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ComD 3700 Basic Audiology
Lesson 6
Pure Tone Audiometry II
Highlighted information refers to a change between the audio
recording (using 10th edition) and the 11th edition of the textbook
1. ComD 3700 for distance education. This is lesson 6, Pure Tone
Audiometry II. This is the second lesson covering pure tone
audiometry. We will be covering chapter 4, pages 81-99.
2. In lesson 5 we learned about the five factors we work with in
audiometric testing: the test equipment, environment, the patient,
the clinician and the test procedure. In this lesson we'll discuss more
specific elements about the testing procedure. We will learn about
proper procedures for air-conduction audiometry, bone-conduction
audiometry and audiogram interpretation.
3. Several techniques to obtain pure-tone thresholds are accepted by
the audiological community, such as the Carhart & Jerger (1959)
modification of the Hughson & Westlake (1944) approach, and the
ANSI (2004) and ASHA (2005) testing methods. These guidelines
present a recommended set of procedures based on existing practice
and research findings. Their intention is not to mandate a single way
of accomplishing a clinical process, but to suggest standard
procedures that in the final analysis should benefit the patient. The
purpose is to improve inter-clinician and inter-clinic comparison of
data, allowing for a more effective transfer of information. The fact
that these methods are not identical, highlights the point that there is
not just one method that is the only right way to test pure tone
thresholds. However, all of these methods agree on the most
important issues, and their similarities reveal a consensus of the
general approach used. The methods we will be reviewing are those
commonly used by most audiologists. The method recommended by
both Carhart & Jerger and ASHA is known as the ascending
technique. Keep that in mind. It can seem confusing because the
procedure involves lowering the intensity as well as increasing the
intensity but overall, the procedure is an ascending approach,
meaning coming from below the client's threshold and intensity and
rising until we find the client's threshold.
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4. The basic procedure for threshold determination consists of first
familiarization with the test signal and then threshold measurement.
The procedure is the same regardless of frequency, output transducer,
or the ear you are testing. Audiologists are encouraged to establish
standard procedures and best practices appropriate to their clinical
population to ensure consistency of approach to each patient and to
minimize the risk of errors.
5. Testing begins by familiarizing the patient with a 1000 Hz test tone
and making a ballpark guesstimate or gross search of approximately
where the threshold might be. The purpose of familiarization is to
assure the clinician that the patient understands and can perform the
response task. Familiarization is a recommended practice for general
populations and should be used whenever warranted by the mental or
physical status of the patient. The participant should be familiarized
with the task before threshold determination by presenting a signal of
sufficient intensity to evoke a clear response. Basically we want to
make sure that the patient understands the task and responds
appropriately, before we start to find threshold. If there are any
problems, this is the time to re-instruct the patient. The following
methods of familiarization is commonly used: A 1000-Hz tone is
presented at a 30 dB hearing level (HL). If a clear response occurs,
begin threshold measurement. If no response occurs, present the tone
at 50 dB HL and at successive additional increments of 10 dB until a
response is obtained. Although this process is usually started at 30
dB, if the clinical history indicates a profound hearing loss, the
audiologist may begin the familiarization process at a higher
presentation level. Actually I have usually seen the testing started at
40 dB, although it isn’t listed that way in any of the formal methods. I
wonder if audiologists just took the 30dB or 50dB starting level
recommendations and decided it might be easier to start at 40dB. I
am not really sure why, but in testing you will see most audiologists
start at 40 dB. I don’t think it really makes any difference on finding
the actual threshold. But I will teach you the textbook method of
starting at 30 dB and if they don’t respond going to 50 dB. If they still
don’t respond the level should be raised in 10 dB steps until they
respond.
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6. After the gross threshold search is complete, then the fine
threshold search begins. The method described, an ascending
technique beginning with an inaudible signal, is recommended as a
standard procedure for manual pure-tone threshold audiometry.
There are some issues that should be standard when testing. First of
all, the tone should be a continuous pure-tone stimuli of 1 to 2
seconds' duration. At the beginning, students worry if they are
presenting the tone for long enough. But really it is an innate thing.
