Dr. Ross on Hearing Loss
Different kinds of implants: Auditory, Penetrating and Hybrid
by Mark Ross, Ph.D.
In my judgment, the advent of cochlear implants has been the most significant prosthetic advance for people with hearing loss since the development of the first wearable electronic hearing aid. Having access to the auditory world around them via cochlear implants has transformed and enriched the lives of thousands upon thousands of adults and children. Furthermore, the technical developments that are continually taking place have permitted a broadening in the criteria for those considered possible candidates. Thus the potential benefits of cochlear implants are now available for more and more people with hearing losses. Still, there are two groups of hearing-impaired people out there, some with profound and some with severe hearing losses, who have been unable to benefit from any new technology. That is, until recently.
The first group are those with a condition known as Neurofibromatosis Type II (or NF2). This is a genetic disease that is characterized by tumors growing on the vestibular portion of both eighth nerves. Symptoms can include hearing loss, tinnitus, vestibular problems, and facial numbness. Treatment includes the surgical removal of the tumors, which often (though not always) necessitates the severing of both acoustic nerves. And this is why conventional cochlear implants will not help; without an intact acoustic nerve to carry neural signals from the cochlear to the brain, there can be no hearing sensation. These are people for whom the Auditory Brain Implant (ABI) and the Penetrating Auditory Brain Implant (PABI) were designed.
The second group of people with hearing loss for whom cochlear implant candidacy was questionable were those with an extreme high frequency hearing loss and relatively good hearing in the lower frequencies. These are people who may have close to normal hearing in the low frequencies, and thus can respond to environmental sounds fairly normally, but who perceive little or none of the higher frequencies. Many of these people appear to function quite adequately, particularly with the complementary use of visual cues (speechreading). However, they just about always have to work twice as hard to perceive around half as much as someone does with normal hearing. Some of these people may indeed be candidates for a cochlear implant based on their poor speech recognition scores, but are reluctant to see their residual hearing destroyed when an electrode is inserted in the cochlear. They know what they have and are unwilling to risk winding up worse off than they were before. This is the population for whom the hybrid cochlear implant is intended.
ABI and PABI
From its external appearance, little differentiates the ABI from a conventional cochlear implant (CI). The individual still wears the external processor, while an internal receiver is still embedded in the mastoid bone behind the ear. The major difference lies in the type and placement of the internal electrodes. With a cochlear implant a very fine electrode about 24 to 28 mm in length is inserted within the cochlea. With an ABI, on the other hand, the electrode is a tiny flat plate, about 8.5 by 2.5 mm in size, composed of 21 tiny disk electrodes. In the surgical procedure, this plate is placed directly on the cochlear nucleus at the base of the brain. This is the structure that the auditory nerves leading from the cochlea normally connect to. By stimulating the cochlear nuclei directly, the intention is to bypass the severed auditory nerve and tap in on the next stage in the neural transmission process. The specific positioning of the ABI on the brain stem is crucial and slight variations in placement can dramatically affect the outcome. Because people differ in terms of their auditory and non-auditory responses to the initial stimulation (perhaps a tingling sensation, some slight dizziness or a "jittering" of vision), it takes much longer to properly program an ABI than a conventional cochlear implant.
But they do work. For people who have no remaining or functioning auditory nerves, the ABI can and does provide welcome and valuable auditory sensations. Most ABI recipients report a general increase in pitch as the electrode stimulation moves from the near to the far end of the plate containing the electrodes. While others may not experience consistent pitch variations, they are all aware of sound during electrical stimulation. The benefits of the ABI are at a minimum no less than those obtained with the original generation of single channel cochlear implants, that is, the awareness of environmental sounds, the ability to monitor one's own vocal output and as an important aid to lip-reading. In some instances, performance may be much better than this, though not yet approaching that possible with traditional cochlear implants. About 16% of the ABI recipients are able to achieve 20% or better score on a sentence identification test. A few have gone as high as 30% or even 50%. Since the year 2000, the ABI has been approved by the FDA as an accepted medical device. Currently, more than 500 ABIs have been implanted worldwide.
The Penetrating Auditory Brain Implant (PABI) is a modification of the existing ABI. It includes an assembly of microelectrodes that are designed to penetrate into the auditory portion of the brain stem, the purpose being to produce a more localized stimulation of the cochlear nuclei than is possible with the ABI. The internal cable portion of the PABI is actually bifurcated, with one portion ending in a conventional ABI plate containing 12 surface electrodes (rather than the 21 in an ABI), while the other portion contains 10 needle type electrodes. Both portions are designed to work together. The intention is to improve pitch perception with the combined use of both surface and penetrating electrodes, and thereby also improve speech perception. The ultimate goal is to bring speech comprehension ability closer to that obtained with a cochlear implant.
