People with a severe or profound hearing loss comprise approximately 11%-13.5% of the hearing-impaired population.1,2 Studies have shown that this population presents numerous challenges to the hearing care professional relative to reduced dynamic range, large variability in speech perception scores, increased distortion, lower audibility of signal, and poor speech perception.3 Fortunately, adherence to fundamental audiological principles when fitting this patient population with high power instruments can achieve good results.3,4
Recent studies and continuing dialog in the field have shed light on the challenges in the fitting of advanced hearing instruments on people who have a severe or profound hearing loss. Four issues stand out in particular:
- Various studies suggest that dispensing professionals should focus on the areas of residual hearing rather than trying to amplify the so called dead zones.
- As universal newborn hearing screening (UNHS) becomes more widespread, the issue of providing a solution for these young infants becomes a more pressing concern.
- We are restricted in methods to measure the outcomes of technologies provided to young children. This is particularly problematic when assisting the lower frequency region of the cochlea.
- Recent studies have highlighted the benefit of providing a hearing aid in the contralateral ear to the cochlear implant.
This paper addresses these issues.
High Frequencies and Adults with Severe or Profound Losses
Fitting hearing aids to people with a severe or profound hearing impairment poses challenges to the dispensing professional. Three main issues arise: gain, output, and frequency response.
Byrne et al.5 found that an extra 10 dB of gain is required for people with a severe or profound hearing loss. In fact, the half-gain rule for sensorineural hearing loss does not appear to apply once the hearing loss becomes greater than 70 dBHL. For severe hearing losses, the required gain to achieve the most comfortable level (MCL) for the listener rises at greater than the half-gain rule.5 Therefore, when fitting people with power and super power instruments, the fitting rationales used should consider the amount of gain required for this population.
Conversely, it should also be remembered that people with severe or profound hearing impairments often have a restricted dynamic range and an extreme loudness growth function. In these cases, the dispensing professional needs to be careful not to over-amplify the individual and/or exceed loudness discomfort levels.
A longstanding maxim of hearing aid fitting is to ensure audibility across the entire speech frequency range.6 While this is excellent advice for individuals with a mild-to-moderate loss, it may not hold true for people with a severe or profound hearing impairment. Unfortunately, many fitting rationales are based on the assumption that useful hearing can be provided across the entire frequency range. The logic used is that speech recognition will be optimized when audibility is maximized. For people with milder losses and typical, gradually sloping hearing loss configurations, we would intuitively expect the best results to occur when the patients are provided with more amplification in the high frequency regionwhere they need it most.
Figure 1. Audiogram highlighting that, for a person with confirmed cochlear dead zones, the best amplification solution may be to increase the low frequency gain while decreasing the high frequency gain.
However, the data of Byrne et al.5 suggest a different strategy for those who have more severe losses. For hearing losses that are greater than 95 dBHTL at 2000 Hz, decreasing the slope of the frequency response should provide more amplification in the low frequencies (Figure 1). Ching et al.7 built on this research and found that people with extreme hearing losses at high frequencies preferred amplification that results in zero or low audibility for the high frequency components of speech. When the audibility in the high frequencies are increased, speech recognition actually decreaseseven though there is an increase in audibility at frequencies where the hearing loss is severe.8
It has been known for decades that there is more to speech recognition than audibility and the ability to listen at high sensation levels.9 More recent research confirms that hearing care professionals need to consider the impact of distortion and lack of surviving hair cells. Regions of few or no surviving inner hairs cells have been labeled dead regions.10,11
Despite this, dispensing professionals should not totally remove the high frequency sounds when hearing loss is greater than 80 dBHL, but should be aware and cautious of the effect of providing audibility in high frequency regions. The best solution may be to reduce the high frequency gain and increase the low frequency gain and/or focus on providing amplification in the areas where there is greater cochlear integrity (eg, for steeply sloping and ski-slope losses, amplification may be applied to the transition area or the area of the greatest slope in the audiogram).
When providing amplification to people with a severe or profound hearing impairment, remember that these people can require more amplification than may be predicted by some fitting strategiesespecially in the low frequencies. Caution also should be observed regarding amplification across the entire frequency range, as applying gain in areas of little residual hearing (ie, the cochlear dead regions) may not improve intelligibility of speech, and may actually lead to worse performance and concomitant discomfort.
Obviously, the difficulty is finding a procedure that provides more audibility in some frequencies, and less audibility in others, while keeping loudness at a comfortable level. Traditionally, limitations in hearing aid designs and capabilities have, at times, prevented us from reaching desired target gain levelsparticularly in the area of 4000 Hz. As the signal processing capability and flexibility of hearing aids have advanced, these issues are increasingly being addressed more successfully in clinical practice.
Fitting Infants Who Have Severe/Profound Losses
Through Auditory Steady State Response (ASSR) and other technologies, profound hearing loss can now be identified and confirmed before 6 months of age. Infants identified with a severe or profound hearing loss need a small, robust, and durable hearing aid that can provide the necessary gain to ensure that the child has the best hearing possible during the critical period for speech and language acquisition.
