Radiosurgical thalamotomy for essential tremor: state of the art, current challenges and future directions

INTRODUCTION

Essential tremor (ET) is the most frequent movement disorder, affecting up to 5% of adults > 65 years old. In 30-50% of cases, optimal medical management provides insufficient tremor relief and surgical options are considered. Thalamotomy is a time-honored intervention, which can be performed using radiofrequency (RF), stereotactic radiosurgery (SRS), or magnetic resonance-guided focused ultrasounds (MRgFUS). While the latter has received considerable attention in the last decade, SRS has consistently been demonstrated as an effective and well-tolerated option.

AREAS COVERED

This review discusses the evidence on SRS thalamotomy for ET. Modern workflows and emerging techniques are detailed. Current outcomes are analyzed, with a specific focus on tremor reduction, complications and radiological evolution of the lesions. Challenges for the field are highlighted.

EXPERT OPINION

SRS thalamotomy improves tremor in > 80% patients. The efficacy appears comparable to other modalities, including DBS, RF and MRgFUS. Side effects result mostly from idiosyncratic hyper-responses to radiation, which occur in up to 10% of treatments, are usually self-resolving, and are symptomatic in < 4% of patients. Future research should focus on accumulating more data on bilateral treatments, collecting long-term outcomes, refining targeting, and improving lesion consistency.

A case report: Dual-lead deep brain stimulation of the posterior subthalamic area and the thalamus was effective for Holmes tremor after unsuccessful focused ultrasound thalamotomy

Holmes tremor is a symptomatic tremor that develops secondary to central nervous system disorders. Stereotactic neuromodulation is considered when the tremors are intractable. Targeting the ventral intermediate nucleus (Vim) is common; however, the outcome is often unsatisfactory, and the posterior subthalamic area (PSA) is expected as alternative target. In this study, we report the case of a patient with intractable Holmes tremor who underwent dual-lead deep brain stimulation (DBS) to stimulate multiple locations in the PSA and thalamus. The patient was a 77-year-old female who complained of severe tremor in her left upper extremity that developed one year after her right thalamic infarction. Vim-thalamotomy using focused ultrasound therapy (FUS) was initially performed but failed to control tremor. Subsequently, we performed DBS using two leads to stimulate four different structures. Accordingly, one lead was implanted with the aim of targeting the ventral oralis nucleus (Vo)/zona incerta (Zi), and the other with the aim of targeting the Vim/prelemniscal radiation (Raprl). Electrode stimulation revealed that Raprl and Zi had obvious effects. Postoperatively, the patient achieved good tremor control without any side effects, which was maintained for two years. Considering that she demonstrated resting, postural, and intention/action tremor, and Vim-thalamotomy by FUS was insufficient for tremor control, complicated pathogenesis was presumed in her symptoms including both the cerebellothalamic and the pallidothalamic pathways. Using the dual-lead DBS technique, we have more choices to adjust the stimulation at multiple sites, where different functional networks are connected. Intractable tremors, such as Holmes tremor, may have complicated pathology, therefore, modulating multiple pathological networks is necessary. We suggest that the dual-lead DBS (Vo/Raprl and Vim/Zi) presented here is safe, technically feasible, and possibly effective for the control of Holmes tremor.

Deep-Brain Stimulation for Essential Tremor and Other Tremor Syndromes: A Narrative Review of Current Targets and Clinical Outcomes

Tremor is a prevalent symptom associated with multiple conditions, including essential tremor (ET), Parkinson's disease (PD), multiple sclerosis (MS), stroke and trauma. The surgical management of tremor evolved from stereotactic lesions to deep-brain stimulation (DBS), which allowed safe and reversible interference with specific neural networks. This paper reviews the current literature on DBS for tremor, starting with a detailed discussion of current tremor targets (ventral intermediate nucleus of the thalamus (Vim), prelemniscal radiations (Raprl), caudal zona incerta (Zi), thalamus (Vo) and subthalamic nucleus (STN)) and continuing with a discussion of results obtained when performing DBS in the various aforementioned tremor syndromes. Future directions for DBS research are then briefly discussed.

Delayed Brain Cyst Formation After Gamma Knife Anterior Capsulotomy

BACKGROUND

Gamma Knife ventral anterior capsulotomy is an effective option to treat refractory obsessive-compulsive disorder. Although well tolerated, complications can develop years after radiosurgery. We describe a case in which abnormal complications induced by very high doses of radiation evolved.

