Tremor in the Degenerative Cerebellum: Towards the Understanding of Brain Circuitry for Tremor

Tremor in Spinocerebellar Ataxia: A Scoping Review

Background

Spinocerebellar ataxia (SCA) denotes an expanding list of autosomal dominant cerebellar ataxias. Although tremor is an important aspect of the clinical spectrum of the SCAs, its prevalence, phenomenology, and pathophysiology are unknown.

Objectives

This review aims to describe the various types of tremors seen in the different SCAs, with a discussion on the pathophysiology of the tremors, and the possible treatment modalities.

Methods

The authors conducted a literature search on PubMed using search terms including tremor and the various SCAs. Relevant articles were included in the review after excluding duplicate publications.

Results

While action (postural and intention) tremors are most frequently associated with SCA, rest and other rare tremors have also been documented. The prevalence and types of tremors vary among the different SCAs. SCA12, common in certain ethnic populations, presents a unique situation, where the tremor is typically the principal manifestation. Clinical manifestations of SCAs may be confused with essential tremor or Parkinson's disease. The pathophysiology of tremors in SCAs predominantly involves the cerebellum and its networks, especially the cerebello-thalamo-cortical circuit. Additionally, connections with the basal ganglia, and striatal dopaminergic dysfunction may have a role. Medical management of tremor is usually guided by the phenomenology and associated clinical features. Deep brain stimulation surgery may be helpful in treatment-resistant tremors.

Conclusions

Tremor is an elemental component of SCAs, with diverse phenomenology, and emphasizes the role of the cerebellum in tremor. Further studies will be useful to delineate the clinical, pathophysiological, and therapeutic aspects of tremor in SCAs.

Home Aerobic Training for Cerebellar Degenerative Diseases: a Randomized Controlled Trial

Balance training has shown some benefits in cerebellar ataxia whereas the effects of aerobic training are relatively unknown. To determine whether a phase III trial comparing home aerobic to balance training in ambulatory patients with cerebellar ataxia is warranted, we conducted a single-center, assessor-blinded, randomized controlled trial. Nineteen subjects were randomized to aerobic training and 17 subjects to balance training. The primary outcome was improvement in ataxia as measured by the Scale for the Assessment and Rating of Ataxia (SARA). Secondary outcomes included safety, training adherence, and balance improvements. There were no differences between groups at baseline. Thirty-one participants completed the trial, and there were no training-related serious adverse events. Compliance to training was over 70%. There was a mean improvement in ataxia symptoms of 1.9 SARA points (SD 1.62) in the aerobic group compared to an improvement of 0.6 points (SD 1.34) in the balance group. Although two measures of balance were equivocal between groups, one measure of balance showed greater improvement with balance training compared to aerobic training. In conclusion, this 6-month trial comparing home aerobic versus balance training in cerebellar ataxia had excellent retention and adherence to training. There were no serious adverse events, and training was not interrupted by minor adverse events like falls or back pain. There was a significant improvement in ataxia symptoms with home aerobic training compared to balance training, and a phase III trial is warranted. Clinical trial registration number: NCT03701776 on October 8, 2018.

Ataxias: Hereditary, Acquired, and Reversible Etiologies

A variety of etiologies can cause cerebellar dysfunction, leading to ataxia symptoms. Therefore, the accurate diagnosis of the cause for cerebellar ataxia can be challenging. A step-wise investigation will reveal underlying causes, including nutritional, toxin, immune-mediated, genetic, and degenerative disorders. Recent advances in genetics have identified new genes for both autosomal dominant and autosomal recessive ataxias, and new therapies are on the horizon for targeting specific biological pathways. New diagnostic criteria for degenerative ataxias have been proposed, specifically for multiple system atrophy, which will have a broad impact on the future clinical research in ataxia. In this article, we aim to provide a review focus on symptoms, laboratory testing, neuroimaging, and genetic testing for the diagnosis of cerebellar ataxia causes, with a special emphasis on recent advances. Strategies for the management of cerebellar ataxia is also discussed.

Is essential tremor a degenerative or an electrical disorder? Electrical disorder

Essential tremor (ET) is one of the most common movement disorders, yet we do not have a complete understanding of its pathophysiology. From a phenomenology standpoint, ET is an isolated tremor syndrome of bilateral upper limb action tremor with or without tremor in other body locations. ET is a pathological tremor that arises from excessive oscillation in the central motor network. The tremor network comprises of multiple brain regions including the inferior olive, cerebellum, thalamus, and motor cortex, and there is evidence that a dynamic oscillatory disturbance within this network leads to tremor. ET is a chronic disorder, and the natural history shows a slow progression of tremor intensity with age. There are reported data suggesting that ET follows the disease model of a neurodegenerative disorder, however whether ET is a degenerative or electrical disorder has been a subject of debate. In this chapter, we will review cumulative evidence that ET as a syndrome is a fundamentally electric disorder. The etiology is likely heterogenous and may not be primarily neurodegenerative.

