'Essential tremor' or 'the essential tremors': is this one disease or a family of diseases?

Gene Expression Analysis of Laser-Captured Purkinje Cells in the Essential Tremor Cerebellum

Essential tremor (ET) is a common, progressive neurological disease characterized by an 8-12-Hz kinetic tremor. Despite its high prevalence, the patho-mechanisms of tremor in ET are not fully known. Through comprehensive studies in postmortem brains, we identified major morphological changes in the ET cerebellum that reflect cellular damage in Purkinje cells (PCs), suggesting that PC damage is central to ET pathogenesis. We previously performed a transcriptome analysis in ET cerebellar cortex, identifying candidate genes and several dysregulated pathways. To directly target PCs, we purified RNA from PCs isolated by laser capture microdissection and performed the first ever PC-specific RNA-sequencing analysis in ET versus controls. Frozen postmortem cerebellar cortex from 24 ETs and 16 controls underwent laser capture microdissection, obtaining ≥2000 PCs per sample. RNA transcriptome was analyzed via differential gene expression, principal component analysis (PCA), and gene set enrichment analyses (GSEA). We identified 36 differentially expressed genes, encompassing multiple cellular processes. Some ET (13/24) had greater dysregulation of these genes and segregated from most controls and remaining ETs in PCA. Characterization of genes/pathways enriched in this PCA and GSEA identified multiple pathway dysregulations in ET, including RNA processing/splicing, synapse organization/ion transport, and oxidative stress/inflammation. Furthermore, a different set of pathways characterized marked heterogeneity among ET patients. Our data indicate a range of possible mechanisms for the pathogenesis of ET. Significant heterogeneity among ET combined with dysregulation of multiple cellular processes supports the notion that ET is a family of disorders rather than one disease entity.

MRgFUS of the nucleus ventralis intermedius in essential tremor modulates functional connectivity within the classical tremor network and beyond

BACKGROUND

Magnetic resonance-guided focused ultrasound (MRgFUS) of the thalamic ventral intermediate nucleus is an incisionless lesional treatment for essential tremor.

OBJECTIVE

To examine relationships between tremor severity and functional connectivity in patients with essential tremor and to assess long-term changes in the tremor network after sonication of the ventral intermediate nucleus.

METHODS

Twenty-one patients with essential tremor (70.33 ± 11.32 years) were included in the final analysis and underwent resting state functional magnetic resonance imaging at 3 T before and 6 months after treatment. Tremor severity (Fahn-Tolosa-Marin Clinical Rating Scale) was evaluated and functional connectivity was investigated using independent component analysis.

RESULTS

MRgFUS of the thalamic ventral intermediate nucleus reduced contralateral tremor effectively. Multiple regression analysis revealed exclusively negative correlations between FC and tremor severity, notably in the right cerebellar lobe VI and the left cerebellar lobe VIIIa (cerebellar network), in the left occipital fusiform gyrus (lateral visual network), the anterior division of the left superior temporal gyrus (fronto-parieto-temporal network), and in the posterior division of the left parahippocampal gyrus and the bilateral lingual gyri (default mode network). Six months after treatment, increased functional connectivity was observed in almost all tremor-associated clusters, except the cluster localized in the left cerebellum.

CONCLUSIONS

Our findings suggest that tremor-related activity in essential tremor extends beyond the classical cerebellar network, additionally involving areas related to visual processing. Functional restoration of network activity after sonication of the ventral intermediate nucleus is observed within the classical tremor network (cerebellum) and notably also in visual processing areas.

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.

Does essential tremor increase risk of cognitive impairment and dementia? Yes

Essential Tremor (ET), by definition, is a disorder of movement. Yet over the years, epidemiologic, clinical, pathologic, and neuroimaging studies have converged to reveal a cognitive side of ET. The cognitive symptoms in ET are heterogeneous and are likely to reflect heterogeneous underlying mechanisms. In this chapter, we review and synthesize a diverse set of studies from both population-based settings to cohorts with more detailed investigations into cognition to consider the various mechanisms by which cognitive symptoms may emerge in a subset of individuals with ET. As part of our analysis, we consider questions surrounding ET diagnosis and the possibility of comorbid disease as potential factors that, upon closer examination, appear to strengthen the argument in favor of ET as a risk factor for dementia. Importantly, we also consider the clinical relevance of cognitive impairment in ET. While ET is not universally characterized by significant cognitive deficits, the data from epidemiological, cognitive, neuroimaging, and postmortem neuropathologic studies converge to reveal an increased risk for cognitive impairment and dementia among individuals with ET. We conclude by offering directions for future research, and a neurocognitive framework with which to consider existing findings and to use in the design of novel studies dedicated to clarifying the basis, nature, and course of cognitive impairments in ET.

The MDS consensus tremor classification: The best way to classify patients with tremor at present

In 2018, the new Consensus Statement on the Classification of Tremors, by the Task Force on Tremor of the International Parkinson Movement Disorder Society, was published. So far, the article has been cited more than 400 times in peer-reviewed international journals and commonly debated in conferences and meetings due to an enthusiastic welcome from the community. Compared to the previous Consensus Statement (1998), the main novelties are: 1) the classification of tremor according to clinical manifestation (Axis 1) and etiology (Axis 2), and therefore the use of a syndromic approach; 2) the definition of essential tremor as a syndrome; 3) the recognition of the new category essential tremor plus, that derives from the uncertain significance of the soft neurological signs often associated with essential tremor. In this paper, we summarise and explain the most important aspects of the new classification of tremors, highlighting the main novelties, their relevance, and application in clinical practice. Moreover, we discuss its possible weakness and reflect on the critical comments made so far. We believe that this new tremor classification is comprehensive, rigorous, and consistent and, considering our current knowledge of tremor syndromes, it is the best we can do at present. This article is part of the Special Issue "Tremor" edited by Daniel D. Truong, Mark Hallett, and Aasef Shaikh.

