by Neil Bauman, Ph.D.
April 4, 2016
If you have lost some or much of your hearing, perhaps you are wondering just how well you might do with a cochlear implant (CI). Wouldn’t you like to know right now whether there are any factors that might help predict how successful you might be if you got a cochlear implant?
If you think a cochlear implant may be in your future, are you also wondering whether there is anything you can do now to increase your degree of success with a cochlear implant when you eventually get one?
Here are some interesting research findings. (Note: the studies quoted here had the objective to test the influences of multiple factors on how well deaf adults—those who had already acquired language before they lost their hearing—understood speech with their cochlear implants.) These data were collected from 15 different cochlear implant centers around the world and comprised 2,251 cochlear implant recipients who were implanted between 2003 and 2011.
For the purposes of this study, researchers determined whether a factor was statistically significant, marginally significant or not significant using the following criteria:
- Significant P < 0.001
- Marginally Significant P > 0.001 and < 0.05
- Not significant P > 0.05
Here are the results.
Factors That Don’t Make Any Difference in Cochlear Implant Performance
Both men and women had equal success with cochlear implants. There was no significant difference between the sexes in this regard. So don’t think the opposite sex has it better than you do.
Level of Education
People with minimal education have just as much success with cochlear implants as do those with college degrees. Thus, lack of formal education does not affect how well you will do with a cochlear implant. You don’t need more education to have better results with a cochlear implant.
Better vs. Worse Ear Implanted
A surprising result was that there was no significant difference between whether you have your better or worse ear implanted. This is good news if you want to keep your better ear for use with a hearing aid and use your worse ear (where you have less/little to lose in the rare case the implant isn’t successful) for your cochlear implant.
What really matters is that you do not deprive your brain of sound before you get your implant. In other words, wearing a hearing aid (or using other sources of amplified sound) before getting a CI is of significant benefit as compared to just living in silence and not using any amplification.
Factors That Make Minimal Difference in Cochlear Implant Performance
There are two basic methods of inserting the cochlear implant electrode array into the inner ear. One method is to go through the round window that is located on the “wall” that separates the middle ear from the inner ear. The second way is to drill a hole though the bone of the cochlea (cochleostomy). There was only a marginal difference in results between these two approaches. Doctors used the cochleostomy method 73% of the time versus 27% of the time for the round window approach.
Angle of Insertion of Electrode Array
Since the cochlea consists of 2½ turns, if the electrode array was fully inserted, it would have completed 900° (2½ x 360). In actual fact, the electrode array is not inserted the full 900°. In this study doctors inserted the electrode arrays varying distances, ranging from 135° to 730°. The good news is that the number of degrees doctors inserted the electrode array was only marginally significant. This means that as long as your doctor inserts the electrode array properly, even if it doesn’t go in all the way, it shouldn’t make much difference in your success with your cochlear implant.
Better predictors of performance success included “a greater number of electrodes within the scala tympani, an absence of translocation from the scala tympani to the scala vestibuli, a not excessively deep insertion and a reduced distance to the modiolus.” Let’s translate that into plain English. The length of the cochlear spiral is divided into two chambers separated by a thin membrane. The scala vestibuli is the upper chamber and the scala tympani is the lower chamber. You get better results if all the implanted electrodes lie within the scala tympani (lower chamber). However, if the electrodes tear through the thin membrane and switch to the scala vestibuli (upper chamber), the results are not as good. Whether this happens or not depends much on the skill of the implant surgeon and the flexibility of the particular electrode array used. The modiolus is the central axis in the cochlea. The closer the electrodes stay to the central axis, the better the results.
Year When Implanted
The year in which you were implanted doesn’t make much difference since this study only used data from CIs implanted between 2002 and 2011. The thinking was that different coding strategies were used in different years. For example, there was a major change in coding strategies just before 2002. However, since then the coding strategies have remained much the same. As a result, not much difference was seen in CIs implanted between 2002 and 2011. Now, if they had included people implanted before 2002 and after 2011, you would expect to see more significant differences.
