March 3, 2009 Feature

Reflections on 40 Years of Amplification

H. Gustav Mueller

One of the founding members of the American Academy of Audiology, Mueller has led developments in amplification and audiology. He has authored nearly 200 articles and book chapters and is an internationally recognized presenter. Mueller also has made his mark as one of the earGuys at earTunes.com.

—Joseph Montano, EdD, CCC-A
Assistant Professor of Otorhinolaryngology, Weill Cornell Medical College

Q: Amplification has demonstrated major developments over the past few years with rapid growth in the dispensing of slim-tube open-canal systems. What are your thoughts about open fit technology and how has it changed over the years?

I started fitting hearing aids in 1971 when I joined the U.S. Army—yes, we were doing open-canal fittings back then—so for me, "over the years" is nearly a 40-year window. In the 1970s we routinely coupled #13 tubing (shaped with the help of a paper clip and hair dryer) to eyeglass CROS instruments, to provide some high-frequency gain with little or no occlusion effect for people with high-frequency hearing loss (we used the CROS style because head-shadow helped prevent feedback).

Today's version of an open-canal (OC) fitting is quite different and much improved. We now have an extended high-frequency range, WDRC and AGCo, multiple channels, directional technology, and DNR. The feedback cancellation is the big player that makes these products so much more effective than those of the 1970s. The system allows substantial gain in the critical speech frequency regions, while still maintaining an open ear canal. The fact that these instruments are rather cute and unobtrusive is another plus, but cosmetic appeal gets old quickly without auditory benefit. Fortunately, there is benefit—the occlusion effect problem is nearly eliminated, and patients typically prefer the sound quality of an OC fitting.

Overall, the modern OC instrument is a great addition to our fitting options—it's encouraging to see patients who want to tell others about their new hearing aids and actually show them off at parties!

Q: Surveys show that many audiologists are fitting more than 50% to 60% of their patients with open-canal products. Does this seem about the right mix to you?

That seems a little high to me, unless your caseload tends to be a lot of people with mild-to-moderate losses. I think with any successful product, we tend to push the fitting range a little. Assuming it's truly an "open" fitting, high-frequency gain will be limited by acoustic feedback (real-world gain no more than 25 dB or so for most models), and of course this design is not geared toward providing substantial low-frequency gain.

It might be true in some offices that many patients are being fitted using a mini-BTE and "slim plumbing," but I'm betting that in many cases a more closed tip is being used to obtain the desired gain. Nothing wrong with this, of course; it's just that in my book it's no longer an OC fitting. The other explanation for these high numbers could be that patients simply are being fitted with less gain than what we commonly believe is appropriate.

Q: That leads directly to the next question—many audiologists rely on the manufacturer's first fit protocol when dispensing hearing aids. Is this a good thing?

It might be okay, but it wouldn't be my first choice. We have validated prescriptive fitting approaches such as the NAL-NL1 and the DSL v5.0, which provide excellent starting points regarding what ear canal SPL is appropriate for different populations and different hearing losses.

To my way of thinking, it doesn't really matter whose product does the processing—the same ear canal fitting targets will apply—so why change philosophy when we change products? In some cases, the manufacturer's default program is one of the validated methods. In other cases, however, the gain and output is very different from the established prescriptive procedures, differing by 15 dB or more.

Numerous studies have shown that casually using an arbitrary default program can be risky. This is especially true for infants and toddlers for whom speech and language development might be at stake. And to state the obvious, when I'm talking about using a validated method, I'm referring to measured ear canal SPL for each individual patient (or calculations using RECD values)—not computer simulations from the fitting software.

Given the research supporting the established fitting algorithms, and the ease with which real-speech inputs can now be used with probe-mic equipment for verification, I can't think of a good reason why all audiologists wouldn't go down this path.

Q: And finally, Gus, we know that the hearing aid is a vital tool for audiologists in the provision of AR services. What are its limitations and what is the value of counseling as a component of amplification dispensing?

Even though we now have many beneficial hearing aid features that were unavailable 10 or even five years ago, it is still often difficult to meet the patient's expectations. Much of what fitting hearing aids is all about is providing high-fidelity audibility, and we've been doing that quite well since the early days of the K-Amp. As a result, many patients do very well in relatively quiet listening situations.

We know, however, that for both peripheral and central processing reasons, even when audibility is maximized and all of today's special noise reduction features are implemented, the typical patient will still have trouble understanding speech in background noise. This is one area where counseling is critical. A basic speech test such as the QuickSIN will provide the dispensing audiologist a general idea of how well a patient performs for different SNRs.

I think it's important to present realistic expectations for the patient, and to get him or her involved at the time of the initial fitting in using the best communication strategies for these listening conditions. That's just one example—there are many other patient-specific areas in which appropriate counseling also is necessary.

Counseling won't make a bad fitting a good one, but it will make a good fitting a more successful one.


  

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