Thalamic physiology of intentional essential tremor is more like cerebellar tremor than postural essential tremor

A Randomized Phase 2 KINETIC Trial Evaluating SAGE-324/BIIB124 in Individuals with Essential Tremor

BACKGROUND

SAGE-324/BIIB124 is an investigational positive allosteric modulator of GABAA receptors.

OBJECTIVE

KINETIC (NCT04305275), a double-blind, randomized, placebo-controlled, phase 2 study, evaluated SAGE-324/BIIB124 in individuals with essential tremor (ET).

METHODS

Individuals aged 18 to 80 years were randomly assigned 1:1 to orally receive 60 mg of SAGE-324/BIIB124 or placebo once daily for 28 days. The primary endpoint was change from baseline in The Essential Tremor Rating Assessment Scale-Performance Subscale (TETRAS-PS) Item 4 (upper-limb tremor) at day 29 with SAGE-324/BIIB124 versus placebo.

RESULTS

Between May 2020 and February 2021, 69 U.S. participants were randomly assigned to receive SAGE-324/BIIB124 (n = 34) or placebo (n = 35). There was a significant reduction from baseline in TETRAS-PS Item 4 at day 29 with SAGE-324/BIIB124 versus placebo (least squares mean [standard error]: -2.31 [0.401] vs. -1.24 [0.349], P = 0.0491). The most common treatment-emergent adverse events included somnolence, dizziness, fatigue, and balance disorder.

CONCLUSION

These results support further development of SAGE-324/BIIB124 for potential ET treatment. © 2024 Sage Therapeutics, Inc and The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Successful MRI-Guided Focused Ultrasound Thalamotomy after Ipsilateral Gamma Knife Radiosurgery for Essential Tremor: A Case Report with Video

We report the case of a 67-year-old left-handed female patient with disabling medically refractory essential tremor who underwent successful right-sided magnetic resonance-guided focused ultrasound (MRgFUS) of the ventral intermediate nucleus after ipsilateral gamma knife radiosurgery (GKRS) thalamotomy performed 3 years earlier. The GKRS had a partial effect on her postural tremor without side effects, but there was no reduction of her kinetic tremor or improvement in her quality of life (QoL). The patient subsequently underwent a MRgFUS thalamotomy, which induced an immediate and marked reduction in both the postural and kinetic tremor components, with minor complications (left upper lip hypesthesia, dysmetria in her left hand, and slight gait ataxia). The MRgFUS-induced lesion was centered more medially than the GKRS-induced lesion and extended more posteriorly and inferiorly. The MRgFUS-induced lesion interrupted remaining fibers of the dentatorubrothalamic tract (DRTT). The functional improvement 1-year post-MRgFUS was significant due to a marked reduction of the patient's kinetic tremor. The QoL score (Quality of Life in Essential Tremor) improved by 88% and her Clinical Rating Scale for Tremor left hand score by 62%. The side effects persisted but were minor, with no impact on her QoL. The explanation for the superior efficacy of MRgFUS compared to GKRS in our patient could be due to either a poor response to the GKRS or to a better localization of the MRgFUS lesion with a more extensive interruption of DRTT fibers. In conclusion, MRgFUS can be a valuable therapeutic option after unsatisfactory GKRS, especially because MRgFUS has immediate clinical effectiveness, allowing intra-procedural test lesions and possible readjustment of the target if necessary.

