28. Ear Function + Testing

Question 1

What is microtia?
What is the innervation of the external ear?
What are the 3 functions of the outer ear?

Answer 1

Microtia - small, underdeveloped outer ear
Innervation: Lateral part (Cartilage) - CN X, V3 (auriculotemporal)
Medial Part (bony) - CN VII, X, V3

Fx: 1. Amplification/Filtering (20dB amplification, strongest at 4000 Hz)

2. Protection
3. Localization of sound/intensity

Question 2

How does the cochlea set up sound wave movement? How does the cochlea perform tuning?

Answer 2

Stapes movement - oval window - vibration in perilymph of scala vestibuli - base to apex of cochlea - vibration in perilymph of scala tympani - apex to base of chochlea - out round window (vest over trousers)
perilymph flow moves membrane of scala media (in b/w)

Tuning: basilar membrane has varying width/stiffness
High frequencies = base of cochlea - stiff membrane
Low frequencies = apex of cochlea - flaccid membrane

Question 3

What is the Central Auditory Nervous System Pathway? Where do crossovers occur?

Answer 3

ECOLI
E - eighth CN (brainstem)
C - cochlear nucleus (brainstem)
O - superior olivary complex (brainstem)
L - lateral meniscus (midbrain)
I - inferior colliculus (midbrain)

Thalamus - Medial Geniculate Body
Temporal Lobe - Primary Auditory Complex

Frequent crossovers in midbrain and up to both sides of brain

Question 4

What are the two different types of hearing loss? What are the two different types of hearing?

Answer 4

Conductive Loss - sound blocked on way to cochlea
Sensorineural Loss - cochlea/CN8 not working

Air Conduction - sound travels through auditory pathway
Bone Conduction - vibrations on bone directly transmit to cochlea

Question 5

What is the difference between Weber and Rinne Tests? How are they reported?

Answer 5

Weber: place tuning fork on middle of forehead or vertex (bone + air conduction test)
Normal = equal to both ears
Sensorineural Loss = lateralizes toward good ear (bad ear dead)
Conductive Loss = lateralizes toward BAD ear (bad ear better bone conduction = louder)

Rinne: place tuning fork on mastoid behind ear (bone conduction), then in air near ear (air conduction)
“+ Rinne” - Air is louder then bone
Sensorineural loss: “+ Rinne” unless dead ear
Conductive Loss: “- Rinne” bone louder than air

Question 6

How to interpret an audiogram?

What are the results of a tympanogram?

Answer 6

Audiogram: headphones for air conduction, headband for bone conduction - find volume at which frequency can be heart - lower on chart = more hearing loss

Tympanogram: check ear pressure, helps establish eustachian tube dysfx
Type A: normal (equal on both sides)
Type As: stiff membrane (tympanosclerosis)
Type Ad: dynamic membrane (post-TM surgery - more flaccid TM)
Type B: blunted, no change in membrane = middle ear full of fluid or TM perforation
Type C: left-shift - negative pressure sucks in eardrum (eustachian tube dysfx)

Question 7

Describe the tests of:
Speech Testing
Acoustic Reflex Test
Otoacoustic Emissions
Child Audiogram Types
Auditory Brainstem Response

Answer 7

Speech Testing: volume when sound heard as speech
Acoustic Reflex: response of stapedius m. to loud sound (reflex: CN 8 - brainstem - both CN 7s - both stapedius m.), absent in severe hearing loss or stapes fixation

OAEs: measure clicks given off by inner ear when cochlea stimulated by a sound - good estimate of conductive hearing in newborns (absence of OAE = block in air conduction or within cochlea, may or may not occur in neural deficits)

child audiograms: behavioral (look for expression changes), visual reinforcement (to lights up if they turn to sound), conditioned play (listening game)

ABR: waveform response measured by surface electrodes, peaks correlate to cell bodies of ECOLI, done in children developmentally delayed or for cochlear implant workup