Considerations on tinnitus retraining therapy and transcranial magnetic stimulation

Introduction. Tinnitus is a subjective perception of sound in the absence of an external acoustic stimulus. It has negative behavioral feelings associated, e.g., depression, insomnia, difficulty of concentration, anxiety, irritability, and panic. The feelings impact negatively on the social and economic life of individuals. Empirical data suggest that disorders in the auditory cortex and its neural pathways give rise to abnormal spontaneous activations associated with tinnitus. Understanding the causes remains challenging. However, the current hypothesis suggests that clusters of neural networks and subnetworks are involved in tinnitus generation. Central dynamic neuroplasticity induced by a peripheral loss of auditory input can cause tinnitus noise. To date, there is no widespread consensus about the most effective therapy for treating tinnitus. Objective. To reflect on two tinnitus therapies: Tinnitus Retraining Therapy (TRT) and Transcranial Magnetic Stimulation (TMS). Method. A narrative review. Explicit and systematic criteria were not adopted in searching for the theoretical framework. Results. TMS

is promising compared to TRT because TMS acts on tinnitus neural mechanisms. TRT is effective on a behavioral level since it relieves mild and moderate tinnitus' negative feelings. Conclusion. TRT does not advance on the neural source, but only on the tinnitus perception. TMS acts directly on the neural causes. Both therapies have limitations and can work for some patients. However, the effect of TMS seems more efficient, although transient. Keywords. Tinnitus Retraining Therapy; Transcranial Magnetic Stimulation; Auditory Cortex; Neuroscience Resumen Introducción. El tinnitus es una percepción subjetiva del sonido en ausencia de estímulos acústicos externos. Tiene sentimientos negativos asociados, por ejemplo, depresión, insomnio, dificultad para concentrarse, ansiedad, irritabilidad y pánico. Los sentimientos impactan negativamente en la vida social y económica de las personas. Los datos empíricos sugieren que los trastornos en la corteza auditiva y sus vías neurales dan lugar a activaciones espontáneas anormales asociadas con el tinnitus. Comprender las causas sigue siendo un desafío. Sin embargo, la hipótesis actual sugiere que grupos de redes neuronales y subredes están involucrados en la generación de tinnitus. La neuroplasticidad dinámica central inducida por una pérdida periférica de entrada auditiva puede causar ruido de tinnitus. Hasta la fecha, no existe un consenso generalizado sobre la terapia más eficaz para tratar el tinnitus. Objetivo. Reflexionar sobre dos terapias para el tinnitus: la terapia de reentrenamiento para el tinnitus (TRT) y la estimulación magnética transcraneal (TMS). Método. Una revisión narrativa. No se adoptaron criterios explícitos y sistemáticos en la búsqueda del marco teórico. Resultados. TMS es prometedor en comparación con TRT porque TMS actúa sobre los mecanismos neurales del tinnitus. La TRT es eficaz a nivel conductual, ya que alivia los sentimientos negativos del tinnitus leve y moderado. Conclusión. TRT no avanza en la fuente neural, sino solo en la percepción del tinnitus. TMS actúa directamente sobre las causas neuronales. Ambas terapias tienen limitaciones y pueden funcionar para algunos pacientes. Sin embargo, el efecto de TMS parece más eficiente, aunque transitorio. Palabras clave. The single criterion for tinnitus evaluation is the patient's perception and report. The intensity of the tinnitus perception varies from person to person, and it is classified as mild, moderate, and severe. An alternative classification was proposed: (i) acute and chronic, (ii) decompensated and compensated 12 . The first refers to the duration and persistence of tinnitus (at least 3 months), the second is related to the degree of habituation to the tinnitus process.
Moreover, the intensity can range from a just noticeable hissing sound to a roaring noise that affects daily life, e.g., sleep, concentration, and several other activities.
This paper discusses two therapies for treating tinnitus: TRT and the potential effect of Transcranial magnetic stimulation (TMS). We consider issues concerning both treatments. We summarized the main positions about the topic.

METHOD
To achieve the objectives, we conducted a literature review on the subject. The focus was on books and texts available on the Google Scholar database since it covers a range of databases. We selected papers that reported studies with TRT or TMS. Moreover, we included texts by their originality and impact on the topic.
We extended the references with the standard texts back in time and added recent research to give support to our reflection. Since it is a narrative review, explicit and systematic criteria were not adopted in searching for the theoretical framework. Finally, we made a critical assessment by considering the positive and negative aspects of TMS and TRT therapies. There are even doubts whether tinnitus is generated in the ears or the head 3 . However, pioneer studies discovered and converged to some extent that the conscious perception of tinnitus results from abnormal neuronal activities in the auditory cortex and its subnetworks [18][19][20][21][22] .

