Alternative Fitting Approaches for Special Populations

Donald J. Schum, Ph.D./CCC-A
Vice President, Audiology & Professional Relations, Oticon, Inc.

Douglas L. Beck Au.D.
Director, Professional Relations

Introduction:

The provision of amplification is dynamic; it evolves and changes over time.

As professionals, our goal is to develop and apply standardized hearing aid fitting and evaluation tools, based on scientifically rigorous and evidenced-based protocols, while accommodating the individual needs of the patients we serve. Ideally, these same protocols should allow the efficient and consistent provision of clinical services; day-to-day, from professional to professional, and across hearing aid dispensing facilities (Cox, 2006).

Professional responsibility dictates clarity regarding the application of standardized treatments and, importantly, the application and circumstance indicating alternative approaches. We believe the client/patient expects the professional to abide by generally agreed upon protocols and techniques in the delivery of professional services and products, while appreciating the necessity of appropriately customizing treatment to best meet their unique and individual needs.

Background:

Over the past decade, our approach to the diagnosis and treatment of sensorineural hearing loss (SNHL) has been affected by an ever-increasing understanding of physiologic mechanisms and interactions. For example, in the presence of mild-to-moderate sensorineural hearing loss, in general, outer hair cells are dysfunctional while inner hair cells remain intact. Secondary to the loss of outer hair cells, sensitivity and selectivity of inner hair cells is compromised. Using this basic model, a decrease in the dynamic range (accelerated loudness growth) can be explained. As hearing loss progresses to moderate and severe, inner hair cell loss becomes more significant, and sensitivity decreases as does signal resolution ability of the auditory system.

Although the above notes explain typical SNHL and common hearing aid candidacy characteristics for relatively uncomplicated presbycusic clients/patients, the physiologic basis of SNHL, as well as the concomitant auditory perceptions of unique individuals is significantly more complicated -- for a minority of hearing aid candidates and wearers.

Peripheral and central auditory systems are incredibly complex. Normal auditory perception is dependent on a fine balance of physiologic and anatomic attributes. For example; adequate and nutritious blood supply, appropriate metabolic balance of inner ear fluids, intact cochlear membranes, normal neural transmitters and neural pathways, an absence of space-occupying lesions, and more, all of which are dependent and interact with cognitive ability and function. Nonetheless, other "sub-clinical" physiologic disturbances may be present, despite apparent SNHL. Schucknect (1974) noted a variety of different physiological disruptions can be present - - even in the presence of clinically defined presbycusis.

Most hearing aid fitting protocols are based on a relatively simple understanding of the nature of SNHL. When disruptions or abnormalities occur in the peripheral and central auditory system, beyond hair cell loss, our understanding and appreciation of the appropriate amplification options and outcomes, diminishes.

The professional has traditionally been provided little guidance related to hearing aid expectations, evaluation, fitting techniques and outcomes as they impact clients/patients with atypical hearing loss presentations. In this article, we refer to clients and patients with atypical audiometric presentations as "Special Populations" including those with; ski-slope, cookie-bite and rising audiograms.

In this article we'll address alternative approaches to help guide treatment for these special populations. Admittedly, large scale clinical research on these small populations of clients/patients is highly desirable, yet lacking. Nonetheless, this paper reminds the professional managing special populations, that atypical presentations, sometimes benefit from alternative solutions.

Amplification's Unspoken Assumptions:

There are many unspoken assumptions which often guide common approaches to hearing aid amplification.

"Measured hearing is useable hearing." This assumption presumes if a threshold can be established and recorded, the ear can make use of this particular amplified sound. However, evidence gathered from ski slope hearing losses has questioned the universal application of this notion (Hogan & Turner, 1998; Ching, Dillon & Byrne, 1998). For patients with ski slope hearing losses, the effectiveness of providing audible speechinformation in regions of severe hearing loss was limited -- and, in some cases, counterproductive. Of course, that doesn't mean that the assumption was not reasonable, but it does mean there are conditions under which the assumption is not correct.
"Full speech audibility is the goal." Often, it is assumed the client/patient will perform better if more of the amplified speech spectrum is made audible. Although this may be an appropriate assumption in the majority of cases, there are conditions under which this assumption does not apply; such as often the case with inner hair cell hearing loss.
"The hearing aids will be a substitute for sound input to the ear." Therefore, it is assumed that all sounds pass through the device and that all fittings are closed, due to the risk of feedback. With the new era of open fittings made possible via effective feedback management algorithms, modern hearing aid fittings often combine amplified sound with direct sound. (Flynn, 2003).

Hearing Loss Correction Model:

In general, we apply a Hearing Loss Correction model when fitting amplification to ears with SNHL. We evaluate the shape (flat, rising, sloping) and degree (mild, moderate, severe, profound) of hearing loss and apply gain proportional to the amount of hearing loss.

