Types and Causes

Now that you understand how we hear, you may have a better understanding of what causes hearing loss. There are essentially 3 main causes of hearing loss:

CONDUCTIVE HEARING LOSS:

This occurs when the sound transmission through the ear canal (outer ear) and middle ear is blocked. Causes of conductive hearing loss within the outer ear canal include impacted cerumen (ear wax), drainage, foreign object (bead, q-tip), or perforated tympanic membrane (hole in the ear drum). In most cases of middle ear-caused conductive loss, fluid develops within the middle ear space when the eustacian tube can not properly ventilate the middle ear space. This is known as middle ear infection. Other middle ear causes of hearing loss include tissue growth or benign tumor (cholesteatoma) filling the middle ear space, damage to the ossicular chain, and in some cases a bony growth within the footplate of the stapes where it attaches to the oval window of the cochlea (otosclerosis).

Most conductive hearing loss can be treated medically or surgically. Obviously, when the blockage of the ear canal or middle ear is removed, or the damaged portion of the middle ear is repaired, most, if not all of the hearing is restored. In cases of middle ear infections, pressure equalizing tubes are inserted to allow for proper middle ear ventilation, preventing middle ear fluid from developing again. In cases of damage or disruption of the ossicular chain, surgery can be done, using a prosthetic device to take the place of the ossicular chain to conduct sound properly to the inner ear.

SENSORINEURAL HEARING LOSS:

With sensori-neural hearing loss, the obstruction of sound transmission occurs within the cochlea and/or auditory nerve. Sensori, or cochlear type of hearing loss occurs when there is damage to hair cells and/or basilar membrane of the cochlea, disrupting the chemical and electrical transmission of sound. This damage is often due to exposure to loud noises, or acoustic trauma. In most cases, the hair cells tuned for high frequencies are damaged first, as they are more vulnerable to violent sound waves due to their location on the membranes of the cochlea. Such noise damage can also result in tinnitus (see tinnitus section), which may accompany the hearing loss.

Cochlear hearing loss can also be caused by inflammation of the cochlea, usually from fungal, bacterial or viral infections. Bacterial, fungal and viral infections may reach the cochlea through the cochlea aqueduct, or via blood supply to the inner ear. Inflammation of the inner ear can also cause tinnitus and dizziness along with hearing loss, as observed in cases of Meniere’s disease (see dizziness).

Exposure to ototoxic chemicals, which enter the inner ear via blood supply, can also cause hearing loss within the cochlea. Circulatory deficiencies, such as high blood pressure, or lack of blood supply to the inner ear, also can cause hearing loss. With lack of blood supply comes lack of adequate oxygen and nutrients to the inner ear to maintain its normal function. Like viral infections, circulatory hearing loss many times is accompanied with dizziness and tinnitus.

Neural, or retrocochlear (beyond the cochlea) causes of hearing loss involve the auditory nerve and/or the neurological stations and auditory cortex of the brain. Structural changes in the retrocochlear portion of the hearing system usually includes demyelination, or the nerve’s loss of ability to transmit neurological impulses to the cortical areas of the brain. However, interruption of blood supply and toxicity can also cause neurological disturbances in hearing. In some cases, a benign tumor known as an acoustic neuroma, can interfere with the neural transmission of sound within the auditory nerve. This tumor usually can be found within the internal auditory canal, where the auditory nerve travels from the cochlea to nerve stations and cortical areas. The tumor will press against the auditory nerve, causing hearing loss to that affective ear. In most cases of acoustic neuroma, unilateral hearing loss is detected (or hearing loss in one ear but not the other).

It must be noted that the most common cause of sensorineural hearing loss (and one that can not be avoided) is Presbycusis, or aging. In most cases of Presbycusis, hearing loss occurs gradually over a period of several years. Sensorineural hearing loss can also be hereditary. A hereditary predisposition may determine the age at which an individual may begin to experience hearing loss, although much research has indicated that hearing begins to deteriorate around the age of 40.

Most sensorineural hearing loss can not be corrected medically or surgically. In some instances, medications can treat sensorineural hearing loss, usually cochlear. Treating viral or bacterial infections via medication such as steroids can improve hearing. Sometimes improving blood circulation through improved diet and medication can improve hearing if the loss is caused by circulatory deficiencies. Also, surgery to remove an acoustic neuroma can restore hearing to that affected ear. However, most sensorineural hearing loss is permanent. In most cases of permanent sensorineural hearing loss, hearing aids can be used to overcome hearing loss and improve communication and safety. More information about hearing aids and how they can improve your communication can be found in the “Hearing Aids” section.

MIXED HEARING LOSS:

This is simply a combination of conductive andsensorineural hearing loss. For example, a person may have fluid within the middle ear ANDdamage to the sensory nerve endings of the cochlea. A combination of middle ear surgery and use of hearing aid would be the treatment for this example of mixed hearing loss.