Bilateral Cochlear Implantation: Achieving a Stereo Effect
Sancak Yuksel, MD
As we all know, normal hearing is binaural. Binaural hearing provides some major benefits: better and targeted hearing in a noisy environment, better localization of sounds, and more natural and balanced hearing quality. If this is the case, would bilateral implantation prove more beneficial than unilateral implantation?
After three decades and thousands of cochlear implants (CIs), we know that CIs are effective and provide better hearing to the patients with profound hearing loss. However, even though unilateral cochlear implants significantly help in understanding speech in a quiet environment, their efficiency is decreased in a noisy environment. Sound localization is also relatively poor with the unilateral implantation. As we mentioned above, these are two important functions of binaural stimulation for normal individuals and hearing aid recipients. For this reason, there is a growing interest in bilateral cochlear implantation.
Three mechanisms in binaural hearing have been demonstrated to achieve improved acoustic results:
- In the head shadow effect, which has the largest impact on binaural hearing in a noisy environment, the head blocks some of the background noise. So, the protected ear achieves a better signal-to-noise ratio.
- In the binaural squelch effect, the central hearing center receives and integrates different signals from both ears, which results in augmented hearing.
- When both ears receive similar signals, this is perceived as louder. This phenomenon is called the binaural summation effect.
Studies have shown that children with bilateral CIs are able to discriminate between the locations of two sound sources better with both, rather than with either one, of their implants. Laszig et al. demonstrated that accuracy of localization was 90° in a unilateral implant. This was reduced to 50° in a bilateral implant. Other studies demonstrated that, in children and adults, speech intelligibility is better under noisy conditions. Peters et al., in a recent study, found significant increase in speech discrimination scores in children with bilateral implants in comparison with their best performing unilateral implants.
There are other benefits of bilateral CIs. Implanting both ears ensures that the ear with better potential receives stimulation, as they might have different levels of neural survival. If there is a technical problem with one of the implants, at least one functional implant provides hearing until the problem is resolved. There are also subjective and qualitative benefits of bilateral implants. Adults feel less stressed in social environments. Additionally, children report that they love having “both ears” like other children, and function better at school.
Besides the advantages, there are also arguments against bilateral CIs, which often focus on the financial cost and risk to the other ear. In the case of failure of an implant, re-implantation is possible, but we need to admit that there is a risk of damaging the cochlea with the implantation, which could preclude the future non-prosthetic hearing. On the other hand, there is a very critical plasticity period in children’s brain when the benefit from CIs, either unilateral or bilateral, is maximized. Speech and language development takes place in this period; therefore, it is argued that the earlier the implantation, the better the benefits. Other concerns are the increased surgical/anesthetic risks and effects on the vestibular system. It should be noted that the risk of the second implant is equal to that of the first implant, but the additional benefit is not nearly as great as the first one. Surgical tolerability is equal in both simultaneous or sequential (in a second surgery) cochlear implantation, although simultaneous implantation seems more cost effective and beneficial. It is estimated that the risk of vestibular deafferentation from bilateral cochlear implant surgery is approximately 10%; in children, this is even smaller. Currently, there is hesitancy in insurance companies to approve patients for second side implant because of the increased cost. However, when improved quality of life measures in patients with bilateral implants were incorporated, this cost seems decreased.
In conclusion, hearing with both ears is better than hearing with one. Bilateral cochlear implantation is a safe and appropriate procedure that provides improved sound localization and enhanced speech perception in noise. Although there are some concerns about it, it appears that it will be the standard of practice in the future for children with profound hearing loss.
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- Laszig R, Aschendorff A, Stecker M, et al. Benefits of bilateral electrical stimulation with the nucleus cochlear implant in adults: 6-month postoperative results. Otol Neurotol 2004; 25:958–968.
- Peters BR, Litovsky R, Parkinson A, et al. Importance of age and postimplantation experience on speech perception measures in children with sequential bilateral cochlear implants. Otol Neurotol 2007; 28(5):649-657.
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