Features of "ET plus" correlate with age and tremor duration: "ET plus" may be a disease stage rather than a subtype of essential tremor

BACKGROUND

Essential tremor (ET) is characterized by considerable clinical heterogeneity. In 2018, the term "ET plus" was introduced to mark a potential stratification point for dividing ET into subtypes - ET vs ET plus (i.e., ET cases with neurological features other than action tremor). However, as ET progresses, patients often develop increasingly severe tremor, spread of tremor, tremor under different activation conditions, and other features. Given this situation, ET plus may represent a disease stage rather than a disease classification or subtype. In theory, if the defining characteristics of a disease subtype fluctuate with age or disease duration, it raises the distinct possibility the "subtype" is a disease stage.

METHODS

A cohort of 241 prospectively enrolled ET cases underwent a detailed motor and cognitive assessment in which the features of ET plus including cerebellar signs (intention tremor, tandem gait difficulty), rest tremor, dystonia, and cognitive performance were evaluated. We determined whether these features of ET plus correlated with action tremor duration and age.

RESULTS

We demonstrated that numerous ET plus features were significantly correlated with both age and action tremor duration (numerous p values < 0.05). The same relationships were observed in a series of sensitivity analyses.

CONCLUSION

We observed that the component parts of ET plus are highly age- and stage-dependent. These features are yearly-changing features conditional on a demographic and disease stage variable. These data support the notion that ET plus may represent a disease stage rather than a distinct disease subtype or disease classification.

Disrupted Brain Network Topology in Drug-naïve Essential Tremor Patients with and Without Depression : A Resting State Functional Magnetic Resonance Imaging Study

PURPOSE

This study was carried out to investigate brain functional connectome and its potential relationships with the disease severity and emotion function in patients with essential tremor with and without depressive symptoms by using resting-state functional magnetic resonance imaging and graph theory approaches.

METHODS

In this study 33 essential tremor patients with depression, 45 essential tremor patients without depression and 79 age and gender-matched healthy controls were recruited to undergo a 3.0‑T imaging scan. The whole brain functional connectome was constructed by thresholding the partial correlation matrices of 116 brain regions, and the topologic properties were analyzed by using graph theory approaches and network-based statistic approaches. Nonparametric permutation test was also used for group comparisons of topological metrics. Correlation analyses between topographic features and the clinical characteristics were performed.

RESULTS

The functional connectome in both essential tremor patients with and without depression showed abnormalities at the global level (decrease in clustering coefficient, global efficiency, and local efficiency but increase in characteristic path length) and at the nodal level (decrease nodal centralities in the cerebellum, motor cortex, prefrontal-limbic regions, default mode network) (p < 0.05, false discovery rate corrected). Moreover, essential tremor patients with depression showed higher node efficiency in superior frontal gyrus and posterior cingulate gyrus compared to essential tremor without depression.

CONCLUSION

Our results may provide insights into the underlying pathophysiology of essential tremor patients with and without depression and aid the development of some potential biomarkers of the depressive symptoms in patients with essential tremor.

Do We Belittle Essential Tremor by Calling It a Syndrome Rather Than a Disease? Yes

Essential tremor (ET) is among the most prevalent neurological diseases. Appreciation in recent years of a richer tremor phenomenology, additional motor and non-motor features, variability in the natural course of tremor, associations with a host of other neurological conditions, and etiological and pathophysiological heterogeneity have resulted in general awareness of the clinical richness of ET. Along with this evolving view of ET have surfaced several conundrums regarding nomenclature. One of these is whether ET should be labeled a "syndrome" or "disease." Here, we revisit the classical definitions of "syndrome" and "disease" and discuss ET in this context. Considering the characteristics of "disease" and "syndrome" and evaluating the characteristics of ET, it seems to fit more into the "disease" construct. There are several reasons: There is considerable knowledge of the underlying etiologies and pathophysiology of ET, in numerous studies ET has been linked with other neurological conditions, the condition is progressive and deteriorative, and therapeutic approaches are grounded in an understanding of disease mechanisms and its associated neuroanatomy. Moreover, the etiological-pathological-clinical heterogeneity suggests that ET should be regarded as a "family of diseases" more appropriately termed "the essential tremors." This nomenclatural issue is not a mere matter of words; public health implications are numerous. A condition with the label "syndrome" may not be recognized as a serious problem, may be plagued by diminished public awareness, and may not garner funds for research that a condition with the label "disease" or "diseases" would. ET should be regarded as a family of diseases.

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.

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.

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.

Gene expression analysis of the cerebellar cortex in essential tremor

Essential tremor (ET) is one of the most common neurological diseases, with a central feature of an 8-12 Hz kinetic tremor. While previous postmortem studies have identified a cluster of morphological changes in the ET cerebellum centered in/around the Purkinje cell (PC) population, including a loss of PCs in some studies, the underlying molecular mechanisms for these changes are not clear. As genomic studies of ET patients have yet to identify major genetic contributors and animal models that fully recapitulate the human disease do not yet exist, the study of human tissue is currently the most applicable method to gain a mechanistic insight into ET disease pathogenesis. To begin exploration of an underlying molecular source of ET disease pathogenesis, we have performed the first transcriptomic analysis by direct sequencing of RNA from frozen cerebellar cortex tissue in 33 ET patients compared to 21 normal controls. Principal component analysis showed a heterogenous distribution of the expression data in ET patients that only partially overlapped with control patients. Differential expression analysis identified 231 differentially expressed gene transcripts ('top gene hits'), a subset of which has defined expression profiles in the cerebellum across neuronal and glial cell types but a largely unknown relationship to cerebellar function and/or ET pathogenesis. Gene set enrichment analysis (GSEA) identified dysregulated pathways of interest and stratified dysregulation among ET cases. By GSEA and mining curated databases, we compiled major categories of dysregulated processes and clustered string networks of known interacting proteins. Here we demonstrate that these 'top gene hits' contribute to regulation of four main biological processes, which are 1) axon guidance, 2) microtubule motor activity, 3) endoplasmic reticulum (ER) to Golgi transport and 4) calcium signaling/synaptic transmission. The results of our transcriptomic analysis suggest there is a range of different processes involved among ET cases, and draws attention to a particular set of genes and regulatory pathways that provide an initial platform to further explore the underlying biology of ET.

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.

