Cochlear implants are surgically implanted devices that provide electrical stimulation to the auditory system, which is perceived within the brain as sound.
What Are Cochlear Implants?
A cochlear implant is a surgically placed device that helps a person with severe hearing loss hear sounds.
The cochlea is a snail-shaped part of the inner ear. It turns sound vibrations into electrical signals that travel along the auditory (hearing) nerve. The brain translates these signals into recognizable sounds.
Cochlear (KOE-klee-er) implants are different from hearing aids:
- A hearing aid makes sounds louder so people with hearing loss can hear.
- Cochlear implants bypass damaged parts of the cochlea to stimulate the auditory nerve directly. They may help when a hearing aid can’t.
How Do Cochlear Implants Work?
Cochlear implants have:
- A microphone and speech processor that sit outside the body. The microphone picks up sound and sends it to the processor. The processor is a minicomputer that changes the sound into digital information. Then, a transmitter sends the digital signal to the receiver/stimulator.
- A receiver/stimulator that’s placed under skin and muscle behind the ear. This gets information from the processor. It sends electrical impulses by a thin wire to electrodes placed in the cochlea. The electrodes stimulate the auditory nerve. The message goes to the brain and the brain can use the information to recognize sounds and understand speech.
Who Can Get a Cochlear Implant?
Doctors consider cochlear implants for children under 12 months of age with profound hearing loss in both ears. Older children with serious hearing loss also may get cochlear implants.
A cochlear implant team will help decide if cochlear implants are a good option. This team includes an audiologist (hearing specialist), an ear-nose-throat (ENT) doctor, a speech therapist, a psychologist, and a social worker.
Kids being considered for the surgery will:
- get hearing tests
- have speech/language evaluations
- use a hearing aid for a while to see if it helps
- get computed tomography (CT) or magnetic resonance imaging (MRI) scans to look at the inner ear and the bones that surround it
Kids might not get the implants if:
- Their hearing is “too good” (they can hear some sound and speech with hearing aids).
- Their hearing loss isn’t due to a problem with the cochlea.
- They’ve been profoundly deaf for a long time.
- The auditory nerve is damaged or absent.
Infants and Children – Their Special Needs:
As has often been said, children are not “little adults.” They are indeed, unique, and to address their CI needs, they require an experienced clinician. Most children are unable to provide accurate feedback while the audiologist programs their cochlear implant and therefore, the clinician must take many things into account:
- The audiologists’ past experiences with other patients
- updated information regarding the child’s progress (from parents, therapists and teachers
- audiometric test measures
- observations of the child during programming
- objective measurements (NRT/NRI, ESRT)
- if age appropriate, the clinician will train the child to participate in programming (Conditioned Play Audiometry (CPA), loudness growth task.
Many of the decisions made during programming appointments come from the clinician’s knowledge and experience, rather than the child’s behavioral responses
Although cochlear implant maps are individualized and set according to the patient’s physiologic responses, there are trends witnessed across patient populations.
(As stated above, T level (threshold) is the least amount of electrical current necessary for a person to perceive a sound. The C or M level is the most comfortable level, or a loud but comfortable level. The CI dynamic range is defined as the difference between T and C/M levels These parameters are referred to as the patient’s “map.”)
With the Cochlear Americas Nucleus 24 device, the amount of electrical stimulation available (using default programming parameters) is between 0 and 240 clinical units (cus) and it is extremely rare to see T levels at or below 100 cus. Additionally, there are not many adult patients that have C levels above 210 cus. With the Advanced Bionics HiRes processing strategy, average M levels are between 100 cus and 300cus. More information on average stimulation levels can be found in Zwolan & Overstreet .
Etiology of hearing loss also appears to influence stimulation levels, possibly due to neural survival. For example, a patient deafened from meningitis may have extremely high stimulation levels, while a patient with Connexin 26 gene deficits may have extremely low stimulation levels.
When a patient’s device is initially activated, stimulation levels change frequently while the patient is learning to listen with their cochlear implant. Figure A shows how cochlear implant maps change over the first two years, as was determined on post-lingually deafened adult. As the graph shows, stimulation levels increased during the beginning of her CI experience, but eventually a plateau was seen and stimulation levels hovered in a certain clinical unit range.
One theory as to why this phenomenon exists could be — At first, the auditory system is extremely sensitive to the electrical stimulus and is learning how to manage the information. After continued use and practice, the brain adjusts (neural plasticity) and learns to effectively use the electrical input. Over time, the person requires higher stimulation levels to hear optimally, and the parameters that become the optimal map generally remain stable for many years of cochlear implant use. It is important to note that the majority of adult CI users do not have stimulation levels that continue to increase over time. If they did, the patient would eventually reach the output limits of the device and no longer benefit from their cochlear implant.
It appears that pediatric maps show higher stimulation levels over time, as compared to adult cochlear implant maps. Researchers hypothesize that children will often adapt to their program settings, even if they are set too high, and therefore, gradual increases in stimulation at subsequent programming visits may not be necessary. Additionally, Overstreet et al found that adults with lower stimulation levels performed better on speech perception tasks. Additionally, negative effects from high stimulation levels exist, including; risk of facial nerve stimulation, increased channel interaction, low battery life, voltage compliance issues and may result in an eventual degradation of the speech signal.
Overstimulation (“overstim”) is the term used to describe cochlear implant map settings with stimulation levels exceeding the amount needed to create the desired percept. Overstim can occur due to a lack of patient participation, pressure from the parents/therapists, and/or inappropriate or inaccurate implementation of objective measures. When stimulation levels are set too high, sound quality and auditory skills can be affected.
Adult patients have previously described how changes in their map impacts their listening experience. If threshold levels are set too high, soft sounds are perceived as loud. For example, a light tapping sound could be perceived as a loud knocking sound, and in effect, consonant sounds could be heavily distorted. Although children may overcome the difficulty in perceiving and distinguishing between consonant sounds when they are distorted in this manner, the child may exhibit poor speech production skills since they are producing the sounds as they are hearing them. Overstim might also exasperate background noise, resulting in poorer listening abilities in noise.
Adult patients have sometimes caused their own overstim by falsely assuming that setting their comfort levels louder results in clearer sound, when in fact, the opposite occurs. When C/M levels are too high, patients describe experiencing echoing, bubbly, or wavering sounds. However, these subjective descriptions can also exist when a person’s comfort levels are set too low. Therefore, the patient may have to listen to a variety of sounds and comfort levels to determine the best map for them.
It is clear that focused and accurate participation during cochlear implant programming is the preferred route to creating an optimal map. However, children cannot usually provide such information due to their current language abilities and the complex nature of discussing concepts of loudness and sound quality. Difficulty expressing their CI experience paired with unreliable behavioral information (due to lack of participation, attention problems or poor behavior) can lead to map settings that are inappropriate. Therefore, case studies are offered here (below) to demonstrate the effects of overstimulation.