20Q: Advances in Middle Ear Implant Amplification

From the Desk of Gus Mueller

If you have ever read an intro to a paper about middle ear implant hearing aids, you know it has to start off with something about the 1930s research of Alvar Wilska from the Physiology Department at the University of Helsinki.  In case you haven’t heard the story, in his 1935 publication, Wilska describes applying iron filings to a human subject’s eardrum, and then generating a magnetic field at the ear.  The magnetic field caused the iron filings to vibrate, which in turn set the eardrum into vibration, and the subject reported hearing a sound although no sound was presented.  Wilska’s research interests primarily were focused on electron microscopy, but you can see how this finding directly relates to the middle ear implant amplification devices of today (or you will see, after you read this month’s 20Q).

Gus Mueller

In the 1970s, we saw devices that were developed to be surgically placed into the middle ear, and in the decades since, we continue to see refinements and improvements in the different techniques and products.  Today, middle ear implants can be divided into the general categories of partially or totally implantable using either piezoelectric or electromagnetic systems.

So why would someone want to undergo surgery when there are much simpler, and less expensive, amplification solutions?  For most, it probably isn’t their first choice.  In many cases, it’s because the traditional solutions didn’t work, or didn’t work to the satisfaction of the patient.  Or, for some reason the patient couldn’t use conventional hearing aids.  The average audiologist won’t be involved in many middle ear implant fittings, but it’s something that our patients do ask about, and a reasonable understanding of these products is therefore necessary.

We happen to know an audiologist who has been involved in the development of middle ear implant products, and also works at a Medical Center where these hearing aids are fitted.  We think these are pretty good credentials for writing a 20Q summary article on the topic.  Our guest author this month is Brad A. Stach, Ph.D. is Director of the Division of Audiology, Department of Otolaryngology-Head and Neck Surgery, at the Henry Ford Hospital in Detroit, Michigan. 

In addition to his responsibilities at Henry Ford, Dr. Stach also serves as Director of Audiology Clinical Education at Wayne State University Department of Communication Sciences and Disorders. He has numerous publications on a wide range of topics, but is perhaps best known for his best-selling book Clinical Audiology, and of course the classic Stach Comprehensive Dictionary of Audiology, which has been translated into several different languages and is used around the world. 

Dr. Stach was instrumental in founding the American Academy of Audiology, served on the inaugural Executive Committee of this organization, later as President, and has also served as Chair of its Foundation’s Board of Trustees. 

A lot has happened in the world of middle ear implants since the day that Wilska sprinkled some iron filing on an eardrum.  In this 20Q article, Brad provides us with an insightful review of the products that are available today, how they work, and how they might benefit our patients.

Gus Mueller, PhD 

Contributing Editor
September 2014

To browse the complete collection of 20Q with Gus Mueller CEU articles, please visit www.audiologyonline.com/20Q

20Q: Advances in Middle Ear Implant Amplification

Brad Stach

1. People have been talking about middle-ear implants as a hearing device solution for nearly 30 years, but these products have not caught on in any meaningful way.  Are there really people out who would choose to undergo extensive and expensive surgery just to get a hearing aid?

Well, considering how few middle ear implants (MEIs) have been implanted worldwide over those three decades in comparison to how many conventional hearing devices have been dispensed, the answer to your question is not very many people.  That said, there have emerged some technologies in the recent past that make implantation more practical and more successful.

2. But why even bother?

Ah, yes, let’s start at the beginning.  I remember meeting Professor Yanagihara from Ehime University in Japan in the early 1980s.  I was a young Ph.D. student, and he was a visiting professor at the Baylor College of Medicine in Houston. He had been working on an implantable MEI with some colleagues at Teikyo University, Japan (Suzuki, Kodera & Yanagihara, 1985). I remember that the surgery at that time was extensive and complicated, and the device was only partially implantable.  So at the end of the day, the patients still had to wear a hearing aid. So we asked him the “why even bother” question. He told us that the most striking outcome was that every patient talked about how much higher in quality the fidelity of sound perception was and how much more natural it sounded than a conventional device.

