The Non-motor Features of Essential Tremor: A Primary Disease Feature or Just a Secondary Phenomenon?

[Essential tremor: are we just splitting hairs? Non-motor symptoms and essential tremor-plus]

INTRODUCTION

The existence of non-motor symptoms in essential tremor (ET) and the appearance of a new condition, ET-plus, are two controversial issues.

AIMS

To offer a review of the current status of these two topics.

DEVELOPMENT

We performed an analysis of the studies conducted on non-motor symptoms in ET and of the articles for and against the use of the term ET-plus.

CONCLUSIONS

Non-motor symptoms have gained increased recognition as a feature accompanying ET. Several studies have documented its presence compared to matched controls. It is not clear, however, whether these non-motor symptoms would be part of the spectrum of ET symptoms (a primary phenomenon) or whether they would be symptoms that appear as a consequence of the physical or psychological disability produced by the clinical signs and symptoms of ET itself (a secondary phenomenon). For the time being, their evaluation and treatment are not included within the standard assessment of patients with ET. In view of the heterogeneous phenotype, the term ET-plus aims to improve phenotypic homogeneity for genetic or therapeutic studies. Yet, there is no pathological basis, and epidemiological, genetic and therapeutic research studies have numerous drawbacks. In the absence of clear objective biomarkers, distinguishing between ET and ET-plus by clinical distinction alone is very complex. We should be cautious about using new terms that are not yet backed by sound scientific evidence.

Graph Theoretical Analysis of Structural Covariance Reveals the Relevance of Visuospatial and Attentional Areas in Essential Tremor Recovery After Stereotactic Radiosurgical Thalamotomy

Essential tremor (ET) is the most common movement disorder. Its pathophysiology is only partially understood. Here, we leveraged graph theoretical analysis on structural covariance patterns quantified from morphometric estimates for cortical thickness, surface area, and mean curvature in patients with ET before and one year after (to account for delayed clinical effect) ventro-intermediate nucleus (Vim) stereotactic radiosurgical thalamotomy. We further contrasted the observed patterns with those from matched healthy controls (HCs). Significant group differences at the level of individual morphometric properties were specific to mean curvature and the post-/pre-thalamotomy contrast, evidencing brain plasticity at the level of the targeted left thalamus, and of low-level visual, high-level visuospatial and attentional areas implicated in the dorsal visual stream. The introduction of cross-correlational analysis across pairs of morphometric properties strengthened the presence of dorsal visual stream readjustments following thalamotomy, as cortical thickness in the right lingual gyrus, bilateral rostral middle frontal gyrus, and left pre-central gyrus was interrelated with mean curvature in the rest of the brain. Overall, our results position mean curvature as the most relevant morphometric feature to understand brain plasticity in drug-resistant ET patients following Vim thalamotomy. They also highlight the importance of examining not only individual features, but also their interactions, to gain insight into the routes of recovery following intervention.

Tremor Syndromes: An Updated Review

Tremor is the most commonly encountered movement disorder in clinical practice. A wide range of pathologies may manifest with tremor either as a presenting or predominant symptom. Considering the marked etiological and phenomenological heterogeneity, it would be desirable to develop a classification of tremors that reflects their underlying pathophysiology. The tremor task force of the International Parkinson Disease and Movement Disorders Society has worked toward this goal and proposed a new classification system. This system has remained a prime topic of scientific communications on tremor in recent times. The new classification is based on two axes: 1. based on the clinical features, history, and tremor characteristics and 2. based on the etiology of tremor. In this article, we discuss the key aspects of the new classification, review various tremor syndromes, highlight some of the controversies in the field of tremor, and share the potential future perspectives.

