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Hearing Journal:
doi: 10.1097/01.HJ.0000325654.95528.53
Page 10

Cochlear implants and hearing aids: Converging/colliding technologies

Fabry, David

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1 For starters, I thought you were a hearing aid guy! What are you doing talking about cochlear implants?

Well, I have spent most of my career working with hearing aids. That said, some of my most important professional role models have straddled the two worlds of hearing aids and cochlear implants. These include Dianne Van Tasell, Darrell Rose, Chris Turner, and especially Margo Skinner, who switched priorities at the top of her game from hearing aids to cochlear implants.

Figure. David Fabry...
Figure. David Fabry...
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Although I never studied with her, Margo was an unofficial mentor to me for over 25 years. We often discussed the similarities and differences between the fields of hearing aids and cochlear implants, and how more recently they appear to be converging, perhaps colliding. Plus, many technologic changes affect both hearing aids and cochlear implants, including digital signal processing, compression, directional microphones, multiple programs for specific listening environments, acoustic scene analysis, and noise cancellation.

There has also been convergence between them in the areas of candidacy, speech-recognition measures, and verification.

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2 I heard that. Now they use the HINT with implant candidates, right? But they use it to evaluate speech recognition in quiet! Doesn't that seem a little oxymoronic?

Don't call me a moron, and I would respectfully request that you refrain from personal insults. Remember, I'm a product of the Wisconsin public school system, not the Ivy League, so I don't have your fancy vocabulary.

Although the HINT was originally developed as an adaptive signal-to-noise ratio measure for hearing aid use, it was adapted and adopted by cochlear implant researchers and clinicians because it has higher “face validity” and is more difficult than some traditional speech measures. For prospective implant patients, the HINT has traditionally been used as a candidacy measure as a sentence test, typically delivered at 60 or 70 dB SPL in quiet. If patients identify fewer than half the sentences with appropriately fitted hearing aids, they will likely do better with an implant than with amplification. For that reason, however, maybe the test should be renamed the HINT-QT or the HIQT to differentiate this application from the original. The bottom line is that these types of measures have helped move cochlear implant candidacy beyond the audiogram.

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3 What do you mean “beyond the audiogram”? Sounds like Star Trek: The Final Frontier to me.

Well, in a sense, you're correct, because communication is the final frontier. We have all observed two patients who have identical hearing thresholds, yet function very differently in real-world situations with hearing aids. The audiogram simply does not serve as a good predictor of disability, and clinicians wanted a better way than using the PTA or SRT to define cochlear implant candidacy. Enter the HINT, which serves as a reliable and efficient tool for measuring how well a person understands connected discourse.

The challenge now is that as implants continue to improve, some researchers and clinicians contend that the traditional HINT-QT (delivered in quiet) is not sensitive enough. They say that new measures are needed to determine candidacy and measure outcomes.

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4 Really? Like what, the HINT administered as it was de-signed to be—in noise?

Exactly, but there are other candidates as well, such as the QuickSIN or the BKB SIN. Other choices could be the AZ-BIO, a test developed by Michael Dorman and colleagues, or the TIMIT materials (developed collaboratively by Texas Instruments and MIT to evaluate automatic speech-recognition systems), which are designed to evaluate more “dynamic” properties of speech in quiet or speech in noise. Each measure uses speech, but some use multiple talkers or presentation levels, different types of background noises, or different amounts of context to stress real-world communication that is important for both hearing aid users and cochlear implant recipients.

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5 Wouldn't it be simpler to de-velop one set of materials that could be used for both groups of patients?

Sure, but the challenge is to avoid floor and ceiling effects; that is, despite the improvements in both hearing aid and cochlear implant technology, it is still difficult to develop a measure challenging enough that the best performers don't score 100% and the poor performers don't score 0%.

In fact, researchers are working diligently to develop measures that may be applied universally to assess candidacy and measure outcomes. Currently, that means looking at adaptive measures (e.g., SRT in noise), subjective measures (e.g., Acceptable Noise Level), or speech materials that don't have a steep performance-intensity function.

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6 Performance-intensity functions? You're probably not a Yankee fan, but why pick on Roger Clemens?

You're right about the Yankees, but no, I was referring to speech-recognition performance as a function of presentation level. If the performance-intensity function is too steep, small increases in audibility will result in large performance improvements. For example, in measuring SRT, we use spondaic words with familiarization because the words are easy and homogeneous; as a result, the performance-intensity function increases at a rate of roughly 10%-12% per dB. In comparison, nonsense syllables are much more challenging, and when used in open-set testing may have an increase of only 4%-5% per dB.

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7 So, are you saying that nonsense syllables are the best stimuli for general purpose?

Theoretically, but they are really boring, quite challenging, and hearing aid and implant users often become very frustrated with their level of difficulty. More research is required, but the ideal speech materials for use with candidacy and verification of outcomes will probably combine multiple talkers, multiple levels, multiple noise sources, and multiple listening environments.