It is crucial when fitting hearing instruments to young children that the real ear to coupler difference (RECD) is measured to overcome differences between the childs ear and the adults ear. This is especially important in neonates for two good reasons:
- The hearing aid set to adult insertion gain targets could be up to 20 dB louder in the infants ear than the desired response;
- The implications of over-amplification and potential damage to the cochlear hair cells is even more of a concern for infants with profound hearing impairment.
This presents a challenge for the hearing care professional, as it is not easy to obtain hearing thresholds using insert earphones and childrens earmolds in neonates. In these cases average values must be used.
While evidence12-14 suggests that providing a cochlear implant as early as possible will lead to the best possible outcomes, it is also quite possible that advances in hearing aid technology could reduce the urgency of fitting a cochlear implant. For these children, there is a time period between diagnosis and surgery when the child is participating in a hearing aid trial, undergoing candidacy evaluations, and the team is being cautious due to the increased risk of general anesthetic in babies less than 12 months old. In this period, it is critical that the infant is fit with the best possible hearing instrument and a fitting method that maximizes amplification where the infant has the most useable hearing.
Figure 2. Mean hearing levels of the aided ear of children with a cochlear implant in the contralateral ear.18
For potential cochlear implant candidates, this will be in the low frequency region of the cochlea. The fitting of advanced hearing instruments to babies who are cochlear implant candidates will remove much of the sense of urgency from the procedure, as the child will have greater access to the speech information than with pervious devices that were more limited in output and frequency response.
With the wide-scale introduction of UNHS, more infants with severe and profound hearing loss are being detected at an earlier age. These children often require a powerful hearing instrument to maximize their speech and language development while they are being assessed for cochlear implant candidacy. For these children with restricted audibility, increased low frequency gain and MPO provides the most access to crucial speech information.
New Technology and Measuring Speech/Language Acquisition
People with a severe or profound hearing losses frequently perform well with amplification, even though they may receive restricted speech cues.3 In fact, models of audibility often underestimate the amount of information that a person using a good hearing instrument receives through auditory cues alone.7 Additionally, people with restricted audibility, particularly in the high frequency regions, often place more reliance on different types of auditory cues. They frequently exhibit a greater reliance on amplitude and low frequency speech cues, and less reliance on frequency specific cues (especially in the high frequency regions).
Inattention to dynamic range issues can lead to rejection of the device, and over-amplification can lead to damage of the auditory structures. Conversely, being overcautious in the fitting can lead to under-amplification. In children, not gaining access to important speech features can result in speech and language acquisition problems.
Similar to hearing aid fitting, the focus in the literature on aural habilitation of children has been on improving access to crucial speech sounds. Teachers of the deaf, as well as auditory verbal therapists, argue passionately for audiologists to boost the higher frequencies so that children can access the /s/ sound. Without question, high frequencies provide important information to the listener (eg, consonant production and important information regarding fricatives). Unfortunately, as discussed above, some people with severe or profound hearing loss may receive little or no benefit from amplification in the high frequency region.
The reality of the situation is that we need to re-address and bolster our knowledge of speech acoustics to ensure that children receive meaningful auditory speech and language information so they have an opportunity to develop these skills during the critical period. Our attention needs to be focused on what benefits (and possible detriments) this population receives from restricting amplification in high frequencies.
People with severe or profound hearing loss receive suprasegmental (duration, frequency, and intensity) and segmental (vowels and consonants) information. With appropriate amplification up to and including 2000 Hz, this population receives the benefit of discriminating most vowel formants, voicing of consonants, transitions between consonants and vowels, and some manner of consonants. Studies have demonstrated that these people often perceive more speech in auditory-alone conditions than would be predicted from models of speech transmission.3,7 Many times, this is because they are relying on different cuesespecially lower frequency cues such as voice onset time, amplitude differences, and vowel formant transitions for consonant identification.
The goal should be to maximize the perception of speech rather than to maximize audibility. Following a model based strictly on audibility from the speech transmission perspective may actually lead to a less accurate perception of speech for people with severe hearing loss. Of course, this is fine at a theoretical level. At a practical level, we do not currently have the technology or tools to evaluate whether a reduction of high frequency gain for these children will be a benefit or a mistake. Long-term studies of speech acquisition and performance take time.
In infants, an increase in low frequency amplification and access to integral speech information can be assessed by various methods that analyze the infant/childs speech production and perception. Simple paradigms can be established that allow comparison of different hearing aid fittings (eg, hearing aid versus no hearing aid) over a short period of time (eg, months rather than years) to shed light on whether a child is experiencing benefit from high frequency amplification or if high frequency amplification should be reduced.
Studies have investigated aspects of speech production in children, including the amount of jaw opening, F1, F2, nasal airflow, voice onset time, and voicing duration.16 Others have assessed phonological patterns including analysis of consonants by place, manner, and voicing and vowels by place and height, tense, lax, open, and closed.17,18 It is crucial that research continues to develop improved measures of hearing aid benefit in infants and children who are fitted with high power instruments so that we can maximize the perception of speech in young children.
