CS 2015

Audiometry TutorialChristian Stricker

Associate Professor for Systems PhysiologyANUMS/JCSMR - ANU

Christian.Stricker@anu.edu.auhttp://jcsmr.anu.edu.au/~stricker/Audiometry.pptx

THE AUSTRALIAN NATIONAL UNIVERSITY

CS 2015

AimsAt the end of this tutorial students should be able to

• describe afferent anatomical pathways for hearing;

• describe efferent pathways of sensory modulation;

• outline the acustic performance range of the ear in

regard to sound intensities;

• explain how mid-ear changes affect lateralisation;

• locate and discuss how mid-year ossicles are

positioned and affect hearing;

• outline principles involved in sound localisation; and

• interpret simple pathologies revealed in audiogram.

CS 2015

Question 1• Why is the threshold for bone conduction typically

higher than that for air conduction? State the

physical reasons why this is the case.

• Sound wave conduction is inversely related to the

density of the material it is traveling through.

Bone is much denser than air and therefore will

absorb much more energy from the traveling

wave than air.

CS 2015

Question 3• After partially filling the middle ear with serous fluid, how

would bone and air conduction be affected in a case of

early otitis media? How would this change after the otitis

transformed into a purulent and chronic form? Give

physical reasons for your answer.

• Early otitis media: kids typically suffer from hyperacusis –

i.e. they perceive the sound much louder since sound

attenuation is reduced in the middle ear due to water

(serious fluid). As the effusate becomes purulent, it is

becoming more viscous and therefore will dampen the

sound waves much more (hypoacusis).

CS 2015

Question 4• If the left middle ear is damaged, why is a test of

bone conduction lateralized to the left ear?

• Because the sensitivity of the hair cells on the side

of the damaged ear is upregulated due to efferent

innervation; therefore, the same sound intensity

from a tuning fork is perceived to be much louder

in the inner ear of the ear affected by middle ear

damage.

CS 2015

Question 5• Describe/draw the afferent pathways from the inner

ear to the brain that are involved in sound

perception. How many synapses are involved from

the hair cells to the thalamic nucleus?

CS 2015

CS 2015

Question 5• Describe/draw the afferent pathways from the inner ear to

the brain that are involved in sound perception. How many

synapses are involved from the hair cells to the thalamic

nucleus?

• (For details consult a neuroanatomical book).

• The answer to the 2nd part is 4 or 5; depending which

anatomical route is taken. In short, the relay stations are:

Hair cells in organ of Corti → cells of spiral ganglion → cells

of the cochlear nucleus → cells of the lateral lemniscus →

cells of the medial geniculate.

CS 2015

Question 6• Describe/draw the efferent pathways and where the

appropriate nuclei are located in the control of

hearing. State how the activity of these control

neurons affect the perception of sound pressures.

CS 2015

CS 2015

Question 6• Describe/draw the efferent pathways and where the

appropriate nuclei are located in the control of

hearing. State how the activity of these control

neurons affect the perception of sound pressures.

• (For details consult a neuroanatomical book).

• The essence is to be familiar with the olivo-cochlear

reflex. The efferent pathway innervates the outer

hair cells in the cochlea. These then set the

sensitivity of the sound receptors.

CS 2015

Question 7• Draw the pathways for reflectic control of the

middle ear hearing muscles.

• (Again consult a neuroanatomical book).

• Crucial is that from the dorsal superior olivary

nucleus, there are two paths; one to the ventral

cochlear nucleus and the other to the inferior

colliculus from where the innervation of the V and

VII nerve is controlled.

CS 2015

Question 8

• Discuss how sound pressure is

transduced into nerve activity of the

cochlear nerve.

• The hair cell is modulating the firing rate

of the spiral ganglion cell.

CS 2015

Question 9 • Discuss the role of the middle ear in relaying air conduction, in

particular the role of M. tensor tympani and M. stapedius. Why can a

paresis of the N. facialis and/or trigeminus result in a loss of air but

not bone conduction? Also comment on the size of the motor unit in

these two muscles in contrast to for example M. soleus.

• There are two muscles: M. tensor tympani, innervated by V3 moves

the malleus; and the M. stapedius, innervated by VII moves the

stapes.

• If inappropriately innervated, sound is attenuated

• Smallest motor units in the body; one or few muscle fibres controlled

by one motoneurone.

CS 2015

Question 10• Does hypokalaemia affect hearing? Justify your answer by giving the

physiological role of K+ in hearing both in the organ of Corti and the

brain.