Etiologic links between environmental and lifestyle factors and Essential tremor

Essential tremor (ET) is one of the most common adult movement disorders, characterized by clinical tremor and other nonmotor symptoms. It is a progressive disease that shares features with other neurodegenerative diseases. ET is a complex disease with both genetic and environmental underpinnings. While genetic forms of ET are well recognized, the role of environmental and lifestyle factors in ET has been debated. Studies suggest that exposure to neurotoxic compounds such as β-carboline alkaloids and ethanol are potential risk factors for ET, while antioxidant intake may be protective. In addition, smoking acts as a protective factor in ET, parallel to its effects in other neurological diseases. New evidence points to pesticide and lead exposure as potential risk factors. There is growing evidence to suggest that environmental and lifestyle factors play a role in ET but additional research is needed in order to completely understand their cause and effect association. There is also a need for larger case-control and prospective cohort studies across different populations to further evaluate the etiological importance of these factors in ET.

White matter measures correlate with essential tremor severity-A pilot diffusion tensor imaging study

BACKGROUND

An evolving pathophysiological concept of essential tremor (ET) points to diffuse brain network involvement, which emphasizes the need to investigate white matter (WM) changes associated with motor symptoms of ET.

OBJECTIVES

To investigate ET-related WM changes and WM correlates of tremor severity using tremor clinical rating scales and accelerometry.

METHODS

Tract-based spatial statistics (TBSS) approach was utilized to compare 3 Tesla diffusion tensor imaging (DTI) data from 12 ET patients and 10 age- and gender-matched healthy individuals. Clinical scales, tremor frequency and amplitude as measured by accelerometry were correlated with DTI data.

RESULTS

ET patients demonstrated mean (MD) and radial diffusivity (RD) abnormalities in tracts involved in primary and associative motor functions such as bilateral corticospinal tracts, the superior longitudinal fascicles, and the corpus callosum but also in nonmotor regions including the inferior fronto-occipital and longitudinal fascicles, cingulum bundles, anterior thalamic radiations, and uncinate fascicles. A combined tremor frequency and amplitude score correlated with RD and MD in extensive WM areas, which partially overlapped the regions that were associated with tremor frequency. No significant relationship was found between DTI measures and clinical rating scales scores.

CONCLUSIONS

The results show that ET-related diffusion WM changes and their correlates with tremor severity are preferentially located in the primary and associative motor areas. In contrast, a relationship between WM was not detected with clinical rating scales. Accelerometry parameters may, therefore, serve as a potentially useful clinical measures that relate to WM deficits in ET.

Evaluation of thickness of retinal nerve fiber layer, ganglion cell layer, and choroidal thickness in essential tremor: can eyes be a clue for neurodegeneration?

The pathology of essential tremor (ET) and underlying mechanisms of the disease are still unclear, but an increasing amount of research has been conducted on the subject. Discussions are ongoing about ET's definition as a neurodegenerative disease. Optic coherence tomography (OCT) provides a window to the brain where direct visualization of central nervous system (CNS) changes may be possible, and it can help us to develop a new point of view on ET. The goal of this study was to examine OCT parameters in ET. 40 ET patients and 40 healthy controls, i.e., a total of 160 eyes were evaluated. Retinal nerve fiber layer (RNFL), ganglion cell layer (GCL), inner plexiform layer (IPL), and choroid thinning were analyzed using spectral domain OCT. The mean age was 25.77 ± 8.98 in the ET group and 27.25 ± 8.22 in the control group. We found a decreased global RNFL thickness for both eyes in ET patients. All GCL and IPL thickness parameters were lower in the patients with ET (P < 0.001, P 0.03). Choroid was significantly thicker in ET patients than the controls (P < 0.001). Our study's results suggest that the usefulness of OCT in detecting neurodegeneration in ET. RNFL, GCL, IPL measurements are highly reproduced findings of neurodegeneration. Increased choroid volume may indicate neuroinflammation. Eyes in ET may shed light on nature of the disease, and may be used as a diagnostic tool.

A Voxel-Based Magnetic Resonance Imaging Morphometric Study of Cerebral and Cerebellar Gray Matter in Patients Under 65 Years with Essential Tremor

BACKGROUND The aim of this study was to compare the morphological changes in cerebral and cerebellar gray matter in patients with essential tremor under 60-years-of-age, with age-matched and gender-matched normal healthy volunteer control subjects, using functional magnetic resonance imaging (fMRI) and voxel-based morphometry (VBM) analysis. MATERIAL AND METHODS A retrospective, controlled, comparative clinical study included 17 patients with essential tremor, <60 years-of-age, and 17 age-matched and gender-matched healthy volunteer control subjects, recruited between June 2010-July 2012. MRI and VBM analysis were used to compare cerebral and cerebellar gray matter density between groups. The Washington Heights-Inwood Genetic Study of Essential Tremor (WHIGET) rating scale was used to assess tremor severity in the patient group. Clinical and demographic characteristics were recorded for all study participants. RESULTS MRI and VBM analysis showed significant bilateral expansion of the cerebellum, occipital fusiform cortices, right inferior temporal gyrus, and precentral lobes in patients with essential tremor (P<0.005); reduction in gray matter was found in the left parietal lobe. The region of interest (ROI) analysis showed volume enlargement in the thalamus, midbrain, and the precuneus (P<0.005). No significant correlation between changes in gray matter and changes in clinical variables, including age, gender, tremor duration, the activity of daily living (ADL) scale, the mini-mental state examination (MMSE) scale, family history, and tremor severity were found. CONCLUSIONS Predominantly cerebellar gray matter expansion in patients less than 60 years-of-age with essential tremor might be the result of compensation for the decline in cerebellar function.

