Speech Effects of Deep Brain Stimulation for Essential Tremor

Deep brain stimulation (DBS) is a treatment for essential tremor (ET) that used implanted electrodes in the brain to intervene in a very small part of the thalamus called the VIM nucleus. This nucleus acts as a “relay station.” It receives messages from the cerebellum, which is an area of the brain that regulates motor movements. These transmissions travel along a physical pathway in the brain. When they reach the VIM nucleus, it forwards them to yet another area called the motor cortex. From there, the signals are directed to the limbs and other body parts involved in movement.

Scientists know that the abnormal signals that lead to ET arise in the cerebellum, but so far no one knows exactly why they start in the first place. However, those who specialize in neuroscience discovered that by isolating and intervening in the VIM nucleus, it’s possible to stop hand tremors. That’s good news. Even better, doing so does not cause any major harm in the brain. A DBS patient regains hand control without any apparent loss of other limb movement.

Speech effect of DBS

However, an unknown percentage of DBS recipients may experience minor speech problems. According to a recent (2018) paper, “Acoustic [speech] studies have revealed that patients with Essential Tremor treated with thalamic Deep Brain Stimulation (DBS) may suffer from speech deterioration in terms of imprecise oral articulation and reduced voicing control.” This means that affected patients have trouble clearly pronouncing or enunciating words and syllables, and their voice may become softer. This condition is called dysarthria. Listeners may have difficulty understanding vowels and consonants, and this will be frustrating both for the speaker and the person who hears them.
The purpose of the paper’s authors was to study ET patients who had DBS to determine if the problem was a general slowing down of the speech motor system (involving the larynx, pharynx, tongue, jaw, lips, soft palate, and breath), or a problem coordinating two or more parts of the system. The question is, how does one measure, depict, and record speech abnormalities in such a way as to identify exactly what’s going wrong? To do so, they used a device called an electromagnetic articulography.

Electromagnetic articulography

Technology, it seems, can be devised to respond to just about any clinical need. The electromagnetic articulography is used in such fields as neurology, neurophysiology, phonetics and even dentistry to analyze both the acoustics (sound) and mechanics of speech problems. Tiny sensors equipped with transmitters are placed in several locations in the mouth and lips. Each of these will capture and send information about the movements of the parts they are connected with. At the same time, the person speaks into a microphone, and the articulograph simultaneously correlates the sound and pronunciation with a 3-D graph of the associated movements.

For purposes of the German study, the research team enrolled 12 ET patients who had DBS at least 4 months previously, and who were observed to have dysarthria following surgical implantation. In addition, 12 age-matched controls with no dysarthria were also enrolled. During the study sessions, the DBS patients were evaluated by articulography (sensors placed on the upper and lower lip, tip of the tongue, tongue blade, and the back of the tongue). While thus connected with the electromagnetic articulography, they were recorded under two alternating conditions: a) their DBS was switched on, using their normal regulation, and b) their DBS was switched off. This gave the researchers a basis for comparing the effects of DBS for each patient, as well as comparing patients with their controls.

Findings

It is not surprising that the team identified two problematic areas:

• Compared with controls, ET patients had more speech problems even when their DBS was switched off, and
• Compared with controls, the ET patients’ speech difficulties worsened when their DBS was switched on, which “likely triggered the additional overall slowing-down of the system…”

The team’s concluding hypothesis was that the dysarthria and slowness contributed to poor performance of syllable production, which may reflect either a pre-exiting cerebellar deficit (which science tells us characterizes ET), and/or the effect of DBS on the pathways between the thalamus and the cortex (outer brain layer containing the motor cortex).

Thanks to the precision of the articulograph, the researchers have contributed a virtual motion picture of speech problems following DBS. However, more such studies need to occur to validate their results.

MRI-guided Focused Ultrasound

The Sperling Neurosurgery Group offers MRI-guided Focused Ultrasound (MRgFUS), a noninvasive alternative to DBS that deadens the VIM by harnessing beams of ultrasound. The outpatient procedure is safe, and results are immediate. For more information, contact Sperling Neurosurgery Associates.

Mücke D, Hermes A, Roettger TB, Becker J et al. The effects of Thalamic Deep Brain Stimulation on speech dynamics in patients with Essential Tremor: An articulographic study. PLoS One. 2018 Jan 23;13(1):e0191359.

Ibid.