The outcomes of hearing aid fitting can be assessed in terms of the technical merit of the device in situ or in terms of the extent to which the device alleviates the daily problems of the hearing-impaired person and his or her family. Early efforts to measure outcomes focused mainly on technical merit. It was assumed that if the instrument was technically superior, real-life outcomes would be proportionately superior. However, this is not always the case. Real-life problems associated with hearing impairment are complicated by issues such as personality, lifestyle, environment, and family dynamics. Thus, it is now recognized that real-life outcomes of a fitting must be assessed separately from the technical adequacy of the hearing aid. These two types of outcomes are evaluated at different times after fitting. Technical merit data are often obtained as part of the verification process conducted immediately after fitting. Sometimes these data may prompt modifications of the fitting. Alleviation of real-life problems is evaluated after the hearing-impaired person has had time to use the hearing aid on a daily basis. This evaluation is usually made after at least 2 weeks of use of the device.
The technical merit of a fitted hearing aid may be reflected in both acoustical and psychoacoustical data. Acoustical outcomes include real ear probe microphone measures (such as insertion gain, aided response, and saturation response) and audibility measures (such as articulation index and speech intelligibility index). Psychoacoustical outcomes include speech recognition scores, aided loudness assessment, and ratings of quality, clarity, pleasantness, or other dimensions.
Real ear probe microphone measures provide information about the sound delivered to the eardrum of the particular patient (e.g., Mueller, Hawkins, and Northern, 1992). This takes into consideration the physical differences among patients and the differences between real ears and standard couplers such as the 2 cm3 couplers used in the ANSI measurement standard (American National Standards Institute [ANSI], 1996). One advantage of these measures is their ability to confirm the extent to which the fitting is congruent with prescriptive fitting target values. Real ear probe microphone data are also valuable for troubleshooting fitting problems.
Audibility measures usually combine information about the availability of amplified acoustical speech cues with weighting factors proportional to the importance of those cues for speech recognition. The availability of cues may be limited by sensitivity thresholds or competing noises, as well as by the speech level. The importance of cues for speech intelligibility varies with frequency and type of speech. Studies of normal-hearing listeners and listeners with mild to moderate hearing impairments have shown that measures of weighted audibility can provide rather accurate predictions of speech intelligibility scores for these individuals in a laboratory setting. There are several well-researched approaches to obtaining audibility measures (e.g., Studebaker, 1992; ANSI, 1997). Some methods incorporate the effects of age, speech level, or hearing loss to improve the accuracy of speech intelligibility predictions for elderly hearing-impaired listeners who are exposed to high-level amplified speech.
Measuring the recognition of amplified speech is perhaps the most venerable approach to evaluating hearing aid fitting outcomes. Many standardized tests are available, in both open-set and closed-set varieties, with stimuli ranging from nonsense syllables to sentences. The popularity of speech intelligibility testing as a measure of outcome is rooted in its high level of face validity. The most frequently cited reason for obtaining amplification is a need to improve communication ability. A measure of improved speech understanding consequent on hearing aid fitting addresses that need in an attractively direct manner. For many years, this measure was the bedrock of hearing aid fitting outcome evaluation. Unfortunately, in order to achieve a useful level of statistical power, speech intelligibility tests must include a large number of test items. This requirement has limited the recent use of speech intelligibility testing mostly to research applications (e.g., Gatehouse, 1998).
The importance of producing amplified sounds that are acceptably loud has long been recognized. Although using loudness data to facilitate fitting protocols has been advocated for many years, interest in measuring the loudness of amplified sounds following the fitting has burgeoned since the widespread acceptance of wide dynamic range compression devices. With these instruments, it is appealing to assess the extent to which environmental sounds, including speech, have been "normalized" by amplification. Interest in this type of outcome data is increasing, and some of the measurement issues have been addressed (e.g., Cox and Gray, 2001).
Formal ratings of aspects of amplified sounds, such as quality, clarity, distortion, and the like, have been used frequently in assessing technical merit in research applications but have not often been advocated for clinical use. These types of measures are commonly performed with listeners supplying a rating on a semantic differential scale, such as the 11-point version developed by Gabrielsson and Sjogren (1979). This approach has the advantage of permitting quantification of dimensions of amplified sounds that are not psychoacoustically accessible via other metrics.
Hearing Aid Fitting: Evaluation of Outcomes 481
The real-life effectiveness of a hearing aid is measured using subjective data provided by the hearing-impaired person or significant others. Numerous questionnaires have been developed and standardized specifically for the purpose of assessing hearing aid fitting outcomes, and many others have been conscripted to serve this application (Noble, 1998; Bentler and Kramer, 2000). In addition to standardized questionnaires, there is strong support for use of personalized instruments in which the patient identifies the items and thus creates a customized questionnaire (e.g., Dillon, James, and Ginis, 1997). Regardless of which type of inventory is used, there are at least seven different domains of subjective outcomes of hearing aid fitting that can be assessed. They include residual activity limitations, residual participation restrictions, impact on others, use, benefit, satisfaction, and quality-of-life changes. Most inventories do not assess all of these domains.
Residual activity limitations are the difficulties the hearing aid wearer continues to have in everyday tasks such as understanding speech, localizing sounds, and the like. The activity limitations experienced by a specific individual will depend on the demands of that person's lifestyle. Residual participation restrictions are the unresolved problems or barriers the hearing aid wearer encounters to involvement in situations of daily life. This also differs with individuals but can include such circumstances as participation in church services, bridge clubs, and so on. ICF (2001) provides a full discussion of activity limitations and participation restrictions.
Hearing impairment often places a heavy burden on the family and friends as well as on the involved individual. In fact, encouragement (or compulsion) by significant others is sometimes the major factor prompting an individual to seek a hearing aid. The relief provided by amplification for the problems in the family constellation (i.e., the impact on others) is an important outcome dimension but one that has received relatively little attention to date.
A measure of benefit quantifies change in a hearing-related dimension of functioning as a result of using amplification. Benefit may be measured directly, in terms of degree of change (small versus large), or it may be computed by comparing aided and unaided performance on a particular dimension. Typical dimensions on which subjective benefit is measured are activity limitations and participation restrictions. Hearing-specific questionnaires are typically used to quantify hearing aid benefit.
Sometimes general, non-hearing-specific questionnaires are used to determine changes that result from hearing aid provision. These types of data tend to be interpreted as reflecting changes in general quality of life. A recent large-scale study found that hearing aid use was significantly associated with improvements in many aspects of life, including social life and mental health (Kochkin and Rogin, 2000). Despite the importance of these effects for individuals, functional health status measures that are often used to gauge quality of life tend not to be sensitive to the changes that result from hearing aid use (Bess, 2000).
It is not unusual to observe that an individual who reports substantial hearing aid benefit is nevertheless not satisfied with the device or does not use amplification very often. These observations suggest that daily use and hearing aid satisfaction are additional, distinct dimensions of real-world outcome that require separate assessment (e.g., Cox and Alexander, 1999).
Relationship Between Technical Merit and Real-World Impact
Numerous studies have shown that measures of technical merit are not strongly predictive of real-world outcomes of hearing aid fitting (e.g., Souza et al., 2000; Walden et al., 2000). Principal components analyses in studies using multiple outcome measures often show that the two types of measures tend to occupy separate factors (e.g., Humes, 1999). Many researchers feel that both types of data are essential for a full description of hearing aid fitting outcome.
See also Hearing aids: prescriptive fitting.
—Robyn M. Cox References
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