By Ted McRackan, MD, co-editor of Otology, Neurotology, and Skull Base Surgery: Clinical Reference Guide
Cochlear implantation is the gold standard for treatment of severe to profound sensorineural hearing loss. Cochlear implants (CIs) were approved by the Food and Drug Administration (FDA) in 1985 and have been suggested to be the most successful neural prosthesis created to date. Over 300,000 cochlear implants have been performed worldwide, with over 50,000 performed in the past year alone. Cochlear implantation involves a surgical procedure whereby an electrode array is placed in the cochlea of the inner ear, which is organized in a tonotopic fashion with decreasing characteristic frequency along its length. Modern CIs contain between 12 and 22 electrodes, which are spaced with the intention of each electrode stimulating a unique area of the spiral ganglia of the auditory nerve. Cochlear implants work by having an external microphone and an external processor convert an acoustic signal to an electrical signal. It is then sent to a speech processor, which is designed to enhance the signal and reduce noise before sending the information to the spiral ganglion through the CI electrode array.
Cochlear implantation is currently at an exciting time point due to the combination of improving technology and proven outcomes that has led to rapid expansion of its indications. The FDA approved the first single-channel CI electrode for adults in 1984, followed by the multichannel electrode in 1987. Cochlear implants were then approved in 1990 for children older than 2 years, in 1998 for children over 18 months, and ultimately in 2000 for children older than 12 months. There has been a recent push to implant children younger than 12 months due to evidence that children implanted at this age are more likely to catch up to normal-hearing peers at an earlier time point. Three major obstacles have hampered this movement. First, obtaining accurate hearing diagnostic testing in a timely manner can often be difficult in those less than 12 months. Second, there is a slight increased risk of surgical complications due to the low blood volume in this age group. Third, it can be extremely difficult to perform cochlear implant programming in this age group. Nonetheless, the clear benefits of early implantation likely outweigh these risks. Pediatricians, audiologists, and otolaryngologists are encouraged to identify infants with hearing loss as soon as possible for hearing rehabilitation. The earlier this is performed, the earlier children with profound hearing loss can be identified, and the earlier they can be implanted, leading to better CI outcomes.
Use of cochlear implantation in patients with residual hearing has been another area of rapid expansion. It was initially thought that all hearing would be lost with cochlear implantation and that if hearing was preserved, patients would not be able to process electrical and acoustic hearing. However, through the trials of the Cochlear Hybrid electrode and the MED-EL EAS electrode, it appears that both are possible. Through these and other trials, most patients had preserved residual hearing after cochlear implantation. Additionally, these patients showed improved hearing outcomes compared to patients without residual hearing. At the present time, it is not clear whether this preserved hearing is sustainable over time. This is an active area of investigation and will continue to be studied for years. Nevertheless, this technology has greatly expanded the indications for cochlear implantation beyond traditional candidacy.
As discussed above, it was previously thought that individuals would not be able to process combined electrical and acoustic hearing. However, cochlear implantation in patients with residual hearing proved this incorrect. This has led to the more widespread use of CIs in individuals with single-sided deafness. Current standard treatment for single-sided deafness includes devices that essentially ignore the deafened ear. However, with cochlear implantation, hearing can be restored to that ear. This was initially performed in patients with severe tinnitus in the deafened ear but is now being more commonly performed in the absence of tinnitus. Further work is certainly needed to develop a more comprehensive understanding of cochlear implantation in this population, but preliminary data show decreased head shadow effect and improvement in binaural summation, spatial release from masking, and potentially sound localization.
Beyond cochlear implantation, the use of auditory brainstem implants (ABIs) in children is another area of expansion. Although this has been performed in Europe for years, it is only more recently being performed in the non-neurofibromatosis type II population in the United States. Several centers have active clinical trials to perform ABIs in children unlikely to benefit from cochlear implantation due to either absent cochlear nerves or cochlear malformations. This is an unfortunate population as they have limited hearing rehabilitation options. Auditory brainstem implants provide an opportunity for hearing in this population, and the neurotology community is excited to hear the results of these trials.
We have come a long way since Bill House developed the first single-channel CI. As outcomes and technology continue to improve, the indications for cochlear implantation will grow. The audiology and otology communities are eager to see what the future holds for cochlear implantation.
About the Author
Dr. Theodore R. McRackan is an assistant professor of otolaryngology at the Medical University of South Carolina. He received his medical degree from the Medical University of South Carolina and completed his otolaryngology residency at Vanderbilt University. Dr. McRackan then completed his fellowship in neurotology-skull base surgery at the House Ear Clinic. His professional interests include neurotologic outcomes and quality of life research. Dr. McRackan and Derald E. Brackmann, MD co-edited Otology, Neurotology, and Skull Base Surgery, which serves as both a study resource for qualifying exams and a portable clinical reference guide. This text features a concise and approachable outline format, contributions by leaders in the field, and key topics such as anatomy and embryology, hearing loss, cochlear implantation, skull base tumors, vestibular disorders, and pediatric otology. View sample pages and place your order at www.PluralPublishing.com.