The first results with the PABI have been sufficiently encouraging for the investigative team at the House Ear Institute to pursue further trials. The results indicated that the device could provide useful hearing sensations safely and effectively. The penetrating electrodes did generate a fairly wide range of pitch sensations at electrical current levels much lower than those typically observed with the surface electrodes of the ABI. According to Dr. Steve Otto of the House Ear Institute, "Penetrating electrodes provided auditory cues that have been beneficial for speech perception, both alone and in combination with the regular surface electrodes that PABI recipients also have." Based on the experiences with the first five subjects, the PABI was somewhat redesigned. Other potential subjects now on the waiting list will receive the redesigned 2nd generation model. Currently the House Ear Institute is the only facility approved by the FDA to perform this procedure on an investigative basis. The PABI is not yet approved as a general medical device for routine clinical use.
The Hybrid Cochlear Implant
For the people who need them, the ABI and the promise of the PABI, are of crucial significance. These implants offer the hope of an auditory connection to the world around them. But the reality is that relatively few hearing-impaired people are potential candidates for these devices. Not so for the hybrid cochlear implant. I doubt that there is an audiologist in the country who does not see one or two potential candidates each and every week. "Ski-slope" hearing losses are a common occurrence in our clinics, and they always present a hearing aid fitting challenge. Because the person may have relatively good low frequency hearing, it is necessary to ensure that over-amplification of the low frequencies does not occur. Because the hearing thresholds in the higher frequencies are so poor, it is difficult for a hearing aid to provide aided audibility without producing unacceptable distortion or discomfort. Furthermore, as some recent research has revealed, the cochlea hair cells that respond to the high frequencies may actually be missing (cochlea "dead spots"). Amplifying these areas may be more than useless; it may actually be counter productive. While some of these people may obtain some help from a hearing aid (at the lower and middle frequencies) significant hearing problems almost always remain.
Traditional cochlear implants have rarely been considered an option for people who fall in this category. Almost always, the insertion of a long electrode into the cochlea results in the destruction of the surviving hair cells. Perhaps some people who elect this route would wind up better off, even with the loss of their low frequency residual hearing, but then again perhaps not. It is not a chance that many people are willing to take. If there were a way in which the person with a severe or profound high frequency hearing loss could preserve their low frequency residual hearing, but simultaneously realize some of the benefits of a cochlear implant, that would seem to be the ideal situation. The hybrid cochlear implant is designed to do exactly that.
Pioneered at the University of Iowa, the hybrid cochlear implant is now undergoing clinical trials. Instead of a long electrode (24 to 28 mm), the hybrid device (in all other respects a conventional cochlear implant) uses only a 10 mm electrode, with six channels of electrical stimulation assigned to transmit sound information corresponding to frequencies above about 1500 Hz. It is inserted into the basal end (that facing the middle ear) of the cochlea and terminates just as the cochlea begins its first spiral (the cochlea is shaped somewhat like a snail). This is the region of the cochlea that responds primarily to the high frequencies.
The surgeon's goal is to insert the short electrode with no or minimal damage to the structures within the inner ear. If this attempt fails, and residual hearing is destroyed during the surgical process (as occurs with the longer electrodes), then this would eliminate the rationale for the entire procedure, i.e. that low frequency hearing can be preserved after implant surgery. Whether residual hearing was indeed preserved was determined by comparing the pre and 3 month post-operative thresholds of 24 recipients of the Hybrid Cochlear implant. Two subjects did experience significant drops in hearing acuity, but this occurred some two and three months post operatively. In one case, some acoustic hearing returned six months later. For the remainder of the subjects, the results indicate that residual hearing was generally preserved, although some (less than 10 dB) average shift did occur at the very lowest frequencies. Generally, it appears that residual hearing was functionally unchanged by the procedure.
Most of the 25 subjects in stage I of this research used in-the-ear (ITE) hearing aids. Some preferred not to use the hearing aid during the testing, complaining that it "blocked" the incoming sounds rather aiding overall perception. This is understandable since their thresholds were normal or close to normal up to about 500-750 HZ; a few others did not use hearing aids at all. The hybrid system can be used whether or not someone wears an ITE hearing aid since acoustic hearing occurs in either case. The implant is programmed so as to present high frequency speech information from the point of the hearing aid or audiogram cut off (around 1500 Hz) to about 6000 Hz. Thus, both acoustic hearing (for the low frequencies) and high frequency hearing (electrical, via the implant) was conveyed to the listener.
Given the preservation of low frequency residual hearing, and the transmission of high frequency information via the implant, now the question is whether the subjects could achieve higher word recognition scores with combined acoustic and electrical stimulation compared to just acoustic stimulation. This is not at all obvious. On one hand, the ear is being stimulated normally with low frequency acoustic energy; on the other hand, there is the direct stimulation of the nerves by the implanted electrodes for the high frequencies only. Up to this point, there was no a priori way of knowing whether the brain could put these two disparate pieces of information together and form a greater whole than each of the parts. Speech perception tests were administered to answer this question. .
