The effects of subthalamic nucleus deep brain stimulation on parkinsonian tremor

Gamma Knife® stereotactic radiosurgery as a treatment for essential and parkinsonian tremor: long-term experience

INTRODUCTION

Unilateral Gamma Knife™ stereotactic radiosurgery on the ventral-intermediate nucleus of the thalamus is a minimally invasive neurosurgical option for refractory tremor. We describe the experience of Gamma Knife™ thalamotomy (GKT) in patients with essential tremor (ET) and tremor-dominant Parkinson's disease (PD) at our specialised stereotactic neurosurgery unit.

METHODS

We reviewed the cases of patients treated with GKT between January 2014 and February 2018 with a minimum of 12 months' follow-up. We analysed clinical and demographic variables, indication, radiation dose, effectiveness (based on subscales of the Fahn-Tolosa-Marin [FTM] scale and the Movement Disorders Society-Unified Parkinson's Disease Rating Scale [MDS-UPDRS] motor score), and adverse events.

RESULTS

Thirteen patients were registered, 6 with a diagnosis of tremor-dominant PD, four with refractory ET, and three with ET and PD. Median age was 78 years (range, 62-83), with seven patients aged over 75 years. Four patients were receiving anticoagulants and two had history of stroke. The maximum radiation dose administered was 130 Gy. Mean (standard deviation) follow-up duration was 30.0 (14.5) months. Significant tremor improvement was observed on the FTM subscales: 63.6% at 12 months and 63.5% at the end of follow-up; MDS-UPDRS tremor items showed improvements of 71.3% at 12 months and 60.3% at the end of follow up. Eleven patients reported significant improvements in quality of life, and 3 reported mild and transient adverse effects.

CONCLUSIONS

This is the largest series of patients with essential and parkinsonian tremor treated with GKT and followed up in the long term in Spain. GKT can be safe and effective in the long term in patients with refractory tremor, including in elderly patients and those receiving anticoagulants.

Comparison of efficacy of deep brain stimulation and focused ultrasound in parkinsonian tremor: a systematic review and network meta-analysis

To compare the efficacy of deep brain stimulation (DBS) and MRI-guided focused ultrasound (MRIgFUS) in parkinsonian tremor. We performed a network meta-analysis based on a Bayesian framework. We searched the literature for articles published between January 1990 and October 2020 using three databases: PubMed, Embase and Cochrane Library (The Cochrane Database of Systematic Reviews). A total of 24 studies were included in our analysis, comprising data from 784 participants. Our findings revealed similar efficacy of DBS and MRIgFUS in parkinsonian tremor suppression. Compared with internal globus pallidus (GPi)-MRIgFUS, GPi-DBS -1.84 (-6.44, 2.86), pedunculopontine nucleus (PPN)_DBS -3.28 (-9.28, 2.78), PPN and caudal zona incerta (cZI)-DBS 0.40 (-6.16, 6.87), subthalamic nucleus (STN)_DBS 0.89 (-3.48, 5.30), STN and cZI-DBS 1.99 (-4.74, 8.65), ventral intermediate nucleus(VIM)_DBS 1.75 (-2.87, 6.48), VIM_FUS 0.72 (-5.27, 6.43), cZI-DBS 0.27 (-4.75, 5.36) were no significantly difference. Compared with VIM-MRIgFUS, GPi-DBS -2.55(-6.94, 2.21), GPi-FUS -0.72 (-6.43, 5.27), PPN_DBS -4.01(-9.97, 2.11), PPN and cZI-DBS -0.32 (-6.73, 6.36), STN_DBS 0.16 (-3.98, 4.6), STN and cZI-DBS 1.31(-5.18,7.87), VIM-DBS 1.00(-3.41, 5.84)and cZI-DBS -0.43 (-5.07, 4.68) were no significantly difference. With respect to the results for the treatment of motor symptoms, GPi-DBS, GPi-MRIgFUS, STN-DBS and cZI-DBS were significantly more efficacious than baseline (GPi-DBS 15.24 (5.79, 24.82), GPi-MRIgFUS 13.46 (2.46, 25.10), STN-DBS 19.62 (12.19, 27.16), cZI-DBS 14.18 (1.73, 26.89). The results from the surface under the cumulative ranking results showed that STN-DBS ranked first, followed by combined PPN and cZI-DBS, and PPN-DBS ranked last. MRIgFUS, an efficacious intervention for improving parkinsonian tremor, has not demonstrated to be inferior to DBS in parkinsonian tremor suppression. Hence, clinicians should distinguish individual patients' symptoms to ensure that the appropriate intervention and therapeutic approach are applied.

