Eye-hand coordination in essential tremor

Beta band oscillations in the motor thalamus are modulated by visuomotor coordination in essential tremor patients

Introduction

Beta oscillations in sensorimotor structures contribute to the planning, sequencing, and stopping of movements, functions that are typically associated with the role of the basal ganglia. The presence of beta oscillations (13-30 Hz) in the cerebellar zone of the thalamus (the ventral intermediate nucleus - Vim) indicates that this rhythm may also be involved in cerebellar functions such as motor learning and visuomotor adaptation.

Methods

To investigate the possible role of Vim beta oscillations in visuomotor coordination, we recorded local field potential (LFP) and multiunit activity from the Vim of essential tremor (ET) patients during neurosurgery for the implantation of deep brain stimulation (DBS) electrodes. Using a computer, patients performed a visuomotor adaptation task that required coordinating center-out movements with incongruent visual feedback imposed by inversion of the computer display.

Results

The results show that, in ET, Vim beta oscillations of the LFP were lower during the incongruent center-out task than during the congruent orientation. Vim firing rates increased significantly during periods of low beta power, particularly on approach to the peripheral target. In contrast, beta power in the subthalamic nucleus of Parkinson's disease (PD) patients did not differ significantly between the incongruent and the congruent orientation of the center-out task.

Discussion

The findings support the hypothesis that beta oscillations of the Vim are modulated by novel visuomotor tasks. The inverse relationship between the power of Vim-LFP beta oscillations and Vim firing rates suggest that the suppression of beta oscillations may facilitate information throughput to the thalamocortical circuit by modulation of Vim firing rates.

Eye movement study in essential tremor patients and its clinical correlates

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

Oculomotor impairments in de novo Parkinson’s disease

Objective

Reliable electrophysiological indicators are urgently needed in the precise evaluation of Parkinson’s disease (PD). It is still elusive whether oculomotor performance is impaired or has clinical value in early PD. This study aims to explore oculomotor performance in newly diagnosed, drug-naïve PD and its correlation with clinical phenotype.

Methods

Seventy-five patients with de novo PD, 75 patients with essential tremor (ET), and 46 gender-and age-matched healthy controls (HCs) were included in this cross-sectional study. All subjects underwent oculomotor test via videonystagmography. Visually guided saccade latency, saccadic accuracy and gain in smooth pursuit eye movement (SPEM) at three frequencies of the horizontal axis were compared among the three groups. Patients with PD also received detailed motor and non-motor evaluation by serial scales. The association between key oculomotor parameters and clinical phenotypes were explored in PD patients.

Results

Both de novo PD and ET patients showed prolonged saccadic latency and decreased saccadic accuracy relative to HCs. SPEM gain in PD was uniformly reduced at each frequency. SPEM gain at 0.4 Hz was also decreased in ET compared with HCs. However, there was no significant difference of oculomotor parameters between de novo PD and ET patients. Furthermore, prolonged saccadic latency was correlated with long disease duration, whereas decreased SPEM gain was associated with severe motor symptoms in de novo PD patients.

Conclusion

Ocular movements are impaired in de novo, drug naïve PD patients; these changes could be indicators for disease progression in PD.

Is essential tremor a disorder of GABA dysfunction? No

Although essential tremor is common, its underlying pathophysiology remains uncertain, and several hypotheses seek to explain the tremor mechanism. The GABA hypothesis states that disinhibition of deep cerebellar neurons due to reduced GABAergic input from Purkinje cells results in increased pacemaker activity, leading to rhythmic output to the thalamo-cortical circuit and resulting in tremor. However, some neuroimaging, spectroscopy, and pathology studies have not shown a clear or consistent GABA deficiency in essential tremor, and animal models have indicated that large reductions of Purkinje cell inhibition may improve tremor. Instead, tremor is increasingly attributable to dysfunction in oscillating networks, where altered (but not necessarily reduced) inhibitory signaling can result in tremor. Hypersynchrony of Purkinje cell activity may account for excessive oscillatory cerebellar output, with potential contributions along multiple sites of the olivocerebellar loop. Although older animal tremor models, such as harmaline tremor, have explored contributions from the inferior olivary body, increasing evidence has pointed to the role of aberrant climbing fiber synaptic organization in oscillatory cerebellar activity and tremor generation. New animal models such as hotfoot17j mice, which exhibit abnormal climbing fiber organization due to mutations in Grid2, have recapitulated many features of ET. Similar abnormal climbing fiber architecture and excessive cerebellar oscillations as measured by EEG have been found in humans with essential tremor. Further understanding of hypersynchrony and excessive oscillatory activity in ET phenotypes may lead to more targeted and effective treatment options.

