Hearing2 - Deafness 1 in 1,000 born deaf worldwide 0.3% of...

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Unformatted text preview: Deafness 1 in 1,000 born deaf worldwide 0.3% of children under 5 years deaf 1 in 1,000 will develop deafness 500,000 to 750,000 Americans deaf No benefit from hearing aids Two types of hearing loss Conductive hearing loss Mechanical deficit in bringing vibrations to inner hear Sensory-neural hearing loss Problem converting sound to electrical signal that travels to the brain 1 Causes of Hearing Loss Presbycusis Gradual loss of hearing as we get older 75% of people over 60 have significant hearing loss Due to loss of hair cells Especially near the oval window (high frequencies) SPL (dB) Threshold of audibility (referred to normal hearing threshold) Consonants contain higher frequencies Inability to hear consonants leads to poor speech discrimination Test: Speech Speech w/o high frequencies 2 Causes of Hearing Loss Excessive noise, as due to: Occupation E.g.: factory noise, pneumatic hammer,... Recreation E.g.: shooting range, hunting,... Noise exposure E.g.: disco, loud music,... Most damaging noise: impulsive E.g. gunshot 3 Causes of Hearing Loss Menier's disease High fluid pressure in the inner ear Gives a low frequency hearing loss Instead of being progressive, it fluctuates Hearing loss may change from day to day Also affects balance (gives vertigo) Otosclerosis Excessive growth of bone surrounding middle and inner ear May block stirrup and pinch auditory nerve Hereditary; also may develop after childhood measles infection Sudden hearing loss Usually afflicts older adults Typically only one ear Can be viral or due to vascular accidents Tinnitus (ringing in the ear) Often accompanies hearing loss Nerve fibers to dying hair cells become irritable and discharge 4 Causes of Hearing Loss Ear infection (Otitis) Meningitis Usher syndrome (also causes blindness) Conductive hearing loss Damage to eardrum or earbones (usually corrected by surgery) Wax or fluid in middle ear Autoimmune diseases E.g., rheumatoid arthritis, lupus Auditory neuropathy Sound enters inner ear normally but transmission from inner ear to brain is impaired May involved damage to hairy cells or faulty connection between hairy cells and auditory nerve When loss is progressive in only one ear, it may be due to causes beyond the inner ear Acoustic nerve or auditory part of the brain 5 Hearing Aids 6 Hearing Aids Components: Microphone Battery-operated amplifier A means of transmitting sound to the user Speaker Direct transmission to bones or skull (requires surgical implant) May selectively amplify high frequencies Some have digital equalizers that can be programmed depending on the environment Some are simple devices that only boost high frequencies Difficult to use with a telephone Some accept Direct Audio Input, which allows an external source (e.g. a telephone) to connect directly to the hearing aid 7 Hearing Aids Common problems Over amplification Occlusion effect Hollow sound due to ear canal blockage Larsen feedback (whistling) Largely eliminated using digital technology Poor speech discrimination in noise Can only amplify signal - won't work for deafness Economic considerations Cost (per ear): $800-$1500 (analogic) $1200-$3000 (digital) Cost mostly due to service (fitting etc.) Not covered by Medicare Partly covered by Medicaid Only some insurances cover it 8 Assistive Listening Devices (ALD) Amplified telephones Alarms/alerts that emit loud signals, flash and shake bed Directional microphones that allow you to hear the person talking to you in a noisy environment FM or infrared link from stereo/TV or microphone E.g. in class: teacher speaks to mic, signal transmitted via FM to student's ALD Reduces problems with reverberation, noise, distance 9 Telecoils Telecoil-equipped hearing aids can receive electromagnetic signal via an induction coil The signal can be generated by: A room loop: an induction loop (wire) surrounding an audience (e.g., in the floor or in the ceiling), connected to the source of sound (e.g., microphone) A neck loop: a necklace-sized wire loop that can be connected to a radio, TV, some telephones, or an ALD and transmits the signal wirelessle to the coil in the hearing aid A silohuette: works like a neck loop but it is kept behind the ear 10 Cochlear Implant Can be used when the auditory nerve is still working but the inner ear isn't Provides electrical signal directly to the auditory nerve by means of multiple electrodes inserted into the cochlea Sound is collected at the ear level and processed by an external module Or via FM, DAI or telecoil from ALD Processor splits sound up into different nerve electrical impulses Electrical impulses transmitted via external coil to internal coil through the skin Electrodes in the cochlea stimulate different auditory nerve fibers 11 Cochlear Implant Up to 24 electrodes wound through the cochlea, to stimulate the auditory nerve Each electrode stimulates a portion of the cochlea The signals transmitted to the electrodes are matched to the corresponding tonotopic organization E.g., for a sound with low frequency, only the electrodes near the apex are stimulated Note: there are > 15,000 hair cells About 100,000 have received an implant so far Roughly half adult, half children Nearly 3000 with bilateral implant Need to decide which ear to implant The anatomy of the cochlea needs to be intact for the implant 12 Performance Depends on: Quality of technology Cause of hearing impairment Amount of functioning nerve fibers Central processing by the brain Here is an acoustic simulation of cochlear implant Transforms from totally deaf to hard of hearing E.g., many can use the telephone Cost: $45K to $70K (all included) Some of this can be covered by health insurance 13 Windows of Opportunity Children: If implanted early enough, a child's brain can learn to make use of the hearing information Otherwise brain used for other sensory modality FDA guideline: 12 months 6 months with special approval Can be educated in regular schools Most are able to engage in conversation at at or near normal level No upper age limit Better if individual was deaf for a short period of time Otherwise it may be difficult to re-adapt to sound 14 ...
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