Candidate Selection for Electric-Acoustic Stimulation Hybrid and Bimodal Cochlear Implantation

By Aaron J Parkinson, Ph.D. and Megan Mears, Au.D., M.S.

In the early days, 20-plus years ago, cochlear implants (CIs) were considered by many to be supplemental aids to lip-reading. This was reinforced by the relatively modest open-set speech perception recipients presented with postimplantation. Despite the time that has passed since implants first became available, it may be a surprise to more than a few that the performance of postlingually deafened CI recipients today goes well beyond that to be expected of a lip-reading aid. In this issue, Dr. Zwolan points to a number of studies that have demonstrated the significant gains in speech perception and quality of life (QoL) to be expected from cochlear implantation over hearing aids alone, which have only increased as indications have broadened. This should be taken as good news for anyone hesitant to explore cochlear implants, or unfamiliar with the potential implantation delivers. The lives of patients and their friends and families are positively impacted by the intervention. In addition, a more positive patient-clinician experience is made possible for those patients for whom hearing aids are not enough. The potential for patients to benefit from both CI and hearing aid technology opens up opportunities for both hearing aid and cochlear implant professionals to develop rich and rewarding partnerships in delivering effective, meaningful auditory (re)habilitation. No better example of this is the development of electric-acoustic implant systems, which will be discussed below.

In the treatment of significant (severe or poorer) high-frequency hearing loss, electrical access to lost sounds through cochlear implantation is the accepted standard of care, which precipitates the question, What about any residual hearing? While many CI recipients are already profoundly deafened at the time of implantation, the evolving landscape of candidacy has shown that people with lesser degrees of hearing loss can also benefit from a CI and particularly from a CI (electric) plus acoustic amplification. Typically, residual hearing refers to low-frequency hearing that can remain intact in the implanted ear after surgery and/or stable in the contralateral ear. Even after speech clarity is regained with a CI, when it is present, the residual low-frequency hearing should not be overlooked, but instead maximized to preserve the completeness of sound in summation of all its discrete and complex details.

As Teresa Ching (Ching, van Wanrooy, & Dillon, 2007) highlights, referring to patients who use a CI in concert with contralateral amplification, low-frequency acoustic information provides cues that point to voice pitch and suprasegmental aspects of the speech signal, and can be useful when listening to speech in background noise. In this way, the addition of low-frequency amplification is complementary to mid- to high-frequency spectral information more readily available via the CI. Encompassing the broad spectrum of speech sounds results in a more accurately reconstructed signal reaching the hearing nerve in such a cohesive fashion that the brain can take advantage of its natural ability to glean cues from each minute feature. These cues can relay the richness and intention of the original sound, resulting in a more normalized listening experience.

The harmonization of electric stimulation and acoustic amplification is germane in two specific patient profiles: the Hybrid™ Hearing patient,* and the CI patient with aidable acoustic hearing in the contralateral ear (bimodal hearing).
Hybrid Cochlear Implantation
With the evolution of technology and the benefits to be derived becoming clear, indications for implantation have broadened, which in turn has contributed to the superior performance observed in more recent recipients. This pattern is most evident in the development of electric-acoustic implant devices such as the Cochlear™ Nucleus® Hybrid™ Implant System.

Hybrid cochlear implantation addresses the needs of a population with severe high-frequency sensorineural hearing loss (often referred to as “ski-slope” hearing loss) underserved by hearing aids. Such individuals typically benefit by the presence of functional low-frequency acoustic hearing but continue to struggle understanding speech due to the extent of their high-frequency hearing loss. This is likely the result of a significantly diminished or absent cochlear reserve (i.e., loss of sensory hair cells) in the basal region of the cochlea. Frequencies corresponding to this region of the cochlea are important for effective speech understanding in quiet and in noisy and/or more complex listening situations (e.g., Amos & Humes, 2007; Hornsby & Ricketts, 2003).

