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COMD 3700 Basic Audiology
Lesson 11
Acoustic Immittance Testing
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 11 on acoustic
immittance testing. This lesson will cover pages 153-157 in Chapter 7
of your textbook.
2. To this point, we've discussed audiometric tests that give us results
as to a person's ability to hear. We've gotten specific numbers that tell
us about a person's threshold and their ability to understand speech
and other material. We’ve discussed that conductive hearing
problems are defined by the relationship of air-conducted thresholds
and bone-conducted thresholds. Let’s talk about what this means at
the basic level. We measure and compare air-conducted thresholds
and bone-conducted thresholds. Air-conducted threshold
measurements are a measure of how efficient the ear is relative to
moving sound through the air. That is, we’re essentially asking, “What
is the ear doing?” Bone-conducted thresholds are measurements of
the efficiency of sound passage through the bone. So, we’re asking,
“What is the ear capable of doing?” If the ear is doing as well as it can,
then that condition is known as sensorineural, whether the person’s
hearing is normal or not normal. Sensorineural indicates that airconducted thresholds equal bone-conducted thresholds; the ear is
doing what it is supposed to be doing. However, when conductive
problems arise, the ear is not doing everything it is supposed to do.
When there is an air bone gap at a given frequency, we say that there
is a loss of efficiency in that system. In fact, we define conductive loss
as a loss of efficiency—or interruption of sound transmission—in the
mechanical system of the middle ear.
3. So audiometric testing gives us some information about the middle
ear. But if there is a problem in the middle ear system, we need more
testing to determine what might be contributing to the problem. We
do this by conducting acoustic immittance tests. Immittance testing is
used to evaluate the status of the middle ear (from the eardrum to the
in1ner ear) and the neural pathways used to transmit sounds to the
brain. During the immittance testing, we generate a specific activity
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across that membrane, then see what type of pattern the ear exhibits.
Researchers and clinicians have provided us with information about
what a “normal” pattern looks like and have linked different types of
patterns to specific disorders and pathologies. So, when we measure
tympanometric responses of an individual’s ear, we gain information
about the structures of the ear and/or conditions likely to be causing
the interruption of sound transmission. Most important,
tympanometry tells us whether that patient needs to be referred for
medical treatment or medical evaluation; it provides a very quick,
very accurate, physiologic test that helps us understand the condition
of the middle ear. Tympanometry is one of the greatest tools we have
at our disposal. It has a high level of reliability and repeatability, it
gives us important information quickly, and it provides us with fairly
easy-to-interpret results. Immittance measures are used to determine
the middle ear’s ability to transfer sound waves to the inner ears, as
well as assess the impact of middle ear mechanisms that alter this
ability. Mass, mobility, and resistance of the outer and middle ear
systems affect the immittance measures. As with audiometry, each
ear is tested separately. These tests present pressure and/or a tone to
each ear and can help identify problems in the ear canal and middle
ear (for example, fluid in the ear), as well as possibly transmission
errors of neural signals to the brain. We now recognize that otitis
media (OM), the general term for middle ear diseases, is the single
most-common health care problem in children. We have also come to
recognize that chronic, recurrent, middle ear disease can potentially
play a significant role in (or lead to) a delay in the acquisition of
language—which leads to delays in speech, reading skills, and
diminished academic performance. So that is why immittance testing
is so vital to ensuring a proper assessment of the patient. Additional
advantages of immittance testing is that it is non-invasive and nonbehavioral. In fact, the patient doesn’t even have to be conscious or
feeling especially participatory—one more reason why this test is
useful and effective, especially for special populations.
4. For those of you planning on becoming SLP’s this information is
relevant to you because when working with the pediatric population,
you will realize that children are very susceptible to ear infections. It
is in the SLP scope of practice to screen individuals for hearing loss or
middle ear pathology using conventional pure-tone air conduction
methods (including otoscopic inspection), otoacoustic emissions
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screening, and/or screening tympanometry. Tympanometry is
becoming one of the most common procedures for hearing screening
in schools. So it is important that you know how to perform an
immittance (mostly tympanometry) screening, understand the results
and make a proper referral. For those of you choosing to continue in
the field of audiology, you will study immittance testing in greater
detail during your graduate studies. This course is designed to give
you an overview in preparation for advanced learning.
5. Immittance is an all encompassing term to describe measurements
made of tympanic membrane impedance, admittance or compliance.
The word immittance, when applied to measurements at the
tympanic membrane, is a combined form of the words impedance and
admittance. Impedance denotes opposition to something. So if we use
impedance, we'll impede someone's progress. We would interfere
with someone's progress. Acoustic impedance is the opposition to the
flow of acoustic energy to the middle ear. We'll take measures of
impedance in the auditory system and use those measures to tell us
about the status of the auditory system. Opposite of impedance is
admittance. We'll also measure admittance or the flow of energy.