3. Did he say why that might occur?

Well, to be fair, recall that the fidelity of conventional hearing aids in the 1980s was not nearly as good as modern devices.  But it also seemed that driving the ossicular chain directly resulted in better sound quality than converting it back into an acoustic signal. That probably relates to elimination of the receiver and the acoustic distortion that can occur in an enclosed ear canal. I experienced it personally about a decade later when I listened through an early iteration of what is now known as the EarLens Hearing Device™. At the time, Dr. Rodney Perkins at the California Ear Institute at Stanford was working on driving the tympanic membrane directly. A magnet, embedded in a silastic material, was placed on the eardrum, and vibration was created through induction via a neck loop. The delivery system was awkward, but the sound fidelity was excellent. This overall perception of how natural hearing sounds with one of these devices holds across implanted technologies and across the decades.

4. Okay, but my patients think that modern, open-fit hearing aid technology sounds fairly natural. Are there any other reasons to undergo surgery?

One of the biggest advantages of an MEI is the amount of gain that can be delivered to an ear without feedback and without occluding the ear canal. In today’s partially implanted systems, a microphone is located in a device that resembles a conventional hearing aid. The amplified signal is then delivered directly to the middle ear ossicles in some manner. So there is no loudspeaker to create sound that can return back to the microphone and cause feedback. As a result, significant acoustic gain can be achieved without occluding the patient’s ear with a tightly fitted earmold. This also eliminates issues relating to the occlusion effect, which can be a problem when a closed earmold is necessary.

5. Any other advantages of an MEI device?

Some of the MEIs are fully implanted systems. That is, the entire device, microphone, amplifier, and receiver, is surgically implanted into the middle ear. These devices offer some important lifestyle advantages.  The ability to hear 24/7 is something those of us without hearing loss take for granted. A fully implanted system makes that possible. My favorite anecdotal example of this was from a patient whose passion was being a triathlete. She talked about being unable to wear conventional hearing aids, and consequently being unable to hear when she ran because of perspiration, when she cycled because of her helmet and feedback, and when she swam for obvious reasons. A fully implantable device allows her to hear when she runs, cycles, and swims. Switching from conventional amplification to a fully implanted device had a very positive impact on her life.

6. Okay, I am beginning to see it. From an audiometric standpoint, who should I consider a candidate?

Well, anyone with a sensorineural hearing loss could benefit from a middle-ear implant. But patients with milder hearing loss are likely to be unwilling to undergo the expense and risk of surgery to address their communication difficulties. The target here is most likely to be the patients with moderate to severe hearing loss who requires sufficient amplification gain to warrant an occluding earmold or in-the-ear (ITE) instrument. As mentioned earlier, one of the very real benefits of MEIs is that substantial amplification gain can be delivered to the ear without acoustic feedback. It is “speakerless” technology in that there is no loudspeaker involved that could route the amplified signal back to the microphone. As a result, the potential for acoustic feedback is eliminated, and considerable amplification gain can be achieved. To achieve the same gain with conventional hearing aids in patients with moderate to severe hearing loss, the ear canal necessarily needs to be occluded, resulting in any number of issues. So the patients who are most likely to be interested in MEIs are those whose hearing losses are severe enough that they cannot use conventional mini-BTEs with thin tubes and therefore must use closed earmolds with a traditional behind-the-ear (BTE) instrument to prevent feedback. On the other end of the spectrum, MEIs are not indicated for those patients with severe to profound hearing loss who would benefit from a cochlear implant. Regardless of the effectiveness of driving the middle ear directly, benefit will not be achieved if the cochlea is impaired enough that an amplified signal will not be useful. So the target of MEIs is the patient with too much hearing loss to use open-fit technology but not enough hearing loss to warrant a cochlear implant.

7. What about patients with conductive losses?  Are they candidates for MEIs too?

In most cases, no. MEIs are intended primarily for patients with normal middle-ear function. Most of them are designed to drive an intact ossicular chain, the effect of which will be hampered by, say, any sort of persistent otitis media. That said, there have been reports in the literature of the clever use of an MEI in cases of chronic middle-ear problems in which traditional surgical solutions were not successful. But the primary target here is sensorineural hearing loss.

8. We all know about the advantages of bilateral amplification.  How does that work with MEIs?  Do you implant both ears, or do people have an implant for one ear and use conventional amplification for the other?

That is clearly two questions. Nothing, except expense and surgical risk, precludes implantation of both ears. There are patients who have bilateral MEIs. In all cases of which I am aware, the surgeries have been sequential rather than simultaneous. Some patients have an MEI on one ear and continue to use a conventional hearing aid on the other ear. Other patients choose to only use a single-sided MEI.