Resolution of tardive tremor after bilateral subthalamic nucleus deep brain stimulation placement

Background:

Tardive tremor (TT) is an underrecognized manifestation of tardive syndrome (TS). In our experience, TT is a rather common manifestation of TS, especially in a setting of treatment with aripiprazole, and is a frequent cause of referrals for the evaluation of idiopathic Parkinson disease. There are reports of successful treatment of tardive orofacial dyskinesia and dystonia with deep brain stimulation (DBS) using globus pallidus interna (GPi) as the primary target, but the literature on subthalamic nucleus (STN) DBS for tardive dyskinesia (TD) is lacking. To the best of our knowledge, there are no reports on DBS treatment of TT.

Case Description:

A 75-year-old right-handed female with the medical history of generalized anxiety disorder and major depressive disorder had been treated with thioridazine and citalopram from 1980 till 2010. Around 2008, she developed orolingual dyskinesia. She was started on tetrabenazine in June 2011. She continued to have tremors and developed Parkinsonian gait, both of which worsened overtime. She underwent DBS placement in the left STN in January 2017 with near-complete resolution of her tremors. She underwent right STN implantation in September 2017 with similar improvement in symptoms.

Conclusion:

While DBS-GPi is the preferred treatment in treating oral TD and dystonia, DBS-STN could be considered a safe and effective target in patients with predominating TT and/or tardive Parkinsonism. This patient saw a marked improvement in her symptoms after implantation of DBS electrodes, without significant relapse or recurrence in the years following implantation.

Gamma Knife® stereotactic radiosurgery as a treatment for essential and parkinsonian tremor: long-term experience

INTRODUCTION

Unilateral Gamma Knife™ stereotactic radiosurgery on the ventral-intermediate nucleus of the thalamus is a minimally invasive neurosurgical option for refractory tremor. We describe the experience of Gamma Knife™ thalamotomy (GKT) in patients with essential tremor (ET) and tremor-dominant Parkinson's disease (PD) at our specialised stereotactic neurosurgery unit.

METHODS

We reviewed the cases of patients treated with GKT between January 2014 and February 2018 with a minimum of 12 months' follow-up. We analysed clinical and demographic variables, indication, radiation dose, effectiveness (based on subscales of the Fahn-Tolosa-Marin [FTM] scale and the Movement Disorders Society-Unified Parkinson's Disease Rating Scale [MDS-UPDRS] motor score), and adverse events.

RESULTS

Thirteen patients were registered, 6 with a diagnosis of tremor-dominant PD, four with refractory ET, and three with ET and PD. Median age was 78 years (range, 62-83), with seven patients aged over 75 years. Four patients were receiving anticoagulants and two had history of stroke. The maximum radiation dose administered was 130 Gy. Mean (standard deviation) follow-up duration was 30.0 (14.5) months. Significant tremor improvement was observed on the FTM subscales: 63.6% at 12 months and 63.5% at the end of follow-up; MDS-UPDRS tremor items showed improvements of 71.3% at 12 months and 60.3% at the end of follow up. Eleven patients reported significant improvements in quality of life, and 3 reported mild and transient adverse effects.

CONCLUSIONS

This is the largest series of patients with essential and parkinsonian tremor treated with GKT and followed up in the long term in Spain. GKT can be safe and effective in the long term in patients with refractory tremor, including in elderly patients and those receiving anticoagulants.

Connectivity profile of thalamic deep brain stimulation to effectively treat essential tremor

Essential tremor is the most prevalent movement disorder and is often refractory to medical treatment. Deep brain stimulation offers a therapeutic approach that can efficiently control tremor symptoms. Several deep brain stimulation targets (ventral intermediate nucleus, zona incerta, posterior subthalamic area) have been discussed for tremor treatment. Effective deep brain stimulation therapy for tremor critically involves optimal targeting to modulate the tremor network. This could potentially become more robust and precise by using state-of-the-art brain connectivity measurements. In the current study, we used two normative brain connectomes (structural and functional) to show the pattern of effective deep brain stimulation electrode connectivity in 36 patients with essential tremor. Our structural and functional connectivity models were significantly predictive of postoperative tremor improvement in out-of-sample data (P < 0.001 for both structural and functional leave-one-out cross-validation). Additionally, we segregated the somatotopic brain network based on head and hand tremor scores. These resulted in segregations that mapped onto the well-known somatotopic maps of both motor cortex and cerebellum. Crucially, this shows that slightly distinct networks need to be modulated to ameliorate head versus hand tremor and that those networks could be identified based on somatotopic zones in motor cortex and cerebellum. Finally, we propose a multi-modal connectomic deep brain stimulation sweet spot that may serve as a reference to enhance clinical care, in the future. This spot resided in the posterior subthalamic area, encroaching on the inferior borders of ventral intermediate nucleus and sensory thalamus. Our results underscore the importance of integrating brain connectivity in optimizing deep brain stimulation targeting for essential tremor.

