From the desk of Gus Mueller
There are times when 20 Questions just aren't enough! We're back again talking about new developments in cochlear implants. Just in case you missed it, last month our 20Q guest author, René Gifford, Ph.D. of Vanderbilt University started things off by telling us that yes indeed, hearing preservation is possible with cochlear implants. She described a procedure that has been used since 1999, usually referred to as electric and acoustic stimulation (EAS) or a hybrid cochlear implant (e.g., implant and hearing aid used in the same ear).
We're pleased to have René back with us. She covered a lot of territory last month, telling us about the specifics of the implant procedure, implant candidacy, what devices are available, the differences among devices, and the research findings from various clinical trials. You can read last month's article here.
When our "Question Man" reached his final 20th question, Dr. Gifford was just starting to explain some interesting findings related to speech recognition with these patients following implantation. Let's join them as they resume their conversation to see how that story ends.
Gus Mueller
Contributing Editor
June 2011
To browse the complete collection of 20Q with Gus Mueller articles, please visit www.audiologyonline.com/20Q
1. Hello again. I enjoyed our talk last month, but I still have a ton of questions that we didn't get to. Can we continue?
Of course, let's get to it. As I recall, we talked about the newest applications of implantable hearing technology that combines electric and acoustic stimulation (EAS) in the same ear via hearing preservation. As I explained, this is accomplished using cochlear implant surgery with electrode arrays of 10 to 20 mm inserted into the scala tympani of patients with bilateral low-frequency hearing with the aim of preserving residual hearing apical to the tip of the array. The importance is that when hearing is preserved, patients can receive acoustic stimulation ipsilateral to the implanted array, contralateral to the implanted array, as well as bilaterally.
2. Right. When we finished you were describing some findings that showed, somewhat unexpectedly, that there was not a measured bilateral advantage when these patients were implanted?
That's correct, and it's important that I point out why. This conclusion was confounded by the fact that the speech recognition measures used for the unilateral/bilateral comparison evaluation only included a single loudspeaker placed at 0 degrees azimuth. So the metrics chosen, unfortunately, were those that did not require (or allow) the listener to take advantage of binaural hearing cues. We know from years and years of hearing aid research that to observe bilateral aided advantages we need spatial separation of the speech and noise so that phase cues can be used. More complex listening tasks such as speech recognition in a diffuse noise background like a cocktail party effect (e.g., uncorrelated noise) and reverberant speech and nose are more likely to showcase the benefits of preserving acoustic hearing in the implanted ear.
3. That makes sense. On a similar topic, didn't you also mention that there have been no studies showing a correlation between degree of hearing preservation and postoperative performance?
Yes, I did. However, there are very few studies that have examined this. Gantz et al. (2009) showed no correlation between the degree of hearing loss and the postoperative improvement on tasks of monosyllabic word recognition for 87 recipients of the Nucleus Hybrid S8 device. However, monosyllabic word recognition is not a task that would necessarily be expected to be sensitive to the potential benefits of binaural acoustic hearing—other than a small binaural summation effect. Our preliminary data are showing that there is a trend that those with the best postoperative hearing in the implanted ear have more of an EAS effect for speech recognition in complex listening environments...but the data are still quite preliminary. Plus, it is unclear how the hearing in the non-implanted ear factors into the equation. This is another area that is being actively investigated.
4. Wouldn't a lack of a correlation between degree of hearing preservation and the magnitude of the EAS benefit argue against the need for hearing preservation surgery?
Good question, and it would at face value; however, there are many other factors worth considering. Research has shown that minimally traumatic surgical techniques (also referred to as "soft" surgery) are more likely to result in hearing preservation in the implanted ear (e.g., Fraysse et al., 2006). Such surgical techniques are also known to result in less trauma to delicate intracochlear structures (e.g., Aschendorff, Kromeier, Klenzer, and Laszig, 2007) which may, in turn, result in higher levels of postoperative speech recognition—even in the absence of hearing preservation. Carlson et al. (in press) completed a retrospective review of 126 implanted patients who had met audiometric criteria for Hybrid/EAS preoperatively but who had received a conventional, long-electrode implant. We used measureable postoperative hearing in the implanted ear as a biomarker for attenuated intracochlear injury. It turned out that those individuals who had some postoperative hearing in the implanted ear—even if it was considered unaidable—achieved significantly higher levels of monosyllabic word recognition and sentence recognition in noise than the group who did not exhibit any measureable postoperative hearing. Thus we concluded that minimizing intracochlear injury during implantation yielded improved postoperative speech recognition in the electric only condition. What was also interesting, however, was that although the same minimally traumatic surgical techniques were employed for all patients, just under 60% demonstrated preserved hearing. Thus, this is yet another area for future research to prospectively examine surgical technique, electrode design, preoperative candidacy criteria, and even pharmacological interventions.