9. I know that there are different manufacturers of MEIs.  Do they all work the same? 

No, they all work a little differently. I can tell you about the four with which I am most familiar.  There are three systems that have FDA approval, two of which are partially implanted devices. A fourth system, not FDA approved, is a fully implanted device.

Among the first MEI hearing device to be FDA approved was the Vibrant Soundbridge, which is a partially implantable device. It was developed by Symphonix and later acquired by MED-EL Corporation. The internal component of this device includes an internal receiver coil and magnet that resembles that of a cochlear implant. A wire runs from the receiver to a transducer that is attached to the long process of the incus. The transducer is a magnet surrounded by an electromagnetic coil that drives the ossicular chain. The external device has a microphone, signal processor, transmitter, and magnet. Amplified sound from the external processor is transmitted to the internal receiver and delivered via the wire to a transducer, delivering amplified vibration to the ossicular chain.

10. So, does it look like a cochlear implant?

The internal device resembles a cochlear implant, except at the end of the wire, of course, where a very small transducer replaces the electrode. The surgery is actually similar to that of cochlear implantation in that it takes a mastoid approach to the middle ear. Rather than an electrode being placed in the cochlea, the MEI transducer is attached to the incus. The external device is held to the head with a magnet similar to a cochlear implant.

11. You said there was a second partially implanted device?  How does it work?

The other partially implanted device is the Maxum (Ototronix, LLC). In contrast to the Soundbridge, the implantable portion of the Maxum is simply a small magnet, encased in titanium, which is attached to the incudostapedial joint. The external processor is housed in a behind-the-ear or in-the-canal device. The magnet is stimulated by an electromagnetic coil placed close to the tympanic membrane. The sound is amplified by the sound processor and delivered to the coil, which sets the implanted magnet into vibration and drives the ossicular chain.

Unlike the other middle-ear implants, surgery for the Maxum is minimally invasive and is done under local anesthesia in a standard outpatient procedure room. Using a transcanal approach, the tympanic membrane is retracted, and the transducer is attached to the ossicles.

12. That sounds simple enough. How about the fully implanted devices?

The two work differently from each other and from the partially implanted devices. One device, developed by Otologics and later acquired by Cochlear, is known as the Carina. It evolved from the early Otologics partially implanted device. The driver in this device is a small electromagnetic transducer, the tip of which is typically attached to a small hole drilled in the incus. The implant consists of a digital signal processor, a rechargeable battery, a microphone, a radiofrequency coil for programming, and a connector for the wire that connects to the transducer. All of these components, including the microphone, are implanted behind the ear in a manner similar to a cochlear implant. Sound is detected from the subcutaneous microphone behind the ear, amplified by the sound processor and delivered to the transducer, which drives the ossicular chain.

The surgery for this device involves a mastoid approach closely resembling that of a cochlear implant.  The driver is different, and so the surgical approach to its placement varies. The other important difference is that the microphone is located subdermally behind the ear and is typically secured to reduce its migration.

13. You said there was another fully implanted device?

Yes, and it is FDA approved.  The Esteem (Envoy Medical) uses piezoelectric tranducers to both sense movement of the tympanic membrane and drive the stapes. A piezoelectric tranducer is made of a crystal aggregate material that has the property of reverse mechanical transduction. That is, when a force is applied to the transducer a voltage is generated, and when a voltage is applied to the transducer, a force is generated.  One transducer, the sensor, is surgically attached to the incus. The other, the driver, is attached to the head of the stapes. When the tympanic membrane vibrates and sets the stapes into motion, the motion moves the sensor and generates a voltage. That voltage is then amplified by the sound processor and delivered to the driver, which vibrates the stapes. Because these vibrations are so small, their amplification requires very little power. As a result the battery that is implanted in the sound processor can last over five years on average.

The surgery for the placement of the battery and sound process is much like that of a cochlear implant, the Soundbridge, and the Carina. Placement of the transducers, however, is a multi-step process that is very intricate.

14. Do all these devices work effectively?  How do they compare to traditional hearing aids?

They all have been shown to provide significant patient benefit. The comparison to conventional hearing aids, however, is a tougher question than you might think.  One important way that we verify our convention hearing aid fittings, of course, is with probe-microphone measurement. With MEIs, there is no acoustic output in the earcanal to measure.  So we end up back to behavioral outcomes such as aided soundfield, functional gain, and aided speech recognition to compare outcomes with MEI to outcomes with hearing aids.  In doing so, the MEI data are mostly comparable to well-fitted conventional hearing aids. That is, given similar gain characteristics, MEIs and conventional devices result in at least comparable performance in terms of functional gain and speech perception. In cases in which more gain can be delivered with an MEI than with a conventional hearing aid, speech cues are more audible, and word recognition scores are enhanced at normal listening levels.

