Eggermont - Hearing loss: causes, prevention, and treatment

1. Tables in Chapter 4
2. Tables in Chapter 5
3. Tables in Chapter 7
4. Tables in Chapter 8
5. Tables in Appendix A

1. Figures in Chapter 1
2. Figures in Chapter 2
3. Figures in Chapter 3
4. Figures in Chapter 4
5. Figures in Chapter 5
6. Figures in Chapter 6
7. Figures in Chapter 7
8. Figures in Chapter 8
9. Figures in Chapter 9
10. Figures in Chapter 10
11. Figures in Chapter 11
12. Figures in Chapter 12
13. Figures in Chapter 13
14. Figures in Appendix A

Jos J. Eggermont

University of Calgary, Calgary, AB, Canada

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Jos J. Eggermont

Hearing is often taken for granted, and so its functions become noted only when they start to deteriorate. Prior to that there may already be warning signs in the form of tinnitus. Suddenly, one faces problems understanding speech, especially in noisy environments. This is annoying because those environments often accommodate social encounters, cocktail parties, sports venues, and bars. Hearing loss is foremost a communicative disorder, but it also diminishes the warning function that it normally has.

Hearing allows us to localize threatening sounds such as a car speeding in our direction or a barking dog close by. Sound localization also allows us to orient to interesting sounds, for instance to distinguish a musical instrument in an orchestra, or a voice in a cocktail party, and focus our attention to it.

The function of hearing is clearly present in all vertebrates. Even if we do not know exactly what they hear, we can infer it from the sounds they make. Bird song can be annoying (magpies) or pleasant (canaries) but we can assume that their conspecifics can hear it. Animals that normally do not make much noise such as cats have sensitive, broad frequency, hearing and use it to localize the scurrying mice and vocal songbirds. One can conversely assume that the potential prey uses hearing to avoid the predators.

Everyone who has heard a chorus of frogs or cicades understands the communication and socialization aspect of sound, particularly if the underlying intention is to call potential mates. Thus, hearing serves to localize and identify sounds emitted by conspecifics that may lead to mating, in humans potentially after socializing and chatting. Hearing warns about danger, from predators to approaching vehicles, and most of these functions are severely diminished in case of hearing loss.

Why do we have hearing loss? If we would be able to ask this to a frog or a finch, the answer would potentially be what is hearing loss? Just as sharks have a batch of teeth in reserve to replace the worn out current set, non-mammalian vertebrates can replace the hair cells in the inner ear when they are damaged by loud sounds or aging. Mammals have lost this gift, because evolution dictated that the ability to hear sounds with higher frequencies had more advantage than loosing some hearing sensitivity. Of course noise trauma was rare in the evolutionary period. The super-numerous replacement cells were converted to form a high-frequency region in the inner ear allowing hearing for frequencies above say 510 kHz, the upper region in birds. Currently, a large effort is put into finding out how to tweak genetic mechanisms to regain the possibility to replace damaged cochlear hair cells and so cure hearing loss and deafness.

My approach to the topic of this book is that the substrate of hearing sensitivity loss is in the ear whereas that for hearing problems is in the brain. The combination of the two aspects forms the communication disorder problem. Any amelioration has to deal with restoring the hearing sensitivity via hearing aids or cochlear implants, but also has to deal with the cognitive problems that result from the hearing loss or may be separate there from as in aging. We also will look at the feedback from the brain to the ear, which may result from attention and stress, through the corticofugal activity to midbrain, brainstem, and even the cochlea. Such central action may include protective effects for noise-induced hearing loss and presbycusis.

The book is comprehensive in the sense that it may be used as a stand-alone text in last year undergraduate and graduate courses in audiology. It starts with three basic science chapters, refreshing and updating knowledge about the auditory system, brain plasticity, and multisensory interaction. These chapters form the basis for training approaches in cochlear-, brainstem-, and midbrain-implant use and may also help understanding acclimatization effects on hearing aid use.