How Safe Is Focused Ultrasound for Essential Tremor?

Focused ultrasound thalamotomy for essential tremor is broadly safe, but it comes with a predictable set of side effects that anyone considering the procedure should understand in detail. The treatment received FDA approval in 2016 and has since accumulated several years of follow-up data from randomized trials and real-world use. A systematic review and meta-analysis found that more than a third of patients experience some kind of procedure-related complaint during or immediately after treatment, though most of those effects are mild and fade within weeks or months. The picture gets more complicated when you look at what persists, what happens when both sides of the brain are treated, and how this procedure stacks up against alternatives like deep brain stimulation.

What Happens During and Right After the Procedure

The most common complaints during the procedure itself are headache and dizziness, driven by the ultrasound energy (called sonication) passing through the skull. These can be intense enough that some patients have had their treatment stopped partway through due to severe head pain. But no sonication-related or head-frame-related effects have been reported to persist beyond three months in the pooled data.

Neurological side effects are a different story and deserve more attention. The immediate pooled rate of ataxia, which covers gait problems and clumsiness in the hands, was around 50% in one meta-analysis. Sensory complications like numbness and tingling appeared in roughly 20% of patients right after treatment.¹ Those numbers sound alarming, and they are high. The reason is anatomical: the target in the brain, called the ventral intermediate nucleus (VIM) of the thalamus, is tiny and sits right next to important fiber pathways. A lesion that slightly overlaps neighboring structures can produce temporary balance problems or numbness.

The landmark randomized trial published in the New England Journal of Medicine reported gait disturbance in about 36% of patients and numbness or tingling in about 38% shortly after the procedure. At one year, those numbers had dropped considerably, but gait problems persisted in roughly 9% and sensory symptoms in about 14%.² So the majority of side effects clear up, but a meaningful minority of patients carry lingering numbness or mild unsteadiness past the one-year mark.

What Persists Over the Long Term

Five-year follow-up data paint a reassuring picture for serious harm. No serious adverse events were recorded at the five-year mark in one of the longest follow-up studies. All remaining side effects at four and five years were classified as mild (about 71%) or moderate (about 29%), and no new procedure-related adverse events appeared after the first year. The lingering effects at five years included numbness or tingling in eight patients, imbalance in six, gait disturbance in two, limb weakness in two, and altered taste in two.³

Another study tracking patients for about four and a half years found that the beneficial effect on tremor is usually maintained, with any worsening of treated-side tremor progressing more slowly than the untreated side. Loss of treatment benefit was described as rare.⁴ That said, the procedure creates a permanent lesion in the thalamus, and the side effects that do stick around are also permanent. You cannot turn them off the way you might adjust the settings on an implanted device.

A separate study focused specifically on gait found that when side effects occurred, 96% were considered mild in severity. The most common persistent complaints were tingling (about 23%) and a subjective feeling of unsteadiness (about 20%). On objective balance testing, scores remained stable for most patients, and only two experienced even mild worsening of balance over the follow-up period.⁵ The gap between what patients reported feeling and what objective tests showed is worth noting: people often felt less steady on their feet even when clinical measurements said their balance was fine.

A Less Common Side Effect Worth Knowing About

Most discussions of focused ultrasound side effects center on numbness and balance, but altered taste (dysgeusia) shows up in the data more often than you might expect. A Japanese case series documented five patients who developed taste changes after the procedure, all of whom also had sensory disturbances. In most cases the taste change was mild, but there have been reports of patients losing enough of their sense of taste to experience significant weight loss.⁶ In the five-year data, taste disturbance lingered in two patients at the final follow-up.³ It is not the most common complaint, but because it affects something as basic as enjoying food, it deserves to be on the radar of anyone weighing the procedure.

Treating Both Sides of the Brain

Essential tremor often affects both hands, which raises the question of whether you can treat both sides. Historically, bilateral thalamotomy (regardless of method) has been considered risky because of higher rates of speech and swallowing problems. Focused ultrasound is no exception, and bilateral treatment is where the safety profile gets the most stretched.

