Download COMD 3700 Basic Audiology Lesson 13 – Acoustic Reflex COMD

COMD 3700 Basic Audiology
Lesson 13 – Acoustic Reflex
COMD 3700 for Distance Education. This is lesson 13 on acoustic
reflexes. This lesson will cover pages 175-181 in Chapter 6 of your
textbook.
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.
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.
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.
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.
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.
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 177 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, threedimensional 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.
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.
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.
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.
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.
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 noncooperative 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.
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.
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.
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.
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.
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 85100 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 (85-100 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.
There are 10 theoretical examples of various results on page 180 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-K) 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.
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.
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.
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.
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.
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.
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.
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).
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.
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.
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 203 is helpful in using the tympanogram
type, static compliance results and the ART to help determine the
type of hearing loss.
For those of you continuing in audiology the acoustic reflex decay
along with additional electroacoustical and electrophysical
procedures will be discussed at length.