Saturday, September 17, 2011

A (boring) Overview of Hearing Loss

This is a fair warning. Before I get started into a bunch of specifics, I wanted to give a quick overview of hearing loss. This will be slightly boring and I have included a few basic terms that I’ll have to use later out of necessity.

First off, what do you call a deaf person? There are several terms used to describe people with hearing loss: deaf, partially deaf, hearing impaired and hard-of-hearing are common phrases. However, the terms deaf and hard-of-hearing are preferred and considered proper. I'll commonly abbreviate hard-of-hearing to just HH on this blog.

I don’t intend to reiterate a lot of info that has already been written by someone more qualified elsewhere, but I will link it and maybe summarize it for you.

You can find a lot of good info on hearing loss on the wiki:

The American Speech-Language-Hearing Association (ASHA) also has a good website for general info on hearing loss, assistive technology, and other subjects.

Now, let’s talk about how humans hear things:

Sound travels through the air in waves, specifically what we call pressure waves. It travels to your ear, into the ear canal, and strikes the eardrum, causing it to vibrate, this is the outer ear. This is called air conduction. Some sound also moves through the bones around your jaw and parts of your skull, this is called bone conduction.

After the eardrum the vibrations enter the middle ear where there is a chain of three tiny bones called the ossicles changes the waves into mechanical vibrations, transmitting it to the opening of the inner ear.
The inner ear contains a fluid that picks up these vibrations and transmits them to a highly sensitive instrument called the cochlea (the little things that usually looks like a snail in most ear diagrams). As the fluid moves through the cochlea it moves little microscopic nerve endings called “hair cells” which turn the movement into electrical pulses so they can be sent over the auditory nerve to the brain. The cochlea is also sensitive to gravity and movement and helps the body balance. Essentially, the cochlea is a digitizer, changing the analog waves into electrical impulses. The little hair cells acting like binary on/off switches. Think of it like a musician playing an electric keyboard. The keyboard is the cochlea, but the keys are the little hair cells. As the musician mechanically hits each key, the keystroke is changed into an electric pulse, and sent on to be translated into music.

The brain then takes this raw electrical information from the auditory nerve and translates it into sounds and assigns the sounds meaning. So you hear things you recognize and understand speech.
That oh-so-beautiful ear diagram you see in doctor’s offices and medical books:

Types of Hearing Loss:

There are two primary types of hearing loss. Conductive hearing loss (CHL) and Sensorineural hearing loss (SNHL). Sometimes it can also be a combination of the two. Hearing loss can be in one ear or both and more severe in one side or the other.

Conductive loss simply means those vibrations don’t transmit properly to the inner ear. Fluid buildup, earwax, ear infections, damaged eardrums, and other minor things can sauce temporary conductive hearing loss. Sometimes the little bones or the eardrum have mechanical issues or are damaged or wear out over time. These issues are mechanical and can often be fixed with surgery. My father-in-law once had the bones in his middle ear start to ossify and had surgery to install a titanium replacement. It worked fairly well and he regained most all of his hearing. Luckily, conductive hearing loss can frequently be fixed with surgery or medical treatment. Aging or loud noise often contribute to this. However, it makes up a small percentage of permanent hearing loss.
Sensorineural hearing loss is more complicated. It is often called permanent since (most of the time) there is no medical process to reverse or heal the damage. SNHL hearing loss happens with the sensitive inner parts of the ear. The inner ear, cochlea, or auditory nerve do not work properly or are damaged. It can also be caused by damage to the brain itself (though this is less common). This loss can be caused or triggered by a large number of things (wiki link) including genetics, developmental problems, inflammatory illnesses, long term exposure to loud noise, physical trauma, and the cochlea simply wearing out with age. The effects of this type of loss vary widely from person to person, from total deafness, to only partial loss.
Solutions involve fitting corrective devices, however as I mentioned, these don't repair the existing damage to the body. Hearing aids can help by boosting volume and changing frequencies of sound into different frequencies that are still being picked up by the user. These are the cheapest and most common solutions. Surgeries have also been developed such as the cochlear implant for profoundly deaf individuals, think of it as a bionic ear. It’s expensive and invasive though, and only restores limited hearing ability.

Hearing loss is often classified into the following categories based on how loud sound must be before a person notices it. It is measured in decibels of hearing loss, or dB HL

Mild for adults: between 26 and 40 dB HL
Mild for children: between 20 and 40 dB HL
Moderate: between 41 and 55 dB HL
Moderately severe: between 56 and 70 dB HL
Severe: between 71 and 90 dB HL
Profound: 91 dB HL or greater

I have a Profound loss, but that’s slightly misleading and oversimplified. I can hear some sounds normally, mostly lower frequency sounds, but for other sounds, high frequency ones specifically, I am profoundly deaf, I can’t hear them at all unless they are painfully loud.

I have hearing aids that help with this. I have since I was a child, with a new pair about every 5 years on average. Technology has been changing a lot and the newer models are now all digital, able to detect sound patterns and shift and change the sound automatically. They also have a ton of neat programmable features. While hearing aides are relatively cheaper then implants, they are still expensive.
However, that’s a subject for another post.                   

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