Valproate-associated Movement Disorder: A Literature Review

Valproate (VPA) was first synthesized in 1882, but it was only in the early 1960s that its anticonvulsant properties were discovered. The aim of this literature review is to evaluate the clinical epidemiological profile, pathological mechanisms, and management of VPA-associated movement disorder (MD). Relevant reports in six databases were identified and assessed by two reviewers without language restriction. A total of 138 reports containing 362 cases of subjects who developed a MD secondary to VPA were reported. The MD identified were parkinsonism (PKN) (252), myoclonus (MCL) (54), dystonia (DTN) (17), dyskinesia (DKN) (16), stutters (4), tics (3), akathisia (AKT) (1). In the not clearly defined group, 15 extrapyramidal symptoms, 3 AKT, 2 DTN, 1 rigidity, 1 unstable gait were assessed. The mean and median age was 55.8 (SD: 16.58) and 61 years (range: 4-87 years). The most common VPA-indication was epilepsy, and 51.36% were males. The mean and median time from the VPA start to the MD onset was 32.75 (SD: 30.05) and 21.15 months (range: 1 day - 20 years). The mean and median time from the VPA withdrawal until the MD recovery was 2.89 (SD: 2.79) and 3 months (1 day - 12 months). The most common management was drug withdrawal. A complete recovery was obtained in 80.61%. VPA-associated MD was extensively reported in the literature. PKN was the most well-described. Future studies need to clearly report the clinical history of the patient, considering the full investigation of other adverse events during their entire life.

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

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

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.

Essential Tremor - A Cerebellar Driven Disorder?

Abnormal tremors are the most common of all movement disorders. In this review we focus on the role of the cerebellum in Essential Tremor, a highly debilitating but poorly treated movement disorder. We propose a variety of mechanisms driving abnormal burst firing of deep cerebellar nuclei neurons as a key initiator of tremorgenesis in Essential Tremor. Targetting these mechanisms may generate more effective treatments for Essential Tremor.

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.

Persons with essential tremor can adapt to new walking patterns

Essential tremor (ET) is a common movement disorder that causes motor deficits similar to those seen in cerebellar disorders. These include kinetic tremor, gait ataxia, and impaired motor adaptation. Previous studies of motor adaptation in ET have focused on reaching while the effects of ET on gait adaptation are currently unknown. The purpose of this study was to contrast locomotor adaptation in persons with and without ET. We hypothesized that persons with ET would show impaired gait adaptation. In a cross-sectional study, persons with ET ( n = 14) and healthy matched controls ( n = 12) walked on a split-belt treadmill. Participants walked with the belts moving at a 2:1 ratio, followed by overground walking to test transfer, followed by a readaptation period and finally a deadaptation period. Step length asymmetry was measured to assess the rate of adaptation, amount of transfer, and rates of readaptation and deadaptation. Spatial, temporal, and velocity contributions to step length asymmetry were analyzed during adaptation. There were no group by condition interactions in step length asymmetry or contributions to step length asymmetry. Regardless of condition, persons with ET walked slower and exhibited lower temporal ( P < 0.001) and velocity ( P = 0.001) contributions to step length asymmetry than controls. Persons with ET demonstrated a preserved ability to adapt to, store, and transfer a new walking pattern. Despite probable cerebellar involvement in ET, locomotor adaptation is an available mechanism to teach persons with ET new gait patterns.

NEW & NOTEWORTHY This study is the first to investigate walking adaptation abilities of people with essential tremor. Despite evidence of cerebellar impairment in this population, people with essential tremor can adapt their walking patterns. However, people with essential tremor do not modulate the timing of their footsteps to meet walking demands. Therefore, this study is the first to report impairments in the temporal aspects of walking in people with essential tremor during both typical and locomotor learning.

Populations Norms for "SLURP"-An iPad App for Quantification of Visuomotor Coordination Testing

Currently the integrity of brain function that drives behavior is predominantly measured in terms of pure motor function, yet most human behavior is visually driven. A means of easily quantifying such visually-driven brain function for comparison against population norms is lacking. Analysis of eye-hand coordination (EHC) using a digital game-like situation with downloadable spatio-temporal details has potential for clinicians and researchers. A simplified protocol for the Lee-Ryan EHC (Slurp) Test app for iPad® has been developed to monitor EHC. The two subtests selected, each of six quickly completed items with appeal to all ages, were found equivalent in terms of total errors/time and sensitive to developmental and aging milestones known to affect EHC. The sensitivity of outcomes due to the type of stylus being used during testing was also explored. Populations norms on 221 participants aged 5 to 80⁺years are presented for each test item according to two commonly used stylus types. The Slurp app uses two-dimensional space and is suited to clinicians for pre/post-intervention testing and to researchers in psychological, medical, and educational domains who are interested in understanding brain function.

Role of cerebellar GABAergic dysfunctions in the origins of essential tremor

Essential tremor (ET) is among the most prevalent movement disorders, but its origins are elusive. The inferior olivary nucleus (ION) has been hypothesized as the prime generator of tremor because of the pacemaker properties of ION neurons, but structural and functional changes in ION are unlikely under ET. Abnormalities have instead been reported in the cerebello-thalamo-cortical network, including dysfunctions of the GABAergic projections from the cerebellar cortex to the dentate nucleus. It remains unclear, though, how tremor would relate to a dysfunction of cerebellar connectivity. To address this question, we built a computational model of the cortico-cerebello-thalamo-cortical loop. We simulated the effects of a progressive loss of GABA_A α₁-receptor subunits and up-regulation of α_2/3-receptor subunits in the dentate nucleus, and correspondingly, we studied the evolution of the firing patterns along the loop. The model closely reproduced experimental evidence for each structure in the loop. It showed that an alteration of amplitudes and decay times of the GABAergic currents to the dentate nucleus can facilitate sustained oscillatory activity at tremor frequency throughout the network as well as a robust bursting activity in the thalamus, which is consistent with observations of thalamic tremor cells in ET patients. Tremor-related oscillations initiated in small neural populations and spread to a larger network as the synaptic dysfunction increased, while thalamic high-frequency stimulation suppressed tremor-related activity in thalamus but increased the oscillation frequency in the olivocerebellar loop. These results suggest a mechanism for tremor generation under cerebellar dysfunction, which may explain the origin of ET.

