Lecture 6 Flashcards
Acoustic admittance measures
A set of objective measures of middle ear function
Based on how energy flows through the OE and ME
Admittance
The ease with which energy will flow through it
Measured in acoustic equivalent volume (mls)
Normal = low SPL
Low = high SPL (stiff or massive system)
Excessive = extremely low SPL (floppy system)
Impedance
The opposition to energy flow through it
How much energy is being reflected back
Reciprocal measures
High admittance = low impedance
Low admittance = high impedance
Admittance changes
Otitis media, otosclerosis: energy of reflected wave is greatest when the middle ear system is stuff or immobile (high impedance, low admittance)
Ossicular chain disruption, floppy TM: a flaccid middle ear system will reflect considerably less energy (low impedance, high admittance)
Immittance meter or middle ear analyzer
Little probe unit with 3 openings
- mini loudspeaker
- microphone to measure the SPL of the reflected energy
- air pressure pump to change the pressure in the canal
Tympanogram
Shows changes in admittance of the ME as we force the TM gently in and out no changing the pressure in the outer ear canal
TPP (tympanometric peak pressure)
The air pressure in daPa at which acoustic admittance is at its maximum
ECV
Ear canal volume
SA (peak compensated static admittance)
The difference between admittance at +200 daPa and the maximum admittance
Type B (flat)
Also need to consider ECV:
Normal- otitis media with effusion
Large- perforated ear drum
Small- impacted cerumen
Type A
Normal tympanogram
- noise induced hearing loss
- profound post meningitis deafness
Type C
Far to the left
- retracted TM (ET dysfunction)
Type As
Very small
- ossicular fixation
Type Ad
Off the charts
-disarticulated ossicles
Acoustic reflex
The middle ear muscle contraction which is elicited by fairly loud sounds
Stapedius muscle
Momentarily decreases the ME admittance
Normal threshold: 70-100dBHL
Bilateral response, loud stimulus in one ear will stimulate the reflex in BOTH ears
-named for the ear in which the reflex in initiated, not the ear it was recorded
Ipsilateral reflex
If we stimulate and measure in the same ear, the stimulus and probe ears are the same
Contralateral reflex
If we stimulate in one ear and measure in the opposite ear, the stimulus and probe ears are different
Acoustic reflex responses
Conductive hearing loss: elevation of the AR threshold by the amount of the air-bone gap
Cochlear hearing loss: below 60dB does not affect the AR reflex, above 60 starts to raise the AR threshold
Retrocochlear: highly elevated or no AR response
Otoacoustic emissions
Low intensity sounds generated by action of the outer hair cells, measurable as they emanate into the outer ear canal
Present in: normal ears and ears with cochlear loss up to 30 or 50 dB
Absent in: ears with cochlear loss above noted levels, and ears with conductive loss
Frequencies below 1kHz are hard to assess
Auditory evoked potentials
Changes in neural activity in response to a sound stimulus (electrical activity of the brain stem and brain)
- exist from VIII nerve up to the cortex
Are small voltage, many responses must be added together to see a response come out of the brains background electrical activity
ABR (auditory brain stem response)
An auditory evoked potential
Neural activity occurs in the brain stem 2-10 msec after auditory stimulation
The ABR reflects activity from the VIII nerve to the inferior colliculus
Present 2000 rapid stimuli and average the responses from the electrodes
May take up to an hour for an adult, babies it could take 3 hours
3 uses of ABR
Infant hearing screening
Threshold ABR: infants, look for wave V
Neurological ABR: typically for adults, high intensity click, look for wave V or other abnormalities- high sensitivity to VIII nerve tumours
