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Types of Hearing Loss

Hearing loss can be conductive (disruption of the transmission of sound within the ear canal or the middle ear), sensorineural (caused by damage to the cochlea or the cochlear nerve), or mixed (both conductive and sensorineural).

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Overview of Types of Hearing Loss

There are several types of hearing loss, conductive, sensorineural, mixed and disorders of central auditory processing. A conductive hearing loss results from anything that impairs the transmission of sound in the external or middle ear. Examples include excessive wax in the ear canal, a hole in the eardrum, fluid in the middle ear, which may impair the vibration of the bones behind the eardrum and a condition called otosclerosis, where the stapes bone behind the eardrum becomes more fixed with time. A sensorineural hearing loss results from conditions that affect the inner ear or the cochlear nerve behind the inner ear. A mixed hearing loss has elements of both conductive and sensorineural hearing loss.

An audiogram is a hearing test conducted in a sound proof room by an experienced audiologist. From this test it can be determined whether a conductive or sensorineural hearing loss exists. The threshold of loudness required to barely hear various frequencies is determined. The audiologist will typically check the hearing through the ear canal, thereby determining hearing through both the conductive and sensorineural pathway, and compare it to the hearing obtained through the mastoid bone, thereby determining the hearing through the sensorineural pathway alone. The difference between the hearing through the air and bone is the conductive hearing.

Conductive Hearing Loss

A conductive hearing loss results from anything that impedes the transmission of sound within the ear canal or middle ear. Problems within the external ear canal that can lead to a conductive hearing loss include earwax, an ear canal infection or external otitis (swimmer's ear), a collapsible ear canal or an ear canal that never formed (aural atresia). Perforations of the eardrum can also cause a conductive hearing loss (for more information see chronic otitis media). The middle ear space occupied by the ossicles is normally an air-containing space. A collection of fluid within the middle ear, as seen with otitis media or middle ear infections, can impair the normal vibratory motions of the ossicles and cause a conductive hearing loss. A growth of skin (cholesteatoma) within the middle ear can erode the ossicles and impede their motion. Otosclerosis is a condition that leads to gradual fixation of one of the ossicles, the stapes bone. Severe trauma to the region around the ear can cause the ossicles behind the eardrum to become disjointed and lead to a conductive hearing loss.

Sensorineural Hearing Loss

The most common cause for sensorineural hearing loss is the natural aging process. This is a process called presbycusis. The cochlea is the part of the inner ear that responsible for transforming the vibratory motions of the stapes into an electrical signal. This signal then travels along the cochlear nerve to the brain. There are rows of hair cells within the cochlea, which specifically detect various frequencies or pitches. As we get older, the hair cells become dysfunctional and cause a hearing loss. Those hair cells that detect higher pitches or frequencies are more sensitive to the aging process. Superimposed upon the aging process is a genetic predisposition to developing age related hearing loss. Another words you can inherit the ability to developing age-related hearing loss from your parents or grandparents.

Viral and bacterial infections of the cochlea may also cause sensorineural hearing loss. Membrane breaks within the cochlea can occur spontaneously and cause mixing of fluids within the cochlea. This can lead to electrical chaos and may be the reason for Sudden Sensorineural Hearing Loss. Trauma to the head can cause a concussion of the cochlea. Hemorrhage within the cochlea can cause sensorineural hearing loss.

Meningitis can also cause sensorineural hearing loss. Meniere's disease is a condition where patients experience hearing loss that fluctuates. It is sometimes associated with a pressure sensation in the ear, ringing and episodes of dizziness. Sensorineural hearing loss in one ear may be the first sign of an acoustic neuroma, a benign tumor affecting the balance and hearing nerves. Diabetes, high cholesterol levels, low or high levels of thyroid hormone may also contribute to the development of sensorineural hearing loss. Autoimmune inner ear disease can lead to sensorineural hearing loss. Malformations of the inner ear and otosclerosis in its later stages may also cause this type of hearing loss. Excessive noise exposure or exposure to toxic medications (chemotherapeutic agents, anti-malarials, and quinine containing medicines)  can damage the hair cells of the cochlea and lead to sensorineural hearing loss.

Mixed Hearing Loss

Here there are elements of both conductive and sensorineural hearing loss. Most commonly this can arise from Otosclerosis.

Central Auditory Processing Disorders

Sometimes patients perceive a hearing loss and the audiogram is completely normal. Here the auditory signals are not being processed in the brainstem. Specialized testing is necessary to make the diagnosis. Special rehabilitative measures can be taken to improve on auditory capability.

About Dr. Prasad

Dr. Sanjay Prasad MD FACS is a board certified physician and surgeon with over thirty-two years of sub-specialty experience in Otology, Neurotology, advanced head and neck oncologic surgery, and cranial base surgery. He is chief surgeon and founder of the private practice, Metropolitan NeuroEar Group, located in the metropolitan Washington D.C. area.