Hearing Loss: What are the different types and what do they mean to you?

Some of you reading this may already know that there are different kinds of hearing loss, but there are many people who don't and a good deal of people who think hearing loss is hearing loss. My hope for this week's post is to shed some light on the three main kinds of hearing loss and to give some insight to what these can mean if you or someone you know is diagnosed with one of them.

1. Sensorineural Hearing Loss (SNHL):
Sensorineural hearing loss (SNHL, as is will be referred to henceforth) is the most common type of hearing loss diagnosed. It is commonly associated with hearing loss due to aging, hearing loss due to noise exposure, and many other causes. A SNHL is one in which the hearing loss is due to damage caused to the tiny sensory hair cells within the inner ear. Unfortunately these hair cells do not grow back after they are damaged and the hearing loss caused by their loss is permanent. Researchers and scientists are currently working toward finding a way to regenerate these tiny sensory hairs, but to date, this has not been accomplished.

People with this type of hearing loss typically complain that they have a hard time hearing soft spoken people, children's and women's voices, and that they have a very difficult time hearing in the presence of background noise. This is because typically SNHL affects the higher frequency region of the inner ear first and these frequencies are necessary for providing clarity to speech and separating speech from background noise. Due to the difficulty experienced in conversations, people with this type of hearing loss often become frustrated to the point that they remove themselves from conversations and from social interactions, isolating themselves. Naturally, this can be very upsetting and often times people with this form of hearing loss become depressed.

Luckily, however, even though there is no way to re-grow the sensory hair cells that have been damaged, and there is nothing medically that can be done to restore the hearing, people with SNHL are prime candidates for help through the use of amplification, or hearing aids. Hearing aids can provide a great deal of help in a myriad of situations, especially in background noise. Current hearing aid technology can provide this benefit in background noise through the use of directional microphones which are designed to analyze the auditory inputs and separate out the speech signal from the noise and turn down the background noise by focusing the microphones toward the speech signal. More on that in a later entry.

2. Conductive Hearing Loss(CHL):
Conductive hearing loss, or CHL, is a form of hearing loss that often afflicts children, but can be seen in any age group. In this kind of hearing loss, there is nothing wrong with the inner ear (cochlea) or the sensory hair cells therein. Instead, there is something wrong in the middle or outer ear, and as the name suggests, there is a reduction in how much sound is conducted to the the inner ear. A quick lesson in the anatomy of the ear will help to explain my point.

The ear is made up of three parts, the outer, middle, and inner ear. The outer ear consists of the pinna or auricle, and the external auditory canal (ear canal). The middle ear consists of the tympanic membrane (eardrum), middle ear cavity, and a chain of three tiny bones, called ossicles, the malleus (hammer), incus (anvil), and stapes (stirrup). These three bones are what conduct the vibrations of the sound from the outer ear to the inner ear. They are connected to the tympanic membrane on one end and the oval window of the cochlea on the other end. When sound waves hit the eardrum, they vibrate the bones which in turn move fluids within the cochlea to stimulate the auditory nerve and hair cells within the cochlea.

A conductive loss indicates that somewhere before the sound reaches the cochlea there is a break down of communication. This can occur in the outer ear, mainly the ear canal, when it becomes blocked by some kind of debris (wax, cotton from q-tips, dirt, etc.), or during testing if the canal wall collapses due to the headphones being used. This can also occur if there is something wrong in the middle ear. Within the middle ear, a number of problems can occur which can cause a conductive loss including, ear infections, perforated eardrum, a disconnection of the three bone chain, fluid build-up from ear infections, dysfunction of the eustachain tube, or a stiffening of the three tiny bones, a condition known as otosclerosis. Of course, these are simply a few of the possible causes of a conductive hearing loss and there are many more that can be responsible.

The typical complaint of a person with a conductive hearing loss that things just aren't loud enough. Once the TV or the person with whom they're speaking get loud enough, they can hear and understand what is being said just fine. That is because in a conductive loss, people lose something called the middle ear transfer function which basically maintains the energy in the original sound as it passed from the outer ear to the tiny inner ear. When this function is lost or somehow dampened, there a loss of energy, which we perceive as loudness.

Additionally, unlike people with SNHL, someone who has a CHL may actually do well in background noise or at a loud party because the people with whom they're interacting will naturally be talking louder to be heard over the noise. This is exactly what the person with a CHL needs to be able to understand the conversation. Generally speaking, a CHL is something that can typically be corrected with some kind of medical intervention and amplification is not necessary. In cases of middle or outer ear infection, medical treatment to clear up the infection and drain the fluid typically alleviates the CHL and after treatment, hearing is restored to normal or near-normal hearing. This, however, is not always the case, and sometimes the CHL cannot be treated medically. In this case, use of amplification which generates enough power to overcome this loss is suggested.

3. Mixed Hearing Loss:
A mixed hearing loss is a combination of both the conductive and sensorineural hearing losses. This means that there is something causing a loss of energy and transmission of the sound waves to the inner ear and that there is structural damage to the sensory hair cells in the inner ear. Unlike someone with a purely conductive loss, these people will not do well at a loud party or in background noise because even though the speech is loud enough to be heard, there is a loss of clarity due to the SNHL. Amplification is usually warranted in these cases to address the SNHL, and medical correction of the CHL may or may not be possible.

A condition which may cause a mixed hearing loss is a stiffening of the three tiny middle ear bones in older adults. In this condition there is a loss of sound transmission to the inner ear due to the reduction of movement on the chain of bones, and in most older adults some SNHL is common due to the aging process. This is not to say that all older adults have a hearing loss, as there are a great number who do not. In general, however, as we age we tend to lose at least some hearing sensitivity. This coupled with the conditions mentioned, otosclerosis, can lead to a mixed hearing loss.

For more information on these three kinds of hearing loss, and other information related to hearing loss, please visit http://www.asha.org/public/hearing/disorders/types.htm.