Neural correlates of dystonic tremor: a multimodal study of voice tremor in spasmodic dysphonia

Tremor, affecting a dystonic body part, is a frequent feature of adult-onset dystonia. However, our understanding of dystonic tremor pathophysiology remains ambiguous as its interplay with the main co-occurring disorder, dystonia, is largely unknown. We used a combination of functional MRI, voxel-based morphometry and diffusion-weighted imaging to investigate similar and distinct patterns of brain functional and structural alterations in patients with dystonic tremor of voice (DTv) and isolated spasmodic dysphonia (SD). We found that, compared to controls, SD patients with and without DTv showed similarly increased activation in the sensorimotor cortex, inferior frontal (IFG) and superior temporal gyri, putamen and ventral thalamus, as well as deficient activation in the inferior parietal cortex and middle frontal gyrus (MFG). Common structural alterations were observed in the IFG and putamen, which were further coupled with functional abnormalities in both patient groups. Abnormal activation in left putamen was correlated with SD onset; SD/DTv onset was associated with right putaminal volumetric changes. DTv severity established a significant relationship with abnormal volume of the left IFG. Direct patient group comparisons showed that SD/DTv patients had additional abnormalities in MFG and cerebellar function and white matter integrity in the posterior limb of the internal capsule. Our findings suggest that dystonia and dystonic tremor, at least in the case of SD and SD/DTv, are heterogeneous disorders at different ends of the same pathophysiological spectrum, with each disorder carrying a characteristic neural signature, which may potentially help development of differential markers for these two conditions.

Essential tremor

Essential tremor (ET) is one of the most common neurologic disorders, and genetic factors are thought to contribute significantly to disease etiology. There has been a relative lack of progress in understanding the genetic etiology of ET. This could reflect a number of factors, including the presence of substantial phenotypic and genotypic heterogeneity. Thus, a meticulous approach to phenotyping is important for genetic research. A lack of standardized phenotyping across studies and patient centers likely has contributed to the relative lack of success of genomewide association studies in ET. To dissect the genetic architecture of ET, whole-genome sequencing will likely be of value. This will allow specific hypotheses about the mode of inheritance and genetic architecture to be tested. A number of approaches still remain unexplored in ET genetics, including the contribution of copy number variants, uncommon moderate-effect alleles, rare variant large-effect alleles (including Mendelian and complex/polygenic modes of inheritance), de novo and gonadal mosaicism, epigenetic changes, and noncoding variation.

Early- and late-onset essential tremor patients represent clinically distinct subgroups

OBJECTIVE

Essential tremor is a very common disease defined by sparse clinical criteria. It is unlikely that essential tremor is an etiologically homogeneous disease. Stratifying broadly defined diseases using clinical characteristics has often aided the etiopathological understanding. Most studies of essential tremor show 2 distinct age at onset peaks: early and late. This study investigates phenotypical differences between early- and late-onset essential tremor patients.

METHODS

We studied a sample of 1137 tremor patients. Of these patients, 978 suffered from definite or probable essential tremor. All of the patients underwent the same standardized examination encompassing, among other items, drawing of the Archimedes spiral and assessment of the Fahn-Tolosa-Marin scale.

RESULTS

Two subgroups of early-onset (≤ 24 years of age, n = 317) and late-onset (≥ 46 years of age, n = 356) patients were selected based on the visual and mathematical analysis of the age-at-onset distribution. Tremor severity in both groups was comparable. Tremor progression measured as Archimedes spiral score and the Fahn-Tolosa-Marin subscales divided by the disease duration in 10-year bins was significantly faster in late-onset patients when compared with early-onset patients. Early-onset patients more frequently reported a positive family history and alcohol sensitivity of the tremor.

CONCLUSIONS

The age-at-onset distribution suggests a distinction between early- and late-onset tremor. Early-onset and late-onset essential tremor differ in the progression rates and the frequencies of a positive family history and history of a positive effect of alcohol on tremor. © 2016 International Parkinson and Movement Disorder Society.

The Olivary Hypothesis of Essential Tremor: Time to Lay this Model to Rest?

BACKGROUND

Although essential tremor (ET) is the most common tremor disorder, its pathogenesis is not fully understood. The traditional model of ET, proposed in the early 1970s, posited that the inferior olivary nucleus (ION) was the prime generator of tremor in ET and that ET is a disorder of electrophysiological derangement, much like epilepsy. This article comprehensively reviews the origin and basis of this model, its merits and problems, and discusses whether it is time to lay this model to rest.

METHODS

A PubMed search was performed in March 2017 to identify articles for this review.

RESULTS

The olivary model gains support from the recognition of neurons with pacemaker property in the ION and the harmaline-induced tremor models (as the ION is the prime target of harmaline). However, the olivary model is problematic, as neurons with pacemaker property are not specific to the ION and the harmaline model does not completely represent the human disease ET. In addition, a large number of neuroimaging studies in ET have not detected structural or functional changes in the ION; rather, abnormalities have been reported in structures related to the cerebello-thalamo-cortical network. Moreover, a post-mortem study of microscopic changes in the ION did not detect any differences between ET cases and controls.

DISCUSSION

The olivary model largely remains a physiological construct. Numerous observations have cast considerable doubt as to the validity of this model in ET. Given the limitations of the model, we conclude that it is time now to lay this model to rest.

The evolving definition of essential tremor: What are we dealing with?

BACKGROUND

Although essential tremor (ET) is commonly encountered in clinical practice, historically, there has been considerable disagreement as how to best define it, and now with a growing sense of its clinical complexity, how to best encapsulate it. Here, I draw attention to five issues of current uncertainty.

METHODS

A PubMed search conducted on June 19, 2017 crossed "essential tremor" with 9 second search terms (e.g., definition, diagnosis).

RESULTS

There are several major issues of clinical and diagnostic uncertainty. Underlying each issue is a larger question about the nature of the underlying pathophysiology of ET. Does age of onset of ET matter? How much dystonia is acceptable in ET? How much in the way of "cerebellar signs" are acceptable? Are non-motor features due to the underlying disease or merely secondary to the clinical features? Is ET a single disease entity or something else?

CONCLUSIONS

We are learning more about ET and, as a by-product of these efforts, are struggling with its definition. Further understanding the nature of the underlying disease pathogenesis as well as the role the cerebellum and cerebellar relays play in this process will likely provide important clues to enable us to bring order to areas of uncertainty.