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

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

Percepção Visual no Tremor Essencial: Uma Revisão Sistemática

RESUMO Investigou-se a relação entre percepção visual e tremor essencial (TE). Realizou-se uma revisão dos estudos publicados sobre o tema nas bases de dados PubMed, BIREME, CINAHL, Web of Science, PsycINFO, SciELO, SCOPUS e ELSEVIER desde a origem até 05 de janeiro de 2017. A triagem, extração de dados e avaliação foram realizadas por dois revisores. Para avaliar a qualidade dos estudos utilizou-se as diretrizes do National Institutes of Health (NIH). De 132 estudos potencialmente elegíveis, 12 foram selecionados de acordo com os critérios de elegibilidade. Os resultados indicaram que pessoas com TE apresentam baixa sensibilidade visual quando comparados com indivíduos saudáveis, apesar de não apresentarem prejuízos na visão de cores. Apesar da heterogeneidade dos dados, observamos que existem prejuízos visuais em pessoas com TE e estes podem ser uma das principais características clínicas não-motoras.

A digital assessment system for evaluating kinetic tremor in essential tremor and Parkinson's disease

Background

Spiral drawing on papers is a common tremor evaluation tool for diagnosing patients with essential tremor (ET) or Parkinson’s disease (PD). No standard drawing methods and parameters that use graphic tablets are yet available for objective evaluation.

Methods

This study established a tremor assessment system for tremor severity by using graphic tablets. Twelve patients with ET and twelve patients with PD were tested to establish system algorithms, and six additional patients were tested with the developed system to evaluate its performance. The patients also performed spiral drawing with three guiding paradigms on a graphic tablet: traced along a given spiral (S1), performed freehand drawing (S2), and traced along a guiding point (S3). Three parameters were calculated to quantify tremor severity: the means of radial difference per radian (|dr/dθ|), the means of radial difference per second (|dr/dt|), and the area under curve (AUC) of the frequency spectrum of the velocity. Each patient’s drawing was also evaluated using a visual rating scale (VRS) by experienced physicians. The interrater reliability was examined to identify the most consistent test paradigm.

Results

The parameter |dr/dθ| and AUC correlated well with the VRS (R > 0.8) in S1, S2 and S3 tests. The S1 test presented the best interrater reliability (Weighted Kappa coefficient, k = 0.80) among three tests. The Weighted Kappa coefficients are 0.67 and 0.71 in S2 and S3 tests, respectively.

Conclusions

We developed three different guiding paradigms for spiral drawing on a digital graphic tablet for clinical tests. Three parameters were calculated to represent the tremor severity in spiral drawing and used to quantify temporal and spatial characteristics of tremor, and provided good correlation with current clinical assessments. The test “traced along a given spiral” is recommended due to its good interrater reliability.

Electronic supplementary material

The online version of this article (10.1186/s12883-018-1027-2) contains supplementary material, which is available to authorized users.

Characterization of graphomotor functions in individuals with Parkinson's disease and essential tremor

In this study, we explored the relationship between the clinical features and motor impairments related to the graphomotor function of individuals with Parkinson's disease (PD) and essential tremor (ET). We recruited 46 participants: 12 with PD, 13 with ET, and 21 controls. All participants were asked to perform six graphomotor tasks on a digitizer tablet: drawing straight lines, cursive-connected loops, discrete circles, and continuous circles, and making goal-aimed movements with a stylus in two different directions with three different accuracy constraints. The results showed that although participants with PD were able to draw straight lines slightly faster than controls, they produced cursive-connected loops much slower than controls. In addition, in contrast to controls and individuals with ET, PDs also drew the cursive loops progressively smaller. In the aiming task, we found that equivalent movements with high accuracy constraints were drawn slower by individuals with ET or PD than by controls. However, when performing the equivalent movements with moderate or low accuracy constraints, PDs performed similarly to controls. In contrast to the equivalent movements, PD and ET participants both performed nonequivalent movements slower than controls, no matter the demands arising from the accuracy constraints. The present study shows that simple graphic tasks can differentiate impairments in fine motor function resulting from ET and PD.

Essential tremor and the cerebellum

Essential tremor (ET) is a progressive and highly prevalent neurologic disease. Along with the tremors, mild to moderate gait ataxia and other signs of cerebellar dysfunction may occur (i.e., subtle saccadic eye movement abnormalities and abnormalities of motor timing) as well as cognitive features, some of which may be due to cerebellar dysfunction. Numerous neuroimaging studies indicate the presence of functional, metabolic, and structural abnormalities in the cerebellum of a patient with ET. In tandem with these clinical and imaging studies, which were gathering increasing support for the notion that the cerebellum and/or cerebellar systems seemed to be at the root of ET, a growing postmortem literature is for the first time beginning to identify microscopic abnormalities in the ET brain, most of which are centered on the Purkinje cells and connected neuronal populations, and are likely to be degenerative. In terms of treatment, most of these pharmacotherapeutic agents serve to enhance GABAergic neurotransmission, further bolstering the notion that ET may very well be a disorder with a primary Purkinje cell dysfunction resulting in reduced cerebellar cortical inhibition. Similarly, the interruption of presumably abnormal cerebellar outflow pathways to the thalamus is the mechanism of deep-brain stimulation surgery, which is highly effective in treating ET.