A staged bilateral trial published in JAMA Neurology reported that about 85% of adverse events were mild, about 13% were moderate, and there was one severe event (a urinary tract infection unrelated to the brain procedure). The most commonly reported issues after both sides were treated included numbness or tingling, speech difficulty (dysarthria), ataxia, unsteadiness, and taste changes. At 12 months, eight patients still had numbness, seven had speech difficulty, and six had ataxia, though all were rated as mild. Swallowing difficulty (dysphagia) was notable: seven patients developed abnormal swallowing within a month of the second treatment, though by six months only three still had a significant abnormality.⁷

A smaller bilateral study found one patient with permanent mild speech difficulty and tongue discomfort, but no cases of swallowing problems or cognitive decline after the second treatment. Four out of five patients in that series were satisfied with the bilateral result.⁸ A study specifically examining speech after second-side treatment found that about half of patients experienced lip or tongue numbness immediately afterward, and roughly 8% reported speech changes, though formal speech assessment showed no deviation from baseline during the procedure itself.⁹

The bottom line on bilateral treatment is that it can be done with a staged approach and careful monitoring, but the risk of speech and swallowing effects goes up compared to one-sided treatment. For people whose tremor is severely disabling on both sides, this tradeoff may be worth it, but it needs to be discussed openly rather than assumed to carry the same risk profile as a single-side procedure.

Cognitive and Psychological Effects

One concern patients raise is whether creating a lesion in the thalamus could affect thinking or memory. The data here are broadly reassuring. A study tracking cognitive outcomes found no meaningful decline in overall cognitive function. In fact, the overall cognitive status of the group improved slightly after treatment, as did anxiety levels and quality of life. Frontal and executive functions, verbal fluency, memory, and problem-solving abilities showed no change.¹⁰

A separate analysis did find that five patients showed minor decline in verbal memory after surgery, but four of those already had mild cognitive impairment before the procedure. The one patient who lost function in both verbal memory and executive ability was an exception in the dataset. Meanwhile, memory function across the whole group actually improved to a statistically significant degree, and four patients with pre-existing mild cognitive impairment showed cognitive improvements in areas like language and visual memory.¹¹ The improvement in cognitive test scores likely reflects the well-known fact that severe tremor interferes with testing itself: when your hand shakes less, you perform better on tasks that require writing, drawing, or steady hand movements.

How It Compares to Deep Brain Stimulation and Radiosurgery

Focused ultrasound does not exist in a vacuum. The two main surgical alternatives for medication-resistant essential tremor are deep brain stimulation (DBS) and stereotactic radiosurgery (such as Gamma Knife). Each has a different risk profile, and understanding the tradeoffs matters for anyone deciding between them.

DBS involves implanting electrodes in the brain and a pulse generator under the skin, which means it carries the risks of any open brain surgery: infection, bleeding, and hardware-related complications. The advantage is that it is adjustable and potentially reversible. If side effects develop, settings can be changed. Focused ultrasound avoids hardware entirely, which eliminates the risk of infection at implant sites and intracranial hemorrhage from electrode placement. But because it creates a permanent lesion, its effects cannot be dialed back if something goes wrong.¹²

A head-to-head comparison found that both treatments produced significant and sustained tremor improvement at one year, along with improved quality of life. The focused ultrasound group had higher rates of persistent neurological side effects (numbness, balance problems), while the DBS group had higher rates of surgery-related and hardware-related adverse events, including intracranial hemorrhage.¹³ The nature of the risks differs: DBS risks are surgical and mechanical, while focused ultrasound risks are neurological and lesion-related.

Compared to stereotactic radiosurgery, focused ultrasound had a higher one-year adverse event rate, particularly for balance and gait disturbances (about 10.5%) and sensory changes (about 8.3%). Radiosurgery’s most common persistent side effects were different in character: weakness on the opposite side of the body (about 2.7%), often accompanied by speech difficulty (about 2.4%).¹⁴ Radiosurgery also has the drawback of delayed effect (it takes weeks to months for the lesion to develop), which means side effects can appear long after the treatment day, making them harder to predict or control in real time.

Skull Density and Who Is a Good Candidate

Not everyone’s skull is equally suited for focused ultrasound. The procedure works by focusing many beams of ultrasound energy through the skull to heat a precise spot inside the brain. If the skull is too thick, too dense in some areas and thin in others, or has an unfavorable composition, the energy does not converge properly. A measurement called the skull density ratio (SDR) is used to screen candidates.

