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The Best of 2009: Cochlear Implants

Zeng, Fan-Gang

doi: 10.1097/01.HJ.0000382728.01909.f3
the best of Audiology Literature
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This daunting task didn't get any easier in 2009. The year produced a record 660 articles related to cochlear implants, compared to 580 in 2008 (try searching “cochlear+implant” at www.pubmed.gov). In addition, we have our “allotted” pages in the Journal, so our Journal Club Captain Gus Mueller has been pounding into our heads, “Less is more.” Thus, the pressure was on from both sides: I had to highlight fewer articles from a pool that was 14% larger this year than last.

Since this is my second year with the Journal Club, I'm not a rookie anymore and I was better prepared for the task. Not only did I subscribe to PubMed to receive its monthly collection of implant-related articles, but I also received alerts on good papers from students, friends, and colleagues all over the world. I especially thank Janice Chang, John Galvin, Tom Lu, Jace Wolfe, and Hsin-I Yang for their help in selecting the best cochlear implant papers of 2009. But I take full responsibility for the final selections you see here.

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GREAT FOR THE CLINICIAN

Everybody knows how important it is to fit a cochlear implant properly. Clinical fitting (or “mapping”) has been traditionally achieved by asking implant users how loud they perceive an electric stimulus. With implantation occurring at earlier and earlier ages, sometimes in infants only several months old, getting reliable subjective responses is often impossible. Clinicians have to rely on alternative cues from eye and head movements to “loud crying” by the baby. Wouldn't it be nice if a computer program could automatically and accurately determine the map for you? Lutz Gärtner and colleagues have made significant progress toward achieving this goal. In a paper published in Acta Oto-Laryngologica, the authors first recorded electrically evoked compound action potentials and then analyzed these recordings to automatically estimate hearing thresholds in 30 cochlear implant patients. Not only were the authors able to show good agreement between automatically estimated thresholds and audiologists' measured thresholds, but they did so with high efficiency—less than a minute per electrode, comparable to the time an audiologist might need.

There are many other worthy papers in this category, but I will restrain my selection to the following two related to management of neurological disorders. First, should we automatically recommend cochlear implantation to children affected with auditory neuropathy/dys-synchrony? The answer is no, according to Gary Rance and Elizabeth Barker in their paper in International Journal of Audiology (IJA) that systematically compared cochlear implants and hearing aids and found them to be equally effective for management of pediatric auditory neuropathy. The challenge is to identify which children are likely to benefit from a hearing aid. Additionally, the hearing aid should be fitted appropriately, not necessarily according to audibility as in a standard fitting, but to the severity of auditory neuropathy. Sound challenging? It does to me.

The second paper concerns neurofibromatosis, a disease characterized by tumors growing in the nervous system that affects one in every 3000 individuals. Specifically, neurofibromatosis Type 2 refers to tumors growing bilaterally on the 8th nerve, and is usually diagnosed in young adults with symptoms of tinnitus or imbalance. Auditory brainstem implants have been used to restore hearing in these patients, but with limited benefit, providing only 10% open-set speech recognition, compared with 70%-80% in the average cochlear implant user. In a paper in Otology and Neurotology, Vittorio Colletti et al. compared brainstem implant performance between tumor and non-tumor patients and found much better performance in the non-tumor patients (∼50 percentage points, on average). Although the study did not find any additional benefit in tumor patients, the encouraging results in non-tumor patients may pave the way toward establishing the auditory brainstem implant as an effective intervention for patients who cannot benefit from cochlear implantation, due to head trauma, cochlear ossification, or other causes. This is an exciting time for clinicians.

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BEST QUICK READ

Following the same criteria as last year, namely, short and interesting, I picked the following six papers as your Best Quick Reads. How well do you think you know the basic ear anatomy? Let's check it out:

  • How many cochlear turns do we have? The textbook answer is 2–1/2 turns. But Slavomir Biedron and colleagues report in Otology and Neurotology that 65% of us have more than 2–1/2 turns, and 11% have 2–3/4 to 3 turns.
  • How thick is our cochlear nerve bundle? The answer is 1.1–1.4 mm in diameter, according to a high-resolution magnetic resonance imaging study by Eric Jaryszak and co-authors writing in Laryngoscope.