Electrical stimulation provides a means to bypass the (absent) sensory component of the cochlea and directly activate residual neural elements that would normally depend on functional hair cells for innervation. However, prior to March of 2014, CI technology was not indicated for patients in the United States (U.S.) with severe high-frequency hearing loss and better than a moderate degree of low-frequency hearing loss. This left individuals “stuck in the middle,” frustrated with hearing aid technology not meeting their needs, yet not able to access implantable technology that might better address their severe high-frequency loss.

Advances in CI electrode array design and surgical technique allowed for the development of the Cochlear™ Nucleus® Hybrid™ L24 Implant. The goal of this design was to provide for electrical stimulation of the diminished high-frequency region of the cochlea while maintaining functional acoustic hearing in the lower frequencies.

Surgery for the Hybrid L24 implant is similar to that for standard CIs, except that a 16 mm electrode array is slowly advanced into the scala tympani rather than a 19 to 24 mm array as would be the case for standard CIs (Roland, Gantz, Waltzman, & Parkinson, 2016). The Hybrid L24 cochlear implant and sound processor are shown in Figure 1. The 16 mm straight array is very thin with 22 half-band modiolar-facing electrode contacts. The intent of this design is to provide stimulation of the basal (high-frequencies) region of the cochlea while maintaining apical cochlear structures responsible for low-frequency hearing. The system includes an external processor that integrates electric and acoustic sound processing when using the either the Cochlear™ Nucleus® 6 or 7 sound processor with the acoustic component attached.

The Hybrid system was approved by the U.S. Food and Drug Administration (FDA) as a first-of-its-kind electric-acoustic device in March 2014. Clinical trial outcomes are provided in a series of publications (Kelsall, Arnold, & Lionnet, 2017; Roland et al., 2016; Roland, Gantz, Waltzman, & Parkinson, 2018). Briefly, these studies show that Hybrid recipients demonstrate:
  1. Restoration of high-frequency hearing sensitivity, so important for good speech understanding, as evidenced by significant improvement on objective measures of speech perception in both quiet and in noise,
  2. Significant improvement on self-reported measures related to hearing speech and sounds in quiet and in complex listening environments encountered in everyday life,
  3. Improved levels of satisfaction with Hybrid technology, relative to hearing aids alone, and
  4. Superior levels of music perception relative to most traditional CI recipients.
Prospective candidates must be counseled regarding the risk of the procedure to residual acoustic hearing in the implanted ear. However, it is equally important that this be considered in balance with the fact that their high-frequency hearing is significantly compromised. While there are benefits to maintaining acoustic low-frequency hearing in the implanted ear, and all efforts are made to achieve this, improved communication ability is only possible if high-frequency speech perception is regained. The level of improvement and the overall capabilities observed in Hybrid recipients is only possible via electrical stimulation to address severe high-frequency sensorineural hearing loss.
Candidacy Criteria
The Nucleus Hybrid L24 cochlear implant system is indicated for unilateral use in patients 18 years and older (see Figure 1 on page 19) with:
  • thresholds ≤ 60 dB HL up to and including 500 Hz, and a threshold average of 2000, 3000, and 4000 Hz ≥ 75 dB HL in the ear to be implanted,
  • a threshold average of 2000, 3000, and 4000 Hz ≥ 60 dB HL in the contralateral ear,
  • an aided CNC word recognition score between 10% and 60%, inclusively, in the ear to be implanted, and
  • an aided CNC word recognition score equal to or better than that of the ear to be implanted but not more than 80% correct.
Prospective candidates should go through a suitable hearing aid trial, unless already appropriately fit with hearing aids. One should also carefully consider ruling out the Hybrid L24 implant for those presenting with evidence of rapidly progressive and/or fluctuating hearing loss and they can be considered for a traditional CI instead. Figure 1 shows an example of a Hybrid CI.

Figure 1. The Cochlear™ Nucleus® Hybrid™ L24 Cochlear Implant and Cochlear™ Nucleus® 7 sound processor with acoustic component.