To present a tone to the patient longer to one or two seconds, it's kind
of weird. If you do this in your own mind, and make your own tone
and present it for a long period of time like beeeeeeeep. Doesn't that
seem too long? On the other hand, why would you present a tone like
beep. Doesn't that sound a little short. Of course it does. What we're
trying to do is present a tone of about beeeep. Which is somewhere
around one to two seconds. It’s not something you need to count
while testing, just a guideline. Another suggestion is that the interval
between successive tone presentations shall be varied but not shorter
than the test tone. So it should be at least 1-2 seconds between tones.
It needs to be varied so the patient doesn’t respond to a pattern or
expectation of a sound, rather than a real tone. So now we are ready
to make our first presentation in the threshold search. The level of the
first presentation of the test tone shall be well below the expected
threshold. This is what we determined in the familiarization portion
of the testing and then the tone is decreased from there until the
patient doesn’t respond. Once the patient doesn’t respond then the
level is increased in 5-dB steps until the first response occurs. After
the response, the intensity is decreased 10 dB, and another ascending
series is begun.
7. So, in other words, after a ballpark estimate is obtained, the
threshold search is then begun. This uses the following method: the
threshold should be approached from below, so testing starts at a
level that is known to be below the patient’s threshold. This can
usually done by presenting the tone to 10 dB below where the patient
responded during familiarization. If they still respond, then you need
to keep decreasing the tone by 10 dB until they no longer respond.
Once the patient doesn’t respond, the level of the tone is raised in 5
dB steps until the patient responds. The tone is then decreased by 10
dB and presented again, in which case it should again be inaudible.
This is done so that the threshold can again be approached from
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below. Sometimes a patient will respond at this lower level. When
that happens the tone is decreased another 10 dB and presented, and
so on, until it is audible. The level of the tone is then raised in 5 dB
steps until the patient responds. Steps 2 and 3 are repeated until a
response is obtained 2 out of 3 times.
8. Repeating this process is necessary to achieve the clinical threshold
criteria. We defined this in lesson 4. But as a reminder the clinical
threshold for a tome is generally defined as the lowest hearing level at
which can be heard for at least 50% of the presentations on ascending
runs. The ANSI & ASHA standards require at least two responses at
this level. That can be a little confusing. But at a minimum we have to
find a 50% response level. 2 out of 4 or 4 out of 6. But most clinicians
take 2 out of 3. That’s 2/3, which is more than ½. But that way you
make sure you have the correct threshold. So if you get 2 responses
out of 3 presentations at the same level, that’s the threshold. When
the threshold is found, it is immediately recorded on the audiogram
using the symbols and placement we learned in lesson 4.
9. We are going to review some examples to hopefully help you
understand how to do a threshold search. Basically the pure-tone
testing procedure can be thought of as having 2 parts. First we raise
or lower the intensity of the tone in fairly large steps to quickly find
the ballpark location of the threshold. Once we know the general
location of the threshold we switch to a more formal threshold
determination strategy in which the threshold is approached from
below in 5 dB steps. An easy way to remember how to properly obtain
a threshold during a fine threshold search is to use 2 tactics:
Whenever the patient does not hear the tone, we increase the level of
the next tone by 5 dB or in other words, Up 5 after a no. Whenever
the patient hears the tone we decrease the level of the next tone by 10
dB or down 10 after a yes. It is no wonder this is known as the up-5
down-10 technique. If you memorize that, you will automatically be
following the proper protocol for pure tone air conduction testing.
Now I want you to look at the audiogram to make sure you
understand what we are referring to when we say up and down. It is a
reference to intensity. But this can be confusing if you are thinking it
is the direction of the audiogram. This is a dilemma we run into in
audiology saying above and below or up and down. When we say that
we want to start at a level below a patients’ threshold, we are actually
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moving upward on the audiogram. So you have to remember that if
we move down in intensity, we’re moving toward the top of the
audiogram. If we’re going up in intensity, we’re moving toward the
bottom of the audiogram. Hopefully that didn’t confuse you more. I
just wanted to make you aware of that to make sure we are all on the
same page.