CASE DESCRIPTION

A 55-year-old man with refractory obsessive-compulsive disorder was treated with Gamma Knife ventral anterior capsulotomy using a dose of 180 Gy. His obsessive-compulsive symptoms improved, but his condition evolved with a manic episode, cognitive memory changes, visual hallucinations, confabulation, and frontal lobe symptoms. Magnetic resonance imaging showed brain edema in the left hemisphere and a 6-mm brain cyst in the right hemisphere at postoperative month 20.

CONCLUSIONS

This case shows the consequences of affecting more fibers related to the anterior frontal region than intended with a radiosurgical procedure and illustrates the importance of careful clinical and imaging follow-up after Gamma Knife ventral anterior capsulotomy.

Correlation between fractional anisotropy changes in the targeted ventral intermediate nucleus and clinical outcome after transcranial MR-guided focused ultrasound thalamotomy for essential tremor: results of a pilot study

OBJECTIVE

This study evaluated changes of fractional anisotropy (FA) in the ventral intermediate nucleus (VIM) of the thalamus after transcranial MR-guided focused ultrasound (TcMRgFUS) thalamotomy and their associations with clinical outcome.

METHODS

Clinical and radiological data of 12 patients with medically refractory essential tremor (mean age 76.5 years) who underwent TcMRgFUS thalamotomy with VIM targeting were analyzed retrospectively. The Clinical Rating Scale for Tremor (CRST) score was calculated before and at 1 year after treatment. Measurements of the relative FA (rFA) values, defined as ratio of the FA value in the targeted VIM to the FA value in the contralateral VIM, were performed before thalamotomy, and 1 day and 1 year thereafter.

RESULTS

TcMRgFUS thalamotomy was well tolerated and no long-term complications were noted. At 1-year follow-up, 8 patients demonstrated relief of tremor (improvement group), whereas in 4 others persistent tremor was noted (recurrence group). In the entire cohort, mean rFA values in the targeted VIM before treatment, and at 1 day and 1 year after treatment, were 1.12 ± 0.15, 0.44 ± 0.13, and 0.82 ± 0.22, respectively (p < 0.001). rFA values were consistently higher in the recurrence group compared with the improvement group, and the difference reached statistical significance at 1 day (p < 0.05) and 1 year (p < 0.01) after treatment. There was a statistically significant (p < 0.01) positive correlation between rFA values in the targeted VIM at 1 day after thalamotomy and CRST score at 1 year after treatment. Receiver operating characteristic curve analysis revealed that the optimal cutoff value of rFA at 1 day after thalamotomy for prediction of symptomatic improvement at 1-year follow-up is 0.54.

CONCLUSIONS

TcMRgFUS thalamotomy results in significant decrease of rFA in the targeted VIM, at both 1 day and 1 year after treatment. Relative FA values at 1 day after treatment showed significant correlation with CRST score at 1-year follow-up. Therefore, FA may be considered a possible imaging biomarker for early prediction of clinical outcome after TcMRgFUS thalamotomy for essential tremor.

Pretherapeutic resting-state fMRI profiles are associated with MR signature volumes after stereotactic radiosurgical thalamotomy for essential tremor

OBJECTIVEEssential tremor (ET) is the most common movement disorder. Drug-resistant ET can benefit from standard stereotactic deep brain stimulation or radiofrequency thalamotomy or, alternatively, minimally invasive techniques, including stereotactic radiosurgery (SRS) and high-intensity focused ultrasound, at the level of the ventral intermediate nucleus (Vim). The aim of the present study was to evaluate potential correlations between pretherapeutic interconnectivity (IC), as depicted on resting-state functional MRI (rs-fMRI), and MR signature volume at 1 year after Vim SRS for tremor, to be able to potentially identify hypo- and hyperresponders based only on pretherapeutic neuroimaging data.METHODSSeventeen consecutive patients with ET were included, who benefitted from left unilateral SRS thalamotomy (SRS-T) between September 2014 and August 2015. Standard tremor assessment and rs-fMRI were acquired pretherapeutically and 1 year after SRS-T. A healthy control group was also included (n = 12). Group-level independent component analysis (ICA; only n = 17 for pretherapeutic rs-fMRI) was applied. The mean MR signature volume was 0.125 ml (median 0.063 ml, range 0.002-0.600 ml). The authors correlated baseline IC with 1-year MR signatures within all networks. A 2-sample t-test at the level of each component was first performed in two groups: group 1 (n = 8, volume < 0.063 ml) and group 2 (n = 9, volume ≥ 0.063 ml). These groups did not statistically differ by age, duration of symptoms, baseline ADL score, ADL point decrease at 1 year, time to tremor arrest, or baseline tremor score on the treated hand (TSTH; p > 0.05). An ANOVA was then performed on each component, using individual subject-level maps and continuous values of 1-year MR signatures, correlated with pretherapeutic IC.RESULTSUsing 2-sample t-tests, two networks were found to be statistically significant: network 3, including the brainstem, motor cerebellum, bilateral thalamus, and left supplementary motor area (SMA) (pFWE = 0.004, cluster size = 94), interconnected with the red nucleus (MNI -2, -22, -32); and network 9, including the brainstem, posterior insula, bilateral thalamus, and left SMA (pFWE = 0.002, cluster size = 106), interconnected with the left SMA (MNI 24, -28, 44). Higher pretherapeutic IC was associated with higher MR volumes, in a network including the anterior default-mode network and bilateral thalamus (ANOVA, pFWE = 0.004, cluster size = 73), interconnected with cerebellar lobule V (MNI -12, -70, -22). Moreover, in the same network, radiological hyporesponders presented with negative IC values.CONCLUSIONSThese findings have clinical implications for predicting MR signature volumes after SRS-T. Here, using pretherapeutic MRI and data processing without prior hypothesis, the authors showed that pretherapeutic network interconnectivity strength predicts 1-year MR signature volumes following SRS-T.

