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Evidence for the Value of Real-Ear Measurement

Abrams, H.B., Chisolm, T.H., McManus, M., & McArdle, R. (2012). Initial-fit approach versus verified prescription: Comparing self-perceived hearing aid benefit. Journal of the American Academy of Audiology, 23(10), 768-778.

Audiology best practice guidelines state that probe microphone verification measures should be done to ensure that hearing aid gain and output characteristics meet prescribed targets for the individual. In the American Academy of Audiology’s Guidelines for the Audiologic Management of Adult Hearing Impairment, an expert task force recommends that “prescribed gain from a validated prescriptive method should be verified using a probe microphone approach that is referenced to ear canal SPL” (Valente, et al., 2006). Similarly, the Academy’s Pediatric Amplification Protocol (AAA, 2003) states that hearing aid output characteristics should be verified with real-ear measures or with real-ear-to-coupler-difference (RECD) calculations when lengthy adjustments subsequent to real-ear measurement are not possible.

In contrast to these recommendations, the majority of hearing aid providers are not routinely conducting real-ear verification measures. In a survey of audiologists and hearing instrument specialists, Mueller and Picou (2010) found that respondents used real-ear verification only about 40% of the time and Bamford (2001) reported that only about 20% of individuals fitting pediatric patients used real-ear measures. The reasons most often cited for skipping probe microphone measures are based on financial, time, or space constraints.

When probe microphone measures are not conducted, other verification techniques may be used such as aided word recognition, but these not likely to provide reliable information (Thornton & Raffin, 1978).  Or, verification may not be attempted at all, with fitting parameters being chosen based on the manufacturer’s initial-fit specifications. Although most fitting software allows for entry of age, experience and acoustic information such as canal length and venting characteristics, their predictions are based on average data and cannot account for individual ear canal effects.

Numerous studies have shown that initial-fit algorithms often deviate significantly from prescribed targets, usually underestimating required gain, especially in the high frequencies. Hawkins & Cook (2003) found that simulated fittings from one manufacturer’s initial-fit algorithm over-estimated the coupler gain and in-situ response by as much as 20dB, especially in the low and high frequencies.  Bentler (2004) compared the 2cc coupler response from six different hearing aids programmed with initial-fit algorithms and found that the responses were different for each manufacturer and deviated from prescriptive targets by as much as 15dB, usually falling below prescribed targets. Similarly, Bretz (2006) studied three manufacturers’ pediatric first-fit algorithms and found that the average output varied by about 20dB and initial-fit gain values were below both NAL-NL1 and DSL (i/o) targets. This is of particular concern because pediatric patients may be less able than adults to provide subjective responses to hearing aid settings, rendering objective measures such as real-ear verification even more important.

These studies and others illuminate the potential difference between first-fit hearing aid settings and those verified by objective measures, but it is not well known how this affects the user’s perceived benefit.  Some early reports using linear amplification targets indicated that verification did not predict perceived benefit (Nerbonne et al., 1995; Weinstein et al., 1995), but more recent work indicates that adults fit to DSL v5.0a targets demonstrated benefit as measured by the Client Oriented Scale of Improvement (COSI, Dillon & Ginis, 1997). A recent survey by Kochkin et al. (2010) found that patients whose fittings were verified with a comprehensive protocol including real-ear verification reported increased hearing aid usage, benefit and satisfaction. Furthermore, these respondents were more likely to recommend their hearing care professional to friends and family than were the respondents who were not fitted with real-ear verification.

The purpose of the study discussed here was to determine if perceived hearing aid benefit differed based on whether the user was fitted with an initial-fit algorithm only or with modified settings based on probe-microphone verification. Twenty-two experienced hearing aid users with mild to moderately-severe hearing loss participated in the study. All were fitted with binaural hearing aids, though a variety of hearing aid styles and manufacturers were represented.  Probe microphone measurements were conducted on all subjects, but  those in the initial-fitting group did not receive adjustments based on the verification measures.

Perceived hearing aid benefit was measured using the Abbreviated Profile of Hearing Aid Benefit (APHAB, Cox & Alexander, 1995). The APHAB consists of 24 items in four subscales: ease of communication (EC), reverberation (RV), background noise (BN) and aversiveness of sounds (AV).  In addition to subscale scores, an average global score can be calculated, as well as a benefit score which represents the difference between unaided and aided responses.

Prior to being fitted with their hearing aids, participants completed the APHAB questionnaire. Because all were experienced hearing aid users, they were asked to base their answers on their experiences without amplification.  Hearing aid fittings and probe microphone verification were then conducted on all subjects, but half of the subjects received adjustments to match prescribed targets and half of the subjects maintained their first-fit settings. Efforts were made to ensure that subjects were not aware of the difference between the initial-fit and verified fitting methods. The only adjustments that subjects in the initial-fit group received were based on issues that could affect their willingness to wear the hearing aids, such as loudness discomfort or feedback.

One month following the first appointment, subjects returned to the clinic and were administered the APHAB again. They were given their initial “unaided” APHAB responses to use as a comparison. After completion of the APHAB, the subjects who had been fitted with the initial-fit algorithms were switched to verified fittings and those had been fitted to prescribed targets were switched to the manufacturer’s initial-fit settings. All subjects were re-tested with probe microphone measures and those with loudness or feedback complaints received minor adjustments.

One month after the second appointment, subjects returned to complete the APHAB and were again allowed to use their original APHAB responses as a basis for comparison. They were not allowed to view their responses to the APHAB that was administered after the first hearing aid trial. Participants were also asked to indicate which fitting method (Session 1 or Session 2) they preferred and would want permanently programmed into their hearing aids.