10. This graph is a hypothetical threshold search for a patient.
Hopefully it will help you to picture what we have been discussing.
This is not an audiogram, but the hearing level in decibels is indicated
on the right similar to an audiogram. However the horizontal line
represents each of the individual presentations of a tone, or trials.
The + indicates the patient heard the presentation and a – shows that
the patient did not hear the tone. Notice that the hearing level of a
trial is raised by 5 dB following a no response. This is the up 5 rule.
Then it is lowered by 10 dB following a response. This is the down 10
rule. This causes the clinician to search for responses in a series of
ascending runs. Okay, so we begin by presenting the tone at 30 dB
HL. The patient does not respond, implying that 30 dB was not heard.
This situation is indicated by the – for trial 1 at 30 dB HL. Because
the 30 dB HL starting level was not audible, we increase the level of
the tone to 50 dB for the next trial. This time the patient does
respond, indicated by the + for trial 2 at 50 dB. We can now estimate
that the threshold is between 30 and 50 dB HL. If the patient did not
hear the 50 dB HL tone, we would have raised the level in 10 dB steps
until they did. On the other hand, if the patient heard the tone at the
initial level of 30 dB HL, we would have lowered it in 10 dB steps
until they could no longer hear the tone. In either case, the idea is to
rapidly find the approximate range of the threshold so that we do not
wasted any effort. Because the tone was heard at 50 dB HL in trial 2,
the tone is lowered by 10 dB and is next presented at 40 dB HL in
trial 3. The patient hears the tone at 40 dB in trial 3, so we drop its
level by 10 dB and present it at 30 dB HL in trial 4. The patient
responds to the tone at 30 dB HL in trial 4, so we again reduce its
level by 10 dB and present the tone at 20 dB HL in trial 5. The patient
does not hear the 20 dB HL tone. The rule now tells us to raise the
level by 5 dB to 25 dB HL for trial 6. The tone is not heard at 25 dB
HL in trial 6. So, it is presented 5 dB higher in trial 7. The patient
does not hear the 30 dB HL tone in trial 7. So, the up 5 rule calls for
trial 8 to be presented at 35 dB HL, which is heard by the patient.
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Notice how trial 5 through 8 constitute an ascending run that ends in
a positive response for trial 8. In other words, we have approached
the response from below. The positive response at 35 dB HL in trial 8
means that the tone must be presented at 10 dB lower, at 25 dB HL,
in trial 9. The patient does not hear the tone at 25 dB HL in trial 9 or
at 30 dB HL in trial 10. But they do hear the tone at 35 dB in trial 11.
So, again we have approached a response from below in 5 dB steps,
completing a second ascending run ending in a + outcome at 35 dB
HL. This completes the threshold search for this tone using the
criterion of 2 responses out of 4 presentations and establishes the
threshold at 35 dB. In other words, 35 dB HL is the patient’s
threshold because it is the lowest level at which they responded to the
tone for at least 50% of the presentations, with at least 2 responses at
that level. We would still need one more ascending run if we wanted
to use a 3 response criterion. This is shown is trials 12-14. Because 35
dB HL was heard in trial 11, we now present trial 12 at 25 dB HL,
where we find no response. Trial 13 is then presented at 30 dB HL.
Because the tone is still not heard at 30 dB HL, it is raised again by 5
dB, to be presented at 35 dB HL in trial 14. The patient hears the 35
dB HL tone in trial 14, thus completing a 3rd ascending run. In other
words, 35 dB HL is the lowest level at which the patient responds to
at least 50% of the 6 presentations, with at least 3 responses at that
level. Like I mentioned earlier, most clinicians use the 2 response
method, but I just wanted to show you how a 3 response criterion
would be met.
