Determining an efficient deep brain stimulation target in essential tremor - Cohort study and review of the literature

Response of neuronal populations to phase-locked stimulation: model-based predictions and validation

Modulation of neuronal oscillations holds promise for the treatment of neurological disorders. Nonetheless, conventional stimulation in a continuous open-loop manner can lead to side effects and suboptimal efficiency. Closed-loop strategies such as phase-locked stimulation aim to address these shortcomings by offering a more targeted modulation. While theories have been developed to understand the neural response to stimulation, their predictions have not been thoroughly tested using experimental data. Using a mechanistic coupled oscillator model, we elaborate on two key predictions describing the response to stimulation as a function of the phase and amplitude of ongoing neural activity. To investigate these predictions, we analyze electrocorticogram recordings from a previously conducted study in Parkinsonian rats, and extract the corresponding phase and response curves. We demonstrate that the amplitude response to stimulation is strongly correlated to the derivative of the phase response ([Formula: see text] > 0.8) in all animals except one, thereby validating a key model prediction. The second prediction postulates that the stimulation becomes ineffective when the network synchrony is high, a trend that appeared missing in the data. Our analysis explains this discrepancy by showing that the neural populations in Parkinsonian rats did not reach the level of synchrony for which the theory would predict ineffective stimulation. Our results highlight the potential of fine-tuning stimulation paradigms informed by mathematical models that consider both the ongoing phase and amplitude of the targeted neural oscillation.Significance Statement This study validates a mathematical model of coupled oscillators in predicting the response of neural activity to stimulation for the first time. Our findings also offer further insights beyond this validation. For instance, the demonstrated correlation between phase response and amplitude response is indeed a key theoretical concept within a subset of mathematical models. This prediction can bring about clinical implications in terms of predictive power for manipulation of neural activity. Additionally, while phase dependence in modulation has been previously studied, we propose a general framework for studying amplitude dependence as well. Lastly, our study reconciles the seemingly contradictory views of pathologic hypersynchrony and theoretical low synchrony in Parkinson's disease.

Strong connectivity to the sensorimotor cortex predicts clinical effectiveness of thalamic deep brain stimulation in essential tremor

INTRODUCTION

The outcome of thalamic deep brain stimulation (DBS) for essential tremor (ET) varies, probably due to the difficulty in identifying the optimal target for DBS placement. Recent approaches compared the clinical response with a connectivity-based segmentation of the target area. However, studies are contradictory by indicating the connectivity to the primary motor cortex (M1) or to the premotor/supplementary motor cortex (SMA) to be therapeutically relevant.

OBJECTIVE

To identify the connectivity profile that corresponds to clinical effective targeting of DBS for ET.

METHODS

Patient-specific probabilistic diffusion tensor imaging was performed in 20 ET patients with bilateral thalamic DBS. Following monopolar review, the stimulation response was classified for the most effective contact in each hemisphere as complete vs. incomplete upper limb tremor suppression (40 assessments). Finally, the connectivity profiles of these contacts within the cortical and cerebellar tremor network were estimated and compared between groups.

RESULTS

The active contacts that led to complete (n = 25) vs. incomplete (n = 15) tremor suppression showed significantly higher connectivity to M1 (p < 0.001), somatosensory cortex (p = 0.008), anterior lobe of the cerebellum (p = 0.026) and SMA (p = 0.05); with Cohen's (d) effect sizes of 0.53, 0.42, 0.25 and 0.10, respectively. The clinical benefits were achieved without requiring higher stimulation intensities or causing additional side effects.

CONCLUSION

Clinical effectiveness of DBS for ET corresponded to a distributed connectivity profile, with the connection to the sensorimotor cortex being most relevant. Long-term follow-up in larger cohorts and replication in out-of-sample data are necessary to confirm the robustness of these findings.

Single-center experience of utilization and clinical efficacy of segmented leads for subthalamic deep brain stimulation in Parkinson's disease

Background

Segmented electrodes for deep brain stimulation (DBS) of the subthalamic nucleus (STN) in Parkinson's disease (PD) enable directional current steering leading to expanded programming options.

