Fitting a cochlear implant together with a hearing aid
Over the last decade audiological criteria for the use of a cochlear implant (CI) have been increasingly relaxed to include many people with residual hearing. This has led to interest in a bimodal approach that combines electrical and acoustic hearing. With a standard cochlear implant, this involves the use of a hearing aid worn on the opposite ear. It has also been possible to combine acoustic and electrical hearing using a so-called “hybrid” approach in the same ear through the use of a short implant electrode This is placed in the basal turn of the cochlea and stimulates only the higher frequency nerve fibres. With careful surgery and electrode array design, trauma to the cochlear structures supporting low frequency acoustic hearing can be minimized and this hearing can be preserved in the implanted ear.
There are many differences between acoustic and electrical hearing that need to be considered when these two modalities are combined. Perhaps the greatest difference is that in CI recipients, the acoustic hearing that remains will almost always be limited to lower sound frequencies, perhaps up to 1000 Hz, while the CI electrode will stimulate higher frequency nerve fibres towards the outer turns of the cochlea. In addition, dynamic range and loudness growth are likely to vary between the two modalities, so that the hearing aid gain and compression may need to be set to match the percepts of loudness between the two devices. A further difference is that even in profound hearing loss, acoustic hearing will usually be considerably more effective than a cochlear implant in providing pitch information for speech and other sounds with fundamental frequencies up to a few hundred Hz.