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Are Two Better than One?

Gifford, René H. PhD

doi: 10.1097/01.HJ.0000472647.60849.93
Journal Club

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

Figure.

Figure.

Bilateral cochlear implantation is considered standard-of-care treatment for children with severe to profound sensorineural hearing loss. Families with children undergoing cochlear implantation who have asymmetric hearing loss or greater than bilateral profound sensory hearing loss, however, are often counseled to consider a single cochlear implant (CI) in combination with a hearing aid for the better hearing ear. This recommendation is primarily based on research outcomes with bimodal (CI + contralateral hearing aid) listening in adult CI recipients and may not be applicable to children.1,2,3

Figure.

Figure.

Approximately 60 percent of adult CI recipients have aidable acoustic hearing in the non-implanted ear with audiometric thresholds better than 85 dB HL at 250 Hz.2 Even ears that yield little-to-no speech understanding when tested alone can add benefit by more than 20 percentage points when added to electric stimulation for speech in noise.2,4 How can we know when to recommend a second CI for our pediatric patients? Are there any data to suggest that bilateral implantation may yield significantly greater outcomes? If so, how much acoustic hearing is too much to sacrifice for a second CI? Unfortunately, none of these questions can be answered unequivocally.

We do not currently have definitive clinical guidelines detailing who is a bilateral CI candidate and who would be better served with bimodal hearing. With adult bimodal listeners, audiometric thresholds in the non-implanted ear provide inconsistent predictive value for the magnitude of this bimodal benefit. A significant correlation is generally seen between low-frequency thresholds and bimodal benefit,5,6 but audiometric thresholds within a given classification of hearing loss severity do not reliably predict bimodal benefit.6 In other words, you could see a patient with moderate sensory loss in the non-implanted ear who exhibits significant bimodal benefit and another patient with a near identical audiogram who derives no bimodal benefit.

A number of studies have shown that very little acoustic hearing is required from the non-implanted ear to yield bimodal benefit. In fact, audibility through 250 Hz can be sufficient to obtain significant bimodal benefit in noise.7,8 Some studies have even shown that bimodal benefit in background noise can be obtained with just F0 information.7,9-11

These studies examining bimodal benefit, however, have all been conducted with adult listeners who had postlingual onset of deafness. Comparable data for pediatric bimodal listeners is absolutely critical from a clinical perspective to aid clinical decision-making on bilateral implant candidacy. We see patients and their families in the clinic every day, and they want data-driven recommendations now.

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ADULTS: TWO VS ONE

The adult literature is full of reports of bilateral benefit. A second CI on average yields an improvement of 10 percentage points for word recognition and sentence recognition in noise over a single CI for adult recipients.3-4,12-16 This benefit is primarily caused by summation. Adult implant recipients also demonstrate significant spatial release from masking and bilateral head shadow, but bilateral implant recipients exhibit little to no evidence of binaural unmasking of speech or squelch.12,13,17,18 Pediatric bilateral implant recipients have been shown to demonstrate similar degrees and ranges of summation,19 head shadow20,19 and spatial release from masking18-22 compared with adult CI users. Like adult bilateral recipients, pediatric bilateral CI users also demonstrate little evidence of binaural unmasking of speech.19-20

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SECOND OUTCOMES

Telling a family that their child will achieve bilateral head shadow, spatial release from masking, and summation with a second CI is not a meaningful counseling tool. Parents and clinicians want to know whether children with bilateral implants will achieve higher language and academic outcomes than unilateral recipients. The results to date are somewhat contradictory.

Nittrouer and Chapman examined the expressive and receptive language capabilities of children at 42 months of age with bimodal hearing and bilateral implants.23 They found no differences across the groups using standardized measures of expressive and receptive language.

Similarly, Ching and colleagues reported no difference between bimodal and bilateral CI groups for expressive and receptive language nor for speech understanding in noise.24 Additional studies, however, have examined children with a single CI (without contralateral amplification) as compared with bilateral CI recipients, demonstrating that the bilateral CI group achieved significantly higher outcomes as compared with the unilateral CI group.25,26 The data in the peer-reviewed literature are not unequivocal.

How are we to proceed with clinical practice based on these current reports? How much acoustic hearing is needed to achieve bimodal hearing equivalent to bilateral implants? Might this be different for children who are developing speech and language and are more reliant on bottom-up processing cues than adults?

Unfortunately, we do not have the data to answer these questions. Language development and speech understanding may not be significantly different across the groups, but bimodal hearing may not be truly equivalent to bilateral implantation for listening effort, academic performance, social communication, spatial hearing, development of the central auditory system, and reorganization of the auditory cortices.

Until we have a better understanding of the differences between bimodal hearing and bilateral cochlear implantation, the best we can do is to stay on top of the literature so that we can provide data-driven recommendations while executing evidence-based practice in the audiology clinic.

Academic Outcomes for School-Aged Children with Severe-Profound Hearing Loss and Early Unilateral and Bilateral Cochlear Implants

Sarant JZ, Harris DC, Bennet LA

J Speech Lang Hear Res

2015;58(3):1017-32 http://jslhr.pubs.asha.org/article.aspx?articleid=2119416

Sarant and colleagues completed a prospective study of language and academic outcomes for 44 8-year-olds with severe to profound sensory hearing loss who had unilateral or bilateral implants. Outcomes selected for investigation included expressive language, receptive language, written language, and composite scores for math and reading. The authors explain that this study carefully controlled for issues that have received criticism in previous studies of language and academic outcomes, including participant selection bias and variables known to predict postoperative outcomes, such as cognitive abilities, age at implantation, birth order, age at initial hearing aid fitting, time spent reading aloud, screen time, and level of education for the primary caregiver. This study did not attempt to compute estimates for missing data points, another criticism of previous studies.