5. Pretty impressive...so are most surgeons employing minimally traumatic surgical techniques for all surgeries?
No. Not everyone is convinced that these techniques will truly result in better outcomes. I would have to agree that even patients who have not had soft surgical insertion of the electrode array achieve considerable benefit. So, clearly more research is needed.
6. Last month you mentioned that you're actively involved in research with Hybrid/EAS. Can you share some late-breaking findings?
The more I work with these patients in the clinic and in the laboratory, the more I am convinced that hearing preservation certainly is efficacious for speech recognition in complex listening environments. We recently have published preliminary data showing that having preserved hearing in the implanted ear does yield significant benefit for speech recognition in a high-level noise when the noise is diffuse (surrounding the listener). In Gifford, Dorman, and Brown (2010) and Dorman & Gifford (2010) we have shown that the mean improvement in the signal-to-noise ratio (SNR) afforded by preserved hearing in the implanted ear—added to the acoustic hearing from the non-implanted ear—was 3.4 dB. Although 3.4 dB may not seem like a large improvement, recall that every 1 dB improvement in the SNR can translate to an improvement of 8 to 15 percentage points, depending on the listening condition (e.g., Nilsson, Soli, & Sullivan, 1994). These findings are consistent with what has been shown in hearing aid research comparing bilateral vs. unilateral amplification (e.g., Hornsby & Ricketts, 2007). We have also been examining reverberant speech recognition, which is also showing a significant benefit of having bilateral acoustic hearing.
7. In these studies, are the patients wearing two hearing aids plus a cochlear implant sound processor—for a total of three external devices?
I know, it does get confusing. Technically Hybrid/EAS patients are making use of three pieces of external equipment, but packaged within just two. The earliest Hybrid patients wore a separate ITE hearing aid in the implanted ear along with the cochlear implant sound processor. The newest generation Hybrid/EAS sound processors have an integrated hearing aid circuit that allows for a combination hearing aid/processor unit—as with the Med El DUET which has had an integrated unit for some time.
8. What about programming the instruments? Is there a specialized prescriptive fitting formula for when a hearing aid and cochlear implant sound processor are used in the same ear?
This is also area of active research interest around the world. There are currently no published studies with large datasets to support one method over the other. For programming the hearing aid component in the implanted ear, the NAL-NL1 prescriptive fitting formula generally has been used. Also, it is generally accepted that frequencies up to the range of 750 to 1000 Hz are amplified—this also is dependent upon the unaided low-frequency thresholds in the implanted ear.
As far as the frequency region over which electrical stimulation will be delivered, there is no general consensus. The options include complete acoustic/electric overlap, no acoustic/electric overlap, and some acoustic/electric overlap. Dorman, Spahr, Loizou, Dana, and Schmidt (2005) examined this issue using simulations of EAS and found that performance was highest when little to no gap between electric and acoustic stimulation was provided. They did not examine a full overlap between electric and acoustic stimulation. Vermeire, Anderson, Flynn, and van de Heyning (2008) reported outcomes for four EAS recipients where having maximum low-frequency amplification with overlap between acoustic and electric frequencies yielded the highest levels of performance for sentence recognition in noise. The small sample size in that study clearly limits the generalizability to the larger EAS population. Thus there is definitely a need for a large-scale study to examine this question.
9. If there is no consensus, how are these devices being fitted in the clinic?
Generally, most clinicians are providing some degree of acoustic/electric overlap. The crossover frequency for this acoustic/electric overlap is generally dependent upon the low-frequency audiometric thresholds. Gstoettner et al. (2008) report acoustic amplification being attempted for all low frequencies where audiometric thresholds are
10. You've mentioned the option of "overlap." Is there any concern of an interaction between the electrical and acoustic signal?
