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Normal aided functioning: Pipe dream or possibility?

Lindley, George

doi: 10.1097/01.HJ.0000293309.08558.31
Page Ten
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1 Before I can answer the question in your title, can you tell me what you mean by “normal aided functioning”?

A person wearing hearing aids can reasonably be described as functioning normally if he or she experiences about the same degree of difficulty as someone with normal hearing in a given listening environment.1 “Normal” does not mean never having trouble understanding in any situation. Everyone has some trouble in difficult listening environments. We all have to say “what” from time to time and sometimes lose track of the conversation.

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2 Okay, then I'd say based on my experiences, I'm leaning toward “pipe dream.” Are you suggesting otherwise?

Absolutely. With today's flexible products, directional-microphone technology, advanced signal processing schemes, and our increasing knowledge of cochlear physiology, normal aided functioning is a reasonable goal for some individuals with hearing loss in the majority of their daily communication environments.

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3So, do you mean that if they understand speech as well as someone with normal hearing in most communication environments, we've achieved our goal?

Well, at least the most critical aspect of our goal, since speech is typically the most important signal to a hearing aid user. We might also want the patient to have normal loudness perception for environmental sounds and overall speech levels. In addition, the sound quality of speech, music, environmental noises, and the user's own voice should be normal and the person should demonstrate normal aided thresholds. Normal localization ability would be nice as well.

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4Is that all? You make it sound so easy! But why should we now expect hearing aid wearers to function normally? Has the technology improved that much?

Technologic improvements are certainly a big factor. We've also seen improvements and refinements in fitting strategies and we are getting better at applying the sophisticated options and signal processing schemes that are available to us. In addition, the growing realization of the beneficial role that counseling and aural rehabilitation play in improving outcomes has helped. But remember, normal hearing is not a realistic goal for all hearing aid wearers (at least not yet), just some.

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5Let's address that last part. Which patients should we expect to demonstrate normal aided functioning?

Unfortunately, there are no specific, well-defined auditory characteristics that allow us to predict consistently how a given patient will function post-fitting. In addition, people with hearing loss are, well, people, and therefore bring with them varying non-auditory characteristics that affect aided performance. We've all had patients who we thought would make excellent hearing aid users but were unsuccessful. Conversely, some individuals with a significant degree of hearing loss end up functioning as if they had bionic ears.

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6 Can we at least predict who might be expected to function normally?

Sure, and I'll even try to back those predictions up with data. But remember, predictions by nature are not 100% accurate. After all, the Patriots won the Super Bowl! Let's just focus on individuals with cochlear hearing loss.

For starters, we need someone with no greater than a mild-to-moderate hearing loss (thresholds of around 55 dB HL or better). For these patients, it is feasible, using wide dynamic range compression (WDRC) processing, a low compression threshold, a wide frequency response, and a binaural fitting, not only to provide normal aided thresholds (i.e., 20 dB HL) through the speech frequencies, but also to restore normal loudness perception. Assuming that these aided thresholds yield sufficient audibility of critical speech information, we would expect understanding in quiet to be normal.

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7Should speech understan-ding in noise also be normal since we've made most speech information audible?

Maybe. This is where our increasing knowledge of inner ear physiology provides some clues. An individual who has primarily outer hair cell loss, with little or no damage to the inner hair cells, may indeed function normally in background noise. Such patients are primarily in need of a “boost” for soft intensities to overcome the outer hair cell damage.2,3

However, this is likely not the case with many of our patients who have some damage to the inner hair cells. The inner hair cells are responsible for transmitting the auditory information to the rest of the auditory system. These patients may exhibit some suprathreshold deficits (e.g., frequency and temporal resolution) that put them at a disadvantage in background noise.4,5 Some elderly individuals also demonstrate suprathreshold deficits, possibly more central in nature, that cannot be overcome through audibility alone.6

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8Is this why options like directional microphones are necessary?

Exactly. A directional microphone might provide the patient with a mild-to-moderate loss the “edge” needed to function normally in background noise, especially when that background noise is speech. In addition, most digital products have signal processing schemes that can recognize and selectively attenuate steady-state, non-speech noise (e.g., fan, airplane engine) to improve listening comfort and intelligibility in certain environments. We can't get inside and fix the cochlea and/or central auditory pathway. We can, however, manipulate the incoming signal to maximize portions that contribute to intelligibility and minimize those that do not.