Here are four more of my favorites:

  • How much acoustic hearing is too much for pediatric cochlear implantation? “Borderline” hearing levels ranged from 70 to 90 dB HL, according to a survey from 11 centers in Canada, as reported by Elizabeth Fitzpatrick et al. in IJA.
  • How much residual hearing is useful for music perception with cochlear implants? 85 dB HL, according to Fouad El Fata and colleagues, who compared song recognition in individuals with bilaterally combined electric and acoustic hearing and reported their findings in Audiology and Neurotology.
  • Is tinnitus prevalent in children with cochlear implants? The answer is yes. Tinnitus was present in 38% of 40 children with cochlear implants, ages 3–15 years old, reported Neil Chadha et al. in International Journal of Pediatric Otorhinolaryngology.
  • Does brain activity at rest tell us about cochlear implant performance? Yes, increased auditory cortical activity coupled with decreased activities in other areas, even in the absence of stimulation, were correlated with better performance. So reported Kuzma Strelnikov and co-authors writing in Cerebral Cortex.
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NOT READY FOR PUBMED

The American Speech-Language-Hearing Association was especially active in 2009, publishing special issues on cochlear implants in Perspectives on Hearing and Hearing Disorders in Childhood. As put forth by the coordinator of these special issues, Gayla Hutsell, there has been a great change in the pediatric cochlear implant landscape, from continuously relaxing candidacy requirements in terms of hearing loss, age, and multiple disabilities to improved monitoring and assessing the outcomes. I especially recommend the following two articles for their immediate utility in objective cochlear implant fitting and evaluation.

Suzanne Purdy and Kirsty Gardner-Berry review the utility of auditory evoked potentials, particularly the cortical potentials, in candidacy determination, implant programming, and outcome prediction. In a follow-up article, Jack King provides detailed information and expands the scope of objective measures and programming cochlear implants. His discussion of electrically evoked stapedial reflex is both informative and relevant.

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MOST THOUGHT PROVOKING

Cochlear implants have earned their money's worth by improving speech recognition in post-lingually deafened adults as well as in promoting relatively normal language development in pre-lingually deafened children. What's the next patient population for the cochlear implant? At present, all eyes (ears?) are focused on hybrid hearing, given its potential to treat sloping hearing loss (the signature of presbycusis) by combining the use of cochlear implants and hearing aids. Two thought-provoking articles argue for very different approaches.

One article, by Bruce Gantz and colleagues in Audiology and Neurotology, summarized the latest results from the Iowa/Nucleus 10-mm hybrid implant clinical trial. Their intervention was to insert a short electrode array (10-mm) into only the basal part of a cochlea, leaving the apical region undisturbed so as to maximize the preservation of low-frequency acoustic hearing. They were able to preserve low-frequency acoustic hearing in 98% of patients (85 of 87) immediately after surgery and in 89% of patients 3–24 months post-surgery. Compared to pre-surgical performance with hearing aids only, most patients (74%) showed improved performance with hybrid hearing for word recognition in quiet and sentence recognition in noise. The most striking finding was that even for patients whose pre-surgical low-frequency acoustic thresholds were elevated by 30 dB or more 3–36 months post-surgery (30% of all patients in the study), speech performance was significantly improved by hybrid hearing. So, should we rush our patients with residual low-frequency hearing to this 10-mm hybrid implant intervention?

Not so fast, says another article in Audiology and Neurotology, in which Michael Dorman and 15 co-authors present an alternative approach. The alternative approach is to implant a conventional long-array (20 mm plus) cochlear implant, which usually destroys any residual hearing in the implanted ear. Not surprisingly, the conventional implant outperformed the short implant because the former not only covers larger cochlear extent (15 vs. 4 mm) but also has more electrode contacts (22 versus 6). Surprisingly, the conventional implant combined with contralateral acoustic stimulation also outperformed the short implant combined with binaural acoustic stimulation. Considering the risk of losing ipsilateral acoustic hearing due to surgery or eventual aging, the conventional implant seems to be a viable, or even better, treatment option for presbycusis.

