Transcranial direct current stimulation for the treatment of tinnitus: a review of clinical trials and mechanisms of action

Day-night hyperarousal in tinnitus patients

Tinnitus, which affects 12-30 % of the population, is associated with sleep disturbances and daytime dysfunction, yet the neural mechanisms that link wake-up states remain unclear. This study investigated electroencephalographic (EEG) characteristics of 51 tinnitus patients and 51 controls across wakefulness (eyes-open, eyes-closed, mental arithmetic) and sleep stages (N1, N2, N3, REM) to clarify day-night pathological mechanisms. The key findings showed persistent hyperarousal in tinnitus: wakefulness revealed enhanced gamma power (30-45 Hz) in eyes-closed and task states, while sleep demonstrated elevated gamma/beta power across all stages accompanied by reduced delta/theta power in deep sleep (N2/N3).). An analysis of sleep structure indicates impaired stability in maintaining the N2 stage among tinnitus patients, corroborating a reduction in N3 duration and an increased proportion of the N2 stage. From the wake states to the sleep stages, group × state interactions for the delta/theta power suggest an impaired state regulation capacity in tinnitus patients. Correlation clustering further revealed aberrant integration of wake-related gamma/beta activity into non-rapid eye movement sleep, indicating neuroplastic overgeneralization of wake hyperarousal into sleep. These results extend the so-called loss-of-inhibition theory to sleep, proposing that deficient low-frequency oscillations fail to suppress hyperarousal, impairing sleep-dependent neuroplasticity, and perpetuating daytime symptoms. Furthermore, this study establishes sleep as a critical therapeutic target to interrupt the 24-h dysfunctional cycle of tinnitus.

Tinnitus Rehabilitation Through Electrical Stimulation as a Neuromodulation Method: A Scoping Review

Background

Tinnitus is the perception of sound without any external sound source, affecting about 10-15% of the world's population. Electrical stimulation, especially transcranial direct current stimulation (tDCS), has shown promising and also heterogeneous results in improving tinnitus symptoms. The present study is a review of the clinical effectiveness and ideal stimulation parameters of the tDCS in tinnitus rehabilitation studies.

Methods

The study was conducted based on PRISMA guidelines and through PubMed and Scopus databases and Google Scholar search engines from 1990 to 2024. Only controlled and randomized trials that used tDCS for tinnitus rehabilitation were considered.

Results

Based on the entry criteria, 14 articles were selected. 12 studies reported some degrees of improvement in symptoms following tDCS sessions, while two did not report any improvement. The maximum number of sessions was 10, and the duration of sessions was 10 to 20 min. The usual current intensities were 1-2 mA. The auditory cortex and dorsolateral prefrontal cortex (DLPFC) were the selected sites for electrical stimulation.

Conclusion

The tDCS can be considered an effective technique for tinnitus management in some patients with tinnitus. One of the main challenges in using tDCS for such patients is its standard stimulus parameters. Randomized, double-blind, placebo-controlled trials with a large homogenous sample size are recommended to reach a definitive conclusion about the standard stimulus parameters of tDCS for tinnitus management.

Effects of tinnitus retraining therapy on patients with tinnitus: a systematic review of randomized controlled trials

PURPOSE

This review aims to explore the influences of tinnitus retraining therapy (TRT) on patients with tinnitus.

METHODS

PubMed, MEDLINE, EMBASE, CINAHL, PEDro, SCOPUS, and Web of Science were screened for randomized controlled trials (RCTs) that explored the effects of TRT on patients with tinnitus from inception to June 30, 2024. The methodological quality of the included RCTs was evaluated using the physiotherapy evidence database (PEDro) scale.

RESULTS

Fifteen studies met our eligibility criteria. A total of 2069 patients with tinnitus (mean age 52.93 years; 66% male) were involved in the present review. The quality of the selected studies ranged from 5 to 8 on the PEDro scale, with a median score of 7. TRT did not provide superior effects in treating patients with tinnitus compared to tinnitus masking (TM), tinnitus educational counselling (TED), TRT with open ear hearing aids, Tailor-made notched music training (TMNMT), partial TRT, usual care, and smart TRT.

CONCLUSIONS

TRT intervention is considered a treatment option for patients with tinnitus. Combining TRT with other interventions, such as cognitive-behavioral therapy, pharmacotherapy, or tDS, may show even greater efficacy. Additional studies are strongly required to recognize the long-term effects of TRT on tinnitus, determine who most likely would benefit from the intervention regarding tinnitus type and severity, and identify the optimal treatment protocol.

