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   Rehabilitation Engineering Research Center
   on Hearing Enhancement

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Dr. Ross on Hearing Loss

Understanding and Managing a Severe Hearing Loss

by Mark Ross, Ph.D.

People with a severe hearing loss represent only about 10% of the total population of those with a hearing impairment. While this may not seem like much, it constitutes about three million of the thirty million people in our country with some degree of hearing impairment. And that is a lot of people. Typically, a severe hearing loss is defined as one falling between the 70 and 90 dB levels in the audiometric range; in practical terms, however,  it also includes those with relatively good hearing in the low frequencies and extremely poor to absent hearing at the higher frequencies.  .    

People with a severe hearing loss fall into a kind of management limbo.  Those with less than a severe hearing loss can usually be fit satisfactorily with hearing aids. The fitting issues with them are straightforward: to select and adjust a specific hearing aid, one that includes potentially helpful and desirable special features. Those with hearing losses in excess of 90 dB are usually candidates for a cochlear implant (although some in this category may still do fairly well with hearing aids). It is the people in the middle, those with a severe hearing loss, who represent the greatest fitting challenge as their ability to use residual hearing varies considerably. In one study the range of word recognition scores in quiet for subjects with severe hearing loss varied from 10% to 100%!  A similar range was found while listening in noise. Those with the poorer scores would likely be considered potential cochlear implant candidates, while those who score above 60% should be able to derive adequate benefit with well-fit hearing aids. Individual differences are always going to occur and have to be taken into account.

The auditory structures most likely to be damaged in cases of severe hearing losses – and indeed all sensori-neural hearing losses – are the tiny hair cells located within the cochlea. As can be seen in the accompanying figure, there are four rows of these hair cells, three outer and one inner row. As the basilar membrane undulates in response to sounds entering the ear, the cilia (the actual “hairs”) embedded on top of the cell body are bent; this triggers the biochemical events that convert sound vibrations into neural impulses.  Normally, as sound inputs increase in intensity there is a smooth growth in the perceived loudness sensations, as first the outer hair cells are activated and later, with increasing sound levels, the inner row of hair cells becomes the primary receptors.

                                                 

Organ of Corti

Figure 1. The four rows of hair cells located in the cochlea. Small movements of the basilar membrane (produced by sound vibrations) will cause the stereocilia of the three outer rows of hair cells to bend, triggering a neural impulse in the auditory nerve.  It takes bigger vibrations of the basilar membrane (louder sounds) to activate the inner row of hair cells. (Figure, courtesy of Dr. John S. Oghalia, Baylor College of Medicine.)

When someone has a hearing loss of about 60-70 dB or more, it is likely that the three rows of outer hair cells - known to be particularly susceptible to all sorts of potentially damaging events - are dead or severely damaged. In such instances, when the outer rows of hair cells are inoperable, it is only the inner row that “tells” the brain about the high level of input sounds stimulating the cochlea.  Therefore, a listener does not experience the sensation of a smooth loudness transition from soft to loud sounds. The resulting subjective experience is that of a rapid growth in the loudness of input sounds, from barely hearing a sound to it quickly being too loud. This phenomenon, known as “recruitment,” is one of the key challenges facing an audiologist when fitting hearing aids to someone with a severe hearing loss

Complicating the situation is the fact that the presumed thresholds of someone with a severe hearing loss may not be accurate because of the presence of cochlear dead regions (which implies damage to the inner as well as the outer hair cells). People with cochlear dead regions often report hearing clicks, hisses, or static during an audiometric examination, rather than a tonal  sensation, suggesting that the specific hair cell(s) usually “tuned” this frequency are damaged or dead. In recent years, a fair amount of research has been conducted on this phenomenon and a simple test has been devised to test for its presence (the Threshold Equalizing Noise test). The likelihood of dead spots increases as a hearing loss exceeds 70 dB, but even then reports suggest that the incidence rate rarely exceeds 60%.

It seems that it is the people with a severe high frequency hearing loss who are the ones most likely to have this problem. The presence of cochlear dead spots will likely affect how well someone does with a hearing aid. For example, if the evidence suggests that dead spots exist in a high frequency range, one should provide amplified sound in this area cautiously and only with suitable comparisons. This can be done, for example, by programming one of the hearing aid’s memories to cut off  the higher frequencies (where the dead spots are found), while another memory can be programmed to include these higher frequencies. This way the user can directly compare the two conditions.  

