So, continuing with our puretone audiometry testing, for air conduction that we'll
begin with for most patients. So there's a certain frequency order that we prefer
and that's recommended by ASHA. So we're gonna be testing from 250 to 8,000 Hz in
octave steps, meaning doubling of frequency. So 250 to 500, 500 to 1,000, 1,000 to 2,000,
4,000 and 8,000. ASHA recommends that we start with 1,000 Hz and I believe
it's because 1,000 Hz has a clear tonal quality, and often individuals have
this high frequency sloping hearing loss and their 1,000 Hz thresholds might
be better than the higher frequencies. The low frequencies, some patients might
find difficult to do readily identify to begin with. So again, that's probably the
reason why we start with 1,000 Hz. As I said, we test in octave steps but if
there is a dramatic difference in the frequency threshold between octaves, then
ASHA recommends that we test even mid-octave frequencies, such as 3,000, that's
between 2,000 and 4,000, and 6,000, that's between 4,000 and 8,000 Hz. In some
test protocols, such as that we do for all the ototoxicity when we're
monitoring if there's any hearing loss from antibiotics or chemotherapy, or if you're testing
individuals who have a propensity for noise induced hearing loss, then in the
protocol itself, it might be testing 3000 and 6000 Hz, along with other
frequencies. The reason is
this type of hearing loss might show up,
or might begin in these mid-octave frequencies, so we don't wanna miss those.
That kind of hearing loss. Again, when we're doing puretone audiometry, we're pretty much
sampling the hearing ability at different points along the basilar
membrane. Of course, as with any study, the more the number of samples we have,
better the representation of one's hearing. Of course, considering the time
efficiency, we restrict ourselves to these frequencies, and it's been shown
that that gives a pretty reliable representation of one's hearing. There's
different ways in which we can narrow down to the threshold. Typically, we start
at a low-level, or what we consider level that close to one's hearing threshold for that
frequency, then we go up in steps and try to narrow down the threshold. So ASHA,
again, recommends that we started at 30 dB HL so we would start at 1000 Hz,
30 dB HL and if they don't hear that that tone for two presentations, then we
go up in 20 dB steps for twice and then we go up in 10 dB steps. The
threshold tracking procedure that's recommended is that whenever we don't see
a response, we go up in 10 dB and when you see a response, meaning that the
patient has press the button or raised his hand, we go down by 5 dB steps. So we
do that until we get about 50% of the presentations, we get a
response. So, the threshold is the lowest level at which the patient correctly
identifies three out of six presentations.
For practical purposes, if you see two responses for three
presentations at the lowest level for that frequency, that would be considered
the patient's threshold for that frequency, for that ear. So when we're
beginning to test, we would prefer to test the best ear first, the better ear
first and we can get an idea of which one is the better ear if the patient
reports during his case history that he hears better on one side and has been
having any trouble listening through the other side, or you might get with
experience, you might get some cues by
observing the patient. For instance, the patient might position himself such that his
one ear favors you when you're talking to him during the case history
process. So that'll give you an idea that that ear is probably the better
ear and you want to test, start testing in the better ear first. So once you
get the thresholds based on this frequency order that ASHA recommends,
you'll be plotting them in the audiogram using appropriate symbols. After you've
got thresholds for all those octave frequencies for both ears,
you want to calculate the the pure tone average. And the pure tone average is the
average of the air conduction thresholds for 500, 1,000 and 2,000 Hz. And the
reason why we calculate the pure tone average is because it's been shown to be a
good
estimate of how his hearing levels are for frequencies that are important for
speech understanding. And you'll be calculating this pure tone average for both
ears. So the pure tone average is important to check our validity for
speech results. So another speech testing that you'll do is known as a speech
reception threshold, in other words, you're trying to find a threshold in
which a person can identify speech about 50% of the time. Typically, we use word list
that has spondees, these are two syllabic words which has equal stress on both
words. And to check the validity of your testing, you wanna see a good
correspondence between the pure tone average and the speech reception
threshold. And that's one of the reasons why the pure tone average is also known
as a three frequency speech average, so in some audiograms, you might see that
as called as a three frequency speech average. Another important purpose of the
pure tone average is it also determines the degree of hearing loss. So when we talk
about the audiogram we'll talk about how ASHA recommends a
classification of hearing loss. And when we use a pure tone average to determine what is
a degree of hearing loss,
individually for each ear.
So once you're done with air conduction testing, you move on to test bone
conduction thresholds. Here the bone conduction thresholds are important to
verify the sensorineural sensitivity for the different frequencies. When we talked
about, earlier, about the difference between the air connection, bone conduction, we talked about
how bone conduction sounds kind of bypass the middle ear and the external
ear and directly stimulate the inner ear. So in some conditions where
the hearing loss is originating from a middle ear or an external ear problem, our
bone conduction thresholds are expected to be within normal limits. So
when you compare your air conduction and bone conduction thresholds, one would be
able to determine where the hearing loss is coming from. Earlier, bone conduction
thresholds were assessed using tuning forks but again, for reasons that we had
noted earlier, tuning forks do not provide us a calibrated level of
intensity and they're limited by the number of frequencies you can test. With the advent
of audiometry, now we can reliably estimate the hearing levels for bone conduction
at different frequencies.
