Withdrawal of visual feedback in essential tremor

[Essential Tremor: Update of Therapeutic Strategies]

In the last decades there has been progress in the treatment of essential tremor (TE) especially in the surgical field and to a lesser extent in the pharmacological field. We carry out a review of the currently available treatments. The first intervention is the use of non-pharmacological and non-surgical strategies (general advice, occupational therapy, speech therapy, psychotherapy). With discrete advances, the pharmacological treatment is not very satisfactory. Only 30-60% of patients have a positive response, and in these the anti-tremor effectiveness is 40-60%. The first-line drugs are still propranolol and primidone. In cases with severe tremor we will consider a surgical option, the method of choice being thalamotomy using high-intensity focused ultrasound. In the future we must continue to study the pathophysiology of TE, develop drugs specifically designed for TE and improve the technology of available invasive techniques.

The interplay of sensory feedback, arousal, and action tremor amplitude in essential tremor

Essential tremor (ET) amplitude is modulated by visual feedback during target driven movements and in a grip force task. It has not been examined yet whether visual feedback exclusively modulates target force tremor amplitude or if other afferent inputs like auditory sensation has a modulatory effect on tremor amplitude as well. Also, it is unknown whether the enhanced sensory feedback causes an increase of arousal in persons with ET (p-ET). We hypothesized that (1) amplitude of tremor is modulated by variation of auditory feedback in the absence of visual feedback in a force tremor paradigm; (2) increase of tremor amplitude coincides with pupillary size as a measure of arousal. 14 p-ET and 14 matched healthy controls (HC) conducted a computer-based experiment in which they were asked to match a target force on a force sensor using their thumb and index finger. The force-induced movement was fed back to the participant visually, auditory or by a combination of both. Results showed a comparable deviation from the target force (RMSE) during the experiment during all three sensory feedback modalities. The ANOVA revealed an effect of the high vs. low feedback condition on the tremor severity (Power 4-12 Hz) for the visual- and also for the auditory feedback condition in p-ET. Pupillometry showed a significantly increased pupil diameter during the auditory involved high feedback conditions compared to the low feedback conditions in p-ET. Our findings suggest that action tremor in ET is firstly modulated not only by visual feedback but also by auditory feedback in a comparable manner. Therefore, tremor modulation seems to be modality independent. Secondly, high feedback was associated with a significant pupil dilation, possibly mirroring an increased arousal/perceived effort.

Misdirected attentional focus in functional tremor

A characteristic and intriguing feature of functional neurological disorder is that symptoms typically manifest with attention and improve or disappear with distraction. Attentional phenomena are therefore likely to be important in functional neurological disorder, but exactly how this manifests is unknown. The aim of the study was to establish whether in functional tremor the attentional focus is misdirected, and if this misdirection is detrimental to the movement, or rather reflects a beneficial compensatory strategy. Patients with a functional action tremor, between the ages of 21-75, were compared to two age and gender matched control groups: healthy controls and patients with an organic action tremor. The groups included between 17 and 28 participants. First, we compared the natural attentional focus on different aspects of a reaching movement (target, ongoing visual feedback, proprioceptive-motor aspect). This revealed that the attentional focus in the functional tremor group, in contrast to both control groups, was directed to ongoing visual feedback from the movement. Next, we established that all groups were able to shift their attentional focus to different aspects of the reaching movement when instructed. Subsequently, the impact of attentional focus on the ongoing visual feedback on movement performance was evaluated under several conditions: the reaching movement was performed with direct, or indirect visual feedback, without any visual feedback, under three different instruction conditions (as accurately as possible/very slowly/very quickly), and finally as a preparatory movement that was supposedly of no importance. Low trajectory length and low movement duration were taken as measures of good motor performance. For all three groups, motor performance deteriorated with attention to indirect visual feedback, to accuracy, and when instructed to move slowly. It improved without visual feedback and when instructed to move fast. Motor performance improved, in participants with functional tremor only, when the movement was performed as a preparatory movement without any apparent importance. In addition to providing experimental evidence for improvement with distraction, we found that the normal allocation of attention during aimed movement is altered in functional tremor. Attention is disproportionately directed towards the ongoing visual feedback from the moving hand. This altered attentional focus may be partly responsible for the tremor, since it also worsens motor performance in healthy controls and patients with an organic action tremor. It may have its detrimental impact through interference with automatic movement processes, due to a maladaptive shift from lower- to higher-level motor control circuitry.

