Thalamic Deep Brain Stimulation Salvages Failed Focused Ultrasound Thalamotomy for Essential Tremor: A Case Report

Essential Tremor - Deep Brain Stimulation vs. Focused Ultrasound

INTRODUCTION

Essential Tremor (ET) is one of the most common tremor syndromes typically presented as action tremor, affecting mainly the upper limbs. In at least 30-50% of patients, tremor interferes with quality of life, does not respond to first-line therapies and/or intolerable adverse effects may occur. Therefore, surgery may be considered.

AREAS COVERED

In this review, the authors discuss and compare unilateral ventral intermedius nucleus deep brain stimulation (VIM DBS) and bilateral DBS with Magnetic Resonance-guided Focused Ultrasound (MRgFUS) thalamotomy, which comprises focused acoustic energy generating ablation under real-time MRI guidance. Discussion includes their impact on tremor reduction and their potential complications. Finally, the authors provide their expert opinion.

EXPERT OPINION

DBS is adjustable, potentially reversible and allows bilateral treatments; however, it is invasive requires hardware implantation, and has higher surgical risks. Instead, MRgFUS is less invasive, less expensive, and requires no hardware maintenance. Beyond these technical differences, the decision should also involve the patient, family, and caregivers.

High-Frequency Ultrasound Ablation in Neurosurgery

Modern transcranial magnetic resonance-guided focused ultrasound is an incisionless, ablative treatment modality for a growing number of neurologic disorders. This procedure selectively destroys a targeted volume of cerebral tissue and relies on real-time MR thermography to monitor tissue temperatures. By focusing on a submillimeter target through a hemispheric phased array of transducers, ultrasound waves pass through the skull and avoid overheating and brain damage. High-intensity focused ultrasound techniques are increasingly used to create safe and effective stereotactic ablations for medication-refractory movement and other neurologic and psychiatric disorders.

Evaluating the Safety of Simultaneous Intracranial Electroencephalography and Functional Magnetic Resonance Imaging Acquisition Using a 3 Tesla Magnetic Resonance Imaging Scanner

Background

The unsurpassed sensitivity of intracranial electroencephalography (icEEG) and the growing interest in understanding human brain networks and ongoing activities in health and disease have make the simultaneous icEEG and functional magnetic resonance imaging acquisition (icEEG-fMRI) an attractive investigation tool. However, safety remains a crucial consideration, particularly due to the impact of the specific characteristics of icEEG and MRI technologies that were safe when used separately but may risk health when combined. Using a clinical 3-T scanner with body transmit and head-receive coils, we assessed the safety and feasibility of our icEEG-fMRI protocol.

Methods

Using platinum and platinum-iridium grid and depth electrodes implanted in a custom-made acrylic-gel phantom, we assessed safety by focusing on three factors. First, we measured radio frequency (RF)-induced heating of the electrodes during fast spin echo (FSE, as a control) and the three sequences in our icEEG-fMRI protocol. Heating was evaluated with electrodes placed orthogonal or parallel to the static magnetic field. Using the configuration with the greatest heating observed, we then measured the total heating induced in our protocol, which is a continuous 70-min icEEG-fMRI session comprising localizer, echo-planar imaging (EPI), and magnetization-prepared rapid gradient-echo sequences. Second, we measured the gradient switching-induced voltage using configurations mimicking electrode implantation in the frontal and temporal lobes. Third, we assessed the gradient switching-induced electrode movement by direct visual detection and image analyses.

Results

On average, RF-induced local heating on the icEEG electrode contacts tested were greater in the orthogonal than parallel configuration, with a maximum increase of 0.2°C during EPI and 1.9°C during FSE. The total local heating was below the 1°C safety limit across all contacts tested during the 70-min icEEG-fMRI session. The induced voltage was within the 100-mV safety limit regardless of the configuration. No gradient switching-induced electrode displacement was observed.

Conclusion

We provide evidence that the additional health risks associated with heating, neuronal stimulation, or device movement are low when acquiring fMRI at 3 T in the presence of clinical icEEG electrodes under the conditions reported in this study. High specific absorption ratio sequences such as FSE should be avoided to prevent potential inadvertent tissue heating.

