CS 2015 Audiometry Tutorial Christian Stricker Associate Professor for Systems Physiology ANUMS/JCSMR - ANU THE AUSTRALIAN NATIONAL UNIVERSITY

CS 2015 Aims At 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

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 2 nd 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

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 V 3 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 (C 4 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 14 This 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 15 The 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 J x is the sound intensity sought and J 0 is the reference intensity value of 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 10 5 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…