Effect of Simultaneous Use of Neuromodulation and Acoustic Stimulation in the Management of Tinnitus

Tinnitus is a relatively common disorder with a heterogeneous nature. Combining methods in its treatment may offergreater effectiveness. We aim to explore the impact of concurrently applying tRNS neuromodulation and acousticstimulation for tinnitus control. Thirty-two tinnitus patients participated in this study, divided into two groups. Thefirst group underwent 8 sessions of electrical stimulation (tRNS) and acoustic stimulation simultaneously, while thesecond group received only tRNS. The outcomes were assessed using psychoacoustic evaluation and tinnitushandicap inventory (THI) and visual analog scale (VAS) for loudness and annoyance of tinnitus. The SF-36questionnaire was utilized to evaluate the quality of life before, and immediately after intervention and at one monthfollow-up. A notable reduction in tinnitus loudness was observed in both groups. There were significant differences inTHI scores before and after the intervention for both groups. However, the first group exhibited larger effect sizes forchanges in loudness and THI scores. The scores of the SF-36 questionnaire improved in both groups, the increase ingeneral health and emotional scores was particularly significant in the first group. According to the results of thisstudy, using electrical and acoustic stimulation simultaneously with dual-modality stimulation is more effective inreducing the loudness and annoyance of tinnitus, compared to the use of electrical stimulation alone.

Personalized Sound Therapy Combined with Low and High-Frequency Electromagnetic Stimulation for Chronic Tinnitus

This study investigates a novel multimodal treatment for chronic tinnitus, a condition that significantly affects quality of life, by combining personalized sound therapy with both low- and high-frequency electromagnetic wave stimulation. Conducted at Tor Vergata University Hospital in Rome, the research involved 55 patients and employed a portable medical device for therapy delivery. Treatment effectiveness was measured through the Tinnitus Functional Index (TFI), Tinnitus Handicap Inventory (THI), Visual Analogue Scale (VAS), Hyperacusis Questionnaire (HQ), and Short Form-36 Health Survey (SF-36), encompassing initial sound therapy and subsequent multimodal treatment phases. Remarkably, 73% of participants experienced notable improvements in TFI scores, with 39% reporting a significant enhancement of 13 points or more. This improvement was mirrored in secondary outcomes like THI, VAS, and HQ scores, along with certain SF-36 domains, indicating enhanced life quality and reduced tinnitus distress. The study underscored high compliance and no adverse effects, suggesting the combined therapy's promising potential in chronic tinnitus management. The findings advocate for further research to discern the distinct contributions of each treatment modality, positing that this innovative approach could ameliorate tinnitus symptoms and improve patient well-being, confirming its safety and efficacy.

Repeated Bilateral Transcranial Direct Current Stimulation over Auditory Cortex for Tinnitus Treatment: A Double-Blinded Randomized Controlled Clinical Trial

Transcranial direct current stimulation (tDCS) is a non-invasive and painless technique of brain neuromodulation that applies a low-intensity galvanic current to the scalp with the aim of stimulating specific areas of the brain. Preliminary investigations have indicated the potential therapeutic efficacy of multisession tDCS applied to the auditory cortex (AC) in the treatment of chronic tinnitus. The aim of this study was to explore the therapeutic effects of repeated sessions of bilateral tDCS targeting the AC on chronic tinnitus. A double-blinded randomized placebo-controlled trial was conducted on patients (n = 48) with chronic intractable tinnitus (>2 years duration). Participants were randomly allocated to two groups: one receiving tDCS (n = 26), with the anode/cathode placed over the left/right AC, and the other receiving a placebo treatment (n = 22). A 20 min daily session of 2 mA current was administered for five consecutive days per week over two consecutive weeks, employing 35 cm2 electrodes. Tinnitus handicap inventory (THI) scores, tinnitus loudness, and tinnitus distress were measured using a visual analogue scale (VAS), and were assessed before intervention, immediately after, and at one-month follow-up. Anodal tDCS significantly reduced THI from 72.93 ± 10.11 score to 46.40 ± 15.36 after the last session and 49.68 ± 14.49 at one-month follow-up in 18 out of 25 participants (p < 0.001). The risk ratio (RR) of presenting an improvement of ≥20 points in the THI after the last session was 10.8 in patients treated with tDCS. Statistically significant reductions were observed in distress VAS and loudness VAS (p < 0.001). No statistically significant differences in the control group were observed. Variables such as age, gender, duration of tinnitus, laterality of tinnitus, baseline THI scores, and baseline distress and loudness VAS scores did not demonstrate significant correlations with treatment response. Repeated sessions of bilateral AC tDCS may potentially serve as a therapeutic modality for chronic tinnitus.