Electrophysiology-aided DBS targeting the ventral intermediate nucleus in an essential tremor patient with MRI-incompatible lead: A case report

Essential tremor (ET) is a common disease in the elderly population. Severe, medication-refractory ET may require surgical intervention via ablation or deep brain stimulation (DBS). Thalamic Vim (Ventral intermediate nucleus), targeted indirectly using atlas-based coordinates, is the classical target in these procedures. We present a case of an ET patient with a non-MR-compatible cardiac orphaned leads who was a candidate for DBS surgery. Due to the lead constraints of MR use, we used a head computed tomography (CT) with contrast media as the reference exam to define the AC, PC, and midline, and to register and indirectly target the Vim. For target validation, we used intraoperative electrophysiological recordings and intraoperative CT. We implanted bilateral directional leads at the target location. We used the-essential-tremor-rating-assessment-scale (TETRAS) pre and postoperatively to clinically evaluate tremor. Intraoperative micro-electrode recordings (MERs) showed individual tremor cells and a robust increase in normalized root mean square (NRMS) indicating entry to the Vim. Postoperative visualization using lead-DBS along with dramatic clinical improvements show that we were able to accurately target the Vim. Our results show that CT-only registration and planning for thalamic Vim DBS is feasible, and that MERs and intraoperative CT are useful adjuncts for Vim target validation.

Clinical Heterogeneity of Essential Tremor: Understanding Neural Substrates of Action Tremor Subtypes

Essential tremor (ET) is a common movement disorder affecting millions of people. Studies of ET patients and perturbations in animal models have provided a foundation for the neural networks involved in its pathophysiology. However, ET encompasses a wide variability of phenotypic expression, and this may be the consequence of dysfunction in distinct subcircuits in the brain. The cerebello-thalamo-cortical circuit is a common substrate for the multiple subtypes of action tremor. Within the cerebellum, three sets of cerebellar cortex-deep cerebellar nuclei connections are important for tremor. The lateral hemispheres and dentate nuclei may be involved in intention, postural and isometric tremor. The intermediate zone and interposed nuclei could be involved in intention tremor. The vermis and fastigial nuclei could be involved in head and proximal upper extremity tremor. Studying distinct cerebellar circuitry will provide important framework for understanding the clinical heterogeneity of ET.

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.

Clinical and Genetic Evaluation of Spinocerebellar Ataxia Type 10 in 16 Brazilian Families

Spinocerebellar ataxia type 10 (SCA10) is an autosomal dominant disorder in which patients have a slowly progressive cerebellar ataxia, with dysarthria, dysphagia, and epilepsy. The aims of this study were to characterize the phenotypic expression of SCA10 and to examine its genotype-phenotype relationships. Ninety-one Brazilian patients with SCA10 from 16 families were selected. Clinical and epidemiological data were assessed by a standardized protocol, and severity of disease was measured by the Scale for the Assessment and Rating of Ataxia (SARA). The mean age of onset of symptoms was 34.8 ± 9.4 years. Sixty-two (68.2%) patients presented exclusively with pure cerebellar ataxia. Only 6 (6.6%) of the patients presented with epilepsy. Patients with epilepsy had a mean age of onset of symptoms lower than that of patients without epilepsy (23.5 ± 15.5 years vs 35.4 ± 8.7 years, p = 0.021, respectively). All cases of intention tremor were in women from one family. This family also had the lowest mean age of onset of symptoms, and a higher percentage of SCA10 cases in women. There was a positive correlation between duration of disease and severity of ataxia (rho = 0.272, p = 0.016), as quantified by SARA. We did not find a statistically significant correlation between age of onset of symptoms and expansion size (r = - 0.163, p = 0.185). The most common clinical presentation of SCA10 was pure cerebellar ataxia. Our data suggest that patients with epilepsy may have a lower age of onset of symptoms than those who do not have epilepsy. These findings and the description of a family with intention tremor in women with earlier onset of symptoms draw further attention to the phenotypic variability of SCA10.

