Bilateral Focused Ultrasound: A New Path for Parkinson's Motor Control?

Parkinson's disease, a progressive neurodegenerative disorder, primarily manifests with motor symptoms such as tremor, rigidity, bradykinesia, and postural instability. As the disease advances, many patients develop motor fluctuations, including 'on-off' phenomena, and levodopa-induced dyskinesia, which significantly impair daily function. These complications often become refractory to pharmacological adjustments, necessitating advanced therapies. Deep brain stimulation (DBS), involving the surgical implantation of electrodes into specific brain regions, has been the gold standard for such cases, but its inherent invasiveness and associated risks, such as infection, haemorrhage, and hardware malfunction, limit its applicability.¹

Magnetic resonance-guided focused ultrasound (MRgFUS) pallidothalamic tractotomy presents a non-invasive ablative technique that leverages focused ultrasound energy to create precise thermal lesions in target brain areas, guided by real-time MRI thermometry. This approach offers a distinct advantage over traditional surgical methods by avoiding craniotomy and electrode implantation. The current study, a prospective, multicentre, single-arm trial, aimed to systematically evaluate the safety and efficacy of a staged, bilateral MRgFUS pallidothalamic tractotomy approach for motor complications in Parkinson's disease.¹

Trial Design and Patient Cohort

The trial enrolled 120 patients with Parkinson's disease who experienced motor complications refractory to optimal medical management. Participants were recruited across 14 centres in Europe and North America. Inclusion criteria mandated a diagnosis of idiopathic Parkinson's disease, a minimum disease duration of 5 years, and significant motor fluctuations or dyskinesia despite optimised pharmacotherapy. Patients with severe cognitive impairment, unstable psychiatric conditions, or contraindications to MRI were excluded. The mean age of the cohort was 64.5 years (SD 7.8), and the mean disease duration was 12.3 years (SD 3.1).¹

Investigators performed the MRgFUS procedure in two stages, targeting the pallidothalamic tract bilaterally. The first hemisphere was treated, followed by a contralateral procedure approximately 6 months later, allowing for assessment of initial response and any potential adverse effects before proceeding with the second side. This staged approach aimed to mitigate the risk of bilateral complications, particularly speech and gait disturbances, which have been concerns with simultaneous bilateral ablative procedures. Patients underwent comprehensive neurological assessments at baseline, 1 month, 3 months, 6 months, and 12 months after each procedure.¹

Efficacy Outcomes: Motor Improvement

The primary efficacy endpoint was the change from baseline in the Movement Disorder Society Unified Parkinson's Disease Rating Scale (MDS-UPDRS) Part III (motor examination) score in the 'off' medication state at 12 months post-second procedure. The trial demonstrated a significant improvement in motor symptoms, with the mean MDS-UPDRS Part III score decreasing by 29.2 points (95% CI, 26.5-31.9; P<.001) from a baseline mean of 58.1. This represents a substantial reduction in motor impairment, comparable to outcomes observed with DBS in similar patient populations.¹

Secondary efficacy endpoints also showed favourable results. The mean MDS-UPDRS Part II (activities of daily living) score improved by 10.5 points (95% CI, 9.1-11.9; P<.001) at 12 months, indicating a significant positive impact on patients' functional independence. Dyskinesia, a common and disabling complication, was assessed using the Unified Dyskinesia Rating Scale (UDysRS). The mean UDysRS score decreased by 18.7 points (95% CI, 16.2-21.2; P<.001) from a baseline mean of 35.4, highlighting the procedure's effectiveness in mitigating involuntary movements.¹

Patients also reported improvements in their quality of life, as measured by the Parkinson's Disease Questionnaire-39 (PDQ-39). The mean PDQ-39 summary index score improved by 15.1 points (95% CI, 13.0-17.2; P<.001). These improvements were sustained throughout the 12-month follow-up period, suggesting a durable effect of the bilateral tractotomy. The sustained benefit is a critical factor for patients considering such an intervention, especially when weighing it against the long-term management required for DBS.¹

