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Combined Electric-and-Acoustic Stimulation: Entering a New Era

Gifford, René H. PhD

doi: 10.1097/01.HJ.0000455835.42735.18
Journal Club

Dr. Gifford is director of the cochlear implant program, associate director of implantable hearing technologies, and associate professor of hearing and speech sciences at the Vanderbilt Bill Wilkerson Center in Nashville.

Figure. C

Figure. C

We have observed an increased incidence of acoustic hearing preservation with cochlear implantation. This rise is likely the result of a number of factors, including the widespread adoption of minimally traumatic surgical techniques, as well as the U.S. Food and Drug Administration (FDA) approval of atraumatic electrodes and of hybrid electric-and-acoustic stimulation (EAS) implant systems.



On March 20, the FDA granted Cochlear's Nucleus Hybrid L24 Cochlear Implant System premarket approval.1 Up to that point, the use of acoustic hearing preservation and combined electric-and-acoustic stimulation in the United States was restricted to approved clinical trials or required that patients purchase an in-the-ear hearing aid to use in conjunction with the cochlear implant processor on the ear with hearing preservation.

What do clinicians need to know about combined electric-and-acoustic stimulation? What levels of hearing preservation can we expect, and how do we fit the hearing aid component? What do we do with acoustic gain and prescriptive targets? What is our target acoustic bandwidth? How do we fit the cochlear implant, and what frequencies should it provide? How do we balance perceived loudness within an ear and across ears?

Right now, we do not have definitive answers to these questions. In essence, our enthusiasm regarding electric–acoustic stimulation has prompted our field to put the cart before the horse.

We have commercial and clinical availability of these implant systems, but our current EAS clinical practices are not evidence based. If we are to see the success of these patients and widespread acceptance of EAS technology from consumers, clinicians, and insurers, we must make every effort to investigate this area thoroughly, from both an empirical and a clinically translational perspective.

These words are meant not to cause alarm, but rather to sound the call to action. The approval of hybrid and EAS systems will encourage us, as a field, to redefine our roles in hearing healthcare and work together to develop data-driven guidelines for the audiologic management of our hearing-preservation patients.

The purpose of this month's column is to give an update on what we can expect with hybrid/EAS systems and to explain the value of combined electric-and-acoustic stimulation for those who are candidates.

European Multicenter Study of the Nucleus Hybrid L24 Cochlear Implant

Lenarz T, James C, Cuda D, et al

Int J Audiol


Thomas Lenarz, MD, PhD, and colleagues described the outcomes of the European multicenter clinical trial of the Nucleus Hybrid L24 Cochlear Implant System for 66 adult recipients implanted across 16 centers. The primary endpoint evaluated outcomes one year after implant activation.

The figure on page 21 is a re-creation of the mean preoperative audiograms for the 66 patients in the study. A number of reports show diminishing amplification benefit for frequency regions where audiometric thresholds exceed 70 dB HL.2-6



Assuming a 70-dB HL cutoff, the mean audiograms for the European Nucleus Hybrid L24 recipients suggest that amplification was successfully provided through approximately 500 Hz to 750 Hz, resulting in low-pass filtered speech that is not sufficient for high levels of speech understanding.

With respect to acoustic hearing preservation, Hybrid recipients had lost an average of 5 dB, 10 dB, 10 dB, and 15 dB in hearing thresholds at 125 Hz, 250 Hz, 500 Hz, and 750 Hz, respectively, upon initial activation. Acoustic audiometric thresholds were assessed again at the one-year mark, revealing that patients had lost additional hearing at 500 Hz and 750 Hz, for a total threshold drop of 15 dB and 28 dB, respectively.

At initial activation, 98 percent of patients had measurable acoustic hearing, compared with 88 percent at one year. Thus, the overwhelming majority of patients retained aidable acoustic hearing in the low-frequency range. In fact, acoustic hearing was most stable in the range of F0 and F1 (up to approximately 500 Hz), which is where acoustic hearing has most to add for combined EAS.7-9

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Dr. Lenarz and colleagues also reported word recognition outcomes for speech in quiet and in noise. Patients were tested at both the pre- and post-implant points with the same material, including disyllabic words in France, Italy, and Spain, and monosyllabic words in Belgium, Germany, the Netherlands, and the United Kingdom.

The word recognition outcomes were quite impressive, showing a median improvement of 40 percentage points both in the implanted ear and best-aided condition for speech understanding in quiet. For word recognition in noise at a 10-dB signal-to-noise ratio (SNR), median performance improved 45 percentage points for the implanted ear and 30 percentage points for the best-aided condition.

Sentence recognition was assessed with adaptive SNR tests in some centers. The patients demonstrated highly significant improvements of 6 dB to 7 dB in the speech reception threshold for the implanted ear and the best-aided condition, respectively.

Though Dr. Lenarz and colleagues reported impressive outcomes for hearing preservation and speech understanding, what tends to be of greatest importance to insurance carriers, patients, and families are subjective responses and the resultant quality of life.

On the Speech, Spatial, and Qualities of Hearing Scale (SSQ),10 the Hybrid recipients reported statistically significant improvements in all three areas, with mean increases of 1.2, 1.3, and 1.8 points for the speech, spatial, and quality subscales, respectively.

