Technical Advances for Bilateral Implantation

The benefits of cochlear implantation have increased progressively with improvements in technology and in clinical practice over the last two decades. Most of the 60,000 people who have received cochlear implants worldwide have received substantial communication benefit from implantation in only one ear. However, advancements in technology now have the potential to add significant benefit through implantation of both ears. In people with normal hearing, having two ears provides two major benefits: 1. The BILATERAL BENEFIT is the ability to listen with the ear that has a better signal-to-noise ratio. The bilateral benefit comes into play when speech and noise come from different directions, and is primarily a result of the head shadow effect. 2. The BINAURAL ADVANTAGE is the ability to combine sounds from the two ears and to hear better than with one ear alone. The binaural advantage enables sound localization (directional hearing) and helps people to hear more clearly in background noise. Bilateral implantation can improve the quality of hearing in many everyday listening situations and can provide significant advantages over unilateral implantation. However, while conventional cochlear implants can provide bilateral benefit in recipients of two implants, they lack several key features that are necessary to enable listeners to experience a binaural advantage. Therefore, the benefit of implanting both ears has only been partially realized with conventional cochlear implant technology. Furthermore, the cost-benefit ratio of providing people with two implants over one implant has not yet been clearly established. Thus, in most cases to date, health insurance carriers have covered the cost of only one implant. In young children, there may be other compelling reasons for considering bilateral implantation. First, a young child’s auditory system is more plastic than that of an adult. Providing sound input to both ears in a young deaf child assures that sound is processed through both sides of the brain. Thus, the right and left auditory cortices can develop in a more normal sequence. On the other hand, if a child is implanted on only one side, the parts of the brain that would have been stimulated by the non-implanted ear will not develop, and eventually plasticity will be greatly diminished. The long-term consequence may be that the pathways that would have been stimulated by the nonstimulated ear will be permanently unresponsive to sound and not available for potential future restoration of hearing through advances in microbiology or genetic engineering. Second, a child who is deafened by meningitis and receives only one implant runs the risk of ossification of the unimplanted cochlea, thereby compromising the ability to benefit from future implantation in that ear. For the first time, the technology used in the HiResolution Bionic Ear has the potential to deliver the critical sound information that can provide improved bilateral benefit and, more importantly, can facilitate a measurable binaural advantage. Adults may experience improved directional hearing and better hearing in background noise. In addition to those benefits, young children have access to sound in both ears that may enable the brain to mature and to learn to process all bilateral and binaural cues during a period where maximum plasticity occurs.