Gray matter density loss in essential tremor: a lobule by lobule analysis of the cerebellum

Background

The pathophysiological basis for essential tremor (ET) remains unclear, although evidence increasingly links it to a disordered and perhaps degenerative cerebellum. Prior imaging studies have treated the cerebellum en bloc. Our hypothesis was that regional differences in cerebellar gray matter (GM) density may better distinguish ET cases from controls.

Forty-seven ET cases and 36 control subjects were imaged using magnetic resonance imaging (MRI). The cerebellum was segmented into 34 lobes using a Spatially Unbiased Infra-Tentorial Template (SUIT) atlas within the Statistical Parametric Mapping (SPM) analysis package. Age, gender and Montreal Cognitive Assessment (MoCA) scores were regressed out from the statistical models to isolate group effects. ET cases were further stratified into phenotypically-defined subgroups. The Benjamini-Hochberg False Discovery Rate procedure (BH FDR) (α = 0.1) was used to correct for multiple comparisons.

Results

When all ET cases and controls were compared, none of the regions met the BH FDR criteria for significance. When compared with controls, ET cases with head or jaw tremor (n = 27) had significant changes in GM density in nine cerebellar lobules, with a majority in the left cerebellar region, and each meeting the BH FDR criteria. Likewise, ET cases with voice tremor (n = 22) exhibited significant changes in 11 lobules in both left and right regions and the vermis. These analyses, in sum, indicated decreases in GM density in lobules I-IV, V, VI, VII and VIII as well as the vermis. ET cases with severe tremor (n = 20) did not show regions of change that survived the BH FDR procedure when compared to controls.

Conclusions

We showed that ET cases with various forms of cranial tremor differed from controls with respect to cerebellar GM density, with evidence of GM reduction across multiple cerebellar regions. Additional work, using a lobule-by-lobule approach, is needed to confirm these results and precisely map the regional differences in ET cases, subgroups of ET cases, and controls.

Rest tremor revisited: Parkinson's disease and other disorders

Tremor is the most common movement disorder characterized by a rhythmical, involuntary oscillatory movement of a body part. Since distinct diseases can cause similar tremor manifestations and vice-versa, it is challenging to make an accurate diagnosis. This applies particularly for tremor at rest. This entity was only rarely studied in the past, although a multitude of clinical studies on prevalence and clinical features of tremor in Parkinson's disease (PD), essential tremor and dystonia, have been carried out. Monosymptomatic rest tremor has been further separated from tremor-dominated PD. Rest tremor is also found in dystonic tremor, essential tremor with a rest component, Holmes tremor and a few even rarer conditions. Dopamine transporter imaging and several electrophysiological methods provide additional clues for tremor differential diagnosis. New evidence from neuroimaging and electrophysiological studies has broadened our knowledge on the pathophysiology of Parkinsonian and non-Parkinsonian tremor. Large cohort studies are warranted in future to explore the nature course and biological basis of tremor in common tremor related disorders.

Awareness of cognitive impairment in individuals with essential tremor

OBJECTIVE

The extent to which individuals with ET who have clinically significant cognitive impairment are aware of their cognitive changes is unclear. Reduced awareness has important implications for everyday function and decision-making.

METHODS

150 individuals with ET (109 Normal Cognition (ET-NC group), and 30 with MCI and 11 dementia (ET-CI group)) completed self-ratings and objective assessments of memory, language, and executive functioning. Discrepancy scores were calculated to assess awareness of cognitive functioning. One sample t-tests evaluated whether mean discrepancy scores in each group were comparable to zero (i.e., accurate). Analyses of covariance (ANCOVA) compared discrepancy scores across two groups controlling for age and education.

RESULTS

In the ET-NC group, discrepancy scores for language (M=-0.08, SD=1.10) and executive functioning (M=-0.01, SD=0.99) were comparable to zero. Memory discrepancy scores (M=0.32, SD=1.22) were greater than zero. In the ET-CI group, memory, (M=0.78, SD=1.01), language, (M=0.46, SD=0.95), and executive (M=0.39, SD=1.14) discrepancy scores were all greater than zero. Discrepancy scores were larger in ET-CI group than in ET-NC group for memory: F(1,148)=4.02, p=0.047, language: F(1,148)=6.16, p=0.014, and executive: F(1,148)=4.51, p=0.035.

CONCLUSIONS

Individuals with ET and normal cognition accurately assessed their language and executive abilities, demonstrating mild overconfidence in memory function. Individuals with ET and cognitive impairment overestimated their performance in all domains of functioning. Since ET is linked to increased risk for cognitive impairment, and such impairment may not be accurately perceived, cognitive functioning should be proactively and regularly screened in ET.

Linking Essential Tremor to the Cerebellum: Neurochemical Evidence

The pathophysiology and the exact anatomy of essential tremor (ET) is not well known. One of the pillars that support the cerebellum as the main anatomical locus in ET is neurochemistry. This review examines the link between neurochemical abnormalities found in ET and cerebellum. The review is based on published data about neurochemical abnormalities described in ET both in human and in animal studies. We try to link those findings with cerebellum. γ-aminobutyric acid (GABA) is the main neurotransmitter involved in the pathophysiology of ET. There are several studies about GABA that clearly points to a main role of the cerebellum. There are few data about other neurochemical abnormalities in ET. These include studies with noradrenaline, glutamate, adenosine, proteins, and T-type calcium channels. One single study reveals high levels of noradrenaline in the cerebellar cortex. Another study about serotonin neurotransmitter results negative for cerebellum involvement. Finally, studies on T-type calcium channels yield positive results linking the rhythmicity of ET and cerebellum. Neurochemistry supports the cerebellum as the main anatomical locus in ET. The main neurotransmitter involved is GABA, and the GABA hypothesis remains the most robust pathophysiological theory of ET to date. However, this hypothesis does not rule out other mechanisms and may be seen as the main scaffold to support findings in other systems. We clearly need to perform more studies about neurochemistry in ET to better understand the relations among the diverse systems implied in ET. This is mandatory to develop more effective pharmacological therapies.