As we have learned over the years, however, these measures can be decidedly insensitive to patient perception. Self-assessment measures of benefit, handicap, and satisfaction all seem to suggest a preference for MEIs in the majority of subjects, although there is no way to control for the bias introduced by having undergone surgery to get the device. Questionnaires that address the issues of quality and naturalness of sound favor the MEIs in most patients, as Yanagihara predicted.

15. Can speech perception be enhanced with an MEI in comparison to a conventional hearing aid?

Theoretically, I suppose, the delivery of high fidelity sound with excellent gain across a wide bandwidth should create a potential advantage for the MEI. To the extent that these factors enhance audibility, they will enhance speech recognition scores. But there is not yet evidence to suggest that similar audibility enhancement by a conventional device would not produce similar outcomes. Again, our outcome measures are probably not sensitive enough to show subtle differences. The question of “naturalness” is a difficult one to address. Patients generally perceive MEIs to sound more natural, but other than self-perception measures, these effects have not been quantified.

16. You presented quite a number of options. How do you distinguish between these different MEI devices and conventional hearing aids when you are talking with patients?

I will talk about the two devices I have the most familiarity with, the Maxum and the Esteem, to help distinguish the trade-offs of the technologies. One obvious advantage of the Maxum is the minimally invasive nature of the surgery. As I said earlier, surgery is carried out as an office procedure under local anesthesia. This, of course, eliminates all of the risk associated with the mastoid procedures of the other devices and all of the risk associated with general anesthesia. It also reduces the cost of the procedure considerably. So patients who have too much loss for open-fit hearing aid strategies, are struggling with ear canal occlusion issues, or just need more gain have a very nice option in use of the Maxum.

17. Why not stop there? Why have a mastoid procedure at all?

Well the Maxum still requires the use of an external device. It can still be seen. It also carries the inherent hassle of changing batteries, cleaning, care, etc. More subtly, as with a conventional hearing aid, this solution moves the natural microphone away from the tympanic membrane. Contrast that to the Esteem. The Esteem uses the tympanic membrane as the microphone. This adds back all of the resonance and peripheral processing done naturally by the pinna, concha, and ear canal, all of which is lost when the microphone is moved.

18. But couldn’t you accomplish the same thing with traditional hearing aid processing?

True, manufacturers today tweak the directional processing of their BTE hearing aids to produce a “pinna effect.”  And open fittings indeed often maintain most of the earcanal resonance. But there is nothing like the real thing. In addition to excellent sound quality, the other advantage of the Esteem, of course, relates to lifestyle changes. A patient can have hearing back 24/7, the patient can hear in the shower, there are no batteries to change, the device is invisible, and so on.

19. Do you have your patients try out conventional hearing aids before they agree to one of the surgical approaches?

I do not know of patients who have MEIs who have not at least tried conventional hearing aids. In some cases, use of a conventional hearing aid is a requirement of the FDA. Again, the target here is the patient with gain needs that preclude the use of an open-fit device. Most of these patients are already hearing aid users.

20. What is the key to success in device selection and patient acceptance?

It does not matter how you drive the ossicular chain or with what fidelity the implant has to the incoming sound, you are still delivering sound to an impaired cochlea. Nothing about that has changed. A patient with cochlear distortion, reduced dynamic range, impaired temporal processing, or whatever else the disordered cochlea is doing, will still have all of those problems whether you are delivering the sound acoustically or mechanically. So the key to success is nothing new. Helping the patient develop realistic expectations for whichever technology is chosen is as important in successful MEI dispensing as it is in conventional hearing aid dispensing.

References

Suzuki, J., Kodera, K., & Yanagihara, N.  (1985).  Middle ear implant for humans.  Acta Oto-laryngologica, 99, 313-317.

Appendix

Links to middle ear implant manufacturers for more information.

• Envoy Medical Esteem Hearing Implant: https://envoymedical.com
• MED-EL Vibrant Soundbridge: https://www.medel.com/us/vibrant-soundbridge
• Ototronix MAXUM: https://www.mymaxum.com