So when you're doing the bone conduction audiometry, you place the bone vibrator
behind the pinna on the mastoid bone. Some researchers advocate using the forehead
instead of the mastoid for placing the bone vibrator. However you need to
remember that when you doing a forehead placement of the bone vibrator, the threshold
you get might be 10 dB poorer than what you would estimate using a mastoid
placement for any given frequency. Again, we also talked about when we talked about
earlier audiometers, how there is a maximum testable intensity for bone conduction,
for bone vibrators because of the limitations of the audiometer. So when you're doing the
forehead placement and the threshold you get is 10 dB poorer, it kind of limits us again, how
far we can estimate the bone conduction thresholds, in the case that this person
has a severe or profound hearing loss. But sometimes you might have to resort
to using a forehead placement, especially with young children and it could be
because most bone vibrators come with a standard size band and with really
small heads, you may not be able to place the bone vibrator correctly. And also, there are some
special cases where like for instance, if it's a congenital syndrome with
some malformations of the head,
you may not be able to place the typical bone vibrator, and also in cases where
the patient has had recent surgery in the middle ear or in their mastoid, so they
might have scar tissue or they might still have inflammation behind the ears. So
that would preclude you from using a bone vibrator on the mastoid, so you
might have to use the forehead.
You need to realize that when you're testing with a bone vibrator, testing bone conduction, the better
cochlea responds, no matter where you place the bone vibrator. Typically, when you
use a earphone or an insert earphone, there is this loss of energy when sound
travels from one side to another, and this is what we call as the interaural
attenuation. Attenuation, any reduction of sound when it travels from one side to
another. And it varies depending upon what kind of air conduction transducer
you use. The problem with the bone vibrator is
the interaural attenuation is almost close to zero. In other words, whatever you giving
in one side, whatever energy you're giving in one side, travels through the bone of the
skull to the other side. The problem with that is now you don't know which ear is
responding, in other words, you don't get ear specific results. So in this case,
you might have the normal hearing side, normal hearing ear, while the other ear
might be completely deaf or profoundly hearing impaired. However, if you would
place a bone vibrator around the deaf side, the person might actually respond
to very low levels because that sound is traveling to the normal side and the
patient responds that he heard it. When you're doing bone conduction testing, the only way we can
get ear specific results, and if you want to get ear specific results, you will
have to do this procedure known as masking. Masking is but a way to engage the better
ear,
while you're testing the poorer ear. The way you do that is you try to give a stimulus,
like narrowband noise or a speech noise and engage one ear, while giving
the tones to the other ear. So you're giving this noise to the non-test ear,
while you're testing the test ear, which is typically the poorer ear. In this course and the audiograms we'll
talking about, we're not gonna be talking in detail about masking, but I just wanted
you guys to know about what it entails to do
masking. When you're doing bone conduction thresholds or when you're trying to estimate
bone conduction thresholds, you use a tracking procedure similar to air conduction.
You test from 250 to 4,000 Hz, you don't test at 8,000 Hz. So
you test 250, 500, 1000, 2000 and 4000 Hz, and you plot those thresholds on the
audiogram using the appropriate symbols.
Unmasked bone conduction thresholds, you need to make sure that the ears are
uncovered or open. The reason for that is a phenomenon known as the occlusion effect.
So when ears that are normal and the ears with sensorineural hearing loss, in other words,
if the hearing loss is originating from the cochlea or the auditory nerve, when you're occluding the ear
with the eaphone or with a simple ear plug, there is an increase
in the intensity of the sound that's delivered by the bone conduction vibrator.
And that's what we call as the occlusion effect. Since there's an increase of
the sound, you get lower thresholds if the ears are occluded.
The reason for that is when you occlude the external auditory canal, it
traps the bone conducted vibrations in the space between the occlusion, or between the ear plug
and the tympanic membrane. Normally, there's a certain amount of the bone
conducted sounds escaping through an open ear canal, but now when we occlude it,
the sounds get trapped inside and we know from our basic acoustics, that as volume
of a space decreases, the pressure increases, the sound of the pressure increases. So
that results in better thresholds, so it seems like the sounds are louder. And
this happens predominantly for the lower frequencies, frequencies that are lower than
1,000 Hz, but the increase or the improvement in the threshold, can be as
much as 20 dB lower. And it also depends upon how deep the insertion is. In this graph,
you're seeing that with deeper insertion of this earplug, the amplitude of the
sound is higher. What's interesting is you don't see this occlusion effect, in
other words, a difference between hearing when your ears are open and when your ears are
occluded in ears that have a conductive hearing loss.
The reason is when you having a conductive hearing loss, the occlusion effect is
in place already. In other words, whatever improvement you expect to see with bone
conduction is already in place, so you doing another occlusion does not make a
difference. In fact, that's been used in some tuning fork tests where they place
the tuning fork, vibrating tuning fork on the forehead and physician or the audiologist
might ask the patient if there's any change in the loudness when they're
manually closing and opening the ear canal.
For those who have a sensorineural hearing loss and if they can hear that
torn, they would report that when they close the ear canal, by pressing on the tragus,
there's an increase in the loudness of the sound. But if the patient has a
conductive hearing loss, such as if there's fluid in the middle
ear or there's a perforation in the eardrum, they don't experience any difference
in the loudness. That was used in earlier days for differential diagnosis of a
conductive and sensorineural hearing loss. And as I said, this increase in the
loudness is predominantly for the lower frequencies. You might have experienced
that, you might have noticed that when you have your ears closed or plugged,
you chewing sounds louder and your voice sounds more booming. And again, that's
because of the increase in the loudness of the lower pitches, lower frequencies
giving you that effect that the voice is more booming.