Genomic Markers for Essential Tremor

There are many reports suggesting an important role of genetic factors in the etiopathogenesis of essential tremor (ET), encouraging continuing the research for possible genetic markers. Linkage studies in families with ET have identified 4 genes/loci for familial ET, although the responsible gene(s) have not been identified. Genome-wide association studies (GWAS) described several variants in LINGO1, SLC1A2, STK32B, PPARGC1A, and CTNNA3, related with ET, but none of them have been confirmed in replication studies. In addition, the case-control association studies performed for candidate variants have not convincingly linked any gene with the risk for ET. Exome studies described the association of several genes with familial ET (FUS, HTRA2, TENM4, SORT1, SCN11A, NOTCH2NLC, NOS3, KCNS2, HAPLN4, USP46, CACNA1G, SLIT3, CCDC183, MMP10, and GPR151), but they were found only in singular families and, again, not found in other families or other populations, suggesting that some can be private polymorphisms. The search for responsible genes for ET is still ongoing.

The Essential Tremors: Evolving Concepts of a Family of Diseases

The past 10 years has seen a remarkable advance in our understanding of the disease traditionally referred to as “essential tremor” (ET). First, the clinical phenotype of ET has been expanded from that of a bland, unidimensional, and monosymptomatic entity to one with a host of heterogeneous features. These features include a broader and more nuanced collection of tremors, non-tremor motor features (e.g., gait abnormalities) and a range of non-motor features, including cognitive, psychiatric, sleep, and other abnormalities. The natural history of these features, as well as their relationships with one another and with disease duration and severity, are better appreciated than they were previously. Studies of disease etiology have identified a number of candidate genes as well as explored several environmental determinants of disease. In addition, the decade has seen the beginnings and expansion of rigorous postmortem studies that have identified and described the postmortem changes in the brains of patients with ET. This emerging science has given rise to a new notion that the disease, in many cases, is one of cerebellar system degeneration. Across all of these studies (clinical, etiological, and pathophysiological) is the observation that there is heterogeneity across patients and that “essential tremor” is likely not a single disease but, rather, a family of diseases. The time has come to use the more appropriate terminology, “the essential tremors,” to fully describe and encapsulate what is now apparent. In this paper, the author will review the clinical, etiological, and pathophysiological findings, referred to above, and make the argument that the terminology should evolve to reflect advances in science and that “the essential tremors” is a more scientifically appropriate term.

Sleep Disorders in Patients with Essential Tremor

PURPOSE OF THE REVIEW

Patients diagnosed with essential tremor (ET) report frequent sleep complaints. This review focuses on the main findings of studies addressing sleep features in patients diagnosed with ET, updating previously reported information. Bad quality of sleep and excessive daytime somnolence are very frequent in patients with ET, although the effects of the drugs used for the therapy of ET could contribute to these complaints. REM sleep behavior disorder, restless legs, insomnia, and nocturia are frequent complaints as well. There is a lack of studies addressing polysomnographic features of ET.

Prospective Longitudinal Study of Gait and Balance in a Cohort of Elderly Essential Tremor Patients

Background: Essential tremor (ET) encompasses a variety of features, including tremor, cognitive dysfunction, and gait and balance impairments. Gait and balance impairments in ET are often mild, but they can be severe and are, in some cases, associated with functional sequelae in terms of increased fall risk and reduced balance confidence. Previous research on gait and balance in ET has been limited to cross-sectional comparisons. There have been no longitudinal studies or prospective studies. As such, our understanding of natural history and possible predictors of declines in ET-related gait and balance impairments is incomplete. Objectives: We (1) present natural history data on the change in gait and balance measures over time, (2) provide estimates of annual rate of change in each gait and balance metric, and (3) examine the relationship between baseline clinical predictors and changes in gait and balance over time. Methods: 149 ET participants (mean age 78.7 years), enrolled in a prospective, longitudinal, clinical-pathological study, underwent an extensive evaluation of cognition, tremor, and gait and balance at three distinct intervals performed every 18 months. Gait and balance measures included a combination of performance-based tests (e.g., tandem gait, tandem stance) and self-reported assessments (e.g., number of falls, use of a walking aid). Results: Between the baseline and final assessments, numerous balance and gait measures showed evidence of decline and annual rates of change were quantified for each. We examined the predictive utility of clinical variables at baseline for five gait and balance outcomes, with global cognition and executive function standing out as the most consistent predictors. Conclusions: We present a much-needed look into the course of disease for elderly patients with ET, focusing on changes observed in gait and balance and the predictors of these changes. These results also add another dimension to the relevance of cognitive impairment observed in ET; such impairment can now be viewed as predictive of poorer gait and balance over time in ET. These findings are a useful tool for clinicians, patients, and their families to better understand and plan for changing disease-features over time.