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8 Wow, that sounds like a lot of work—and clinical effort. Just what we need—another time-consuming test battery for which we are not paid.

True, but we are partially to blame for the situation that has been created by the “bundled” pricing structure used for both the hearing aid and cochlear implant service-delivery models. Unless and until we use evidence-based outcomes to establish clinical best practice test batteries and demonstrate the benefits of our services, the focus of compensation will be on the device, rather than the assessment and treatment strategies.

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9 Okay, you've made your point. What other similarities and differences are there in how hearing aids and cochlear implants are measured?

Well, the first thing that comes to mind is how initial program settings are verified. With hearing aids, probe-microphone measures serve as an objective technique for measuring insertion gain and mapping the output of real speech as a function of frequency. Although this doesn't assess perception, it is an excellent way to assess speech audibility and ensure that prescriptive targets are met in situ. Of course, we know that audibility is a necessary but not sufficient prerequisite to speech intelligibility, and probe-microphone measures provide a very effective objective tool for ensuring that speech is audible without requiring a behavioral response. This is, of course, very useful when working with very young and very old patients, as well as those who cannot easily complete speech recognition or functional gain testing.

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10 I agree, but you and I both know that most people dispensing hearing aids don't do this testing. So what really is the difference?

Good point. The main difference is that at present there is no well-established objective method, like matching prescriptive gain targets via ear canal SPL, for objectively verifying that initial cochlear implant gain targets are being met, because it is an electrical signal, not sound in the ear canal. Imaging telemetry (e.g., NRT, NRI) may be used to verify that electrodes are working and also to help set initial map characteristics, but these are still mostly a research tool at present.

In the future, telemetry may assist with automatically optimizing the signal processing to ensure that soft sounds are soft, moderate sounds are comfortable, and loud sounds are not uncomfortable over as broad a frequency response as possible.

Also, some researchers have found that electrically evoked stapedius reflex thresholds (ESRT) can be used to provide frequency-specific information for setting cochlear implants. In this respect, the potential for this technology is even better than for our ear canal probe-microphone measures because it theoretically ensures both audibility and perception.

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11 Could these techniques potentially eliminate the need for measuring aided and unaided audiograms?

Possibly, but there are probably better reasons to consider abandoning functional gain measurements as a verification measure. The trend with both hearing aids and cochlear implants has been toward increased use of non-linear amplification (compression), which means that gain varies as a function of stimulus input. Functional gain measures were sort of okay when dinosaurs roamed the earth and hearing aids were linear, but they are inappropriate when you literally have a “moving” target.

I'm not here to say there is no utility in measuring “optimal aided thresholds” (assuming you do it in a manner that provides reliable results) to give a relative indication of aided performance, but functional gain usually will overestimate how much gain a hearing aid or cochlear implant provides for conversational speech.

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12 Let's change the subject. What can you tell me about combined electric and acoustic hearing in the same ear? Is this really possible?

Yes, it is, and there are a number of global experimental trials going on right now with electroacoustic (hybrid) stimulation. The results are quite exciting, particularly in terms of speech recognition in noise and music perception. Think of hybrid stimulation as the ultimate auditory dead region solution for patients with precipitous high-frequency hearing loss (Figure 1).

Figure 1
Figure 1
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Over the years, we've worked with a lot of these patients, as they have suffered from noise trauma, ototoxicity, or even presbycusis. Many of these patients obtain minimal benefit from traditional hearing aids because they need little or no amplification in the low frequencies and they often have severe to profound high-frequency loss. Although frequency-compression hearing aids have the potential to provide benefits, they aren't for everyone.

Another approach developed by coch-lear implant companies involves implanting an electrode array into the ear without damaging the residual auditory structures, so that the patient can hear acoustically and electrically in the same ear.

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13 Wait a minute! I thought that most patients lose their residual hearing when an implant is placed in their cochlea.

You are correct, sir. That said, if the surgeon uses a shorter electrode array or a less invasive insertion technique, the patient can use electric stimulation for the high-frequency information and acoustic hearing for low frequencies.

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14 Whoa! Can people make sense of that combined electric and acoustic information?

Yes. In the past, on rare occasions, some patients had post-operative hearing after a traditional cochlear implant surgery, but for this approach to be successful the trick is to preserve hearing routinely. Also, years ago, Schulman and House used a shallow-insertion cochlear implant for tinnitus suppression. Although the results (in terms of tinnitus suppression) were mixed, the patients reported being able to understand speech and other sounds normally with the device in place.

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15 So, is the premise that preserving the acoustic information will provide better results than electric hearing alone?

Yes. In fact, results from several publications suggest that patients with combined acoustic and electric stimulation have improved speech recognition in noise and better sound quality when listening to music than do most patients with traditional cochlear implants. This makes sense, because the additional acoustic fine structure augments the excellent intensity and temporal coding provided by electrical stimulation to provide a more robust stimulus to the brain.