Objective

This retrospective study covering a longitudinal period of up to 7 years compares the efficacy of segmented and non-segmented leads in motor symptom alleviation and reduction of dopaminergic medication in PD patients treated in a specialized center and assesses the long-term use of directional steering in clinical routine.

Methods

Demographic data and clinical scores before surgery and at 12-month follow-up (12MFU) as well as stimulation parameters at 12MFU and last follow-up (LFU) were assessed in all patients implanted with segmented leads between 01/2016 and 12/2019 and non-segmented leads in a corresponding time-period. Patients were classified as very good (>60 %), good (30-60 %) and poor (<30 %) responders according to DBS-induced motor improvement.

Results

Clinical data at 12MFU was available for 61/96 patients with segmented (SEG) and 42/53 with non-segmented leads (N-SEG). Mean DBS-induced motor improvement and reduction of medication at 12MFU did not differ significantly between SEG and N-SEG groups or in a subgroup analysis of steering modes. There was a lower proportion of poor responders in the SEG compared with the N-SEG group (23% vs. 31%), though not statistically significant. At LFU, the percentage of patients set at directional steering increased from 54% to 70%.

Conclusion

Efficacy in reduction of motor symptoms and medication does not differ between electrode types for STN-DBS at 12 months follow-up. The use of directional steering increases over time and may account for a lower proportion of poor responders.

Reduced-Reference Learning for Target Localization in Deep Brain Stimulation

This work proposes a supervised machine learning method for target localization in deep brain stimulation (DBS). DBS is a recognized treatment for essential tremor. The effects of DBS significantly depend on the precise implantation of electrodes. Recent research on diffusion tensor imaging shows that the optimal target for essential tremor is related to the dentato-rubro-thalamic tract (DRTT), thus DRTT targeting has become a promising direction. The tractography-based targeting is more accurate than conventional ones, but still too complicated for clinical scenarios, where only structural magnetic resonance imaging (sMRI) data is available. In order to improve efficiency and utility, we consider target localization as a non-linear regression problem in a reduced-reference learning framework, and solve it with convolutional neural networks (CNNs). The proposed method is an efficient two-step framework, and consists of two image-based networks: one for classification and the other for localization. We model the basic workflow as an image retrieval process and define relevant performance metrics. Using DRTT as pseudo groundtruths, we show that individualized tractography-based optimal targets can be inferred from sMRI data with high accuracy. For two datasets of 280×220/272×227 (0.7/0.8 mm slice thickness) sMRI input, our model achieves an average posterior localization error of 2.3/1.2 mm, and a median of 1.7/1.02 mm. The proposed framework is a novel application of reduced-reference learning, and a first attempt to localize DRTT from sMRI. It significantly outperforms existing methods using 3D-CNN, anatomical and DRTT atlas, and may serve as a new baseline for general target localization problems.

Deep Brain Stimulation with Double Targeting of the VIM and PSA for the Treatment of Rare Tremor Syndromes

INTRODUCTION

In tremor syndromes, pharmacological therapy is the primary treatment, but deep brain stimulation (DBS) is used when it is insufficient. We explore the use of DBS, focusing on the globus pallidus internus for dystonia and the ventral intermediate nucleus (VIM) for tremor conditions. We introduce the posterior subthalamic area (PSA) as a potential target, suggesting its efficacy in tremor reduction, particularly in rare tremor syndromes. We aim to evaluate the efficacy and safety of double targeting the VIM and PSA in rare tremor conditions, highlighting the limited existing data on this.

METHODS

Between 2019 and 2023, 22 patients with rare tremor syndromes were treated with bilateral DBS of the VIM and PSA. This case series consisted of 7 isolated head tremor, 1 hepatic encephalopathic tremor due to Abernethy syndrome, 2 voice tremor, 4 dystonic tremor, and 8 Holmes tremor (2 multiple sclerosis, 2 cerebellar insult, and 4 posttraumatic) patients. Patients' preoperative and 12-month postoperative tremor scores were compared, and the optimum VIM and PSA stimulation areas were investigated.