Neuropsychiatric Symptoms and Quality of Life in Patients With Adult-Onset Idiopathic Focal Dystonia and Essential Tremor

Background: While idiopathic focal dystonia (IFD) and essential tremor (ET) have been considered pure movement disorders, they reportedly induce neuropsychiatric manifestations and may thus be more accurately described as network disorders.

Methods: The present multi-center, cross-sectional, case-control study evaluated the severity of depression and anxiety with the Beck Depression Inventory (BDI) and Beck Anxiety Inventory (BAI), respectively; the frequency of neuropsychiatric disorders with the Korean-Mini International Neuropsychiatry Interview; and QoL with the Short-Form 36 (SF-36).

Results: Seventy-four subjects participated in this study (IFD, 27; ET, 24; controls, 23). The BDI and BAI scores were higher in the IFD and ET groups than in the control group. Although the frequency of neuropsychiatric disorders diagnosed according to the Diagnostic and Statistical Manual of Mental Disorders, 4th Edition Axis I was comparable among the groups, the prevalence of major depressive disorder tended to be high among patients with IFD. QoL was correlated with the severity of depression and anxiety across the groups.

Conclusions: Depression and anxiety are more severe in patients with IFD and ET compared to healthy controls, while their severity is similar among patients with IFD and ET. Axis I major depressive disorder is relatively more prevalent in patients with IFD. Neuropsychiatric symptoms affect QoL regardless of the affected individual's condition, addressing neuropsychiatric symptoms in patients with movement disorders may be crucial to improving their QoL.

Clinical Profile of Non-Motor Symptoms in Patients with Essential Tremor: Impact on Quality of Life and Age-Related Differences

Background

Identifying the clinical phenotypes of non-motor symptoms (NMSs) of essential tremor (ET) among different populations is necessary due to their impact on the quality of life (QoL). This study aimed to investigate the clinical phenotype and impact of NMSs on QoL in Egyptian patients with ET.

Methods

Thirty ET patients were compared to 30 matched controls. Subjects were evaluated by the Fahn-Tolosa-Marin Tremor Rating Scale, Non-Motor Symptoms Scale (NMSS), Montreal Cognitive Assessment, Hamilton Anxiety Rating Scale, Beck Depression Inventory, Pittsburgh Sleep quality Index, and the Short Form 36 Health Survey Questionnaire. Both groups were divided into two subgroups of younger (<45 years, 14 patients) and older age (>45 years, 16 patients) groups, to investigate age-related differences.

Results

ET patients showed significantly worse cognition, depression, anxiety, sleep and NMSS domains (p < 0.001), compared to controls, that negatively affected and predicted QoL. Older patients had more cognitive impairment (p = 0.003) and worse sleep/fatigue (p = 0.032) and sexual functions (p = 0.006), compared to younger group.

Discussion

The study supports that NMSs are integral part of ET, negatively affect QoL, and similarly affect younger and older patients. Therefore, NMSs should be explored for proper care of ET patients.

Mind the gaps: What we don't know about cognitive impairment in essential tremor

INTRODUCTION

Although the hallmark feature of essential tremor (ET) is tremor, there is growing appreciation that cognitive impairment also occurs, including increased prevalence of mild cognitive impairment (MCI) and increased prevalence and incidence of dementia. With emerging knowledge of ET-cognitive impairment, come fundamental questions regarding its course, bases, predictors and clinical outcomes. Studies in the general population and in Parkinson's disease (PD), a related movement disorder, offer a starting point from which to begin filling these clinically important knowledge gaps.

METHODS

A PubMed search (June 2018) identified articles for this review.