Current and Emerging Treatment Modalities for Spinocerebellar Ataxias

INTRODUCTION

Spinocerebellar ataxias (SCA) are a group of rare neurodegenerative diseases that dramatically affect the lives of affected individuals and their families. Despite having a clear understanding of SCA's etiology, there are no current symptomatic or neuroprotective treatments approved by the FDA.

AREAS COVERED

Research efforts have greatly expanded the possibilities for potential treatments, including both pharmacological and non-pharmacological interventions. Great attention is also being given to novel therapeutics based in gene therapy, neurostimulation, and molecular targeting. This review article will address the current advances in the treatment of SCA and what potential interventions are on the horizon.

EXPERT OPINION

SCA is a highly complex and multifaceted disease family with the majority of research emphasizing symptomatic pharmacologic therapies. As pre-clinical trials for SCA and clinical trials for other neurodegenerative conditions illuminate the efficacy of disease modifying therapies such as AAV-mediated gene therapy and ASOs, the potential for addressing SCA at the pre-symptomatic stage is increasingly promising.

Phase I Single-Blinded Randomized Controlled Trial Comparing Balance and Aerobic Training in Degenerative Cerebellar Disease

INTRODUCTION

Primary deficits in individuals with cerebellar degeneration include ataxia, unstable gait, and incoordination. Balance training is routinely recommended to improve function whereas little is known regarding aerobic training.

OBJECTIVE

To determine the feasibility of conducting a randomized trial comparing balance and aerobic training in individuals with cerebellar degeneration.

DESIGN

Assessor blinded randomized control phase I trial.

SETTING

Assessments in medical center, home training.

PARTICIPANTS

Twenty participants with cerebellar degeneration were randomized to home balance or aerobic training.

INTERVENTION

Aerobic training consisted of 4 weeks of stationary bicycle training, five times per week for 30-minute sessions. Home balance training consisted of performing the same duration of easy, moderate, and/or hard exercises.

OUTCOME MEASURES

Scale for the Assessment and Rating of Ataxia (SARA), maximal oxygen consumption (VO2 max), Dynamic Gait Index, Timed Up and Go, gait speed.

RESULTS

All 20 participants completed assigned training with no major adverse events. Seven of each group attained target training duration, frequency, and intensity. Although both groups had significant improvements in ataxia severity, balance, and gait measures, there were greater improvements in individuals who performed aerobic training in ataxia severity and maximal oxygen consumption when compared to balance training. The effect size for these outcome measures was determined to be large, indicating a phase II trial comparing the benefits of aerobic and balance training was feasible and required 26 participants per group. Improvements in SARA score and VO2 max remained in the aerobic training group at 3 months posttraining, but these improvements were trending back to baseline. In contrast, all balance group measures for pretraining and 3 months posttraining were statistically similar.

CONCLUSIONS

A phase II trial comparing balance and aerobic training in individuals with cerebellar degeneration is feasible. Benefits trended back toward baseline after training stopped, although benefits of longer duration exercise programs still need to be determined.

Collaborative Efforts for Spinocerebellar Ataxia Research in the United States: CRC-SCA and READISCA

Spinocerebellar ataxias are progressive neurodegenerative disorders primarily affecting the cerebellum. Although the first disease-causing gene was identified nearly 30 years ago, there is no known cure to date, and only a few options exist for symptomatic treatment, with modest effects. The recently developed tools in molecular biology, such as CRISPR/Cas9 and antisense oligonucleotides, can directly act on the disease mechanisms at the genomic or RNA level in disease models. In a nutshell, we are finally just one step away from clinical trials with therapies targeting the underlying genetic cause. However, we still face the challenges for rare neurodegenerative diseases: difficulty in obtaining a large cohort size for sufficient statistical power and the need for biomarkers and clinical outcome assessments (COA) with adequate sensitivity to reflect progression or treatment responses. To overcome these obstacles, ataxia experts form research networks for clinical trial readiness. In this review, we retrace our steps of the collaborative efforts among ataxia researchers in the United States over the years to study and treat these relentless disorders and the future directions of such research networks.

Dysphagia in spinocerebellar ataxias type 1, 2, 3 and 6

BACKGROUND

Dysphagia is a common symptom and may be a cause of death in patients with spinocerebellar ataxias (SCAs). However, little is known about at which disease stage dysphagia becomes clinically relevant. Therefore, our study aims to investigate the prevalence of dysphagia in different disease stages of SCA 1, 2, 3 and 6.

METHODS

We studied 237 genetically confirmed patients with SCA 1, 2, 3, 6 from the Clinical Research Consortium for SCAs and investigated the prevalence of self-reported dysphagia and the association between dysphagia and other clinical characteristics. We further stratified ataxia severity and studied the prevalence of dysphagia at each disease stage.

