Cochlear Implants, Residual Hearing, and Hybrid CIs

“When you get a cochlear implant, all of your natural hearing is destroyed.”
This used to be the common wisdom: get a cochlear implant, forgo any residual hearing (hearing that you have without the use of hearing devices).  Today, however, less traumatic surgical techniques and improved electrode arrays have proven this to be untrue.  What is the most current information?  

  

The latest research on residual hearing post-implant shows that quite a few patients do retain what hearing they had before the procedure (see James et al., 2005, Gstoettner et al., 2004Kiefer et al., 2004, and Hodges et al., 1997, as a small sampling to start).  Why is this possible now when we thought cochlear implants equaled no residual hearing “back then”?  It’s a combination of factors.  First, the electrode arrays themselves have improved significantly.  They are more flexible and have a contoured design that causes less damage to the hair cells in the cochlea.  Surgical techniques have improved as well, making the whole experience less traumatic to the inner ear.  Because of these advances, some, if not all, of the patient’s residual hearing can be preserved.  The studies of residual hearing preservation that we have currently are still limited in the number of patients assessed, so we do not have good percentages on the likelihood of any one patient having preserved residual hearing post-surgery.  (If you are a CI user and interested to know if your residual hearing (if you had any) has been preserved, ask your audiologist to do unaided threshold testing in the booth at your next mapping appointment.)

  

Patients with some residual hearing post-CI may benefit from electro-acoustic stimulation, that is electric (cochlear implant) and acoustic (hearing aid) hybrids.  In the past, I’ve heard case studies of patients who had residual hearing and were simply fitted with an ITE (in the ear) hearing aid in addition to wearing their CI processor.  Today, two companies, Cochlear and MED-EL, each have hybrid hearing solutions.  To understand how a hybrid implant works, it is important to understand that the cochlea is tonotopically arranged.  This means that the hair cells in each region of the cochlea respond to different frequencies (pitches) of sound.  High frequency sounds are registered at the basal end (the entrance to the “snail shell”), and low frequencies are registered at the apex (the top of the spiral).  Unlike the preservation of residual hearing with a traditional CI electrode array, the electrode arrays made specifically for hybrid implants are shorter.  They enter the cochlea and stimulate the high frequency cells at the base, but do not go up to the apex, where the residual low frequency hearing is found.  HERE is more information on hybrid CI candidacy, though note that the information provided on hybrid devices is out of date and does not reflect the current market (see instead the links to Cochlear and MED-EL above).  Note for US patients: currently, the Cochlear device is the only hybrid implant with FDA approval.

  

Yet people continue to cite the “destruction of residual hearing” as a reason not to implant, or to delay implantation, for themselves or their children.  Why does this persist?  I think the vast majority of people who still buy into the CI = no more residual hearing myth are simply misinformed.  They’ve read this information on old websites, or (even scarier) been told this by hearing health professionals who are not up to date with the literature.  A small but vocal minority who argue against CIs on the basis of destruction of residual hearing are anti-CI overall, and use this outdated and inaccurate argument to scare people, but I think/hope that this is the motivation of only a minority of those who mistakenly share this information.

  

 Another thing to consider when debating the cost of lost residual hearing versus the benefit of CI surgery is this: How much is that residual hearing “worth” anyway?  While it is great to have the unaided ability to hear low frequency tones at loud levels, what does this buy you in terms of overall language development (for a young child) or conversational ability (for teens and adults)?  That low frequency residual hearing, as nice as it may be, is not going to give the brain the access to sound it needs for developing listening and spoken language.  It’s a no-brainer, and the great part is, you might not even lose that hearing anyway!

  

The last objection I hear about the potential loss of residual hearing is, “What about future technologies?”  If I get the cochlear implant now, and it destroys my residual hearing (which it may not), does that mean I can’t benefit from new innovations, like hair cell regeneration, down the line?  Based on the state of the field of speech and hearing science today, while the promise of hair cell regeneration to reverse/correct sensorineural hearing loss is under investigation and may someday be a reality, it is far, far, far from ready to be used in humans anytime soon.  Truly, it may not happen in our lifetime.  It certainly isn’t worth the gamble of depriving the brain of auditory input (remember, it’s all about the brain, and if you don’t get the input, that brain changes forever!).  If and when hair cell regeneration does become available, it’s unlikely that it won’t be able to repair hair cells damaged by the insertion of the cochlear implant — after all, the point of hair cell regeneration is to regenerate hair cells — the premise of the thing is to fix what’s been destroyed!  Do not wait to benefit from hearing technology that is available today in hopes of an uncertain “cure” in the future!

  

So what’s the take-home message?

  1. Insertion of a cochlear implant no longer means the destruction of residual hearing.

  2. Preservation of residual hearing is not a good reason to decide against a cochlear implant.  A small amount of low frequency hearing is nothing compared to the access to sound (and the brain development that facilitates) that a cochlear implant provides.

  3. Having a cochlear implant does not preclude benefitting from future hearing technology.  Don’t sacrifice your present for the hope of the future.

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One thought on “Cochlear Implants, Residual Hearing, and Hybrid CIs

  1. We chose a surgeon with an excellent reputation for residual hearing preservation (it’s his area of research) for my daughter but unfortunately saw a 50 dB drop in the lows (from 30 dB at 500 & 1000 Hz down to 80 dB and from 65 dB at 1500 down to 85 dB). So it basically did destroy the residual hearing in that ear. But the flip side is that she now has access to those high frequencies that even the best digital hearing aids couldn’t give her. So it was worth sacrificing the natural lows to gain access to the full range of frequencies necessary for speech.

    It’s possible that the hybrid CI might’ve done a better job at preserving the residual low hearing but there’s no guarantee and as the mutation means my daughter’s loss will progress, it made more sense to go with the full-length traditional CI.

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