RESULTS

There was a significant reduction in the mean TRS score from 3.70 (±0.57) to 0.45 (±0.68) after 12 months of surgery. Specific outcomes for different indications were observed: for head tremor, 6 of 7 patients showed a reduction in TRS scores to 0 points; the vocal tremor patients demonstrated improvement; this change was not statistically significant, which is likely to be due to the low number of patients in this subgroup; the dystonic tremor patients showed either complete tremor abolition or a reduction in TRS scores; the Holmes tremor patients showed an 80% reduction in TRS scores; and the hepatic encephalopathy tremor and Abernethy syndrome patients showed a 75% improvement in TRS scores. The stimulation parameters converged on the VIM and dorsal PSA. Complications included the need for electrode repositioning, infections requiring electrode removal and re-implantation, dysarthria, and stimulation-induced ataxia, which was resolved by adjusting the stimulation parameters.

DISCUSSION

The literature on DBS for rare tremors is limited. Double targeting of the VIM and PSA appears to produce promising improvements on the outcomes reported in the existing literature on VIM-only DBS. The proximity of the VIM and PSA allows for flexible electrode placement, contributing to the potential success of the dual-target approach. We also discuss the theoretical advantages of targeting the PSA based on the distribution of tremor circuits, emphasizing the need for further research and electrophysiological studies.

The role of the motor thalamus in deep brain stimulation for essential tremor

The advent of next-generation technology has significantly advanced the implementation and delivery of Deep Brain Stimulation (DBS) for Essential Tremor (ET), yet controversies persist regarding optimal targets and networks responsible for tremor genesis and suppression. This review consolidates key insights from anatomy, neurology, electrophysiology, and radiology to summarize the current state-of-the-art in DBS for ET. We explore the role of the thalamus in motor function and describe how differences in parcellations and nomenclature have shaped our understanding of the neuroanatomical substrates associated with optimal outcomes. Subsequently, we discuss how seminal studies have propagated the ventral intermediate nucleus (Vim)-centric view of DBS effects and shaped the ongoing debate over thalamic DBS versus stimulation in the posterior subthalamic area (PSA) in ET. We then describe probabilistic- and network-mapping studies instrumental in identifying the local and network substrates subserving tremor control, which suggest that the PSA is the optimal DBS target for tremor suppression in ET. Taken together, DBS offers promising outcomes for ET, with the PSA emerging as a better target for suppression of tremor symptoms. While advanced imaging techniques have substantially improved the identification of anatomical targets within this region, uncertainties persist regarding the distinct anatomical substrates involved in optimal tremor control. Inconsistent subdivisions and nomenclature of motor areas and other subdivisions in the thalamus further obfuscate the interpretation of stimulation results. While loss of benefit and habituation to DBS remain challenging in some patients, refined DBS techniques and closed-loop paradigms may eventually overcome these limitations.

Combined Short-Pulse and Directional Deep Brain Stimulation of the Thalamic Ventral Intermediate Area for Essential Tremor

OBJECTIVE

Novel deep brain stimulation (DBS) systems allow directional and short-pulse stimulation to potentially improve symptoms and reduce side effects. The aim of this study was to investigate the effect of short-pulse and directional stimulation, in addition to a combination of both, in the ventral intermediate thalamus (VIM)/posterior subthalamic area (PSA) on tremor and stimulation-induced side effects in patients with essential tremor.

MATERIALS AND METHODS

We recruited 11 patients with essential tremor and VIM/PSA-DBS. Tremor severity (Fahn-Tolosa-Marin), ataxia (International Cooperative Ataxia Rating Scale), and paresthesia (visual analog scale) were assessed with conventional omnidirectional and directional stimulation with pulse width of 60 μs and 30 μs.

