Mapping Essential Tremor to a Common Brain Network Using Functional Connectivity Analysis

Heterogeneous brain abnormalities in subjective cognitive decline converge on a common network and their transcriptional signature

INTRODUCTION

Subjective cognitive decline (SCD) is increasingly recognized as closely related to future Alzheimer's disease (AD). Numerous neuroimaging findings in SCD are inconsistent. We tested whether the various findings localize to a common brain network.

METHODS

Using a novel coordinate network mapping approach, we delineated common brain damage networks that were functionally connected to reported neuroimaging findings. We then decoded these common networks using microscale transcriptomic and chemo-architectures and psychological processes.

RESULTS

We enrolled 45 studies comprising 2453 SCD patients and 3017 healthy controls. The identified SCD networks were largely localized in the somatosensory network (SMN) and default mode network (DMN). Both were robust to perturbations of analyzed parameters and in an independent validation dataset. Neurobiology correlation analyses identified some key biological pathways and neurotransmitters linked to these networks.

DISCUSSION

Our findings reconcile heterogeneous neuroimaging abnormalities in SCD and provide a richer neurobiological underpinning, which has implications for understanding patients with SCD.

HIGHLIGHTS

The heterogeneous neuroimaging findings on SCD were reconciled in a coordinate network mapping framework. The SCD-related functional network involves changes in the DMN, while the SCD-related structural network has changes mainly in primary sensory areas. The identified genes in the functional network were predominantly enriched in biological processes related to synaptic structure, calcium ion binding, and cellular metabolism. An ALE meta-analysis was conducted for comparison.

Essential Tremor and Mortality in Older Adults: The Role of Word Recall, a Measure of Episodic Memory, in a 23-Year Follow-Up Study

Background: The association between essential tremor (ET) and mortality risk remains uncertain. This study investigated the impact of episodic memory performance, measured through a word recall task, on mortality risk in ET within the Neurological Disorders in Central Spain (NEDICES) cohort, a population-based study of older adults. Methods: Participants were followed until death or 31 December 2017, and divided into four groups based on ET status and memory performance (errors in the 37-Minimental Examination's three-word recall task). Cox proportional hazards models estimated mortality hazard ratios (HRs), and the Relative Excess Risk due to Interaction (RERI) assessed additive interactions. Results: Among 3998 participants, 3432 (85.8%) died over a median follow-up of 11.2 years. ET patients with episodic memory impairments had a higher mortality risk (HR = 1.25, 95% CI: 1.06-1.46) compared with controls with similar deficits (HR = 1.19, 95% CI: 1.09-1.28), whereas no significant increase was observed for ET patients without memory impairments (HR = 0.95, 95% CI: 0.74-1.21). RERI analysis revealed no significant additive interaction between ET and memory impairment (fully adjusted RERI: 0.11 [95% CI: -0.19-0.41]). Episodic memory impairments, regardless of ET status, were strongly associated with Alzheimer's disease as a primary cause of death. Conclusions: These findings highlight the independent contribution of episodic memory impairment to increased mortality risk, with ET modestly amplifying this effect without significant interaction. Further research is needed to explore shared pathophysiological mechanisms between ET and neurodegenerative conditions.

Targeting the fundamentals for tremors: the frequency and amplitude coding in essential tremor

Essential tremor (ET) is one of the most common movement disorders with heterogeneous pathogenesis involving both genetic and environmental factors, which often results in variable therapeutic outcomes. Despite the diverse etiology, ET is defined by a core symptom of action tremor, an involuntary rhythmic movement that can be mathematically characterized by two parameters: tremor frequency and tremor amplitude. Recent advances in neural dynamics and clinical electrophysiology have provided valuable insights to explain how tremor frequency and amplitude are generated within the central nervous system. This review summarizes both animal and clinical evidence, encompassing the kinematic features of tremors, circuitry dynamics, and the neuronal coding mechanisms for the two parameters. Neural population coding within the olivocerebellum is implicated in determining tremor frequency, while the cerebellar circuitry synchrony and cerebellar-thalamo-cortical interactions play key roles in regulating tremor amplitude. Novel therapeutic strategies aimed at tuning tremor frequency and amplitude are also discussed. These neural dynamic approaches target the conserved mechanisms across ET patients with varying etiologies, offering the potential to develop universally effective therapies for ET.