Exploring the effects of deep brain stimulation and vision on tremor in Parkinson's disease - benefits from objective methods

Background

Tremor is a cardinal symptom of Parkinson’s disease (PD) that may cause severe disability. As such, objective methods to determine the exact characteristics of the tremor may improve the evaluation of therapy. This methodology study aims to validate the utility of two objective technical methods of recording Parkinsonian tremor and evaluate their ability to determine the effects of Deep Brain Stimulation (DBS) of the subthalamic nucleus and of vision.

Methods

We studied 10 patients with idiopathic PD, who were responsive to _L-Dopa and had more than 1 year use of bilateral subthalamic nucleus stimulation. The patients did not have to display visible tremor to be included in the study. Tremor was recorded with two objective methods, a force platform and a 3 dimensional (3D) motion capture system that tracked movements in four key proximal sections of the body (knee, hip, shoulder and head). They were assessed after an overnight withdrawal of anti-PD medications with DBS ON and OFF and with eyes open and closed during unperturbed and perturbed stance with randomized calf vibration, using a randomized test order design.

Results

Tremor was detected with the Unified Parkinson’s Disease Rating Scale (UPDRS) in 6 of 10 patients but only distally (hands and feet) with DBS OFF. With the force platform and the 3D motion capture system, tremor was detected in 6 of 10 and 7 of 10 patients respectively, mostly in DBS OFF but also with DBS ON in some patients. The 3D motion capture system revealed that more than one body section was usually affected by tremor and that the tremor amplitude was non-uniform, but the frequency almost identical, across sites. DBS reduced tremor amplitude non-uniformly across the body. Visual input mostly reduced tremor amplitude with DBS ON.

Conclusions

Technical recording methods offer objective and sensitive detection of tremor that provide detailed characteristics such as peak amplitude, frequency and distribution pattern, and thus, provide information that can guide the optimization of treatments. Both methods detected the effects of DBS and visual input but the 3D motion system was more versatile in that it could detail the presence and properties of tremor at individual body sections.

Current Practice and the Future of Deep Brain Stimulation Therapy in Parkinson's Disease

Deep brain stimulation (DBS) is an effective therapy for Parkinson's disease patients experiencing motor fluctuations, medication-resistant tremor, and/or dyskinesia. Currently, the subthalamic nucleus and the globus pallidus internus are the two most widely used targets, with individual advantages and disadvantages influencing patient selection. Potential DBS patients are selected using the few existing guidelines and the available DBS literature, and many centers employ an interdisciplinary team review of the individual's risk-benefit profile. Programmed settings vary based on institution- or physician-specific protocols designed to maximize benefits and limit adverse effects. Expectations should be realistic and clearly defined during the evaluation process, and each bothersome symptom should be addressed in the context of building the risk-benefit profile. Current DBS research is focused on improved symptom control, the development of newer technologies, and the improved efficiency of stimulation delivery. Techniques deliver stimulation in a more personalized way, and methods of adaptive DBS such as closed-loop approaches are already on the horizon.

Comparison of VIM and STN DBS for Parkinsonian Resting and Postural/Action Tremor

Background

Resting tremor is common in Parkinson’s disease (PD), but up to 47% of PD patients have action tremor, which is sometimes resistant to medications. Deep brain stimulation (DBS) of the ventral intermediate nucleus (VIM) of the thalamus or subthalamic nucleus (STN) is effective for medication-refractory tremor in PD, though it remains unclear whether STN DBS is as effective as VIM DBS for postural/action tremor related to PD.

Methods

We carried out a single-center retrospective review of patients with medication-refractory resting, postural, and action PD tremor, treated with either VIM or STN DBS between August 2004 and March 2014. We assessed the degree of improvement using items 20 and 21 of the Unified Parkinson’s Disease Rating Scale (UPDRS) motor scale and examined the proportion of patients achieving tremor arrest.