RESULTS

Much of our knowledge of cognitive impairment in ET is of the static condition (e.g., prevalence of cognitive impairment in ET), with nearly no information on its bases, predictors and dynamics (i.e., course, and clinical outcomes). In PD, where such data have been published, rates of cognitive decline and conversion to MCI/dementia are higher than in the general population. Predictors of cognitive change in PD and the general population have also been identified, yet they only partially overlap one another.

CONCLUSION

The predictors and dynamics of cognitive impairment have been investigated fairly extensively in the general population, to a somewhat lesser extent in PD, and are emerging only now in ET. We suggest that longitudinal studies specific to ET are needed, and we outline variables to be considered in these investigations. Increased knowledge of ET-cognitive impairment will facilitate meaningful counseling of patients and their families.

Essential Tremor is not Only a Movement Disorder; Its Relationship with Sleep and Anxiety

Introduction

Essential tremor (ET) is the most common movement disorder, and some studies suggest that there are non-motor symptoms as well as motor symptoms. This study aims to investigate the relationship between ET and anxiety and sleep disorder.

Methods

The study was conducted with 38 healthy individuals in the control group and 40 patients who admitted to the neurology clinic of our hospital and had definite ET according to the Consensus statement of the Movement Disorder Society on Tremor. Pittsburgh Sleep Quality Index (PSQI) and Hamilton Anxiety Rating Scale (HAM-A) were applied to both groups. Statistical analysis was carried out with SPSS 22.0 for Windows.

Results

The patient and control groups were similar in terms of age, gender and educational status. The PSQI and HAM-A scores in the patient group were significantly higher than the control group (p<0.000, p<0.000, respectively). Both scores were higher in female patients compared to male patients (p<0.05, p<0.05, respectively), and in married patients compared to single patients (p<0.05, p<0.05, respectively).

Conclusion

Sleep disorder and anxiety are more common in patients with ET than in healthy individuals. We believe that more severe sleep disorders and anxiety in female and/or married patients are indicative of the psychosocial component of the disorder, and that this condition should not be overlooked and should be studied more comprehensively in this regard.

Psychological Suffering in Essential Tremor: A Study of Patients and Those Who Are Close to Them

Background

Although the motor and non-motor features of essential tremor (ET) have been characterized in detail, it is not known whether ET patients suffer psychologically and whether those who are close to them consider them to be suffering in this way.

Methods

Fifty ET patients and 50 “close others” (COs), identified by patients “as someone who knows you well and sees you often” and who can “provide a different perspective on your well-being”, reported their own depressive symptoms, daily stress, and perceptions of patient psychological suffering and patient overall suffering with validated scales. ET patients’ tremor severity, duration, disability, cognition, and number of medications were also assessed.

Results

ET patients reported levels of psychological suffering within the range documented in arthritis and dementia patients from previous studies, and COs perceived significantly more psychological suffering in patients than patients reported themselves. Regression models, controlling for tremor severity, duration, and disability revealed that patients’ greater psychological suffering was associated with greater patient depression. The greater perceptions of COs of patient psychological and overall suffering were associated with greater CO depression and daily stress. Sensitivity analysis showed that patients’ cognitive status or number of medications did not affect the results.

Discussion

Multidisciplinary teams caring for ET patients should look beyond simple clinical ET indicators. They should be aware of patient experiences and perceptions of COs of psychological and overall suffering. This will help guide the development of evidence-based, supportive interventions that improve communication about the needs of ET patients and those who are close to them.

Is there a Premotor Phase of Essential Tremor?

BACKGROUND

Essential tremor (ET) is the most common tremor disorder. In addition to its hallmark feature, kinetic tremor of the upper limbs, patients may have a number of non-motor symptoms and signs (NMS). Several lines of evidence suggest that ET is a neurodegenerative disorder and certain NMS may antedate the onset of tremor. This article comprehensively reviews the evidence for the existence of a "premotor phase" of ET, and discusses plausible biological explanations and implications.

METHODS

A PubMed search in May 2017 identified articles for this review.

