Download ComD 3700 Basic Audiology Lesson 13 Acoustic Reflex Highlighted

ComD 3700 Basic Audiology
Lesson 13
Acoustic Reflex
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 13 on acoustic
reflexes. This lesson will cover pages 162-167 in Chapter 6 of your
textbook.
2. We have been discussing immittance testing. We learned about
static compliance and tympanometry. The last immittance test we will
cover in this course is acoustic reflex testing. Acoustic reflex testing
consists of threshold testing as well as decay testing. However, we will
only be discussing the acoustic reflex threshold test in this course. We
will also not be covering testing for Eustachian tube dysfunction in
this course. I just want to mention briefly that the presence of ET
dysfunction can be determined subjectively through a history of
patient symptoms or objectively by means of direct or indirect
measures. In the most basic sense, Eustachian tube function can be
assessed with conventional tympanometry. As we discussed in lesson
12, if the tympanogram demonstrates greater than -250 daPa middle
ear pressure ET function is considered abnormal.
3. In the last lesson we discussed tympanometry. The diagnostic
immittance machines we discussed are also used for acoustic reflex
testing. After the tympanometry testing is completed, then the next
test administered is the acoustic reflex. So the patient stays seated
with the probe in their ear. The same probe tip that was used for
tympanometry is used for acoustic reflex testing. In addition a
headphone or insert earphone is placed in the opposite ear. On this
slide are 2 examples of an immittance machine where you can see the
probe tip for tympanometry and acoustic reflex testing and also the
earphone or headphone that is placed in or over the opposite ear.
Most of the time the patient is set up initially with the insert earphone
in the opposite ear from the probe tip. Then after the tympanometry
testing, acoustic reflex testing is started. You should always conduct a
tympanometric test before making any acoustic reflex measurements.
Some machines automatically perform the acoustic reflex test
following the tympanogram. The patient instructions for the AR
testing would be similar to the tympanometry testing. You should
inform the patient that they will hear a series of beeps or tones in
their ear. They do not need to respond to the tones in any way and
should try not to move or startle during the loud measurement.
4. Before we can begin to study the acoustic reflex, you have to
understand the anatomy and physiology associated with the acoustic
reflex threshold. This includes the outer, middle and inner ear
structure, the vestibulocochlear (VIIIth cranial nerve) and structures
of the central auditory nervous system (CANS); specifically the
auditory structures located at the level of the lower brainstem
including the cochlear nuclei, superior olivary complex and facial
nerve (VIIth nerve) nuclei. If you have taken COMD 3400 or another
anatomy of hearing course, then you will have studied this. It is
important to remember that there are two muscles associated with
the middle ear, the tensor tympani and the stapedius muscle. The
facial nerve innervates the stapedius muscle and the stapedius muscle
contraction is responsible for the acoustic reflex threshold.
5. The acoustic reflex is defined as the contraction of one or both of
the middle ear muscles in response to an intense sound. The
contraction of these muscles results in increased impedance or
lowered compliance. It is important to understand the nature of a
reflex. A reflex occurs below the level of cognitive control. In other
words, we do not have to think about it. For example, this is why you
pull your hand away quickly when you touch a hot stove. You don’t
have to first think, “Wow, my hand is burning”. Your reflexes pull
your hand away instantly. In this manner, the stapedius muscle
contracts in response to a loud sound, even though the patient does
not control the action. This is to protect the ear from the damage
associated with loud sounds. As I mentioned earlier, there are two
muscles associated with the middle ear, the tensor tympani and the
stapedius muscle. When a loud sound enters a normal ear, the
stapedius muscle will contract on both sides, regardless of which ear
is stimulated. So, the AR is a bilateral (two side) reflex. Adequate
stimulation in one ear results in an acoustic reflex occurring in both
ears at the same time. There is a very small time difference. But for all
intents and purposes, stimulus in one ear results in a response in both
ears at the same time. Therefore, in AR testing we measure the
pathway stimulated in both ears. The words ipsilateral and
contralateral are used to report the test results. The ear with the
immittance probe is called the probe ear, while the ear receiving the
stimulus is called the stimulus ear. The word ipsilateral, sometimes
referred to as ipsi, means the same side. The probe ear and the
stimulus ear are one in the same. The stimulus and the measurement
occur in the same ear. Contralateral, or contra, means opposite side.
The probe ear and the stimulus ear are different. The stimulus is
presented to one ear, while measurements occur in the opposite ear.
So, these terms refer to where the AR is measured relative to where
the loud sound is presented. If the AR is measured on the same side
in which a loud sound is presented, then it is an ipsilateral AR. If the
signal is measured in the opposite side to that in which the loud
sound is presented, then it is a contralateral AR. The results are
reported by the ear that is stimulated by the signal.