Approval of the Hybrid L24 implant and the above indications represent the most significant broadening of indications for cochlear implantation in more than 15 years. While speech perception requirements for traditional cochlear implantation are predicated on sentence recognition scores, rather than word recognition scores, the preoperative speech perception abilities of Hybrid L24 candidates are considerably higher than typical CI candidates. The mean preoperative aided CNC score for Hybrid L24 clinical trial participants was 28% for the ear to be implanted, with scores typically in the range of 10% to 60%. Across a number of clinical trials with traditional CIs, mean preoperative aided word scores were under 10% for the implanted ear. For example, in a recent trial of the Nucleus 5 cochlear implant system (Runge, Henion, Tarima, Beiter, & Zwolan, 2016) the mean score preoperatively was 5.6% in the implanted ear, with scores ranging from 0% to 31% (the majority, 30/38 subjects, scored < 10%). Clearly, Hybrid implantation opened up CI technology to a much broader range of hearing impairment than previous to 2014.
Referring for Hybrid L24 Cochlear Implantation
The first step, audiometrically, is confirming that a patient presents with the audiometric profile described above, under Candidacy Criteria. Cochlear implantation should always be in mind when discussing treatment options for those with severe high-frequency sensorineural hearing loss, regardless of low-frequency hearing status. In the case of Hybrid L24 cochlear implantation, the level of residual hearing for low frequencies (up to and including 500 Hz) also needs to be considered, as delineated above. That is, patients with better levels of low-frequency hearing are more suited to Hybrid L24 implantation, should electrical stimulation be appropriate, whereas those with poorer low-frequency hearing thresholds are possible traditional CI candidates. Individuals with thresholds poorer than 60 dB HL up to 500 Hz, should be referred for traditional cochlear implantation evaluation.

Assuming audiometric requirements are met, the next step is to assess aided speech perception performance. For Hybrid L24 implantation, this assessment is based on aided monosyllabic word recognition, typically using CNC word materials (Peterson & Lehiste, 1962). Should a patient demonstrate aided word scores in the ranges referred to above, in addition to the audiometric requirements being met, referral for Hybrid L24 implantation would be appropriate.

If a patient presents with very poor word recognition (e.g., < ~30%) the patient may be a traditional CI candidate instead of, or in addition to, a Hybrid L24 candidate, particularly if low-frequency thresholds are poorer than ~40 dB HL. In this case, since selection criteria include sentence recognition scores, further testing should be completed to assess aided performance using sentence materials. If additional testing and/or sentence materials are not readily available the recommendation is to refer on to a CI facility. This topic is discussed more fully by Dr. Zwolan in this issue of Audiology Practices.

If in doubt about a patient’s eligibility for either traditional or Hybrid L24 cochlear implantation, and the patient presents with severe high-frequency hearing loss, referral is recommended for further evaluation. Fostering a positive and collaborative relationship with a local CI clinic is highly encouraged. Together, hearing aid and CI professionals can develop effective treatment plans for patients with significant high-frequency sensorineural hearing loss.

Whether one is considering traditional CI or Hybrid L24 implantation, the goal should be to maximize hearing outcomes for both ears. It is noteworthy, that all patients who participated in the Hybrid L24 clinical trial continued to make use of a hearing aid in the contralateral ear and optimal outcomes were observed in this listening mode (Roland et al., 2016). Whether patients were able to make use of acoustic amplification on the implanted ear or not, the best speech perception scores were observed in quiet and in noise when patients used a contralateral hearing aid and this was their preferred listening configuration. The importance of contralateral amplification for traditional CI similarly applies and will be discussed more below.
Hybrid and Bimodal Hearing
Another way that patients can benefit from the combination of electric and acoustic hearing is through Hybrid™ Hearing, which refers to use of any Nucleus cochlear implant model coupled to either a Nucleus® 6 or a Nucleus® 7 sound processor and an acoustic component – both on the implanted ear.

And, finally, one more way in which recipients can benefit from the combination of electric and acoustic hearing is through a bimodal configuration. It is this bimodal wearing paradigm that we will focus on now. Most CI users (both traditional and Hybrid L24) are unilaterally implanted and likely will benefit by continued use of amplification in conjunction with their CI. With broadening indications for implantation, the potential for patients to benefit by continued use or addition of a hearing aid contralaterally is not to be underestimated, simply because more patients present with aidable hearing in the non-implanted ear.