Even when applying wide dynamic range compression based upon measured loudness growth characteristics, the result inevitably leads to greater gain in regions with more hearing loss. We basically provide the listener with sounds rendered inaudible, due to hearing loss. Prescriptive approaches balance the amount of amplification in different frequency regions (Dillon, 1999). However, the underlying philosophy is to "correct" what is lost, essentially based on the established pure-tone thresholds.

Unfortunately, Hearing Loss Correction simply does not account for the unique communication needs of some special populations.

Residual Capabilities Model:

An alternative approach is the Residual Capabilities model. In this model, rather than "correcting" for elevated thresholds, the goal of the hearing aid fitting is to make best use of the patient's remaining (i.e., residual) hearing capabilities. The impaired auditory system is viewed as possessing a limited set of capabilities, and these capabilities are viewed in relation to signals of most interest, presumably speech. In this model, the hearing aid serves as an "interface" between the client/patient's residual auditory capabilities and the acoustic environment the client/patient is interested in. Amplification created within this paradigm allows the client/patient access to the important signals of interest.

Figure 1 depicts the concept of the hearing aid as an interface between the residual capabilities of the patient and the world of sound. The interface approach to hearing aid fitting places the focus not on what has been lost (hearing) but rather, what capabilities remain. In this model, hearing aids repackage sound for impaired auditory systems. The Residual Capabilities approach provides flexibility based on the individual's needs and it encourages a broader view of hearing loss than what is captured and presented on the audiogram. Importantly, the Residual Capabilities approach not only considers the remaining measurable hearing, but also, the quality of remaining hearing and the signals of greatest interest to the listener.

Alternative Clinical Strategies:

There are two implications to adopting a Residual Capabilities approach to fitting special populations. First, for some groups, this approach will call for systematic changes to prescribed fitting algorithms. In this paper, we will suggest alternative fittings for three specific groups: ski-slope loss, rising audiograms and irregular audiograms. Hearing Loss Correction simply does not appropriately account for the communication needs of some groups.

The second implication is that there is a greater need for adaptive adjustments to standard prescribed settings. There are two other special population groups not considered in this paper: those with severe and profound loss and those with medically complex disorders. For these groups, there is a higher expectation of patient-to-patient variability in the reaction to amplification. This expectation should trigger an intervention approach that puts less emphasis on the initial prescribed settings and more emphasis on systematic exploration of changes in gain and compression settings. We will explore alternative approaches for these groups in upcoming papers in this series.

Ski-slope Hearing Loss: This is the largest of the special populations. It includes patients with hearing within normal limits through, at least, 1000 Hz with a rapid drop in sensitivity in the higher frequencies. The most common cause is of course noise exposure.

The best guidance in fitting these patients is actually over 20 years old. Skinner (1980) reported on the benefit of high frequency amplification for patients with ski-slope losses (at a time where few of the patients were being fit due to the limited bandwidth of hearing aids). However, she also pointed out that full high frequency audibility was not a reasonable goal due to comfort and sound quality problems. She also pointed out the need to achieve a balance between gain provided in the mid and high frequencies.

A Hearing Loss Correction approach would call for little or no gain except in the extreme high frequencies. However, these patients may not be able to make use of high frequency audibility (Hogan & Turner, 1998; Ching, Dillon & Byrne, 1998).

An alternative fitting approach for this group is to reduce the gain in the extreme high frequencies and, instead, target a modest audibility enhancement in the mid-frequency, transition region. Although these patient have the most loss in the region above 2000 Hz., they also have a partial hearing loss somewhere in the region between 1000 and 3000 Hz. This is also the area where the greatest amount of speech information falls (Kryter, 1962). Providing a modest gain enhancement here not only will boost speech audibility, it will also provide a better to chance to avoid the sound quality concerns that accompany a response with an excessive amount of high frequency gain. It will also allow for a more open fitting and a reduced chance for acoustic feedback.

Figure 2 provides the effect of the gain prescribed by the DSL i/o approach (left panel) and by an Oticon fitting algorithm, Voice Aligned Compression, (right panel) that specifically applies a correction for ski-slope losses. In each panel, the unaided moderate speech spectrum is show in the background in grey and the aided spectrum is shown in the foreground in red (DSL i/o) or blue (VAC). Notice the difference in gain levels for mid versus high
frequency regions.

Low Frequency Sensorineural Hearing Loss: These patients typically have a moderate sensorineural hearing loss in the low frequencies rising to normal hearing in the region above 2000 Hz. The losses are almost always congenital and often genetically linked. Thornton and Abbas (1980) point out that some of these patients may actually show false low frequency thresholds during routine clinical testing due to the nature of the traveling wave within the cochlea.

A Hearing Loss Correction model for these losses calls for significant gain in the low frequencies and little or no gain in the high frequencies. This approach enhances audibility specifically in a region where there is a minimal amount of speech information. In addition, it greatly increases the risk of upward spread of masking.

Schum & Collins (1992) report on the subjective and objective assessment of a range of alternative fitting strategies for patients with rising audiograms. Performance for most patients improved when some amount of gain was provided in the mid to high frequency region where thresholds were returning to normal. Many patients preferred little or no gain in the lower frequencies despite the fact that this is precisely the area of greatest hearing loss.

