Bilateral Cochlear Implants

Robbi Bishop began his workshop on bilateral cochlear implants (CIs) by introducing the Listen for Life Center at Virginia Mason Hospital, where he works. They offer a full range of hearing healthcare services, which includes state of the art CI services. This center is the first and largest CI center in the northwest, with 21 years of CI experience and over 230 current CI patients. Their CI services include aural rehabilitation and support groups following implantation.

A CI consists of an external part and an internal part. The external part contains a microphone, a speech processor, and a transmitter. The internal part contains a receiver/stimulator and an electrode array. Sound enters the microphone and is sent to the speech processor, which codes the sound using one of several speech processing strategies. The coded signal is sent to the transmitter, which sends it through the skin on a radio frequency (rf) signal to the internal receiver/stimulator. The receiver/stimulator sorts out the signal and sends the sounds to the appropriate electrodes, which stimulate the auditory nerve to send signals to the brain.

Recent advances in CI technology include:
- Improved coding strategies, which assist a user to better understand speech
- Miniaturization of speech processors, which allows the recent introduction of behind-the-ear (BTE) speech processors
- Improved microphone technology.
- Expansion of CI candidacy criteria, which allows more people to benefit from CI technology. Initial criteria included 0% comprehension on a standard test; the current limit is 50%.

Current candidacy criteria include:
- Severe to profound hearing loss in both ears as measured by a pure tone test without hearing aids. Note that this degree of hearing loss is not required at all frequencies. For example, many implantees have better hearing at low frequencies, but it rapidly falls to severe or profound as the frequency increases.
- Candidates can be either prelingually or postlingually deafened.
- Limited measurable benefit from hearing aids.
- Desire to be part of the hearing community.
- Medical criteria including a healthy cochlea
- Psychological criteria including reasonable expectations of results

The perceived benefit of a CI varies widely among implantees. Some users are able to perceive environmental sounds, but not comprehend speech. Others report that they are better able to understand speech with the assistance of lipreading. Still others are able to understand speech without lipreading, including the ability to use a telephone.

But users also report some limitations with the use of an implant. Perhaps most troublesome is speech comprehension in a noisy environment. A second difficulty with a single implant is sound localization. People can hear and comprehend a sound, but have no idea where it's coming from. A final reported limitation is a subjective feeling of being disconnected from environmental sounds,

People with normal hearing in two ears do not report these problems, because two ears offer significant advantages, particularly for locating the source of a sound. Sound arrives at the two ears at slightly different times and at different loudness, and these clues allow a person with two ears to pinpoint sound location. In addition to improved sound localization, the use of two ears improves understanding in noisy environments and removes the feeling of disconnection.

There have been two major studies of bilateral (both ear) CIs. One study by Cochlear Corporation and the University of Iowa involved ten patients with the Nucleus 24 straight electrode array who were simultaneously implanted (both ears at the same time). Tests on these patients demonstrated that, when noise was presented to the left ear, they were better able to understand speech coming from the front using their right ear than their left. Reversing the direction of the noise source reversed the better ear. This is a good argument for improved speech understanding in the presence of noise when equipped with bilateral CIs.

The second study was done primarily in Europe by MedEl, and included both simultaneous and sequential implantees. Both groups were tested for word understanding in a quiet environment, and both improved by about 20 db with both CIs on, as compared to their results with either ear alone.

Robbi also pointed out that there are some potential disadvantages to bilateral CIs, including:
- The loss of residual hearing in both ears.
- There is a slightly higher chance of surgical complications, because of more surgical procedures.
- Because the hair cells in both ears are damaged or destroyed, bilateral CIs may preclude potential future treatments, such as hair cell regeneration or improved CI technology.
- These procedures are not currently FDA approved. They are currently being done on an experimental basis.

Cochlear Corporation is currently conducting two bilateral CI studies. The simultaneous study involves the simultaneous implantation of both ears in patients who have never had a cochlear implant and who have had severe or profound hearing loss for less than fifteen years. There are currently 41 patients in this study at 17 CI centers throughout the US. The sequential study involves the implantation of a second CI in patients who have had a Nucleus CI for at least 3 months. This study involves 11 patients at 6 centers. Note that Cochlear is providing the second implant at no cost for people in the simultaneous study, but they are not providing any assistance for the second CI in the sequential study. This means that the patients or their insurance must pay for the second CI in the sequential study; that's the reason for the low participation.

MedEl is also currently conducting a study of both simultaneous and sequential bilateral CIs. They currently have eight subjects in 11 participating centers.

Note that there are several issues to consider with bilateral implantation, including:
- Limited insurance coverage. Most insurance companies won't pay for two CIs, so paying for the second CI is difficult.
- Increased out of pocket expense. Because insurance companies don't cover this, the patient is required to pay for it.
- Additional time commitments. Because a patient is learning to use two CIs and to use them together, she must anticipate more and longer appointments with her audiologist.

What does the future hold for CIs? Here's Robbi's guesses:
- Improved microphone technology, including the possibility of implanting it in the ear canal to take advantage of the acoustical properties of the outer ear.
- One sound processor for both CIs.
- Fully implantable CIs.
- Whatever else we can think of!

Part Two