As described earlier, a CI will allow the damaged portions of the cochlea to be bypassed and provide electrical representation of sound directly to the hearing nerve for interpretation by the brain. While the cues that are most necessary for speech understanding are extracted and preserved, full representation of the signal can be limited. Namely, mid- and high-frequency cues of consonant articulation manner and place are well conveyed by the CI, while low-frequency cues of voicing and fundamental frequency (less crucial for perception in stress-timed languages) are less-well conveyed. So what can be done to improve the quality of this signal?

Patients who have a CI on one ear and a hearing aid on the other (bimodal hearing) demonstrate improved hearing in background noise, improved sound quality, and improved satisfaction (Fitzpatrick, Seguin, Schramm, Chenier, & Armstrong, 2009; Illg, Bojanowicz, Lesinski-Schiedat, Lenarz, & Buchner, 2014; Morera et al., 2012). Clearly, patients can benefit by the best of both worlds – acoustic and electric hearing.

As a general rule, hearing aid patients scoring 70% word recognition or better in both ears are probably appropriately amplified with hearing aids and may not qualify for a CI though only a complete CI referral can determine candidacy. Continued monitoring of these patients is recommended, with referral for a CI if a decrease in hearing thresholds or speech perception is reported by the patient. CI patients who have been appropriately fit with hearing aids prior to implantation tend to do better than those who spent extended periods of time being deprived of auditory stimulation (Blamey et al., 2013; Lazard et al., 2012).

Patients with speech perception scores of 50-60% or worse: This is the perfect time to begin the cochlear implant discussion with these patients. Ask them how well they are managing in their classroom/job settings, find out if hearing in background noise is burdensome, or if they struggle to use the telephone. Other valuable details to find out are the importance of music in these patients’ lives, how often they find themselves in challenging listening environments, and what their expectations are for their hearing technology. These patients may benefit from a formal CI evaluation and it is best to take a whole-picture approach and figure out how the hearing loss affects their lifestyle. If the patients report significant difficulty hearing in their daily lives, a CI evaluation may begin to prepare them for future options whether or not they qualify for implantation at this time. If they do not qualify, continue to monitor their hearing performance, and refer them again if they experience increasing challenges in their daily lives. Many people who choose to get a CI in one ear to improve their speech understanding, but still have an average low-frequency PTA of 70 dB HL or better in the non-implanted ear, do well wearing a hearing aid in that non-implanted ear (Choi et al., 2016). These patients will continue to need hearing aid services and may be able to take advantage of bimodal features offered by CI/hearing aid joint partners. Because of the known benefits of listening with two ears, patients with useful residual hearing in the non-implanted ear should continue to take advantage of that bimodal benefit with a hearing aid (Dunn, Tyler, & Witt, 2005; Firszt et al., 2018; Tyler et al., 2002).

Patients with asymmetric speech perception scores: What if one ear is just below the qualifying speech recognition score and the other ear scores less than 20%? You already know that the poorer ear can have a high probability of showing improvement with a CI, so a referral is absolutely warranted for this patient. However, for the other ear, the decision may not be as clear. These patients likely still have aidable thresholds and could benefit from an acoustic balance on the better ear. There are a host of bimodal benefits that a CI user can reap from continuing to wear a hearing aid in the contralateral ear such as better hearing in background noise and music appreciation.