Protocol for a Phase 2 randomized controlled patient-assessor blinded study: efficacy and safety of combined cortical and cerebellar dual-target transcranial magnetic stimulation for the treatment of essential tremor

Background

Essential tremor (ET) is the most common neurological movement disorder with few treatments and limited therapeutic efficacy, research into noninvasive and effective treatments is critical. Abnormal cerebello-thalamo-cortical (CTC) loop function are thought to be significant pathogenic causes of ET, with the cerebellum and cortex are common targets for ET treatment. In recent years, transcranial magnetic stimulation (TMS) has been recognized as a promising brain research technique owing to its noninvasive nature and safety. In this study, we will use left M1 cortex continuous theta-burst stimulation (cTBS) combined with right cerebellar hemisphere 1 Hz repetitive transcranial magnetic stimulation (rTMS) dual-target stimulation to explore the Safety, feasibility and efficiency of this dual-target stimulation mode, and the mechanism of its therapeutic effect.

Methods

Twenty-four patients with ET will be randomly assigned to three groups: dual-target stimulation, single-target stimulation, or sham stimulation. The single-target stimulation group will receive stimulation of the right cerebellar hemisphere for 10 days, whereas the dual-target stimulation group will be given stimulation of both the left M1 cortex and the right cerebellar hemisphere. The sham stimulation group will be given sham stimulation for 10 days. Tremor will be assessed using both the subjective The Essential Tremor Rating Assessment Scale (TETRAS) and objective accelerometer-based tremor analysis. at baseline (before stimulation), after the first, fifth, tenth days of treatment (D1, 5, 10), 24 h after 10 days of treatment (D10-24 h), and 1, 2, 3, and 4 weeks after stimulation (W1, 2, 3, 4).

Discussion

This is a Phase 2 randomized, controlled, patient-assessor blinded clinical trial. The goal of this study is to investigate the Safety, feasibility and efficiency of TMS for the treatment of ET.

The research focus and frontiers in surgical treatment of essential tremor

Background

Essential tremor (ET) is one of the most prevalent neurodegenerative disorders, with surgery serving as the principal treatment option. This paper presents a bibliometric analysis of research in the field of ET surgery from 2004 to 2024, aiming to identify current research hotspots and inform future research directions.

Methods

This study employs CiteSpace to analyze publication trends, countries/institutions, authors, keywords, and co-cited references in ET surgery, using the Web of Science core database from 2004 to 2024 to delineate the research pathways.

Results

A total of 1,362 publications were included in this study. The number of publications has shown steady growth over the analyzed period from 2004 to 2024. Research in this field was carried out in 58 countries and by 371 institutions. The United States had the highest volume of publications, with the University of California System identified as the most prolific institution. Dr. Michael S. Okun from the University of Florida was the most prolific author, also based in the United States. This study identified 879 keywords, with significant citation bursts noted in areas such as the caudal zona incerta, ventral intermediate nucleus, location, and MR-guided focused ultrasound. Among the top ten highly cited articles, five pertained to MR-guided focused ultrasound thalamotomy, two addressed localization techniques, and one focused on surgical targets.

Conclusion

This study employs comprehensive bibliometric and visualization analyses to elucidate the evolution of research and identify emerging hotspots. The identified hotspots are as follows: First, deep brain stimulation (DBS), the most advanced technology in ET surgery, has room for improvement, especially in neuromodulation automation. Second, MR-guided focused ultrasound thalamotomy is a new surgical approach that requires further research on efficacy, safety, and side effect management. Third, novel surgical targets have demonstrated some efficacy, yet further research is essential to validate their effectiveness and safety. Lastly, localization techniques are fundamental to ET surgery, with ongoing efforts directed towards achieving more precise, individualized, and intelligent localization.