An experimental examination of neurostimulation and cognitive restructuring as potential components for Misophonia interventions

Misophonia is a disorder of decreased tolerance to certain aversive, repetitive common sounds, or to stimuli associated with these sounds. Two matched groups of adults (29 participants with misophonia and 30 clinical controls with high emotion dysregulation) received inhibitory neurostimulation (1 Hz) over a personalized medial prefrontal cortex (mPFC) target functionally connected to the left insula; excitatory neurostimulation (10 Hz) over a personalized dorsolateral PFC (dlPFC) target; and sham stimulation over either target. Stimulations were applied while participants were either listening or cognitively downregulating emotions associated with personalized aversive, misophonic, or neutral sounds. Subjective units of distress (SUDS) and psychophysiological measurements (e.g., skin conductance response [SCR] and level [SCL]) were collected. Compared to controls, participants with misophonia reported higher distress (∆SUDS = 1.91-1.93, ps < 0.001) when listening to and when downregulating misophonic distress. Both types of neurostimulation reduced distress significantly more than sham, with excitatory rTMS providing the most benefit (Cohen's dSUDS = 0.53; dSCL = 0.14). Excitatory rTMS also enhanced the regulation of emotions associated with misophonic sounds in both groups when measured by SUDS (dcontrol = 1.28; dMisophonia = 0.94), and in the misophonia group alone when measured with SCL (d = 0.20). Both types of neurostimulation were well tolerated. Engaging in cognitive restructuring enhanced with high-frequency neurostimulation led to the lowest misophonic distress, highlighting the best path forward for misophonia interventions.

Non-invasive brain stimulation for patients and healthy subjects: Current challenges and future perspectives

Non-invasive brain stimulation (NIBS) techniques have a rich historical background, yet their utilization has witnessed significant growth only recently. These techniques encompass transcranial electrical stimulation and transcranial magnetic stimulation, which were initially employed in neuroscience to explore the intricate relationship between the brain and behaviour. However, they are increasingly finding application in research contexts as a means to address various neurological, psychiatric, and neurodegenerative disorders. This article aims to fulfill two primary objectives. Firstly, it seeks to showcase the current state of the art in the clinical application of NIBS, highlighting how it can improve and complement existing treatments. Secondly, it provides a comprehensive overview of the utilization of NIBS in augmenting the brain function of healthy individuals, thereby enhancing their performance. Furthermore, the article delves into the points of convergence and divergence between these two techniques. It also addresses the existing challenges and future prospects associated with NIBS from ethical and research standpoints.

Personalized Neuromodulation: A Novel Strategy for Improving Tinnitus Treatment

This study evaluated the efficacy of personalized neuromodulation, where treatment modalities are chosen based on the patient's responses in a pilot trial. A total of 71 patients with tinnitus were divided into two groups: a personalized group and a randomized neuromodulation group. In the personalized group (n = 35), repetitive transcranial magnetic stimulation (rTMS) and transcranial direct-current stimulation (tDCS) were assessed in a pilot trial, and responsive modalities were administered to 16 patients, while the non-responders (n = 19) were randomly assigned to rTMS, tDCS, or combined modalities. Patients in the randomized group (n = 36) were randomly allocated to rTMS, tDCS, or combined modalities. The Tinnitus Handicap Inventory (THI) score improvement after 10 sessions of each neuromodulation was significantly greater in the personalized group than in the randomized group (p = 0.043), with no significant differences in tinnitus loudness, distress, or awareness. The treatment success rate was highest in the personalized responder subgroup (92.3%), and significantly greater than that in the non-responder subgroup (53.0%; p = 0.042) and the randomized group (56.7%; p = 0.033). Personalized neuromodulation, where the treatment modality is chosen based on the patient's responses in a pilot trial, is an advantageous strategy for treating tinnitus.

