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Deafness or hearing loss that is caused by impairment of the cochlea or problems with the auditory nerve is classified as nerve deafness or sensorineural hearing loss. If either the cochlea or the auditory nerve is completely destroyed, permanent loss of hearing occurs. The auditory nerve is a bundle of nerve fibers that transmits sound information between the cochlea and the brain. Nerve hearing problems can cause a loss of volume or clarity in sounds as well as complete deafness.
The auditory nerve and the vestibular nerve which also transmits information from the semicircular canals of the ears to the brain, join together as they pass through the bony canals of the skull, forming the eighth cranial nerve or Vestibulocochlear nerve. These nerve fibers carry sound information to the brain and many of the fibers transmit information back to the cochlea in the ear.
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Adult Onset Hearing Loss is Usually Associated With the Cochlea
Causes of nerve deafness are often linked to the cochlea, a sense organ in the inner ear that translates sound waves into nerve impulses that can be sent along the auditory nerve to the temporal lobe of the brain.
The cochlea is fluid filled and contains tiny hair cells that cause the fluid to vibrate when receiving sound and generate nerve impulses. The cochlea also receives sound information from the brain which it can use to suppress information that requires tuning out such as background noise. This helps to explain why hearing aids which can amplify sounds cannot completely correct hearing loss due to damage to the cochlea. Often adult onset nerve deafness is caused by problems in the cochlea and not the vestibulocochlear nerve.
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Nerve Deafness and Hearing Loss Caused By Medications, Disease, Age and Genetics
Only a small percentage of hearing loss or deafness that occurs later in life is directly due to auditory nerve problems. These are usually related to acoustic neuromas or tumors on the covering that wraps around the auditory or vestibulocochlear nerve, called the Schwann cell or myelin sheath.
A rare genetic disorder that causes nerve deafness is called Opticoacousitc nerve atropy-dementia. This disease affects the entire central nervous system including the optic nerve causing loss of vision or blindness. It causes hearing loss and eventually death by about the age of forty years due to calcification or hardening of the nerves.
Loss of the ability to hear high frequency sounds occurs to some extent in almost all older individuals. Other patterns of nerve deafness include decrease in the ability to hear low-frequency sounds due to excessive exposure to very loud sounds such as loud music or mechanical engine noises. Deafness to all sound frequencies can result due to a drug sensitivity such as to the antibiotics streptomycin, kanamycin and chloramphenicol. Other medications can have a teratogenic (defect causing) effect on the fetus in the womb. An example of this is the antibiotic Gentamicin which can cause developmental problems in the auditory nerve resulting in deafness.
Nerve deafness that result from a defect in the auditory nerve that is structural or functional may also be caused by illnesses and disease. Conditions that may result in loss of hearing due to nerve and related hearing organ problems include multiple sclerosis, cerebral palsy, Meniere disease, Barakat Syndrome, Downs Syndrome and maternally transmitted Diabetes-deafness syndrome.
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Clark & Kumar: Clinical Medicine Fifth Edition. WB Saunders, London 2002