Factors That Make Significant Differences in Cochlear Implant Performance
Pure Tone Average of the Better Ear
The better the pure tone average of the better ear, regardless of which ear was implanted, the better the results. Your Pure Tone Average (PTA) is calculated by averaging your hearing loss at 500, 1000 and 2000 Hz.
Closely associated with this is the more residual hearing you have in your better ear at the time you get a CI is significant in how well you will hear with your CI. Typically, the more residual hearing you have, the higher your PTA, but this is not always true. For example, a person with a reverse-slope loss may have a lower PTA, but greater residual hearing.
It seems that speech results with a CI do not depend so much on how well your cochlea (inner ear) works, but on how well the auditory circuits in your brain work. Thus, what is important is that no matter which ear is implanted, what really matters is that your brain was not deprived of sounds before you got your cochlear implant.
This is why people with residual hearing better than 50 dB in their better ear had better CI speech scores.
Brand of Cochlear Implant
The differences between which brand of CI you have implanted is significant—amounting to a difference of about 14% between the highest performing cochlear implant and the lowest performing implant. (I explain this more fully in my article, “Which Cochlear Implant Brand Gives the Best Performance”.)
Note that the above only applies to hearing in quiet. In noisy situations, interestingly enough, the effects of the various brands is only marginally significant. So no matter which CI brand you chose, in noisy situations they all perform about the same.
Percentage of Active Electrodes
The higher the percentage of active electrodes you have working in your CI determines how well you will understand speech with your CI. For example, using 85% of electrodes working as the base result, if you have less than 70% of the electrodes working, you will score about 9% lower in percentile rank. Similarly, if you have more than 85% of working electrodes, you could expect an 8% increase in percentile rank. In other words, the more electrodes (expressed as a percentage) you have working, the better you will understand speech.
Unfortunately, sometimes electrodes have to be turned off because they cause unwanted side effects—such as causing your face to twitch, etc.
Note: the above is only true in quiet situations. In noisy situations the results showed that the percentage of active electrodes made no difference.
Use of Hearing Aids During the Period of Profound Hearing Loss
Let’s assume you are the hypothetical “average” cochlear implant recipient in this study. The results of this study showed that if you started wearing two hearing aids while your pure tone average hearing loss was still in the moderate range (41 to 55 dB), as your hearing loss slipped into the severe/profound range your rate of hearing loss was slower as compared to those that did not wear hearing aids. By the time you got implanted, the difference in auditory performance (percentile rank) was 52.5% if you did not wear hearing aids as opposed to a score of 56.5% (a difference of 4%) if you did wear two hearing aids.
Not only that, but the corresponding results after getting a cochlear implant showed an auditory performance (percentile rank) of 70% vs. 74%, also a 4% increase.
Looking at it another way, not wearing hearing aids before getting a cochlear implant resulted in a loss of speech performance of 0.83% per year for every year you had a hearing loss in the severe/profound category (greater than 70 dB). Thus, for example, if you had a hearing loss in the severe/profound category for 10 years before you got a cochlear implant, and did not wear hearing aids, your resulting speech scores would typically be 8.3% less than they would have been if you had been implanted 10 years earlier.
However, if you had worn two hearing aids your decrease in speech performance would have been slower at 0.45% per year. Thus, if you had waited 10 years after your hearing loss slipped into the severe/profound category, your resulting speech scores would only be 4.5% less.
Note that this is roughly only half as fast. Therefore, wearing 2 hearing aids from the time you have a moderate hearing loss until you get a cochlear implant will not only slow your hearing loss down, but will also give you better CI performance when you do get one. So you win both ways.
If you wore only one hearing aid, what would happen? Here’s the results. If you wore a hearing aid in your future implanted ear, the loss of speech performance would be 0.64% per year. And if you wore a hearing aid in your future non-implanted ear, the loss of speech performance would be 0.49% per year.
Thus, wearing two hearing aids when your hearing drops to a moderate level is the best for future CI performance (0.45% per year). Next is wearing just one hearing aid in the ear opposite to your future implanted ear (0.49% per year). Third best is wearing just one hearing aid in your future implanted ear (0.64% per year). Finally the worst is not wearing any hearing aids at all (0.83% per year).