These results are a clear example of the difference between a Hearing Loss Correction model and a Residual Capabilities model. These patients seem to perform better when the emphasis of amplification is placed in a region where there is important speech information and hearing is functioning in a near-normal manner.For these patients, we recommend a minimum of 10 to 15 dB insertion at 2kHz. and above, even if hearing is normal. We also recommend that the insertion in the low and mid frequencies be set at no more than 15 to 20 db. This initial response should be evaluated subjectively by the patient, with the first attention paid to making adjustments either up or down in the amount of low frequency gain based on a judgment of clearness and loudness. Some patients will want more, but many may want less.

Irregular Shaped Audiograms: These patients have sensitivity patterns that change in a non-monotonic manner across frequency. Within the group are the "cookie bite" and "W" audiograms. Although these patients have received very little attention in the fitting literature, there is one simple observation to make. Figure 3 provides the thresholds, most comfortable levels and uncomfortable levels for a patient tested by the first author. As can be seen, the suprathreshold loudness measures show a flatter pattern than the audiogram. These results are completely consistent with our understanding of loudness growth in sensorineural hearing loss.

The implication for fitting is as follows. Hearing aid patients do not typically listen at threshold. Rather, most of the day is spent listening to moderate and louder inputs that are amplified into at least the middle of the dynamic range. Since the MCL curve for a patient with an irregular audiogram is likely flatter than the threshold curve, a flatter aided signal is appropriate. The frequency response of the hearing aid does not need to follow the dramatic irregularities of the audiogram. A Hearing Loss Correction approach will specify a response that will follow the shape of the threshold curve. It makes better sense for these patients to manually smooth the response of the device, trying to provide gain and compression such that the aided speech spectrum falls smoothly within the remaining dynamic range. Figure 4 provides the NAL-NL1 response (left panel) for the patient in Figure 3 along with an alternative suggestion (right panel) consistent with the focus on a smoother, suprathreshold response. In each panel, the lower section provides the prescribed insertion gain for 50, 65 and 80 dB SPL inputs. The upper section shows the effect on a moderate speech input.

Additional Signal Processing:

The guidelines discussed in this paper are focused on the provision of gain and compression in multi-channel, wide dynamic range circuitry. Modern amplification also offers advanced circuitry such as feedback cancellation, noise reduction and directionality. These other features generally provide benefit to most patients, as long as the practical capabilities of these systems are acknowledged by the patient and professional.

There is every reason to expect that these systems should also provide benefit to patients within the Special Population subgroups. There may be some specific limitations. Note that the modifications suggested in this paper often call for a reduction in gain compared to traditional prescribed settings. With less gain especially in the low frequencies, the perceptual effect of noise reduction and directionality may not be as apparent to the user. However, in general, these systems should also be considered as part of the fitting solution for the patient groups discussed above.

Final Thoughts:

The recent focus within our profession of applying structure and standardized approaches to hearing aid fittings has been a positive and welcome step. Excellence in professional practice requires the professional to have a vast understanding of common and atypical audiologic presentations, as well as prescribed and alternative solutions. SNHL and it's plethora of manifestations clearly indicate that simple rule-based hearing aid fittings are not enough. Sometimes, excellent hearing aid fittings cannot be prescribed.

References

Ching, T, Dillon, H. & Byrne, D. (1998). Speech recognition of hearing-impaired listeners : Predictions from audibility and the limited role of high-frequency amplification. Journal of the Acoustical Society of America, 103:1128-1140.

Cox, R. (2006). Evidence-based practice practice in provision of amplification. Journal of the American Academy of Audiology, 16:419-438.

Dllon, H. (1999). NAL-NL1: A new prescriptive fitting procedure for non-linear hearing aids. The Hearing Journal, 52(4):10-16.

Flynn, M. (2003). Open ears: The scientific basis for an Open Ear Acoustics system. The Hearing Review, May:34-37, 67.

Hogan, C. & Turner, C. (1998). High frequency audibility: benefits for hearing impaired listeners. Journal of the Acoustical Society of America, 104:432-441.

Kryter, K. (1962). Methods for the calculation and use of the articulation index. Journal of the Acoustical Society of America, 34:1689-1697.

Schuknecht, H. (1974). Pathology of the Ear. Cambridge, MA: Harvard University Press.

Schum, D. & Collins, M.J. (1992). Frequency response options for people with low-frequency sensorineural hearing loss. American Journal of Audiology, Nov: 56-62.

Skinner, M. (1980). Speech intelligibility in noise-induced hearing loss: Effects of high-frequency compensation. Journal of the Acoustical Society of America, 67:306-317.

Thornton, A. & Abbas, P. (1980). Low-frequency hearing loss: perception of filtered speech, psychophyscial tuning curves, and masking. Journal of Speech & Hearing Research, 67:638-643.