Mean standard scores for the unilateral and bilateral implant groups were not significantly different when analyzed using a simple t-test, which is consistent with some previous studies.23,24 Sarant and colleagues, however, completed multiple regression to develop a statistical model for predicting student academic outcomes based on implant status (unilateral vs. bilateral), age at implant activation (first and second), the predictor variables chosen from the hearing-aid use, parenting style, child characteristics, and family background.

Regression analysis revealed that having bilateral implants was a significant predictor of expressive language and mathematical abilities. Combining bilateral implantation and age at activation for the second CI provided additional significant predictive power for expressive language, math, and written language abilities. Bilateral implantation, however, was not significantly associated with reading abilities for these children.

The results presented by Sarant et al. suggest that children with bilateral CIs are more likely to achieve higher academic performance in expressive language, written language, and math as compared with children who have a single implant. This bilateral benefit will be greatest for children who receive their second CI at a younger age.

What we do not know, however, is whether the hearing in this non-CI ear was classified based on the full audiometric profile (125 Hz through 8,000 Hz) or the pure-tone average. This could influence outcomes because a child with sloping sensory loss may be better equipped to benefit from bimodal hearing than a child with a flat loss in the range of 70 dB HL to 90 dB HL.

The Effects of Asymmetric Hearing on Bilateral Brainstem Function: Findings in Children with Bimodal (Electric and Acoustic) Hearing

Polonenko MJ, Papsin BC, Gordon KA

Audiol Neurootol

2015;20(Suppl 1):13-20 https://www.karger.com/Article/FullText/380743

Polonenko et al. examined auditory brainstem responses (ABRs) in the implanted and non-implanted ears of 21 children with bimodal hearing. The primary purpose of this study was to investigate whether there were differences in brainstem function across the implanted and non-implanted ears. This holds functional clinical significance because asymmetries at the brainstem level can yield abnormal and asymmetric reorganization of the central auditory system.

They noted that ABRs were observed in both ears for all 21 participants, and considerable differences were seen in interwave III-V latencies for electrically evoked compared with acoustically evoked ABR regardless of the degree of hearing in the non-implanted ear.

All 21 participants had wave III-V latencies in the normative range for the implanted ear, but only 14 of the 21 participants had interwave latencies within the normative range for the non-implanted ear. These differences in interwave latency were not attributed to hearing modality differences and modality/transduction speed. In fact, interwave latencies for the non-implanted ears were still delayed compared with implanted ears even when normalizing latency for the method of transduction (electric vs. acoustic). These differences were not necessarily related to the degree of loss in the non-implanted ear.

Of the 13 participants with severe to profound sensory hearing loss, half had wave III-V latencies in the normative range, and the other half had significantly delayed latencies. This means that some children with bimodal hearing will have asymmetric bilateral brainstem function, the presence of which cannot be predicted by the audiogram.

Asymmetries in brainstem function may have functional hearing consequences for spatial hearing abilities and even speech understanding in complex listening environments. As mentioned, asymmetrical brainstem function can drive abnormal central auditory reorganization for the developing auditory system. This holds real-world significance for clinical recommendations regarding a second CI.

Gordon and colleagues showed that an 18-month separation between CI surgeries was the critical window during which sequential implants could be placed and still facilitate symmetrical brainstem development.27 That gives us a relatively limited period following the placement of the first CI to recommend a second CI and possibly ameliorate the effects of abnormal reorganization of the developing auditory system, which is thought to contribute to a strong preference for the first CI and an asymmetry in performance.

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CLINICAL IMPLICATIONS

Are two implants better than one? The answer is most certainly “yes” for children with severe to profound sensory hearing loss when considering symmetry of hearing performance, bilateral neural development, spatial hearing, language, and academic performance. (This does not take into account music perception and naturalness of sound quality, both of which may be better with bimodal hearing.)

This question would best be served with a double-blind study investigating the outcomes of thousands of children who are randomly assigned to bimodal or bilateral implantation, but the reality is that this will not be done, so we must make important clinical decisions with the information we have at hand.

Based on the findings of the studies summarized here, we can safely recommend bilateral cochlear implantation for our youngest patients provided that the child has bilateral severe to profound sensory hearing loss. Even children with sloping losses or less severe hearing losses may be better served with two implants depending on the degree and extent of auditory dysfunction (e.g., spectral resolution and/or presence of cochlear dead regions) in the non-implanted ear.

The likelihood of having acoustic hearing preservation following cochlear implantation is greater than it ever has been with the selection of atraumatic electrodes and widespread adoption of minimally traumatic surgical techniques for cochlear implantation plus the ability to fit a child with acoustic amplification with an integrated implant processor and hearing aid, such as the Nucleus 6 processor. A child could wind up with bilateral CIs plus acoustic hearing from one or both ears.28 For children with less severe hearing losses in the non-implanted ear, however, we will need to monitor speech understanding, language development, and academic performance closely and make clinical recommendations case by case.

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