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Hearing Anatomy and Hearing Loss

Ear Anatomy

Each of our ears collects sound waves and funnels them down our ear canals, to our tympanic membranes, or eardrums. When sound waves strike the tympanic membrane, it vibrates like a drum (hence the term “eardrum”). Behind the eardrum are three tiny bones that vibrate in response its movements, thereby sending the signal to the inner ear. Our inner ears are comprised of the cochlea - our hearing and balance organ - together with the auditory (hearing) nerve. Microscopic sensory hair cells within the cochlea convert the vibrations into an electro-chemical signal that is carried by the auditory nerve to the brain, where the signal is heard and recognized as sound.

Ear anatomy middle outer inner ear cochlear americas

Figure 1.  Ear anatomy including 1) Outer ear 2) Middle ear 3) Inner Ear 4) Hearing Nerve.  Image courtesy of Cochlear Americas.

Hearing Loss

Hearing loss occurs when there is blockage or disruption in the path of sound from the outer ear to the brain. In approximately 10% of cases, hearing loss originates from outer ear and/or middle ear conditions that disrupt the path of sound. This type of hearing loss is called conductive hearing loss and can usually be medically treated. A few examples of conditions that may cause conductive hearing loss are: excessive ear wax, ear infection, or stiffening of the middle ear bones.

When medical approaches cannot restore hearing, the use of hearing aids or bone conduction implants such as the BAHA (bone anchored hearing aid) may provide effective means to better hearing for people with conductive hearing loss.

Approximately 90% of all hearing losses originate in the inner ear or auditory nerve. This type of hearing loss is called sensorineural hearing loss. Most commonly, a sensorineural hearing loss is caused by damaged inner ear hair cells. Hair cells that are damaged can’t effectively send complete sound information to the brain. Sensorineural hearing loss is permanent, as once damaged, hair cells can’t repair themselves nor be medically restored. Sensorineural hearing loss can range in severity from mild to profound. In more than 95% of cases of sensorineural hearing loss, hearing aids or cochlear implants are the recommended course of treatment.

Degree of Hearing Loss

 

Audiogram showing degrees of hearing loss

Figure 2. The audiogram.

Degree of hearing loss is determined by a comprehensive hearing evaluation. A comprehensive hearing evaluation will determine the softest levels you can hear sounds of different pitches. These levels are called your thresholds, and are measured in an intensity scale called “decibels hearing level” or dB HL. Normal hearing is defined as thresholds for all sounds within the 0 dB HL to 25 dB HL range. Degrees of hearing loss are defined as follows:

Mild Hearing Loss: Thresholds in the 26 dB–40 dB range. People with a mild hearing loss have difficulty hearing and understanding soft sounds and soft speech. Hearing aids are recommended when mild hearing loss cannot be medically treated. A wide range of styles, including some that are nearly invisible when worn, is available. New, open ear hearing aid models offer benefits for people with mild and moderate high frequency hearing loss.

Moderate Hearing Loss: Thresholds in the 41 dB–70 dB range. With a moderate loss, conversations can be difficult to follow, especially in noisy environments. People with moderate hearing loss often perceive that other people are mumbling, because their hearing loss prevents them from hearing speech clearly. Even in quiet environments, people with moderate hearing loss find it hard to have a conversation in a group of people, or if the person speaking has their back turned or has a soft voice. They may often rely on visual cues or lipreading to help fill in what they don’t hear, without even realizing it. Hearing aids are recommended for moderate hearing loss that cannot be medically treated. A wide range of styles is available.

Severe Hearing Loss: Thresholds in the 71 dB–90 dB range. People with severe hearing loss cannot hear soft or moderate sounds, birds singing, or conversational speech. They require the person speaking to them to use a very loud voice in order to hear speech at all. In addition, when volume is increased, words or sounds may sound unclear and distorted.

Profound Hearing Loss: Thresholds greater than 90 dB. Profound hearing loss is sometimes referred to as “deafness”. People with profound hearing loss can typically only hear very loud environmental sounds.

In almost all cases of severe and profound hearing loss, hearing aids or cochlear implants are recommended.

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