11. Let’s do this again to make sure you understand. This time the
patient hears the tone at 30 dB HL, so the tone is decreased to 20 dB
HL. Since the patient heard the tone, it is decreased to 10 dB. They
did not respond, so the tone is increased to 15 dB. A response is given,
so the tone is then presented at 5 dB. The patient does not respond at
10 or 15 dB HL, but does respond at 20 dB HL. So the tone is then
presented at 10 dB and increased by 5 dB steps until they respond at
25 dB HL. The tone is then presented at 15 dB, with no response and
then increased to 20 dB. This time the patient responds. So they have
met the 2/3 criteria and the threshold is recorded at 20 dB HL.
12. So, without the chart I want you to be able to picture this process
on the audiogram. I want you to look at his and be able to picture
where the presented tones would be on the audiogram and which
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direction you are moving and why.
13. We have been learning how to perform a threshold search in
general. But you also need to realize that a separate threshold is
needed for every test frequency for both ears and for both air and
bone conduction. Pure tone thresholds are routinely tested separately
for each ear, followed by bone conduction. So, the threshold search
procedure is performed many times for each patient. Clinical pure
tone thresholds using the diagnostic technique are routinely tested in
the frequency range from 250 to 8000 Hz. According to the current
(2005) ASHA standards, threshold assessment should be made at
250, 500, 1000, 2000, 3000, 4000, 6000, and 8000 Hz, except when
a low-frequency hearing loss exists, in which case the hearing
threshold at 125 Hz should also be measured. When a difference of 20
dB or more exists between the threshold values at any two adjacent
octave frequencies from 500 to 2000 Hz, inter-octave measurements
at 750 Hz and 1500 Hz should be made.
14. There is also a proper order to the frequency testing. When
appropriate information is available, the better ear should be tested
first. The initial test frequency should be 1000 Hz. Following the
initial test frequency, the audiologist should test, in order, 2000,
3000, 4000, 6000, and 8000 Hz, followed by a retest of 1000 Hz
before testing 500, 250, and 125 Hz (if needed). A retest at 1000 Hz is
not necessary when testing the second ear. Although the order of
frequencies is not likely to significantly influence test results,
presentation of frequencies in the order described may help ensure
consistency of approach to each test participant and minimize the risk
of errors. So the question is, why retest 1000 Hz? The answer is that
is the most frequently heard frequency. It has the best test, retest
reliability. In other words, if we test 1000 Hz one time then we come
back and retest the second time, those results are probably going to
be closer and more easily reproduced than another frequency. We
really want the patient to perform the same way the second time as
they did the first time. If the threshold is within 5 dB the second time
of the first time we found, that's good test/retest reliability. If the
second trial or threshold difference is 10 dB or more, particularly
more, that's going to present a problem because it's difficult to
understand why the patient would not give you the threshold or
somewhere close to the same threshold the second time. That would
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have to be investigated closely and may require retesting of the other
frequencies as well.
15. After air conduction testing is completed, the testing needs to be
done to determine the bone conduction thresholds. So the
headphones or insert receivers are removed and the bone oscillator is
placed on the mastoid process. A bone conduction test determines the
threshold of the cochlea directly, and establishes if a conductive
component exists at any frequency in the hearing loss of the patient.
Procedures for finding bone conduction thresholds are essentially
identical to air conduction thresholds, with a slight exception. The
differences deal with the frequencies to be tested and whether or not
to test certain frequencies. This might involve a slightly different
approach than air conduction. You recall that with the pure tone
threshold procedure first performed a gross threshold search. They
protocol is to start at 30 dB then 50 dB then go down in 10 dB steps if
we get a response. However, If you're looking at an audiogram where
you have already obtained a pure tone threshold and you note that
the threshold for pure tone audiometry conduction are somewhere
about 25 dB, for example. Then that gives you a hint as to where to
start your bone conduction search. Surely you wouldn't give a first
response at 50 dB or 70 dB. That far above the pure tone threshold.
You would go below and start the search at 15 dB. So by looking at the
pure tone results it helps us understand where to begin with our bone
conduction treatment. Bone conduction testing is performed in the
similar order of 1000, 2000, 4000, retest at 1000, 500 and 250 Hz.