Deep brain stimulation in the caudal zona incerta modulates the sensorimotor cerebello-cerebral circuit in essential tremor

Essential tremor is effectively treated with deep brain stimulation (DBS), but the neural mechanisms underlying the treatment effect are poorly understood. Essential tremor is driven by a dysfunctional cerebello-thalamo-cerebral circuit resulting in pathological tremor oscillations. DBS is hypothesised to interfere with these oscillations at the stimulated target level, but it is unknown whether the stimulation modulates the activity of the cerebello-thalamo-cerebral circuit during different task states (with and without tremor) in awake essential tremor patients. To address this issue, we used functional MRI in 16 essential tremor patients chronically implanted with DBS in the caudal zona incerta. During scanning, the patients performed unilateral tremor-inducing postural holding and pointing tasks as well as rest, with contralateral stimulation turned On and Off. We show that DBS exerts both task-dependent as well as task-independent modulation of the sensorimotor cerebello-cerebral regions (p ​≤ ​0.05, FWE cluster-corrected for multiple comparisons). Task-dependent modulation (DBS ​× ​task interaction) resulted in two patterns of stimulation effects. Firstly, activity decreases (blood oxygen level-dependent signal) during tremor-inducing postural holding in the primary sensorimotor cortex and cerebellar lobule VIII, and activity increases in the supplementary motor area and cerebellar lobule V during rest (p ​≤ ​0.05, post hoc two-tailed t-test). These effects represent differences at the effector level and may reflect DBS-induced tremor reduction since the primary sensorimotor cortex, cerebellum and supplementary motor area exhibit less motor task-activity as compared to the resting condition during On stimulation. Secondly, task-independent modulation (main effect of DBS) was observed as activity increase in the lateral premotor cortex during all motor tasks, and also during rest (p ​≤ ​0.05, post hoc two-tailed t-test). This task-independent effect may mediate the therapeutic effects of DBS through the facilitation of the premotor control over the sensorimotor circuit, making it less susceptible to tremor entrainment. Our findings support the notion that DBS in essential tremor is modulating the sensorimotor cerebello-cerebral circuit, distant to the stimulated target, and illustrate the complexity of stimulation mechanisms by demonstrating task-dependent as well as task-independent actions in cerebello-cerebral regions.

A functional micro-electrode mapping of ventral thalamus in essential tremor

Deep brain stimulation enables the delivery of therapeutic interventions to otherwise inaccessible areas of the brain while, at the same time, offering the unique opportunity to record from these same regions in awake patients. The posterior ventrolateral thalamus has become a reliable deep brain stimulation target for medically-refractory patients suffering from essential tremor. However, the contribution of the thalamus in essential tremor, and even whether posterior ventrolateral thalamus is the optimal target, remains a matter of ongoing debate. There are several lines of evidence supporting clusters of activity within the posterior ventrolateral thalamus that are important for tremor emergence. In this study we sought to map the functional properties of these clusters through microelectrode recordings during deep brain stimulation surgery. Data were obtained from 10 severely affected patients with essential tremor (12 hemispheres) undergoing deep brain stimulation surgery. Our results demonstrate power and coherence maxima located in the inferior posterior ventrolateral thalamus and immediate ventral region. Moreover, we identified distinct yet overlapping clusters of predominantly efferent (driving) and afferent (feedback) activity, with a preference for more efferent contributors, consistent with a net role in the driving of tremor output. Finally, we demonstrate that resolvable thalamic spiking activity directly relates to background activity and that the strength of tremor may be dictated by phase relationships between efferent and afferent pockets in the posterior ventrolateral thalamus. Taken together, these results provide important evidence for the role of the inferior posterior ventrolateral thalamus and its border region in essential tremor pathophysiology. Such results progress our mechanistic understanding and promote the adoption of next-generation therapies such as high resolution segregated deep brain stimulation electrodes.

Frequency-dependent antidromic activation in thalamocortical relay neurons: effects of synaptic inputs

OBJECTIVE

Deep brain stimulation (DBS) generates action potentials (APs) in presynaptic axons and fibers of passage. The APs may be antidromically propagated to invade the cell body and/or orthodromically transmitted to downstream structures, thereby affecting widespread targets distant from the electrode. Activation of presynaptic terminals also causes trans-synaptic effects, which in turn alter the excitability of the post-synaptic neurons. Our aim was to determine how synaptic inputs affect the antidromic invasion of the cell body.

