Introduction:
A hearing aid treatment program may follow five logical processes, namely: evaluation, selection, fitting, verification and validation. Evaluation refers to information needed for the selection of an appropriate device and/or counseling. Selection refers to identifying the most appropriate device based on information obtained during the evaluation process. Fitting reflects the process of orienting the patient to the selected device. Verification refers to objective assessments of the features chosen in the selection process. Validation may include the assessment of the patient's perceived benefit from the device and professional services. Although there were over 150 articles related to hearing aids published in 2002, we reviewed articles that address some part of the total hearing aid treatment program and have immediate clinical relevance.
Evaluation of Site of Lesion:
Audiologists evaluate or measure some function of the auditory system before fitting any type of amplification. Typically a measure of pure tone sensitivity across frequencies serves as the minimum pre-fitting evaluation. Pure tone audiometric assessments provide a small portion of the total information needed for the evaluation process. In the past, approaches to pre-fitting evaluation of site of lesion were concerned with differentiating conductive from sensorineural hearing loss. The complexity of a hearing aid evaluation is further increased by the need to differentiate different types of sensory (cochlear) loss.
The two dominant types of cochlear hearing loss are sensory transmission and sensory transduction loss (Kemp, 2002). Sensory transmission loss refers to damage of the basilar membrane (B), outer hair cells (O) and the tectorial membrane (T) complex or the "BOT" complex (Cheatham & Dallos, 2000). Here the function of the cochlear amplifier is impaired leading to less than a 60 dB hearing loss. Conventional amplification using wide dynamic range compression (WDRC) addresses this type of cochlear loss (Berlin, Hood, Hurley & Wen, 1996). Sensory transduction loss refers to dysfunction of the inner hair cells (IHC). Here IHC forward transduction of mechanical outputs from the BOT complex is impaired. Speech discrimination (particularly in noise) even when audible may be abnormal (Killion, 1996). Therefore, the success of amplification may depend on adequate evaluation of the degree of transmissive vs. transductive loss or both.
Articles On Evaluation of Site of Lesion:
In 2002 the issue of sensory transduction loss and its evaluation was addressed. This type of loss leads to poor discrimination of amplified speech and reduced signal-to-noise ratio, both of which influence the success of hearing aid fittings. Killion (2002) proposed that equation 1 (see below) is sufficient in determining a perceptive articulation index (pAI) that is believed to indirectly represent reserved IHC transduction:
pAI = (AI)(CC)
where p means "perceived" and AI is the usual "count the dot" AI percentage score based on stimulus detection or audibility, while CC is "channel capacity" and represents the percentage of speech cues that could be discriminated. Previously Shannon, Galvin and Baskent (2001) applied the AI to normal hearing and cochlear implant users and concluded that AI was a good measure of missing or "dead" IHC, which they described as "holes in hearing." Rankovic (2002) analyzed data from a previous study that examined IHC loss with Flecher's articulation index and found that Flecher's AI calculations corresponded with consonant recognition scores. Therefore, Rankovic questioned the need for additional special calculation or testing of IHC damage or dead regions, since such information is already determined indirectly from the AI.
Moore (2002) replied to Rankovic's article by noting that AI is erroneous in determining benefit of amplification above the edge frequency (Fe) of a dead region. Moore points out that amplifying frequencies up to 1.7Fe provides the listener with benefit and AI predictions may suggest overamplification for frequencies>Fe. Baer, Moore and Kluk (2002) examined speech intelligibility among 10 subjects with high frequency hearing loss, five with and without dead regions. The dead regions were identified with Moore's Threshold Equalizing Test (TEN). These authors showed that performance increased up to 1.5-2 Fe but showed no benefit beyond this point.
Killion (2002) suggests that the calculation of a pAI is needed to measure IHC loss, Moore (2002) introduced a new test in order to assess IHC loss (dead regions), while Rankovic (2002) suggested that the usual Fletcher AI without additional calculations is appropriate to estimate benefit from amplification. This discrepancy leads to confusion and engenders concern about the validity of AI in determining consequences of IHC loss.
Although this area of study is interesting and important to the understanding of the auditory system, the day-to-day evaluation or application of dead regions is questionable given Baer, Moore and Kluk's (2002) findings. Considering the typical hearing loss an audiologist sees, dead regions are probably at 2000 Hz or higher. If individuals receive benefit at two times the edge frequency, then this means amplification is effective out to 4000 Hz which practically is the useful range of many current hearing aids.
