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Bimodal Hearing Or Bilateral Cochlear Implants: a Review of the Research Literature

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Bimodal Hearing Or Bilateral Cochlear Implants: a Review of the Research Literature Bimodal Hearing or Bilateral Cochlear Implants: A Review of the Research Literature Carol A. Sammeth, Ph.D.,1,2 Sean M. Bundy,1 and Douglas A. Miller, M.S.E.E.3 ABSTRACT Over the past 10 years, there have been an increasing number of patients fitted with either bimodal hearing devices (unilateral cochlear implant [CI], and hearing aid on the other ear) or bilateral cochlear implants. Concurrently, there has been an increasing interest in and number of publications on these topics. This article reviews the now fairly voluminous research literature on bimodal hearing and bilateral cochlear implantation in both children and adults. The emphasis of this review is on more recent clinical studies that represent current technology and that evaluated speech recognition in quiet and noise, localization ability, and perceived benefit. A vast majority of bilaterally deaf subjects in these studies showed benefit in one or more areas from bilateral CIs compared with listening with only a unilateral CI. For patients who have sufficient residual low-frequency hearing sensitivity for the provision of amplification in the nonimplanted ear, bimodal hearing appears to provide a good nonsurgical alternative to bilateral CIs or to unilateral listening for many patients. KEYWORDS: Bimodal hearing, bimodal devices, bilateral cochlear Downloaded by: SASLHA. Copyrighted material. implants, binaural hearing Learning Outcomes: As a result of this activity, the participant will be able to (1) compare possible benefits of bimodal hearing versus bilateral cochlear implantation based on current research, and (2) identify when to consider fitting of bimodal hearing devices or bilateral cochlear implants. Multichannel cochlear implants (CIs) tion for persons with hearing loss that is too have proven to be a highly successful interven- severe for good performance with conventional 1Cochlear Americas, Centennial, Colorado; 2Division of Centennial, CO 80111 (e-mail: [email protected]). Speech, Language, and Hearing Sciences, University of Bimodal Hearing and Bilateral Implantation; Guest Colorado, Boulder, Colorado; 3Department of Electrical Editor, Teresa Y.C. Ching, Ph.D. and Computer Engineering, University of Denver, Semin Hear 2011;32:3–31. Copyright # 2011 by Denver, Colorado. Thieme Medical Publishers, Inc., 333 Seventh Avenue, Address for correspondence and reprint requests: Carol New York, NY 10001, USA. Tel: +1(212) 584-4662. A. Sammeth, Ph.D., Senior Regulatory/Clinical Specialist, DOI: http://dx.doi.org/10.1055/s-0031-1271945. Cochlear Americas, 13059 East Peakview Avenue, ISSN 0734-0451. 3 4 SEMINARS IN HEARING/VOLUME 32, NUMBER 1 2011 amplification.1 As CI technology has evolved Although most individuals with bilateral over the past 25 years, outcomes for implant deafness are still only fitted with one CI, more recipients also have significantly improved.2 CI recipients are now wearing a hearing aid This has resulted in an expanding candidacy (HA) in the opposite ear (bimodal hearing) criteria in terms of how severe the hearing loss because they have sufficient low-frequency must be for a patient to be approved for residual hearing sensitivity to benefit from implantation. For example, in the United acoustic amplification. In addition, an increas- States the Nucleus multichannel CI (Cochlear ing number of patients are receiving bilateral Ltd, Sydney, Australia) was initially approved CIs, either in sequential (one ear at a time) or by the Food and Drug Administration for use simultaneous (both ears at the same time) in the 1980s only for adults with bilaterally surgeries. profound hearing loss (thresholds 90 dB HL) There is good reason for the increase in across the frequency range. Today, however bimodal fittings and bilateral cochlear implan- (see Fig. 1), hearing loss in the low frequencies tation. The psychoacoustics and HA literature (below 1000 Hz) can be of only a moderate speaks to the benefits of having two-eared degree in adults (40 dB HL) or only a severe rather than one-eared input.3–8 Bilateral listen- degree in children aged 2 and older (70-dB ing benefits can range from improved speech HL). recognition ability for separated sound sources Downloaded by: SASLHA. Copyrighted material. Figure 1 Current U.S. Food and Drug Administration-approved audiometric and speech recognition candidacy criteria by age group for the Nucleus 5 (Cochlear Ltd, Sydney, Australia) cochlear implant. HL, hearing loss; ANSI, American National Standards Institute; MLNT, Multisyllable Lexical Neighborhood Test; LNT, Lexical Neighborhood Test. BIMODAL HEARING OR BILATERAL COCHLEAR IMPLANTS/SAMMETH ET AL 5 and listening in noise, to better sound