A Digitally-Controlled Integrated Circuit Solution for Tinnitus Treatment with Charge Balancing

An integrated circuit specified for tinnitus treatment is described. This chip, realized using a 0.18um BCD high-voltage CMOS process, is capable of generating current stimulus with any wave shape directly into the inner-ear tissue without the need for off-chip control circuitry. Used as part of a multi-chip module that can be implanted into the inner ear, this core chip contains an 8-bit digital-to-analog converter, an amplitude control block, a novel high-voltage drive and charge balance circuit, a high-voltage level shifter, an SRAM, a ROM, and an on-chip central control unit. The chip can achieve ±0.1 mV charge-balance precision.

The Evaluation of Effects of Electrical Stimulation in Treatment of Patients with Chronic Tinnitus with Normal Hearing Sensitivity

Tinnitus is usually associated with different comorbidities such as anxiety, annoyance and depression. Evidences have targeted two main places for tinnitus treatment, namely the auditory cortex and the dorsolateral prefrontal cortex (DLPFC). Transcranial direct current stimulation (tDCS) has been reportedly associated with improvement of cognitive functions in individuals. This study was conducted to evaluate the therapeutic effects of repeated sessions of anodal bifrontal tDCS on tinnitus symptoms. Furthermore, the tDCS impacts on the comorbid depression and anxiety of the patients were investigated. Forty-two voluntaries that suffers from chronic tinnitus were randomly assigned into "real tDCS" (n = 21) and "sham tDCS" (n = 21) groups. The tDCS group, received tDCS with the protocol consisted of 2 mA current, daily one session of 20 min, 6 consecutive days per week and for 4 consecutive weeks. The tinnitus handicap inventory (THI) scale, was measured before the first tDCS session and at one-week and two weeks follow-up. With the same intervals; the distress-related tinnitus was evaluated using visual analogue scale. Depression and anxiety scores were also measured using the Beck depression inventory and Beck anxiety inventory scales, respectively. Our findings indicated that THI score, depression and anxiety level has been gradually diminished across subsequent measurement intervals. We also find significant reduction of distress-related tinnitus in the real-tDCS group after treatment. We conclude that application of tDCS to the bilateral DLPFC region alleviates chronic tinnitus and it should be considered in patients with refractory tinnitus.

The Comorbidity of Depression and Anxiety Symptoms in Tinnitus Sufferers: A Network Analysis

OBJECTIVE

Sufferers of tinnitus, especially of the prolonged type, frequently suffer from comorbid depression and anxiety. From the perspective of the network model, this comorbidity is thought to be an interacting system of these two symptoms. In our study, we conducted a network analysis of depression and anxiety comorbidity in tinnitus sufferers, aiming to identify the central and bridge symptoms and make informed suggestions for clinical interventions and psychotherapy.

METHOD

A total of 566 tinnitus sufferers were enrolled in our study. The Patient Health Questionnaire-9 (PHQ-9) and the Generalized Anxiety Disorder 7-Item Questionnaire (GAD-7) were selected to evaluate depression and anxiety symptoms, respectively, followed by network analysis to construct the interacting networks.

RESULTS

The findings identified six edges of strongest regularized partial correlations in this network. Of these, three were depression symptoms and three were anxiety symptoms. The anxiety symptoms "Unable to control worry" and "Relaxation difficulty" and the depression symptom "Feeling depressed or hopeless" had the highest expected influence centrality. The analysis results also revealed three bridge symptoms: "Afraid something awful might happen", "Feeling of worthlessness", and "Trouble concentrating". As for "Suicidal ideation", the direct relations between this symptom and "Afraid something awful might happen" and "Feeling depressed or hopeless" were the strongest.

CONCLUSIONS

The central and bridge symptoms of the interacting network of depression and anxiety symptoms in tinnitus sufferers can be considered a significant transdiagnostic intervention target for the management of this comorbidity. In particular, clinical prevention and psychotherapy should be implemented, targeting the symptoms that have the strongest associations with suicidal ideation.

Integrative Treatment for Tinnitus Combining Repeated Facial and Auriculotemporal Nerve Blocks With Stimulation of Auditory and Non-auditory Nerves

Background

Tinnitus is a prevalent condition (&gt;10% of the population) affecting the quality of life of 0.5–3% of the population. Although several treatments have been proposed, most of these lack evidence of efficacy in the treatment of chronic tinnitus. Thus, we aimed to evaluate an integrative treatment strategy for subacute and chronic tinnitus.

