Article of interest:
Variation in Preferred Gain with Experience for Hearing-Aid Users
Keidser, G., O’Brien, A., Carter, L., McLelland, M., and Yeend, I. (2008)
This editorial discusses the clinical implications of an independent research study. The original work was not associated with Starkey Laboratories and does not reflect the opinions of the authors.
Auditory plasticity, or the reorganization of neural connections in the auditory system, has been documented in studies with human and animal subjects (Palmer et. al., 1998; Philibert et. al, 2005; Willott, 1996). These studies found that the representation of acoustic stimuli along auditory pathways can change based on auditory experience. The concept of auditory plasticity may relate to hearing aid patients in two ways: first, neural reorganization is likely to occur in response to the hearing loss itself, then subsequent reorganization may occur in response to amplification. Indeed, most clinicians observe that new hearing aid users experience an adjustment period in which they prefer less gain, but that over time they are able to accept increases in gain.
The clinical manifestation of auditory plasticity is sometimes associated with acclimatization to amplification and has been studied in hearing aid users in numerous ways: preferred gain for new and experienced users (Marriage, et al., 2004; Cox and Alexander, 1992; Horwitz & Turner, 1997), speech performance over time (Bentler, et al.1993a, Gatehouse, 1992), subjective benefit or sound quality over time (Bentler, et al., 1993b; Ovegard, et al., 1997), loudness perception and intensity discrimination over time (Olsen, et al., 1999; Philibert et al., 2002), even changes in ABR wave V latency (Philibert, et al, 2005). Most studies have found small but significant changes over time as participants adjusted to amplified sound. Others, however, have found no significant difference between new and experienced hearing aid users (Smeds et al, 2006a, 2006b). Some recent work suggests that acclimatization may occur specifically in relation to high level, high frequency sounds (Munro & Lutman, 2003).
The authors of the current study were specifically interested in comparing gain preferences and loudness perception in new hearing aid users and users with more than 3 years of experience with hearing aids. Fifty new users and twenty-six experienced users, most with mild to moderate sensorineural hearing loss, were fitted with digital, two-channel, WDRC instruments equipped with volume controls. Compression attack and release characteristics were set identically for all subjects and a noise reduction algorithm was turned off. The hearing aids had three independent programs:
- NAL-NL1 target response
- NAL-NL1 with a 6dB high-frequency cut at 3000Hz (HFC)
- NAL-NL1 with a 6dB low-frequency cut at 500Hz (LFC)
Subjects were asked to compare the programs in everyday environments and record their preferred overall program. Follow-up testing was conducted at 1 month, 4 months and 13 months post-fitting and subjects were specifically instructed to arrive at each appointment with the device on their overall preferred program and volume control setting. At each appointment, 2-cc coupler and real-ear measurements were obtained with hearing instruments at the preferred settings. Questionnaires were administered to record hearing aid usage time, ranking and performance of the three programs and to what extent the volume control was used. Loudness perception tests were performed using a categorical loudness scale test (Cox & Alexander, 1997) to determine the median SPL levels that were categorized as “comfortable”.
The authors found that new and experienced users preferred the high-frequency cut (HFC) program most often. Initially, about 60% of the new users preferred the HFC program, but by 13 months post-fitting the preferences of new and experienced users were very similar with approximately half of the subjects still preferring the HFC program. Fewer than 10% of the users preferred the LFC program across the duration of the study.
On average, overall preferred gain was 3dB lower for new users and increases were noted at subsequent appointments. By the time of the final appointment, new users reported higher gain settings than they did before, but did not reach the preferred levels of experienced users. This suggests that the gain acclimatization process for some users may continue beyond the 13-month point. Degree of hearing loss had a significant effect, as subjects with moderate hearing loss preferred 6dB lower overall gain than those with mild hearing loss.
The findings of Keidser and colleagues offer important implications for clinical practice. First, it appears that new hearing aid users experience acclimatization with regard to comfortable loudness and preferred gain settings. This supports the use of adaptation levels in hearing aid software, though some adaptation managers may provide larger decrements in overall gain (5-10dB) than many hearing aid users require. The fact that new users with mild hearing loss did not prefer as much initial gain reduction as those with moderate losses indicates that audiometric thresholds should be considered. The authors noted that more information is needed about acclimatization effects in new hearing aid users with more than moderate hearing loss. However, it is probably appropriate for clinicians to assume that for patients with moderate to severe hearing loss, lower initial gain settings may be needed and an extended period of adjustment may be expected.
Recent emphasis on evidence-based clinical practice underscores the importance of verification measures to ensure adequate gain and frequency response from new hearing aids. However, study of auditory acclimatization demonstrates that it is equally important to evaluate the patient’s perception of the amplified sound to ensure satisfaction. Ultimately, a patient with excellent aided test results may still reject new hearing aids if they are not comfortable.
Hearing aid acclimatization can be measured several ways, many of which are not viable for busy clinical practices. But there are two simple ways in which it should be addressed during the hearing aid fitting and follow-up appointments. At the fitting appointment, patients should be counseled about appropriate expectations for the adjustment process. For instance, patients who know ahead of time that it is normal to notice, and perhaps be slightly annoyed by, newly amplified sounds are less likely to be disheartened when this occurs. They should be advised to wear their hearing aids as consistently as possible and to report any discomfort or pain so it can be addressed with programming adjustments.
Initially, high frequency gain in particular may need to be reduced relative to target settings. However, care should be taken to determine each individual’s comfort with high frequency sounds. In the current study, individuals were free to reduce high frequency gain at any time by selecting the HFC program. It seems appropriate to question whether this affected their ability to adjust to high-frequency gain and if they would have eventually been able to tolerate, even prefer, more high-frequency amplification had they not been able to switch at will into the HFC program. The importance of high-frequency information for speech intelligibility, especially in noise, is well established (Turner & Henry, 2002; Hornsby & Ricketts, 2003). To avoid detrimental effects on speech perception, high frequency gain should approach targets as closely as possible, while still maintaining patient comfort.
At follow-up appointments, patients should be questioned in detail about their comfort and overall progress with the new aids. Formal questionnaires like the APHAB (Cox and Alexander, 1995) can be used to determine specific sound preferences so that appropriate adjustments can be made. The more precise information a clinician obtains from a patient, the more likely they are to zero in on necessary programming changes. One important point to note is that this study evaluated gain preferences only. Most hearing instruments have several adjustable parameters, and some new users might respond as favorably to increased compression ratios or lowered compression kneepoints, thereby reducing louder sounds but maintaining gain for low to moderate level sounds.
Because it appears that the acclimatization process may continue beyond a year, follow-up care after the initial trial period should be planned accordingly. It may be appropriate to schedule check-ups at 4-months, 8-months and 12-months post-fitting. This way, final target settings can be approached systematically for each individual.
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