While more difficult, people with severe hearing loss have been successfully fit with hearing aids for years. The goal is to “package” the amplified sound between the person’s severely impaired hearing thresholds and the loudness tolerance levels (the “dynamic range”). We want someone to be able to hear soft input sounds as well as possible, while at the same time not making sounds too loud. The narrower the dynamic range, the more difficult the hearing aid fitting. One way to package a wide range of input sounds into a restricted dynamic range is to use a hearing aid feature called wide dynamic range compression, or WDRC (included now in most hearing aids). What this feature does is increase the degree of amplification (gain) of low input sounds – to ensure that they can be heard – and decrease the gain of higher level input sounds - to be sure that sounds do not become unpleasantly loud.

Professionals hold different views regarding the feasibility of using this feature for people with severe hearing losses. Decisions that have to be made concern the degree of compression to utilize, at what intensity level it should begin, and how quickly or slowly it should be activated to incoming speech signals (the time constants). These are not yet settled issues as far as I can see. When one “compresses” a relatively large range of speech inputs into a small area, the signal will consequently be distorted (albeit intentionally) to a certain extent.  Often this is a desirable outcome; some people with severe hearing loss can utilize the acoustic information provided in this narrow range to increase their speech perception capabilities. Other people with severe hearing loss will not be able to benefit from highly compressed speech, possibly because in addition to the elevated hearing thresholds, the damage to their hair cells also produces various kinds of psychoacoustic distortions.
This latter group of people apparently prefer a linear setting,  where a similar amount of gain is provided for all input levels until the output limits of the hearing aid is reached, It is only at this point that the hearing aid’s gain is reduced. While softer sounds may not be audible in this approach, that portion of the input sounds that can be perceived is relatively undistorted. There are studies that support both approaches, as we would expect because of the usual range of individual differences. The solution to this dilemma, similar to the one made above in regards to frequency range, is to devote one hearing aid program to wide dynamic range compression (WDRC)  and the other to a linear mode of amplification (called compression limiting).  That way a person can compare both approaches and use the one he or she prefers.

There are several other potentially helpful features that can be included in a hearing aid intended for a person with a severe hearing loss. A few years ago, it was thought that directional microphones would not be helpful to such people. Some recent research , however, suggests that those with a severe hearing loss can derive benefit from directional microphones, perhaps not as much as someone with less of a hearing loss, but significant nonetheless. A necessary feature to include in any hearing aid meant to be worn by people with a severe hearing loss is a feedback management program. For many years, such people have disturbed themselves and those around them with the constant presence of acoustic squeals emanating from their hearing aids. An effective feedback management circuit, while perhaps not completely eliminating feedback, will permit the user to realize at least 15 dB or so of increased amplification before the feedback occurs. Finally, and this hasn’t changed from the early days of personal hearing aids, is the need to have a well-fitting earmold that completely occludes the ear. Not for them, unfortunately, are vented earmolds (except perhaps for a tiny pressure vent) or any sort of non-occluding tube or earmold.

In the severe hearing loss category, the most difficult hearing aid candidates are those with sharply sloping audiograms, where thresholds at 500 or 1000 Hz are relatively good (perhaps about 20 to 30 dB) while those at higher frequencies fall into the severe and profound categories. The fitting challenge is not over-amplifying the low frequencies while at the same time delivering some perceptually useful sounds to the higher frequencies. Much depends upon the steepness of hearing slope, the frequency point where the thresholds begin to drop off, and the extent of the hearing loss at the higher frequencies. In these instances, it is useful to measure the hearing thresholds at the inter-octave points. In the usual audiogram, the hearing thresholds are measured at 250, 500, 1000, 2000, and 4000 and 8000 Hz. Depending upon the frequency where the thresholds appear to drop off,  the Audiologist should also measure the inter-octave frequency, as at 750 Hz if the drop off occurs at 500 Hz. This detailed information may be very useful when fitting a hearing aid. Generally, people with a severe high frequency hearing loss are often discouraged from trying a hearing aid. Still, for a motivated person, it is often worth a try. Sometimes even a little   hearing aid benefit can make a big real-life difference.

Unfortunately, a possible solution to the audiological dilemma these people present is not yet available as a routine clinical procedure. This is the hybrid cochlear implant, one that uses a short (6 to 10 mm electrode) in order to foster the preservation of acoustic hearing in the low frequencies. The goal with this arrangement to is provide the person with acoustic low frequency sounds (either directly or with a hearing aid) and high frequency auditory information via the cochlear implant. This combination of acoustic and electric hearing has been found to result in higher speech recognition scores than with either mode alone (acoustic or implant). Although research on hybrid implants has been going on for about ten years, FDA approval has not yet been obtained. So, for the present, this management alternative for people with severe hearing frequency loss is not available.  

In brief, hearing assistance is available for the person with a severe hearing loss, either through well-fit hearing aids or a cochlear implant. In order to realize the most assistance, it is necessary to ensure particularly careful preliminary testing and, usually, more attention to various amplification possibilities than is usually the case. While such a fitting may take more time,  the results are worth it.

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Last modified: 07/01/2013

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