Outcomes from stereotactic surgery for essential tremor

There are several different surgical procedures that are used to treat essential tremor (ET), including deep brain stimulation (DBS) and thalamotomy procedures with radiofrequency (RF), radiosurgery (RS) and most recently, focused ultrasound (FUS). Choosing a surgical treatment requires a careful presentation and discussion of the benefits and drawbacks of each. We conducted a literature review to compare the attributes and make an appraisal of these various procedures. DBS was the most commonly reported treatment for ET. One-year tremor reductions ranged from 53% to 63% with unilateral Vim DBS. Similar improvements were demonstrated with RF (range, 74%-90%), RS (range, 48%-63%) and FUS thalamotomy (range, 35%-75%). Overall, bilateral Vim DBS demonstrated more improvement in tremor reduction since both upper extremities were treated (range, 66%-78%). Several studies show continued beneficial effects from DBS up to five years. Long-term follow-up data also support RF and gamma knife radiosurgical thalamotomy treatments. Quality of life measures were similarly improved among patients who received all treatments. Paraesthesias, dysarthria and ataxia were commonly reported adverse effects in all treatment modalities and were more common with bilateral DBS surgery. Many of the neurological complications were transient and resolved after surgery. DBS surgery had the added benefit of programming adjustments to minimise stimulation-related complications. Permanent neurological complications were most commonly reported for RF thalamotomy. Thalamic DBS is an effective, safe treatment with a long history. For patients who are medically unfit or reluctant to undergo DBS, several thalamic lesioning methods have parallel benefits to unilateral DBS surgery. Each of these surgical modalities has its own nuance for treatment and patient selection. These factors should be carefully considered by both neurosurgeons and patients when selecting an appropriate treatment for ET.

Clinical response to Vim's thalamic stereotactic radiosurgery for essential tremor is associated with distinctive functional connectivity patterns

INTRODUCTION

Essential tremor (ET) is the most common movement disorder. Drug-resistant ET can benefit from standard surgical stereotactic procedures (deep brain stimulation, thalamotomy) or minimally invasive high-intensity focused ultrasound (HIFU) or stereotactic radiosurgical thalamotomy (SRS-T). Resting-state fMRI (rs-fMRI) is a non-invasive imaging method acquired in absence of a task. We examined whether rs-fMRI correlates with tremor score on the treated hand (TSTH) improvement 1 year after SRS-T.

METHODS

We included 17 consecutive patients treated with left unilateral SRS-T in Marseille, France. Tremor score evaluation and rs-fMRI were acquired at baseline and 1 year after SRS-T. Resting-state data (34 scans) were analyzed without a priori hypothesis, in Lausanne, Switzerland. Based on degree of improvement in TSTH, to consider SRS-T at least as effective as medication, we separated two groups: 1, ≤ 50% (n = 6, 35.3%); 2, > 50% (n = 11, 64.7%). They did not differ statistically by age (p = 0.86), duration of symptoms (p = 0.41), or lesion volume at 1 year (p = 0.06).

RESULTS

We report TSTH improvement correlated with interconnectivity strength between salience network with the left claustrum and putamen, as well as between bilateral motor cortices, frontal eye fields and left cerebellum lobule VI with right visual association area (the former also with lesion volume). Longitudinal changes showed additional associations in interconnectivity strength between right dorsal attention network with ventro-lateral prefrontal cortex and a reminiscent salience network with fusiform gyrus.

CONCLUSIONS

Brain connectivity measured by resting-state fMRI relates to clinical response after SRS-T. Relevant networks are visual, motor, and attention. Interconnectivity between visual and motor areas is a novel finding, revealing implication in movement sensory guidance.

