Clinically, hearing loss is categorized into one of the following three categories:
- Conductive hearing loss
- Sensorineural hearing loss
- Mixed hearing loss
A fourth category, central deafness, is much more uncommon. Central deafness occurs when the sound is successfully transmitted by the lower auditory pathways, but the brain cannot recognize the signal as sound. This type of deafness generally occurs in conjunction with other neurological conditions.
- Hearing Loss - Video Clip courtesy of Cochlear Corporation
Conductive Hearing Loss
Conductive hearing loss occurs when something in the outer ear or middle ear blocks or impedes the passage of sound waves to the inner ear. You can simulate a conductive hearing loss by wearing earplugs. Conductive hearing loss can be caused by a number of things, including fluid in the middle ear, excessive cerumen (earwax) buildup in the ear canal, perforated eardrum, damaged ossicles or tumors in the ear canal or middle ear. Conductive hearing losses are more likely to be temporary and can often be corrected medically or surgically. However, if medication or surgery does not work or is not an appropriate treatment, then many people with conductive hearing loss can be fit with hearing aids. Conductive hearing losses can range up to a maximum of about 50-60 dB HL (mild to moderate hearing loss). People with conductive hearing losses, who use hearing aids, generally do very well.
Sensorineural Hearing Loss
Sensorineural hearing loss (SNHL) occurs when there is a problem in the inner ear. “Sensorineural” hearing loss is an umbrella term that refers to problems with either the cochlea (sensory hearing loss) or the auditory nerve (neural hearing loss). Most people with SNHL actually have just sensory hearing loss, which means the source of hearing loss is in the cochlea. Sometimes people who have SNHL also have balance problems because the cochlea and the semicircular canals (for balance) are both part of the inner ear; therefore, the source of the hearing loss can also cause balance problems too. SNHL can be caused by many different things, including certain kinds of medications, lack of oxygen at birth, excessive noise exposure, tumors, degenerative diseases, auto-immune diseases, genetic factors, viruses or bacteria. Sensorineural hearing losses are more likely to be permanent and often cannot be treated with surgery or medication. Hearing aids are typically recommended for people with SNHL.
The amount of benefit from hearing aids varies widely for people with SNHL. Unlike conductive losses, which essentially create a reduction in volume, individuals with SNHL sometimes also experience the added disadvantage of a distorted signal. Those individuals often comment that, “I can hear you but I can’t understand what you’re saying.” Think of a radio that is perfectly tuned in to your favorite station but the volume has been turned way down. This would sound like what a person with a conductive hearing loss hears. For those individuals, putting on a hearing aid would be like turning up the volume on the radio. Now suppose the radio is not quite tuned all the way in to the station, AND the volume has been turned way down. This would sound like what some people with SNHL hear. For those individuals, putting on a hearing aid would be like turning up the volume on the radio, but the station still isn’t tuned all of the way in. It is important to understand that not everybody with SNHL has trouble with distortion. But for people with severe or profound hearing losses, hearing aids are often inadequate because of the perception of distortion. Those individuals are typically considered good candidates for a cochlear implant. Cochlear implants are generally appropriate when the hearing loss is extensive enough that hearing aids do not help and the loss cannot be treated with medication or other surgery.
Mixed Hearing Loss
Mixed hearing loss occurs when there is a conductive component on top of a sensorineural component. For example, someone who has a noise-induced hearing loss (sensorineural) who subsequently gets an ear infection with fluid in the middle ear (conductive) would have a mixed hearing loss.