Great question! There are actually two distinct possibilities for an interaction: acoustic stimulus masking electrical stimulation or electrical stimulation masking the acoustic stimulus. Generally when it comes to electrical stimulation, we often think of channel interaction. Channel interaction is the spread of electrical excitation within the cochlea from one electrode affecting detection of an electrical stimulus from another, typically adjacent, electrode. With the simultaneous acoustic and electrical signals present for EAS patients, there will certainly be some level of spread of electrical stimulation but, it is unclear whether or not there will be significant masking from either the electrical stimulus or the acoustic stimuli. There are currently no published studies that have examined this issue, but there is certainly a good possibility. In fact, such a phenomenon has the potential to influence programming of the hearing aid and implant within the same ear.
11. It would seem that many, if not most of these implant candidates would have cochlear dead regions? True?
They probably do, and again, this is something that is gaining a lot of attention worldwide but is not routinely being examined in the clinics nor as part of the clinical trials. As we know, cochlear dead regions are intracochlear locations where inner hair cells (IHCs) are largely dysfunctional or completely absent. Since IHCs are the primary sensory transducer cells, if they are not functional, even the world's best hearing aid couldn't transmit those frequencies to the auditory nerve. I tell my patients that it's like having a world-class, high tech stereo system with a broken speaker.
Research has shown that individuals with 1) precipitously downward-sloping hearing losses and 2) thresholds in excess of 70 dB HL are significantly more likely to have cochlear dead regions (e.g., Vinay & Moore, 2007). This is likely why this configuration of hearing loss is one of the most difficult to fit with hearing aids and also why these individuals are generally not satisfied with high-frequency amplification. This is something that should be receiving more clinical attention as it relates to implants. It's certainly possible, for example, that cochlear dead regions could act at a biomarker for Hybrid/EAS candidacy, but I'm not aware of any published studies that have examined this topic.
12. Is there any reason to think that cochlear dead regions would be associated with poorer results with cochlear implantation?
Interesting question, as maybe you're thinking about the possible association with absent spiral ganglion cells. There are no data, however, to suggest that the degree of spiral ganglion cell survival is in any way correlated with postoperative speech perception (e.g., Nadol et al. 2001; Gassner, Shallop, & Driscoll, 2005; Fayad & Linthicum, 2006). In humans, spiral ganglion cell atrophy and death is a very slow process that occurs over decades following onset of severe-to-profound deafness. In fact, there have been histological studies with human temporal bones that have shown no correlation between degree of inner or outer hair cell loss and spiral ganglion cell survival (e.g., Linthicum & Fayad, 2009).
13. Regarding other options, what about using hearing aids with frequency lowering for this population?
Well yes, there are commercial hearing aids available with frequency compression and frequency transposition. I would always advocate the least invasive treatment option before a pursuing surgical intervention. But, this is an area that has not been studied extensively, and actual real-world benefits observed for adults, on average, appear to be equivocal (See Benter & Mueller, 2011, for review).
We did a study with this population of individuals several years back that showed modest improvement on speech recognition tasks using proportional frequency compression (Gifford, Dorman, Spahr, & McKarns, 2007). We described the outcomes of one patient who opted for cochlear implantation - without hearing preservation-and the degree of improvement was significantly higher with the cochlear implant over the frequency compression hearing aid. Further I can give you anecdotal evidence that many of these patients have already tried these technologies and by the time they come in for a Hybrid/EAS evaluation and are still struggling with communication. Nevertheless, this is another obvious area for future research. On another note, it is important for all readers to be aware that frequency transposition (e.g., Widex Inteo) and frequency compression (e.g., Phonak Naida) are completely different technologies and thus far, it is not clear which may be more useful for this population.
14. What about the possibility of bilateral EAS? Is this something you've looked into?
This is the best case scenario! The bilateral EAS recipient would have binaural acoustic hearing to take advantage of interaural time difference cues as well as loudness balancing across ears—the latter of which is something that can be difficult, if not impossible, to overcome with a single implant. Theoretically bilateral EAS recipients would have the greatest possibility for localization, speech recognition in diffuse noise, equivalent head shadow across the ears, and also possibly binaural squelch. There has been one case study published by Klein Punte, Vermeire, and van de Heyning (2010) out of Belgium and a few reports of bilateral EAS at the cochlear implant meetings. The Klein Punte et al. (2010) case study reported ceiling level performance for sentences in quiet and noise following bilateral EAS as well as significant subjective benefit on the APHAB. This is an area that I expect to gain considerable momentum in the very near future!
