Bilateral Cochlear Implants - Audiological Perspective

Joan Hewitt, Au.D. discussed the audiological aspects of bilateral cochlear implants at the January meeting of the San Diego Cochlear Implant Group. Her practice has 18 bilateral patients between the ages of two years and nineteen years old. Her presentation will focus on what they have learned from these patients, so she began by providing some information about them.

Patient age at first implantation ranged from one year to fifteen years and ten months. Age at second implantation ranged from one year and four months to nineteen years. The time between implants ranged from three months to six years and five months. (Note that this means none of the patients received simultaneous implants.) The patients have had their second implant from two weeks (and not yet activated) to two years and eleven months, with many just under a year. All patients are prelingually deaf.

Dr. Hewitt next discussed some of the limitations of traditional audiological testing and recommended changes that she feels would greatly increase the value of these tests. She focused on the following tests:
1. Sound field warble tone testing
2. Detection of Ling sounds
3. Sentences in noise (e.g. HINT) testing
4. Speech discrimination testing

The sound field warble tone test is the test used to construct an audiogram. It determines the volume required for various tones to be detectable by the patient. It therefore measures access to sound, rather than intelligibility or understanding. Dr. Hewitt described one patient who was being tested after getting a new mapping. His warble tone test showed that his hearing ranged from 20dB to 25dB across the spectrum, which is excellent! But subsequent testing revealed a speech discrimination score of 12%, compared to 80% with the previous map. The excellent results indicated by the warble tone testing disguised a serious problem with the new map!

Ling sounds include some of the sounds that make up speech (phonemes). One might think that they would provide a better indication of speech intelligibility than warble tones, but that's not necessarily the case. The problem with Ling sounds remains the same. They indicate access to sound, rather than intelligibility. One patient, for example, was able to detect many of the sounds at a low volume. Unfortunately, the patient was unable to differentiate the sounds, even at a higher volume.

Sentence testing provides a better measure of speech intelligibility, but suffers from the fact that it also relies a lot on a patient's language ability. Dr. Hewitt believes that language ability and hearing ability contribute about equally to these scores. One patient with awesome language skills was able to score 90% on the HINT test, while scoring only 36% on a speech discrimination (individual words) test. This indicates that she was able to reconstruct the sentence with very little acoustic information.

Another patient had the opposite experience, getting no sentences correct despite a speech discrimination score of 50%. The problem was the child's poor language skills, which caused him to order words incorrectly, form tenses incorrectly, etc. These errors resulted in his missing every sentence, even though he obviously understood them.

Word discrimination tests have a couple of problems. The first is that the test that is administered is not always age-appropriate to the patient. If the word list is intended for a much younger child, an older patient will probably do much better on it than he would on a more age-appropriate list. A second issue is that discrimination is often not tested at various speech volumes. A patient who scores around 70% for normal volumes may score 0 at quiet volume. For a CI user, such disparity in scores probably means that the map should be adjusted.

Dr. Hewitt believes that a few modifications and additions would make these tests more effective in evaluating the hearing of CI patients.

One modification would be to test all phonemes, rather than just the Ling sounds. This test would still be measuring access rather than intelligibility, but would at least indicate the accessibility of all sounds that comprise speech.

A second modification would be to add a speech reception threshold test, which would be a better intelligibility indicator.

Finally, speech discrimination testing should always be done at low, medium, and high volumes.

Dr. Hewitt spent the remainder of her presentation considering some common questions from patients who get a second implant.

Q. I just live my current map on my first implant. Can I use the same one on my second implant?
A. No. Even if your second implant hardware is identical to your first, it will require a different map.

A related topic is the volume setting of the two implants. It's common for a patient to report, after she's been using both implants for a couple of weeks or so, that they're too loud. Virtually all patients want the volume of their second implant reduced, but the correct response is almost always to reduce the volume of the first implant. The reason for this is binaural summation - the brain adds the sounds from both ears. Because the first CI was programmed when there was no input from the other ear, the volume was set without accounting for binaural summation. When the second CI is activated, the map of the first should be adjusted.

Q. How will the new implant sound?
A. Experiences differ, but it will probably not sound like the first one did. But it will also probably not sound "normal". Patients often forget how the first implant initially sounded, and are initially disappointed in the sound of the second.

Q. Should I not use my first implant while I'm getting used to the second?
A. There is a lot of disagreement about this. We do not recommend removing the first implant while you get used to the second. It's just too hard to function at work or school without the first one. And we think it's important to use your implant every day. But we also think that you should practice with just your second implant for at least a short time every day.

Q. How should I practice with the second implant?
A. Go back to the very beginning, like you did with your first implant. (Again, patients often forget how far back the beginning is!) We have found that patients generally acclimate to the second implant quite rapidly.