantified independently of tremor using fast, reverse-at-target goal-directed movements. These results have important implications for the understanding of ET and other cerebellar and tremor disorders. Future research should examine the neurophysiological mechanisms underlying each symptom and characterize their independent contribution to disability. ANN NEUROL 2020;88:375-387.

Auditory and Olfactory Deficits in Essential Tremor - Review of the Current Evidence

Background:

Essential tremor (ET) is the most common adult movement disorder, characterized by several motor and increasingly well recognized non-motor symptoms. Sensory deficits, such as hearing impairment and olfactory dysfunction, are amongst them. This review analyzes the available evidence of these sensory deficits and their possible mechanistic basis in patients with ET.

Method:

A PubMed literature search on the topic was performed in the May 2019 database.

Results:

Nineteen articles on hearing impairment and olfactory dysfunction in ET patients were identified. The prevalence of hearing impairment is higher in ET patients than healthy controls or Parkinson disease. Cochlear pathologies are suggested as the underlying cause, but there is still a lack of information about retrocochlear pathologies and central auditory processing. Reports on olfactory dysfunction have conflicting results. The presence of mild olfactory dysfunction in ET was suggested. Conflicting results may be due to the lack of consideration of the disease’s heterogeneity, but according to recent data, most studies do not find prominent evidence of olfactory loss in ET.

Conclusion:

Although there is increasing interest in studies on non-motor symptoms in ET, there are few studies on sensory deficits, which are of particularly high prevalence. More studies are needed on to investigate the basis of non-motor symptoms, including sensory deficits.

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.

Detectability of cerebellar activity with magnetoencephalography and electroencephalography

Electrophysiological signals from the cerebellum have traditionally been viewed as inaccessible to magnetoencephalography (MEG) and electroencephalography (EEG). Here, we challenge this position by investigating the ability of MEG and EEG to detect cerebellar activity using a model that employs a high‐resolution tessellation of the cerebellar cortex. The tessellation was constructed from repetitive high‐field (9.4T) structural magnetic resonance imaging (MRI) of an ex vivo human cerebellum. A boundary‐element forward model was then used to simulate the M/EEG signals resulting from neural activity in the cerebellar cortex. Despite significant signal cancelation due to the highly convoluted cerebellar cortex, we found that the cerebellar signal was on average only 30–60% weaker than the cortical signal. We also made detailed M/EEG sensitivity maps and found that MEG and EEG have highly complementary sensitivity distributions over the cerebellar cortex. Based on previous fMRI studies combined with our M/EEG sensitivity maps, we discuss experimental paradigms that are likely to offer high M/EEG sensitivity to cerebellar activity. Taken together, these results show that cerebellar activity should be clearly detectable by current M/EEG systems with an appropriate experimental setup.

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.

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.

Prevalence and Risk Factors of Depression and Anxiety in Essential Tremor Patients: A Cross-Sectional Study in Southwest China

Background: Essential tremor (ET) is one of the most common movement disorders, and it has been associated with anxiety and depression, but risk factors for these mental symptoms are unclear. This study aimed to evaluate the prevalence and potential risk factors of depression and anxiety in ET patients in China. Methods: This cross-sectional study involved 245 Han Chinese patients with ET. Depression was assessed using the Hamilton Depression Rating Scale-24 Items, and anxiety was assessed using the Hamilton Anxiety Rating Scale. Clinico-demographic characteristics were compared between patients with or without depression or anxiety. Results: We revealed that 63.3% of patients had at least mild anxiety and 54.3% had at least mild depression. Severity of anxiety or depression was correlated with lower Mini-Mental State Exam score (P = 0.028 and P = 0.002, respectively), higher self-report functional disability score (P = 0.027 and P = 0.003, respectively), as well as the presence of tremor in the neck (both P < 0.001), face (P = 0.025 and P < 0.001, respectively), or voice (both P < 0.001). Binary logistic regression identified female sex, presence with cranial tremor, and greater functional disability score as risk factors of anxiety; the last two factors were also the determinants of depression. Neither anxiety nor depression correlated with physician-assessed severity of arm or leg tremor. Conclusion: Chinese ET patients show high prevalence of depression and anxiety. ET patients should be screened for these mental symptoms, especially women and those with cranial tremor and self-reported functional disability.

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.

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.

Neuroimaging depression and anxiety in essential tremor: A diffusion tensor imaging study

OBJECTIVE

Patients with essential tremor (ET) may exhibit non-motor features, including those that are neuropsychiatric. Depression and anxiety are the most common among these. This study aims to investigate the possible relationship between microstructural brain changes and symptoms of depression and anxiety in ET.

