FUS for Essential Tremor: How to Get the Same Success with Less Power

When you need to get a job done, you want the best equipment available to accomplish the task as effortlessly and efficiently as possible. Why take more strength and time than you need? But suppose the right gear isn’t available. You’re going to have to use a little creativity and apply some patience, but you’re rewarded when you achieve success.

A similar process recently occurred in the use of MRI-guided Focused Ultrasound (MRgFUS) for the treatment of hand tremors.

How MRgFUS stops hand tremors

Hand tremors originate in an area of the brain called the cerebellum and are transmitted along what is called the tremor pathway. The abnormal signals eventually reach the thalamus. The thalamus has a small center called the VIM nucleus that acts as a relay station, forwarding the tremor signals to the part of the brain that controls hand movement.

MRgFUS is a noninvasive treatment that interrupts the signals by precisely destroying the tiny VIM nucleus. The signals are stopped in their tracks without affecting any other part of the brain! One of the greatest advantages of MRgFUS is that it is noninvasive: it uses 1,000 beams of ultrasound that harmlessly pass through the scalp and the skull until they meet at the VIM nucleus. There, they create enough heat to deaden the VIM nucleus (temperatures in centigrade ranging from 55° to 64° maximum). Each “shot” of ultrasound is called a sonication, and doctors plan the most powerful and efficient number of sonications at the strongest temperatures for each individual case.

When the best-laid plan won’t work

However, the “best” plan isn’t always the right plan for treating an individual’s tremors, and a 2019 paper by a team of Canadian researchers illustrates their creative solution: more sonications at lower temperatures.

Jones, et al. (2019)i conducted a study of 75 essential tremor (ET) patients whose tremors did not respond to medication. All patients were to be followed up and evaluated for tremor control at 3 months and 12 months after MRgFUS. However, 19 of the participants could not be treated by full strength sonications due to “unfavorable skull characteristics” that would not tolerate high power sonications, and/or patient discomfort during sonications. Here was a problem in which the best plan was not the right plan for these 19 individuals.

The research team used this situation as an opportunity to apply a greater number of sonications at lower focal temperatures (from 50° to 54°) in hopes that the ultimate thermal dose based on an accumulated effect would result in the equivalent effectiveness of fewer sonications at higher temperatures. The treatment will take a bit longer, but the outcomes are the same. A good metaphor would be bundling twigs together to achieve the same strength as a sturdy branch. Less efficient, but just as strong.

Thanks to multiparametric MRI imaging before, during and after treatment—and MRI thermometry to monitor temperatures in tissue during the procedure—the team could measure the size of the treatment zone in each patient, and correlate it with the number of lower temperature sonications. When compared with the patients who received the standard higher thermal ultrasound dose, the treatment zones appeared equivalent.

The real proof, however, came through the 3-month and 12-month follow-up evaluations. There were no significant tremor score differences in terms of successful tremor control between the two groups. Furthermore, the accumulated thermal dose at lower temperatures could be used to predict the size of the treatment zones as well as the standard thermal dose could. The authors concluded, “These data will serve to guide future clinical MRgFUS brain procedures, particularly those in which focal temperatures are limited to below 55°C.”

Such research adds important knowledge to the rapidly expanding use of MRgFUS for the noninvasive treatment of ET that does not respond to medication. Particularly in this case, we are assured that creative options can be developed to treat persons with ET who might otherwise be excluded. We salute the Canadian clinicians whose patience and applied science have added a new solution to this treatment.

iJones RM, Kamps S, Huang Y, Scantlebury N et al. Accumulated thermal dose in MRI-guided focused ultrasound for essential tremor: repeated sonications with low focal temperatures. J Neurosurg. 2019 May 10:1-8.

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