Linking Essential Tremor to the Cerebellum: Neuropathological Evidence

A fundamental question about essential tremor (ET) is whether its associated pathological changes and disease mechanisms are linkable to a specific brain region. To that end, recent tissue-based studies have made significant strides in elucidating changes in the ET brain. Emerging from these studies is increasing neuropathological evidence linking ET to the cerebellum. These studies have systematically identified a broad range of structural, degenerative changes in the ET cerebellum, spanning across all Purkinje cell compartments. These include the dendritic compartment (where there is an increase in number of Purkinje cell dendritic swellings, a pruning of the dendritic arbor, and a reduction in spine density), the cell body (where, aside from reductions in Purkinje cell linear density in some studies, there is an increase in the number of heterotopic Purkinje cell soma), and the axonal compartment (where a plethora of changes in axonal morphology have been observed, including an increase in the number of thickened axonal profiles, torpedoes, axonal recurrent collaterals, axonal branching, and terminal axonal sprouting). Additional changes, possibly due to secondary remodeling, have been observed in neighboring neuronal populations. These include a hypertrophy of basket cell axonal processes and changes in the distribution of climbing fiber-Purkinje cell synapses. These changes all distinguish ET from normal control brains. Initial studies further indicate that the profile (i.e., constellation) of these changes may separate ET from other diseases of the cerebellum, thereby serving as a disease signature. With the discovery of these changes, a new model of ET has arisen, which posits that it may be a neurodegenerative disorder centered in the cerebellar cortex. These newly emerging neuropathological studies pave the way for anatomically focused, hypothesis-driven, molecular mechanistic studies of disease pathogenesis.

The essential tremor syndromes

PURPOSE OF REVIEW

Essential tremor has not been defined or used consistently in clinical diagnosis and research. Other monosymptomatic disorders are often referred to as essential tremor variants.

RECENT FINDINGS

There is now solid evidence that essential tremor, however defined, is a syndrome with multiple causes.

SUMMARY

A new tremor classification scheme is being developed by the International Parkinson and Movement Disorder Society Task Force on Tremor. In this scheme, tremor in the absence of other neurological signs is called isolated tremor, and tremor in combination with other signs is called combined tremor. Many isolated and combined tremor syndromes can be defined on the basis of commonly recurring or unique clinical symptoms and signs, including historical features (age at onset, family history, and temporal evolution) and tremor characteristics (body distribution, activation condition, associated features). Essential tremor, however defined, is simply a syndrome and not a specific disease. Essential tremor should be defined and used consistently, or this term should be abandoned. As essential tremor is an arbitrarily defined syndrome, it makes no sense to refer to other tremors as variants of essential tremor or essential tremor syndromes.

Daytime sleepiness and nighttime sleep quality across the full spectrum of cognitive presentations in essential tremor

There is increasing evidence that essential tremor (ET) is a complex and heterogeneous disorder with nonmotor features including cognitive deficits and sleep problems. We are unaware of a study that has examined sleep deficits in ET across the full spectrum of cognitive presentations. Cross-sectional (baseline) data on self-reported nighttime sleep dysfunction and excessive daytime sleepiness were collected using the Pittsburgh Sleep Quality Index (PSQI) and the Epworth Sleepiness Scale (ESS) in 96 ET cases enrolled in a prospective study. Cases underwent a comprehensive neuropsychological assessment, and were classified as ET with normal cognition (ET-NC), ET with mild cognitive impairment (ET-MCI), and ET with dementia (ET-D). PSQI scores did not significantly differ across the three ET cognitive groups (p=0.22). ESS scores were highest (more daytime sleepiness) in the ET-MCI group, followed by the ET-D and ET-NC groups, respectively (p=0.016). We examined sleep dysfunction across the cognitive spectrum in ET. We demonstrate for the first time that excessive daytime sleepiness is greater in ET-MCI than ET-NC. Unpredicted low ESS scores in the dementia group raises two possibilities: a self-report bias related to cognitive impairment and/or the possibility that currently undefined pathological heterogeneity in ET may map onto multiple presentations of non-motor deficits.

Knowledge gaps and research recommendations for essential tremor

Essential tremor (ET) is a common cause of significant disability, but its etiologies and pathogenesis are poorly understood. Research has been hampered by the variable definition of ET and by non-standardized research approaches. The National Institute of Neurological Disorders and Stroke (USA) invited experts in ET and related fields to discuss current knowledge, controversies, and gaps in our understanding of ET and to develop recommendations for future research. Discussion focused on phenomenology and phenotypes, therapies and clinical trials, pathophysiology, pathology, and genetics. Across all areas, the need for collaborative and coordinated research on a multinational level was expressed. Standardized data collection using common data elements for genetic, clinical, neurophysiological, and pathological studies was recommended. Large cohorts of patients should be studied prospectively to collect bio-samples, characterize the natural history of the clinical syndrome including patient-oriented outcomes, investigate potential etiologies of various phenotypes, and identify pathophysiological mechanisms. In particular, cellular and system-level mechanisms of tremor oscillations should be elucidated because they may yield effective therapeutic targets and biomarkers. A neuropathology consortium was recommended to standardize postmortem analysis and further characterize neuropathological observations in the cerebellum and elsewhere. Furthermore, genome-wide association studies on large patient cohorts (>10,000 patients) may allow the identification of common genes contributing to risk, and whole exome or genome sequencing may enable the identification of genetic risk and causal mutations in cohorts and well-characterized families.

Current concepts of essential tremor

Essential tremor is clinically defined but there is increasing evidence that it is not a unique entity. Its pathophysiology has been studied with many methods but may also vary between subtypes. Neurophysiologically, there is strong evidence that a specific cerebello-thalamo-cortical loop is abnormally oscillating. The cause of its uncontrolled oscillation is not yet understood. The clear proof of a degenerative cause is still lacking and abnormal receptors or other causes of altered non-progressive functional disturbance cannot be excluded. Strong evidence supports the major involvement of the cerebellum and there is ample evidence that GABA is the main neurotransmitter involved in the pathophysiology in ET. Genetics have provided so far only a few rare subtypes which are due to specific mutations but there is no doubt that it is mostly a hereditary condition. There is evidence that the large subgroup of late onset tremor is a separate condition and this tremor is an independent risk factor for earlier mortality and comes with signs of premature aging (aging-related tremor). It will be important to improve phenotyping of patients in more detail possibly to include not only features of the tremor itself but also other clinical assessments like force measurements or cognitive testing. Based on these variables, we may be able to better understand the presumably different mechanisms underlying different variants of the disease.