Linking Essential Tremor to the Cerebellum: Clinical Evidence

Essential tremor (ET) might be a family of diseases unified by the presence of kinetic tremor, but also showing etiological, pathological, and clinical heterogeneity. In this review, we will describe the most significant clinical evidence, which suggests that ET is linked to the cerebellum. Data for this review were identified by searching PUBMED (January 1966 to May 2015) crossing the terms "essential tremor" (ET) and "cerebellum," which yielded 201 entries, 11 of which included the term "cerebellum" in the article title. This was supplemented by articles in the author's files that pertained to this topic. The wide spectrum of clinical features of ET that suggest that it originates as a cerebellar or cerebellar outflow problem include the presence of intentional tremor, gait and balance abnormalities, subtle features of dysarthria, and oculomotor abnormalities, as well as deficits in eye-hand coordination, motor learning deficits, incoordination during spiral drawing task, abnormalities in motor timing and visual reaction time, impairment of social abilities, improvement in tremor after cerebellar stroke, efficacy of deep brain stimulation (which blocks cerebellar outflow), and cognitive dysfunction. It is unlikely, however, that cerebellar dysfunction, per se, fully explains ET-associated dementia, because the cognitive deficits that have been described in patients with cerebellar lesions are generally mild. Overall, a variety of clinical findings suggest that in at least a sizable proportion of patients with ET, there is an underlying abnormality of the cerebellum and/or its pathways.

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.

Eye movement abnormalities in essential tremor

Essential tremor (ET) is the most prevalent movement disorder, characterized mainly by an action tremor of the arms. Only a few studies published as yet have assessed oculomotor abnormalities in ET and their results are unequivocal. The aim of this study was to assess the oculomotor abnormalities in ET patients compared with the control group and to find the relationship between oculomotor abnormalities and clinical features of ET patients. We studied 50 ET patients and 42 matched by age and gender healthy controls. Saccadometer Advanced (Ober Consulting, Poland) was used to investigate reflexive, pace-induced and cued saccades and conventional electrooculography for evaluation of smooth pursuit and fixation. The severity of the tremor was assessed by the Clinical Rating Scale for Tremor. Significant differences between ET patients and controls were found for the incidence of reflexive saccades dysmetria and deficit of smooth pursuit. Reflexive saccades dysmetria was more frequent in patients in the second and third phase of ET compared to the first phase. The reflexive saccades latency increase was correlated with severity of the tremor. In conclusion, oculomotor abnormalities were significantly more common in ET patients than in healthy subjects. The most common oculomotor disturbances in ET were reflexive saccades dysmetria and slowing of smooth pursuit. The frequency of reflexive saccades dysmetria increased with progression of ET. The reflexive saccades latency increase was related to the severity of tremor.

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.

Essential tremor: from bedside to bench and back to bedside

PURPOSE OF REVIEW

The last several years have witnessed a remarkable increase in research on essential tremor, with consequent advances in our understanding of this entity. An attempt to both summarize and frame this work has not been undertaken.

RECENT FINDINGS

Here, I show that observations on essential tremor arising from clinical practice/clinical studies have guided scientific studies of this disorder. In turn, the results of scientific studies are beginning to be translated back to the bedside to improve treatment. Recent essential tremor research has given rise to several novel and intriguing ideas about the disease. These include the following: essential tremor may represent a family of diseases rather than a single disease; essential tremor seems to be a disease of the cerebellum or cerebellar system; essential tremor may be neurodegenerative; low gamma aminobutyric acid tone seems to be a central feature of essential tremor. As with many emerging ideas, there is significant discussion and debate over these emerging ideas, and this fuels additional scientific studies.

SUMMARY

The flow of ideas from clinical observations about essential tremor, to their translation into scientific studies, and their translation back to the bedside, is expected to eventually lead to improvements at the patient interface.

From neurons to neuron neighborhoods: the rewiring of the cerebellar cortex in essential tremor

Remarkably little has been written on the biology of essential tremor (ET), despite its high prevalence. The olivary model, first proposed in the 1970s, is the traditional disease model for ET; however, the model is problematic for a number of reasons. Recently, intensive tissue-based studies have identified a series of structural changes in the brains of most ET cases, and nearly all of the observed changes are located in the cerebellar cortex. These studies suggest that Purkinje cells are central to the pathogenesis of ET and may thus provide a focus for the development of novel therapeutic strategies. Arising from these studies, a new model of ET proposes that the population of Purkinje cells represents the site of the initial molecular/cellular events leading to ET. Furthermore, a number of secondary changes/remodeling observed in the molecular and granular layers (i.e., in the Purkinje cell "neighborhood") are likely to be of additional mechanistic importance. On a physiological level, the presence of remodeling indicates the likely formation of aberrant synapses and the creation of new/abnormal cortical circuits in ET. Specific efforts need to be devoted to understanding the cascade of biochemical and cellular events occurring in the Purkinje cell layer in ET and its neuron neighborhood, as well as the physiological effects of secondary remodeling/rewiring that are likely to be occurring in this brain region in ET.

