damaged hair cells & hearing loss
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HIS 150 Objective 1TRANSCRIPT
Damaged Hair Cells & Hearing Loss
Effects of Outer Hair Cell Damage
Outer hair cells are very “mechanical” and they require a lot of oxygen to maintain this mobility.
These highly mobile hair cells are very sensitive to the insults that life throws at them.
Damaged Hair Cells & Hearing Loss
Effects of Outer Hair Cell Damage
If a sensorineural hearing loss is fifty decibels HL or less in degree, it can usually be assumed that the damage is mostly confined to the outer hair cells.
DamagedHair Cells
(mostlyouter)
Fig 1-8, Venema, T. Compression for Clinicians 2nd edition, Thomson Delmar Learning 2006
Damaged Hair Cells & Hearing Loss
Effects of Outer Hair Cell Damage
Outer hair cell damage results in the most common type of hearing loss—a moderate sensorineural hearing loss where soft sounds below conversational speech are inaudible—yet loud sounds are perceived as loud—just as they would to someone with normal hearing.
Damaged Hair Cells & Hearing Loss
Effects of Outer Hair Cell Damage
The Threshold Equalizing Noise (TEN) test has been developed to specifically identify dead hair cell regions.
The stimulation of dead hair cells by amplification can result in an unpleasant distortion artifact experienced by the hearing instrument user.
Damaged Hair Cells & Hearing Loss Effects of Outer Hair Cell Damage
Severe precipitous high frequency sensorineural hearing loss may reflect cochlear “dead spots”.
Generally audiogram regions greater than eighty-five to ninety decibels reflect these “dead hair cell regions”.
Damaged Hair Cells & Hearing Loss
Effects of Outer Hair Cell Damage
When one has dead regions in their cochlea, they may actually respond to sound by means of other “remote” living hair cells. This is sometimes called “off-frequency hearing”.
Damaged Hair Cells & Hearing Loss
Effects of Outer Hair Cell Damage
When turning the decibel level loud enough to obtain a response, it may be so loud that it is actually stimulating a portion of the basilar membrane further toward the apex of the cochlea.
A Traveling Wave
BaseApex
Basilar MembraneFig 1-4, Venema, T. Compression for Clinicians 2nd edition, Thomson Delmar Learning 2006
HF
LF
Damaged Hair Cells & Hearing Loss
Effects of Outer Hair Cell Damage
With the intense high frequency stimulation, the steep traveling wave front will spread into those living mid-frequency hair cells; and those hair cells will respond to the high frequency tone presented.
Damaged Hair Cells & Hearing Loss
Effects of Outer Hair Cell Damage
The “Cookie-bite” sensorineural hearing loss may represent dead regions within the mid-frequency hair cells.
Again, if the HL threshold is below eighty-five to ninety decibels, it is a safe assumption to believe those hair cells are dead.
Damaged Hair Cells & Hearing Loss
Damaged Hair Cells & Hearing Loss
Revealed reverse-slope sensorineural hearing loss would also follow the same rule regarding HL decibel levels of stimulation and dead hair cell regions.
Damaged Hair Cells & Hearing Loss
Damaged Hair Cells & Hearing Loss
Multiple-channel digital hearing instruments can certainly assist in avoiding stimulation of these “dead spot” regions in the cochlea.
Amplification does not sharpen the traveling wave of the basilar membrane.
Hearing Instruments ?They
don’t
grow
new
hair
cells
Damaged Hair Cells & Hearing Loss
Damaged Hair Cells & Hearing Loss
The implications for amplification and cochlear dead regions are important; namely, don’t focus amplification on the very worst thresholds. It is best to amplify at the transition point of the audiogram. We will discuss this in further detail tomorrow.