I should point out a couple of caveats when considering these two options. As suggested by Dorman and colleagues, the brain may re-map to the 10-mm electrode array if it is given enough time to adapt. Second, the value of preserving residual low-frequency hearing in both ears may have not been fully considered, particularly in complex and challenging listening environments. Let's closely monitor progress in this exciting area of hybrid hearing research. Be sure to remind me if I forget to revisit this issue in a couple of years.

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ALL-AROUND FAVORITES

Let's now turn our attention to bilateral cochlear implantation, as all of my All-Around Favorites involve bilateral implants. To give you an idea how hot this area has become, bilateral implant research accounted for nearly 20% of the 660 articles published in 2009.

First of all, I have noticed a significant position shift in the argument against bilateral implantation. Although it is still debatable whether or not bilateral implantation is cost effective, a series of studies has documented the benefit of bilateral implants over unilateral implants in sound localization and speech recognition in noise. The most notable reversal comes from Lovett et al., who previously questioned the cost effectiveness of bilateral implantation. Their recently published study in Archives of Disease in Childhood contributed to a change in the UK guidelines that now recommend bilateral implantation for children with severe-profound deafness.

At a mechanistic level, the bilateral benefit is still derived mostly from the head-shadow or better-ear effect rather than true binaural interaction enjoyed by normal-hearing listeners, according to a study on bilateral implant speech recognition in a cocktail-party setting, published by Philipos Loizou and colleagues in Journal of the Acoustical Society of America.

If we probe the mechanism further, we find that the lack of true binaural benefit is due to at least two factors. One is the implant design. Currently, bilateral implants operate independently, unlike the two ears of normal-hearing listeners. The other factor is the time required for the bilateral implant users to (re)learn binaural listening skills. In a study published in Otology and Neurotology, Rose Eapen et al. showed that the binaural interaction capability continued to improve 4 years after bilateral implantation.

Finally, it would be a disservice if I didn't mention the current controversy regarding the effects of bilateral implantation and bimodal stimulation on language development. Writing in Trends in Amplification,Susan Nittrouer and Christopher Chapman found no difference in language performance in 58 children who had either one implant or bilateral implants, or were bimodal using an implant plus a hearing aid. However, when these children were grouped according to whether or not they had any bimodal experience, the authors found a significant advantage for children with bimodal experience in generative language skills, such as the mean length of utterance and the number of pronouns. The authors believe that this advantage is due to pitch-related prosodic cues that can be accessed by hearing aids but not by cochlear implants. The implication is that bilateral implantation at very young ages, particularly simultaneous bilateral implantation, may not be good for language development, at least the generative part of language. Bilateral implantation has won the battle over unilateral implantation, but not yet the war.

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IN CLOSING

Looking back at how much the cochlear implant field has advanced, I cannot help noticing two significant trends. First, the field is becoming—if it hasn't already become—truly “multidisciplinary,” an enterprise rivaled by few but envied by many. At first glance, many of the articles in 2009 were published in traditional audiology and otology journals. However, more and more articles began to appear in other specialty and general-audience journals. Isn't it cool to highlight a cochlear implant paper in Cerebral Cortex?

The second trend is internationalization, as evidenced by the wide geographic representation of the articles highlighted here, including Australia, Canada, France, Germany, Italy, New Zealand, U.K., and U.S. While the U.S. still leads the way, both in the number of cochlear implant users and the number of publications, other countries are catching up. Among the 660 papers published last year, Germany contributed 66, Australia 48, the U.K. 38, China 27, Canada and Italy, 25 each, and France 21. The 27 articles from China were a bit surprising, surpassing 17 in 2008, largely due to a special issue on cochlear implants in China edited by Daqing Li in ORL; Journal for Oto-rhino-laryngology and Its Related Specialties. While the cochlear implant field has expanded, the world has become smaller, demanding our attention.

