Bilateral Cochlear Implants
Robert Battista, M.D., & Kathleen Highhouse, Au.D., Ear Institute of Chicago, LLC
| Editor’s note: This article is an edited transcript of the live expert e-seminar presented on November 3, 2010. To register to view the recorded course, click here.|
Robert Battista: Today I’ll talk about bilateral cochlear implantation and the benefits of having two implants in terms of speech perception in quiet and in noise, localization ability and implications on language development in children. I’ll review the auditory perceptual disadvantages caused by unilateral hearing loss, and the advantages of bilateral hearing, which is the rationale for bilateral cochlear implantation. I’ll also contrast simultaneous and sequential implantation, talk about pros and cons of each, and review some published literature. My associate, Dr. Kathleen Highhouse, will review clinical case studies of three adults who use bilateral cochlear implants (CI), two of whom were implanted simultaneously.
Disadvantages of unilateral hearing
We know that children who are deaf in one ear often have speech perception and language acquisition difficulties (Bess & Tharpe, 1984; Culbertson & Gilbert, 1986; Ruscetta, Arjmand, & Pratt, 2005), and that those with unilateral hearing loss have difficulty hearing in the presence of noise, and are also unable to localize sound.
Advantages of bilateral hearing
Having good hearing in each ear gives us optimal access to sound arriving at either ear. Speech comprehension in quiet and in noise is improved in a binaural listening condition for three reasons: 1) binaural redundancy effects due to summation, 2) the squelch effect, and 3) most importantly - the head shadow effect (Litovsky, Parkinson, Arcaroli & Sammeth, 2006; Dunn, Tyler, Oakley & Gantz, 2008). Summation contributes to better speech recognition in noise because the brain receives some redundant info from each ear, and can draw more meaningful acoustic information from those two sources than it could from a single ear’s input. The head shadow effect is the most important thing about binaural hearing in terms of the ability to help localize sound and also to eliminate background noise. In the context of bilateral cochlear implants, we note better speech comprehension in noise, better sound localization, and improved ease of listening.
Before I continue, I should review some terms that are important in discussing the topic of bilateral cochlear implantation. These terms are: bilateral, bimodal, and binaural hearing, as well as simultaneous and sequential bilateral implantation. The term bilateral is used to describe sound being presented to both ears at once. Bimodal refers to sound being presented to each ear using a different modality: a cochlear implant on one ear (electrical stimulation) and a hearing aid on the other ear (acoustical stimulation). Binaural hearing refers to the integration of bilateral sound input as it travels up the auditory pathway. Simultaneous cochlear implantation involves the implantation of a cochlear implant into each ear during the same surgery. Sequential implantation involves two surgeries, and these may be weeks, months or many years apart. I’ll talk about the pros and cons of simultaneous versus sequential implantation a little later.
Findings: Bilateral cochlear implants
Let’s review some of the literature to see what kind of results we’re getting with bilateral cochlear implants.
Speech recognition in the presence of noise
In a study of 34 simultaneously-implanted bilateral cochlear implant users, Litovsky et al. (2006) found that they were able to sustain a 50% correct score on the BKB-SIN test at a lower signal-to-noise (SNR) ratio using their bilateral implants than when using only one of their implants. When only one implant was used, the SNR had to be increased to more favorable levels in order for the subjects to sustain that same performance level. In each of the three test conditions (noise from the left, noise from in front, noise from the right), these results were statistically significant after 3-months of bilateral implant use, with scores increasing further at the 6 month test. The bilateral fitting condition was superior regardless of the location of the noise. This confirms the superiority of bilateral cochlear implant use in recognizing speech in the presence of noise.
Sound localization ability using bilateral cochlear implants
Dunn et al. (2008) conducted a study of sound localization in sequentially-implanted cochlear implant users who had been using their bilateral implants for an average of 59 months. They found that sound location ability was 25⁰ more accurate using both implants rather than using just one implant.
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