Methods

This retrospective chart review study included 55 patients with tinnitus (subacute, n = 15; chronic, n = 40) who underwent repeated nerve blocks after stimulation of the trigeminal (V) and facial (VII) nerves to modulate the auditory and non-auditory nervous systems via the vestibulocochlear (VIII) cranial nerve pathways. We used a simplified smiley tinnitus-visual analog scale (T-VAS) with scores ranging from 0 to 10 combining the effect of tinnitus loudness, distress, and quality of life as the outcome measure to evaluate the efficacy of our treatment method. Statistical analyses were performed using SPSS (version 18.0, SPSS Inc., Chicago, IL, United States), one-way and two-way analysis of variance.

Results

In more than 87.5% of patients (14/15 subacute, 35/40 chronic), tinnitus disappeared or had significantly reduced by the end of the treatment. The mean T-VAS score reduced significantly from 7.13 to 0.60 in the subacute group and from 7.73 to 1.53 in the chronic group by the end of treatment (p &lt; 0.05). The benefits were maintained after treatment cessation and at the 1-year follow-up. The average number of treatment procedures was 9.8 ± 3.589 (range, 5–15) in the subacute group and 9.775 ± 3.717 (range, 5–18) in the chronic group.

Conclusion

Our results show that the proposed integrative approach is highly effective in treating subacute and chronic tinnitus and represents a promising therapeutic approach.

The Effect and Mechanism of Transcranial Direct Current Stimulation on Episodic Memory in Patients With Mild Cognitive Impairment

OBJECTIVE

This study aimed to investigate the efficacy of transcranial direct current stimulation (tDCS) on episodic memory in patients with mild cognitive impairment (MCI) and analyze the neural mechanism of tDCS therapy from the perspective of neuroelectrophysiological parameters.

METHODS

Forty MCI patients were recruited and randomly divided into a sham group (n = 20) and a tDCS group (n = 20). Patients in the tDCS group were treated with a tDCS instrument for 20 min, once a day, for 5 days. Patients in the sham group were treated with sham stimulus. Montreal Cognitive Assessment Scale (MoCA), Wechsler Memory Scale (WMS), and event-related potential (ERP) (amplitude and latency of P300 wave) were comparatively assessed between the two groups at pre-treatment, 5 days and 4 weeks post-treatment points.

RESULTS

The two groups showed no significant difference in any of the assessed parameters at pre-treatment (P > 0.05). At 5 days post-treatment, memory quotient (MQ) score in the tDCS group significantly increased (P < 0.05), scores of picture memory, visual regeneration, logical memory, memory span, visual regeneration-delay, and logical memory-delay were significantly increased compared to pre-treatment (P < 0.01). The P300 amplitude significantly increased, and its latency significantly shortened (P < 0.01). Four weeks post-treatment, the scores of MQ and visual regeneration-delay in the tDCS group increased, compared to pre-treatment (P < 0.05); picture memory, visual regeneration, logical memory, memory span, and logical memory-delay improved (P < 0.01); the P300 amplitude increased, and its latency shortened (P < 0.01). At 5 days and 4 weeks post-treatment points, the tDCS group, compared with the sham group (P < 0.01), exhibited greater scores of MQ, picture memory, visual regeneration, logical memory, memory span, visual regeneration-delay, and logical memory-delay, increased P300 amplitude, and shortened P300 latency. Similarly, the tDCS group showed higher MQ scores at 5 days post-treatment (P < 0.05) and 4 weeks post-treatment (P < 0.01). Before treatment and after 5 days of treatment, P300 amplitude and latency difference were positively correlated with MQ difference (P < 0.05).

CONCLUSION

tDCS improved episodic memory in MCI patients, and the effect lasted for 4 weeks. Changes in ERP (P300) suggested that tDCS could promote changes in brain function.

Modulation of Brain Networks during MR-Compatible Transcranial Direct Current Stimulation

Transcranial direct current stimulation (tDCS) can influence performance on behavioral tasks and improve symptoms of brain conditions. Yet, it remains unclear precisely how tDCS affects brain function and connectivity. Here, we measured changes in functional connectivity (FC) metrics in blood-oxygenation-level-dependent (BOLD) fMRI data acquired during MR-compatible tDCS in a whole-brain analysis with corrections for false discovery rate. Volunteers (n=64) received active tDCS, sham tDCS, and rest (no stimulation), using one of three previously established electrode tDCS montages targeting left dorsolateral prefrontal cortex (DLPFC, n=37), lateral temporoparietal area (LTA, n=16), or superior temporal cortex (STC, n=11). In brain networks where simulated E field was highest in each montage, connectivity with remote nodes decreased during active tDCS. During active DLPFC-tDCS, connectivity decreased between a fronto-parietal network and subgenual ACC, while during LTA-tDCS connectivity decreased between an auditory-somatomotor network and frontal operculum. Active DLPFC-tDCS was also associated with increased connectivity within an orbitofrontal network overlapping subgenual ACC. Irrespective of montage, FC metrics increased in sensorimotor and attention regions during both active and sham tDCS, which may reflect the cognitive-perceptual demands of tDCS. Taken together, these results indicate that tDCS may have both intended and unintended effects on ongoing brain activity, stressing the importance of including sham, stimulation-absent, and active comparators in basic science and clinical trials of tDCS.