Identification of candidate genes for familial early-onset essential tremor

Essential tremor (ET) is one of the most common causes of tremor in humans. Despite its high heritability and prevalence, few susceptibility genes for ET have been identified. To identify ET genes, whole-exome sequencing was performed in 37 early-onset ET families with an autosomal-dominant inheritance pattern. We identified candidate genes for follow-up functional studies in five ET families. In two independent families, we identified variants predicted to affect function in the nitric oxide (NO) synthase 3 gene (NOS3) that cosegregated with disease. NOS3 is highly expressed in the central nervous system (including cerebellum), neurons and endothelial cells, and is one of three enzymes that converts l-arginine to the neurotransmitter NO. In one family, a heterozygous variant, c.46G>A (p.(Gly16Ser)), in NOS3, was identified in three affected ET cases and was absent in an unaffected family member; and in a second family, a heterozygous variant, c.164C>T (p.(Pro55Leu)), was identified in three affected ET cases (dizygotic twins and their mother). Both variants result in amino-acid substitutions of highly conserved amino-acid residues that are predicted to be deleterious and damaging by in silico analysis. In three independent families, variants predicted to affect function were also identified in other genes, including KCNS2 (KV9.2), HAPLN4 (BRAL2) and USP46. These genes are highly expressed in the cerebellum and Purkinje cells, and influence function of the gamma-amino butyric acid (GABA)-ergic system. This is in concordance with recent evidence that the pathophysiological process in ET involves cerebellar dysfunction and possibly cerebellar degeneration with a reduction in Purkinje cells, and a decrease in GABA-ergic tone.

Tremor entities and their classification: an update

PURPOSE OF REVIEW

This review focuses on important new findings in the field of tremor and illustrates the consequences for the current definition and classification of tremor.

RECENT FINDINGS

Since 1998 when the consensus criteria for tremor were proposed, new variants of tremors and new diagnostic methods were discovered that have changed particularly the concepts of essential tremor and dystonic tremor. Accumulating evidence exists that essential tremor is not a single entity rather different conditions that share the common symptom action tremor without other major abnormalities. Tremor is a common feature in patients with adult-onset focal dystonia and may involve several different body parts and forms of tremor. Recent advances, in particular, in the field of genetics, suggest that dystonic tremor may even be present without overt dystonia. Monosymptomatic asymmetric rest and postural tremor has been further delineated, and apart from tremor-dominant Parkinson's disease, there are several rare conditions including rest and action tremor with normal dopamine transporter imaging (scans without evidence of dopaminergic deficit) and essential tremor with tremor at rest.

SUMMARY

Increasing knowledge in the last decades changed the view on tremors and highlights several caveats in the current tremor classification. Given the ambiguous assignment between tremor phenomenology and tremor etiology, a more cautious definition of tremors on the basis of clinical assessment data is needed.

Resting State fMRI Demonstrates a Disturbance of the Cerebello-Cortical Circuit in Essential Tremor

Individuals with essential tremor (ET) have postural and active movement abnormalities. Disturbances in the cerebello-thalamo-cortical circuit may contribute to the several motor symptoms of ET. Resting state fMRI provides a valuable, noninvasive tool to study intrinsic activation in the human brain, particularly in the brains of individuals with neuropsychiatric diseases. To investigate the low frequency oscillation features of intrinsic activation in ET in this study, we performed a resting state fMRI analysis in 24 patients with ET and 23 healthy controls. The amplitudes of low frequency fluctuation (ALFF) were analyzed. When compared with healthy controls, patients showed significantly enhanced ALFF in the bilateral cerebral cortex, which is related to motor function, including the pre- and post-central gyrus, supplementary motor area and paracentral lobule. The larger ALFF value in the right precentral gyrus is related to a longer duration of tremor. The decreased ALFF in the bilateral cerebellum was also observed in patients. In addition, aberrant ALFF in the right cerebellar tonsil was negatively associated with the duration of tremor. Our findings suggest that abnormalities exist in the intrinsic activation of brain regions in patients with ET. These findings provide noninvasive evidence that supports the hypothesis that the abnormality of intrinsic activity in the cerebello-cerebral cortex pathway could be associated with the motor-related symptoms of ET. Furthermore, the duration of a tremor might relate to the severity of the alterations to the motor system of ET.

Multiple Resting-State Networks Are Associated With Tremors and Cognitive Features in Essential Tremor

INTRODUCTION

The heterogeneous clinical features of essential tremor indicate that the dysfunctions of this syndrome are not confined to motor networks, but extend to nonmotor networks. Currently, these neural network dysfunctions in essential tremor remain unclear. In this study, independent component analysis of resting-state functional MRI was used to study these neural network mechanisms.

METHODS

Thirty-five essential tremor patients and 35 matched healthy controls with clinical and neuropsychological tests were included, and eight resting-state networks were identified. After considering the structure and head-motion factors and testing the reliability of the selected resting-state networks, we assessed the functional connectivity changes within or between resting-state networks. Finally, image-behavior correlation analysis was performed.

RESULTS

Compared to healthy controls, essential tremor patients displayed increased functional connectivity in the sensorimotor and salience networks and decreased functional connectivity in the cerebellum network. Additionally, increased functional network connectivity was observed between anterior and posterior default mode networks, and a decreased functional network connectivity was noted between the cerebellum network and the sensorimotor and posterior default mode networks. Importantly, the functional connectivity changes within and between these resting-state networks were correlated with the tremor severity and total cognitive scores of essential tremor patients.

CONCLUSIONS

The findings of this study provide the first evidence that functional connectivity changes within and between multiple resting-state networks are associated with tremors and cognitive features of essential tremor, and this work demonstrates a potential approach for identifying the underlying neural network mechanisms of this syndrome.

Comparative Incidence of Conformational, Neurodegenerative Disorders

Background

The purpose of this study was to identify incidence and survival patterns in conformational neurodegenerative disorders (CNDDs).