Research has shown that patients with a higher SDR are more likely to reach the target temperature needed to create an effective lesion. One study found that the target temperature was achieved significantly more often in patients with an SDR of 0.45 or above. Interestingly, adverse event rates were actually lower in the group with an SDR below 0.45, but that is likely because the ultrasound energy was not penetrating as effectively, meaning fewer patients in that group got a full-strength treatment.¹⁵ A case-matched analysis confirmed that patients with low SDR needed higher power, more energy, and longer sonication times, and still ended up with lower peak temperatures and smaller lesion volumes.¹⁶

What this means practically is that if you are being evaluated for focused ultrasound, your skull composition will be assessed with a CT scan beforehand. A low SDR does not necessarily rule you out, but it means the procedure may be less effective or may require more aggressive energy delivery, and the team should discuss that candidly.

The Operator Learning Curve

Focused ultrasound thalamotomy is a technically demanding procedure, and the skill of the treating team matters. A study that tracked one center’s experience across three chronological groups of patients found a clear learning effect: complication rates and tremor relapse rates both dropped as the team gained experience. In the earliest group of patients, nine developed complications and seven experienced tremor recurrence. In the most experienced group, five had complications and only two had a relapse, even though the length of hospital stay stayed about the same across all three periods.¹⁷

This is relevant because focused ultrasound for tremor is still a relatively new procedure at many centers. If you are choosing where to be treated, the volume of cases a center has performed is a reasonable proxy for how refined their technique is. A center early in its learning curve is more likely to have the complication rates seen in earlier studies, while a high-volume, experienced team may achieve results closer to the best-case data.

When Tremor Comes Back

Focused ultrasound does not always provide a permanent fix. Some patients experience tremor recurrence months to years after treatment. In one case report, a patient saw initial improvement that faded within a year, with significant progression of tremor on both the treated and untreated sides.¹⁸ Another study looking specifically at early re-emerging tremor found that while tremor intensity dropped by about 85% at 24 hours, that benefit had already started to erode at one month, with some measures of tremor reduction falling substantially.¹⁹

The longer-term data suggest that early recurrence is not the norm but is not vanishingly rare either. The learning curve data mentioned earlier showed that relapse rates dropped as operators became more experienced, suggesting that targeting precision plays a role. When tremor does return, the options include a repeat focused ultrasound treatment or conversion to deep brain stimulation. The choice depends on the individual’s anatomy, the reason for the recurrence, and what the patient is willing to go through again.

Quality of Life Gains and What They Mean for the Risk Calculation

Safety data are only half the equation. The reason people accept these risks is that essential tremor can be profoundly disabling, and focused ultrasound can make a dramatic difference in daily function. The multicenter randomized trial found that hand tremor improved by about 47% at three months compared to just 1% for patients who received a sham procedure. That benefit decreased to about 40% improvement at one year, but quality of life and disability scores improved meaningfully alongside tremor reduction.²⁰ Earlier open-label work showed even larger improvements in hand tremor, on the order of about 75%.

The cognitive outcome study found that anxiety dropped significantly and quality of life scores improved dramatically after treatment.¹⁰ These improvements are not trivial. Essential tremor can make it impossible to eat with a spoon, write a signature, or drink from a glass without spilling. When medication stops working, the choice is not between a risk-free life and a procedure with side effects. It is between living with worsening disability and accepting a treatment whose most common persistent effects are mild numbness and slight unsteadiness. For most patients, the math favors treatment, but only after a thorough conversation about what “mild” and “persistent” actually look like in daily life.

Lessons From Parkinson’s Disease Treatment

Though this article focuses on essential tremor, focused ultrasound thalamotomy has also been studied in tremor-dominant Parkinson’s disease, and the safety findings from that population offer additional perspective. In a randomized trial of Parkinson’s patients, early experience revealed that heating spread to a nearby structure called the internal capsule, causing mild weakness on one side of the body in two patients (about 8%). That complication improved, and it prompted the research team to add an additional monitoring axis during the procedure to watch for off-target heating. Other persistent effects in that trial included facial tingling in about 20%, finger tingling in about 5%, and balance problems in about 5%. The Parkinson’s experience illustrates an important point about the procedure’s safety trajectory: complications from early treatments fed directly into protocol refinements that reduced risk for subsequent patients. The procedure is still evolving, and the safety data from 2016 do not fully represent the safety of the procedure as practiced in 2025.