Selection:
In 2002 a number of good articles on selection of amplification were published. We have chosen to address a few that revealed interesting findings.
Digital Delay:
Directional microphones continue to be an important feature in the selection process and key articles are addressed below. Hearing aid signal processing of acoustic inputs from the microphone leads to a delay in the transduction of that input to the receiver. Theoretically, this delay may lead to perceptual consequences to the listener. In 2002 the issue of digital delay was addressed but additional research is needed before it becomes an issue of concern in the selection process.
Articles on Directional Microphones:
Cord, Surr, Walden and Olson (2002) reported on the performance of directional microphone use in everyday listening situations. These authors were interested in the frequency with which wearers used directionality, ability to use it appropriately, and the frequency of encountering noisy situations. Hearing aid wearers with switchable (omni to directional) technology were surveyed. Out of 112 subjects contacted, 48 individuals indicated they switched between microphones in everyday use. These individuals completed the abbreviated profile of hearing aid benefit (APHAB, Cox & Alexander, 1995) and a microphone performance questionnaire. The results showed that respondents used directionality ¼ of the time and reported being mostly in situations requiring omni- directional microphones. Respondents also reported having fewer communication problems with directional microphones and knew in which situations to use directionality. This study demonstrates that it is important that users perceive a need for directionality based on the environments they encounter and that they are instructed on use. One of the most interesting findings in this study was that over half the people originally contacted no longer switched between the two microphone settings and therefore, were not included in the study. Since no objective pre or post test measures were conducted one cannot be sure if all of these instruments were actually producing a directional response.
Surr, Walden, Cord and Olson (2002) assessed the influence of environmental factors on omni vs. directional microphone preference. Hearing instruments with switchable modes (omni vs. adaptive directional) were fitted to experienced hearing aid users who used a journal checklist to describe situations where one mode was better than the other. These subjects reported difficulty perceiving a difference in benefit between the two modes. A subject's decision on which microphone to use was based on: position of talker, presence and type of noise and reverberation. Interestingly, although the presence of noise was significant, location of the noise was not. This may speak to the adaptive nature of the directional microphone tested in this study. Speech testing in the lab at the end of the study indicated that the microphones remained directional.
In light of the results from both studies, it is important to verify that the instruments are truly directional. For example, defects in manufacturing may lead to delivery of a nonfunctioning directional instrument, which leads to difficulty perceiving a difference in microphone performance. Dirt and foreign debris resulting from everyday use may limit the performance of directional microphones. Therefore, experiments as well as clinical practices should verify the functionality of directionality post-fitting. The success of directional microphones is dependent on environmental conditions, such as reverberation, distance from the talker and degree of separation between the signal and competing noise, and patient education regarding appropriate use.
Article on Temporal Delay:
Stone and Moore (2002) obtained objective and subjective measures of the effect of hearing aid induced delay on speech production and perception among normal hearing subjects. Using different signal processing schemes, these investigators created delays of 7, 18, 30 and 43 msec. The results revealed that word production rates decreased as delay increased from 7 to 43 msec. Additionally, the fundamental frequency of the subject's voice decreased as delay increased and the effects of delay were independent of signal processing. However, delays up to 30 msec could be used before speech production was disrupted among normal hearing subjects. Commercially available hearing instruments typically have delays from 1 to 10 msec. Based on the results of Stone and Moore, current delays are unlikely to significantly affect speech perception or production. However, it is still unclear how delays affect patients with sensory transmission, transduction and/or neural hearing loss, since the subjects in this study all had normal hearing. Also unclear is the effect of delay as a function of earmold style and venting, since all subjects were fitted with similar unvented earmolds.
Articles on Verification:
Scollie and Seewald (2002) examined the difference between the results of electroacoustic testing with aided test signals and aided speech. Three test signals (Fonix pure tone, Fonix composite noise and Audioscan swept) were compared to running speech. Various types of hearing aids were used, for example, analog vs. digital, linear vs. nonlinear, WDRC of different release times, single vs. multichannel, and systems with and without temporal modulation detection (TMD). Forty-one hearing aids configured to desired sensation level (DSL, Seewald, 1992) for moderate, severe, and profound hearing loss were used in this study. Results showed that speech weighted or temporally modulated test signals more accurately matched aided levels of speech for all types of hearing aids. Therefore, pure tone test signals and hearing aids with TMD yielded the worst match to aided speech. Scollie and Steinberg (2002) examined three possible correction calculations for the work of Scollie and Seewald. Their correction calculations improved the match between aided test signals and aided speech output. TMD continued to be a challenge. The authors recommended that TMD should be turned off during electroacoustic verification.