Essential tremor: a neurodegenerative disease?

Background

Essential tremor (ET) is one of the most common neurological disorders among adults, and is the most common of the many tremor disorders. It has classically been viewed as a benign monosymptomatic condition, yet over the past decade, a growing body of evidence indicates that ET is a progressive condition that is clinically heterogeneous, as it may be associated with a spectrum of clinical features, with both motor and non-motor elements. In this review, I will describe the most significant emerging milestones in research which, when taken together, suggest that ET is a neurodegenerative condition.

Methods

A PubMed search conducted in June 2014 crossing the terms “essential tremor” (ET) and “neurodegenerative” yielded 122 entries, 20 of which included the term “neurodegenerative” in the article title. This was supplemented by articles in the author's files that pertained to this topic.

Results/Discussion

There is an open and active dialogue in the medical community as to whether ET is a neurodegenerative disease, with considerable evidence in favor of this. Specifically, ET is a progressive disorder of aging associated with neuronal loss (reduction in Purkinje cells) as well as other post-mortem changes that occur in traditional neurodegenerative disorders. Along with this, advanced neuroimaging techniques are now demonstrating distinct structural changes, several of which are consistent with neuronal loss, in patients with ET. However, further longitudinal clinical and neuroimaging longitudinal studies to assess progression are required.

Understanding essential tremor: progress on the biological front

For many years, little was written about the underlying biology of ET, despite its high prevalence. Discussions of disease mechanisms were dominated by a focus on tremor physiology. The traditional model of ET, the olivary model, was proposed in the 1970s. The model suffers from several critical problems, and its relevance to ET has been questioned. Recent mechanistic research has focused on the cerebellum. Clinical and neuroimaging studies strongly implicate the importance of this brain region in ET. Recent mechanistic research has been grounded more in tissue-based changes (i.e., postmortem studies of the brain). These studies have collectively and systematically identified a sizable number of changes in the ET cerebellum, and have led to a new model of ET, referred to as the cerebellar degenerative model. Hence, there is a renewed interest in the science behind the biology of ET. How the new understanding of ET will translate into treatment changes is an open question.

Neuroimaging essentials in essential tremor: a systematic review

Background

Essential tremor is regarded to be a disease of the central nervous system. Neuroimaging is a rapidly growing field with potential benefits to both diagnostics and research. The exact role of imaging techniques with respect to essential tremor in research and clinical practice is not clear. A systematic review of the different imaging techniques in essential tremor is lacking in the literature.

Methods

We performed a systematic literature search combining the terms essential tremor and familial tremor with the following keywords: imaging, MRI, VBM, DWI, fMRI, PET and SPECT, both in abbreviated form as well as in full form. We summarize and discuss the quality and the external validity of each study and place the results in the context of existing knowledge regarding the pathophysiology of essential tremor.

Results

A total of 48 neuroimaging studies met our search criteria, roughly divided into 19 structural and 29 functional and metabolic studies. The quality of the studies varied, especially concerning inclusion criteria. Functional imaging studies indicated cerebellar hyperactivity during rest and during tremor. The studies also pointed to the involvement of the thalamus, the inferior olive and the red nucleus. Structural studies showed less consistent results.

Discussion and conclusion

Neuroimaging techniques in essential tremor give insight into the pathophysiology of essential tremor indicating the involvement of the cerebellum as the most consistent finding. GABAergic dysfunction might be a major premise in the pathophysiological hypotheses. Inconsistencies between studies can be partly explained by the inclusion of heterogeneous patient groups. Improvement of scientific research requires more stringent inclusion criteria and application of advanced analysis techniques. Also, the use of multimodal neuroimaging techniques is a promising development in movement disorders research. Currently, the role of imaging techniques in essential tremor in daily clinical practice is limited.

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We conducted a systematic review of neuroimaging studies in essential tremor.

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Cerebellar involvement is the most consistent finding.

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GABAergic dysfunction is worthwhile investigating more intensively.

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We encourage multimodal neuroimaging focussing on brain networks.

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

There is accumulating evidence that the entity referred to as 'essential tremor' (ET) is not a single disease. It may be a family of diseases better referred to as 'the ETs'. This review will summarize the following evidence: (1) the presence of etiological heterogeneity; (2) the heterogeneity of findings in postmortem studies, thus suggesting several diseases; (3) the recent discussion that age of onset may be an important marker of disease heterogeneity; (4) the clinical expansion of the concept of ET in recent years to include a broader range of tremor phenomenology, other motor features (gait ataxia), other involuntary movements (dystonia), and nonmotor features (cognitive problems, psychiatric problems), some of which could be primary; (5) the heterogeneity of pharmacological response profiles and clinical progression, and (6) the association of ET with Parkinson's disease, Alzheimer's disease, and possibly progressive supranuclear palsy, with the possibility that some ET patients are more predisposed to develop one of these. © 2013 S. Karger AG, Basel.

The cognitive side of essential tremor: what are the therapeutic implications?