Connectivity derived thalamic segmentation in deep brain stimulation for tremor

The ventral intermediate nucleus (VIM) of the thalamus is an established surgical target for stereotactic ablation and deep brain stimulation (DBS) in the treatment of tremor in Parkinson's disease (PD) and essential tremor (ET). It is centrally placed on a cerebello-thalamo-cortical network connecting the primary motor cortex, to the dentate nucleus of the contralateral cerebellum through the dentato-rubro-thalamic tract (DRT). The VIM is not readily visible on conventional MR imaging, so identifying the surgical target traditionally involved indirect targeting that relies on atlas-defined coordinates. Unfortunately, this approach does not fully account for individual variability and requires surgery to be performed with the patient awake to allow for intraoperative targeting confirmation. The aim of this study is to identify the VIM and the DRT using probabilistic tractography in patients that will undergo thalamic DBS for tremor. Four male patients with tremor dominant PD and five patients (three female) with ET underwent high angular resolution diffusion imaging (HARDI) (128 diffusion directions, 1.5 mm isotropic voxels and b value = 1500) preoperatively. Patients received VIM-DBS using an MR image guided and MR image verified approach with indirect targeting. Postoperatively, using parallel Graphical Processing Unit (GPU) processing, thalamic areas with the highest diffusion connectivity to the primary motor area (M1), supplementary motor area (SMA), primary sensory area (S1) and contralateral dentate nucleus were identified. Additionally, volume of tissue activation (VTA) corresponding to active DBS contacts were modelled. Response to treatment was defined as 40% reduction in the total Fahn-Tolosa-Martin Tremor Rating Score (FTMTRS) with DBS-ON, one year from surgery. Three out of nine patients had a suboptimal, long-term response to treatment. The segmented thalamic areas corresponded well to anatomically known counterparts in the ventrolateral (VL) and ventroposterior (VP) thalamus. The dentate-thalamic area, lay within the M1-thalamic area in a ventral and lateral location. Streamlines corresponding to the DRT connected M1 to the contralateral dentate nucleus via the dentate-thalamic area, clearly crossing the midline in the mesencephalon. Good response was seen when the active contact VTA was in the thalamic area with highest connectivity to the contralateral dentate nucleus. Non-responders had active contact VTAs outside the dentate-thalamic area. We conclude that probabilistic tractography techniques can be used to segment the VL and VP thalamus based on cortical and cerebellar connectivity. The thalamic area, best representing the VIM, is connected to the contralateral dentate cerebellar nucleus. Connectivity based segmentation of the VIM can be achieved in individual patients in a clinically feasible timescale, using HARDI and high performance computing with parallel GPU processing. This same technique can map out the DRT tract with clear mesencephalic crossing.

The role of diffusion tensor imaging tractography for Gamma Knife thalamotomy planning

OBJECTIVE

The role of tractography in Gamma Knife thalamotomy (GK-T) planning is still unclear. Pyramidal tractography might reduce the risk of radiation injury to the pyramidal tract and reduce motor complications.

METHODS

In this study, the ventralis intermedius nucleus (VIM) targets of 20 patients were bilaterally defined using Iplannet Stereotaxy Software, according to the anterior commissure–posterior commissure (AC-PC) line and considering the localization of the pyramidal tract. The 40 targets and tractography were transferred as objects to the GammaPlan Treatment Planning System (GP-TPS). New targets were defined, according to the AC-PC line in the functional targets section of the GP-TPS. The target offsets required to maintain the internal capsule (IC) constraint of < 15 Gy were evaluated. In addition, the strategies available in GP-TPS to maintain the minimum conventional VIM target dose at > 100 Gy were determined.

RESULTS

A difference was observed between the positions of both targets and the doses to the IC. The lateral (x) and the vertical (z) coordinates were adjusted 1.9 mm medially and 1.3 mm cranially, respectively. The targets defined considering the position of the pyramidal tract were more medial and superior, based on the constraint of 15 Gy touching the object representing the IC in the GP-TPS. The best strategy to meet the set constraints was 90° Gamma angle (GA) with automatic shaping of dose distribution; this was followed by 110° GA. The worst GA was 70°. Treatment time was substantially increased by the shaping strategy, approximately doubling delivery time.

CONCLUSIONS

Routine use of DTI pyramidal tractography might be important to fine-tune GK-T planning. DTI tractography, as well as anisotropy showing the VIM, promises to improve Gamma Knife functional procedures. They allow for a more objective definition of dose constraints to the IC and targeting. DTI pyramidal tractography introduced into the treatment planning may reduce the incidence of motor complications and improve efficacy. This needs to be validated in a large clinical series.