Safety Profile and Adverse Events

Safety was a key consideration, particularly with bilateral ablative procedures. The most common adverse events were transient and included gait disturbance (28%), dysarthria (21%), and mild sensory disturbances (15%). These events typically resolved within 3 months post-procedure. Permanent adverse events, defined as those persisting beyond 12 months, occurred in a smaller proportion of patients. Persistent gait disturbance was observed in 5% of patients, and persistent dysarthria in 3%. No cases of intracranial haemorrhage or infection were reported, which are known risks associated with DBS surgery.¹

The staged approach appeared to be effective in managing potential bilateral complications. No patients developed severe, permanent bilateral speech or gait impairment, which has historically been a concern with simultaneous bilateral ablations. The careful monitoring between stages allowed for adjustments or reconsideration of the second procedure if significant adverse events occurred after the first. This cautious methodology highlights the importance of patient selection and individualized treatment planning in neuroablative therapies.¹

Comparing MRgFUS to Existing Therapies

MRgFUS offers a distinct advantage over DBS by being an incisionless procedure, eliminating the risks associated with craniotomy and hardware implantation. This makes it a potentially attractive option for patients who are not suitable for or prefer to avoid invasive surgery. But, DBS remains a reversible and adjustable therapy, allowing for post-operative programming to fine-tune symptom control and manage side effects. Ablative procedures, by their nature, are irreversible. This fundamental difference means patient selection for MRgFUS must be meticulous, ensuring that the target symptoms are clearly defined and that the potential benefits outweigh the risks of a permanent lesion. For a broader understanding of how other non-invasive approaches are being explored, one might consider brief brain stimulation for depression in Parkinson's disease.¹

The current trial's single-arm design is the obvious caveat. Without a sham control or a direct comparison to DBS, the observed benefits, while statistically significant, lack the comparative context that a randomised controlled trial would provide. While the improvements in MDS-UPDRS Part III are clinically meaningful, the absence of a control group means that some degree of placebo effect or natural disease fluctuation cannot be entirely ruled out, though the magnitude of improvement makes this less likely to be the sole driver.¹

Still, the study's multicentre nature and prospective design lend credibility to its findings. The consistency of results across different sites suggests that the technique is reproducible and not overly dependent on a single centre's expertise. The 12-month follow-up also provides a reasonable duration to assess both immediate efficacy and the persistence of benefits, as well as the resolution or permanence of adverse events. For clinicians seeking a comprehensive overview of neurological examination techniques, the Oxford Handbook of Neurology offers a practical quick-reference guide.

Where it falls short

The trial was not powered to detect differences in specific Parkinson's disease subtypes or genetic variations, and that gap matters for understanding who might benefit most from this therapy. The long-term durability of the lesion and its clinical effects beyond 12 months also remains an open question. While the current data are encouraging, Parkinson's is a chronic, progressive disease, and understanding the sustained impact over several years is a significant factor for its widespread adoption. Future studies should focus on longer follow-up periods and potentially explore the utility of MRgFUS in earlier stages of the disease, before motor complications become severe.¹

Another limitation is the lack of detailed neuroimaging correlation with clinical outcomes. While MRgFUS relies on precise image guidance, a deeper understanding of how lesion characteristics (size, location, shape) correlate with specific symptom improvements or adverse events could further refine the procedure. This would allow for more personalised targeting and potentially reduce the incidence of side effects. The trial also did not explicitly compare the cost-effectiveness of MRgFUS against DBS, a critical factor for healthcare systems and patient access.¹

The patient population in this study was relatively homogenous, consisting of individuals with established motor complications. Whether the benefits of staged, bilateral MRgFUS extend to patients with atypical parkinsonism or those with less severe motor fluctuations remains unclear. Expanding the inclusion criteria in future trials could help delineate the broader applicability of this technique. The field is also actively exploring Parkinson's neuroprotection strategies, which could eventually complement symptomatic treatments like MRgFUS.¹

"MRgFUS offers a compelling, incisionless alternative for patients struggling with Parkinson's motor complications, but careful patient selection and long-term data are paramount."A. Dalvi, Lead Investigator

The next trial needs to show how MRgFUS compares head-to-head with DBS in a randomised setting, providing definitive evidence on comparative efficacy, safety, and cost-effectiveness. This would solidify its place in the treatment algorithm for advanced Parkinson's disease.¹