Admittance is the ease of sound flow. It is the acoustic energy passed
by the TM into the middle ear. So, as the flow of energy increases
without impedance, admittance increases. These are reciprocal terms.
As impedance increases, admittance decreases and vice versa. Note
impedance is designated as capital Z, admittance with capital Y.
6. So, as I mentioned, immittance tests help diagnose middle ear
disorders, lesions in the seventh (facial) or eighth (acoustic) cranial
nerve, and Eustachian tube dysfunction. Immittance measures may
indicate not only a middle-ear disorder but also its probable cause.
Acoustic immittance tests evaluate middle ear function by measuring
sound energy’s flow into the ear (admittance) and the opposition to
that flow (impedance). Acoustic immittance tests include static
compliance (which is a sub test of tympanometry), tympanometry,
acoustic reflex testing and Eustachian tube function testing. We will
discuss these tests in greater detail in lessons 12 and 13.
7. To truly understand immittance testing, you need to have
knowledge of the anatomy and physiology of the ear and the basic
physics of sound. This was covered in the ComD 3400, Anatomy and
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Acoustics of Hearing course. However, for those of you who have not
taken the course or may not remember everything from the course, I
will do a quick review. This drawing shows a front view of the right
ear. On the far left side of the illustration is the auricle (the external
ear). Proceeding medially from the concha toward the tympanic
membrane is the external auditory meatus, or ear canal. The outer
third of the ear canal is skin over cartilage, and the inner two-thirds is
skin over bone. Within the ear canal, there is very little fatty tissue,
but there does exist a highly vascular network, as well as a network of
neural end organs (over the bone and under the skin). The ear canal
of most adults is roughly about 1 inch in length from the concha
opening to the tympanic membrane, but the canal can vary in
diameter over that length from about a half to three-quarters of an
inch. The ear canal is not round, but rather ovoid, meaning it changes
over its length. At the medial end of the ear canal is the tympanic
membrane or TM. The TM is not flat, but rather shaped more like a
cone. As you look down the canal toward the TM, the apex of the cone
is pointed away from you. So this means that the “point” of the cone is
the most medial part of the external ear canal. The umbo is located at
this “point”. Attached to the medial surface of the TM are the ossicles:
the malleus, incus, and stapes. Trivia buffs can tell you that these are
the smallest skeletal bones in the body. However, what most lay
people don’t realize is that these bones are fully “adult size” at birth,
and therefore consume a large percentage of the space in the middle
ear of a newborn. A single drop of fluid in the ear of a newborn may
have a major effect on that child, while that same drop of fluid may
have little or no effect in the ear of an adult. The ossicles are not only
connected to the medial wall of the TM, but are also connected to, and
articulate with, each other. Each of these bones is suspended
independently in the middle ear; if you removed one, the others
would retain their position. Together, they perform a mechanical
system that acts as a Class II lever: the ratio of the work that goes in,
compared to that of the work that goes out, is improved. So, when you
deliver sound energy to the TM, it passes directly through and into
the cochlea. This whole surface is connected to the medial surface of
the tympanic membrane and inserts into the oval window. The
footplate of the stapes moves in and out of the oval window so that
the force exerted on the TM is exerted directly through the oval
window.
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8. This drawing is of the middle ear. The purpose of the middle ear is
to enhance the incoming sound wave in order to overcome the higher
impedance of the inner ear fluids, thus allowing the air-borne sound
wave to be transferred into a liquid wave without being deflected. The
key landmarks here are the tympanic membrane, the three ossicular
bones (malleus, incus, stapes), the Eustachian tube, the oval window,
and the round window, and the promontory between them. Sound is
introduced through this middle ear system into the inner ear. It is
introduced via pressure waves at the oval window membrane by the
motion of the footplate of the stapes, which moves in and out of the
recess. The pressure wave is then released back out into the middle
ear space at the round window. So the main mechanism used to
achieve the sound wave transformation is the difference in surface
area of the TM and the stapes footplate. Also, the lever action of the
three middle ear bones adds to the sound pressure at the oval
window. Any alteration to the middle ear system, such as fluid or air
pressure build up in the middle ear cavity or inhibition of the
ossicular chain movements, will result in an inefficient transfer of
acoustic energy through the middle ear. This results in sound waves
being reflected back through the outer ear.