Many audiologists do not perform the 1000 Hz reliability check or
test semi-octaves by bone conduction unless there is reason to do so.
However, a 3000 Hz bone conduction threshold is recommended if
that frequency was tested by air-conduction. Here is an important
point in testing using bone conduction. When you put the receiver on
the right mastoid process and complete an unmasked bone
conduction test for that ear, you may not have a bone conduction
threshold for the right ear. This is due to inter-aural attenuation,
which is the loss of energy of a sound presented by either air
conduction or bone conduction as it travels from the test ear to the
non-test ear. It is the number of decibels lost in cross hearing. We will
review this further when we talk about masking in lesson 7. But I just
want to point out that inter-aural attenuation by bone conduction is 0
dB. So nothing is lost when the signal crosses from the right cochlea
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to the left cochlea. This means you have tested the best cochlea only,
and do not have any idea which ear answered for sure without
masking. So with bone conduction testing, whichever cochlea is better
at that frequency will respond. There is a figure on page 88, figure
4.11 that demonstrates this.
16. Hearing by bone conduction is actually an extremely complex
phenomena. When we stimulate bone conduction, we actually have
three contributors to the total bone conduction perception process.
They are distortional bone conduction, inertial bone conduction and
osseotympanic bone conduction. Distortional bone conduction is a
major contributor to the bone conduction perception. It involves the
inner ear. It is the primary determiner of bone conduction thresholds.
As the skull is set into vibration this causes the bone surrounding the
cochlea to vibrate. This causes a travelling wave identical to that
produced by the tone to be received into the cochlea by way of air
conduction. Another contributor is inertial bone conduction. This
involves the middle ear. As the ear is stimulated by bone conduction,
the ossicular chain suspended in the middle ear lags behind the
stimulation by bone conduction. Therefore, the lagging behind will
cause the stapes footplate to move and give compressions and
rarefactions at the oval window. A minor contributor to the bone
conduction experience is called osseotympanic. Tympanic referring to
the tympanic membrane. Osseous referring to bone. When we
compress and release, compress and release, and expand the bone by
a bone oscillating device, we set up some vibratory motion. If we're
vibrating that bone, it causes oscillations or changes in air pressure in
the ear canal. And now--this is kind of ironic--we have conducted an
air conduction signal from our bone conduction stimulation. That air
conduction simulation goes through the tympanic membrane just as
pure tone stimulation or sounds from our environment goes through
oscillation and increases our perception of sound. But it originates as
a bone conducted phenomenon. It is transduced into an air
conducted phenomenon in the ear canal then passed into the air
canal, into the tympanic membrane, and into the middle ear. So the
three together, compressional or distortional bone conduction,
inertial lag, and osseotympanic bone conduction, they all contribute
to the bone conduction experience. But the distortional bone
conduction is the main player.
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17. I’d like to say a few words about the occlusion effect. Do you recall
our discussion with the tuning fork tests? One was referred to as the
Bing. We took and pressed the tragus in to occlude the ear canal then
released it to unocclude or open the ear canal. We occluded then
unoccluded it. When we unoccluded it, we noticed that if it were a
conductive hearing loss, there was an enhancement of the perception
of the stimulus in terms of intensity. The occlusion effect in the bing
also comes into play in audiometric testing.
When we place an earphone, particularly a supra aural earphone,
we're occluding the external auditory canal with the earphone. A
stronger signal reaches the cochlea when bone conduction signals are
presented with the ears occluded compared with the unoccluded.