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9What about patients with a more severe hearing loss, say between 60 dB HL and 70 dB HL? Is it possible to provide normal aided thresholds for them as well?

It's certainly possible with a well-fitted hearing aid of an appropriate style. But that doesn't necessarily mean they will function completely normally in quiet or noise, even when noise-reduction technology is employed. Typically, a greater degree of hearing loss is associated with a greater degree of cochlear damage and, thus, greater deficits in suprathreshold functioning.

In fact, some recent research suggests there is a point of diminishing returns to providing audibility, especially in the higher frequencies, when hearing loss reaches the moderate-severe range.7,8 For these individuals, performance may not improve as a function of increasing audibility. The same technology and signal processing schemes that improve performance for individuals with less hearing loss will provide benefit, but absolute performance may not be normal.

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10 Are you suggesting that maximizing audibility is not always appropriate? I was taught the wider the frequency response, the better.

We're not at the point where we can reliably predict whether or not a given patient should receive less than optimal amplification in a certain frequency region based on degree of loss. And we still need to determine the role of deprivation and acclimatization in these cases. The main issue for this discussion is that there is a point where, even if you make most speech sounds audible, the individual may not function normally. That doesn't mean we should abandon the goal of normal aided functioning. Rather, we shouldn't be surprised if it doesn't happen.

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11 What you've said so far sounds reasonable, but I'm not yet convinced normal aided functioning is possible. Can you back this up with research?

I'll do my best. But first, we need to outline methods for determining if aided performance is normal. Numerous tests for measuring aided performance are conducted in the clinic or lab. Examples include standard word-recognition testing, speech-in-noise testing, loudness-rating tasks, and formal sound-quality-rating tasks.9-12 Normative data often exist for these measures, so determining if a hearing aid wearer is functioning within normal limits is relatively straightforward.

Self-assessment tools also play a critical role in evaluating aided performance.13 We can recreate various challenging test environments in the clinic or lab, but recreating all the daily communication environments of a given patient is not possible. That's where self-assessment tools can provide additional valuable, subjective information on not only speech intelligibility in a variety of situations but also loudness and sound quality. While these tools are often used to assess benefit by comparing aided with unaided functioning, they can also be used to assess aided performance by comparing aided scores with scores obtained from persons with normal hearing.

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12I'm more familiar with using these tools to assess benefit. Can you provide an example of how to use a self-assessment tool to show how close to normal a hearing aid wearer is functioning?

Sure. Take a look at Figure 1. It provides data from the Profile of Hearing Aid Benefit (PHAB) obtained from 37 persons with normal hearing.14-16 The aided results from a subject with mild-to-moderate sensorineural hearing loss also is provided. This person was a participant in a study conducted at Towson University on adaptation and was fitted with a two-channel, analog, programmable hearing aid with WDRC processing.17 The instrument did not have a directional microphone.

Significant benefit (i.e., aided scores minus unaided scores) was found in quiet, noisy, and reverberant environments. In addition, the hearing aid user's aided results fall along the 50th percentile of normally hearing individuals (i.e., half of those with normal hearing reported a higher percentage of problems and half reported lower). Based on the PHAB data and other objective test findings, this individual is essentially functioning within normal limits when aided. While these results are not representative of the study group as a whole, they do demonstrate that normal aided functioning is possible for some individuals.

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13 What about the “average” hearing aid user? Can we realistically expect enough benefit so that performance is normal even in background noise?

Assuming we can restore audibility, it will depend on how much of an improvement the individual needs in the signal-to-noise ratio (SNR) to function normally. Recent data suggest that, on average, persons with hearing loss need a 1.5-dB improvement in the SNR for every 10 dB of hearing loss when unaided.18 Thus, someone with a mild-to-moderate loss might need an improvement of 2 dB to 6 dB (or more) in the SNR to function normally. Simply restoring audibility of critical speech sounds will help, but will likely not overcome this deficit entirely. That's where directional microphones and noise reduction via signal processing come into play.

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14 So, the obvious question is, do we have the capability to provide this degree of improvement in the SNR?