While essential tremor (ET) has traditionally been categorized as a pure motor disease, cross-sectional and longitudinal studies of cognition in ET have demonstrated that these patients may have cognitive dysfunction. Recent epidemiological studies demonstrate an association between ET (particularly with onset after age 65) and increased risk for cognitive impairment and dementia. Although existing studies have generally conceptualized cognitive changes in ET as consistent with a 'frontosubcortical' or 'corticocerebellar' profile, results from these same studies suggest that cognitive impairment in ET may in fact be heterogeneous. Furthermore, the underlying mechanisms remain uncertain. Cognitive changes could be a byproduct of the cerebellar dysfunction of ET itself; alternately, they may be a feature of concomitant neurodegenerative diseases that have been associated in several studies with ET, including Alzheimer's disease, Parkinson's disease or progressive supranuclear palsy. While the study of cognitive dysfunction in ET has received research attention in recent years, the results of these studies have not been translated into the clinical domain and clinical practice. This review first summarizes the current literature on the potential relationships between ET and cognitive change. We then suggest areas of further clinical evaluation and treatment; these suggestions are directed at physicians caring for ET patients who may demonstrate or complain of cognitive impairment. As we discuss, clinicians should ideally screen ET patients for possible signs or symptoms of cognitive impairment in addition to assessing for psychiatric comorbidity and quality of life. These recommendations are in contrast to most current clinical practice, which does not routinely include such assessment among ET patients. To our knowledge, there have been no pharmacotherapeutic trials to date of any agent for cognitive change associated with ET. We believe that studies for this indication are now called for. Future efforts in this direction will also need to take into account the pathobiology of cognitive changes in ET, which itself is an area that is ripe for future investigations.

Slowed saccades and increased square wave jerks in essential tremor

BACKGROUND

Eye movements in essential tremor (ET) are poorly described and may present useful information on the underlying pathophysiology of the disorder.

METHODS

Sixty patients with ET, including 15 de novo untreated patients, and 60 age-matched controls constitute the study population. A video-based eye tracker was used to assess binocular eye position. Oculomotor function was assessed while subjects followed random horizontally and vertically step-displaced targets.

RESULTS

For all reflexive saccades, latencies were increased in ET subjects by a mean of 16.3% (p<0.01). Saccades showed reduced peak velocities with a lengthy, wavering velocity plateau, followed by slowed decelerations. For larger 30°+ saccades, peak velocities were decreased by a mean of 25.2% (p<0.01) and durations increased by 31.8% (p<0.01). The frequency of square wave jerks (SWJs) in patients was more than triple that of controls (p<0.0001). Despite frequent interruptions by SWJs, fixations were otherwise stable and indistinguishable from controls (root mean square [RMS] velocity, p = 0.324). The abnormal eye movement parameters were independent of disease duration, tremor severity, and medication therapy.

DISCUSSION

In contrast to normally swift onset and efficient acceleration/deceleration movements, saccades in ET are characterized by abnormally prolonged latencies and slowed velocity profiles. Although ET subjects maintain highly stable fixations, they are interrupted by increased numbers of SWJs. This study reveals novel oculomotor deficits in ET, which are distinct from the eye movement dysfunction of other movement disorders, supporting a role for eye tracking to assist in the differential diagnoses of not only atypical, but also more common movement disorders.

Lies tell the truth about cognitive dysfunction in essential tremor: an experimental deception study with the guilty knowledge task

BACKGROUND

Research conducted in the past decade challenges the traditional view that essential tremor (ET) is characterised exclusively by movement disorder, and increasingly shows that these patients have deficits in cognitive and behavioural functioning. The available evidence suggests that this impairment might arise from dysfunction in either the fronto-subcortical or cortico-cerebellar circuits. Although abnormalities in the fronto-subcortical circuits could imply difficulty in lying, no study has investigated deception in patients with ET.

AIMS

To examine the cognitive functions regulating deception in patients with ET, we used a computerised task, the Guilty Knowledge Task (GKT). We also tested a group of patients with Parkinson's disease (PD), a disease associated with a known difficulty in lie production, and a group of healthy subjects (HS).

RESULTS

In the GKT for deception, patients with ET responded less accurately than HS (p=0.014) but similarly to patients with PD (p=0.955). No differences between groups were found in truthful responses (p=0.488).

CONCLUSIONS

Besides confirming impaired deception in patients with PD, our results show a lie production deficit in patients with ET also. These findings suggest that difficulty in lying is an aspecific cognitive feature in movement disorders characterised by fronto-subcortical circuit dysfunction, such as PD and ET. Current knowledge along with our new findings in patients with ET--possibly arising from individually unrecognised extremely mild, cognitive difficulties--should help in designing specific rehabilitative programmes to improve cognitive and behavioural disturbances in patients.