APPROACH

We used a biophysically-based multi-compartment model to simulate antidromic APs in thalamocortical relay (TC) neurons. We applied distributed synaptic inputs to the model and quantified how excitatory and inhibitory inputs contributed to the fidelity of antidromic activation over a range of antidromic frequencies.

MAIN RESULTS

Antidromic activation exhibited strong frequency dependence, which arose from the hyperpolarizing afterpotentials in the cell body and its respective recovery cycle. Low-frequency axonal spikes faithfully invaded the soma, whereas frequent failures of antidromic activation occurred at high frequencies. The frequency-dependent pattern of the antidromic activation masked burst-driver inputs to TC neurons from the cerebellum in a frequency-dependent manner. Antidromic activation also depended on the excitability of the cell body. Excitatory synaptic inputs improved the fidelity of antidromic activation by increasing the excitability, and inhibitory inputs suppressed antidromic activation by reducing soma excitability. Stimulus-induced depolarization of neuronal segments also facilitated antidromic propagation and activation.

SIGNIFICANCE

The results reveal that synaptic inputs, stimulus frequency, and electrode position regulate antidromic activation of the cell body during extracellular stimulation. These findings provide a biophysical basis for interpreting the widespread inhibition/activation of target nuclei during DBS.

Task-dependent variability of Essential Tremor

INTRODUCTION

In Essential Tremor (ET), tremor characteristics and the impairment caused by tremor may vary from task to task. A variability of tremor frequency between postural and kinetic tasks has been proposed in ET, suggesting either multiple central oscillating networks, or peripheral or proprioceptive feedback-mechanisms. This electrophysiological study aimed to assess tremor frequencies and amplitudes in tasks involving postural and kinetic tremor, and compare findings within and across tasks, to delineate physiological differences underlying individually affected manual tasks in ET.

METHODS

40 ET patients were included in the study. Tremor was characterized clinically, as well as electrophysiologically using accelerometry and digitizing tablet tasks. Tremor amplitude measures and frequencies were extracted for tasks involving kinetic (digital spiral drawing, handwriting), as well as postural tremor. Tremor was compared between and within tasks.

RESULTS

Digital spiral tremor frequencies were significantly higher compared to postural tremor frequencies, as measured by accelerometry, with a mean difference of >2 Hz (p < 0.001). Within-task variability of repeated digital spirals revealed a significant amplitude reduction over time in both hands (p < 0.001), with an up to 32% reduction compared to the first spiral.

CONCLUSION

ET exhibited a frequency variability, which was dependent on activation condition, suggesting neurophysiologically distinct pathways between postural and kinetic tremor. The reduction of tremor amplitudes observed in repeated digital spiral drawing may be explained by a learning effect or adaptation, and should be considered as non-random factor of variability when using spirals in ET to assess effects of interventions.

Incisionless transcranial MR-guided focused ultrasound in essential tremor: cerebellothalamic tractotomy

Background

Already in the late 1960s and early 1970s, targeting of the “posterior subthalamic area (PSA)” was explored by different functional neurosurgical groups applying the radiofrequency (RF) technique to treat patients suffering from essential tremor (ET). Recent advances in magnetic resonance (MR)-guided focused ultrasound (MRgFUS) technology offer the possibility to perform thermocoagulation of the cerebellothalamic fiber tract in the PSA without brain penetration, allowing a strong reduction of the procedure-related risks and increased accuracy. We describe here the first results of the MRgFUS cerebellothalamic tractotomy (CTT).

Methods

Twenty-one consecutive patients suffering from chronic (mean disease duration 29.9 years), therapy-resistant ET were treated with MRgFUS CTT. Three patients received bilateral treatment with a 1-year interval. Primary relief assessment indicators were the Essential Tremor Rating Scale (Fahn, Tolosa, and Marin) (ETRS) taken at follow-up (3 months to 2 years) with accent on the hand function subscores (HF16 for treated hand and HF32 for both hands) and handwriting. The evolution of seven patients with HF32 above 28 points over 32 (group 1) differentiated itself from the others’ (group 2) and was analyzed separately. Global tremor relief estimations were provided by the patients. Lesion reconstruction and measurement of targeting accuracy were done on 2-day post-treatment MR pictures for each CTT lesion.

