Patients
often ask me if they will ever end up in a cochlear implant. Generally, most people with hearing loss do
not reach that point. Your hearing has
to reach the bottom of the audiogram (hearing test graph) before a cochlear
implant will help you hear better than with your hearing aids. Sometimes a patient may qualify for an
implant even though their "graph" portion of the audiogram is not in
the profound range. That may occur if
your ability to "understand" speech becomes very poor, even when
wearing your hearing aids. The portion
of the cochlear implant that is visible to others is the "speech
processor", which looks like a large behind-the-ear hearing aid. The speech processor has a wire coming out of
the bottom of it which leads to a round disk which attaches magnetically to the
patient's skin, above their ear. The
rest of the device is the "implanted" part and is not visible to others. Speech processors have all decreased in size,
and have greatly improved over the years.
A more in-depth discussion of cochlear implants follows:
What is a Cochlear Implant?
A
cochlear implant (CI) is a surgically implanted electronic device that can help
provide a sense of sound to a severely hard of hearing or deaf person. Cochlear implants may help provide hearing in
patients who are deaf due to damage to the sensory hair cells
in the cochlea. An implant does not restore normal hearing.
Instead, it can give a deaf person a useful representation of environmental
sounds and assist them in understanding speech.
The surgery is performed by a Neurotologist
(ear specialist), and the pre-operative testing, and post-operative programming
of the device are performed by an audiologist.
There is typically a four to six week delay between surgery and device
hook-up to allow the surgical incision to heal.
A cochlear implant consists of two main
parts:
- An external portion that sits behind the patient's ear and consists
of:
- A microphone which picks up sounds from the environment.
- A speech processor which selects, filters, and arranges sounds from
the environment to be sent to electrodes in the cochlea.
- A transmitter which transmits power and the processed signal across
the skin to the internal portion of the device.
- An internal portion that is surgically implanted under the skin and
in the cochlea:
- A receiver/stimulator which receives the signal from the
transmitter and converts it into electrical impulses.
- An electrode array which collects the electrical impulses and sends
them to different regions of the auditory nerve.
How does a Cochlear Implant work?
Cochlear
implants are very different from hearing aids.
Hearing aids amplify sound and sends the signal from the outer ear
through the middle ear into the cochlea.
The cochlea contains hair cells which have frequency (or
"pitch") representation from high pitch to low pitch (think about a
piano keyboard). A cochlear implant
bypasses the damaged hearing hair cells and directly simulates the auditory
nerve in a frequency appropriate manner.
These electrical impulses on the auditory nerve are then sent directly
to the brain for processing. It takes
time for patients to learn or re-learn how to process sound. Some individuals may only learn to use the
sounds for environmental awareness and safety, while others are able to use
these sounds to have full conversations in person or on the phone.
What is the candidacy criteria for a Cochlear
Implant?
The
candidacy criteria for children and adults differ: Current cochlear implant candidacy criteria
for adults: (Boys Town Research Hospital
Website)
- Bilateral (both ears) moderate to
profound sensorineural hearing loss (thresholds
of 40 dB HL or greater)
- Limited benefit with
appropriately fit hearing aids, defined as pre-operative test scores of: ≤50% understanding of sentences in
the ear that is to be implanted or ≤60% understanding of sentences
when tested bilaterally (best aided condition)
- No physical contraindications for
placement of the implant (i.e., CT scan results)
- Medically cleared to undergo
surgery
- Realistic expectations
- Commitment to follow-up
appointments
What are some Contraindications to receiving
a Cochlear Implant?
- Deafness due to lesions of the
acoustic nerve or central auditory pathway
- Active middle ear infection
- Absence of cochlear development
- Tympanic membrane (ear drum)
perforations in the presence of active middle ear disease
- Medically unstable to undergo
surgery
- Unrealistic expectations
- Not committed to follow-up
appointments
Information
from the NIH: National Institute of Deafness and Other Communication Disorders
(NIDCD)
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