Evidence of cochlear neural degeneration in normal-hearing subjects with tinnitus, 2023, Maison et al

Abstract
Tinnitus, reduced sound-level tolerance, and difficulties hearing in noisy environments are the most common complaints associated with sensorineural hearing loss in adult populations. This study aims to clarify if cochlear neural degeneration estimated in a large pool of participants with normal audiograms is associated with self-report of tinnitus using a test battery probing the different stages of the auditory processing from hair cell responses to the auditory reflexes of the brainstem. Self-report of chronic tinnitus was significantly associated with (1) reduced cochlear nerve responses, (2) weaker middle-ear muscle reflexes, (3) stronger medial olivocochlear efferent reflexes and (4) hyperactivity in the central auditory pathways. These results support the model of tinnitus generation whereby decreased neural activity from a damaged cochlea can elicit hyperactivity from decreased inhibition in the central nervous system.

Conclusion
This study clarifies the association between biomarkers of peripheral neural deficits with tinnitus and is consistent with the idea that CND may serve as a peripheral trigger for excess central gain43,122,123. Future psychophysical measures of tinnitus and sound-level intolerance may help interpret the pathology underlying the changes in physiological responses including at higher stages of the auditory system. They may also clarify the role of CND in the development and maintenance of central hyperactivity and the engagement of autonomically driven changes in the affective responses to sound. In a noise-damaged mouse model, neurotrophin overexpression via gene therapy or supplementation via local delivery can elicit regeneration of ANF connections with IHCs124,125. Developing diagnostic assays of CND in humans and clarifying its link to the genesis and/or maintenance of the tinnitus percept is therefore key to identify candidates for future therapeutics and to track the efficacy of any treatments designed to rebuild a damaged inner ear and perhaps reverse the tinnitus percept.

Nature Scientific Reports
Open access: https://www.nature.com/articles/s41598-023-46741-5#Sec10

 

Tinnitus Linked to Hidden Undetected Auditory Nerve Damage – A Step Towards a Cure
https://scitechdaily.com/tinnitus-l...-auditory-nerve-damage-a-step-towards-a-cure/

Many individuals with hearing loss report a buzzing, humming, ringing, or even roaring sound in their ears. It’s been a longstanding idea that these symptoms, known as tinnitus, arise as a result of a maladaptive plasticity of the brain. In other words, the brain tries to compensate for the loss of hearing by increasing its activity, resulting in the perception of a phantom sound, tinnitus. Until recently though, this idea was disputed as some tinnitus sufferers have normal hearing tests.
...
“Our work reconciles the idea that tinnitus may be triggered by a loss of auditory nerve, including in people with normal hearing,” said Maison.

In terms of future directions, the investigators aim to capitalize on recent work geared toward the regeneration of auditory nerve via the use of drugs called neurotrophins.​

 

Tinnitus is commonly reported in Long Covid, which would be consistent with COVID damaging nerves. It also rather commonly resolves with time, which suggests a natural recovery from the immune system's ability to repair the damage. SARS-CoV-2 is clearly neurotropic so this adds to the evidence that viral infections could be the likely cause of many, if not most, cases of tinnitus.

Related, but Henrik Vogt's, of RecoveryNorway fame, origin story is that he cured his tinnitus using "mind-body techniques", fueling his certainty that it's possible to universally do so whenever no clear damage can be identified. Of course to find the damage, you have to look in the right place the right way.

Tinnitus is also common after concussions, so there are clearly more possible causes, likely no single unique pathology with a single unique biomarker, which is a typical, and unrealistic, requirement in medicine. The model of a disease-based approach with unique universal causes and pathologies has long reached its limit.

 

One of my medications causes it too. I had to reduce the dose by two thirds because it was driving me nuts.

 

but dont worry about that pesky neural degeneration, because CBT will sort it out

 

An overview of auditory brainstem responses in tinnitus is —

Auditory Brainstem Responses in Tinnitus: A Review of Who, How, and What? (2017, Frontiers in Aging Neuroscience)

Some recent publications include —

Abnormal Spontaneous Neural Activity of the Central Auditory System Changes the Functional Connectivity in the Tinnitus Brain: A Resting-State Functional MRI Study (2019, Frontiers in Neuroscience)

Alterations in auditory brain stem response distinguish occasional and constant tinnitus (2022, The Journal of Clinical Investigation)

Acoustic and optoacoustic stimulations in auditory brainstem response test in salicylate induced tinnitus (2023, Nature Scientific Reports)

 

A new paper (small numbers) from Poland is —

Patogenesis of tinnitus in patients with post-COVID syndrome – preliminary report
Paweł Rozbicki, Sandra Krzywdzińska, Michał Kaczmarczyk, Jacek Usowski, Arkadiusz Lubas, Dariusz Jurkiewicz

Introduction: Similarly to hearing loss and dizziness, tinnitus is a frequently reported complication of COVID-19 and remains the subject of numerous scientific reports. However, the exact impact of SARS-CoV-2 on the pathophysiology of tinnitus observed in post-COVID syndrome remains unclear. One suspected cause behind the development of vestibulocochlear symptoms is the inflammation of neural tissue triggered by SARS-CoV-2 infection.

Aim: The aim of this study was to analyze the results of Brainstem Auditory Evoked Potentials (BAEP) in the context of tinnitus development among patients diagnosed with post-COVID syndrome.

Material and methods: This retrospective study involved the analysis of BAEP test results of patients admitted to the Audiology Clinic of the Military Institute of Medicine at the National Research Institute and diagnosed with the post-COVID syndrome. The study compared the average latency values of waves I, II, III, IV, V, and intervals I–III, III–V, I–V. The statistical analysis of the obtained data was performed.

Results: Out of 18 patients (9 female, 9 male) with an average age of 54.22 years (±9.65) diagnosed with post-COVID syndrome, tinnitus was identified in 5 individuals (27.78%). A statistically significant increase in wave V latency (5.98 vs. 5.63 [ms]; P < 0.05) and interval III–V (1.99 vs. 1.71 [ms]; P<0.05) was observed between the groups of patients with and without tinnitus.

Discussion: Study results analyzing BAEP changes in patients with post-COVID syndrome in the context of tinnitus are insufficient in current literature. The only available report linking post-COVID syndrome with sensorineural hearing loss demonstrated a statistically significant increase in latency values of waves III, V, and an increase in intervals I–III, III–V.

Conclusions: Tinnitus in patients suffering from post-COVID syndrome may be related to prolonged conduction of nerve impulses within the brainstem auditory pathway. Detailed pathophysiology of these changes requires further research.

Link | PDF (Polish Journal of Otolaryngology)

(I don't think their discussion adds much.)