9. To explain this let’s use this schematic of acoustic impedance.
Basically this is a representational picture of sound waves striking the
TM. The piston represents the TM and the black box equals the mass
of all the bones, muscles and ligaments in the middle ear. So when the
sound waves strike the TM, the piston moves the mass of all the
bones, muscles, and ligaments in the middle ear. This mass is
connected to the piston on the left to the spring on the right, which
represents the resistance the mass encounters as it moves. The spring
can be influenced by pressure on the footplate of the stapes or the
stretching or movement of the ligaments and muscles of the middle
ear. The saw tooth wave underneath the lack box represents the
reactance of the system, or conversely, the tendency of the system to
remain at rest. This is the inertia—the tendency of a body to remain at
rest—that has to be overcome, if the mass is to be set in motion. As I
mentioned earlier, when hearing care professionals use the word
impedance, they are referring to this relationship in the flow of
energy. Impedance is the tendency of any system to slow down or
impede the flow of energy moving through it. Compliance, on the
other hand, refers to the facilitation of the flow of energy through a
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system. So, impedance and compliance are essentially two sides of the
same coin.
10. So now let’s take this energy or sound source and introduce it to a
variety of closed cavities representing the middle ear. We need to
understand the concept of equivalent volumes. This shows four boxes
into whic11h four identical sound sources are introduced, and a meter
allows us to measure the sound pressure level (SPL) within each box.
If we put a signal of 85 dB SPL at 226 Hz into a closed cavity of figure
A, it is then possible to adjust the output reading until it reads “0”. So
the center of the dial of the sound level meter is at 0. But if you put
the same sound into a larger box, the sound pressure will be lower.
This is what is happening in figure B. In figure C, the box is much
smaller, making the volume much higher—driving the needle to the
right. Finally, if we let some of the sound leak out like in figure D, the
cavity acts like a larger box, and the sound pressure goes down.
Basically this is what we are doing in tympanometry measurement. In
tympanometry, a probe covered with a soft plastic or silicone probe
tip is inserted into the ear canal, permitting a hermetic seal to form.
This creates a closed cavity that resembles a tube with its two ends
blocked off: the ear canal is the tube, and the ends are the tympanic
membrane (at the medial end) and the probe tip. So, whatever sound
is inserted into this cavity essentially stays in this cavity. This is
happening in all of the examples except for figure D. In this figure
we're losing some sound pressure through that wall because it is in
this case perforated. The gauze is representing a perforated eardrum.
We could lose sound pressure through the membrane. As you can see
there is a relationship between the size of a cavity or volume and
sound pressure levels. Let me give you another example of this, if you
take a drop of blue food coloring and you put it in a small glass. You
would note after a while the color of the water in the small glass. This
would be rather dense. You'd see the blue color very well. Let's take a
larger value and fill the kitchen sink full of water and put a drop in
there like you did the small glass, you may see a slight blue tint. It
would be a lot less dense than in the small glass. Then if we put one
drop of coloring in a full bathtub, I'm certain we wouldn't see any
discoloring of the water because it would have dispersed through the
volume to the point you couldn't detect it. This is a good analogy to
how sound pressure works, if you put a small amount of sound in a
small, medium, and large cavity, you measure more sound pressure in
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the small cavity than in the medium cavity and more sound pressure
in the medium cavity than in the large cavity. Keep that in mind as we
continue to look at the immittance tests and results.
11. When we want to measure the acoustic admittance characteristics
of the ear we use an electroacoustic immittance meter. This is a
diagram of the device. The acoustic immittance of the ear is measured
by inserting an ear piece call the probe tip into the ear canal. The
probe tip is encased in a flexible plastic cuff to create an airtight
connection between the ear canal and the probe tip. This is called a
hermetic seal. The probe tip includes 4 tubes. One tube is connected
to a receiver or a miniature loudspeaker, which is used to deliver a
tone into the ear canal. This sound is called the probe tone. Usually
this is an 85 dB SPL, 226 Hz pure-tone sound. The second tube is
connected to a tiny measuring microphone and is used to monitor the
probe sound within the ear canal. This sound compromises both the
incident wave introduced to the ear from the speaker and the
reflected wave as it returns from the TM. This is how we measure the
sound pressure in the cavity. The third tube is connected to an air
pressure pump and manometer, or a pressure meter. We create and
remove pressure in the cavity to get a dynamic measure of the
movement of the TM. The fourth tube connects to another receiver
used to present stimuli for testing the acoustic reflex. In addition to
the probe tip in one ear, a second earphone goes to the opposite ear
and is used for acoustic reflex tests.
12. The slide shows a picture of the immittance equipment inserted
into the ear of a patient. Again, the equipment includes the probe
tone oscillator and loudspeaker, Monitor microphone, Pressure or air
pump and manometer, ipsilateral reflex oscillator and loudspeaker,
contralateral earphone and probe tip.
