Audiology Lesson 3: Hearing Tests Flashcards
Discuss Sound, sound pressure, frequency and decibels
Sound waves are small fluctuations of atmospheric air pressure caused by the alternating condensation and rarefaction of atoms and molecules.
Sound pressure is the amplitude of pressure variations -
measured in Pascal. However in audiology we use decibel to express sound pressure. Decibel (dB) is a logarithmic unit.
Frequency is the number of occurrences of a repeating event per unit of time - in Hertz (Hz)
What is the Decibel hearing threshold level?
If for each frequency we set P0 equal to the minimum audible sound at that frequncy, we have obtained the dB hearing threshold for our patient.
dB HL is used in pure tone audiometry, which is one of the most common tests performed in audiology
Note: P0 = reference pressure 0.0002 Pascal
Describe the 2 ways to hear
Air conduction:
• the normal way
• Involves the external ear, middle ear and the cochlear and retro-cochlear structures
Bone conduction:
• by placing an audio processor over the skull, we bypass the external and middle ear and provide a direct stimulation of the cochlear hair cells
Fully describe Pure Tone Audiometry and the two ways to hear
The technique of Pure tone audiometry: an electric tone generator is used to determine the “hearing threshold” for pure tones.
We check two types of hearing thresholds:
a. The air-hearing threshold is evaluated by means of earphones
b. The bone-hearing threshold (hair cells of the cochlea) is evaluated by means of bone audio processor placed over the skull.
The results are expressed in a graph
o frequencies expressed in Hz on the horizontal axis, while on the vertical axis we have the dB HL
o The red color stands for the right ear, while the blue/black color stands for the left ear
o When we are evaluating the air-hearing threshold, the signs are circles for the right ear and crosses for the left ear
o When we are evaluating the bone-hearing threshold, the signs are a blue arrow for the left ear and a red arrow for the right ear
Describe conductive hearing loss
A patient has conductive hearing loss when they have a problem in the conductive system (external and middle ear)
The bone-hearing threshold is normal (as conductive system is bypassed)
o Stimulation of the cochlear system by an audio processor that bypasses the conductive system will show normal result
o Evaluation of the hearing threshold through the normal airway will show a deficit
o a gap between the air-hearing threshold and the bone-hearing threshold is the expression of a conductive deficit
o e.g. earwax
Can be a maximum of -60dB hearing loss
Describe sensorineural hearing loss
Problem is at the level of the cochlea or retrocochlear structures.
- No difference between the air-hearing threshold and the bone-hearing threshold
- the nervous system is the problem and not the conductive system
- e.g. Hearing loss of older age
Describe Mixed hearing loss
o conductive and a sensorineural hearing loss
o e.g. chronic middle otitis
Note: Remember it’s not possible to have a bone hearing threshold higher than an air hearing threshold
State the degrees of hearing loss by hearing threshold
Normal: 0-10 dB Mild hearing loss: when the threshold is between 20 and 40 dB Medium: between 40 and 70 dB Severe: between 70 and 90 dB Profound: between 90 and 120 dB Complete
Fully describe Impedance Audiometry (basis and how it is performed)
It is an OBJECTIVE TEST - doesn’t require patient’s attention. Very important to reveal the site of the impairment, and tell us if we have a conduction problem or not.
Impedance is the resistance to the flow of sound pressure waves through a medium
• Resistance - the frictional resistance in the joints, ligaments and muscles of the sound-conducting system.
• Reactance - represents the mass and stiffness of the sound conducting system.
Compliance is the flexibility of the sound conductive system (the opposite of impedance)
How the test is performed:
o A probe, composed of three tubes, is placed in the external acoustic meatus, hermetically closing it.
i. the first one carries the tone probe, which is a tone of about 220 Hz
Part of the energy of the tone probe is absorbed by the system and part of it is reflected
ii. The reflected part is captured and measured by a microphone, which is carried by the second tube.
If the system presents with high impedance, most of the energy is reflected.