Analysis of the probe microphone measurements indicated, not surprisingly, that the verified fittings were more closely matched to prescriptive targets than the fittings based on the first-fit algorithms, even after minor adjustments based on comfort and user preferences.  For three of the APHAB subscales – ease of communication, reverberation and background noise – scores obtained with verified fittings were superior to those obtained with the initial-fit approach and the main effect of fitting approach was found to be statistically significant. There was no interaction between fitting approach and APHAB subscale, indicating that the better outcomes obtained with verified fittings were not related to any specific listening environment.

When asked to indicate their preferred fitting method, 7 of the 22 participants selected the initial-fit approach, whereas more than twice as many subjects, 15 out of 22, selected the verified fitting. For all but 5 subjects, the global difference score on the APHAB predicted their preferred fitting method, and the relationship between global score and final preference was statistically significant.

The findings of this study and of related reports bring up some philosophical and practical considerations for audiologists. One of our primary goals is to provide effective rehabilitation for hearing-impaired patients and this is most often accomplished by fitting and dispensing quality hearing instruments. Clinical and research data repeatedly indicates the importance of probe microphone verification. It serves the best interest of our patients to offer them the most effective fitting approach, so it follows that probe microphone verification measures should be a routine, essential part of our clinical protocol.

The reports that a minority of hearing aid fittings are being verified with real-ear measures indicates that many clinicians are not following recommended best practices. Indeed, Palmer (2009) points out that failure to follow best practice guidelines is a departure from the ethical standards of professional competence. Failure to provide the recommended objective verification for hearing aid fittings does run counter to our clinical goals and as Palmer suggests may even be damaging to our “collective reputation” as a profession.

Philosophical arguments notwithstanding, there are also practical reasons to incorporate real-ear measures into the fitting protocol. In the MarkeTrak VIII survey, Kochkin reported that hearing aid users who received probe microphone verification testing as part of a detailed fitting protocol were more satisfied with their hearing instruments and were more likely to refer their clinician to friends. In the current field of hearing aid service provision, it is important for audiologists to consider ways that they can meaningfully distinguish themselves from online, mail-order and big-box retail competitors. Hearing aid users are becoming well-informed consumers and it is clear that establishing a base of satisfied patients who feel they have received comprehensive, competent care is crucial for growing a private practice. Probe microphone verification is a brief yet effective part of ensuring successful hearing aid fittings and it benefits our patients and our profession to provide this essential service.

References

Abrams, H.B., Chisolm, T.H., McManus, M., & McArdle, R. (2012). Initial-fit approach versus verified prescription: Comparing self-perceived hearing aid benefit. Journal of the American Academy of Audiology, 23(10), 768-778.

American Academy of Audiology (2003). Pediatric Amplification Protocol. www.audiology.org, (accessed 3-3-13).

Bamford,  J., Beresford, D., Mencher, G.(2001). Provision and fitting of new technology hearing aids: implications from a survey of some “good practice services” in UK and USA. In: Seewald, R.C., Gravel, J.S., eds. A Sound Foundation Through Early Amplification: Proceedings of an International Conference. Stafa, Switzerland: Phonak AG, 213–219.

Bentler, R. (2004). Advanced hearing aid features: Do they work? Paper presented at the convention of the American Speech-Language-Hearing Association, Washington, D.C.

Bretz, K. (2006). A comparison of three hearing aid manufacturers’ recommended first fit to two generic prescriptive targets with the pediatric population. Independent Studies and Capstones, Paper 189. Program in Audiology and Communication Sciences, Washington University School of Medicine. http://digitalcommons.wustl.edu/pacs_capstones/189.

Cox, R. & Alexander, G. (1995). The abbreviated profile of hearing aid benefit. Ear and Hearing 16, 176-183.

Dillon, H. & Ginis, J. (1997). Client Oriented Scale of Improvement (COSI) and its relationship to several other measures of benefit and satisfaction provided by hearing aids. Journal of the American Academy of Audiology 8: 27-43.

Hawkins, D. & Cook, J. (2003). Hearing aid software predictive gain values: How accurate are they? Hearing Journal 56, 26-34.

Kochkin, S., Beck, D., & Christensen, L. (2010). MarkeTrak VIII: The impact of the hearing health care professional on hearing aid user success. Hearing Review 17, 12-34.

Mueller, H., & Picou, E. (2010). Survey examines popularity of real-ear probe-microphone measures. Hearing Journal 63, 27-32.

Nerbonne, M., Christman, W. & Fleschner, C. (1995). Comparing objective and subjective measures of hearing aid benefit. Poster presentation at the annual convention of the American Academy of Audiology, Dallas, TX.

Palmer, C.V. (2006). Best practice: it’s a matter of ethics. Audiology Today, Sept-Oct.,31-35.

Thornton, A. & Raffin, M. (1978) Speech-discrimination scores modeled as a binomial variable. Journal of Speech and Hearing Research 21, 507–518.

Valente, M., Abrams, H., Benson, D., Chisolm, T., Citron, D., Hampton, D., Loavenbruck, A., Ricketts, T., Solodar, H. &  Sweetow, R. (2006). Guidelines for the Audiological Management of Adult Hearing Impairment. Audiology Today, Vol 18.

Weinstein, B., Newman, C. & Montano, J. (1995). A multidimensional analysis of hearing aid benefit. Paper presented at the 1st Biennial Hearing Aid Research & Development Conference, Bethesda, MD.

One thought on “Evidence for the Value of Real-Ear Measurement

  1. Thank you very much for this valuable post. Sometime performing a probe microphone measurement for paediatric patient is challenging but it is worthy to try it.

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