Advances in computer technology as well as in ear surgery itself have given us the opportunity to offer the world of sound, music, and speech to those who would otherwise have spent their lives in a soundless cocoon.
Those who are born deaf (prelingual deafness), as well as children and adults who lost their hearing from either disease or trauma (post lingual deafness), can experience the gift of hearing thanks to cochlear implants. Even Seniors; we recently implanted a grandmother who had been told she was too old for a device. Nonsense. Please read her diary for a wonderful outcome and a heartwarming story.
Development of the devices came about through the surgical experiments of a group of French medical scientists as well as, the pioneering work of William House of Los Angeles, Dr. Robert Schindler of San Francisco and a cochlear implant team from Melbourne Australia. Miniaturization and advances in computer technology over the last 30 years set the stage for the further development of more sophisticated and effective cochlear implant devices. Today, over 30,000 individuals have been implanted worldwide, over 3,000 cochlear implants were performed in 1999 alone.
Initial experiments with deaf participants indicated that if an electrode was placed on the bony wall of the inner ear and an electric current was passed through the electrode, the deaf individual could perceive the sensation of sound. From this very basic observation the concept of placing an active electrode into the inner ear by way of the round window (the only non-bony entry into the cochlea of the inner ear) was born.
Initial implants were thus placed against the outer wall of the inner ear. Soon, they were introduced into the cochlear chamber itself through the round window. Not only was this introduction more effective in stimulating the residual nerve endings of the inner ear, but it also allowed the surgeon to better anchor the implant into the inner ear without risk of loosing contact with the nerve endings.
Individuals with deafness usually have lost most of their “hair cell” nerve endings which normally conduct sound from the ear to the brain. Significant residual nerve endings are needed to be present in the deaf to allow for their direct stimulation by the neighboring electrode(s). The cochlear implant is thought to function by directly stimulating the surviving nerve population in the cochlea and spiral ganglion.
The earliest cochlear implants consisted of a single active electrode, introduced into the inner ear, through the cochlear wall next to the round window. Today, most investigators in the cochlear implant field agree that stimulating the residual nerve cell population with multiple electrodes at different sites within the cochlea, combined with sophisticated computer assisted processing strategies allows for better recognition and understanding of speech.
The Nucleus 5 cochlear implant has further refined this concept with a multiple array of electrodes that stimulate cochlea at different sites. These multiple electrodes allow better differentiation of speech.
Long lasting batteries requiring little electric current will allow for a completely implantable cochlear device in the future. For the present, the implantable portion includes the electrode array and wire from the electrodes to an implanted portion of the device which is behind the ear. The remaining hardware of the device includes the microphone, receiver, speech processor and magnetic link between the outer scalp and the implanted portion of the device.
The external portion of the unit receives and processes sounds which are then fed into a specific electrode pair. These, in turn, stimulate the auditory nerve and finally the auditory center of the brain.
The brain of a newborn can be compared to a new computer without its software or operating system installed. During both infancy and earlier fetal life, the auditory (hearing) portion of the brain forms its neural connections, and learns how to process incoming sounds. Neurophysiologists consider the first few years of life as critical in establishing these connections because the brain is highly receptive to making new connection at this time: i.e. very “plastic”.
Originally implants were first allowed only in children four years of age or older. It has now become clear that the window of greatest hearing development may be the first two years of life, probably because of the high degree of neural plasticity. Children who are implanted earlier than two years of age seen to be able to “wire in” their neural connections with greater facility; the early results of cochlear implantation in very young children now appear to surpass those results obtained in older children.
In summary the first implants were usually performed on those who had become deaf AFTER they had acquired speech (post lingual deafness). These individuals derived significant benefit from their cochlear implants. Congenitally deaf children, or adults, (deaf from birth), did not have as much success with the first implants. Today we are quite certain that the younger a congenitally deaf child receives a cochlear implant, the better the long-term results will be.
The FDA has now approved cochlear implants in children as young as 12 months of age. Excellent speech and hearing results are expected to be part of their future with proper follow-up and training.
Dr. Mark Levenson has performed cochlear implants since 1984 both at Manhattan Eye, Ear, and Throat Hospital where he was chief of ear surgery, and currently at Albany Medical Center in Albany, New York.
The operation generally takes about an hour and a half, depending on which device is to be implanted. All implants require the opening of the mastoid bone which leads to the middle ear.
You can see an actual operation here.
The device itself has a microphone (outside the ear itself), a processor which selects and codes useful sounds (for understanding speech) and a receiver/stimulator which converts the coded sound into electric signals (delivered to the electrodes). These stimulate the nerve of hearing and can now be recognized by the brain as sound.
Prior to the operation, a detailed evaluation by the cochlear implant team must be made for those who are considered as candidates for the cochlear implant. In addition, both the pre-operative and post-operative training are important to the overall success of the device.
The surgical procedure of implantation itself is obviously very important. In a lighter vein, sometimes we compare our job to that of a piano mover. We bring the instrument to the “pupil”, but success also depends on a many people: the Cochlear implant team, ear surgeon, audiologist, psychologist, and ultimately the “practicing” and performance of the “pupil”. Success of the cochlear implant is truly the result of that team effort of audiologist, psychologists, deaf instructors, the ear surgeon and of course most importantly, dedicated “pupils” and parents.
Who is eligible for cochlear implants today?
* Prelingual deafness: (children or adults deafened before acquiring speech)
* Postlinqual deafness: (children or adults who lost their hearing after acquiring speech via trauma or a disease such as meningitis)
* Certain patients (adults) with serious hearing loss who are not deriving much benefit from hearing aids