All the recipients of the Hybrid system were administered word recognition tests under three conditions: Acoustic in one ear, then combined with the implant, and finally binaural (one ear implanted and the other not). Of the first 11 subjects, 10 achieved substantially higher (39%) average scores in speech recognition scores with the addition of the implant. The range of improvement across patients was 18 to 68%. Scores further improved when the test included the non-implanted ear (the binaural condition) and they continued to improve with time (from 3 months post op to 12 months). At this point in time, the average word recognition score of these subjects was 79% in the implanted ear.
An analysis of the errors in the implant condition showed a marked improvement in the perception of the high frequency consonants. These are precisely the types of speech perception errors usually made by people with extreme high frequency hearing losses. Evidently, the high frequency information conveyed by the implant was able to provide the necessary acoustic cues for their perception. Subjectively, many of the subjects reported that speech sounded "much like it used to, only clearer."
The researchers noted that while people wearing conventional cochlear implants may do well in quiet, their performance often decays markedly in the presence of competing speech babble (as occurs in a cocktail party or restaurant). Because in a hybrid implant some low frequency natural hearing is preserved, thus also preserving some of the normal analytic powers in the cochlea, the researcher's hypothesized that hybrid users would demonstrate better speech perception in a voice babble than would subjects with conventional implants. The researchers compared the speech to noise (S/N) ratio at which listeners obtained a 50% speech understanding score for subjects using both hybrid implants and traditional implants. They also examined a group of people with bilateral mild-to-moderate hearing losses as well as a control group of normally hearing people. (Note: Lower S/N ratios to obtain a 50% word recognition score means that people can better understand speech in a noisy place.)
The results indicate that the subjects wearing the hybrid system were able to achieve lower S/N ratios than those obtained by the group with conventional cochlear implants. Their scores were nearly equal to those obtained by the mild to moderate hearing loss group, which is actually quite encouraging. So now we see that hybrid implants not only improve speech perception ability compared to that obtained with just hearing aids, but that they also increase speech perception scores in a competing noise situation compared to what people using traditional cochlear implant could achieve.
Another important issue for people wearing conventional implants is the problems they report regarding the recognition and enjoyment of music. While they are thankful for being connected again to the world of sound, still many of them miss the fact that music can no longer be a part of their lives. The inability to fully appreciate music is attributed to the poor pitch resolution shown by people wearing cochlear implants. For example, a person with a cochlear implant requires a larger separation between adjacent pitches before they recognize that a second tone is different from the preceding one (poorer frequency resolution). In order to respond appropriately to music, a listener must be able to recognize melodies made up of sequential pitch patterns. This requires a listener be able to perceive the direction of fine pitch changes (higher or lower) as well as the magnitude of the pitch change. Most people with conventional implants can't do this.
As part of this ongoing research at the University of Iowa, the question was posed was whether the residual hearing existing in a hybrid implant patient could improve their comprehension and enjoyment of music. This was examined using a melody recognition test. The hybrid implant subjects obtained an 80% score on this test, or close to the 87% that the normally hearing people achieved. In contrast, the score of the group with the traditional cochlear implant (long electrode) was only 27%. The higher scores obtained by the hybrid group was attributed to the fact that the fundamental frequency of nearly all musical instruments lies below 750 to 1000 Hz. Being able to naturally perceive sounds below this frequency permits listeners to perform considerably better than traditional implant users in discriminating musical intervals and perceiving the direction of pitch changes. Now, according to hybrid implant users, music sounds pleasant again.
Not everybody with a high frequency hearing loss can qualify as a candidate for a hybrid cochlear implant. The initial selection criteria for potential subjects included a post-lingual hearing loss, word recognition scores between 10 and 50% in the ear to be implanted (while using an "appropriately" fit hearing aid) and no more than a 60% score in the better ear. The pure-tone thresholds had to drop sharply after 500 Hz with the thresholds at the low frequencies between 0 and 60 dB. According to Dr. Christopher Turner of the University of Iowa medical school, stage 1 of the trials has been so encouraging that the criteria for inclusion have been extended and expanded. In stage 2 of the trials, for which subjects are now being recruited in ten centers around the country, the limits are 10 to 60% word recognition scores in the ear to be implanted, plus no more than an 80% score in the non-implanted ear. Instead of the audiogram dropping off at 500 Hz and lower, now subjects will be accepted with the drop off frequency up to 1500 Hz. While the initial study was conducted using the cochlear corporation device, I've been informed that other companies will also be conducting clinical trials. This is one development that I'll be watching very closely. Once approved by the FDA, I would not be surprised to learn that hybrid implant users would shortly be exceeding the number receiving traditional implants.
I'd like to thank Steve Otto of the House Ear Institute and Dr. Christopher Turner of the University of Iowa Medical School for providing me with up to date information during the drafting of this article.