RESULTS

All stimulation conditions reduced tremor. The best directional stimulation with 60 μs reduced more tremor than did most other stimulation settings. The best directional stimulation, regardless of pulse width, effectively reduced stimulation-induced ataxia compared with the conventional stimulation (ring 60 μs) or worst directional stimulation with 60 μs. All new stimulation modes reduced occurrence of paresthesia, but only the best directional stimulation with 30 μs attenuated paresthesia compared with the conventional stimulation (ring 60 μs) or worst directional stimulation with 60 μs. The best directional stimulation with 30 μs reduced tremor, ataxia, and paresthesia compared with conventional stimulation in most patients. Correlation analyses indicated that more anterior stimulation sites are associated with stronger ataxia reduction with directional 30 μs than with conventional 60 μs stimulation.

CONCLUSION

Directional and short-pulse stimulation, and a combination of both, revealed beneficial effects on stimulation-induced adverse effects.

Aberrant dentato-rubro-thalamic pathway in action tremor but not rest tremor: A multi-modality magnetic resonance imaging study

AIMS

The purpose of this study was to clarify the dentato-rubro-thalamic (DRT) pathway in action tremor in comparison to normal controls (NC) and disease controls (i.e., rest tremor) by using multi-modality magnetic resonance imaging (MRI).

METHODS

This study included 40 essential tremor (ET) patients, 57 Parkinson's disease (PD) patients (29 with rest tremor, 28 without rest tremor), and 41 NC. We used multi-modality MRI to comprehensively assess major nuclei and fiber tracts of the DRT pathway, which included decussating DRT tract (d-DRTT) and non-decussating DRT tract (nd-DRTT), and compared the differences in DRT pathway components between action and rest tremor.

RESULTS

Bilateral dentate nucleus (DN) in the ET group had excessive iron deposition compared with the NC group. Compared with the NC group, significantly decreased mean diffusivity and radial diffusivity were observed in the left nd-DRTT in the ET group, which were negatively correlated with tremor severity. No significant difference in each component of the DRT pathway was observed between the PD subgroup or the PD and NC.

CONCLUSION

Aberrant changes in the DRT pathway may be specific to action tremor and were indicating that action tremor may be related to pathological overactivation of the DRT pathway.

How Sample Size Impacts Probabilistic Stimulation Maps in Deep Brain Stimulation

Probabilistic stimulation maps of deep brain stimulation (DBS) effect based on voxel-wise statistics (p-maps) have increased in literature over the last decade. These p-maps require correction for Type-1 errors due to multiple testing based on the same data. Some analyses do not reach overall significance, and this study aims to evaluate the impact of sample size on p-map computation. A dataset of 61 essential tremor patients treated with DBS was used for the investigation. Each patient contributed with four stimulation settings, one for each contact. From the dataset, 5 to 61 patients were randomly sampled with replacement for computation of p-maps and extraction of high- and low-improvement volumes. For each sample size, the process was iterated 20 times with new samples generating in total 1140 maps. The overall p-value corrected for multiple comparisons, significance volumes, and dice coefficients (DC) of the volumes within each sample size were evaluated. With less than 30 patients (120 simulations) in the sample, the variation in overall significance was larger and the median significance volumes increased with sample size. Above 120 simulations, the trends stabilize but present some variations in cluster location, with a highest median DC of 0.73 for n = 57. The variation in location was mainly related to the region between the high- and low-improvement clusters. In conclusion, p-maps created with small sample sizes should be evaluated with caution, and above 120 simulations in single-center studies are probably required for stable results.

Deep brain stimulation, lesioning, focused ultrasound: update on utility

Procedures for neurological disorders such as Parkinsons Disease (PD), Essential Tremor (ET), Obsessive Compulsive Disorder (OCD), Tourette's Syndrome (TS), and Major Depressive Disorder (MDD) tend to overlap. Common therapeutic procedures include deep brain stimulation (DBS), lesioning, and focused ultrasound (FUS). There has been significant change and innovation regarding targeting mechanisms and new advancements in this field allowing for better clinical outcomes in patients with severe cases of these conditions. In this review, we discuss advancements and recent discoveries regarding these three procedures and how they have led to changes in utilization in certain conditions. We further discuss the advantages and drawbacks of these treatments in certain conditions and the emerging advancements in brain-computer interface (BCI) and its utility as a therapeutic for neurological disorders.