Dysgeusia induced and resolved by focused ultrasound thalamotomy: case report

Dysgeusia, or distorted taste, has recently been acknowledged as a complication of thalamic ablation or thalamic deep brain stimulation as a treatment of tremor. In a unique patient, left-sided MR-guided focused ultrasound thalamotomy improved right-sided essential tremor but also induced severe dysgeusia. Although dysgeusia persisted and caused substantial weight loss, tremor slowly relapsed. Therefore, 19 months after the first procedure, the patient underwent a second focused ultrasound thalamotomy procedure, which again improved tremor but also completely resolved the dysgeusia. On the basis of normative and patient-specific whole-brain tractography, the authors determined the relationship between the thalamotomy lesions and the medial border of the medial lemniscus-a surrogate for the solitariothalamic gustatory fibers-after the first and second focused ultrasound thalamotomy procedures. Both tractography methods suggested partial and complete disruption of the solitariothalamic gustatory fibers after the first and second thalamotomy procedures, respectively. The tractography findings in this unique patient demonstrate that incomplete and complete disruption of a neural pathway can induce and resolve symptoms, respectively, and serve as the rationale for ablative procedures for neurological and psychiatric disorders.

Microelectrode Recording and Radiofrequency Thalamotomy following Focused Ultrasound Thalamotomy

Magnetic resonance imaging-guided focused ultrasound (MRgFUS) is a novel method for stereotactic brain lesioning and has primarily been applied for thalamotomies to treat essential tremor (ET). The electrophysiological properties of previously MRgFUS-sonicated thalamic neurons have not yet been described. We report on an ET patient who underwent an MRgFUS thalamotomy but experienced tremor recurrence. We expanded the MRgFUS-induced thalamic cavity using radiofrequency (RF), with good effect on the tremor but transient sensorimotor deficits and permanent ataxia. This is the first report of a patient undergoing RF thalamotomy after an unsuccessful MRgFUS thalamotomy. As we used microelectrode recording to guide the RF thalamotomy, we could also study for the first time the electrophysiological properties of previously sonicated thalamic neurons bordering the MRgFUS-induced cavity. These neurons displayed electrophysiological characteristics identical to those recorded from nonsonicated thalamic cells in ET patients. Hence, our findings support the widespread assumption that sonication below the necrotic threshold does not permanently alter neuronal function.

Case Report on Deep Brain Stimulation Rescue After Suboptimal MR-Guided Focused Ultrasound Thalamotomy for Essential Tremor: A Tractography-Based Investigation

Essential tremor (ET) is the most prevalent movement disorder in adults, and can often be medically refractory, requiring surgical intervention. MRI-guided focused ultrasound (MRgFUS) is a less invasive procedure that uses ultrasonic waves to induce lesions in the ventralis intermedius nucleus (VIM) to treat refractory ET. As with all procedures for treating ET, optimal targeting during MRgFUS is essential for efficacy and durability. Various studies have reported cases of tremor recurrence following MRgFUS and long-term outcome data is limited to 3–4 years. We present a tractography-based investigation on a case of DBS rescue for medically refractory ET that was treated with MRgFUS that was interrupted due to the development of dysarthria during the procedure. After initial improvement, her hand tremor started to recur within 6 months after treatment, and bilateral DBS was performed targeting the VIM 24 months after MRgFUS. DBS induced long-term tremor control with monopolar stimulation. Diffusion MRI tractography was used to reconstruct the dentatorubrothalamic (DRTT) and corticothalmic (CTT) tracts being modulated by the procedures to understand the variability in efficacy between MRgFUS and DBS in treating ET in our patient. By comparing the MRgFUS lesion and DBS volume of activated tissue (VAT), we found that the MRgFUS lesion was located ventromedially to the VAT, and was less than 10% of the size of the VAT. While the lesion encompassed the same proportion of DRTT streamlines, it encompassed fewer CTT streamlines than the VAT. Our findings indicate the need for further investigation of targeting the CTT when using neuromodulatory procedures to treat refractory ET for more permanent tremor relief.

Surgical Treatment of Parkinson Disease

Surgery in Parkinson disease is effective for a select group of patients when optimal medical management is not sufficient. Functional neurosurgery can be used as either a salvage therapy in patients with disabling symptoms or to maintain quality of life and independence before progression to severe disability in high-functioning patients. With recent technological advancements in imaging and targeting as well as novel neuromodulation paradigms, there are numerous options for targeted brain lesions and deep brain stimulation. Surgical decision making and postoperative management in Parkinson disease therefore often requires a multidisciplinary team effort with neurology, neurosurgery, neuropsychology, and psychiatry.