local- ization ability, to a more satisfactory and ‘‘nat- ural’’ sound perception, and even to a better perceived quality of life. There are three main areas in which benefits are ascribed to bilateral listening: the head shadow effect, binaural summation and redundancy effects, and the binaural squelch effect.3 HEAD SHADOW (OR HEAD DIFFRACTION) EFFECT This effect results from the fact that the head blocks transmission of higher-frequency sounds due to their small wavelength. Persons fitted with only a unilateral CI often must turn their implanted ear toward whomever they want to hear and may miss important sounds on the other side. With two-eared listening (bimodal hearing or bilateral CIs), the ear with the more favorable signal-to-noise ratio (SNR) in the particular listening situation is always available. In the research literature, the head shadow effect is commonly tested by presenting speech from a frontal loudspeaker (0 degrees azimuth) and noise from a side loudspeaker (often 90 degrees right or left, also referred to as either Æ90 degrees or 90 and 270 degrees azimuths). Performance is compared for unilateral listening with the ear ipsilateral to the noise source to that for the bilateral listening condition (adding the ear contralateral to the noise source). Any Downloaded by: SASLHA. Copyrighted material. measured performance increase with addition of the second ear represents benefit primarily from Figure 2 Schematic illustration of the loudspea- overcoming the head shadow effect. This is ker configurations commonly used to measure the three binaural/bilateral listening effects. illustrated in Fig. 2A. Shown is a patient whose unilateral listening condition is a right ear cochlear implant (CI), with the left CI added for the bilateral listening condi- BINAURAL SUMMATION tion. (A) Speech emanates from the front and AND REDUNDANCY EFFECTS noise from the unilateral CI side. Head shadow When both ears are listening rather than only benefit occurs when the CI for the ear opposite one, sound is perceived as louder due to binaural the noise is added for bilateral listening. (B) Both summation. This is evidenced by the fact that speech and noise emanate from a single frontal clinicians often have to adjust comfort levels loudspeaker to measure binaural summation and/ down for mapping of bilateral CIs compared or redundancy benefits of listening with both CIs with the patient listening with a unilateral CI. compared with only one. (C) Speech emanates from the front and noise from the side opposite Binaural redundancy refers to the fact that there the unilateral CI. Binaural squelch benefit occurs are cues that can be picked up when both ears when the CI on the side of the noise is added for are participatory (redundancies or overlapping bilateral listening. information) that result in improved sensitivity to fine differences in the intensity and frequency 6 SEMINARS IN HEARING/VOLUME 32, NUMBER 1 2011 domains. This may translate into improved trated in Fig. 2C. Even though the ear added speech perception ability in more difficult lis- actually has a poorer SNR, it also is receiving tening situations. The binaural redundancy ef- timing and intensity cues from the frontal loud- fect is typically measured in research studies by speaker that the central auditory nervous system presenting either speech alone or both speech can use to help ‘‘separate’’ the desired speech and noise from a single frontal loudspeaker. signal from the undesired noise signal. Performance is compared for unilateral listening versus bilateral listening. Any measured per- formance increase with addition of the second LOCALIZATION (DIRECTIONALITY) ear is assumed to represent primarily benefit In addition to the possibility for improved from binaural redundancy (and also binaural speech recognition with bilateral inputs, there summation if loudness balancing between the is another practical benefit of having two func- conditions was not done). This is illustrated in tional ears: improvement in the ability to deter- Fig. 2B. Unlike the head shadow effect, which mine the direction that a sound is coming from. is purely an external physical phenomenon, Persons with input from only one ear can attest binaural summation and redundancy effects to the frustration of hearing their name spoken are the result of binaural processing via central but not knowing which direction to turn to find auditory system integration of information ar- the person calling them. Localization ability also riving at each ear. can be a safety consideration; for example, when crossing a busy street, it is important to know the direction that a car is coming from. Localization BINAURAL SQUELCH (OR ability depends on the auditory system’s percep- UNMASKING) EFFECT tion of interaural differences in time and inten- When both ears participate for listening to sity. Interaural timing differences provide
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