13. I want to discuss in greater detail the air pump. Through the air
pump we create and remove pressure in the cavity to get a dynamic
measure of the movement of the TM. By introducing air pressure, we
can cause the TM to move. We can exert positive pressure, pushing
the TM away from us, or negative pressure, creating a partial vacuum
and pulling the TM toward us. The air pump is calibrated in milliliters
(ml), milliliters of water (MM H20), or dekapascals (daPa). The units
of measurements are very similar- 1 daPa = 1.02 mm H20, and 1 mm
H20 = 0.98 daPa-when measurements are made at standardized
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conditions of temperature and pressure. Because they are so similar,
the measurements can be used interchangeably. So you may see
milliliter (ml) in a measurement or milliliters of water (mmho) or
dekapascals (daPa), but it is essentially the same. The devices must be
calibrated so that 0 daPa (or mm H2O or ml) is equal to atmospheric
pressure at the site where measurements are to be made. We have a
limited amount of pressure that can be inserted into an ear when
performing tympanometry, and most testing instruments use +200
ml to -200 ml. How much pressure is this? To give you an idea, a
pneumo-otoscope with a little squeeze bulb can deliver up to 1000 ml
of pressure against the TM. So the ±200 ml of pressure delivered
from the probe tip is not “dangerous” or even seriously uncomfortable
for the patient. While this is not much pressure, it is enough of a
range to obtain essential information on the performance of the
mechanical system that we’ve been discussing. In basic
tympanometry, we insert +200 ml pressure against the TM,
effectively pushing it away from us (into the middle ear space). When
we do that, we make it stiffer. As we make it stiffer, it reflects more
sound back into the cavity and this allows less energy (less sound)
through the TM. So, the sound is soft when we push the TM away
from us. Then, we begin to remove the pressure in the cavity, a bit at a
time. As we do, the TM becomes more compliant, lets more sound
through, and the perception is that the sound gets louder. Each ear is
tested separately, then they are compared to each other and to our
established norms. However, some caution should be applied here.
All immittance measurements are based on inferences generated
from sound pressure level readings taken in the external ear canal.
There is nothing magical about immittance, including tympanometry;
they are simply measures inferred from the SPL measurements in the
ear canal, between the probe tip and the tympanic membrane. We
also need to recognize that these measurements are made in dB SPL.
If the stimulus comes along with the probe tone through the probe
tip, the measurements are in SPL. The bottom line is that we are
looking at the dynamic performance of the TM as we exert positive
and negative pressures upon it, and the subsequent reactions of the
middle ear’s mechanical system in response to TM movement.
14. These are some pictures of immittance equipment in use today. So
this equipment is completely separate from the audiometer. The two
larger pictures are of diagnostic immittance equipment also known as
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a middle ear analyzer or an immittance bridge. It is also called a
tympanometer. The smaller picture on the top right is a screening
tympanometer. It is useful as a portable device that could be taken to
a school screening or health fair.
15. So now that we reviewed a little bit about the anatomy and physics
of sound and the immittance equipment, you are prepared to start
learning about the immittance testing protocol. The first step to the
testing is to prepare the patient. It is easiest if you seat the patient
facing the immittance meter. Immittance testing can be performed
prior to the audiometric testing or following the testing. Different
clinics prefer it to be done before or after knowing the audiometric
results for various reasons. But prior to any testing, an otoscopic
examination should be done on the patient. An otoscopic examination
is just a visual inspection of the outer ear, ear canal and TM. This will
be discussed further in lesson 14. Then you give instructions to the
patient, which I’ll go over in the next slide. Then you select a probe tip
to fit the ear canal tightly. They usually range in size from 4mm to
15mm in size. There are some drawings of probe tips in the corner of
this slide. You must get an air-tight seal. So it’s best to use a tip
slightly larger than you expect. You usually use 9mm-11mm for most
adults and 7mm-9mm for children. Then you place a probe tip on the
end of the probe and insert it into ear canal; you want it to stay in
place without holding it. Pull up and back on the pinna to straighten
the ear canal and rotate the probe slightly when inserting it if
necessary. Now the patient is ready for you to run the immittance
test. In total, tympanometry takes about 5 minutes for both ears of a
patient, and it does not require the patient to be awake or
participatory. In fact, in my opinion, the hardest thing about
tympanometry involves situating the probe tube or picking the right
probe tip to obtain the hermetic seal.
16. Before you begin you need to give the patient instruction to
explain the test and let them know what to expect. Tell them the
purpose of the test and then explain what is going to happen and how
they should respond. This is an example of possible patient
instructions, “This is a test to see how well the ear drum is moving.
I’m going to put a soft plug into your ear canal so that it will fit tightly.
We need to get an air-tight seal. You will hear a tone, which may vary
in loudness. You will feel changes in air pressure. It will feel similar to
when your ears don’t pop when going in an airplane. Try to avoid
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talking, swallowing or yawning while the test is running. You just
need to sit quietly without moving; the test is automatic.”
After you instruct the patient, you will begin testing. We will continue
to discuss the testing procedure and interpreting the results of
immittance testing, focusing on tympanometry, in the next lesson.
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