RESULTS

The existence of a premotor phase of ET gains support primarily from longitudinal data. In individuals who develop incident ET, baseline (i.e., premotor) evaluations reveal greater cognitive dysfunction, a faster rate of cognitive decline, and the presence of a protective effect of education against dementia. In addition, baseline evaluations also reveal more self-reported depression, antidepressant medication use, and shorter sleep duration in individuals who eventually develop incident ET. In cross-sectional studies, certain personality traits and NMS (e.g., olfactory dysfunction) also suggest the existence of a premotor phase.

DISCUSSION

There is preliminary evidence supporting the existence of a premotor phase of ET. The mechanisms are unclear; however, the presence of Lewy bodies in some ET brains in autopsy studies and involvement of multiple neural networks in ET as evident from the neuroimaging studies, are possible contributors. Most evidence is from a longitudinal cohort (Neurological Disorders of Central Spain: NEDICES); additional longitudinal studies are warranted to gain better insights into the premotor phase of ET.

Essential tremor then and now: How views of the most common tremor diathesis have changed over time

BACKGROUND

Essential tremor (ET) is the most prevalent tremor diathesis. In this "then" and "now" piece, I detail how views of this disorder have changed over time.

METHODS

A PubMed search was conducted on June 15, 2017. The term "essential tremor" was crossed in sequential order with 14 s search terms (e.g., genetics, clinical).

RESULTS

The traditional view of ET was that it was monosymptomatic. An emerging view of ET is as a more clinically-complex entity with a range of possible motor and non-motor features. Traditionally, ET was viewed as autosomal dominant with complete penetrance by age 65. Current thinking is that, in addition to monogenic forms of ET, the disease is likely to be complex, with incomplete penetrance into advanced age. While non-genetic factors were traditionally presumed to exist, epidemiological studies have explored several potential environmental toxicants. The traditional olivary model of ET posited the existence of a central oscillator; an alternative is the cerebellar degenerative model, which is now under consideration.

CONCLUSIONS

For ET, there is clearly a "then" and "now" when one assesses changes in our understanding of the disease with time. Indeed, concepts relating to the clinical features, disease etiology and pathogenesis have all changed substantially.

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.

A computerized tablet system for evaluating treatment of essential tremor by magnetic resonance guided focused ultrasound

Background

Transcranial magnetic resonance guided focused ultrasound is an emerging technology under evaluation for treatment of essential tremor, a prevalent movement disorder. A qualitative evaluation is performed by a clinician periodically during the procedure to maximize treatment effects and minimize adverse effects. The present work demonstrates a magnetic resonance-compatible method to enable more precise, quantitative measurement of tremor severity.

Methods

Tremor severity was measured in 12 patients pre-, post-, and intra-operatively, using a magnetic resonance-compatible tablet and a computerized adaptation of drawing tasks from the widely-used Fahn-Tolosa-Marin Tremor Rating Scale. Tremor metrics based on spectral analysis were calculated for each drawing and compared using Wilcoxon signed rank tests.

Results

Tremor metrics in the dominant (treated) hand were significantly and consistently lower post-operatively compared to pre-operatively, but there was no significant difference in the non-dominant (untreated) hand, as expected. Intra-operative metrics were intermediate between pre- and post-operative metrics.

Conclusions

Use of the tablet for quantitative tremor measurement was demonstrated pre-, post-, and intra-operatively during treatment of essential tremor, complementing standard qualitative assessment. With additional work, the system has potential to add objectivity to clinical trials and to aid treatment decision-making by providing a metric for optimization during the procedure, which may eventually lead to more optimal treatment. Enhancements and further studies are suggested, and extensions to fMRI studies of essential tremor and Parkinson’s disease are also likely.

Fragile X-associated tremor/ataxia syndrome: phenotypic comparisons with other movement disorders

OBJECTIVE

The purpose of this paper is to review the typical cognitive and motor impairments seen in fragile X-associated tremor/ataxia syndrome (FXTAS), essential tremor (ET), Parkinson disease (PD), spinocerebellar ataxias (SCAs), multiple system atrophy (MSA), and progressive supranuclear palsy (PSP) in order to enhance diagnosis of FXTAS patients.