Variability of Movement Disorders: The Influence of Sensation, Action, Cognition, and Emotions

Patients with movement disorders experience fluctuations unrelated to disease progression or treatment. Extrinsic factors that contribute to the variable expression of movement disorders are environment related. They influence the expression of movement disorders through sensory-motor interactions and include somatosensory, visual, and auditory stimuli. Examples of somatosensory effects are stimulus sensitivity of myoclonus on touch and sensory amelioration in dystonia but also some less-appreciated effects on parkinsonian tremor and gait. Changes in visual input may affect practically all types of movement disorders, either by loss of its compensatory role or by disease-related alterations in the pathways subserving visuomotor integration. The interaction between auditory input and motor function is reflected in simple protective reflexes and in complex behaviors such as singing or dancing. Various expressions range from the effect of music on parkinsonian bradykinesia to tics. Changes in body position affect muscle tone and may result in marked fluctuations of rigidity or may affect dystonic manifestations. Factors intrinsic to the patient are related to their voluntary activity and cognitive, motivational, and emotional states. Depending on the situation or disease, they may improve or worsen movement disorders. We discuss various factors that can influence the phenotypic variability of movement disorders, highlighting the potential mechanisms underlying these manifestations. We also describe how motor fluctuations can be provoked during the clinical assessment to help reach the diagnosis and appreciated to understand complaints that seem discrepant with objective findings. We summarize advice and interventions based on the variability of movement disorders that may improve patients' functioning in everyday life. © 2020 International Parkinson and Movement Disorder Society.

Normalization of aberrant pretherapeutic dynamic functional connectivity of extrastriate visual system in patients who underwent thalamotomy with stereotactic radiosurgery for essential tremor: a resting-state functional MRI study

OBJECTIVE

The tremor circuitry has commonly been hypothesized to be driven by one or multiple pacemakers within the cerebello-thalamo-cortical pathway, including the cerebellum, contralateral motor thalamus, and primary motor cortex. However, previous studies, using multiple methodologies, have advocated that tremor could be influenced by changes within the right extrastriate cortex, at both the structural and functional level. The purpose of this work was to evaluate the role of the extrastriate cortex in tremor generation and further arrest after left unilateral stereotactic radiosurgery thalamotomy (SRS-T).

METHODS

The authors considered 12 healthy controls (HCs, group 1); 15 patients with essential tremor (ET, right-sided, drug-resistant; group 2) before left unilateral SRS-T; and the same 15 patients (group 3) 1 year after the intervention, to account for delayed effects. Blood oxygenation level-dependent functional MRI during resting state was used to characterize the dynamic interactions of the right extrastriate cortex, comparing HC subjects against patients with ET before and 1 year after SRS-T. In particular, the authors applied coactivation pattern analysis to extract recurring whole-brain spatial patterns of brain activity over time.

RESULTS

The authors found 3 different sets of coactivating regions within the right extrastriate cortex in HCs and patients with pretherapeutic ET, reminiscent of the "cerebello-visuo-motor," "thalamo-visuo-motor" (including the targeted thalamus), and "basal ganglia and extrastriate" networks. The occurrence of the first pattern was decreased in pretherapeutic ET compared to HCs, whereas the other two patterns showed increased occurrences. This suggests a misbalance between the more prominent cerebellar circuitry and the thalamo-visuo-motor and basal ganglia networks. Multiple regression analysis showed that pretherapeutic standard tremor scores negatively correlated with the increased occurrence of the thalamo-visuo-motor network, suggesting a compensatory pathophysiological trait. Clinical improvement after SRS-T was related to changes in occurrences of the basal ganglia and extrastriate cortex circuitry, which returned to HC values after the intervention, suggesting that the dynamics of the extrastriate cortex had a role in tremor generation and further arrest after the intervention.

CONCLUSIONS

The data in this study point to a broader implication of the visual system in tremor generation, and not only through visual feedback, given its connections to the dorsal visual stream pathway and the cerebello-thalamo-cortical circuitry, with which its dynamic balance seems to be a crucial feature for reduced tremor. Furthermore, SRS-T seems to bring abnormal pretherapeutic connectivity of the extrastriate cortex to levels comparable to those of HC subjects.

Network-level connectivity is a critical feature distinguishing dystonic tremor and essential tremor