Essential tremor: New advances

Background

Essential Tremor (ET) is one of the most common movement disorders but many controversies still exist in regards to its definition and pathophysiology. In view of the recent published criteria by the Tremor Task Force of the International Parkinson's and Movement Disorders Society (IPMDS), we intended to analyze if this has changed our view of ET and if new developments have arisen since.

Methods

A Medline search for English-written articles was done on June 15, 2019 using the keyword "Essential Tremor". Publications from November 2017 (publication date of the new tremor classification) were taken into account. Reviews, letters and original studies relevant to the subject were selected and reviewed according to the following themes: clinical characteristics, epidemiology, genetics, pathology, biomarkers and treatment.

Results

Out of 132 publications the most relevant articles were selected and reviewed (total of 65 articles). The great majority of these studies focused on surgical treatments (new targets, new technologies) while relatively few articles addressed epidemiology, pathology and pathophysiology.

Conclusions

The use of the new classification is not commonly used still, excepting more recent studies on therapeutics. This is in keeping with diverse opinions and criticisms reported by the IPMDS task force members themselves. One important change has been validating ET as a heterogeneous condition and defining the ET-plus category. We propose a further sub-group classification derived from the new definition of ET-plus.

Predictors of Outcomes After Focused Ultrasound Thalamotomy

BACKGROUND

Magnetic resonance-guided focused ultrasound thalamotomy (FUS-T) is an emerging treatment for essential tremor (ET).

OBJECTIVE

To determine the predictors of outcomes after FUS-T.

METHODS

Two treatment groups were analyzed: 75 ET patients enrolled in the pivotal trial, between 2013 and 2015; and 114 patients enrolled in the postpivotal trials, between 2015 and 2016. All patients had medication-refractory, disabling ET, and underwent unilateral FUS-T. The primary outcome (hand tremor score, 32-point scale with higher scores indicating worse tremor) and the secondary outcome variables (Clinical Rating Scale for Tremor Part C score: 32-point scale with higher scores indicating more disability) were assessed at baseline and 1, 3, 6, and 12 mo. The operative outcome variables (ie, peak temperature, number of sonications) were analyzed. The results between the 2 treatment groups, pivotal and postpivotal, were compared with repeated measures analysis of variance and adjusted for confounding variables.

RESULTS

A total of 179 patients completed the 12-mo evaluation. The significant predictors of tremor outcomes were patient age, disease duration, peak temperature, and number of sonications. A greater improvement in hand tremor scores was observed in the postpivotal group at all time points, including 12 mo (61.9% ± 24.9% vs 52.1% ± 24.9%, P = .009). In the postpivotal group, higher energy was used, resulting in higher peak temperatures (56.7 ± 2.5 vs 55.6 ± 2.8°C, P = .004). After adjusting for age, years of disease, number of sonications, and maximum temperature, the treatment group was a significant predictor of outcomes (F = 7.9 [1,165], P = .005).

CONCLUSION

We observed an improvement in outcomes in the postpivotal group compared to the pivotal group potentially reflecting a learning curve with FUS-T. The other associations of tremor outcomes included patient age, disease duration, peak temperature, and number of sonications.

Is Deep Brain Stimulation still an option for tremor recurrence after Focused Ultrasound thalamotomy? A case report

With the development of MRI-guided focused ultrasound (FUS), there is an ongoing renewal of interest for ablative procedures as a surgical option for tremor disorders. One of the main criticisms regarding FUS thalamotomy is the potential recurrence of tremor symptoms during follow-up. In case of tremor reappearance, repeating the ultrasound ablation may represent a reasonable option. However, tremor is often perceived as a highly disabling condition and patients may be reluctant to undergo the same unsuccessful treatment again. In this context, few data are available about the feasibility of Deep Brain Stimulation (DBS) in case of tremor recurrence after FUS. Moreover, concerns exist that FUS lesioning could preclude or limit the effectiveness of future DBS. Here we present the case of a 73-year-old right-handed man with a disabling, right-hand, mixed tremor recurring after initial successful FUS thalamotomy and that was properly managed in the end with thalamic Deep Brain Stimulation. Our case suggests that DBS and FUS thalamotomy are not mutually exclusive, but rather they represent complementary tools in the surgical approach to tremor.