From these results it seems that if you only wore one hearing aid prior to getting a CI, you should have your CI implanted in the opposite ear for better results.
Furthermore, the longer you wait before getting a CI after you need one, the poorer your results will be. This means that if you should get a CI at a certain point, but you put it off for 10 years, your ultimate success with a CI will be 4.5% less than it would have been if you had gotten a CI when you were first eligible for one.
The Duration of Moderate Hearing Loss
The longer you go without correcting your hearing loss, the worse your results will be when you finally do get a CI. This factor is significant because once you lose a significant amount of hearing (more than 40 dB PTA) your brain starts to reorganize itself to use those areas for other things since they are no longer needed for processing sound as a result of your hearing loss. The results of this study showed that the duration of your moderate hearing loss before you got a cochlear implant reduced your cochlear implant speech performance by 0.23% for every year you had a moderate hearing loss. Thus for every 4 years you have an uncorrected moderate hearing loss, you could expect roughly a 1% reduction in ultimate speech performance with a CI.
Incidentally, the relative importance of the duration of a moderate hearing loss was greater in noise with a 50% steeper slope of decrease in auditory performance per year as compared to the slope for auditory performance in quiet. In other words, the longer you had a moderate hearing loss, the worse you would understand speech in noise when you get a cochlear implant. However, there was no change in slope for the duration of severe/profound hearing loss in noise. This is another reason to get hearing aids when you first notice you need them—typically by the time you have a moderate hearing loss.
The Cause of Hearing Loss
The cause of your hearing loss may affect how well you understand speech in quiet with a cochlear implant. Here are a number of causes of hearing loss arranged from those causing the worst performance with a cochlear implant to those giving the best performance. They are arranged in three groups—those that perform significantly less than average, those that are average, and those that perform significantly better than average. Within each of these groups they are arranged in ascending order from worst to best.
Below Average Performance: The condition that caused the worst performance was Auditory Neuropathy Spectrum Disorder (ANSD) followed by people who had acoustic neuromas, temporal bone fractures and meningitis.
Average Performance: Ear conditions that caused average cochlear implant performance, still in ascending order from worst to best, were people who had hearing loss caused by chronic otitis media (middle ear infections), labyrinthitis (inflammation of the inner ear), ototoxicity (hearing loss from taking drugs), acoustic trauma (typically hearing loss from exposing your ears to loud noise) and hearing loss from unknown causes.
Better Than Average Performance: People with ear conditions that performed significantly better than average, still in ascending order, included otosclerosis, Meniere’s disease and genetic hearing losses. At the top of the pile—the people that had the best performance with their CIs—were those that had sudden idiopathic hearing loss (sudden hearing loss from unknown causes).
Hearing in Noisy Situations
When ranking the various factors in quiet situations as compared to noisy situations, the factor that had the greatest effect on how well you would understand speech in noise was the length of time you had your cochlear implant. The longer you had your CI, the better (on the average) you understand speech in noisy situations. This shows that your brain is plastic and over time continually learns how to better separate speech from background noise.
Note: Don’t read too much into the above factors because these various factors only account for 22% of the variance in the data studied. This means that there are still other factors that account for the remaining 78% of the variance. Future studies may drastically change (or not) their relative importance. However, at this time this is the best data we have.
The One Thing You Can Do Now to Improve Future Cochlear Implant Results
If you have learned nothing else in this article, remember this one thing. If you want to improve your success with a future cochlear implant, get and wear properly fitted hearing aids and/or use effective assistive listening devices (ALDs) as soon as (if not before) your hearing loss drops into the moderate range (greater than 40 dB). Not only will this let you hear better now, but it will slow down your rate of hearing loss as you slip into the severe to profound classes. Furthermore, it will improve your success with a cochlear implant when you finally get one. Sounds like a winning strategy to me.
Lazard, Diane S., et. al. 2012. Pre-, Per- and Postoperative Factors Affecting Performance of Postlinguistically Deaf Adults Using Cochlear Implants: A New Conceptual Model over Time. PLOS One. http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0048739.