METHODS

We assessed 62 ET patients (40 women and 22 men, mean age 46.0 ± 20.4) for symptoms of depression and anxiety using the Beck Depression Inventory (BDI) and Beck Anxiety Inventory (BAI). Thirty-two patients had severe or moderate symptoms of anxiety, and 15 patients had severe or moderate depressive symptoms. Microstructural brain changes were evaluated using diffusion tensor imaging (DTI), which was reported using fractional anisotropy (FA), mean diffusivity (MD), apparent diffusion coefficient (ADC), radial diffusivity (RD), and axial diffusivity (AD) values calculated for 17 regions of interest including the prefrontal cortex, paralimbic and limbic structures and cerebellar peduncles. We evaluated the relationship between observed changes in brain regions and symptoms of depression and anxiety.

RESULTS

Decreased left amygdala FA (p = 0.003) and increased left amygdala RD (p = 0.04) were detected in depressed vs. non-depressed ET patients. Left ventrolateral prefrontal cortex (VLPFC) FA (p = 0.02) and left precuneus FA (p = 0.02) values differed between anxious patients vs. non-anxious ET patients. BDI scores were correlated with left amygdala FA and left RD, while BAI scores were correlated with left VLPFC FA and left precuneus FA.

DISCUSSION

Our results provide evidence that symptoms of depression and anxiety could be based in structural brain changes observed in patients with ET.

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%.

A widespread visually-sensitive functional network relates to symptoms in essential tremor

Essential tremor is a neurological syndrome of heterogeneous pathology and aetiology that is characterized by tremor primarily in the upper extremities. This tremor is commonly hypothesized to be driven by a single or multiple neural oscillator(s) within the cerebello-thalamo-cortical pathway. Several studies have found an association of blood-oxygen level-dependent (BOLD) signal in the cerebello-thalamo-cortical pathway with essential tremor, but there is behavioural evidence that also points to the possibility that the severity of tremor could be influenced by visual feedback. Here, we directly manipulated visual feedback during a functional MRI grip force task in patients with essential tremor and control participants, and hypothesized that an increase in visual feedback would exacerbate tremor in the 4-12 Hz range in essential tremor patients. Further, we hypothesized that this exacerbation of tremor would be associated with dysfunctional changes in BOLD signal and entropy within, and beyond, the cerebello-thalamo-cortical pathway. We found that increases in visual feedback increased tremor in the 4-12 Hz range in essential tremor patients, and this increase in tremor was associated with abnormal changes in BOLD amplitude and entropy in regions within the cerebello-thalamo-motor cortical pathway, and extended to visual and parietal areas. To determine if the tremor severity was associated with single or multiple brain region(s), we conducted a birectional stepwise multiple regression analysis, and found that a widespread functional network extending beyond the cerebello-thalamo-motor cortical pathway was associated with changes in tremor severity measured during the imaging protocol. Further, this same network was associated with clinical tremor severity measured with the Fahn, Tolosa, Marin Tremor Rating Scale, suggesting this network is clinically relevant. Since increased visual feedback also reduced force error, this network was evaluated in relation to force error but the model was not significant, indicating it is associated with force tremor but not force error. This study therefore provides new evidence that a widespread functional network is associated with the severity of tremor in patients with essential tremor measured simultaneously at the hand during functional imaging, and is also associated with the clinical severity of tremor. These findings support the idea that the severity of tremor is exacerbated by increased visual feedback, suggesting that designers of new computing technologies should consider using lower visual feedback levels to reduce tremor in essential tremor.

No reliable gray matter changes in essential tremor

BACKGROUND

Voxel-based morphometry (VBM) has been used to study human brain gray matter (GM) alterations in essential tremor (ET) for over one decade. However, the literature revealed heterogeneous findings.

METHODS

We therefore conducted a coordinate-based meta-analysis to synthesize the VBM studies to examine which brain regions show the most reliable GM alterations in patients with ET relative to healthy controls.

RESULTS

A total of 16 original VBM studies, comprising 387 patients with ET and 355 healthy controls, were included in this meta-analysis. This quantitative meta-analysis revealed no evidence of robust and reliable alterations in regional brain GM structures in ET. Meta-regression analyses indicate that many moderators (e.g., MR field strength, statistical methodology, age, onset age, gender, illness severity, illness duration, and family history) account for some of the heterogeneity in GM across studies.

CONCLUSIONS

High heterogeneity in GM alterations across studies may reflect true heterogeneity in ET regarding the clinic, etiology, and pathology, as well as possibly the VBM methodological variations. Currently, this heterogeneity limits the use of VBM as a reliable tool to distinguish ET from healthy controls. In order to improve reproducibility of VBM results in ET, future research may benefit from increasing the sample size, comprehensively subtyping ET phenotypes, and using well-designed and standardized imaging acquisition and analytical protocols. Furthermore, data sharing should be considered as a high priority.

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.

Action Control Deficits in Patients With Essential Tremor

OBJECTIVES

Essential tremor (ET) is a movement disorder characterized by action tremor which impacts motor execution. Given the disrupted cerebellar-thalamo-cortical networks in ET, we hypothesized that ET could interfere with the control mechanisms involved in regulating motor performance. The ability to inhibit or stop actions is critical for navigating many daily life situations such as driving or social interactions. The current study investigated the speed of action initiation and two forms of action control, response stopping and proactive slowing in ET.

