Hearing loss or hearing impairment can be categorized as either conductive hearing loss, sensorineural hearing loss or a mixed type that exhibits underlying symptoms of both conductive and sensorineural hearing loss. Auditory neuropathy is also a condition that can result in hearing loss.
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The Pathophysiology of Hearing
Hearing begins at the pinna, which is the anatomical term used to describe the cartilaginous outer ear structure. The hearing pathway continues through the pinna to the external auditory canal, where soundwaves will eventually reach the tympanic membrane, which is more commonly referred to as the eardrum. The movement of incoming soundwaves causes the eardrum to vibrate according to the frequency of the sounds. For example, high-frequency sounds will cause the eardrum to move back and forth quickly, whereas low-frequency sounds will cause the eardrum to move slowly.
The vibrations generated by the eardrum eventually reach the ossicles, which are the three smallest bones in the body that are located within the middle ear. The three ossicles, which include the malleus, incus and stapes, move to amplify the vibrations and help transmit these stimuli to the inner ear.
Within the inner ear is the bony labyrinth that contains a shell-shaped organ called the cochlea. Of the three scalae that make up the cochlea includes the middle scala that houses the organ of Corti. Once soundwaves reach the organ of Corti, the thousands of hair cells within this structure convert the physical stimulus of hearing into an electrical signal. This signal will eventually reach the auditory cortex of the temporal lobe in the brain for sound processing.
Decibels Hearing Loss (dB HL)
The normal hearing loss range is -10 to 15 decibels (dB), which is a unit used to describe loudness. To classify the degree of an individual’s hearing loss, the following decibels hearing loss (dB HL) ranges are utilized.
- Slight Hearing Loss: 16 – 25 dB HL
- Mild Hearing Loss: 26 – 40 dB HL
- Moderate Hearing Loss: 41 – 55 dB HL
- Moderately Severe Hearing Loss: 56 – 70 dB HL
- Severe Hearing Loss: 71 – 90 dB HL
- Profound Hearing Loss: > 91 dB HL
To put these values into perspective, whispers are approximately 30 dB, whereas average noises in the home can reach around 50 dB and conversational speech is typically around 60 dB. In contrary, louder sounds, such as those produced by jet engines, can reach up to 140 dB and are more painful to hear upon exposure.
Types of Hearing Loss
The types of hearing loss include conductive, sensorineural and mixed hearing loss, as well as auditory neuropathy.
Conductive Hearing Loss
Conductive hearing loss occurs when sound waves are unable to pass from the external ear into the inner ear. The most common causes of this type of hearing loss include:
- Blockage of the ear canal by ear wax or some type of object
- Perforation of the ear drum
- Accumulation of fluid in the middle ear as a result of colds, allergies or ear infections
- Altered function of the Eustachian tube
- Benign tumors
- Deformed outer or middle ear structures (typically discovered at birth)
Sensorineural Hearing Loss
Sensorineural hearing loss (SNHL) occurs as a result of some type of damage that occurs within the inner ear. Damage to this area can affect the function of hair cells, the auditory nerve as well as other nerves that play a role in transmitting sound information to the brain. SNHL is the most common type of permanent hearing loss. SNHL can be the result of an illness, a birth defect of the inner ear, hereditary factors, aging, previous ear injury or the use of ototoxic medications, which are medications that can cause damage to the ear. Some examples of potentially ototoxic medications include:
- Some aminoglycoside antibiotics
- Cancer chemotherapy drugs
- Salicylate pain relivers
- Loop diuretics (used to treat heart and kidney conditions)
Hearing loss and aging
Hearing loss that occurs as a result of aging is known as presbyacusis. As people between the ages of 30 to 40 grow older, many experience gradual hearing loss in small amounts that continues to increase with age. Therefore, by 80 years old, many people may have significant hearing impairment.
Presbuacusis occurs when the sensitive hair cells inside the cochlea gradually become damaged or die. Initial symptoms include loss in the ability to hear high-frequency sounds, such as female or children’s voices, as well as difficulty in hearing consonants, which can make hearing and understanding speech difficult.
Hearing loss and ear injury
Ear injury is another common cause of SNHL that is commonly the result of damage caused by chronic exposure to loud noises. As a result of constant exposure to loud noises, the hair cells within the cochlea become inflamed and damaged.
Mixed Type Hearing Loss
Mixed hearing loss can occur when an individual experiences both conductive and SNHL simultaneously as a result of damage to both the outer and/or middle ear and the inner ear and/or auditory nerves. Any cause of both conductive hearing loss and SNHL can therefore contribute to mixed hearing loss.
Auditory neuropathy involves the inability of sound waves to reach the nerves that exist past the inner ear, thereby preventing this information from reaching the brain. Auditory neuropathy can result from damage to the hair cells of the inner ear, hereditary factors or some other type of damage to the nerve cells that transmit sound information. While SNHL and auditory neuropathy are similar in their pathophysiology, research has shown that these disorders differ patients’ response to pure-tone hearing thresholds.
- “Degree of Hearing Loss” – American Speech-Language-Hearing Association
- “Types of Hearing Loss” – Deafness Foundation
- “Conductive Hearing Loss” – American Speech-Language-Hearing Association
- “Sensorineural Hearing Loss” – American Speech-Language-Hearing Association
- “Ototoxic Medications (Medication Effects)” – American Speech-Language-Hearing Association
- “Mixed Hearing Loss” – American Speech-Language-Hearing Association
- “Auditory Neuropathy” – National Institute on Deafness and Other Communication Disorders (NIDCD)
- Wang, S., Dong, R., Liu, D., Wang, Y., Mao, Y., Zhang, H., Zhang, L., & Xu, L. (2016). Perpetual separation of sensorineural hearing loss and auditory neuropathy spectrum disorder. Laryngoscope 126(6); 1420-1425. DOI: 10.1002/lary.25595.