Harnessing Bimodal Stimulation to Treat Somatic Tinnitus
Summary by John Coverstone, AuD
Bimodal stimulation for tinnitus has been studied for years by a handful of research groups around the world and shows some promise in reducing tinnitus symptoms. One such group was led by Susan Shore, PhD, at the University of Michigan. Her lab previously showed increased treatment benefit from using bimodal stimulation, which refers to simultaneous auditory and electrical stimulation. The research team calls this bisensory stimulation in their most recently published paper.1
As bimodal stimulation investigations continue, using it with more specific types of tinnitus will be investigated. This may help develop more targeted treatments for different types of tinnitus, which may make the treatment process more efficient and effective. Shore’s group has studied the effect of bimodal stimulation for individuals with somatic tinnitus. Somatic tinnitus is affected by movement of the head or neck, sometimes including facial movements. Movement may change tinnitus loudness, pitch, or even the type of sound. Certain nerves in our bodies provide sensory awareness of our movements—basically, a sense of touch for movements we make. This sensory feedback for our movements is called the somatosensory system (hence, “somatic” tinnitus). Somatosensory signals are known to integrate with the auditory system in the brainstem, specifically in the dorsal cochlear nucleus. This is also a location that neuroscience research has implicated in generating—or at least being involved in—tinnitus.
Therefore, scientists have theorized that sensory signals from the somatosensory system affect or modulate tinnitus in the cochlear nucleus, altering what people hear. Because bimodal stimulation provides both auditory and sensory (mild electrical) stimulation, it may have applications to improve somatic tinnitus.
For this study, a double-blind, randomized clinical trial, recruited 99 participants with somatic tinnitus. Participants needed to have no more than moderate hearing loss and sufficiently bothersome tinnitus with no prior treatment during the last six months. Twenty-six participants were dropped from the study owing to unavailability, changes in eligibility, or noncompliance. In addition, 10 participants stopped treatment, 3 because no benefit was found and 7 because it made their tinnitus worse.
Auditory stimuli were matched to each participant’s tinnitus and presented at 40 dB above their hearing threshold. Participants then took home a portable device for treatment. The device included two electrodes that were placed at C1 and C2 (the first two vertebrae of the cervical spine, just below the skull), with exact placement determined by evaluation during the fitting and training process. Electrical stimulation was presented every 10 seconds during the treatment session.
The participants were separated into two groups, with one group receiving bimodal stimulation and the other group receiving auditory stimuli only—something previously shown to have no residual effect and therefore used as a control. Daily treatments lasted 30 minutes each and continued for six weeks. (This is longer than the four-week treatment period that had been used in a pilot study, which had shown improvement in tinnitus symptoms after four weeks, with a return to baseline during the subsequent rest period.) After six weeks of treatment, each group had a six-week resting period to remove any influence of the previous stimulation on the second phase and to monitor the effects of treatment afterward. During the second six-week treatment period, groups swapped treatments from active bimodal stimulation to auditory-only (control) stimulation, and vice versa. By doing this, half the participants received bimodal stimulation first, and half received it second, with the other group serving as a control during that time. This allowed the researchers to look for differences when people received bimodal stimulation first versus those who received it second.
When analyzing results, the researchers found that the additional two weeks of treatment (from four weeks to six weeks in each phase) resulted in longer-lasting effects as measured by loudness of tinnitus and improvement of scores on the Tinnitus Functional Index (TFI). In fact, the improvements in tinnitus continued after treatment and throughout the rest period. As a result, the researchers used only the first 12 weeks (six weeks of treatment, six weeks of rest) in their primary analysis because they were concerned that the continuing benefits of treatment would affect the results of the second phase of the study.
Overall, data showed that both the auditory-only and the bimodal stimulation treatments resulted in lower TFI scores, but only bimodal stimulation showed clinically significant decreases in TFI scores of 12 points or greater. The control group receiving only auditory stimulation had improvements of approximately 8 points on the TFI, whereas those receiving bimodal stimulation improved approximately 17 points among the 56 people who completed treatment. When those who dropped out of the study (those not following up or those showing no or adverse benefit) were included in the analysis, improvement was closer to 14 points.
Looking further at the data, 53 percent of people receiving bimodal treatment had a 13-point or greater improvement on the TFI. Among those receiving auditory-only treatment, 20 percent had a significant improvement on the TFI. This demonstrates the possibility of either treatment benefiting people with somatic tinnitus but with bimodal stimulation having a greater chance of improving tinnitus symptoms. Without further studies and other controls in a larger population, it cannot be known why these effects exist. Is the entire effect of the auditory-only group a result of the placebo effect or is there some benefit to auditory-only stimulation that is enhanced when properly directed electrical stimulation is also used?
The Tinnitus Handicap Inventory (THI) was also used to measure tinnitus improvement, and participants showed a positive change through treatment by maintaining a stable score over the six-week rest period. For all three measures—loudness scale, TFI, and THI—the largest improvement in tinnitus, by far, occurred in the first week for each measure.
These results may demonstrate a potential greater effect for longer treatment periods. However, it should be noted that none of the participants in this study were enrolled in a four-week treatment course, as per the pilot study, and therefore some question regarding the true effect of treatment length remains for future research.
The study showed the continuing promise of a bimodal protocol for tinnitus treatment, but with certain caveats. It should not be ignored that, although 53 percent of people receiving bimodal stimulation experienced improvement in tinnitus, 7 percent of people experienced a worsening of their tinnitus. This illustrates the concerns of many researchers and clinicians that we lack information to fully qualify individuals for specific treatments. It was noted that the number of people dropping out because their tinnitus worsened was roughly equal (3 versus 4) in the bimodal treatment group and the control group, suggesting that factors other than the treatment itself may have played a role. Participants were screened for medical conditions and medications, including those associated with mental health disorders. However, no measure of anxiety or depression was given to screen participants for undiagnosed or subclinical existence of either condition, which are well known to have significant effects on people with bothersome tinnitus and may be one reason why 7 percent of participants experienced adverse reactions.
Differences between those with bilateral (both ears) versus unilateral tinnitus were not explored and it may not have been possible to do so in a meaningful way without recruiting a larger group of participants. Also, no differentiation was made of the kind of somatic stimulation that modulated tinnitus, perhaps also because of the smaller sample size. However, it is worth considering that someone who can modulate tinnitus only through neck movements may not respond the same as someone who modulates tinnitus only through facial movements.
The fact that over half of participants with somatic tinnitus experienced benefit from bimodal treatment is encouraging. A larger study may be able to look at other effects and help clinicians using this emerging technology to provide more directed treatment for those who are the best candidates for this therapy.
Dr. Shore and the co-first author of the study, David Martel, PhD, patented the device and cofounded Auricle to commercialize it. Auricle is working to receive FDA approval before releasing the device for tinnitus treatment.
Reference
1. G. R. Jones, D. T. Martel, T. L. Riffle, et al. (2023). Reversing synchronized brain circuits using targeted auditory-somatosensory stimulation to treat phantom percepts. JAMA Network Open, 6 (6). https://doi.org/10.1001/jamanetworkopen.2023.15914