Results

A total of 18 patients were analyzed, 10 treated with STN and eight treated with VIM, with similar off-medication motor UPDRS scores. There was no significant difference in improvement in tremor scores or in the proportion of patients experiencing tremor arrest between the two stimulation sites. Overall, 56% and 72% of patients experienced complete absence of postural/action tremor and resting tremor, respectively, at last follow-up.

Discussion

This study demonstrated excellent outcomes on both resting and postural/action tremor after either VIM or STN DBS. Resting tremor improved to a greater degree than postural/action tremor in both groups. These results suggest that a large randomized controlled trial is needed to show a superior effect of one target on PD tremor.

Subthalamic stimulation improves motor function but not home and neighborhood mobility

OBJECTIVE

Subthalamic (STN) deep brain stimulation (DBS) is a recognized therapy for alleviating motor symptoms of Parkinson's disease (PD). However, little is known about its impact on mobility, an important component of quality of life (QoL). To address this issue, we assessed the impact of STN DBS on life-space mobility and QoL.

METHODS

Twenty surgical patients with PD were assessed using mobility and QoL scales and the United Parkinson's disease rating scale, and results were compared before surgery and 6 to 9 months postoperatively.

RESULTS

STN DBS significantly improved motor dysfunction but had a limited impact on measures of life-space mobility and QoL.

INTERPRETATION

STN DBS improves motor function and some components of QoL. However, motor recovery does not translate into improved life-space in the intermediate term. In addition to a focus on motor function, multidisciplinary attention to increasing mobility may further improve QoL in the intermediate and long-term.

Deep brain stimulation for the treatment of Parkinson's disease: efficacy and safety

Deep brain stimulation (DBS) surgery has become increasingly utilized in the treatment of advanced Parkinson's disease. Over the past decade, a number of studies have demonstrated that DBS is superior to best medical management in appropriately selected patients. The primary targets for DBS in Parkinson's disease include the subthalamic nucleus and the internal segment of the globus pallidus, both of which improve the cardinal motor features in Parkinson's disease. Recent randomized studies have revealed that both targets are similarly effective in treating the motor symptoms of Parkinson's disease, but emerging evidence suggests that the globus pallidus may be the preferred target in many patients, based on differences in nonmotor outcomes. Here, we review appropriate patient selection, and the efficacy and safety of DBS therapy in Parkinson's disease. Best outcomes are achieved if the problems of the individual patient are considered when evaluating surgical candidates and considering whether the subthalamic nucleus or the globus pallidus internus should be targeted.

Novel applications of deep brain stimulation

The success of deep brain stimulation (DBS) surgery in treating medically refractory symptoms of some movement disorders has inspired further investigation into a wide variety of other treatment-resistant conditions. These range from disorders of gait, mood, and memory to problems as diverse as obesity, consciousness, and addiction. We review the emerging indications, rationale, and outcomes for some of the most promising new applications of DBS in the treatment of postural instability associated with Parkinson's disease, depression, obsessive–compulsive disorder, obesity, substance abuse, epilepsy, Alzheimer′s-type dementia, and traumatic brain injury. These studies reveal some of the excitement in a field at the edge of a rapidly expanding frontier. Much work still remains to be done on basic mechanism of DBS, optimal target and patient selection, and long-term durability of this technology in treating new indications.

Revisiting the relationship between essential tremor and Parkinson's disease

BACKGROUND

The relationship between essential tremor and Parkinson's disease has been a subject of reviews and debates for long time, but there is now growing evidence that the two common movement disorders are pathogenically related, at least in some patient populations.

METHODS

PubMed as well as authors' own files were searched for relevant keywords regarding overlap between the disorders in clinical features as well as on epidemiologic, genetic, imaging, and pathological studies.

RESULTS

New findings in each of these categories are critically reviewed and placed in the context of previously published data.

DISCUSSION

Although we believe that there is compelling evidence for the notion that some patients with "pure" ET evolve into PD, the biologic nature of the association is not well understood. Furthermore, it is not clear what factors predict which ET patients later develop PD and whether patients with PD are more likely to develop ET. Further epidemiologic, clinical, genetic, imaging, and pathological studies are needed to better understand this mixed, ET-PD phenotype.