Case report: Left gaze and facial nerve palsies after ventral intermediate thalamic nucleus deep brain stimulation implantation

Deep brain stimulation (DBS) to the ventral intermediate nucleus (VIM) of the thalamus has become a common procedure for some refractory, medication-resistant movement disorders like essential tremors. The most common adverse effects from this surgery include dysarthria and gait disturbances. This case report details a left gaze and ipsilateral facial nerve palsy following overshot cannula insertion into the pons during a VIM DBS procedure. Initial patient presentation after surgery revealed significant impairment of horizontal gaze to the left. This improved during follow-up visits and after the recession of the bilateral medial recti. When considering complications of the VIM DBS procedure, surgeons should be aware of the risks of cannula overshot given the anatomic proximity between the thalamus and brainstem. Furthermore, patients should be aware of this risk when making their surgical decision. All patients who undergo VIM DBS should be assessed for cranial nerve deficits after placement.

Probabilistic maps for deep brain stimulation - Impact of methodological differences

BACKGROUND

Group analysis of patients with deep brain stimulation (DBS) has the potential to help understand and optimize the treatment of patients with movement disorders. Probabilistic stimulation maps (PSM) are commonly used to analyze the correlation between tissue stimulation and symptomatic effect but are applied with different methodological variations.

OBJECTIVE

To compute a group-specific MRI template and PSMs for investigating the impact of PSM model parameters.

METHODS

Improvement and occurrence of dizziness in 68 essential tremor patients implanted in caudal zona incerta were analyzed. The input data includes the best parameters for each electrode contact (screening), and the clinically used settings. Patient-specific electric field simulations (n = 488) were computed for all DBS settings. The electric fields were transformed to a group-specific MRI template for analysis and visualization. The different comparisons were based on PSMs representing occurrence (N-map), mean improvement (M-map), weighted mean improvement (wM-map), and voxel-wise t-statistics (p-map). These maps were used to investigate the impact from input data (clinical/screening settings), clustering methods, sampling resolution, and weighting function.

RESULTS

Screening or clinical settings showed the largest impacts on the PSMs. The average differences of wM-maps were 12.4 and 18.2% points for the left and right sides respectively. Extracting clusters based on wM-map or p-map showed notable variation in volumes, while positioning was similar. The impact on the PSMs was small from weighting functions, except for a clear shift in the positioning of the wM-map clusters.

CONCLUSION

The distribution of the input data and the clustering method are most important to consider when creating PSMs for studying the relationship between anatomy and DBS outcome.

Deep brain stimulation for arm tremor: A randomized trial comparing two targets

OBJECTIVE

Deep brain stimulation (DBS) of the thalamic ventral intermediate nucleus (VIM) effectively suppresses arm tremor. Uncontrolled studies suggest the posterior subthalamic area (PSA) may be superior. We compared the intra-individual efficacy of VIM- versus PSA-DBS on tremor suppression and arm function.

METHODS

We performed a randomized, double-blind, crossover trial at Oslo University Hospital in patients (18-80 years) with isolated or combined action tremor affecting at least one arm. Four-contact DBS leads were implanted (bi- or unilaterally) with a trajectory to cover the VIM (upper two contacts) and PSA (lower two contacts). Patients were randomized (1:1 ratio) post-surgery to: Group 1, VIM-stimulation months 0-3 (period 1), then PSA-stimulation months 4-6 (period 2); Group 2, PSA-stimulation first, then VIM-stimulation. Primary endpoint was the difference in improvement from baseline to the end of the VIM- versus PSA-period in the sum of the dominant arm tremor scores of the Fahn-Tolosa-Marin Tremor Rating Scale (FTMTRS), items 5/6 + 10-14.