The Health Utilities Index Mark 3 (HUI-3)11 is a validated 15-item instrument that provides a health-related quality-of-life index based on the patient's responses for the following domains: hearing, vision, speech, emotion, pain, ambulation, dexterity, cognition, and self-care.

The mean HUI-3 difference between pre- and post-implant scores was 0.117, which was statistically significant. As expected, the greatest benefit was noted for questions gauging hearing-related quality of life.

Effects of Hearing Aid Settings for Electric-Acoustic Stimulation

Dillon MT, Buss E, Pillsbury HC, Adunka OF, Buchman CA, Adunka MC

J Am Acad Audiol


Margaret T. Dillon, AuD, and colleagues completed an EAS optimization study for nine adult hearing-preservation patients who had been implanted in the Med-El EAS clinical trial and fitted with the Duet sound processor.

The purpose of their study was to compare the EAS benefit obtained with the manufacturer's prescribed hearing aid fitting formula12 and that obtained with National Acoustic Laboratories’ nonlinear fitting procedure, version 1 (NAL-NL1),13 for low-frequency acoustic hearing in the implanted ear.

The manufacturer's prescriptive formula is based on the half-gain rule, and the slope is calculated as half of the difference in audiometric thresholds at 250 Hz and 500 Hz. As a result, the manufacturer's formula tends to prescribe less gain than NAL-NL1 and a more restricted bandwidth, Dr. Dillon and coauthors wrote. In fact, across the nine subjects in the study, the mean low-frequency gain was approximately 5 dB greater for NAL-NL1 compared with Med-El's fitting formula.

With NAL-NL1 targets used as the standard, the manufacturer's formula achieved target audibility through 417 Hz, on average, whereas NAL-NL1 achieved target audibility through 611 Hz for the same nine subjects. Because greater audibility and bandwidth do not necessarily translate to better EAS outcomes,7,9,14 Dr. Dillon and colleagues completed a controlled comparative investigation.

The researchers assessed speech understanding using the adult minimum speech test battery (MSTB) for both hearing aid settings. The EAS patients achieved significantly better outcomes on all speech measures, with a mean improvement of 10 percentage points for consonant-nucleus-consonant (CNC) words,15 12 percentage points for AzBio sentences16 at 10-dB SNR, and a 2-dB improvement in the SNR-50 for Bamford-Kowal-Bench sentences in noise (BKB-SIN).17

Despite the reported efficacy for acoustic hearing preservation in the implanted ear and additive speech understanding, the current literature may underestimate the degree of electric-and-acoustic stimulation benefit, as patients in the published studies may not have had optimized EAS parameters, Dr. Dillon and colleagues concluded.

These suboptimal parameters may be even more of an issue in complex listening environments, where the value of hearing preservation is more evident.18-21

Also important to note is that Dr. Dillon and colleagues did not adjust cochlear implant settings, which is another EAS variable that potentially could affect outcomes. Though there have been studies examining the cochlear implant frequency allocation with hybrid/EAS patients,22-26 sample sizes have been quite small—three to 10 patients—and limited in diversity with respect to device type, insertion depth, and aidable bandwidth. Clearly, there is much work to do in this area.

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These two studies and others have demonstrated significant improvements in speech understanding, subjective hearing quality, and overall quality of life following cochlear implantation with hearing preservation.

In the few months since the FDA approved the Nucleus Hybrid L24 system, those of us working in the cochlear implant and hearing aid clinic have witnessed history in the making. Acoustic and electric technologies are merging at an increasing rate.

As shown in the figure, the mean audiograms for people receiving the Nucleus Hybrid L24 system would have put them completely outside the range of cochlear implant candidacy just a few years ago. Based on the evidence showing the diminishing benefit of amplification for spectral regions with audiometric thresholds higher than 70 dB HL, the hybrid/EAS indications are data driven and arguably overdue.

Despite the tremendous benefit that can be derived from hybrid/EAS implant systems, this technology is not for everyone. It is important to note that individuals with hybrid/EAS-qualifying audiograms who derive significant benefit from conventional or nonlinear frequency compression amplification are not candidates.

We have just begun to scratch the surface in determining clinical guidelines for fitting these systems and optimizing EAS parameters for each patient. Even with the current cochlear implant and hearing aid settings, though, patients demonstrate significant communication and quality-of-life benefits from hybrid/EAS systems, with a median improvement of 40 to 45 percentage points for word recognition in quiet and noise.

I would encourage all clinicians to begin thinking about hearing aid patients in a different light, particularly those with ski-slope losses who are unhappy with their current amplification. These are the patients who tend to be the most unsatisfied with hearing aids but, up until this past year, were furthest from cochlear implant candidacy. We are heading into a new era of rehabilitative audiology.

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1. Food and Drug Administration. Nucleus Hybrid L24 Cochlear Implant System approval letter. March 20, 2014
2. Ching TY, Dillon H, Byrne D Speech recognition of hearing-impaired listeners: predictions from audibility and the limited role of high-frequency amplification. J Acoust Soc Am 1998;103(2):1128-1140
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