High width variability during spiral drawing: further evidence of cerebellar dysfunction in essential tremor

Essential tremor (ET) is among the most prevalent neurological diseases, yet the location of the primary disease substrate continues to be a matter of debate. The presence of intention tremor and mild gait ataxia suggests an underlying abnormality of the cerebellum and/or cerebellar pathways. Uncovering additional signs of cerebellar dysfunction would further substantiate the proposition that ET is a disease of the cerebellar system. We evaluated 145 ET cases and 34 normal controls clinically and by computerized spiral analysis. Spiral analysis is a program that objectively characterizes kinematic and physiologic features of hand-drawn spirals using specific calculated spiral indices that correlate with spiral shape and motor execution. We used the spiral width variability index (SWVI), a measure of loop-to-loop spiral width variation with the influence of tremor removed, as a metric of drawing ataxia. The SWVI was higher in cases than controls (0.91 ± 1.94, median=0.46 vs. 0.40 ± 0.29, median=0.30, p<0.001). Cases with higher SWVI also had greater intention tremor during the finger-nose-finger maneuver, r=0.27, p=0.001), and cases with intention tremor of the head had the highest SWVI (1.57 ± 3.44, median=0.51, p<0.001). There was a modest association between SWVI and number of missteps during tandem gait (r=0.16, p=0.06). The primary anatomical substrate for ET continues to be a matter of speculation, yet these and other clinical data lend support to the notion that there is an underlying abnormality of the cerebellum and/or its pathways.

Cortical control of saccades in Parkinson disease and essential tremor

A number of studies suggest that some features of essential tremor (ET) and Parkinson disease (PD) overlap. Besides tremor, also some cognitive features have been implicated in ET and PD. There is recent evidence that a common genetic mutation occurs in ET and PD. Saccadic eye movements could provide an easily quantifiable procedure to help in the differential diagnosis in early PD and ET. Being able to distinguish early on the two diseases may help in tailoring therapy. Cortical control of saccades and antisaccades as they pertain to the potential discrimination of PD and ET is reviewed. Imaging and electrophysiological studies are highlighted; however, there are still few studies. Hopefully this review will stimulate further research, in particular in the direction of differences and similarities in the neural circuits involved in PD and ET.

Essential tremor: phenotypes

Classically, essential tremor (ET) was defined by the Movement Disorder Society Consensus Statement on Tremor (1998) as "a bilateral, largely symmetric postural or kinetic tremor involving hands and forearms that is visible and persistent". Additional or isolated tremor of the head may occur but in the absence of abnormal posture. Duration is more than 5 years and the neurological examination is normal, with exception of the cogwheel phenomenon. In the last years ET has evolved into two different meanings. First of all, the classical ET, as a monosymptomatic disorder, and second, a heterogeneous disorder, the Essential Tremors, or a family of diseases. Nowadays, ET can be classified with both motor and non-motor elements. Tremor may occur also in the legs, feet, trunk, jaw, chin, tongue, and voice. Although postural and kinetic tremors are the main features of ET, intentional tremor and tremor at rest may also occur in some patients. Other motor features described in patients with ET are gait ataxia, postural instability and eye-motion abnormalities. Non-motor features include cognitive (memory and executive problems and dementia), psychiatric (anxiety, depression and social phobia), and sensory abnormalities (olfactory deficits, hearing loss).

Milestones in clinical neurophysiology

Over the last 25 years, clinical neurophysiology has made many advances in the understanding, diagnosis, and even treatment of different movement disorders. Transcranial magnetic stimulation has been the biggest technical advance. Progress in pathophysiology includes improved knowledge about bradykinesia in Parkinson's disease, loss of inhibition and increased plasticity in dystonia, abnormal startle in hyperekplexia, and various features of psychogenic movement disorders that can aid diagnosis. Studies have been done looking at the use of noninvasive brain stimulation for therapy, but effects are generally small.

Association between essential tremor and other neurodegenerative diseases: what is the epidemiological evidence?

BACKGROUND

The possible links between essential tremor (ET) and Parkinson's disease (PD) and, more recently, between ET and Alzheimer's disease (AD) have been of great interest, particularly with the emergence of postmortem data suggesting that ET itself may be a neurodegenerative disease. Given the very high population prevalence of ET, its possible role as a risk factor for PD and AD is of special significance. At the center of this debate is the burgeoning evidence from epidemiological studies, examining whether there is an increased co-occurrence of these conditions within the same individuals or within families.

METHODS

We conducted a PubMed search from 1966 to March 2011 and reviewed the epidemiological evidence, restricting our analyses to studies that generated quantifiable measures of association (odds ratios or relative risks), thereby being able to assess the role of chance.

RESULTS

The most robust evidence, from case-control, prospective and familial aggregation studies, indicates that ET is associated with increased odds and increased risks of both PD and AD.

CONCLUSIONS

There is reasonable epidemiological evidence to support a link between ET and these neurodegenerative diseases. Further studies are needed to corroborate the current results, provide additional estimates of effect and begin to explore the mechanistic underpinnings for these intriguing associations.

Combined measures of movement and force variability distinguish Parkinson's disease from essential tremor

OBJECTIVE

To examine whether behavioral and electrophysiological measures of motor performance accurately differentiate Parkinson's disease (PD) and essential tremor (ET).

METHODS

Twenty-four patients (12 PD; 12 ET) performed isometric force, ballistic movements, and tremor tasks. Receiver operating characteristic (ROC) analyses were conducted on all dependent measures that were significantly different between the two patient groups.