Recent data suggest we're at least in the ballpark. As an example, one study demonstrated that the directional-microphone option on several hearing aids provided a mean SNR improvement of close to 3 dB as assessed using the Hearing in Noise Test (HINT).19 This occurred in a relatively difficult test environment designed to simulate communication in a restaurant. So, potentially, a directional microphone could provide a 3-dB improvement in a moderately difficulty environment (more or less, depending on reverberation and configuration of noise sources).

Bray and Nilsson evaluated how much of an improvement in SNR could be achieved through multichannel amplification, directional-microphone technology, and noise reduction via signal processing.20Their findings demonstrated an overall improvement in the SNR of 6.0 dB with 2.5 dB, 1.0 dB, and 2.5 dB attributed to amplification, directional-microphone technology, and advanced signal processing, respectively. Of course, the benefit obtained in a given laboratory setting will not necessarily occur to the same degree in various real-world environments. Clearly, though, the potential for normal to near-normal aided performance exists in many communication environments.

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15 It seems possible that we may be giving some patients “superhearing” in certain situations. Is this appropriate?

If our goal is to provide normal hearing functioning, it seems reasonable to expect that for some patients we'll meet that goal, for some we won't, and for some we may exceed it. Most audiologists probably would not have a problem with providing better than normal hearing to their patients in certain environments. It seems unreasonable to limit performance because a hearing aid wearer exceeds normal functioning. Realistically, any “advantage” afforded to the hearing aid wearer will quickly disappear in certain extreme situations where there are very high levels of reverberation and background noise.

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16You mentioned that performance in the laboratory does not necessarily parallel performance in the real world. What do studies using self-assessment tools indicate?

As we discussed earlier, normal aided functioning can realistically be achieved only when several conditions are met: The degree of hearing loss should be no greater than moderate, the fitting should be binaural, and technology that improves the SNR is typically necessary. Unfortunately, all of these conditions are rarely met in a given study and, therefore, we would not expect most subjects to demonstrate normal aided functioning.

For example, Figure 2 provides aided mean PHAB data from some (but not all) studies published between 1990 and 2000.14,21–25 This figure is modeled after one provided by Cox in addressing the issue of hearing aid performance limits.1 Included in the figures are 50th and 95th percentile scores obtained from normally hearing individuals.16

The mean scale scores from all but one of the selected studies are above the 95th percentile for normally hearing individuals. This is not to suggest that none of these participants performed within normal limits on one or more of the scales. Rather, on average, the participants in these studies did not demonstrate normal aided functioning. These studies varied in subject characteristics (e.g., degree of loss, age), hearing aid technologies employed, and percentage of monaural versus binaural fittings, and did not meet our conditions for achieving normal aided functioning. (In all fairness, they were not designed to do so, since they were addressing other issues.)

The Walden et al. study comes closest to meeting the previously outlined conditions and also yields the best results. Subjects were fitted binaurally and wore digital hearing aids with noise-reduction technology (directional microphone and signal processing). Some subjects, however, had high-frequency thresholds greater than 60 dB HL and the mean age was 67.8 years. Despite these characteristics, mean aided performance fell between the 80th and 95th percentile for normally hearing individuals on three of the four subscales, indicating that 5% or more of persons with normal hearing reported a greater percentage of problems. Similar findings using the APHAB as an outcome measure have been demonstrated when advanced technology is employed.26,27

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17 Will we ever reach a point where mean performance for hearing aid users is similar to mean performance for normally hearing individuals?

Let's hope so, at least for most patients with mild-to-moderately-severe hearing loss. As evident in Figure 2, we have a long way to go if we want the hearing aid users' mean performance to fall along the 50th percentile line. But we do have reason for hope. The percentage of individuals fitted with directional-microphone and digital technology will likely increase dramatically over this decade. After all, today's advanced technology is tomorrow's standard technology. This fact, coupled with future technologic advances, can be expected to lead to continued increases in the percentage of hearing aid users approaching and reaching normal communication functioning.

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18 Let's visit some issues beside speech intelligibility. Can we realistically provide normal loudness percep-tion and sound quality for many hearing aid users?

