Essential tremor (ET) is the most common movement disorder, affecting millions of people. Roughly half of those with ET either find that medications like Primidone or Propranolol aren’t effective, or they don’t like the side effects. Therefore, many of them consider a neurosurgery procedure called thalamic Deep Brain Stimulation (DBS).
The thalamus is two side-by-side gray masses located approximately centrally in the brain. According to brainmadesimple.com,
The thalamus is similar to a doctor that diagnoses, or identifies, a patient’s disease or sickness. It diagnoses different sensory information that is being transmitted to the brain including auditory (relating to hearing or sound), visual, tactile (relating to touch), and gustatory (relating to taste) signals. After that, it directs the sensory information to the different parts and lobes of the cortex.
Each sense is mediated by its own center, or nucleus, in the thalamus. There are several of them, and each one sends messages to its own dedicated area of the brain’s outer surface (cortex). One such nucleus, the ventral intermediate (VIM) nucleus, is a part of what is called the tremor pathway. When the VIM nucleus is electrically stimulated by DBS or ablated (destroyed) by MRI-guided Focused Ultrasound (MRgFUS), the abnormal signals that travel along the tremor pathway from their starting point in the cerebellum to the VIM nucleus are effectively interrupted so the tremor messages never make it to the motor cortex. This stops tremors from occurring without affecting other movement functions. Therefore, both thalamic DBS and MRgFUS are aimed very specifically at the VIM nucleus.
What is thalamic Deep Brain Stimulation?
Thalamic DBS is an invasive two-part surgery. Since DBS uses electrical stimulation, one part of the surgery involves implanting a battery unit (similar to a pacemaker) under the skin, usually in the chest. Wires that run through the neck connect it to tiny electrodes inserted through a hole in the skull to the VIM nucleus, which is the other part of the surgery. Both the electrodes and battery pack remain in the patient’s brain and chest. The battery pack can be programmed and periodically adjusted for optimum tremor control. While thalamic DBS is highly effective, as with any surgery involving implantation there is a hospital stay, and the surgery has risks of adverse effects, including infection and movement of the electrode from its most effective location. Also, the battery occasionally needs to be replaced.
DBS for one side of the thalamus, or both?
You probably know that the left side of the brain controls the rights side of the body, and vice versa. For a person with tremors in both hands, this means that the left VIM nucleus is relaying tremor signals to the right hand, and the right VIM nucleus is doing so for the left hand.
Someone who has been living with severe, life-impairing hand tremors naturally wishes for normal movement in both hands. To achieve this with DBS would mean having electrode implants on both sides of the thalamus at the VIM nucleus. This approach is called bilateral (“bi” means “both” and “lateral” means “side”) instead of unilateral (just the side that controls the dominant hand). However, bilateral DBS has a higher rate of adverse effects. A recent study of 115 patients who had unilateral DBS and were followed for 6 months found significant tremor improvement that remained durable during the follow-up period. However, a second cohort of 39 patients received a second implant on the opposite side after six months. As it turned out, 32 of them (82%) had surgery-related adverse effects and additional stimulation in unwanted areas.i For now, it appears that unilateral DBS is the safest course.
An alternative to DBS: MRgFUS
While those with medication-resistant tremors may want “the shakes” ended through a neurosurgical procedure, not all of them are comfortable with how invasive DBS is, or with the idea of implants. Now, there is a noninvasive approach to tremor control by destroying, rather than stimulating, the VIM nucleus. Rather than surgically entering the brain through the skull, it uses advanced MRI scanning to identify the target and to plan a treatment using focused beams of ultrasound. It is called MRI-guided Focused Ultrasound, or MRgFUS. When over 1000 tiny rays or beams of high frequency ultrasound are directed to meet at the VIM nucleus, they create a point of heat that is intense enough to ablate the tiny area, yet precise enough to avoid damage to nearby tissue. No hospital stay is required, and there is no risk of infection from cutting or implants.
The same question is raised with MRgFUS: should we treat one or both sides? At this time, the answer is the same – just the side that controls the dominant hand (unilateral MRgFUS). In fact, MRgFUS is FDA-approved only for unilateral treatment. There are clinical trials in Europe to explore a second treatment to the opposite side after a longer period of adjustment from the first treatment. Time will tell whether bilateral MRgFUS has fewer adverse effects than bilateral DBS. However, some neurologic experts are of the opinion that unilateral MRgFUS will ultimately appeal more than unilateral DBS, because MRgFUS is outpatient, effective, and has fewer risks.
Again, only time will tell. For now, Sperling Neurosurgery Associates welcomes and encourages your questions. Please contact us with them!