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If my understanding is correct, tinnitus is in most cases caused by damage to the inner ear hair cells, especially on the basal parts of the cochlea, which correspond to high frequencies. So, for example, you lose a few hair cells on the high frequency range (such as 14,000 Hz). This causes your brain to over-compensate the lost signal by essentially down-regulating the activation threshold of that range so much that it ends up assuming the cell is always on, rather than always off.

If that is correct, wouldn't a simple cochlear implant that replaced the lost cells by a dummy electrode that sent a "silent" signal 24/7 solve tinnitus? That electrode would emulate the signal that the lost hair cell would send when silent. That way, as soon as the brain receives that "silent" signal, it would up-regulate the activation threshold, and the ringing would stop.

There are ongoing trials that involve injecting stem cells, or small molecules such as FX-322, in order to re-grow lost ear hair cells, and these trials have shown some success against tinnitus, so, there is some evidence that restoring the absent signals lets the brain renormalize. The catch is, we shouldn't need fully functional hair cells to cure tinnitus. Replacing these cells with a dummy "always silent" cell would do the same job. Of course, the patient wouldn't be able to hear on these ranges, but I don't think most tinnitus sufferers want the ability to hear snapping shrimps, they just want to shut up the ringing.

Is my logic or understanding flawed?

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    $\begingroup$ How would you attach the electrodes without occluding the opening of the cochlea or otherwise interfering with the rest of the hearing range? It's also highly invasive and expensive as the demands of asepsis and precision required are enormous. Apart from those concerns, do you have evidence that an approach like "sending a silent signal" might work? (Perhaps tested on other less-invasive parts). $\endgroup$ Commented Sep 11, 2022 at 23:20
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    $\begingroup$ @JiminyCricket. aren't cochlear implants a common procedure? Except these need to cover the entire hearing spectrum, and be connected to an outside device that sends precise signals, and decode these into the right impulses to emulate cells. So, we have these relatively complex devices as a common procedure already! A dummy electrode that just sends a 24/7 "silent" signal looks trivial in comparison. I could be (and probably am) wrong, but I'd love to learn why/how! $\endgroup$
    – MaiaVictor
    Commented Sep 11, 2022 at 23:24

1 Answer 1

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Short answer
You are right, with a few caveats.

Background
Most tinnitus cases are caused by sensorineural hearing loss, as you rightfully indicate, namely due to a loss of hair cells in the cochlea. The deafferentation of the auditory cortex and subsequent chronic inactivity of this region in the brain ultimately leads to the generation of intrinsic brain activity in the auditory cortex, which is perceived as a 'ringing in the ears' (PloS Med, 2005).

Cochlear implantation has been shown in a multitude of studies to ameliorate the symptoms of tinnitus; for a review see Linhares de Freitas Borges et al., (2021). CIs basically mask the tinnitus sounds by swamping it with electrically evoked activity in the auditory system.

A CI is a medical device for treating severe-to-profound hearing loss. It bypasses the damaged hair cells in the cochlea, and restores activity in the auditory nerve by direct electrical stimulation of auditory nerve fibers (Fig. 1). CI is, however, not a first-line treatment for tinnitus and remains in the experimental phase for that purpose, at least as far as I know. Tinnitus has been mainly addressed as a secondary outcome to hearing performance after CI for treating deafness, although prospective randomized controlled trials with tinnitus relief being the primary outcome are underway (Assouly et al., 2021).

However, your idea of a single-electrode 'dummy implant' isn't very feasible, because it is difficult to insert a cochlear implant such that one or more electrodes are in direct electrical contact with the cochlear region corresponding to the frequency (or frequencies) of the tinnitus (Fu & Shannon, 2002). The cochlear regions with hair cell damage differs between patients and depends on the type of hearing loss. Therefore, a regular CI with a multi-electrode array is inserted that covers the better part of the cochlea (barred the low frequencies).

Furthermore, a 'silent' (i.e., below threshold) electrical stimulus would make not much sense, as you wouldn't know when sufficient electrical activity was generated. Rather, these folks are treated with a CI and fitted with a with a clinical program to restore hearing function in that ear.

Another note of importance is the fact that CI results in substantial loss of the residual hearing in the implanted ear due to insertion trauma, and scarring later on. That's why treating an ear with tinnitus due to localized trauma (e.g., noise-induced hearing loss, for instance due to occupation or in professional musicians) may not be a good idea, because a CI doesn't restore acoustic hearing, it allows for the restoration of some hearing function, but it will never be anything like a normal ear. That's why, in general, only ears with severe-to-profound hearing loss should be treated with a CI, including for tinnitus patients (Punte et al., 2011. In the end, it will be a cost-benefit consideration between physician and patient to determine whether the loss of residual hearing weighs up against the chance and extent of treating the tinnitus.

CI
Fig. 1. Cochlear implant system. Source: Mayo Clinic

References
- Assouly et al., BMJ Open (2021); 11: e043288
- Fu Shannon, Ear Hear (2002); 23(4): 339-48
- Linhares de Freitas Borges et al., Brazilian J Otorhinolaryngol (2021); 87(3): 353-65
- PLoS Med (2005); 2(6): e194
- Punte et al., Cochlear Implants Int. (2011); S1: S26-9

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    $\begingroup$ Thanks for taking your time to provide this extremely information-rich, well-researched answer. Just a small clarification, if I may: as we know, cochlear implants are designed to restore hearing, not to treat tinnitus. If treating tinnitus requires a simpler mechanism (i.e., constant stimulus, and covering just the outer part of the cochlea), it is still possible that there are simpler interventions that don't require drilling the cochlea, right? Or would anything in that space absolutely need to be internal? $\endgroup$
    – MaiaVictor
    Commented Sep 16, 2022 at 0:45
  • $\begingroup$ @MaiaVictor thanks, happy to write about this stuff! I happen to work in the field :) I have clarified the answer (CIs are indeed designed to treat hearing loss, no more, no less). I however am afraid I don't understand what you mean with 'the outer part of the cochlea'. The cochlea is a bony structure and the auditory nerve cannot be accessed electrically easily through this bony capsule because of resistance issues. The reason that it is inserted in the cochlea is to maintain tonotopic (frequency specific) stimulation... $\endgroup$
    – AliceD
    Commented Sep 16, 2022 at 8:10
  • $\begingroup$ ...this way functional hearing can be more easily restored. Because often tinnitus is caused in deafened ears, treating deafness and collaterally tinnitus is an interesting option. In case tinnitus is caused by causes other than profound hearing loss, a simpler solution may work, yet you have to mask the tinnitus by providing perceivable sounds. Therefore you could generate electrical white noise, but that would impair normal hearing, effectively making you hard of hearing. Inserting a CI at least provides you with meaningful stimuli. I have added the 'masking' effect in the answer, thanks! $\endgroup$
    – AliceD
    Commented Sep 16, 2022 at 8:14
  • $\begingroup$ I was thinking of stimulating the auditory nerve as it comes out from the cochlea, but now I see why that would be impossible. Thanks again for your answer. This disease is debilitating, and the fact it doesn't have a non-psychological treatment is depressing. I hope this will change soon. $\endgroup$
    – MaiaVictor
    Commented Sep 16, 2022 at 15:55

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