History of CI:
The idea of generating noise by directly stimulating the auditory nerve is not a new one. Volta in 1790’s stimulated his ear with metal probes and besides giving himself an electric jolt he was able to hear what he called “sound of boiling soup”. Others did similar experiments till the French surgeon Eyries in 1957 placed an alternating current electrode on a patient’s auditory nerve who developed some sound perception. Soon after Bill House from Los Angeles and Blair Simmons from Stanford implanted patients with early cochlear implants. Simultaneous efforts at few institutions all over the globe resulted in creation of more sophisticated implants and the US FDA approved the House/3M implant in 1984. Since then three major manufacturers have been producing nearly all the implants used worldwide.
Unfortunately much misinformation and controversy surrounds cochlear implantation. Here are a few questions for you to ponder about while reading the rest of this article:
- Cochlear implants are for people with absolutely no hearing
- People with cochlear implants have a difficult time using the phone
- There is an age limit for implantation
- Every deaf patient would do very well with an implant
- Once a person is implanted they can easily appreciate music and hear in difficult listening environments
- Cochlear implants have a very low failure rate and rarely need to be changed
- Cochlear implantation would preclude the person from getting MRI scans
- After cochlear implantation one would lose the residual hearing in the implanted ear.
How does a cochlear implant function?
Unlike all other hearing rehabilitation options, from hearing aids to surgical options, cochlear implants are not dependant on most of the normal ear physiology. The electrode of a cochlear implant is implanted into the cochlea and directly stimulates the nerve of hearing. Thus, it bypasses any pathology present in every part of hearing before the nerve. In select cases the implantation is not possible or very challenging when the cochlea is either abnormal or scarred from infection.
Cochlear implants (CI’s) have two major components. The outer component captures the sound, processes it, and then transmits it through a magnetic coil to the internal component. The internal component is implanted under the skin and communicates with the outer component via a magnet.
Who is a good candidate?
The duration of deafness is one of the best predictors of who would do well with an implant. Children who are born deaf and never develop language (pre-lingual deaf) do very well with cochlear implants as long as they are implanted very young. Adults who have never heard and have no language skills do not perform as well with an implant and usually would not develop language abilities. The main reason to do an implantation for this group is better sound awareness and improvement of lip-reading. The adults who lose hearing after developing language (post-lingual deaf) usually do very well with cochlear implants.
The other factor is cause of deafness. Patients who have lost hearing due to cochlear toxicity such as Gentamicin or other antibiotic exposures do very well with cochlear implants. However, patients with history of meningitis usually do not function as well. In many of these patients the inside of the cochlea is scarred and implantation is more challenging.
Please refer here for information about surgical steps of the internal device.
Life after implantation:
The surgery for implantation is only the first step in the long process of re-training the brain to learn how to hear again. Most patients develop excellent hearing ability compared to pre-implantation however they are still far away from normal hearing. Great majority of CI patients are able to use the phone without difficulty but are not able to appreciate music.
Like every other machine, implants may fail. However, the rate of failure for this medical grade device has been extremely low. There have been a few issues with certain types of implants but most never need to be changed. For example, the failure rate of Cochlear Crop implants is between 1-3% per year dependant on the type of implant.
The new generation of implants offer many advantages such as special programming for adverse hearing environments. The Cochlear Corp’s latest generation of outer processors are water proof so children can keep them on while in the shower or in the pool.
At this time according to the FDA guidelines, CI’s are not compatible with MRI’s. The reason is presence of a magnet inside the implant that might shift during an MRI scan. However, in Europe and select US academic centers MRI’s are performed for implant patients when the risk of not-performing the MRI is too high. The other issue is that the implant casts a large shadow on the scan in the areas near it. CT scans are not contraindicated for implant patients although there is still the issue of the shadow that the implant casts around itself on the images.
There has been a major endeavor in protecting the patients’ residual hearing during implantation. Depending on the circumstance and level of hearing, in many cases the residual hearing can be protected. Because of this advances, new hybrid (cochlear implant and hearing aid) devices are under investigation that use a hearing aid for certain frequencies and the implant for others to amplify and process the sound.
Images courtesy of Cochlear Corp.