As an audiologist, I mainly deal with adults with progressive, sensorineural, age-related hearing loss. Most patients see me when they develop a moderate, high-frequency (pitch) hearing loss. This audiogram shows a typical hearing loss, with most of the loss affecting the highest frequencies we test (4 and 8kHz).
These typical patients are fitted with hearing aids, which I update as their hearing deteriorates. Gradually, over the years, that moderate high-frequency loss becomes severe. It also starts to impact more and more on the middle frequencies (1 and 2 kHz), at which point people are struggling much more to understand speech.
When we get a severe sensorineural loss, there are several difficulties people have.
They usually have inner hair cell damage, as well as outer hair cell damage. Outer hair cells deal with amplification of sound, whereas the inner ones are responsible for sending signals to the brain. When someone has a conductive hearing loss, these inner hair cells are usually fine. All you need to do is amplify, and the patient can identify speech clearly and easily. With a sensorineural loss - seen in age-related loss and noise damage as well as other types - just amplifying is not enough. The patients still can’t distinguish speech from background noise. They struggle to get a clear signal, no matter the volume of the hearing aids.
With mild/moderate loss, it’s doable. We amplify, but we also clean up the speech by focusing the microphones of the hearing aids. Cutting down the background noise the patient receives in the first place, they just have to focus then on discriminating the speech of the person in front of them.
Severe sensorineural loss, like the image above, is very difficult to fix. Life has background noise! It’s everywhere. People don’t always talk one at a time, they don’t always speak clearly. With this type of loss, even with the best hearing aids you can buy, you are not going to hear perfectly with aids in noise. You’ll probably have subtitles on the TV, and it will be mentally draining to have conversation in noise. This is the inner hair cell damage at work.
When the hearing test shows hearing loss greater than 80dBHL, and the hearing loss slopes steeply on the audiogram, the person likely has cochlear dead regions. This is when there are no inner hair cells at all in the corresponding part of the cochlea. I have dead regions myself at 2kHz and above. What this means is that you are not receiving sound at certain pitches. Not only that, but the cochlea is confused, and it can generate sound at the wrong frequency. So when I amplify 4kHz (a loud, single, high pitched note) in my test, I actually hear a white noise*, a rushing sound. For speech, this is very bad indeed, because it means that fricatives (S, F etc) are blurred. I can’t hear speech clearly even with all the volume in the world.
Finally, by the time someone has a severe/profound hearing loss, they are probably 80+ years old, and their working memory is not so good. This means they have issues with processing what sounds they receive, after they receive them. It means that a fast conversation or a mumbled conversation is impossible. The brain simply cannot process it all fast enough, in real time, while the speaker is talking.
The audiologist should be aware that a patient might have cochlear dead regions, based on the audiogram and the feedback from the patient. If so, they will avoid adding amplification at the affected frequencies. The patient gets a duller hearing experience, but it’s clearer. This is similar to hearing without the aids in, but is still well worth doing, because you can hear the lower frequencies much better.
Audiologists also use a method of frequency lowering, to mitigate the dead regions. I’ve written about that in the past, but essentially it offers sharper, clearer sound. It has its limits though when a patient is elderly.
The only way to make hearing clearer when a patient has significant cochlear dead regions is to have a cochlear implant, and this is what the next post will consider.
*If you don’t hear tones on your hearing test, tell your audiologist.
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