Bagatto, Scollie, Seewald, Moodie and Hoover (2002) investigated real-ear-to-coupler difference (RECD) predictions as a function of age for two coupling procedures. Using custom earmolds and immittance probe tips, these investigators developed predictive RECD values on 392 children and infants, ranging in age from 1-16 yrs. Significant between-subject-variability confounded their objective. As a result these investigators favored individual measurement of RECD over predictive estimates. Their predictive estimates were found to be more accurate than available normative RECD values and should be used when individual measurements are not available.
Stelmachowicz, Pittman, Hoover and Lewis (2002) assessed the accuracy with which children wearing hearing aids could detect the morphemes /s/ and /z/. These inflectional morphemes have high linguistic importance in English, are the 3rd to 4th most frequently occurring phoneme, and indicate plurality, tense, and possessiveness. Normative data were first developed on 36 normal hearing youngsters and then 40 bilateral sensorineural hearing-impaired children served as the experimental group. The investigators found poorer performance for the female talker, poorer performance for plural words, and the poorest performance for plural items spoken by the female talker. For the male talker, subjects needed a hearing aid response from 2000 to 4000 Hz whereas they needed a frequency response of 2000 to 8000 Hz for the female talker. The results of this investigation should encourage clinicians to choose bandwidth carefully for their young patients and to verify these choices with probe microphone measures to insure the frequency response is achieved.
Articles on Validation:
Kochkin (2002) found that overall satisfaction for hearing aid users increased if any type of post-fitting survey was used. Cox, Stephens and Kramer (2002) reported on the International Outcome Inventory for Hearing Aids (IOI-HA), a self-report outcome measure for audiologic treatment. This inventory allows the reporting of self-perceived hearing aid benefit on a unified template across 20 different languages, that allows easy comparison of data across researcher groups. Noble (2002) presented an alternative use of the IOI-HA. He suggested that in addition to being a self-report the questionnaire could be used to assess the difficulty of significant others and use of listening aids other than hearing aids. An example of the psychometric data of the IOI-HA comes from Kramer, Goverts, Dreschler, Boymans and Festen (2002). These authors reported results from the Netherlands where the IOI-HA was used as a post-fitting tool in a national study. High internal consistency was found with the IOI-HA.
Humes, Wilson, Barlow and Garner (2002) evaluated changes in hearing aid benefit after one or two years of hearing aid use among older adults. Subjects were fitted binaurally with linear class-D amplifiers with output-limiting compression. Validation scales included the Hearing Aid Performance Inventory and the Hearing Handicap Inventory for the Elderly. The results showed that less benefit was perceived by the subjects at six months and one year after fitting compared to 1 month after fitting. At 2 years after fitting, participants perceived only minimal benefit. This study showed that perceived benefit at one month post fitting may predict perceived benefit for up to two years. For example if at one month a patient perceived high benefit then this benefit will remain high in comparison to a person who had an initial low perceived benefit at one month post fitting. However, perceived benefit decreases over time regardless of whether the initial perception was high or low.
Abrams, Chisolm and McArdle (2002) conducted a cost-utility analysis of adult hearing aid fitting only vs. hearing aid fitting along with educational sessions. Veterans with sensorineural hearing loss were enrolled in this study. The analysis showed that hearing aid fitting along with informational sessions was a more cost-effective treatment than hearing aid fitting alone.
Recommendations for Clinicians from Articles of 2002:
(What to do Monday morning...)
Abrams, H., Chisolm, T. H. & McArdle, R. (2002). A cost-utility analysis of adult group audiologic rehabilitation: Are the benefits worth the cost? Journal of Rehabilitation Research and Development 39, 549-558.
Baer, T., Moore, B. C. J. & Kluk, K. (2002). Effects of low pass filtering on the intelligibility of speech in noise for people with and without dead regions at high frequencies. Journal of the Acoustical Society of America 112, 1133-1144.