Bilateral subthalamic deep brain stimulation in Parkinson disease patients with severe tremor

BACKGROUND

Previous studies have shown that subthalamic nucleus (STN) deep brain stimulation (DBS) improves tremor in Parkinson disease (PD). However, the patients included in those studies were unselected for tremor severity.

OBJECTIVE

We specifically assessed the effect of STN DBS on tremor in selected PD patients with severe tremor.

METHODS

Seventy-two PD patients who had received bilateral STN DBS were included. The effects of STN DBS on the off-medication tremor, the on-medication tremor, and the off-medication action tremor in patients selected as the worst one-third in each category at baseline were evaluated after a mean duration of > 2 years.

RESULTS

In patients with severe off-medication tremor, off-medication tremor score improved from 12.28 +/- 2.80 at baseline to 1.93 +/- 2.85 at the last follow-up (P < .001). The off-medication tremor in the off-stimulation state at the last follow-up was less severe than the preoperative off-medication tremor. In patients with severe on-medication tremor, on-medication tremor score improved from 6.17 +/- 2.45 to 1.35 +/- 2.58 (P < .001). In patients with severe off-medication action tremor, off-medication action tremor score improved from 5.08 +/- 1.35 to 1.24 +/- 1.42 (P < .001).

CONCLUSION

STN DBS is effective for severe off- and on-medication tremor and off-medication action tremor in PD. Our findings suggest that STN DBS reduces PD tremor through, at least in part, its effect on the tremor-generating mechanism independent of dopaminergic transmission and that long-term electrical stimulation of STN might induce a structural or neurochemical change leading to the improvement of tremor.

Subregional 6-[18F]fluoro-ʟ-m-tyrosine uptake in the striatum in Parkinson's disease

BACKGROUND

In idiopathic Parkinson's disease (PD) the clinical features are heterogeneous and include different predominant symptoms. The aim of the present study was to determine the relationship between subregional aromatic l-amino acid decarboxylase (AADC) activity in the striatum and the cardinal motor symptoms of PD using high-resolution positron emission tomography (PET) with an AADC tracer, 6-[18F]fluoro-ʟ-m-tyrosine (FMT).

METHODS

We assessed 101 patients with PD and 19 healthy volunteers. PD was diagnosed based on the UK Brain Bank criteria by two experts on movement disorders. Motor symptoms were measured with the Unified Parkinson's Disease Rating Scale (UPDRS). FMT uptake in the subregions of the striatum was analyzed using semi-automated software for region-of-interest demarcation on co-registered magnetic resonance images.

RESULTS

In all PD patients, FMT uptake was decreased in the posterior putamen regardless of predominant motor symptoms and disease duration. Smaller uptake values were found in the putamen contralateral to the side with more affected limbs. The severity of bradykinesia, rigidity, and axial symptoms was correlated with the decrease of FMT uptake in the putamen, particularly in the anterior part. No significant correlation was observed between tremors and FMT uptake.

CONCLUSIONS

Decrease of FMT uptake in the posterior putamen appears to be most sensitive in mild PD and uptake in the anterior putamen may reflect the severity of main motor symptoms, except for tremor.

Deep brain stimulation emerging indications

There are a number of emerging surgical indications for deep brain stimulation. We have shown that modulation of activity within motor, mood, and cognitive circuits has beneficial effects in patients with Parkinson's disease, treatment-resistant depression, and perhaps Alzheimer's type dementia. We review the rationale, safety, and efficacy for each of these indications, focusing on disease mechanisms and relevant data that are necessary to document therapeutic value in each case. The review closes with some thoughts on possible future directions for deep brain stimulation. It is likely that applications for deep brain stimulation will continue to expand as accumulating data establish its safety and efficacy profile in these and other conditions.

Tremor

PURPOSE OF REVIEW

Tremor continuously attracts the attention of clinicians and basic researchers in search of pathophysiological, molecular and genetic mechanisms of the oscillatory activity.

RECENT FINDINGS

A widespread dynamic network of cortical and subcortical oscillators taking part in tremor generation intermittently has been postulated. Essential tremor is accompanied by functional deficits but may also occur along with subtle cerebellar changes. According to recent epidemiological studies there may be a link of essential tremor with Parkinson's disease. Many of the epidemiologic studies suffer from small cohorts, small effects or the lack of a definite test for essential tremor leaving the diagnosis a pure clinical one. A very recent large genome-wide association study has revealed that the LINGO1 gene is associated with an increased risk for essential tremor. Topiramate is becoming the best-established second line treatment for essential tremor. Targets for deep brain stimulation in the grey matter below the ventral intermediate nucleus of the thalamus seem to be most effective.