• Yes, it does. K+ is essential for the receptor potential of the hair cells.

The apical, sensory surfaces of the hair cells are exposed to the K+-rich

endolymph of the scala media and their lateral and ventral surfaces are

exposed to the perilymph of the scala tympani whose chemical

composition resembles that of other extracellular fluids. The high K+

concentration of the endolymph (150 mM) results from the activity of

electrogenic K+ pumps located in the stria vascularis, which lines the

lateral walls of the cochlea. In hypokalaemia, the receptor potential is

reduced and, therefore, the patient may perceive a hearing loss (not all

patients perceive a hearing loss).

CS 2015

Question 11• Sound localization is a phenomenon on which we rely daily to identify

sources of sound (traffic, conversation), however, the neuronal

mechanisms that allow the localization are not well understood. The

problem is as follows: auditory nerve discharge is maximally around 400

Hz, which would allow a temporal resolution of 2.5 ms between action

potentials. However, the interaural distance is of the order of 20 cm with

a sound velocity of 330 m/s. This results in a maximal timing difference

of ≤ 0.6 ms, which is scaled even more given the angular resolution of a

few degrees. State potential mechanisms that would allow the nervous

system to still perceive this difference even though it is ≥ 4 times faster

than the resolution allowed between two action potentials.

• There is no correct answer to this question. It is a topic of intense

research (delay line? correlation between two ear signal?).

CS 2015

Question 12• The following clinical audiogram was

obtained from a 25 yo man who had a

6-year exposure of loud noises at a

metal factory. He has repeatedly

refused to wear earmuffs thinking that

his ears would tough it out. His

girlfriend had noticed that he on

occasion could not follow normal

conversation, in particular consonants.

Describe and discuss his audiogram.

Are all findings consistent with a single

diagnosis?• Reduction of high frequency perception L > R (C4 reduction) resulting in

“high frequency deafness”. Consistent with single diagnosis (likely more

exposure on the left).

CS 2015

Normal Hearing

CS 2015

Question 13

This is an audiogram of a 50 yo female

who regularly needs her ears checked.

The audiologist commented that the

hearing was the same at an earlier check

some 6 months ago. Describe and

interpret the audiogram and comment on

the female’s ability to listen and

comprehend normal conversation. What

would help this patient?

• Increased sound perception threshold over the whole range, particularly

accentuated for the higher frequencies. Since her perception of speech

is significantly reduced, she is an excellent candidate for a hearing aid.

CS 2015

Question 14This audiogram of a 23 yo male was

obtained 10 days after an altercation in

which the man received a blow to his right

year. The man complained that he partially

lost his ability to locate a sound source in

his office when the background noise was

increased. Describe and interpret the

audiogram and comment on the male’s

ability to listen and comprehend normal

conversation. What would help this

patient?

• Right sound perception threshold lowered over the whole range. If this

persists and since his perception of speech is significantly reduced on

the right ear, he may become a candidate for a one sided hearing aid.

CS 2015

Question 15The following audiogram was obtained

from a 75 yo male. He visited the

audiologist because his family members

complained that he could not follow normal

conversation. Describe and interpret this

audiogram. Comment on the patient’s

ability to listen to normal conversation, if

and how it is different from presbyacusis

and explore avenues by which this person

could be helped. In particular, discuss

factors involved in peripheral and central

forms of hearing loss.

• Bilateral hearing loss, more at higher frequencies. Slightly affected in the

normal speech range. This is presbyacusis. At the moment, does

unlikely require hearing aid (yet…).

CS 2015

Question 2

• For the following sound pressure levels,

calculate the corresponding sound intensities:

a. 25 dB (a whisper):

b. 75 dB (a car horn)

c. 125 dB (Australian hornet flying over the

War Memorial on

ANZAC day)

d. 175 dB (space shuttle launch)

CS 2015

Answer 2• Crucial is the relationship between sound pressure and

intensity levels: where Jx is the

sound intensity sought and J0 is the reference intensity

value of 10-12 Wm-2. We therefore solve for

• Inserting the correct numbers, you obtain for

a. 25 dB:

b. 75 dB:

c. 125 dB:

d. 175 dB:

CS 2015

Question 2’• Explain why even though the sound levels

increase by 50 dB in each case from a – d,

the sound intensities are so different. What

are the implications for the performance of

the human ear?

• The sound intensities grow by 105 from a – d.

This comes simply out of the formalism. The

human ear therefore spans a performance

range over 12 orders of magnitude.

CS 2015

That’s it folks…