RESULTS
Studies have shown that tinnitus experience is associated with functional and structural changes at the neural level [23][24][25][26][27][28][29][30] . Neurophysiological tools could signal the source of tinnitus. With this in mind, empirical finds suggest that the auditory cortex is the possible basis, where the neural stimuli are transformed into a subjective sensation.
Besides, disorders of the somatosensory system and abnormal neural activities are related to the generation of tinnitus 2,12,31 . As far as can be seen, there is an agreement that tinnitus is neurally realized. However, the difficulty is the accurate identification of the neural correlates of tinnitus 32 . Since brains are adaptive systems with several levels of interconnections, the task of identifying the correct neuronal networks mainly active during the experience of tinnitus is challenging.
Some data suggest that tinnitus can be caused by maladaptive neuroplasticity and deafferentation of auditory neural pathway 19,[33][34][35][36] . The changes (triggered by several factors, e.g., physical injuries) give rise to abnormal spontaneous activity in the auditory cortex, abnormal neurotransmitter release, abnormalities in the transduction processes, cortical reorganization of the tonotopic sensory map, hypersynchrony of neural discharges, and enhanced sensitivity of the auditory pathways 3 . Further empirical finds from neuroimaging suggest that tinnitus is related to functional and structural brain abnormalities in distributed auditory and non-auditory brain regions 22,37 . There is a difficulty in the identification of tinnitus etiology. However, there are therapies and treatments which aim to relieve such negative sensation produced by tinnitus.
One is the TRT 8,10,14,41 , a clinical implementation with a focus on the habituation of tinnitus signal noise. The other therapy that emerges as an alternative, is TMS. However, both have issues to take into account.

Tinnitus retraining therapy
TRT can be clinically implemented as an attempt to relieve the tinnitus perception 8,10,14,41 . TRT is based on two properties of the brain: plasticity and habituation. Two main systems in the brain play central roles: the limbic and the autonomic nervous systems. TRT aims to induce changes in these systems by removing the association of tinnitus with negative feelings. The core idea is that the limbic system regulates tinnitus perception. Therefore, it can be modulated by a specific intervention.
Misophonia (aversion to sound) is also a frequent symptom in patients with tinnitus since they have a negative attitude toward sounds. Some studies showed a relationship between negative feelings and the activation of the limbic system 42 .
The limbic system regulates several functions e.g., emotion, behavior, motivation, long-term memory, and olfaction. The limbic system also controls the emotional life. Indeed, there is an urgent demand for an effective treatment for tinnitus. TRT seems to be an effective therapy.
It was reported 8  The kissing or rubbing on the painful spot applies the same principles as sound therapy for tinnitus. The therapy decreases the contrast between the perceived pain and the background neural activity, thus interfering with the brain's ability to detect the painful signal. Furthermore, the treatment reconditions the pain perception with a positive reinforcer, a kiss, or any other sign of affection. However, this "hush and kiss" approach seems to be efficient only for moderate pain; failing to do so when a leg is broken, a tumor Up to now, there is no objective method of evaluation of tinnitus. Even though the acquisition of "data" through the patient's reports, which are subjective and work as support for medical diagnostic, raises a concern about reliability. TMS is an experimental tool and it requires further studies. However, it seems effective since TMS acts directly on specific brain areas by changing its neuronal frequencies.
Empirical support is available as follows.
Repetitive TMS (rTMS) of 1Hz was applied daily for 10 consecutive workdays in 64 participants with chronic tinnitus 56   Despite this, the studies reported that the application of TMS was effective in tinnitus relief. The challenging task is to induce long-duration after-effects since the result is transient. The nature of the rTMS after-effects depends on the number, intensity, and frequency of stimulation pulses, a task whose settings are still challenging.

CONCLUSION
In sum, TRT does not seem a suitable treatment for chronic tinnitus, since it tackles the symptoms, rather than the cause. TRT can be useful in treating mild cases. TRT is not recommended when the patient has severe or chronic tinnitus.
The research should focus on the neuronal basis underlying the tinnitus experience. Through the identification of the neural mechanisms active during the tinnitus perception, researchers and clinicians can apply effective tools such as TMS associated with fMRI, MEG, and EEG to track the etiology.
The utilization of TMS seems to be more effective in comparison to the TRT. TMS acts on the neural source.
However, the after-effect of TMS is temporary and this is a challenge in TMS therapy. Besides, there are doubts concerning how to set a methodological account that could make the relief last longer, since, after the TMS sessions, the phantom perception returns. Despite this experimental constraint, there are prospects in the attempt to find a solution for the tinnitus problem in a neuroscientific way 66 .
In summary, TMS still has a transitory effect on tinnitus, but this is due to the lack of knowledge concerning the cortical sensory area TMS should be applied, the duration, intensity, and frequency of the stimuli. TMS is a new tool that needs further investigation. Nonetheless, it is the beginning of a new phase in tinnitus research, from which tinnitus patients can benefit in the future.
TRT is a beneficial therapy for mild tinnitus, but as a solution for severe or chronic tinnitus, it is inappropriate.
TMS seems to be promising, however, there are issues to be solved. Despite the limitations, both therapies have advantages in treating types of tinnitus disorder: half a loaf is better than none.