Editor: Here's Mark Ross with a provocative article on managing a severe hearing loss. This article originally appeared in "Hearing Loss" magazine and is reprinted with the author's kind permission.

This is part one of two parts.

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July 2010

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.

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.

Here's Part Two