Vagus nerve stimulation for tinnitus: A review and perspective

Current status of transcutaneous auricular vagus nerve stimulation for tinnitus: a narrative review of modern research

Tinnitus, characterized by phantom sound perception, is a highly disruptive disorder lacking definitive and effective treatments. Its intricate neural mechanisms are not fully understood. Transcutaneous auricular vagus nerve stimulation (taVNS) has demonstrated potential as a substitute or supplementary treatment by activating central vagal pathways. However, standardized therapeutic protocols and objective tests to assess efficacy are lacking. Therefore, taVNS shows promise as a therapy for tinnitus, and treatment protocols should be optimized in future clinical trials.

Effectiveness of transcutaneous vagus nerve stimulation (tVNS) on salicylate-induced tinnitus

INTRODUCTION

Tinnitus is the most common symptom of auditory system disorders. It affects the quality of life of millions of people, but it is still incurable in most cases. Vagus nerve stimulation (VNS) therapy is a potential new treatment for subjective tinnitus. In this study, transcutaneous vagus nerve stimulation (tVNS) combined with tones was utilized to treat salicylate-induced tinnitus since salicylate is a reliable and convenient approach for rapidly inducing tinnitus.

METHODS

Wistar rats were divided into acoustic stimulation alone (AS, n = 6), tVNS alone (n = 6), and tVNS with AS (n = 6) groups for behavioral and electrophysiological tests. They were assessed by auditory brainstem response (ABR), prepulse inhibition (PPI), gap prepulse inhibition of the acoustic startle (GPIAS), social interactions, and aggressive behavior tests at baseline and seven days' post-salicylate (175 mg/kg, twice a day) injection.

RESULTS

The inhibition percentage of the GPIAS test was significantly reduced post-salicylate injection in the tVNS and AS alone groups, while it was not significant in the tVNS with AS group. There was no significant difference in the mean percentage of the GPIAS test between the tVNS groups (with or without AS) after salicylate injections. Social interactions were significantly different in the AS alone group pre- and post-salicylate injections, but they were not significant in other groups. Moreover, the results of aggressive behavior tests showed significantly increased post-salicylate injections in the AS alone group, while they were not significant in the tVNS groups (with or without AS).

CONCLUSIONS

The current study revealed that the application of tVNS alone produced improved social interaction and mood and alleviated salicylate-induced tinnitus severity. Moreover, combining tVNS with acoustic stimulation can prevent salicylate-induced tinnitus.

Transcutaneous cervical vagus nerve stimulation improved motor cortex excitability in healthy adults: a randomized, single-blind, self-crossover design study

Purpose

To investigate the effect of transcutaneous cervical vagus nerve stimulation (tcVNS) on motor cortex excitability in healthy adults.

Method

Twenty eight healthy subjects were assigned to receive real and sham tcVNS for 30 min. The interval between the real and sham conditions was more than 24 h, and the sequence was random. The central and peripheral motor-evoked potential (MEP) of the right first dorsal interosseous (FDI) muscle was measured by transcranial magnetic stimulation (TMS) before and after stimulation. MEP latency, MEP amplitude and rest motor threshold (rMT) were analyzed before and after stimulation.

Results

MEP amplitude, MEP latency and rMT had significant interaction effect between time points and conditions (p < 0.05). After real stimulation, the MEP amplitude was significantly increased (p < 0.001). MEP latency (p < 0.001) and rMT (p = 0.006) was decreased than that of baseline. The MEP amplitude on real condition was higher than that of sham stimulation after stimulation (p = 0.027). The latency after the real stimulation was significantly shorter than that after sham stimulation (p = 0.005). No significantly difference was found in rMT after stimulation between real and sham conditions (p > 0.05).

Conclusion

tcVNS could improve motor cortex excitability in healthy adults.

State of the art: non-invasive electrical stimulation for the treatment of chronic tinnitus

Subjective tinnitus is the perception of sound in the absence of external stimulation. Neuromodulation is a novel method with promising properties for application in tinnitus management. This study sought to review the types of non-invasive electrical stimulation in tinnitus to provide the foothold for further research. PubMed, EMBASE, and Cochrane databases were searched for studies on the modulation of tinnitus by non-invasive electrical stimulation. Among the four forms of non-invasive electrical modulation, transcranial direct current stimulation, transcranial random noise stimulation, and transauricular vagus nerve stimulation yielded promising results, whereas the effect of transcranial alternating current stimulation in the treatment of tinnitus has not been confirmed. Non-invasive electrical stimulation can effectively suppress tinnitus perception in some patients. However, the heterogeneity in parameter settings leads to scattered and poorly replicated findings. Further high-quality studies are needed to identify optimal parameters to develop more acceptable protocols for tinnitus modulation.

Too Blind to See the Elephant? Why Neuroscientists Ought to Be Interested in Tinnitus

A curative therapy for tinnitus currently does not exist. One may actually exist but cannot currently be causally linked to tinnitus due to the lack of consistency of concepts about the neural correlate of tinnitus. Depending on predictions, these concepts would require either a suppression or enhancement of brain activity or an increase in inhibition or disinhibition. Although procedures with a potential to silence tinnitus may exist, the lack of rationale for their curative success hampers an optimization of therapeutic protocols. We discuss here six candidate contributors to tinnitus that have been suggested by a variety of scientific experts in the field and that were addressed in a virtual panel discussion at the ARO round table in February 2021. In this discussion, several potential tinnitus contributors were considered: (i) inhibitory circuits, (ii) attention, (iii) stress, (iv) unidentified sub-entities, (v) maladaptive information transmission, and (vi) minor cochlear deafferentation. Finally, (vii) some potential therapeutic approaches were discussed. The results of this discussion is reflected here in view of potential blind spots that may still remain and that have been ignored in most tinnitus literature. We strongly suggest to consider the high impact of connecting the controversial findings to unravel the whole complexity of the tinnitus phenomenon; an essential prerequisite for establishing suitable therapeutic approaches.