Results

The mean ETRS score for all patients was 57.6 ± 13.2 at baseline and 25.8 ± 17.6 at 1 year (n = 10). The HF16 score reduction was 92 % in group 2 at 3 months and stayed stable at 1 year (90 %). Group 1 showed only an improvement of 41 % at 3 months and 40 % at 1 year. Nevertheless, two patients of group 1 treated bilaterally had an HF16 score reduction of 75 and 88 % for the dominant hand at 1 year after the second side. The mean patient estimation of global tremor relief after CTT was 92 % at 2 days and 77 % at 1-year follow-up.

Conclusions

CTT with MRgFUS was shown to be an effective and safe approach for patients with therapy-refractory essential tremor, combining neurological function sparing with precise targeting and the possibility to treat patients bilaterally.

Painful cutaneous laser stimuli induce event-related gamma-band activity in the lateral thalamus of humans

Although the thalamus is an important module in "pain networks," there are few studies of the effect of experimental pain upon thalamic oscillations. We have now examined the hypothesis that, during a series of painful cutaneous laser stimuli, thalamic signals will show stimulus-related gamma-band spectral activity, which is modulated by attention to vs. distraction from the painful stimulus. When the series of laser stimuli was presented, attention was focused by counting the laser stimuli (count laser task), while distraction was produced by counting backward (count back plus laser task). We have studied the effect of a cutaneous laser on thalamic local field potentials and EEG activity during awake procedures (deep brain stimulation implants) for the treatment of essential tremor. At different delays after the stimulus, three low gamma- (30-50 Hz) and two high gamma-band (70-90 Hz) activations were observed during the two tasks. Greater high-gamma activation was found during the count laser task for the earlier window, while greater high-gamma activation was found during the count back plus laser task for the later window. Thalamic signals were coherent with EEG signals in the beta band, which indicated significant synchrony. Thalamic cross-frequency coupling analysis indicated that the phase of the lower frequency activity (theta to beta) modulated the amplitude of the higher frequency activity (low and high gamma) more strongly during the count laser task than during the count back plus laser task. This modulation might result in multiplexed signals each encoding a different aspect of pain.

Intention Tremor of the Legs in Essential Tremor: Prevalence and Clinical Correlates

OBJECTIVE

To estimate the prevalence and assess the clinical correlates of intention tremor in the legs in essential tremor (ET) patients.

BACKGROUND

The cerebellar features of ET are of growing interest to clinical neurologists. Arm tremor has an intentional component in many ET patients. Intention tremor in the legs, however, has never been systematically evaluated.

METHODS

One-hundred-twenty-eight ET patients were enrolled in a clinical-epidemiological study at Columbia University. A videotaped neurological examination included 10 toe-to-target movements with each foot. Videotapes were independently reviewed by two movement disorder neurologists who noted the presence vs. absence of intentional leg tremor. Two patients underwent quantitative computerized tremor analysis to study the physiological characteristics of the tremor.

RESULTS

Thirty-five patients (27.3%) had intentional leg tremor; in 21, tremor was unilateral and in 14 it was bilateral. The 35 patients with intentional leg tremor did not differ from the remaining 93 patients in their clinical characteristics. Analyses comparing the 14 patients with bilateral intentional leg tremor to the 93 patients with no intentional leg tremor showed trends towards longer disease duration and more severe intentional arm tremor in the former. Tremor analysis showed a 3-fold increase in average tremor amplitude from movement onset to the point just prior to touching the target.

CONCLUSIONS

Our data suggest that intentional leg tremor, another cerebellar feature, is common in ET patients. The tremor may be associated with longer disease duration and more severe intentional arm tremor, but these preliminary trends need to be assessed in larger study samples.