What that means is if we present bone conduction stimuli to an ear
occluded by an earphone, there will be an increase of sound delivered
by a bone-conduction vibrator to the cochlea. This is why I said
earlier that when you begin bone conduction testing you need to
remove the earphones first. However, there are times when we need
an earphone in the opposite ear to deliver masking to the non test ear
while we are testing the test ear. So if we're using bone conduction on
the test ear and occluding the non test ear, we have to be aware that
the non test ear might be enhanced with the occlusion effect. As a
result, occluded bone conduction results are lower or better than
occluded ones, and a given bone conduction signal will sound louder
with the ears covered compared with when the ears are open. This
chart shows the amount of increase the OE will produce. It can
increase the values by as much as thirty dB at 250, 20 at 500, 10 dB at
1000. There is not much observable occlusion effect at 2000 or 4000.
This comes to play as we discuss masking for bone conduction and
more as you become a graduate audiologist. But at this level, I just
want you to be aware of it.
18. Once you have found the thresholds and recorded them on the
audiogram, you can begin to interpret the results to determine the
type and severity of hearing loss. Results are looked at for each
frequency in terms of: the amount of hearing loss by air conduction,
the amount of hearing loss by bone conduction and the relationship
between AC and BC. Remember that the outer and middle ears
collectively make up the conductive mechanism, and the cochlea and
auditory nerve compose the sensorineural mechanism, or from a
combination of the two. The entire ear is tested by air conduction
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because the signal from an earphone must be processed through the
outer, middle, and inner ear and the auditory nerve. All of these parts
must be working properly for the air conduction threshold to be
normal, and a problem in any one, or more, of these locations would
cause a hearing loss by air conduction. So, the AC thresholds show the
total amount of hearing loss that is present. It tests the whole ear. But
it can’t distinguish between a problem coming from one part of the
ear vs. another. In contrast, the BC signal bypasses the outer and
middle ears and directly stimulates the cochlea. So BC is considered
to test only the sensorineural mechanism. So, if there is a difference
between the air and bone conduction thresholds, then that implies
that there is a problem with the conductive system. The difference
between the AC thresholds and the BC thresholds at the same
frequency is called an air-bone gap. If you take the threshold of the
AC results and subtract the BC thresholds then that would equal the
ABG. Another way to look at this is that the pathway of the whole ear
minus the sensorineural hearing loss will equal the conductive part of
the hearing loss.
19. Now let’s review some audiograms that would be typical of the
different types of hearing loss. Hopefully the visual imagery,
immediate feedback and information you get from observing these
audiograms will help to put everything together that we have
discussed in this lesson and lesson 5. We will be putting together into
meaningful information what we will use to determine the type of
hearing loss an individual has. From the type of hearing loss, we can
also think about what part of the auditory pathway is causing the
hearing loss. In future lessons we’ll add in determining a disease
associated with that hearing loss. But for now, we’ll look at the air
conduction threshold and bone conduction threshold, and make a
determination as to whether the hearing results are consistent with
normal hearing, conductive hearing loss, sensorineural hearing loss,
or a mixed hearing loss. This is an audiogram depicting normal
hearing. Here we have the air conduction and bone conduction
results from a person. These are text book normal results. This person
has normal AC and BC results. The outer and inner ear are
functioning properly. They don't have a hearing disorder. They have
normal hearing acuity. This is what normal would possibly look like
on the audiogram.
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20. This audiogram shows a mild conductive hearing loss in the right
ear. Mild means that the hearing loss can range from 26 dB to 40 dB.
Conductive hearing loss is when sound can’t reach the inner ear.
Notice that the BC thresholds are higher than AC thresholds, this
means that something is not working right in the middle or outer ear.
In the right ear, when we start the test at the diaphragm of the ear
canal, and send it through the auditory pathway, something in there
is inhibiting the sound from reaching the auditory cortex. When we
put a bone oscillator on the same ear and stimulate it by bone
conduction, then there is normal hearing acuity by way of bone
conduction. Think back to the anatomy we reviewed of the ear canal,
the tympanic membrane, the middle ear, the cochlea, and the nerve to
the brain. If there's an impairment in the outer ear or tympanic
membrane or middle ear, the air conduction can be impaired. If you
stimulate bone conduction and bypass the ear canal and middle ear
and go right to the cochlea and the hearing is normal, then you have
to conclude the problem was in the middle or outer ear because it
affects air conduction, but not bone conduction. Also notice that
There is an Air-Bone Gap in the right ear ranging from 10 dB at 2000
Hz to 35 dB at 250 Hz.