15. As I've been listening to you talk about different studies, I've noticed that there seems to be a lot of research in this area out of Europe...why is that?
Europe tends to be the leader in many types of biomedical technology research and clinical practice as their regulatory practices are much less stringent than that in the U.S. The result is that medical technology reaches clinical trial and marketability much sooner. Whether this a good thing is a topic that could take its own 20 questions!
16. So, if I am an audiologist or hearing aid dispenser in private practice, how do I know when I should refer someone for an EAS evaluation?
This is a question that is not easily answered. For conventional cochlear implant candidacy, I generally recommend that once an individual reports that they are no longer able to talk on the phone, it is time to refer for a cochlear implant evaluation. For EAS candidates, they are often still able to talk on the phone, though with some difficulty. EAS candidates are those patients who are generally dissatisfied with amplification and are struggling to communicate due to unavailability of high-frequency speech cues. Usually they have exhausted all available options with modern hearing aid technology. If you have a difficult-to-fit patient who has relatively good low-frequency hearing and severe-to-profound hearing frequency hearing loss, it would be a good idea to at least mention EAS.
17. Where would I refer my patients for a Hybrid/EAS evaluation?
As I mentioned, these devices are currently only available via select centers across the country involved in the FDA clinical trial. For information on Med El Flex EAS, the following website provides a list of clinical trial sites: www.medel.com/us/show/index/id/445/titel/Take+the+Next+Step+
For information on Nucleus Hybrid S12 and L24, you could contact Cochlear Americas customer service at 1 800 523 5798.
18. You mention two different products. Does it matter which one I refer to?
Recall, last month we discussed that in terms of device selection, the choice will likely depend upon the risk/benefit ratio with which the individual patient and their surgeon feel most comfortable. For patients most concerned about degree of hearing preservation, they would likely be best served with a shorter array—like the Hybrid S12. For patients wanting to preserve hearing but also interested in maximizing the likelihood that should considerable hearing be lost that the implanted device will provide high levels of speech perception without requiring a second surgery, then a longer electrode array is probably best - like the Hybrid L24 or the Flex EAS. Luckily there are multiple available options out there!
19. What if the patient doesn't want or cannot travel to one of the clinical trial sites...what would you recommend?
Again, as we discussed before, increasingly more patients are experiencing preservation of hearing even with conventional, long-electrode cochlear implantation. Thus it would be in the patient's best interest to inquire with their neuro-otologist about the possibility of hearing preservation surgery using a standard implant. At issue is the diameter of the electrode array, the amount of hearing to be preserved, use of intraoperative steroids, size of the cochleostomy, and a number of other factors. Research has shown that for individuals with Hybrid/EAS-like hearing, almost 60% will have measurable postoperative hearing in the implanted ear (Carlson, Gifford, Archibald, & Driscoll, in press). Of course, they would need to meet conventional cochlear implant criteria which is no greater than 60% correct for open-set sentence recognition in the best aided condition (and no greater than 40% correct for Medicare patients).
20. So it seems as if hearing aids and cochlear implants are truly becoming merging technologies...
Absolutely! The Hybrid/EAS cochlear implant system with the goal of hearing preservation is really indicated for those in-between patients who struggle with amplification due to the severity of high-frequency hearing loss, but who are generally performing too well with their hearing aids to qualify for a cochlear implant. And I believe you'll see even more blending of the technologies in the next few years. We might want to have another discussion!
About the Author
Dr. René Gifford is an assistant professor at Vanderbilt University and the Director of the Cochlear Implant Program and Pediatric Audiology at the Vanderbilt Bill Wilkerson Center. She is also the Director of the Cochlear Implant Research Laboratory at Vanderbilt where she carries out her NIH funded research on combined electric and acoustic stimulation (EAS). She got her undergraduate and Ph.D. degrees at Arizona State University and her M.S. at Vanderbilt University—where she also completed her CFY (back when we had CFY's). She was happy to join the faculty back "home" in Nashville and has been teaching AuD, SLP, and MDE students as well as mentoring students in the lab. Dr. Gifford has authored over 20 peer-reviewed articles, multiple book chapters, and is currently working on a book for Plural Publishing's Core Clinical Concepts series entitled "Cochlear implant patient assessment: from candidacy to postoperative performance and outcomes." When she's not working, you can often find her with her husband and three sons at the skate park, football or baseball field, or just hanging out at home where she would like to think of herself as an aspiring master gardener.
References
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