6. One additional way to remember this is, if the tone is presented on
the probe side, then it is an ipsilateral AR. If the tone is presented on
the earphone (which may be an insert earphone) side, it is considered
a contralateral AR. So, right and left results in AR testing refer to the
ear that is stimulated by the loud sound. If the signal goes into the
right ear and the AR is measured in the right ear then it is called a
right ipsi AR. If the signal goes into the right ear and the AR is
measured in the left ear, it is called the right contralateral AR. This
can be confusing when reviewing the results. Remember that the
results refer to the stimulated ear. So in this “patient” we are testing
the right ipsilateral ART and the left contralateral ART. We will
discuss this in more detail when we review the testing process.
7. To understand the implications of AR testing, it is important to
have a basic knowledge of the acoustic reflex pathway also called the
acoustic reflex arc. This is the path of the acoustic stimulus. There is a
diagram on page 164 of your textbook of this pathway. To me, it is
easier to understand when viewed as a model like I posted on this
slide. This is the acoustic reflex pathway. It bears just about no
resemblance to the actual anatomical structures which are extremely
small, three-dimensional and much more complex in terms of the
nerve projections. However, this figure illustrates the main AR
pathways and most of the key structures. The stimulus ascends from
the outer ear to the brainstem and then descends via the facial nerve
on both sides of the head to innervate the stapedius muscles in both
middle ears. In this simple model of the acoustic reflex pathway the
acronyms are as follows: ME = middle ear, IE = inner ear, VIII =
vestibulocochlear or auditory nerve, CN = cochlear nucleus, SOC =
superior olivary complex, VII = facial nerve. It is important to note
that two of the structures in the pons (SOC and VII nucleus) are
shown together for simplicity. The book just refers to the SOC at this
level. They are actually separate structures. A branch of the facial
nerve ends at the stapedius muscle and the stapes is shown as a
stirrup-shaped stick figure. So, imagine first a normal right ear and
trace the pathway of a loud signal. The signal enters the right ear,
travels through the outer, middle (ME) and inner ear (IE), along the
VIII nerve, to the brainstem. When the signal reaches the brainstem,
the signal arrives first at the cochlear nucleus (CN). From here, the
signal travels to both right and left superior olivary complexes and
both right and left facial nerve (VII) nuclei. The signal is sent from
both facial nerve nuclei to both facial (VII) nerves, which results in a
contraction of both stapedius muscles. Thus, both stapes bones are
pulled outward and downward, in a direction away from the inner
ear. This action makes it harder for energy to travel through the
middle ear (increase in impedance/decrease in admittance). The
lowest intensity level at which this contraction is measurable is the
Acoustic Reflex Threshold (ART). We will study the ART after we
study the various pathways in greater detail.
8. This model represents the right ipsilateral pathway. Remember
that in ipsilateral testing the stimulus and the measurement occur in
the same ear. So the stimulus is presented in the right ear through the
probe, it travels through the outer, middle (ME) and inner ear (IE),
along the VIII nerve, to cochlear nucleus (CN). The signal then travels
to the right superior olivary complex and right facial nerve (VII)
nerves. It is then measured by the probe as a contraction made by the
right stapedius muscle in response to the sound.
9. In the right contralateral pathway, the probe ear and the stimulus
ear are different. The stimulus is presented to the right ear through
the earphone, while measurements occur in the left ear. So the signal
enters the right ear, travels through the outer, middle (ME) and inner
ear (IE), along the VIII nerve, to the cochlear nucleus (CN). From
here, the signal travels to the left superior olivary complex and is sent
from the left facial (VII) nerve, to the left middle ear, which results in
a contraction of the stapedius muscle. The result is then recorded by
the probe placed in the left ear.
10. The left ipsilateral pathway will be the same as the right (except
on the other side). So the stimulus is presented in the left ear, it
travels through the outer, middle (ME) and inner ear (IE), along the
VIII nerve, to cochlear nucleus (CN). The signal then travels to the
left superior olivary complex and left facial nerve (VII) nerves. It is
then measured as a contraction made by the left stapedius muscle in
response to the sound.
11.The final scenario is the left contralateral pathway. Here the
stimulus is presented to the left ear through the earphone, while
measurements occur in the right ear. So the signal enters the left ear,
travels through the outer, middle (ME) and inner ear (IE), along the
VIII nerve, to the cochlear nucleus (CN). From here, the signal travels
to the right superior olivary complex and is sent from the right facial
(VII) nerve, to the right middle ear, which results in a contraction of
the stapedius muscle. The result is then recorded by the probe placed
in the right ear. Now that we have learned the reflex pathway, we will
discuss the specific reflex tests.
