Would you trust your brain to a neurosurgical treatment you can’t see, hear, and that doesn’t require cutting or drilling? That describes a revolutionary way to control hand tremors caused by a movement disorder called essential tremor (ET). This innovative treatment is MRI-guided Focused Ultrasound (MRgFUS or simply FUS), and it works by deadening a very tiny area of the thalamus deep in the brain. The precise location, the VIM nucleus, has the job of forwarding movement messages from their origin in the cerebellum to parts of the brain that coordinate movement.
Unfortunately, the VIM nucleus can’t tell the difference between normal and abnormal messages. This means that if the signals have been corrupted or distorted at their source (no one knows exactly why this happens) they can result in tremors of the hands, head, voice or other part of the body.
Blocking abnormal signals
For decades, neurosurgeons have known that intervening in the VIM nucleus can stop tremor messages to the hands by blocking its function. Until FUS came along, the only approach was to drill a hole in the skull and physically insert something into the brain. There were two methods:
- Radiofrequency thalamotomy – Uses image guidance to place a slender probe with electric wires through the brain into the VIM nucleus. When activated, electric current ablates (destroys) the VIM nucleus with intense heat. The probe is then removed. Although largely effective, there was a relatively high rate of complications.
- Deep Brain Stimulation (DBS) – Uses image guidance to place electrodes in the thalamus. They are connected by wires to a “powerpack” implanted below the collarbone in the chest wall, which sends electric messages to the electrodes to block tremor signals. Where thalamotomy destroys tissue, DBS does not, and it is reversible if necessary. Better long-term effectiveness than thalamotomy, but about 20% risk of complications even at 1 year after placement.
FUS is invisible and noninvasive
Unlike the physical penetration of probes or electrodes, FUS uses invisible “beams” of ultrasound energy (soundwaves that can’t be heard by human ears). There is no physical invasion of skin or skull. Instead, a special type of helmet over the top and sides of the head produces 1,000 beams, all aimed from different directions at the VIM nucleus. Each of these treatment soundwaves moves through the skull and the brain’s tissues as harmlessly as the imaging soundwaves used to “see” a fetus developing in the womb; however, different wavelengths are used for treatment than for imaging. When the treatment beams are simultaneously focused on the VIM nucleus, they precisely destroy it by generating heat. You can think of it as a “virtual thalamotomy” but without the complications of radiofrequency thalamotomy since the rest of the brain is not physically disturbed, and the tiny destruction zone is more precise.
Location, location, location
But if it’s invisible, how can the neurosurgeon trust the accuracy of FUS to destroy the VIM nucleus without doing collateral damage to adjacent brain tissue? A December 2018 published study addresses that question.i The paper introduces the issue by acknowledging that since FUS for ET is a recent neurosurgical innovation, the “size and location of therapeutic lesions producing the optimal clinical benefits while minimizing adverse effects” had not been firmly established.
Knowing that location is all-important, the authors studied the case records of 66 persons with ET who had been treated with FUS between 2012 and 2017. They turned to the awesome imaging ability of multiparametric MRI of the brain to correlate the size and location of the treatment areas with treatment effectiveness (degree of tremor control) and any acute (short term) post-treatment complications. They also used the imaging scans to analyze whether the treatment lesions had overlapped with nearby major brain structures.
Thanks to their work, they were able to confirm that the best location for maximum benefit with minimum-to-no side effects is the posterior (rear) portion of the VIM nucleus. In addition, they identified the adjacent areas linked with possible side effects immediately following treatment – most of which diminish over time (e.g. sensory, motor, gait or speech effects). In addition, they also determined that lesions larger than 170 mm3 increased the risk of side effects. The authors conclude with confidence that their findings will help “refine current MRgFUS treatment planning and targeting, thereby improving clinical outcomes in patients.”
This is a very important study because it defines both what to treat, and what not to treat. In other words, it’s all about location, location, location. But since the ultrasound itself is invisible, how can its precision be confirmed in real time? Credit goes to MRI scanning during treatment activation. Very specialized software called thermometry allows the neurosurgeon to “see” exactly the area that is heating as the soundwaves converge (meet) at the target. The software is designed to register the temperature of the heat itself, so it’s not too little (less tremor control) or too much (risk of side effects).
Trusting your brain to experience
Indeed, the MRgFUS essential tremor treatment has already undergone many refinements and improvements since the FDA approved it in July 2016. Patients who undergo FUS can trust it is founded on research, advanced imaging technology, device development, and science. Just as important is the experience of the neurosurgeon. At the Sperling Neurosurgery Associates, our team is made of up expert physicians, physician assistants, nurses and technicians. We are confident that you can trust your brain to FUS and to US! For more information or to set up a consultation, visit our website.
iBoutet A, Ranjan M, Zhong J, Germann J et al. Focused ultrasound thalamotomy location determines clinical benefits in patients with essential tremor. Brain. 2018 Dec 1;141(12):3405-3414.