SUMMARY

New concepts of the central origin of tremors stimulate the search for new therapeutic targets for tremor suppression outside the basal ganglia and thalamus (e.g. cortex). The role of structural neurodegenerative changes in essential tremor remains an open question. Further studies on specific subgroups of patients are necessary.

Long-term suppression of tremor by deep brain stimulation of the ventral intermediate nucleus of the thalamus combined with pallidotomy in hemiparkinsonian patients

Deep brain stimulation (DBS) of the ventral intermediate nucleus of the thalamus (VIM) is a powerful surgical option in the treatment of tremor-predominant Parkinson's disease. However, its therapeutic efficacy depends on the tremor distribution. DBS is highly efficient in relief of distal appendicular tremor but not other types of tremor. Also, it is generally thought that DBS of the VIM has no significant beneficial effects on other motor symptoms of Parkinson's disease. We report two hemiparkinsonian patients, in whom unilateral VIM DBS combined with posteroventral pallidotomy produced long-lasting suppression of not only hand tremor, but also leg or jaw tremor and other motor symptoms.

[Can subthalamic nucleus stimulation reveal parkinsonian rest tremor?]

INTRODUCTION

Rest tremor, one of the main symptoms in Parkinson's disease (PD), is dramatically improved following subthalamic nucleus stimulation (STN). Results are often better than after l-dopa treatment. The occurrence of rest tremor after neurosurgery in patients without preoperative tremor is uncommon.

AIM

The aim of this work was to investigate the role of subthalamic nucleus stimulation in the appearance of parkinsonian rest tremor. PATIENTS-RESULTS: Thirty PD patients (14%) out of 215 undergoing STN deep brain stimulation had an akinetorigid form of the disease, without preoperative tremor 11 years after onset of the disease. Six of them experienced the appearance of tremor six months after bilateral STN stimulation when the stimulator was switched off in the Off medication state. This de novo parkinsonian tremor was improved by l-dopa treatment and disappeared when the stimulator was turned on.

CONCLUSION

This finding suggests that infraclinical parkinsonian tremor is probably present in all PD patients.

Current approaches to the treatment of Parkinson's disease

Enormous progress has been made in the treatment of Parkinson's disease (PD). As a result of advances in experimental therapeutics, many promising therapies for PD are emerging. Levodopa remains the most potent drug for controlling PD symptoms, yet is associated with significant complications such as the "wearing off" effect, levodopa-induced dyskinesias and other motor complications. Catechol-o-methyl-transferase inhibitors, dopamine agonists and nondopaminergic therapy are alternative modalities in the management of PD and may be used concomitantly with levodopa or one another. The neurosurgical treatment, focusing on deep brain stimulation, is reviewed briefly. Although this review has attempted to highlight the most recent advances in the treatment of PD, it is important to note that new treatments are not necessarily better than the established conventional therapy and that the treatment options must be individualized and tailored to the needs of each individual patient.

Mechanisms and targets of deep brain stimulation in movement disorders

Chronic electrical stimulation of the brain, known as deep brain stimulation (DBS), has become a preferred surgical treatment for medication-refractory movement disorders. Despite its remarkable clinical success, the therapeutic mechanisms of DBS are still not completely understood, limiting opportunities to improve treatment efficacy and simplify selection of stimulation parameters. This review addresses three questions essential to understanding the mechanisms of DBS. 1) How does DBS affect neuronal tissue in the vicinity of the active electrode or electrodes? 2) How do these changes translate into therapeutic benefit on motor symptoms? 3) How do these effects depend on the particular site of stimulation? Early hypotheses proposed that stimulation inhibited neuronal activity at the site of stimulation, mimicking the outcome of ablative surgeries. Recent studies have challenged that view, suggesting that although somatic activity near the DBS electrode may exhibit substantial inhibition or complex modulation patterns, the output from the stimulated nucleus follows the DBS pulse train by direct axonal excitation. The intrinsic activity is thus replaced by high-frequency activity that is time-locked to the stimulus and more regular in pattern. These changes in firing pattern are thought to prevent transmission of pathologic bursting and oscillatory activity, resulting in the reduction of disease symptoms through compensatory processing of sensorimotor information. Although promising, this theory does not entirely explain why DBS improves motor symptoms at different latencies. Understanding these processes on a physiological level will be critically important if we are to reach the full potential of this powerful tool.