This post is also available in Dutch.
You may have heard of the ultrasound technique before. It is particularly well known as a tool to visualize fetuses during routine prenatal examinations. Ultrasound is also more broadly used for medical imaging as it can be used to visualize muscles, tendons, and many internal organs. However, apart from being a diagnostic tool, a specific form of ultrasound, focused ultrasound, is also used to treat different forms of tremor.
Many treatments for tremor aren’t enough
Tremor, which is the involuntary, rhythmic movement of a part or limb of the body, is often caused by a problem in brain areas that control movements. These brain regions lie deep within the brain, which is exactly where focused ultrasound comes in handy. While there are other techniques that can be used to manipulate activity in certain brain areas (e.g., some types of brain stimulation), these are usually either limited to superficial areas of the brain or are rather invasive (i.e., require open brain surgery). Another huge problem that tremor treatments have in general, and that includes oral medication, is that they are often ineffective and have many side effects. For example, medication available for Parkinson’s disease usually does improve overall motor symptoms, but can be relatively ineffective against tremor compared to other symptoms.
What is focused ultrasound and how does it work?
Focused ultrasound targets a brain nucleus at the base of the brain called the thalamus. The thalamus is involved in coordinating movements, but, in the case of tremor, its activity is disrupted. Faulty signals in the thalamus cause problems and need to be counteracted or suppressed to reduce tremor. Focused ultrasound can be used to inhibit faulty signals in the thalamus without classical open brain surgery, which means it is non-invasive. But, if no real surgery is required, how does focused ultrasound work? As the name suggests, the procedure uses sound waves of very high frequencies (hence ultrasound), too high to be audible for humans. Many sound waves or sound beams of high intensity are emitted by a transducer, which surrounds the head. Each single sound wave can pass through the scalp, skull and brain tissue without exerting any effect. However, all the sound waves are concentrated or focused on a small subpart of the thalamus. At that particular point, the convergence of the many sound beams causes the tissue to heat up. As a consequence, the brain tissue of this small nucleus, containing the cells that cause tremor, will be destroyed and a small brain lesion is created.
What are the pros and cons of focused ultrasound?
Focused ultrasound is often the last option for patients if they don’t respond to medication or other treatments. It can be applied with high spatial precision, leads to minimal discomfort, has fewer side effects than other treatments, and patients undergoing focused ultrasound therapy can often leave the hospital on the same day. Additionally, focused ultrasound is very effective in reducing tremor. Although not every patient benefits from this treatment, most patients show a significant reduction of tremor and there are a number of cases where a severe tremor disappears almost completely (watch this video for a real-life example).
Unfortunately, there are also some downsides to this otherwise promising technique. First, focused ultrasound is currently only applied to the thalamus in one half of the brain, thereby only treating tremor on one side of the body. Second, because focused ultrasound has only been applied in humans for a few years now, one of the big concerns of neurologists and neurosurgeons is that we don’t know all possible long-term consequences. There could be unforeseen negative consequences, which will only become apparent after years. Last, we know by now that the tremor reduction achieved with focused ultrasound usually does not last forever. After a few years, the tremors start to reappear and gradually worsen (in some patients faster than in others).
We still don’t know why the effectiveness of focused ultrasound decreases over time and tremor reoccurs. Though we still haven’t found the perfect treatment for tremor yet, focused ultrasound is a promising technique that has improved the lives of many tremor patients.