Transcranial direct current stimulation (tDCS), a treatment option for tinnitus in profound hearing loss

A 45-year-old man presented with a history of sudden sensory neural hearing loss and severe tinnitus in his left ear. Audiological investigations revealed a profound hearing loss on his left ear and mild conductive hearing loss on his right. Tinnitus pitch and loudness were matched to a 4 kHz narrow-band noise at 50dBHL and subjective tinnitus questionnaires revealed that he had a catastrophic handicap (grade IV). Traditional audiological treatment approaches (tinnitus maskers, hearing aid and sound therapy) that stimulate the cochlea to induce cortical reorganisation were futile. Hence, a top-down approach (transcranial direct current stimulation (tDCS)) to directly modulate the cortical centres was attempted. tDCS was provided for a sum of 15 sessions across 2 phases. There was a substantial improvement in the tinnitus loudness, distress and depression scores which maintained for 3 months post-treatment. tDCS is a potential treatment for phantom perceptions (tinnitus) in cases of profound sensory neural hearing loss where there is no residual sensory ability. Tailor-made approaches seem to be more appropriate until a standard protocol for tDCS in tinnitus is established.

Evaluation of Spin in the Abstracts of Systematic Reviews and Meta-Analyses Focused on Tinnitus

HYPOTHESIS

The objective was to investigate the prevalence of spin in abstracts of systematic reviews and meta-analyses covering the treatment of tinnitus. We hypothesized that spin would be present in these articles and a significant relationship would exist between spin usage and extracted study characteristics.

BACKGROUND

Spin, the misrepresentation of study findings, can alter a clinician's interpretation of a study's results, potentially affecting patient care. Previous work demonstrates that spin is present in abstracts of randomized clinical trials.

METHODS

Using a cross-sectional analysis, we conducted a systematic search using MEDLINE and Embase databases on June 2, 2020, for systematic reviews focused on tinnitus treatment. Investigators performed screening and data extraction in a masked, duplicate fashion.

RESULTS

Forty systematic reviews met inclusion criteria, and spin was identified in four of them. Spin in abstracts most frequently occurred when conclusions claimed the beneficial effect of the experimental treatment despite high risk of bias in primary studies (n = 3). The other form of spin found was the conclusion claims safety based on nonstatistically significant results with a wide confidence interval (n = 1). There was no significant association between spin and any of our extracted study characteristics.

CONCLUSION

Spin was observed in 10% of abstracts of systematic reviews and meta-analyses covering the treatment of tinnitus. Although this percentage may be small, we recommend that medical journals provide a more detailed framework for abstract structure and require the inclusion of risk of bias assessment results in abstracts to prevent the incorporation of spin.

Animal Models of Tinnitus Treatment: Cochlear and Brain Stimulation

Neuromodulation, via stimulation of a variety of peripheral and central structures, is used to suppress tinnitus. However, investigative limitations in humans due to ethical reasons have made it difficult to decipher the mechanisms underlying treatment-induced tinnitus relief, so a number of animal models have arisen to address these unknowns. This chapter reviews animal models of cochlear and brain stimulation and assesses their modulatory effects on behavioral evidence of tinnitus and its related neural correlates. When a structure is stimulated, localized modulation, often presenting as downregulation of spontaneous neuronal spike firing rate, bursting and neurosynchrony, occurs within the brain area. Through anatomical projections and transmitter pathways, the interventions activate both auditory- and non-auditory structures by taking bottom-up ascending and top-down descending modes to influence their target brain structures. Furthermore, it is the brain oscillations that cochlear or brain stimulation evoke and connect the prefrontal cortex, striatal systems, and other limbic structures to refresh neural networks and relieve auditory, attentive, conscious, as well as emotional reactive aspects of tinnitus. This oscillatory neural network connectivity is achieved via the thalamocorticothalamic circuitry including the lemniscal and non-lemniscal auditory brain structures. Beyond existing technologies, the review also reveals opportunities for developing advanced animal models using new modalities to achieve precision neuromodulation and tinnitus abatement, such as optogenetic cochlear and/or brain stimulation.

