Focused ultrasound
A thousand ultrasound beams cross the bone without damaging it and converge on a point a few millimetres across. It is one of the few things in functional neurosurgery tested against a sham procedure. And it is an ablation: there is no going back.
The principle has a physical elegance uncommon in medicine. A helmet with more than a thousand transducers emits ultrasound beams that cross the skull from different directions1. Each beam on its own is harmless. Only where they all converge does the energy accumulate enough to raise tissue temperature and produce a lesion a few millimetres across. Magnetic resonance measures that temperature in real time throughout the procedure and feeds the information back to the system, which corrects as it goes. The patient is awake and responsive throughout.
Why this evidence is different
In functional neurosurgery, sham-controlled trials are extremely rare, and for good reason: it is difficult to subject someone to a simulated operation. Focused ultrasound, requiring no incision, made them possible. And they were done three times.
In essential tremor, a randomised double-blind trial published in 2016 compared ultrasound thalamotomy against a sham procedure2. The most compelling argument in that work is not the size of the treated group's improvement: it is what happened in the control group. At three months, it had barely moved. Two trials with the same methodology followed in Parkinson's disease, one targeting the subthalamic nucleus3 and one the globus pallidus4, both positive and both — this time — with a considerable response in the sham arm, an uncomfortable reminder of how much placebo the motor scales of this disease carry.
Durability, told properly
At five years, tremor in the treated hand remains improved by more than two-thirds from baseline5. But the composite score including global motor function falls from somewhat over half at one year to around forty per cent at five, and functional disability deteriorates in parallel. This is not a cure: it is a stable lesion inside a disease that keeps advancing.
The reassuring side of the ledger: that follow-up showed no new, late or progressive adverse effects. Those already present — paraesthesia, some imbalance — remained mild or moderate. An independent series with four years of follow-up reaches equivalent conclusions6.
The second side is not the first again
For years treatment was unilateral, and the obvious question was what happened to the other hemisphere. There is now an answer, and it is nuanced. Treating the second side in staged fashion improves tremor substantially7, and since July 2025 it also carries regulatory approval in Parkinson's disease8. But the risk profile changes: speech and gait disturbances appear markedly more often after the second procedure than after the first, and some persist at one year78. In Parkinson's disease, moreover, the improvement contributed by the second side is considerably smaller than that of the first8.
The skull filter
Not every patient is a candidate, and the reason is purely physical. The ratio between compact and cancellous bone — the skull density ratio — determines how much energy crosses the vault. Around a quarter of assessed patients fall below the historical threshold9, and the current regulatory criterion still excludes those below a certain value8.
The nuance matters, though: in that series, when the thermal target was reached despite an unfavourable ratio, the clinical result was not significantly worse9. The ratio predicts whether the skull will allow heating, not whether the patient will improve.
The other half of the field
There is an entirely different use of the same technology, and it may well have the greatest long-term reach. At much lower intensities, combined with microbubbles injected into the bloodstream, focused ultrasound can open the blood-brain barrier transiently, reversibly and locally. The barrier, which exists to protect the brain, is also why most drugs effective against tumours never reach it.
What is proven is that the barrier opens, closes on its own, can be repeated — even every two days in a paediatric trial10 — and that more drug does get in: several times more in treated than in untreated tissue11. What no controlled trial has yet demonstrated is that this translates into longer survival in a brain tumour or better cognition in Alzheimer's disease. The available studies are early phase, with three, seventeen, twenty patients101112. It is a well-founded promise. It is not a treatment.
What to take away
The phrase "surgery without a scalpel" is both exact and misleading. It is exact: there is no incision, no implant, the patient walks out. And it is misleading because it implies that without an incision there is neither risk nor irreversible consequence. Precisely the opposite is true. Because no electrode remains, there are no parameters to reprogramme, nothing to switch off if the target landed a millimetre away, and no tissue left to study. Nor is it a procedure with nothing to it: it requires shaving the head completely and fixing a stereotactic frame to the skull1.
It is ablation, not neuromodulation. That distinction is not a technicality: it is the difference between a reversible treatment and a definitive one, and it should sit at the centre of the conversation with every patient considering it.