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16 What if the patient has a traditional “long-insertion” implant and wears a hearing aid on the non-implanted ear?

You have hit on the key issue of the importance of having the electrical and acoustic stimulation in the same ear versus “bimodal” stimulation where the acoustic and electric information enters the periphery in separate channels and then meet in the higher pathways. The results of studies that have evaluated this are equivocal, but remember that they do require the non-implanted ear to have some residual auditory function.

The bigger issue is that studies seem to suggest strongly that the acoustic signal contains valuable information that cannot easily be presented electrically, and so it stresses the importance of preserving hearing whenever possible.

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17 Interesting. It sounds as if there could be a dilemma. Should we implant with a hybrid early or wait until the patient's hearing is bad enough to qualify for a “traditional” implant?

Exactly. That is the ongoing debate now, along with whether bilateral implants should become the standard of care.

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18 Wait a minute, bilateral? What percentage of coch-lear implant users have one in each ear?

Currently, the binaural fitting rate for implants is around 15%, compared with 78% for hearing aids. Of course, implants cost a lot more, require surgery, and don't come with a 30-day trial period, but the trend has been the same as for hearing aids.

Furthermore, cochlear implants are considered a Class III Medical device by the FDA, so they are a little particular about changing the standard of care. That, in combination with the fact that cochlear implants cost over $50,000 per ear, compared with approximately $2000 per ear for hearing aids, and you can understand why caution is advised. The results, however, suggest that localization and speech recognition in noise are improved when two ears are implanted instead of one.

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19 What about directional microphones? Have they taken the cochlear implant world by storm, as they have with hearing aids?

I think the latest information is that directional microphones are now used in about 40% of all hearing aids sold.

Directional microphones are definitely used with cochlear implants, but not to the same extent, and many clinicians are more skeptical of their benefits for implant wearers than for hearing aid users. I'm not sure, but I suspect this is due to concerns that young children may be better off with omnidirectional microphones due to their need for incidental listening. For older children and adults, however, the trend has been increasingly toward using directional microphones when background noise is present. Interestingly, cochlear implant engineers are thinking more and more about preserving pinna and concha cues for “preprocessing” of sound than they have in the past, with impressive results.

Another area ripe for convergence is increased use of wireless FM technology by both adult hearing aid and cochlear implant users. There's no question that this technology remains the best way to preserve or enhance signal-to-noise ratio for a single talker in background noise or reverberant listening environments.

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20 Well, that covers a lot of ground. It looks as if some day the same patient might have her hearing aid and implant adjusted by the same person! That would be great, because right now in many facilities, the only time hearing aid audiologists and CI audiologists get together is for lunch or a beer.

We'll see. It seems as though the patient's best interests are served if the same practitioner is comfortable with both technologies, but until now that has been the exception rather than the rule.

Like a thick boiling soup.” That's how Alessandro Volta described what he heard in 1790 following what may have been the first experimental electrical stimulation of the auditory system. More than a century and a half later, in the 1950s, reports were published about the use of an electrode to directly stimulate the 8th nerve. Wearable devices became available in the 1960s, and today cochlear implants are a routine treatment for hearing loss for both adults and children.

If you have followed the research and progress of cochlear implants, you've probably noticed, not surprisingly, that many of the questions raised are similar to those we ask about hearing aids. Candidacy? Best compression technology? Advantages of directional microphones? Speech measures as verification strategies? Bilateral benefits?

While hearing aids and cochlear implants have a lot in common, for various reasons (some not entirely logical), audiologists who are “hearing aid people” don't tend to be “cochlear implant people,” and vice versa. So, to write this article about the convergence of these two technologies, we looked for someone who works both sides. Such a person is David Fabry, PhD.

Dr. Fabry is chief of audiology at the University of Miami, Miller School of Medicine, Miami, FL. He is a past-president of the American Academy of Audiology and recently was named editor of one of the academy's journals, Audiology Today. Dave Fabry also has faculty appointments at the University of Minnesota, Nova Southeastern University, and the University of Miami. He previously served as vice-president of professional relations and education for Phonak Hearing Systems and director of audiology at Mayo Clinic in Rochester, MN. Some of you might even remember his days at the research section of Walter Reed Army Medical Center.

Dr. Fabry is known internationally for his informative, insightful, and often innovative workshops. He is also among a select group of professional lecturers who has had an audience response keypad (#45) write its own yearlong blog about its adventures (see www.earTunes.com).

Despite spending the majority of his life on the [frozen] tundra (even attending “Lombardi” Junior High), the word on the street (and beach) is that Dave has quickly adapted to the subtropical climate in Miami. His ice-hole auger was sold at a Rochester yard sale, and he is now learning how to snag the elusive barracuda, sea trout, and snapper. But don't be fooled—this article is not a Florida “early bird special.” Dave has put together a great buffet of what's happening in the world of cochlear implants.

GUS MUELLER

Page Ten Editor

© 2008 Lippincott Williams & Wilkins, Inc.

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