Tinnitus and Brain Stimulation

The pathophysiological mechanisms that underlie the generation and maintenance of tinnitus are being unraveled progressively. Based on this knowledge, a large variety of different neuromodulatory interventions have been developed and are still being designed, adapting to the progressive mechanistic insights in the pathophysiology of tinnitus. rTMS targeting the temporal, temporoparietal, and the frontal cortex has been the mainstay of non-invasive neuromodulation. Yet, the evidence is still unclear, and therefore systematic meta-analyses are needed for drawing conclusions on the effectiveness of rTMS in chronic tinnitus. Different forms of transcranial electrical stimulation (tDCS, tACS, tRNS), applied over the frontal and temporal cortex, have been investigated in tinnitus patients, also without robust evidence for universal efficacy. Cortex and deep brain stimulation with implanted electrodes have shown benefit, yet there is insufficient data to support their routine clinical use. Recently, bimodal stimulation approaches have revealed promising results and it appears that targeting different sensory modalities in temporally combined manners may be more promising than single target approaches.While most neuromodulatory approaches seem promising, further research is required to help translating the scientific outcomes into routine clinical practice.

Transcranial direct current stimulation improves tinnitus perception and modulates cortical electrical activity in patients with tinnitus: A randomized clinical trial

OBJECTIVES

This study aims to determine whether transcranial direct current stimulation (tDCS): a) is effective in the treatment of tinnitus by decreasing its annoyance and severity; b) modulates the cortical electrical activity of such individuals.

METHODS

A double-blind, placebo-controlled clinical trial was conducted with 24 patients with tinnitus, randomized into two groups: Group 1 (n = 12) received anodal tDCS over the left temporoparietal area (LTA) and cathodal tDCS over the right dorsolateral prefrontal cortex (DLPFC) and Group 2 (n = 12) received placebo intervention. Tinnitus perception using a visual analog scale (VAS) and the Tinnitus Handicap Inventory (THI) questionnaire, in addition to electroencephalogram (EEG) was measured with eyes opened and closed at baseline and after the intervention. For the treatment, patients were subjected to five consecutive sessions of tDCS with the anodal electrode over the LTA and cathodal electrode over the right DLPFC (7 × 5 cm, 2 mA for 20 min). tDCS was turned off after 30 s in the sham group.

RESULTS

Active tDCS significantly improved tinnitus annoyance and severity. It was associated with decreased beta and theta EEG frequency bands with eyes opened and decreased alpha frequency with eyes closed. sLORETA identified changes in frequency bands in the frontal, temporoparietal, and limbic regions. Finally, there were negative correlations between baseline EEG frequency bands and tDCS-induced change in tinnitus annoyance and severity.

CONCLUSIONS

These results demonstrate that tDCS modulates the EEG activity and alleviates tinnitus perception. This effect may be related to baseline EEG activity.

Auditory Neural Plasticity in Tinnitus Mechanisms and Management

Tinnitus, which is the perception of sound in the absence of a corresponding external acoustic stimulus, including change of hearing and neural plasticity, has become an increasingly important ailment affecting the daily life of a considerable proportion of the population and causing significant burdens for both the affected individuals and society as a whole. Here, we briefly review the epidemiology and classification of tinnitus, and the currently available treatments are discussed in terms of the available evidence for their mechanisms and efficacy. The conclusion drawn from the available evidence is that there is no specific medication for tinnitus treatment at present, and tinnitus management might provide better solutions. Therapeutic interventions for tinnitus should be based on a comprehensive understanding of the etiology and features of individual cases of tinnitus, and more high quality and large-scale research studies are urgently needed to develop more efficacious medications.