Methods

We identified 2563 reports on the incidence of eight conditions representing sporadic, acquired and genetic, protein-associated, i.e., conformational, NDD groups and age-related macular degeneration (AMD). We selected 245 papers for full-text examination and application of quality criteria. Additionally, data-collection was completed with detailed information from British, Swedish, and Spanish registries on Creutzfeldt-Jakob disease (CJD) forms, amyotrophic lateral sclerosis (ALS), and sporadic rapidly progressing neurodegenerative dementia (sRPNDd). For each condition, age-specific incidence curves, age-adjusted figures, and reported or calculated median survival were plotted and examined.

Findings

Based on 51 valid reported and seven new incidence data sets, nine out of eleven conditions shared specific features. Age-adjusted incidence per million person-years increased from ≤1.5 for sRPNDd, different CJD forms and Huntington's disease (HD), to 1589 and 2589 for AMD and Alzheimer's disease (AD) respectively. Age-specific profiles varied from (a) symmetrical, inverted V-shaped curves for low incidences to (b) those increasing with age for late-life sporadic CNDDs and for sRPNDd, with (c) a suggested, intermediate, non-symmetrical inverted V-shape for fronto-temporal dementia and Parkinson's disease. Frequently, peak age-specific incidences from 20–24 to ≥90 years increased with age at onset and survival. Distinct patterns were seen: for HD, with a low incidence, levelling off at middle age, and long median survival, 20 years; and for sRPNDd which displayed the lowest incidence, increasing with age, and a short median disease duration.

Interpretation

These results call for a unified population view of NDDs, with an age-at-onset-related pattern for acquired and sporadic CNDDs. The pattern linking age at onset to incidence magnitude and survival might be explained by differential pathophysiological mechanisms associated with specific misfolded protein deposits.

Accumulation of amyloid-β in the cerebellar cortex of essential tremor patients

The accumulation of insoluble amyloid-beta (Aβ) peptides is associated with neurodegenerative disorders, such as Alzheimer's disease (AD). As essential tremor (ET) could involve neurodegenerative processes in the cerebellum, we quantified soluble and insoluble Aβ in cerebellar cortices from patients diagnosed with ET (n=9), compared to Controls (n=16) or individuals with Parkinson's disease (n=10). Although ante-mortem cognitive performance was not documented, all individuals included had the diagnosis of AD ruled out by a neuropathologist. ELISA-determined concentrations of insoluble Aβ42 in ET patients displayed a bimodal distribution, with a median 246-fold higher than in Controls (P<0.01, Kruskal-Wallis). Higher Aβ42 concentrations were measured in the parietal cortex of the same ET patients, compared to Controls (107-fold median increase, P<0.01, Kruskal-Wallis), but similar phosphorylated tau levels were detected. The rise in cerebellar insoluble Aβ42 concentrations is not associated to APP expression and processing or the ApoE4 status. However, Aβ42 levels in ET individuals were correlated with cerebellar insoluble phosphorylated tau (r(2)=0.71, P=0.005), unphosphorylated neurofilament heavy chain (NF-H; r(2)=0.50, P=0.030) and Lingo-1 (r(2)=0.73, P=0.007), indicative of a generalized neurodegenerative process involving the cerebellum. Our results suggest prevalent accumulations of insoluble Aβ42 in the cerebellum of ET, but not in age-matched PD. Whether this anomaly plays a role in ET symptoms warrants further investigations.

Essential Tremor: A Common Disorder of Purkinje Neurons?

Essential tremor (ET) is one of the most common neurological diseases, with an estimated 7 million affected individuals in the United States. Postmortem studies in the past few years have resulted in new knowledge as well as a new formulation of disease pathophysiology. This new formulation centers on the notion that ET might be a disease of the cerebellum and, more specifically, the Purkinje cell (PC) population. Indeed, several investigators have proposed that ET may be a "Purkinjopathy." Supporting this formulation are data from controlled postmortem studies demonstrating (1) a range of morphological changes in the PC axon, (2) abnormalities in the position and orientation of PC bodies, (3) reduction in the number of PCs in some studies, (4) morphological changes in and pruning of the PC dendritic arbor with loss of dendritic spines, and (5) alterations in both the PC-basket cell interface and the PC-climbing fiber interface in ET cases. This new formulation has engendered some controversy and raised additional questions. Whether the constellation of changes observed in ET differs from that seen in other degenerative disorders of the cerebellum remains to be determined, although initial studies suggest the likely presence of a distinct profile of changes in ET.

Challenges in essential tremor genetics

The field of essential tremor (ET) genetics remains extremely challenging. The relative lack of progress in understanding the genetic etiology of ET, however, does not reflect the lack of a genetic contribution, but rather, the presence of substantial phenotypic and genotypic heterogeneity. A meticulous approach to phenotyping is important for genetic research in ET. The only tool for phenotyping is the clinical history and examination. There is currently no ET-specific serum or imaging biomarker or defining neuropathological feature (e.g., a protein aggregate specific to ET) that can be used for phenotyping, and there is considerable clinical overlap with other disorders such as Parkinson's disease (PD) and dystonia. These issues greatly complicate phenotyping; thus, in some studies, as many as 30-50% of cases labeled as "ET" have later been found to carry other diagnoses (e.g., dystonia, PD) rather than ET. A cursory approach to phenotyping (e.g., merely defining ET as an "action tremor") is likely a major issue in some family studies of ET, and this as well as lack of standardized phenotyping across studies and patient centers is likely to be a major contributor to the relative lack of success of genome wide association studies (GWAS). To dissect the genetic architecture of ET, whole genome sequencing (WGS) in carefully characterized and well-phenotyped discovery and replication datasets of large case-control and familial cohorts will likely be of value. This will allow specific hypotheses about the mode of inheritance and genetic architecture to be tested. There are a number of approaches that still remain unexplored in ET genetics, including the contribution of copy number variants (CNVs), 'uncommon' moderate effect alleles, 'rare' variant large effect alleles (including Mendelian and complex/polygenic modes of inheritance), de novo and gonadal mosaicism, epigenetic changes and non-coding variation. Using these approaches is likely to yield new ET genes.