The Five Senses and Beyond: The Encyclopedia of Perception - Jennifer L. Hellier 2017
Deafness is a functional loss in the ability to hear. It varies along a spectrum from mild hearing impairment to profound deafness and total deafness where little to no sound can be heard, even with amplification. There are two categories of deafness—conductive and sensorineural. Conductive hearing loss is the result of a disruption in sound wave conduction within the outer and middle ear. Sensorineural deafness is the result of damage or injury to the cochlea or the auditory nerve that prevents the transduction of the electrical impulse created by sound waves. A combination of both conductive and sensorineural hearing loss is called mixed deafness. Deafness can be unilateral, affecting one ear, or bilateral, affecting both ears. Tuning fork tests, such as the Weber test and the Rinne test, are used to distinguish between the type of hearing loss, the extent of the loss, as well as the differentiation between unilateral and bilateral. It is estimated that in the United States about 13 percent of the population have some form of hearing loss ranging from total deafness to hearing impairment. Deafness affects all age groups and all races equally. Typically, hearing assistive devices, including hearing aids and cochlear implants, are used for sensorineural loss, while local medical treatments are used when the hearing loss is conductive.
Conductive deafness is caused by decreased conductivity of sound waves at any point between the outer ear and the middle ear. The majority of pathologies that cause conductive hearing loss can be treated with standard medical interventions, generally with the full recovery of hearing. Causes of conductive hearing loss can include normal processes that go into overproduction. Earwax secretion is an example of a normal and beneficial process that can cause conductive deafness. When earwax buildup becomes excessive, it can collect and block the auditory canal, which thereby inhibits the passage of sound waves to the middle ear. The buildup usually causes sudden hearing loss accompanied by localized pain and irritation. To treat this issue, the wax buildup must be removed. Health care providers can either scoop the wax out of the ear with a special instrument or syringe the wax out with a warm saline lavage.
Other common causes of conductive hearing loss are otitis media and otitis externa, which are ear infections of the middle ear and outer ear, respectively. Bacteria such as Streptococcus pneumonidae as well as Haemophilus influenzae introduced to the middle ear via the auditory tubes typically cause ear infections of the middle ear and the subsequent inflammation in the outer ear. Hearing loss with ear infections is most commonly triggered by excess fluid within the middle ear, which inhibits movement of the ossicles and tympanic membrane. Ear infections are typically treated with a medicated eardrop that is combination of antibiotics and steroids.
Tympanic membrane injury is another common cause of conductive hearing loss. It can range from perforations to ruptures in the membrane. If the eardrum is injured, the conduction of sound waves to the middle ear can be decreased because the tympanic membrane is less responsive to sound waves. Eardrum trauma can be caused by direct injury to the membrane via a foreign body, significant air compression within the outer ear, or by middle ear infections when fluid buildup causes a rupture. Treatment of eardrum injury depends on the severity and cause. If the injury is mild, the eardrum should heal without direct treatment; if the injury is severe, it may have to be repaired surgically. Any injury to the eardrum accompanied by an infection should be treated with antibiotic and steroid eardrops.
Sensorineural hearing loss is caused by a decrease or complete inability to transduce sound waves into electrical impulses in the cochlea or cochlear nerve. This type of deafness is sometimes referred to as nerve deafness because the location of the deficit is at the level of the cochlear nerve or hair cells. There are various causes of sensorineural deafness including old age, loud noise, congenital defects, and infection. The pathologies that cause sensorineural hearing loss typically damage the hair cell cilia directly, or they damage the cochlear nerve in such a manner that neural impulses cannot be sent to the brain. In instances such as old age and prolonged exposure to loud noises, the damage occurs at the level of the cilia. Hearing loss caused by old age or prolonged exposure to loud noises is the result of cilia that are no longer attached to the hair cells. Without the cilia on hair cells, action potentials cannot be generated and sent to the cochlear nerve. This form of sensorineural hearing loss typically affects only some frequencies, so individuals affected can still hear other frequencies of sound. The use of a standard hearing aid can help compensate for the hearing loss caused by old age or loud noise exposure.
Congenital hearing loss occurs while a baby is still in the womb and is caused by a variety of factors. Some common causes of congenital sensorineural hearing loss are maternal infections such as rubella (German measles) or toxoplasmosis (a parasite that is often found in cat feces). In other cases, congenital hearing loss can be caused by a hereditary disease that is passed from the parents to the child genetically. Another cause of sensorineural hearing loss is infection that leads to inflammation of the inner ear. Infection-induced hearing loss occurs when an infection such as the mumps causes inflammation and swelling around the cochlea, which causes damage to the hair cell cilia. Treatment for any type of sensorineural deafness requires the use of hearing assistive devices such as hearing aids or cochlear implants.
See also: Americans with Disabilities Act; Assistive Technology; Auditory System; Cochlea; Cochlear Implants; Vestibulocochlear Nerve
Gallaudet University Library LibGuides. (2012). Deaf population in the U.S. (data file). Retrieved from http://libguides.gallaudet.edu/content.php?pid=119476&sid=1029111
Leighton, S., A. Robson, & J. Russell. (2000). Hall & Colman’s diseases of the ear, nose and throat (15th ed., M. Burton, Ed.). London: Harcourt Brace.