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ARTICLES CITED

Gartner L, Lenarz T, Joseph G, Buchner A: Clinical use of a system for the automated recording and analysis of electrically evoked compound action potentials (ECAPs) in cochlear implant patients. Acta Otolaryngol Dec. 4.
    Rance G, Barker EJ: Speech and language outcomes in children with auditory neuropathy/dys-synchrony managed with either cochlear implants or hearing aids. IJA 48(6):313–320.
      Colletti V, Shannon R, Carner M, Veronese S, Colletti L: Outcomes in nontumor adults fitted with the auditory brainstem implant: 10 years experience. Otol Neurotol Aug;30(5):614–618.
        Biedron S, Westhofen M, Ilgner J: On the number of turns in human cochleae. Otol Neurotol Apr;30(3):414–417.
          Jaryszak EM, Patel NA, Camp M, Mancuso AA, Antonelli PJ: Cochlear nerve diameter in normal hearing ears using high-resolution magnetic resonance imaging. Laryngoscope Oct;119(10):2042–2045.
            Fitzpatrick E, Olds J, Durieux-Smith A, McCrae R, Schramm D, Gaboury I: Pediatric cochlear implantation: How much hearing is too much? IJA Feb;48(2):91–97.
              El Fata F, James CJ, Laborde ML, Fraysse B: How much residual hearing is ÒusefulÓ for music perception with cochlear implants? Audiol Neurootol 14 Suppl 1:14–21.
                Chadha NK, Gordon KA, James AL, Papsin BC: Tinnitus is prevalent in children with cochlear implants. Int J Pediatr Otorhinolaryngol May; 73(5):671–675.
                  Strelnikov K, Rouger J, Demonet JF, Lagleyre S, Fraysse B, Deguine O, et al.: Does brain activity at rest reflect adaptive strategies? Evidence from speech processing after cochlear implantation. Cereb Cortex. May;20(5):1217–1222.
                    Purdy SC, Gardner-Berry K: Auditory evoked potentials and cochlear implants: Research findings and clinical applications in children. Perspectives on hearing and hearing disorders in childhood. 2009;19(1):14–21.
                      King J. Objective measures and programming cochlear implants. Perspectives Hear Hear Dis Childhood 19(2):54–62.
                        Gantz BJ, Hansen MR, Turner CW, Oleson JJ, Reiss LA, Parkinson AJ: Hybrid 10 clinical trial: Preliminary results. Audiol Neurootol 14 Suppl 1:32–38.
                          Dorman MF, Gifford R, Lewis K, McKarns S, Ratigan J, Spahr A, et al.: Word recognition following implantation of conventional and 10-mm hybrid electrodes. Audiol Neurootol 14(3)181–189.
                            Lovett RE, Kitterick PT, Hewitt CE, Summerfield AQ: Bilateral or unilateral cochlear implantation for deaf children: An observational study. Arch Dis Child Feb;95(2):107–112.
                              Loizou PC, Hu Y, Litovsky R, Yu G, Peters R, Lake J, et al.: Speech recognition by bilateral cochlear implant users in a cocktail-party setting. J Acoust Soc Am Jan;125(1):372–383.
                                Eapen RJ, Buss E, Adunka MC, Pillsbury HC 3rd, Buchman CA: Hearing-in-noise benefits after bilateral simultaneous cochlear implantation continue to improve 4 years after implantation. Otol Neurotol Feb;30(2):153–159.
                                  Nittrouer S, Chapman C: The effects of bilateral electric and bimodal electric acoustic stimulation on language development. Trends Amplif Sept; 13(3):190–205.
                                    Li D: Cochlear implants in China. Preface. ORL J Otorhinolaryngol Relat Spec 71(4):183.
                                      © 2010 Lippincott Williams & Wilkins, Inc.