If the system presents with low impedance, most of the energy is absorbed.
iii. The third tube is connected to a pressure regulator that changes the pressure of the system from -400 mmH2O to +400 mmH2O.
impedance audiometry is a dynamic test because we are able to change the pressure of the system
The test is made up of TWO parts: tympanometry and stapedius reflex.
Describe Typmpanometry
Evaluation of the compliance changes due to a change in pressure (created by the 3rd probe) and plot the results on a curve.
The results are expressed in a graph where in the horizontal axis we have the pressure (mmH2O), and on the vertical axis the compliance
There are three possible results:
i. In the case of a normal compliance, the peak of our curve is comprised between -100mmH2O and +100mmH2O (type A tympanogram - middle)
This curve is called tympanogram type A
ii. If we have a mild impairment of the system, the result is a curve where the peak is lower than -100mmH2O (type C - middle)
e.g. fluids in the middle ear
iii. If we have a severe impairment of our system, the result is a flat tympanogram (type B - on the right)
the system is completely blocked
e.g. otosclerosis (abnormal growth of bone near the middle ear)
Describe the Stapedial Reflex, how it’s generated and all the situations in which it is abnormal
The stapedial reflex decreases the transmission of vibrational energy to the cochlea by responding to high intensity sounds by contracting the stapedius and tensor tympani muscles.
It’s composed of afferent and efferent pathways
The afferent pathway is composed of:
1. the cochlear nerve
2. cochlear nucleus
The efferent pathway is composed of
the stapedial nerve (a branch of the facial nerve)
provides innervation to the stapedius muscle
- The integration of the signals occurs in the superior olivary complex in the brainstem
- The stapedial reflex is a bilateral reflex - stimulation of one ear will induce the reflex also in the opposite one.
- We can observe the stapedial reflex if a sound is higher than 70-80 dB over the hearing threshold
ABNORMAL IN THE FOLLOWING CONDITIONS
Middle ear disorders
the stapedial reflex will be absent if the system is completely blocked
Cochlear lesions
a positive Metz’s Test or reduction of the stapedial reflex threshold
Auditory nerve lesion
a positive Reflex Decay Test or an absent stapedial reflex
Involvement of the brainstem
reflex present on the ipsilateral side but absent on the contralateral side
Facial nerve lesion
stapedial reflex is absent
Atrophy of stapedial muscle (very rare, 1% of population)
stapedial reflex is absent bilaterally.
Explain Metz’s test
The decrease in the stapedial reflex threshold. The normal stapedius reflex threshold is equal to the hearing threshold + 70~80 dB HL.
When there is cochlea damage - a phenomenon called ‘recuitment’ occurs and lowers the threshold (e.g. to +30/40)
A positive Metz’s test helps us understand that the site of the lesion is the cochlea (damaged hair cells)
Explain the Reflex decay test (aka Anderson’s Test)
A (loud) test tone is presented for 10 seconds at a stimulus level of 10 dB above the individual reflex thresholds of 500 and 1000 Hz
o The result is the expression of the stapedial reflex
o In case of an auditory nerve lesion - the response amplitude declines by more than 50% in 5 seconds.
Pathologic auditory fatigue = decreasing of loudness when a sound is presented continuously
Now describe how to precisely locate a type of sensorineural hearing loss
By means of pure tone audiometry we can record a sensorineural hearing loss
o it means that the problem can be in:
i. the cochlea
ii. the auditory nerve
iii. another part of the auditory pathway
The question remains ‘where is the problem located?’
• positive Metz test - problem is mainly in the cochlea
• positive Anderson test - problem is in the cochlear nerve
Compare Auditory Evoked Potentials and EEG and describe the purpose of AEPs (particularly ABR)
EEG - the expression of the spontaneous and random electrical activity of the CNS
AEP - the expression of the electrical activity in response to an acoustic stimulation
The AEP response reflects the auditory response of an individual. The most common AEP test is the ABR - Auditory brainstem potential.
Purposes:
- Objective measurements of hearing threshold (legal reasons for adults and for finding hearing loss in kids)
- Finding the side of auditory lesion (between cochlear and retro-cochlear lesions
- Intraoperative test to measure hearing loss during neural surgery
or in MS diagnosis