12. When conducting AR testing, the first test we will perform is the
Acoustic Reflex Threshold (ART). As I mentioned, this is the lowest
level at which an AR can be obtained. So similar to finding the
threshold for pure tone air and bone conduction testing and speech
testing, we will now be finding the threshold for the acoustic reflex
response. Of course, the difference is that this threshold test is
objective. An acoustic reflex threshold is a middle ear measurement
of stapedius muscle response to higher intensity and adequate
duration sounds for individual frequencies. Consider the softest
sound that elicits a reflex contraction of the stapedius muscle as the
acoustic reflex threshold. When the stapedius muscle contracts in
response to a loud sound, that contraction changes the middle ear
immittance. This change in immittance can be detected as a
deflection in the recording. We have learned that tympanometry
records changes in middle ear immittance, while air pressure is varied
in the ear canal. Acoustic reflexes are recorded at a single air pressure
setting. It is tested at the pressure setting that provided the peak
immittance reading for that particular ear on the tympanogram. Ear
canal pressure is maintained at that specific setting, while tones of
various intensities are presented into the ear canal and immittance is
recorded. A significant change in middle ear immittance immediately
after the stimulus is considered an acoustic reflex. Acoustic reflex
thresholds generally are determined in response to stimuli of 500,
1000, 2000, and 4000 Hz. For screening purposes, or for a general
check of the pathway's integrity, clinicians usually test at 1000
Hz.
The book states that the normal acoustic reflex responses occur
with a stimulus of 85 to 100 dB SPL. To determine the ART, a pure
tone signal is introduced at 70 dB. If no compliance change is
measured, then the level is raised in 5 dB steps until a response is
seen. The highest level at which the AR should be tested is 115 dB HL.
If there is a measurable response at 70 dB, then the level is lowered
by 10 dB until no response is measured and then raised in 5 dB steps
until the threshold is determined. The criteria to constitute a
response varies depending on the equipment you are using. But a
response amplitude of .02 ml - .03 is very commonly used. The ART
results can help in provide information about the site of pathology in
a hearing loss, the approximation of degree and type of hearing loss
in non-cooperative patients, the facial nerve integrity and more. The
AR is used to help differentiate cochlear from retrocochlear
pathology. Retrocochlear refers to abnormalities along the auditory
pathway beyond the cochlea, including the auditory (VIIIth cranial)
nerve or within the auditory brainstem. Other tests are necessary for
confirmation of a site of lesion suggested by the AR. One can suspect,
but cannot diagnose, site of lesion based solely on the results of AR
testing.
13. Hopefully seeing an actual printout of the ART responses will help
make this clearer. This is one example of the ART results. As you can
see the top row is being tested using ipsilateral (same ear) stimulus
and the bottom row represents contralateral (opposite ear)
stimulation. The stimulus frequency is 1000 Hz. The probe is in the
right ear. The stimulus intensity is listed for each presentation and
the response amplitude is listed below the presentation.
14. So, in this test, the probe is in the left ear. We are testing 1000 Hz.
Testing begins at 80 dB HL. A response amplitude of .05 is measured,
so the stimulus is lowered to 75 dB and a response of .03 is measured.
0.2 ml is considered a pass for this equipment, so the stimulus is
lowered to 70 dB. This time the response amplitude is only .01 ml.
This is not considered a response, so the intensity is raised to 75 dB
and a response of .02 ml is measured. 75 dB would now be considered
the ART for the left ipsilateral test at 1000 Hz. The patient would
have heard 4 tones or beeps in their ear during the testing, but would
be unaware of any reflex occurring or measurements being made.
Next contralateral testing would begin. On this printout, the results
are reported by the probe ear, so the left ear is still reported as the
probe ear. But this is actually a right contralateral AR. The stimulus is
presented from the right earphone, although the probe tone is still
heard in the left ear though the probe. The change in compliance is
measured in the probe in the left ear. So at 80 dB the response
amplitude is .04 ml. So the level is lowered to 75 dB, but a response of
.01 is not an adequate response, so it is raised to 80 dB HL. A
response of .03 is recorded. So the right contralateral ART is 80 dB at
1000 Hz for this patient.
15. The results are usually recorded on a graph located on the
audiometric worksheet. So this patient had ipsilateral reflexes at 85
dB at 1000 Hz and at 80 dB at 2000 Hz in the left ear. The picture is
representing that the probe is placed in the left ear. The stimulus is
also in the left ear. The left contralateral results are 100 dB at 500 Hz,
90 dB at 1000 Hz, 85 dB at 2000 Hz and 90 dB at 4000 Hz. Note that
the picture shows the probe in the right ear, but the stimulus is being
presented through the left earphone and measured in the probe in the
right ear. So this is the left contralateral response. This is important
to remember before we move on.