It is probably easier to meet this goal than any other. Multichannel, WDRC processing, coupled with flexible programming software, affords us the ability to restore normal loudness perception to whatever degree the audiologist deems appropriate. Today's fitting strategies yield prescriptions that more often than not yield normal loudness perception for speech in the sound booth and normal subjective ratings of loudness in real-world environments.28 Even when loudness perceptions are abnormal after a period of hearing aid use, addressing user complaints is relatively easy given today's flexible programmable technology.

One of the primary advantages of today's hearing aids over past generations is improved sound quality. Output limiting via peak clipping is now the exception to the rule. Most new hearing aids demonstrate very little distortion over a wide operating range. Audiologists have the flexibility needed to quickly address sound-quality issues based on user feedback. Multiple memories are standard and programs can be designed for non-speech signals such as music.

This is not to say that sound quality is no longer a major issue. One of the common reasons for rejection of hearing aids is the occlusion effect. In addition to standard options for addressing this problem (e.g., deep fittings, venting), efforts are being made to address the occlusion effect through signal processing.

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19Earlier you mentioned localization. Can we expect this ability to be normal in our patients?

Not completely. Configuration and degree of hearing loss, acclimatization, earmold style, and binaural versus monaural fitting are all factors that play a role in aided localization ability.29,30 Some types of localization (e.g., horizontal versus vertical) are more affected by hearing loss and hearing aids than others. Increasing audibility through amplification undoubtedly improves localization ability. On the other hand, sophisticated signal processing schemes and directional-microphone technology can adversely affect localization when phase characteristics are manipulated between ears.30

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20What is the take-home message?

We're starting to see a new generation of digital benefits beyond flexibility, feedback reduction, and steady-state noise reduction. Technologic advances coupled with an increasing knowledge base will cause clinicians and consumers to redefine what they mean by a successful fitting. In evaluating new technology, researchers need to include normative data when possible for benchmark purposes. Over time, this will allow us to see how far we've come and how far we need to go.

In short, we should no longer be satisfied with simply providing some degree of benefit for our patients. Rather, normal or near-normal aided functioning should serve as a realistic goal for many persons with a mild-to-moderate degree of hearing loss.

Your Monday morning patient is a 65-year-old male with a bilateral, symmetrical, downward-sloping hearing loss ranging from 20 dB to 25 dB in the lows to 60 dB to 65 dB in the 3000-Hz to 4000-Hz region. Immittance results are normal, LDLs are in the range of 100 dB HL to 105 dB HL across frequencies, the QuickSin shows an SNR loss of 6 dB, and DSI results suggest no significant APD. He is a first-time hearing aid user, but motivated to make hearing aids work.

You fit him with a pair of custom, half-shell products that have multichannel WDRC, AGCo, and directional-microphone technology. You program the hearing aids appropriately, conduct the usual first-time-user orientation and counseling, and send him out into the real world. How well will he do? Better than unaided? I would hope so. How about nearly as well as someone with normal hearing? Is that too much of a stretch?

How do we define “normal” hearing performance for people wearing hearing aids? Normal aided pure-tone thresholds? Normal loudness perceptions? Normal localization ability? Normal sound quality? Normal speech understanding in quiet? Normal speech understanding in background noise? And, is it reasonable to believe that with today's hearing aid technology we have a shot at attaining “normal” in most or all of these categories for at least some of our patients?

Here to answer these questions is this month's Page Ten author, George Lindley, PhD, faculty member at the School of Audiology, Pennsylvania College of Optometry and staff audiologist at St. Luke's Hospital in Bethlehem, PA. Television game show addicts probably know Dr. Lindley for his first-place finish on Wheel of Fortune a few years back, but you probably recall his name from his numerous publications in the areas of auditory adaptation, prescriptive fitting approaches, and directional-microphone technology. When he's not planning AuD curricula, seeing patients, writing papers, or buying a vowel, George tells me he enjoys hunting and fishing near his home in rural Pennsylvania.

In this Page Ten article, Dr. Lindley encourages us not only to look at the benefit patients obtain from hearing aids, but also to compare their aided performance to that of normally hearing listeners. After reading his review, you just might conclude that maybe, with today's hearing aid technology, providing “normal” hearing for our Monday morning patient is not as far-fetched as we sometimes believe.

Gus Mueller

Page Ten Editor

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© 2002 Lippincott Williams & Wilkins, Inc.