RESULTS

Dysphagia was present in 59.9% of SCA patients. Patients with dysphagia had a longer disease duration and more severe ataxia than patients without dysphagia (patients with dysphagia vs. without dysphagia, disease duration (years): 14.51 ± 8.91 vs. 11.22 ± 7.82, p = .001, scale for the assessment and rating of ataxia [SARA]: 17.90 ± 7.74 vs. 13.04 ± 7.51, p = .000). Dysphagia was most common in SCA1, followed by SCA3, SCA 6, and SCA 2. Dysphagia in SCA1 and 3 was associated robustly with ataxia severity, whereas this association was less obvious in SCA2 and 6, demonstrating genotype-specific clinical variation.

CONCLUSION

Dysphagia is a common clinical symptom in SCAs, especially in the severe disease stage. Understanding dysphagia in SCA patients can improve the care of these patients and advance knowledge on the roles of the cerebellum and brainstem control in swallowing.

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

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

The impact of ethnicity on the clinical presentations of spinocerebellar ataxia type 3

BACKGROUND

For a variety of sporadic neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis, it is well-established that ethnicity does affect the disease phenotypes. However, how ethnicity contributes to the clinical symptoms and disease progressions in monogenetic disorders, such as spinocerebellar ataxia type 3 (SCA3), remains less studied.

METHODS

We used multivariable linear and logistical regression models in 257 molecularly-confirmed SCA3 patients (66 Caucasians, 43 African Americans, and 148 Asians [composed of 131 Chinese and 17 Asian Americans]) to explore the influence of ethnicity on age at onset (AAO), ataxia severity, and non-ataxia symptoms (i.e. depression, tremor, and dystonia).

RESULTS

We found that Asians had significantly later AAO, compared to Caucasians (β = 4.75, p = 0.000) and to African Americans (β = 6.64, p = 0.000) after adjusting for the pathological CAG repeat numbers in ATXN3. African Americans exhibited the most severe ataxia as compared to Caucasians (β = 3.81, p = 0.004) and Asians (β = 4.39, p = 0.001) after taking into consideration of the pathological CAG repeat numbers in ATXN3 and disease duration. Caucasians had a higher prevalence of depression than African Americans (β = 1.23, p = 0.040). Ethnicity had no influence on tremor or dystonia.

CONCLUSIONS

Ethnicity plays an important role in clinical presentations of SCA3 patients, which could merit further clinical studies and public health consideration. These results highlight the role of ethnicity in monogenetic, neurodegenerative disorders.

Phase I randomized single-blinded controlled study investigating the potential benefit of aerobic exercise in degenerative cerebellar disease

OBJECTIVES

To investigate whether people with cerebellar degeneration can perform rigorous aerobic exercise and to assess the clinical impact of training.

DESIGN

Randomized single-blinded controlled, feasibility study comparing aerobic training to no training.

SETTING

Home intervention, assessments conducted at an academic medical center.

SUBJECTS

Twenty individuals with cerebellar degeneration caused by a range of genetic disorders.

INTERVENTION

Aerobic training consisted of four weeks of stationary bicycle training, five times per week for 30-minute sessions. Intensity ranged from 65% to 80% of the participant's maximal heart rate determined during cardiopulmonary exercise testing.

MAIN MEASURES

Primary outcome measure was change in the Scale for the Assessment and Rating of Ataxia scores. Recruitment rate, adherence, drop-out, and adverse events were also determined. The treatment was considered technically feasible if participants achieved target training frequency, duration, and intensity.

RESULTS

The 20 participants mean age was 50 years (standard deviation 15.65 years) and average Scale for the Assessment and Rating of Ataxia score was 9.6 (standard deviation 3.13). Ten participants were randomized to aerobic training and 10 to no training. Seven participants in the aerobic group attained target training duration, frequency, and intensity. There was a mean reduction in ataxia severity of 2.1 points (standard deviation 1.26) with four weeks of aerobic training, whereas ataxia severity increased by 0.3 (standard deviation 0.62) in the control group over the same period. Walking speed, balance measures, and fitness also improved in individuals who performed aerobic exercise.

CONCLUSIONS

Rigorous aerobic training is feasible in people with cerebellar degeneration. Improvements in ataxia, balance, and gait are promising.

Molecular Mechanisms and Therapeutics for Spinocerebellar Ataxia Type 2

The effective therapeutic treatment and the disease-modifying therapy for spinocerebellar ataxia type 2 (SCA2) (a progressive hereditary disease caused by an expansion of polyglutamine in the ataxin-2 protein) is not available yet. At present, only symptomatic treatment and methods of palliative care are prescribed to the patients. Many attempts were made to study the physiological, molecular, and biochemical changes in SCA2 patients and in a variety of the model systems to find new therapeutic targets for SCA2 treatment. A better understanding of the uncovered molecular mechanisms of the disease allowed the scientific community to develop strategies of potential therapy and helped to create some promising therapeutic approaches for SCA2 treatment. Recent progress in this field will be discussed in this review article.