Patients with poor speech perception scores in both ears: These patients will likely have a severe-profound audiogram. When patients are missing over half of the spoken message while using their hearing aids, they should be evaluated for a CI to improve their speech perception. In this situation, even if a patient qualifies for two CIs, and even if they might benefit from two CIs, they do not always choose that option. Most choose to proceed with one CI initially, while maintaining hearing aid use in the non-implanted ear. There are certain populations of patients who, based on a long history of hearing aid use, prefer the ‘boomy’ sensation of a super-power hearing aid in the non-implanted ear to round out speech cues from the CI. Some patients find the contribution of the hearing aid to be comforting, even if the hearing aid is not providing them clarity and would never be sufficient on its own.
It would be a mistake to assume that the line dividing patients who benefit from a hearing aid and those who benefit from a CI is hard and fast. In reality, there are many patients whose outcomes are maximized by the combined use of both technologies. Cochlear implant candidacy is no longer restricted to only the profoundly deaf, but encompasses those with residual hearing who can wear an acoustic component in their implanted ear (Hybrid™ Hearing), and/or a hearing aid on the opposite side (bimodal hearing). Recognizing these potential Hybrid Hearing and bimodal patients is a skill that can be mastered by audiologists in both hearing aid and hearing implant specialties. Tips for providing comprehensive patient care:
  1. Establish a professional network with one or two CI surgeons or clinics in your area that you feel confident referring your patients to for evaluation. A good working relationship will ensure that patients you refer who do not qualify for a CI will be referred to you for continued service. See the article by Kaplan and Brown in this issue of Audiology Practices for additional details on how to bring cochlear implantation into your practice as a differentiator in your market.
  2. In addition to the FDA approved indications for cochlear implants, all patients require a whole-person approach. Please consider:
    • The needs and priorities of the patient: job requirements, lifestyle preferences, social interactions, impact of hearing loss on activities of daily living, goals for intervention, etc.
    • Hearing performance in a variety of situations: an audiogram cannot tell the whole story. How does the patient hear in a crowded room? Or on the phone? Or using both ears together?
  3. When in doubt, refer for a CI evaluation. An evaluation is not a commitment, and it can set the stage for future expectations for the patient. As Dr. Zwolan points out in this issue of Audiology Practices, patients often report that they appreciate the referral and the opportunity to learn more about their treatment options. It would be far worse for a patient to continue to struggle with their hearing performance because their hearing healthcare provider did not know when it was time to refer or assess them for the next option.    
Aaron Parkinson, Ph.D. is a Principal Clinical Project Manager with Cochlear Americas. He can be contacted at

Megan Mears, Au.D. is a Clinical Project Manager with Cochlear Americas. She may be reached at
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Blamey, P., Artieres, F., Baskent, D., Bergeron, F., Beynon, A., Burke, E., . Lazard, D. S. (2013). Factors affecting auditory performance of postlinguistically deaf adults using cochlear implants: an update with 2251 patients. Audiol Neurootol, 18(1), 36-47. doi:10.1159/000343189

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Firszt, J. B., Reeder, R. M., Holden, L. K., Dwyer, N. Y., & the Asymmetric Hearing Study Team. (2018). Results in adult cochlear implant recipients with varied asymmetric hearing: a prospective longitudinal study of speech recognition, localization, and participant report. Ear Hear, Epub ahead of print. doi:10.1097/AUD.0000000000000548

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Hornsby, B. W., & Ricketts, T. A. (2003). The effects of hearing loss on the contribution of high- and low-frequency speech information to speech understanding. J Acoust Soc Am, 113(3), 1706-1717.

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Kelsall, D. C., Arnold, R. J. G., & Lionnet, L. (2017). Patient-reported outcomes from the United States clinical trial for a hybrid cochlear implant. Otol Neurotol, 38(9), 1251-1261. doi:10.1097/MAO.0000000000001517

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Morera, C., Cavalle, L., Manrique, M., Huarte, A., Angel, R., Osorio, A., Morera-Ballester, C. (2012). Contralateral hearing aid use in cochlear implanted patients: multicenter study of bimodal benefit. Acta Otolaryngol, 132(10), 1084-1094. doi:10.3109/00016489.2012.677546

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Runge, C. L., Henion, K., Tarima, S., Beiter, A., & Zwolan, T. A. (2016). Clinical outcomes of the Cochlear™ Nucleus® 5 Cochlear Implant System and SmartSound™ 2 signal processing. J Am Acad Audiol, 27(6), 425-440. doi:10.3766/jaaa.15021

Tyler, R. S., Parkinson, A. J., Wilson, B. S., Witt, S., Preece, J. P., & Noble, W. (2002). Patients utilizing a hearing aid and a cochlear implant: speech perception and localization. Ear Hear, 23(2), 98-105.
*The Acoustic Component should only be used when behavioral audiometric thresholds can be obtained and the recipient can provide feedback regarding sound quality. The Hybrid L24 Implant is approved in the US for adults ages 18 and older.

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