16. Here are 2 more examples of how the ART may be recorded. The
NR responses on the graph represent No Response. So there was no
recordable AR. The lower graph just lists the results as normal or
absent. It may also be recorded as elevated or present at high
sensation levels. Some clinicians refer to normal results as present,
present at normal sensation levels or within normal limits (WNL) as
well. We will discuss what the results indicate next.
17. Once you have obtained the ART results, then they must be
interpreted. There are a number of result outcomes for the acoustic
reflex threshold. The textbook highlights four. Most responses fall
into one of these categories. The four categories are present at normal
sensation level, absent, present at low sensation level or present at
high sensation level. Notice that the results are categorized according
to sensation level (SL). Remember that the sensation level is the
number of decibels above the hearing threshold of a given subject for
a given signal. So it varies by patient dependent on their hearing level.
If a patient has a threshold of 0 dB HL and a stimulus is presented at
85 dB HL, then it would also be 85 dB SL for them. They would
perceive the stimulus as being 85 dB. However, if they have a hearing
loss of 20 dB, then a signal of 85 dB would only sound like 65 dB to
them. So most normal hearing individuals will have a reflex when
pure tones are introduced to either ear at 85-100 dB SPL. This is
because their pure tone threshold is 0-15 dB. So the SL for them
would be the same as the normal range of 85-100 dB for the ART
response. However, if they have a pure tone threshold of 50 dB, then
the ART will not be measured because it would not be present until
85 dB SL or 135 dB HL. The results are reported as dB HL. Most of
the recording and interpretation of the ART are done using the actual
dB HL results. But you can also interpret the ART based on the SL. So
in order to do this the audiometric (pure tone) thresholds would have
to be obtained. The pure tone thresholds would then be subtracted
from the dB HL ART to obtain the SL of the ART. So if at 1000 Hz
patient had a ART of 90 dB HL and a audiometric threshold at 1000
Hz of 40 dB, then the sensation level of the ART would be 50 dB SL
(90 dB response of ART-40 dB HL audiometric threshold=50 dB ART
SL). You would then determine what 50 dB SL is considered.
Hopefully that helps rather than confuse you. I just wanted to make
sure you understood what is meant by the descriptions of normal, low
and high sensation levels. Okay, so now we will go over each outcome
briefly so that when you see the results you will understand what
might be causing them. The first is outcome is that the ART are
present at normal SL. This occurs when the acoustic reflex threshold
at about or near 85 dB SL or 85-100 dB SPL. A normal ipsilateral
acoustic reflex threshold suggests that a large conductive component
is not present in that ear. If reflexes are present, sensorineural
hearing loss is probably no worse than moderate in degree, and the
ipsilateral acoustic reflex pathway is largely intact. The next outcome
is absent results. This means that there were no reflexes recorded at a
great enough amplitude, even if tested at 115 dB. This indicates that
the patient has a hearing loss. It may be a conductive hearing loss or a
moderate to severe SNHL. The absence of acoustic reflexes warrants
investigation. If the ART are present at very low sensation levels, less
than 60 dB SL, this is due to the fact that the person is recruiting
loudness, which narrows the sensation level or the distance between
audiometric threshold and the acoustic reflex threshold. So for
example, if a patient had a 50 dB hearing loss they may show a reflex
at 95 dB HL. This would be 45 dB SL. This can indicate a mild to
moderate SNHL associated with cochlear lesions. In this situation it is
important to realize that the ARTs occur at about normal levels (85100 dB). This is because the acoustic reflex threshold in an ear with a
cochlear (SNHL) loss may resemble the results of a normal ear when
the air conduction thresholds are below about 50dB HL. As the
hearing threshold increases above this level, the chance of recording
an absent or elevated acoustic reflex increases. In the last category,
ARTs are present at a high sensation level. This is also referred to as
elevated ART responses. If the ART are present at high sensation
levels, greater than 100 dB above the audiometric threshold, it can be
due to a conductive hearing loss in the stimulus ear. If abnormally
elevated ARTs are not caused by a conductive disorder then the
patient is considered to be at risk for a retrocochlear pathology in the
ear that receives the stimulus. This is usually due to damage in the
auditory (VIIIth) nerve. Again, it is not advised to introduce a signal
greater than 115 dB HL. So this occurs when the results are present at
a level higher (105-115 dB) than we would expect for the degree of
SNHL or in a patient with normal hearing.