METHODS

Thirty-three ET patients and 25 healthy controls (HCs) completed a choice reaction task and a stop-signal task, and measures of going speed, proactive slowing and stop latencies were assessed.

RESULTS

Going speed was significantly slower in ET patients (649 ms) compared to HCs (526 ms; F(1,56) = 42.37; p <.001; η 2 = .43), whereas proactive slowing did not differ between groups. ET patients exhibited slower stop signal reaction times (320 ms) compared to HCs (258 ms, F(1,56) = 15.3; p <.00; η 2 = .22) and more severe motor symptoms of ET were associated with longer stopping latencies in a subset of patients (Spearman rho = .48; p <.05).

CONCLUSIONS

In line with previous studies, ET patients showed slower action initiation. Additionally, inhibitory control was impaired whereas proactive slowing remained intact relative to HCs. More severe motor symptoms of ET were associated with slower stopping speed, and may reflect more progressive changes to the cerebellar-thalamo-cortical network. Future imaging studies should specify which structural and functional changes in ET can explain changes in inhibitory action control. (JINS, 2019, 25, 156-164).

Hybrid PET-MRI Applications in Movement Disorders

Even before the success of combined positron emission tomography and computed tomography (PET/CT), the neuroimaging community was conceiving the idea to integrate the positron emission tomography (PET), with very high molecular quantitative data but low spatial resolution, and magnetic resonance imaging (MRI), with high spatial resolution. Several technical limitations have delayed the use of a hybrid scanner in neuroimaging studies, including the full integration of the PET detector ring within the MRI system, the optimization of data acquisition, and the implementation of reliable methods for PET attenuation, motion correction, and joint image reconstruction. To be valid and useful in clinical and research settings, this instrument should be able to simultaneously acquire PET and MRI, and generate quantitative parametric PET images comparable to PET-CT. While post hoc co-registration of combined PET and MRI data acquired separately became the most reliable technique for the generation of "fused" PET-MRI images, only hybrid PET-MRI approach allows merging these measurements naturally and correlating them in a temporal manner. Furthermore, hybrid PET-MRI represents the most accurate tool to investigate in vivo the interplay between molecular and functional aspects of brain pathophysiology. Hybrid PET-MRI technology is still in the early stages in the movement disorders field, due to the limited availability of scanners with integrated optimized methodological models. This technology is ideally suited to investigate interactions between resting-state functional/arterial spin labeling MRI and [¹⁸F]FDG PET glucose metabolism in the evaluation of the brain "hubs" particularly vulnerable to neurodegeneration, areas with a high degree of connectivity and associated with an efficient synaptic neurotransmission. In Parkinson's disease, hybrid PET-MRI is also the ideal instrument to deeper explore the relationship between resting-state functional MRI and dopamine release at [¹¹C]raclopride PET challenge, in the identification of early drug-naïve Parkinson's disease patients at higher risk of motor complications and in the evaluation of the efficacy of novel neuroprotective treatment able to restore at the same time the altered resting state and the release of dopamine. In this chapter, we discuss the key methodological aspects of hybrid PET-MRI; the evidence in movement disorders of the key resting-state functional and perfusion MRI; [¹⁸F]FDG PET and [¹¹C]raclopride PET challenge studies; the potential advantages of using hybrid PET-MRI to investigate the pathophysiology of movement disorders and neurodegenerative diseases. Future directions of hybrid PET-MRI will be discussed alongside with up-to-date technological innovations on hybrid systems.

Animal Models of Tremor: Relevance to Human Tremor Disorders

Background

Tremor is the most common movement disorder; however, the pathophysiology of tremor remains elusive. While several neuropathological alterations in tremor disorders have been observed in post-mortem studies of human brains, a full understanding of the relationship between brain circuitry alterations and tremor requires testing in animal models. Additionally, tremor animal models are critical for our understanding of tremor pathophysiology, and/or to serve as a platform for therapy development.

Methods

A PubMed search was conducted in May 2018 to identify published papers for review.

Results

The methodology used in most studies on animal models of tremor lacks standardized measurement of tremor frequency and amplitude; instead, these studies are based on the visual inspection of phenotypes, which may fail to delineate tremor from other movement disorders such as ataxia. Of the animal models with extensive tremor characterization, harmaline-induced rodent tremor models provide an important framework showing that rhythmic and synchronous neuronal activities within the olivocerebellar circuit can drive action tremor. In addition, dopamine-depleted monkey and mouse models may develop rest tremor, highlighting the role of dopamine in rest tremor generation. Finally, other animal models of tremor have involvement of the cerebellar circuitry, leading to altered Purkinje cell physiology.

Discussion

Both the cerebellum and the basal ganglia are likely to play a role in tremor generation. While the cerebellar circuitry can generate rhythmic movements, the nigrostriatal system is likely to modulate the tremor circuit. Tremor disorders are heterogeneous in nature. Therefore, each animal model may represent a subset of tremor disorders, which collectively can advance our understanding of tremor.