RESULTS

Patients with PD were more impaired on measures of movement deceleration than ET. Patients with ET were more impaired on measures of force variability than PD. ROC analyses revealed that sensitivity and specificity were excellent when combining measures during the isometric force task (torque rise time and force variability; 92% sensitivity and 92% specificity; AUC = 0.97). When combining measures across the force and movement tasks, the ROC analysis revealed improved sensitivity and specificity (force variability and peak deceleration; 92% sensitivity and 100% specificity; AUC = 0.99).

CONCLUSIONS

Combining measures of force variability and movement deceleration accurately differentiate patients with PD from those with ET with high sensitivity and specificity.

SIGNIFICANCE

If validated in a larger sample, these measures can serve as markers to confirm the diagnosis of PD or ET and thus, enhance decision making for appropriate treatments for patients with these respective diseases.

Essential tremor: evolving clinicopathological concepts in an era of intensive post-mortem enquiry

Essential tremor (ET) is one of the most common neurological disorders. In recent years, as a result of systematic post-mortem examinations, our knowledge of the pathophysiology of this disease has grown substantially. Clearly identifiable structural changes (ie, Purkinje cell loss, Lewy bodies) have been observed in the brains of individuals with ET. These changes are not uniform and seem to follow several patterns, localising to the cerebellum itself or to a collection of brainstem neurons that synapse directly with Purkinje cells. Furthermore, these changes are similar to those seen in degenerative diseases. A wealth of clinical, epidemiological, and now post-mortem data indicate that this disease, or perhaps this family of diseases, is likely to be neurodegenerative. The molecular mechanisms that underlie these structural changes in ET are unknown. However, with more controlled, tissue-based studies being done, it is hoped that these mechanisms will be elucidated, thereby laying the foundation for the development of more targeted, effective pharmacotherapeutic interventions.

Epidemiology and management of essential tremor in children

Essential tremor (ET) is a common, often familial, movement disorder characterized by tremor of the limbs, head, and voice. Epidemiological surveys indicate that up to 5% of the adult population has ET, and 5-30% of adults with ET report symptom onset during childhood. There is, however, little published regarding ET in the pediatric population, and no prospective studies targeted specifically to children. Retrospective studies from subspecialty movement disorder clinics indicate that childhood-onset ET is usually hereditary, begins at a mean age of 6 years, and affects boys three times as often as girls. While ET occasionally results in disability during childhood, only one-quarter of children seeing a neurologist for ET require pharmacotherapy. Small case series suggest that propranolol is effective in approximately 50% of children with ET, but controlled treatment trials are lacking.

Update on pathogenesis and treatment of essential tremor

PURPOSE OF REVIEW

This review focuses on recent findings on the aetiological, clinical, pathological and genetic heterogeneity of essential tremor and new therapeutic approaches.

RECENT FINDINGS

Although essential tremor is one of the most common movement disorders, understanding of the causes and mechanisms of the disease is still very limited. Studies on the clinical presentation of essential tremor have expanded the clinical dimension, now including nontremor manifestations such as cerebellar signs, neuropsychological characteristics, distinct personality traits and behavioural symptoms. Results of neuropathologic and imaging studies are conflicting, with hints of neurodegeneration or a nondegenerative disturbance of functional circuits or receptors. Genetic heterogeneity of essential tremor has been demonstrated by linkage to three different chromosomal loci so far, and several negative genetic studies. New animal models are reinforcing previous hypotheses about gamma-aminobutyrate (GABA)-ergic mechanisms in essential tremor. New therapeutic agents for essential tremor have been tested and demonstrated to be partly effective.

SUMMARY

The traditional view of essential tremor as a single disease entity has been replaced with the concept that this disorder is a complex and heterogeneous disease. Heterogeneity of the condition, and lack of diagnostic criteria and objective diagnostic tests add to this problem. Many conflicting results may be due to differences in patient selection.

Functional dissociation of saccade and hand reaching control with bilateral lesions of the medial wall of the intraparietal sulcus: Implications for optic ataxia

The posterior parietal cortex (PPC) is essential for the integration of visuomotor information during visually guided reaching. Studies in macaque monkeys have demonstrated a functional specialisation around the intraparietal sulcus (IPS) with a more medial representation of hand movements ("parietal reach region") and a more lateral representation of saccadic eye movements (lateral intraparietal area, LIP). Here we present evidence for the validity of this concept with respect to the human parietal cortex. We recorded isolated and combined goal-directed eye-hand movements in normal control subjects and in a patient with bilateral parieto-occipital lesions and incomplete Balint's syndrome including severe optic ataxia (misreaching to visual targets). Brain lesions in the patient were caused by acute posterior leucoencephalopathy in association with aortic surgery because of Takayasu's arteritis. MRI scans showed bilateral line-shaped hemorrhagic lesions, restricted to the cortex at the medial banks of the intraparietal sulcus, but leaving its lateral banks largely intact. In the patient visually guided reaching was significantly dysmetric, whereas the metrics of visually guided saccades were within normal limits. Dysmetria was more pronounced for the right visual field, with a gross hypermetria. Variability of the movement improved when a delay of 5 or 10 s was introduced between target presentation and movement execution. Lesion data support the concept of a functional specialisation around the human IPS: The cortex medial to the IPS predominantly controls rapid goal-directed reaching movements, comparable to the parietal reach region in monkeys, whereas saccadic eye movements appear to be controlled rather by the cortex lateral to the IPS.