Hearing impairment classification
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Overview
Classification
Conductive
Conductive hearing loss occurs when sound is not normally conducted through the outer or middle ear (or both)[1]. Since sound can be picked up by a normally sensitive inner ear even if the ear canal, ear drum, and ear ossicles are not working, conductive hearing loss is often only mild and is never worse than a moderate impairment. Hearing thresholds will not rise above 55-60 dB from outer or middle ear problems alone. Generally, with pure conductive hearing loss, the quality of hearing (speech discrimination) is good, as long as the sound is amplified loud enough to be easily heard.
A conductive loss can be caused by any of the following:
- Ear canal obstruction
- Middle-ear abnormalities
Sensorineural hearing loss
A sensorineural hearing loss is due to insensitivity of the inner ear (the cochlea), or to impairment of function in the auditory nervous system. It can be mild, moderate, or severe, including total deafness. This is classified as a disability under the ADA and if unable to work is eligible for disability payments.[2]
The great majority of human sensorineural hearing loss is caused by abnormalities in the hair cells of the organ of Corti in the cochlea. There are also very unusual sensorineural hearing impairments that involve the VIIIth cranial nerve (the Vestibulocochlear nerve) or the auditory portions of the brain. In the rarest of these sorts of hearing loss, only the auditory centers of the brain are affected. In this situation, central hearing loss, sounds may be heard at normal thresholds, but the quality of the sound perceived is so poor that speech can not be understood.
Most sensory hearing loss is due to poor hair cell function. The hair cells may be abnormal at birth, or damaged during the lifetime of an individual. There are both external causes of damage, like noise trauma and infection, and intrinsic abnormalities, like deafness genes.
Sensory hearing loss (also called sensorineural hearing loss) may also result from abnormalities of the VIII cranial nerve.
Sensory hearing loss that results from abnormalities of the central auditory system in the brain is called Central Hearing Impairment. Since the auditory pathways cross back and forth on both sides of the brain, deafness from a central cause is unusual.
References
- ↑ "Types of Hearing Loss". Retrieved 2007-09-24.
- ↑ http://www.eeoc.gov/facts/deafness.html Hearing impairment and the Americans with Disabilities Act