Bagatto, M. P., Scollie, S. D., Seewald, R. C., Moodie, K. S. & Hoover, B. M. (2002). Journal of the American Academy of Audiology 13, 407-415.
Berlin, C. I., Hood, L. J., Hurley, A. & Wen, H. (1996). Hearing aids: Only for hearing impaired patients with abnormal otoacoustic emissions. In C. I. Berlin (Ed.), Hair Cells and Hearing Aid (pp. 99-111). San Diego: Singular Press.
Cord, M., Surr, R., Walden, B. & Olson, L. (2002). Performance of directional microphone hearing aids in everyday life. Journal of the American Academy of Audiology 13, 295-307.
Cox, R. & Alexander, G. (1995). The abbreviated profile of hearing aid benefit. Ear and hearing 16, 176-183.
Cox, R., Stephens, D., Kramer, S. (2002) Translation of the international outcome inventory for hearing aids. International Journal of Audiology 41, 3-26.
Cheatham, M. A. & Dallos, P. (2000). The dynamic range of inner hair cell and organ of Corti responses. Journal of the Acoustical Society of America 107, 1508-1520.
Humes, L. E., Wilson, D. L, Barlow, N. N. & Garner, C. (2002) Changes in hearing aid benefit following 1 or 2 years of hearing aid use by older adults. JSLHR 45, 772-782
Kochkin, S. (2002). Factors impacting consumer choice of dispenser & hearing aid brand; Use of ALDs & computers. Hearing Review 9 (12), 14-23.
Kemp, D. T. (2002). Otoacoustic emissions, their origin in cochlear function, and use. British Medical Bulletin 63, 223-241.
Killion, M. C. (2002). New thinking on hearing in noise: A generalized articulation index. Seminars in Hearing 23, 57-75.
Killion, M. C. (1996). Talking hair cells: What they have to say about hearing aids. In C. I. Berlin (Ed.), Hair Cells and Hearing Aid (pp. 125-172). San Diego: Singular Press.
Kramer, S., Goverts, T., Dreschler, W., Boymans, M., Festen, J. (2002). International Outcome Inventory for Hearing Aids (IOI-HA): results from the Netherlands. International Journal of Audiology, 41(1), 36-41.
Moore, B. C. J. (2002). Response to "articulation index predictions for hearing impaired listeners with and without cochlear dead regions." Journal of the Acoustical Society of America 111, 2549-671.
Munro, K. J., Salisbury, V. A. (2002). Is the real-ear to coupler difference independent of the measurement earphone? International Journal of Audiology, 41(7), 408-413.
Noble, W. (2002). Extending the IOI to significant others and to non-hearing-aid-based interventions. International Journal of Audiology, 41(1), 27-29.
Rankovic, C. M. (2002). Articulation index predictions for hearing impaired listeners with and without cochlear dead regions. Journal of the Acoustical Society of America 111, 2545-2550.
Ricketts, T. & Henry, P. (2002). Evaluation of an adaptive directional microphone hearing aid. International Journal of Audiology 41, 100-112.
Santarelli, R. & Arslan, E. (2002). Electrocochleography in auditory neuropathy. Hearing Research 170, 32-47.
Seewald, R. C. (1992). The desired sensation level method for fitting children: Version 3.0. Hearing Journal 45 (4), 36-41.
Scollie, S. D. & Seewald, R. C. (2002). Evaluation of electroacoustic test signals I: Comparison with amplified speech. Ear and Hearing 23, 477-487.
Scollie, S. D., Steinberg, M. J. & Seewald, R. C. (2002). Evaluation of electroacoustic test signals II: Development and cross-validation of correction factors. Ear and Hearing 23, 488-498.
Shannon, R. V., Galvin, J. J. & Baskent, D. (2001). Holes in hearing. JARO 3, 185-199.
Stelmachowicz, P.G., Pittman, A.L., Hoover, B.M., & Lewis, D.E. (2002). Aided perception of /s/ and /z/ by hearing-impaired children. Ear and Hearing, 23, 316-324.
Stone, M. A. & Moore, B. C. J. (2002). Tolerable Hearing aid delays. II. Estimation of limits imposed during speech production. Ear and Hearing 23, 325-338.
Surr, R., Walden, B., Cord, M. & Olson, L. (2002). Influence of environmental factors on hearing aid microphone preference. Journal of the American Academy of Audiology 13, 208-322.