21. Here is a bilateral mild conductive hearing loss. Bilateral means
that both ears have a hearing loss. Conductive hearing loss occurs
with pathology in the outer or middle ear. The bone conduction
thresholds are normal, but air conduction results suggest a decrease
in hearing sensitivity. The patient with a conductive hearing loss
typically demonstrates decreased sensitivity across all frequencies.
Sometimes hearing is better for the higher frequencies than it is for
the lower ones, as seen in this audiogram. Again, you can see the airbone gap in all the frequencies.
22. This is a bilateral mild to moderate sensorineural hearing loss.
Mild to moderate means that the HL can range between 15-70 dB.
Notice that the BC results are the same as the AC results. They are
almost superimposed on each other. That's the indication that you
have a sensorineural hearing loss. If the bone conduction were
separated or above the air conduction by 15dB or greater, we'd have a
mixed or conductive hearing loss. But when air and bone conduction
are within 10 dB of each other, we have a sensorineural hearing loss.
That should be understandable when you think about the anatomy. If
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the inner ear is poor, the air conduction both air and bone conduction
have to be equally poor. This is the case here. We have equally poor
air and bone conduction in both ears indicative of a sensorineural
hearing loss. Audiograms can also be further classified by shape. We
won’t go over the shapes in too much detail in this course, but just
notice the different shapes of sensorineural hearing loss that we’ll
review in the next few slides.
23. Now we have a bilateral sloping mild to profound SNHL. This
type of HL is most often seen in older adults. Because of the severity
of the HL in the HF, this patient may not be able to hear some speech
sounds at all.
24. This is a bilateral mild precipitously sloping profound hearing
loss. Mild precipitously sloping to profound means that hearing loss
is mild for the lower frequencies. But it suddenly gets a lot worse for
the higher frequencies. Patients with this kind of hearing loss can
work just fine in quiet rooms. But they may have a lot more trouble
working in big or noisy rooms. Many people with sensorineural losses
experience a loss only in the high frequency region. These individuals
have no difficulty understanding speech at normal intensities in a
quiet environment since low-frequency hearing is unimpaired.
However, they do experience difficulty in understanding speech in a
noisy environment. Generally, the low frequencies are defined as the
range from 250 Hz to 750 Hz, the middle frequencies as 1,000 Hz to
3,000 Hz, and the high frequencies as 4,000 Hz to 8,000 Hz on the
standard audiogram.
25. This audiogram shows a bilateral profound sensorineural hearing
loss. Profound means that the hearing loss is 90dB or greater. This
means that the patient may not be able to hear anything softer than
90dB. This kind of hearing loss is sometimes called a "Left Corner"
audiogram.
26. The final audiogram is of a mixed hearing loss. A mixed hearing
loss consists of a conductive and a sensorineural component in the
same ear. The patient's results will reflect attributes of both a
conductive and a sensorineural disorder. The pure tone audiometric
pattern for a mixed hearing loss will include bone conduction
thresholds below 20 dB HL at some or all frequencies. A
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sensorineural component is also present in the hearing loss. So, the
sensorineural component is the loss by bone conduction, and the
conductive component is the air-bone gap.
Before leaving this module, I want you to make sure you understand
the procedure for conducting a pure tone air conduction and bone
conduction threshold search to determine a patients’ audiogram. And
then review until you have in your mind a good understanding of
normal, conductive, sensorineural, and mixed hearing losses and
what those conditions look like on this audiogram. If I were to say to
you sensorineural audiogram, can you picture it? What about a
conductive hearing loss? You can picture the bone up near the
audiogram and the air conduction line further down. If it is a mixed
hearing loss, both the air and the bone are impaired. You’ll have an
air bone gap and a sensory component to the hearing loss. Practice
that so you feel good about looking about the types of audiograms, the
four conditions and relating them one to another.
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