Exploring White Matter Microstructure with Symptom Severity and Outcomes Following Deep Brain Stimulation in Tremor Syndromes

Background

Essential tremor (ET) and dystonic tremor (DT) are movement disorders that cause debilitating symptoms, significantly impacting daily activities and quality of life. A poor understanding of their pathophysiology, as well as the mediators of clinical outcomes following deep brain stimulation (DBS), highlights the need for biomarkers to accurately characterise and optimally treat patients.

Objectives

We assessed the white matter microstructure of pathways implicated in the pathophysiology and therapeutic intervention in a retrospective cohort of patients with DT (n = 17) and ET (n = 19). We aimed to identity associations between white matter microstructure, upper limb tremor severity, and tremor improvement following DBS.

Methods

A fixel-based analysis pipeline was implemented to investigate white matter microstructural metrics in the whole brain, cerebello-thalamic pathways and tracts connected to stimulation volumes following DBS. Associations with preoperative and postoperative severity were analysed within each disorder group and across combined disorder groups.

Results

DBS led to significant improvements in both groups. No group differences in stimulation positions were identified. When white matter microstructural data was aligned according to the maximally affected upper limb, increased fiber density, and combined fiber density & cross-section of fixels in the left cerebellum were associated with greater tremor severity across DT and ET patients. White matter microstructure did not show associations with postoperative changes in cerebello-thalamic pathways, or tracts connected to stimulation volumes.

Discussion

Diffusion changes of the cerebellum are associated with the severity of upper limb tremor and appear to overlap in essential or dystonic tremor disorders.

Multimodal neuroimaging to characterize symptom-specific networks in movement disorders

Movement disorders, such as Parkinson's disease, essential tremor, and dystonia, are characterized by their predominant motor symptoms, yet diseases causing abnormal movement also encompass several other symptoms, including non-motor symptoms. Here we review recent advances from studies of brain lesions, neuroimaging, and neuromodulation that provide converging evidence on symptom-specific brain networks in movement disorders. Although movement disorders have traditionally been conceptualized as disorders of the basal ganglia, cumulative data from brain lesions causing parkinsonism, tremor and dystonia have now demonstrated that this view is incomplete. Several recent studies have shown that lesions causing a given movement disorder occur in heterogeneous brain locations, but disrupt common brain networks, which appear to be specific to each motor phenotype. In addition, findings from structural and functional neuroimaging in movement disorders have demonstrated that brain abnormalities extend far beyond the brain networks associated with the motor symptoms. In fact, neuroimaging findings in each movement disorder are strongly influenced by the constellation of patients' symptoms that also seem to map to specific networks rather than individual anatomical structures or single neurotransmitters. Finally, observations from deep brain stimulation have demonstrated that clinical changes, including both symptom improvement and side effects, are dependent on the modulation of large-scale networks instead of purely local effects of the neuromodulation. Combined, this multimodal evidence suggests that symptoms in movement disorders arise from distinct brain networks, encouraging multimodal imaging studies to better characterize the underlying symptom-specific mechanisms and individually tailor treatment approaches.

Updates in essential tremor

Essential tremor (ET) is one of the most common tremor disorders and can be disabling in its affect on daily activities. There have been major breakthroughs in the treatment of tremor and ET is the subject of important ongoing research. This review will present recent advancements in the epidemiology, genetics, pathophysiology, diagnosis, comorbidities, and imaging of ET. Current and future treatment options in the management of ET will also be reviewed. The need for continued innovation and scientific inquiry to address the unmet needs of persons of ET will be highlighted.

Mapping Essential Tremor to a Common Brain Network Using Functional Connectivity Analysis

In the Research Article "Mapping Essential Tremor to a Common Brain Network Using Functional Connectivity Analysis" by Younger et al.,1 the heat bars for Figure 4, panels B and C, should have been labelled as "p value". In addition, the first author's name should have been listed as "Ellen F.P. Younger" in the byline. The article has been replaced by a corrected version. The original version with the changes highlighted is available from a link in the corrected article. The publisher regrets the errors.