METHODS

We compared the cognitive and motor phenotypes of FXTAS with each of these other movement disorders. Relevant neuropathological and neuroimaging findings are also reviewed. Finally, we describe the differences in age of onset, disease severity, progression rates, and average lifespan in FXTAS compared to ET, PD, SCAs, MSA, and PSP. We conclude with a flow chart algorithm to guide the clinician in the differential diagnosis of FXTAS.

RESULTS

By comparing the cognitive and motor phenotypes of FXTAS with the phenotypes of ET, PD, SCAs, MSA, and PSP we have clarified potential symptom overlap while elucidating factors that make these disorders unique from one another. In summary, the clinician should consider a FXTAS diagnosis and testing for the Fragile X mental retardation 1 (FMR1) gene premutation if a patient over the age of 50 (1) presents with cerebellar ataxia and/or intention tremor with mild parkinsonism, (2) has the middle cerebellar peduncle (MCP) sign, global cerebellar and cerebral atrophy, and/or subcortical white matter lesions on MRI, or (3) has a family history of fragile X related disorders, intellectual disability, autism, premature ovarian failure and has neurological signs consistent with FXTAS. Peripheral neuropathy, executive function deficits, anxiety, or depression are supportive of the diagnosis.

CONCLUSIONS

Distinct profiles in the cognitive and motor domains between these movement disorders may guide practitioners in the differential diagnosis process and ultimately lead to better medical management of FXTAS patients.

Non-Motor Symptoms of Essential Tremor Are Independent of Tremor Severity and Have an Impact on Quality of Life

Background

Several publications have focused on accompanying non-motor symptoms (NMS) in essential tremor (ET) patients; however, it remains unclear if NMS are an intrinsic part of the disease or secondary phenomena. We present the results of several neuropsychiatric tests and their impact on quality of life (QoL) in community-dwelling patients with ET.

Methods

Participants were recruited via a newspaper article about ET published in the local media and on the internet. All participants completed several standard neuropsychiatric tests, including those that assess QoL. To compare differences between cases and controls, Student’s t-tests with Bonferroni-Holm post hoc tests were performed. Spearman’s correlation coefficients were also calculated.

Results

We enrolled 110 patients with definite or probable ET. Highly significant changes were observed for apathy, anxiety, and cognition and negatively impacted QoL. Most aberrations were independent of tremor severity and duration.

Discussion

The significant neuropsychiatric deficits and reduced QoL demonstrate a degree of illness that appears to be a non-motor phenotype rather than a secondary effect of ET. In the future, NMS should carefully be explored in ET patients as they may have an impact on QoL and treatment.

Non-motor symptoms in essential tremor: A review of the current data and state of the field

BACKGROUND

The motor features of essential tremor (ET) include its hallmark element, kinetic tremor, yet non-motor features are increasingly being recognized as an accompanying part of what was previously viewed as a solely motor disorder. Given the evolving state of the ET field with respect to these non-motor features, the purpose of this manuscript is to critically review the current data.

METHODS

A PubMed search was conducted on July 1, 2015. The term "essential tremor" was crossed in sequential order with 13 additional search terms (e.g., "cognitive", "dementia", "depression"). The total number of unique hits was 322.

RESULTS

Numerous studies seem to substantiate the presence of a range of non-motor features occurring in excess in ET cases compared to age-matched controls. These comprise cognitive features (including a full spectrum from mild cognitive difficulty through to frank dementia), psychiatric (including depression, apathy, anxiety, and personality characteristics), sensory (hearing and possibly olfactory abnormalities), and other non-motor features (e.g., sleep dysregulation). Emerging evidence suggests that some of these features could be primary disease features that pre-date motor features of ET.

CONCLUSIONS

The presence of numerous non-motor features in ET is increasingly evident. The biological basis of these features deserves additional study.