Providing a Hearing Aid to the Contralateral Ear in CI wearers
Cochlear implants are recommended when there is no useable hearing bilaterally. Typically, this entails a lack of speech information received at and above 2000 Hz following the hearing aid trial. As implant candidacy criteria have widened to encompass patients with less severe hearing loss, there have become more cochlear implant users who have sufficient useable hearing in their contralateral ear.
Recent studies have shown the utility of providing a hearing aid in the contralateral ear for adults19,20 and children7 who use cochlear implants. Collectively, these studies provide increasing support for contralateral fitting of hearing aids (bimodal amplification) to be the standard of care for adults and children with cochlear implants. As with bilateral hearing aid fittings, the fitting of a contralateral hearing aid provides benefits in terms of speech perception, localization, and speech intelligibility over the hearing aid or cochlear implant alone.22-24
Importantly, the hearing care professional should remember that the performance with the hearing aid alone will be pooror even non-existent. For example, in the Ching et al21 study, the children had open-set sentence scores below 15% in the hearing aid alone condition. It is when the cochlear implant and the hearing aid are combined that the maximum benefit is obtained. Bimodal/bilateral stimulation also provides the benefit of preventing the deterioration of speech recognition in that ear with no stimulation.
An important question is, How should the audiologist fit the contralateral hearing aid, especially when performance in the aided alone condition is difficult to observe? While current hearing aid fitting rationales for severe or profound losses (eg, NAL-RP & ASA2sp) do provide a good match with the cochlear implant in terms of frequency response, Ching et al.21 found that, on average, the 16 children in their study required an extra 6 dB of overall gain to match the loudness of the cochlear implant. In fact, 8 of the 16 children required 6-16 dB more overall gain. It was unclear from the study how the increase in gain was related to the overall hearing loss. For example, did the children with less useable high frequency thresholds require more gain than the theoretical rationale or vice versa? An answer to this question and more specific guidelines to the fitting of contralateral hearing aids will emerge as the weight of research evidence increases.
What we do know is that performance is optimized when the hearing aid and cochlear implant are adjusted and balanced in terms of loudness and bimodal/binaural function. As children with a cochlear implant have minimal (if any) hearing in the contralateral ear relative to high frequencies, hearing aid selection should focus on the performance of the aid for low frequency sounds in particular, because this is where the most useable hearing resides.
Additionally, extra information may be provided by acoustical hearing in the low frequencies due to the position of the electrode array within the cochlea. Electrodes are located in the apicalor high frequency portionof the cochlea while coding for the full frequency range; it is possible that further focusing on the low frequencies with the hearing aid may provide additional speech information, as well as provide greater access to musical cues. It is for these reasons the selected hearing aid should have sufficient headroom and gain in the low frequencies to provide the best aided response for the child or adult.
Some people have argued for the use of bilateral cochlear implants, citing the potential benefits of improved localization, speech understanding in noise, and speech production. However, recent research by Summerfield25 indicates that the second implant must cost less than 10% of the cost of the first to achieve the same cost-benefit ratio. With a surgically implanted medical device, such as a cochlear implant, this is not possible due to the high cost. The fitting of a hearing aid on the contralateral ear to provide bimodal input allows similar benefits of bilateral implants but at 2%-3% the cost of an implant.
Bilateral cochlear implants should be considered in cases where binaural stimulation is seen as a benefit and where the patient, after extensive clinical trial, has been shown to experience no benefit from the contralateral fitting of a hearing aid. The contralateral fitting of a hearing instrument for people with cochlear implants should be the standard of care with all patients with useable aided residual hearing.
Providing sound amplification solutions to adults and children with a severe or profound hearing impairment presents many challenges. The extent of damage to the cochlea in these cases requires the hearing care professional to be more circumspect in selecting fitting rationales. No longer does the sensible maxim of providing audibility across the speech frequencies hold true in all cases. Instead, one should balance audibility with the realization that providing too much amplification in the areas of maximum hearing loss may actually result in poorer speech outcomes and comfort.
Universal newborn hearing screening will result in a greater proportion of babies with severe and profound hearing loss being identified earlier, which creates a series of challenges in terms of the required amplification and how to measure benefit. In these infants, the hearing care professional more than ever is going to be working in a team using new and improved measures of outcomes to determine adequacy of the hearing aid fitting (including auditory, speech, and language development).
The overlap between cochlear implants and hearing aids continues to grow, and should now be seen as a continuum. No longer is it the case that dispensing professionals should view people with cochlear implants as not being potential candidates for amplification. Following the same arguments that led to binaural hearing aid fittings becoming a standard of care, the hearing care field now has the opportunity to fit a hearing aid on the contralateral ear of a person with a cochlear implant so that person can benefit from bimodal stimulation.
The new class of advanced hearing instruments offer more powerful gain and output, feedback controls, and new algorithms for those clients with severe/profound hearing loss. However, we should continue to be mindful of the additional challenges, as well as what new research tells us about the provision of amplification for successful client outcomes.
|Mark C. Flynn, PhD, and Traci E. Schmidtke, MA, are research audiologists at Oticon A/S, Hellerup, Denmark.|
Correspondence can be addressed to HR or Mark C. Flynn, PhD, Oticon A/S, Strandvejen 58, Hellerup, DK 2900, Denmark; email: [email protected]
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