Inner Ear - Hearing Flashcards
What is included in the hearing mechanism?
Outer ear, middle ear, inner ear, and central connections
What is included in the inner ear?
Semi circular canals
Cochlea
What is the function of the middle ear?
Transforms acoustic energy from medium of air to medium of fluid
Acts as sound amplifier
How does the middle ear act as a sound amplifier?
Area effect of the tympanic membrane - ratio of TM to staples footplate is 7:1
Lever action of ossicular chain - ration of pressure on staples footplate to on malleus is 1.3:1
Describe otitis media with effusion
Glue ear
Affects the movement of the tympanic membrane
Causes conductive hearing loss
Describe a small perforation and hearing loss
Effect on hearing is variable so may not impact greatly
Main indication for repair is recurrent infections
Describe subtotal perforation and hearing loss
If sensorineural function is maintained then max. hearing loss is 60dB
Describe otosclerosis and hearing loss
Depression of new bone where footplate of staples fits into oval window - lever action lost
Conductive hearing loss
Describe the structure of the cochlea
Pressure wave flows up scala vestibuli from piston action of staples - through helictotrema at apex and down scala tympani
Pressure differential deflects the basilar membrane of scala media
Explain the hearing mechanism in the cochlea
Footplate moves in and out of oval window creating a traveling wave in the scala vestibuli and scala tympani of the cochlea
Causes movement of basilar membrane and movement of inner and outer hair cells in organ of corti
Explain the movement of cilia in the hearing mechanism of cochlea
Cilia of hair cells are deflected by movement and ion channels open
Cations flow from endolymph into hair cells
Depolarisation takes place
Explain depolarisation in hearing mechanism of cochlea
Impulse is then sent up to cochlea nerve
Inner hair cells activate afferent nerves
Outer hair cells modify the response of inner hair cells
Describe the tonotopic arrangement
For every frequency there is specific place on basilar membrane where hair cells are maximally sensitive to that frequency
More sensitive closer to middle ear
Where is the tonotopic arrangement?
Continues through auditory pathway up to acoustic area of temporal lobe (ear to brain)
How is hearing assessed?
Clinical testing
Tuning fork tests
Audiometry
Objective testing
What are the 2 tuning fork tests?
Weber
Rinne
Help differentiate between conductive (blocking/stopping) loss or sensorineural loss (cochlea or nerve)
What is the Weber test?
Test for lateralisation
Tests both ears
Conductive - louder in ear with problem
What is the Rinne test?
Test which compares loudness of perceived air conduction to bone conduction in one ear at a time
Conductive - louder at bone
What are the different types of audiometry?
Pure tone
Visual reinforcement
Play
Tympanometry
What physical properties are useful in assessment of hearing ability?
Frequency and volume
Describe pure tone audiometry
Determines the faintest tones a person can hear at selected pitches from high to low
Headphones are worn
Looking for threshold
What is presbycusis?
High tone hearing loss which is age related
What is the air-bone gap?
Bone conduction perception is greater than air conduction perception - conductive loss
What is is used for objective testing?
Otoacoustic emissions - OAEs
Describe otoacoustic emissions
Sounds given off by inner ear when cochlea is stimulated by sound.
When sound stimulates cochlea - outer hair cells vibrate
This causes nearly inaudible sound which echoes to middle ear and can be measured with small probe
When is the OEA test used?
Often part of new born hearing screening test
Normal - produce emissions
If hearing loss grater than 25-30dB then don’t produce emissions
Describe tympanometry
It assists in detection of fluid in middle ear, perforation of eardrum, and wax blocking the ear canal
How does tympanometry work?
Pushes air pressure into ear canal making eardrum move back and forth
Test measures pressure in middle ear and mobility of eardrum
Can show stiff eardrum, hole in eardrum, or eardrum moves too much
What is the management for hearing loss?
Surgery
Sound amplification
Direct stimulus of cochlear nerve cells
Describe bone anchored hearing aid?
Osseous integrated screw into skull
Microphone and amplifier clip onto abutment or attach by magnet
Sound conducts from skull to cochlea
Requires 50dB sensorineural threshold
When is bone anchored hearing aid (BAHA) used?
When anatomy makes conventional aid difficult
Persistent otitis externa
External canal atresia
Describe a cochlear implant
Very strong hearing aid
Electrode placed inside cochlea
For those with profuse hearing loss
