Have you ever sat under a household lamp, reading a book by its light, and suddenly the light begins to flicker? At first, the flickering light is only occasional – say, every 10 minutes for about 15 second at a time – so you ignore it and go on reading. But soon the flickering lasts longer, with less time between episodes. Now it’s getting on your nerves. Eventually, the flickering is nearly constant.
You wonder why it’s flickering. Is it the electricity itself? You rule that out by turning on another lamp, and the light is steady. You wonder if it’s the bulb. You replace the flickering bulb with a new one. Darn it, that bulb also flickers. Now you rise to the challenge, determined to figure it out using logic.
You’re not an electrician, but as you analyze the situation you come up with two possibilities:
- Something is wrong in the circuit from the plug to the switch (perhaps the plug itself or worn out wires in the cord), or
- Something is wrong from the switch to the bulb (the switch itself, or the circuit between the switch and the base of the bulb).
While you may not feel confident about taking the lamp apart and putting it back together, you pat yourself on the back for having been smart enough to identify two pathways. To stabilize the light, you just have to figure out which pathway needs attention.
The two brain pathways of essential tremor
Stabilizing the common movement disorder called essential tremor (ET) isn’t exactly as simple as fixing a lamp because the brain is far more complex. However, just as an electric current drives the shine of the light bulb when the pathway is open, the brain has millions of currents that drive all kinds of things, like thoughts, emotions, heartbeats, digestion, and of course movement—when the right current is flowing and the right pathway is open.
Scientists have identified that ET arises from a misfiring current, but they haven’t been able to figure out how to prevent it in the first place. However, they have identified two pathways along which the abnormal current runs, plus an important relay switch that links one pathway to the other.
- The cerebellothalamic tract – The cerebellum (“little brain”) directs motor function (movement programs). It transmits signals along a fiber pathway (tract) to a small center in the brain’s center called the thalamus. Imagine the signals as an electric current and the thalamus as a relay station from the cerebellum to the brain’s outer layer (cortex). The thalamus has been likened to an orchestra conductor, since it relays a great number of different inputs, not just the motor signals, to many areas in the cortex. One component of the thalamus, the VIM nucleus, is responsible for forwarding certain movement signals from the cerebellum to the motor cortex. If The VIM receives dysfunctional signals, that’s what it forwards.
- The thalamocortical pathway – The thalamus has many pathways, not just motor function, that radiate to various parts of the cortex. These pathways act like circuits. They forward sensory information to the cortex, and also integrate information from different sensory functions. The regions thus influenced and coordinated by the thalamus include movement, emotion, auditory (hearing), visual, our sense of our inner selves and the space we occupy, and associations.
What does this have to do with essential tremor?
Knowing about the two pathways helps explain how ET is treated by intervening in the brain itself. We know that the medication prescribed for ET only works for about half of ET patients (even if medication is initially effective, often the dosage is increased as ET progresses, leading to side effects that cause people to stop using drugs).
When drugs fail, the most widely used intervention is an invasive or noninvasive manipulation of the VIM nucleus. Preventing the VIM from forwarding the dysfunctional signals that cause “the shakes” is a physical way to control tremors. It’s like “turning off the switch” without causing any other movement problems! Cutting off the VIM relay either by incisionless MRI-guided Focused Ultrasound (MRgFUS) that deadens the VIM nucleus, or using surgically-implanted electrodes to electrically control the VIM activity (Deep Brain Stimulation, or DBS). Thus, these two methods control tremor by interrupting the thalamocortical pathway.
A different approach is in development at a center in Solothurn, Switzerland. The facility, called SoniModul, uses MRgFUS to treat several functional brain disorders. One of them is ET, but instead of targeting the VIM nucleus, their approach targets the cerebellothalamic tract. In short, they halt the dysfunctional signals from the cerebellum before they reach the thalamus. Both methods appear to give similar results, but it will take years to determine if one is truly superior to and safer than the other.
Fixing the flickering lamp
ET is like the flickering light. One can tolerate it until it becomes more frequent and disruptive. If that happens, is it best to fix the wiring from the plug to the switch (more or less like the cerebellothalamic tract) or the circuit between the switch and the base of the bulb (similar to the thalamocortical pathway)? If either approach safely and lastingly makes the “flickering” go away, then it may come down to which method is most available and easiest to access. In the U.S., the FDA has approved MRgFUS of the thalamus (VIM nucleus) for the control of ET.
The Sperling Neurosurgery Associates offers MRgFUS for essential tremor. Contact us for information.