Parkinson’s Disease (PD) is a progressive neurological condition characterized primarily by movement symptoms such as tremor, rigidity, and slowed movement. While medication is the initial standard approach, some individuals with advanced symptoms require surgical intervention. Focused Ultrasound (FUS) has emerged as a non-invasive alternative for treating motor symptoms associated with PD, particularly tremor. This incision-free procedure addresses neurological circuits responsible for movement dysfunction when conventional pharmacological treatments are inadequate.
The Mechanism of Focused Ultrasound
Focused ultrasound uses multiple low-energy ultrasonic waves precisely directed through the skull. Individually, these waves are harmless to the brain tissue they pass through. A specialized helmet focuses the acoustic energy beams to converge at a single point deep within the brain.
This convergence generates intense, localized heat, known as thermal ablation. The temperature at the focal point rapidly elevates, typically reaching 55 to 60 degrees Celsius, which is sufficient to destroy the targeted tissue. Surrounding tissue remains undamaged because it absorbs only the low-energy beams. This targeted destruction interrupts the abnormal neural circuits that cause movement symptoms.
Magnetic Resonance Imaging (MRI) ensures precision by providing real-time, high-resolution images to locate the anatomical target. The MRI also monitors temperature changes at the focal point during the treatment, allowing physicians to control the ablation process without surgical incision.
Applying Focused Ultrasound to Parkinson’s
For patients with tremor-dominant Parkinson’s Disease, the primary target is the Ventral Intermediate Nucleus (VIM) of the thalamus. The VIM is a cluster of neurons that plays a significant role in transmitting signals that result in involuntary movements like tremor. Ablating this nucleus, a procedure called a thalamotomy, effectively interrupts the abnormal circuit causing the tremor.
The FUS procedure requires the patient to remain fully awake and communicative throughout the session. This allows for immediate, functional feedback, which is particularly beneficial in a movement disorder. Physicians begin by applying a very low-power test pulse to the target site.
After each test pulse, the physician asks the patient to perform specific motor tasks, such as drawing spirals, to assess the immediate effect on the tremor. This real-time assessment confirms correct target location and helps predict potential side effects before permanent ablation. Once optimal placement is confirmed, the therapeutic, high-power ablation is delivered, creating a permanent lesion in the VIM.
Because the lesion is permanent, the procedure is typically performed unilaterally, treating only one side of the brain per session. Treating the left side alleviates symptoms on the right side of the body, and vice versa. This unilateral approach minimizes the risk of balance and speech side effects associated with bilateral lesions. However, staged bilateral treatments are now possible with an interval of several months between procedures.
Patient Eligibility and Expected Outcomes
Not every individual with Parkinson’s Disease is a candidate for focused ultrasound. Candidates typically have tremor-dominant PD that has not responded adequately to standard medications like levodopa. They must also be able to tolerate the long duration of the procedure, remaining still inside the MRI machine.
Patients with severe cognitive impairment, advanced balance issues, or skull properties that impede ultrasound transmission are usually excluded. The treatment is approved for adults who are at least 22 years old.
The expected outcome following a successful FUS procedure is an immediate and substantial reduction in the treated tremor. Studies show many patients experience sustained tremor relief for several years post-treatment, with an average improvement of 50 to 75%. While immediate results are often dramatic, the long-term durability of the effect beyond five years is still being studied due to the relative newness of the procedure.
Potential side effects exist, though many are temporary. Common temporary side effects include:
- Gait disturbances
- Mild numbness or tingling (paresthesia) on the treated side of the body
- Balance issues
In clinical studies, permanent unsteadiness or gait disturbance was reported in up to 10% of patients, and permanent sensory disturbance occurred in 10 to 15% of cases.
Comparing Focused Ultrasound to Other Therapies
Focused ultrasound is a distinct option compared to medication and Deep Brain Stimulation (DBS). Medication management provides broad symptom relief but often results in motor fluctuations or dyskinesia as the disease progresses. When medication proves insufficient, patients typically consider either FUS or DBS.
The primary difference is invasiveness. FUS is an incision-free procedure that does not require hardware implantation. DBS requires invasive surgery to implant electrodes and a pulse generator device. DBS is also adjustable and reversible, allowing physicians to fine-tune the electrical stimulation over time. In contrast, FUS creates a permanent, non-reversible thermal lesion in the brain.
FUS is currently most effective for patients with severe, tremor-dominant Parkinson’s Disease. DBS addresses a wider spectrum of motor symptoms, including rigidity, slowness of movement, and dyskinesia, making it a more comprehensive option. While both treatments show similar efficacy in suppressing parkinsonian tremor, the choice depends on the patient’s overall symptom profile and tolerance for invasive surgery.