RESULTS

Forty-five patients were randomized to Group 1 (n = 23) or 2 (n = 22). In the primary endpoint per-protocol analysis (mixed model, n = 40), mean difference in the sum FTMTRS score improvement for the dominant arm was -2.65 points (95% CI -4.33 to -0.97; p = 0.002). The difference in favour of PSA stimulation was highly significant in period 2, but not period 1.

INTERPRETATION

Our randomized trial demonstrated that PSA stimulation provided superior tremor suppression compared with VIM stimulation. A period effect reducing tremor for up to three months in both groups was most likely attributed to a post-surgery stun effect. This article is protected by copyright. All rights reserved.

Case Report: Dual Target Deep Brain Stimulation With Externalized Programming for Post-traumatic Complex Movement Disorder

Introduction: Movement disorders can be common, persistent, and debilitating sequelae of severe traumatic brain injury. Post-traumatic movement disorders are usually complex in nature, involving multiple phenomenological manifestations, and can be difficult to control with medical management alone. Deep brain stimulation (DBS) has been used to treat these challenging cases, but distorted brain anatomy secondary to trauma can complicate effective targeting. In such cases, use of diffusion tractography imaging and inpatient testing with externalized DBS leads can be beneficial in optimizing outcomes.

Case Description: We present the case of a 42-year-old man with severe, disabling post-traumatic tremor who underwent bilateral, dual target DBS to the globus pallidus internus (GPi) and a combined ventral intermediate nucleus of the thalamus (Vim)/dentato-rubro-thalamic tracts (DRTT) target. DRTT fiber tracts were reconstructed preoperatively to assist in surgical targeting given the patient’s distorted anatomy. Externalization and survey of the four leads extra-operatively with inpatient testing allowed for internalization of the leads that demonstrated benefit. Six months after surgery, the patient’s tremor and dystonic burden had decreased by 67% in the performance sub-score of The Essential Tremor Rating Scale (TETRAS).

Conclusion: A patient-tailored approach including target selection guided by individualized anatomy and tractography as well as extra-operative externalized lead interrogation was shown to be effective in optimizing clinical outcome in a patient with refractory post-traumatic tremor.

Connectivity correlates to predict essential tremor deep brain stimulation outcome: Evidence for a common treatment pathway

BACKGROUND AND PURPOSE

Deep brain stimulation (DBS) is the most common surgical treatment for essential tremor (ET), yet there is variation in outcome and stimulation targets. This study seeks to consolidate proposed stimulation "sweet spots," as well as assess the value of structural connectivity in predicting treatment outcomes.

MATERIALS AND METHODS

Ninety-seven ET individuals with unilateral thalamic DBS were retrospectively included. Using normative brain connectomes, structural connectivity measures were correlated with the percentage improvement in contralateral tremor, based on the Fahn-Tolosa-Marin tremor rating scale (TRS), after parameter optimization (range 3.1-12.9 months) using a leave-one-out cross-validation in 83 individuals. The predictive feature map was used for cross-validation in a separate cohort of 14 ET individuals treated at another center. Lastly, estimated volumes of tissue activated (VTA) were used to assess a treatment "sweet spot," which was compared to seven previously reported stimulation sweet spots and their relationship to the tract identified by the predictive feature map.

RESULTS

In the training cohort, structural connectivity between the VTA and dentato-rubro-thalamic tract (DRTT) correlated with contralateral tremor improvement (R = 0.41; p < 0.0001). The same connectivity profile predicted outcomes in a separate validation cohort (R = 0.59; p = 0.028). The predictive feature map represented the anatomical course of the DRTT, and all seven analyzed sweet spots overlapped the predictive tract (DRTT).

CONCLUSIONS

Our results strongly support the possibility that structural connectivity is a predictor of contralateral tremor improvement in ET DBS. The results suggest the future potential for a patient-specific functionally based surgical target. Finally, the results showed convergence in "sweet spots" suggesting the importance of the DRTT to the outcome.