Dystonia is a movement disorder characterized by involuntary muscle co-contractions that give rise to disabling movements and postures. A recent expert consensus labelled the incidence of tremor as a core feature of dystonia that can affect body regions both symptomatic and asymptomatic to dystonic features. We are only beginning to understand the neural network-level signatures that relate to clinical features of dystonic tremor. At the same time, clinical features of dystonic tremor can resemble that of essential tremor and present a diagnostic confound for clinicians. Here, we examined network-level functional activation and connectivity in patients with dystonic tremor and essential tremor. The dystonic tremor group included primarily cervical dystonia patients with dystonic head tremor and the majority had additional upper-limb tremor. The experimental paradigm included a precision grip-force task wherein online visual feedback related to force was manipulated across high and low spatial feedback levels. Prior work using this paradigm in essential tremor patients produced exacerbation of grip-force tremor and associated changes in functional activation. As such, we directly compared the effect of visual feedback on grip-force tremor and associated functional network-level activation and connectivity between dystonic tremor and essential tremor patient cohorts to better understand disease-specific mechanisms. Increased visual feedback similarly exacerbated force tremor during the grip-force task in dystonic tremor and essential tremor cohorts. Patients with dystonic tremor and essential tremor were characterized by distinct functional activation abnormalities in cortical regions but not in the cerebellum. We examined seed-based functional connectivity from the sensorimotor cortex, globus pallidus internus, ventral intermediate thalamic nucleus, and dentate nucleus, and observed abnormal functional connectivity networks in dystonic tremor and essential tremor groups relative to controls. However, the effects were far more widespread in the dystonic tremor group as changes in functional connectivity were revealed across cortical, subcortical, and cerebellar regions independent of the seed location. A unique pattern for dystonic tremor included widespread reductions in functional connectivity compared to essential tremor within higher-level cortical, basal ganglia, and cerebellar regions. Importantly, a receiver operating characteristic determined that functional connectivity z-scores were able to classify dystonic tremor and essential tremor with 89% area under the curve, whereas combining functional connectivity with force tremor yielded 94%. These findings point to network-level connectivity as an important feature that differs substantially between dystonic tremor and essential tremor and should be further explored in implementing appropriate diagnostic and therapeutic strategies.

A widespread visually-sensitive functional network relates to symptoms in essential tremor

Essential tremor is a neurological syndrome of heterogeneous pathology and aetiology that is characterized by tremor primarily in the upper extremities. This tremor is commonly hypothesized to be driven by a single or multiple neural oscillator(s) within the cerebello-thalamo-cortical pathway. Several studies have found an association of blood-oxygen level-dependent (BOLD) signal in the cerebello-thalamo-cortical pathway with essential tremor, but there is behavioural evidence that also points to the possibility that the severity of tremor could be influenced by visual feedback. Here, we directly manipulated visual feedback during a functional MRI grip force task in patients with essential tremor and control participants, and hypothesized that an increase in visual feedback would exacerbate tremor in the 4-12 Hz range in essential tremor patients. Further, we hypothesized that this exacerbation of tremor would be associated with dysfunctional changes in BOLD signal and entropy within, and beyond, the cerebello-thalamo-cortical pathway. We found that increases in visual feedback increased tremor in the 4-12 Hz range in essential tremor patients, and this increase in tremor was associated with abnormal changes in BOLD amplitude and entropy in regions within the cerebello-thalamo-motor cortical pathway, and extended to visual and parietal areas. To determine if the tremor severity was associated with single or multiple brain region(s), we conducted a birectional stepwise multiple regression analysis, and found that a widespread functional network extending beyond the cerebello-thalamo-motor cortical pathway was associated with changes in tremor severity measured during the imaging protocol. Further, this same network was associated with clinical tremor severity measured with the Fahn, Tolosa, Marin Tremor Rating Scale, suggesting this network is clinically relevant. Since increased visual feedback also reduced force error, this network was evaluated in relation to force error but the model was not significant, indicating it is associated with force tremor but not force error. This study therefore provides new evidence that a widespread functional network is associated with the severity of tremor in patients with essential tremor measured simultaneously at the hand during functional imaging, and is also associated with the clinical severity of tremor. These findings support the idea that the severity of tremor is exacerbated by increased visual feedback, suggesting that designers of new computing technologies should consider using lower visual feedback levels to reduce tremor in essential tremor.

Cortical dynamics within and between parietal and motor cortex in essential tremor

BACKGROUND

Evidence from functional imaging in essential tremor suggests that activity within parietal and motor cortices may be associated with worsening of tremor at increased visual feedback.

OBJECTIVES

Examine how cortical oscillations within these regions and the connectivity between these regions is associated with worsening of tremor in essential tremor in response to high visual feedback.

METHOD

The study included 24 essential tremor participants and 17 controls. We measured cortical activity and tremor magnitude at low and high feedback conditions. Cortical activity was measured using high-density electroencephalogram and isolated using source localization.

RESULTS

Changes in power across feedback in the 4-12 Hz and 12-30 Hz bands were reduced within the contralateral motor cortex of essential tremor patients compared to controls. The 12-30 Hz bidirectional connectivity between the parietal and contralateral motor cortex was decreased in essential tremor patients. Worsening of tremor from low to high visual feedback was associated with 4-12 Hz activity in contralateral motor cortex. The greatest separation between groups was found when using the difference of the contralateral motor cortex activity at high and low feedback, rather than either feedback condition alone.

CONCLUSION

Our findings provide new evidence that tremor in essential tremor is associated with reduced power across feedback in the motor cortex and reduced connectivity between the parietal and motor cortices. Combined with previous work on the cerebellar-thalamo-cortical motor circuit, our findings suggest that the network level disturbances associated with essential tremor extend to the cortico-cortical pathway between the parietal cortex and motor cortex. © 2018 International Parkinson and Movement Disorder Society.