Non-Invasive Neuromodulation for Tinnitus

Tinnitus is a prevalent disorder that has no cure currently. Within the last two decades, neuroscientific research has facilitated a better understanding of the pathophysiological mechanisms that underlie the generation and maintenance of tinnitus, and the brain and nerves have been identified as potential targets for its treatment using non-invasive brain stimulation methods. This article reviews studies on tinnitus patients using transcranial magnetic stimulation, transcranial electrical stimulation, such as transcranial direct current stimulation, alternating current stimulation, transcranial random noise stimulation as well as transcutaneous vagus nerve stimulation and bimodal combined auditory and somatosensory stimulation. Although none of these approaches has demonstrated effects that would justify its use in routine treatment, the studies have provided important insights into tinnitus pathophysiology. Moreover bimodal stimulation, which has only been developed recently, has shown promising results in pilot trials and is a candidate for further development into a valuable treatment procedure.

Avenue for Future Tinnitus Treatments

Tinnitus is a common symptom. Standard therapies aim at improving the quality of life and reducing the psychological stress associated with tinnitus. Most interventions have little or no effect on the main symptom. Those affected subjects, however, want such a change and prefer a specific solution, such as pharmacologic therapy to other modalities. Scientific efforts have not yet led to significant improvement in the range of therapies. This article outlines existing efforts and develops ideas on how research for improved tinnitus therapy might look in the future.

Transcranial electrical stimulation motor threshold can estimate individualized tDCS dosage from reverse-calculation electric-field modeling

Background:

Unique amongst brain stimulation tools, transcranial direct current stimulation (tDCS) currently lacks an easy or widely implemented method for individualizing dosage.

Objective:

We developed a method of reverse-calculating electric-field (E-field) models based on Magnetic Resonance Imaging (MRI) scans that can estimate individualized tDCS dose. We also evaluated an MRI-free method of individualizing tDCS dose by measuring transcranial magnetic stimulation (TMS) motor threshold (MT) and single pulse, suprathreshold transcranial electrical stimulation (TES) MT and regressing it against E-field modeling. Key assumptions of reverse-calculation E-field modeling, including the size of region of interest (ROI) analysis and the linearity of multiple E-field models were also tested.

Methods:

In 29 healthy adults, we acquired TMS MT, TES MT, and anatomical T1-weighted MPRAGE MRI scans with a fiducial marking the motor hotspot. We then computed a “reverse-calculated tDCS dose” of tDCS applied at the scalp needed to cause a 1.00 V/m E-field at the cortex. Finally, we examined whether the predicted E-field values correlated with each participant’s measured TMS MT or TES MT.

Results:

We were able to determine a reverse-calculated tDCS dose for each participant using a 5 × 5 x 5 voxel grid region of interest (ROI) approach (average = 6.03 mA, SD = 1.44 mA, range = 3.75–9.74 mA). The Transcranial Electrical Stimulation MT, but not the Transcranial Magnetic Stimulation MT, significantly correlated with the ROI-based reverse-calculated tDCS dose determined by E-field modeling (R² = 0.45, p < 0.001).

Conclusions:

Reverse-calculation E-field modeling, alone or regressed against TES MT, shows promise as a method to individualize tDCS dose. The large range of the reverse-calculated tDCS doses between subjects underscores the likely need to individualize tDCS dose. Future research should further examine the use of TES MT to individually dose tDCS as an MRI-free method of dosing tDCS.

Bifrontal transcranial direct current stimulation modulates fatigue in multiple sclerosis: a randomized sham-controlled study

Fatigue is a frequent and debilitating symptom in patients with central nervous system diseases. Up to 90% of patients with multiple sclerosis (MS) suffer from fatigue that drastically affects the quality of life. MS patients also complain of anxiety and depressive symptoms and these three manifestations tend to cluster together in this clinical population. The objective of this work was to assess the effects of transcranial direct stimulation (tDCS), a noninvasive brain stimulation technique, on fatigue as well as anxiety and depressive symptoms. Eleven fatigued MS patients randomly received two blocks (active and sham tDCS) of five consecutive daily sessions of bifrontal tDCS (anode/cathode over the left/right prefrontal cortices, respectively) in a crossover manner, separated by a 3-week washout interval. Evaluation took place at day 1, day 5 (right after each block) and 1 week later. Active but not sham tDCS resulted in a significant improvement of fatigue at day 5 (p < 0.05), an effect that seems to last at least 1 week following the stimulation (p = 0.05). Active tDCS also significantly improved anxiety symptoms, but the effect emerged 1 week later (p < 0.05). No significant effects were obtained regarding depression (p > 0.05). Bifrontal tDCS seems to modulate fatigue in PwMS. The observed anxiolytic effects could constitute delayed after effects of tDCS or might be mediated by fatigue improvement. These findings merit to be addressed in large-scale controlled trials.