18. There are 10 theoretical examples of various results on page 167 of
your textbook. I do not want you to worry about memorizing this
chart. I am more concerned with making sure that you can look at the
ART results and understand what area of the pathway is affected. In
looking at the chart the “patient” is set up for 4 different test
conditions. From left to right, they are right contralateral testing, Left
ipsilateral testing, right ipsilateral testing and left contralateral
testing. Each test can have different results (present, absent, present
at low SL and present at high SL). As you look at the conditions down
the side (A-J) you can determine what ART results you would expect
to find with each condition. I want to go over a few of these, to get you
used to determining what the results indicate. In the first condition
the patient has normal hearing in both ears. So all results should be
normal. This means that the acoustic reflex pathway stimulated both
ipsilateral and contralateral from both the right and left ear are all
functioning normal.
19. In this situation the right ear has normal hearing and the left ear
has a mild to moderate cochlear hearing loss. So, are the ART results
consistent with this condition? The right ipsilateral and contralateral
results to are normal, so we can rule out any problems in this area.
The left ipsilateral and contralateral results are present at low SL. Due
to the common loudness recruitment issues found in cochlear (SN)
hearing loss, this is to be expected.
20. So when we take the results and put them on the chart, it points
to normal hearing in the right ear and mild to moderate SNHL in the
left ear. So yes, the findings are consistent.
21. In this situation there are no results. No acoustic reflexes were
measured in the right ear or the left ear by contralateral or ipsilateral
stimulation. So there are no acoustic reflex pathways that are
functioning normally. This is consistent with a bilateral severe SNHL.
22. As you can see if you take the results and place them on the chart,
it confirms that the patient has a moderate to severe SNHL.
23. With this slide just look at the results first and then we will
determine where the damage has occurred in the pathway. This is
normally all you will have, just the numerical results. So first we have
to determine the category they belong in. The right ipsilateral reflex is
85 dB, this is considered normal. The contralateral right ear and
ipsilateral left ear reflexes are absent. The left contralateral threshold
is 105 dB. This is considered elevated. So what do these results
indicate? To determine this, I think it is easiest to sketch in the
pathways for ARTs that are normal. After obtaining an individual
patient’s ARTs, the pathology can be localized to the area that is not
highlighted. So, the right ipsilateral ART responses are present at
normal levels. So we know this pathway is intact. Everything else is
affected. The right contralateral ARTs are absent. We have
determined that the ipsilateral pathway is normal, so we know the
problem must be occurring at some point beyond the right cochlear
nucleus. This points to the left ear. In looking at the left ear, the left
ipsilateral responses are absent, so we know the problem is occurring
in between when signal is presented to the middle ear and when it is
measured in the middle ear of the left ear. The left contralateral is
elevated or absent. We can determine that this is because the signal
must cross whatever is causing the problem in the left ear and reach
the cochlea of the right ear loud enough to cause a reflex.
24. When we put the results on the chart we can determine that the
right ear is normal. The absent reflexes in the right contra and left
ipsi (probe ear) stimulation indicate a conductive hearing loss. In
addition the elevated reflexes in the left contralateral (stimulus ear)
stimulation point to a conductive hearing loss as well.
25. In this scenario, you have a conductive hearing loss in the left ear.
With a ME Pathology anything that goes through the left ear or is
measured in the left ear can be affected, depending on the severity of
the pathology. The ART may be affected for signals traveling through
the left ear (L ipsi or L contra) or signals measured in the left ear (L
ipsi, L contra).
26. In this slide the results have been provided, but as practice I
would like you to go through the pathways and the chart to determine
the condition for the right and left ear.
27. In this example, the site of lesion has been provided for you. What
you need to be able to do is predict ipsilateral and contralateral
acoustic reflex thresholds for the right and left ear based on this
information. You should be able to look at the results of the ART and
determine the condition and also predict the ARTs based on the
pathology of the ear.
28. In conclusion I want to let you know that as clinicians, we need to
combine findings from the entire audiological test battery to make
appropriate recommendations for further testing, medical referrals,
amplification, and so forth. Always keep in mind when conducting
ART testing, that ARTs are not meant to be used alone, but as part of
a battery of tests to help in the evaluation process. This chart, located
in your textbook on page 187 is helpful in using the tympanogram
type, static compliance results and the ART to help determine the
type of hearing loss.
29. For those of you continuing in audiology the acoustic reflex decay
along with additional electroacoustical and electrophysical
procedures will be discussed at length.