Focused Ultrasound (FUS) Thalamotomy
FUS thalamotomy is an incisionless, MRI-guided thermal ablation procedure that uses hundreds of converging ultrasound beams to precisely destroy tissue in the ventral intermediate nucleus (VIM) of the thalamus, primarily used to treat medication-refractory essential tremor without requiring skin incisions, skull openings, or physical brain penetration. 1
How the Technology Works
MRgFUS combines high-intensity focused ultrasound with real-time MRI guidance, allowing continuous visualization and temperature monitoring during tissue ablation. 1 The procedure heats and destroys the targeted VIM tissue at the focal point where hundreds of ultrasound beams converge, while thermographic imaging superimposed on patient-specific anatomy provides real-time control by monitoring tissue temperature throughout the ablation. 1
The key advantage over traditional radiofrequency thalamotomy is avoiding open brain surgery—no skin incision, no bone craniostomy (twist drill hole), and no physical probe (typically 1-2 mm diameter) traversing brain tissue toward the target. 1
Clinical Indications
FUS thalamotomy is indicated for patients meeting ALL of the following criteria: 1
- Confirmed diagnosis of essential tremor
- Failed treatment with at least 2 medications (one must be first-line: propranolol or primidone) due to lack of efficacy, intolerance, or medical contraindication 1
- Appendicular tremor significantly interfering with quality of life 1
Surgical intervention should be considered when medications fail at maximum doses, cause dose-limiting side effects, are contraindicated due to medical comorbidities (such as beta-blockers in COPD patients), or create occupational limitations. 1, 2
Absolute Contraindications
The following are strict contraindications: 1
- Bilateral MRgFUS thalamotomy (cannot be performed bilaterally)
- Contralateral to a previous thalamotomy
- Inability to undergo MRI for medical reasons
- Skull density ratio <0.40 (ratio of cortical to cancellous bone)
Efficacy and Durability
Tremor improvement is immediate and sustained long-term. In the pivotal randomized controlled trial, tremor scores improved by 53% at 1 year and 56% at 2 years, with similar sustained improvements in disability scores. 1 At 4-year follow-up, hand tremor improvement remained at 56% with sustained improvement in disability (63%), postural tremor (70%), and action tremor scores (63%). 1
In clinical practice studies, immediate cessation of tremor in the treated hand occurred in all patients following treatment. 3 At 6 months, CRST scores in essential tremor patients decreased from 40.7 to 8.2 (p<0.001), and quality of life scores improved dramatically. 3
For Parkinson's disease tremor, FUS thalamotomy also demonstrates efficacy, with UPDRS motor scores decreasing from 24.9 to 13.4 at 6 months (p=0.009) and quality of life improvements sustained over follow-up periods extending to 5 years. 3, 4, 5
Safety Profile and Adverse Events
MRgFUS has a superior safety profile compared to other surgical tremor treatments, with a complication rate of 4.4% versus 11.8% for radiofrequency thalamotomy and 21.1% for deep brain stimulation. 2, 6
Early adverse effects are common but mostly resolve: 1
- Gait disturbance: 36% initially, decreasing to 9% by 1 year
- Paresthesias: 38% initially, decreasing to 14% by 1 year
- Most adverse events are mild or moderate (98.4%), with serious adverse events rare (1.6%)
Transient procedural effects include headache, vertigo, dizziness, nausea, and burning scalp sensation. 3 Post-procedural effects that may persist temporarily include gait ataxia, unsteady feeling, taste disturbances, asthenia, and hand ataxia, but no adverse events lasted beyond 3 months in reported series. 3
Importantly, there are no incidents of hemorrhage or infection, which can occur with more invasive procedures. 1
Treatment Algorithm for Surgical Candidates
For unilateral tremor or patients with medical comorbidities, MRgFUS thalamotomy is preferred due to its lower complication rate. 2, 7
For bilateral tremor involvement, deep brain stimulation should be considered instead, as bilateral MRgFUS is contraindicated. 2, 7
For patients requiring adjustable or reversible treatment (such as younger patients), DBS may be preferred as it offers programmability and reversibility, whereas MRgFUS effects cannot be adjusted or reversed once the ablation is complete. 2
Important Clinical Considerations
The procedure requires adequate skull density (ratio ≥0.40) for ultrasound beam penetration and focusing. 1 Patients undergo an average of 21 sonications with maximal temperatures reaching approximately 56.5°C to create the therapeutic lesion. 3
Tremor recurrence can occur in approximately 20% of patients, though typically to a lesser degree than pre-treatment levels. 3 In long-term follow-up of Parkinson's disease patients, FUS thalamotomy may delay the need for initiating or increasing levodopa therapy. 8, 5
Neuropathological examination after FUS thalamotomy demonstrates a focal lesion characterized by demyelination and neuropil loss with lipid-laden macrophages, but relatively preserved neurons and axons, without lymphocytic infiltration. 9