So in this lecture, we'll continue with
our middle ear immittance testing. So in
the previous lecture, we talked about
tympanometry, how tympanometry is an
objective way of assessing the middle
ear, and we do so by bouncing sounds off
the tympanic membrane, and depending upon
how much sound is reflected bac, we
indirectly assess how well energy, or sound
energy travels through the middle ear.
And based on our knowledge of how
different clinical conditions affect the
stiffness of the tympanic membrane, we'll be
able to differential diagnosis of what,
whether the middle ear is normal
or if not, what is likely the
clinical condition that makes it
abnormal. Some conditions make the
tympanic membrane stiff and some
conditions might make it more lax than
it's expected. When we talked about the
middle ear physiology, we also talked
about the acoustical reflex, which is
because of the action of two middle ear
muscles, namely the tensor tympani which
is attached to the malleus and then
we've got the stapedius, which is
attached to the stapes.
Both of these muscles are independently
innervated by to cranial nerves. The tensor
tympani is innervated by the fifth or the
trigeminal nerve, while the stapedius
is innervated by the seventh cranial
nerve. Together these muscles are
believed to serve a protective function,
and they are affected, in other words, or
their action is, results in stiffening
the tympanic membrane and the ossicular
chain. In humans, the stapedius muscle it
plays a more active role and it's
consistently and reliably is affected by
loud stimulus.
And that's what we call as acoustical
reflex. And in normal, normal middle ear
system and normal auditory structures,
this acoustic reflex is activated by
moderately loud sounds can be as
effective as reducing the loudness of
the sound by 20 decibels. So it serves a
protective function because it, it
protects the delicate structures within
the cochlea from this loud, damaging
loud, or potentially damaging loud sound,
and, and realistically, the acoustical
reflex might not be effective for
abrupt sounds because it takes a few
milliseconds for this reflex to be
activated. So it might not work well if
it, if it was a, like in quick sounds, such
as a gunshot or even the blast of,
the slam of a door. Some investigators
believe that if we have been trained to
activate this acoustic reflex a few,
even a few milliseconds before you are
expecting a loud sound, actually,
potentially, that might be used as a, as a
good protection against these loud
sounds. So there are actually some
commercial devices, which falls under the
category of ear protective devices
which gives this moderately loud sound
to keep this acoustic reflex
perpetually activated, especially in, and
if these people were working in
a noisy environment.
The acoustic reflex, just like many other
reflexes in our human body, involves a
reflex arc. So the acoustic reflex goes
beyond just the middle ear structures
actually, it involves structures in the inner
ear and in, in the central auditory
pathway. Some structures that are in the
brainstem or the stalk of the brain. So
effictively, when you're activating this
acoustical reflex, all these structures
are, are, are involved. So indirectly, are
assessing the dependency, about how well
these structures work. So some of the
structures involved in the simplest
stapedial reflex arc is the spiral
ganglion neurons, which is within the
cochlea, those are those ends of the
auditory nerve. Then, some structures in
the brain stem, such as a cochlear
nucleus, the superior olivary complex and
then the stapedius muscle is actually
innervated by the facial nerve. So the facial
nerve is of course activated and then this
activation results in contracting the
stapedius muscles. So when you're
activating this reflex, all these
structures are involved, and so for
clinically, that's important because now
you're assessing beyond the middle ear
and if you see a reflex, that means that these
structures involved are normal or we'll
be able to assess them.
Here's a schematic illustration of the
reflexive arc. So you've got the acoustic
reflex was actually a bilateral
reflex. In other words, even if you give
a loud sound in one ear, it
results in activating, if the system is
normal on both sides, activates reflexes
in both, both middle ears. So here, we've got
the ipsilateral reflex, meaning that the arc
is, let's say from the right ear, started
by a loud sound of the right ear,
results in effecting the right stapedius
muscle over here, in the middle ear. And
then, we've got this contralateral
pathway, where if the sound is given to
the right ear,
it still results and crossing over to the
left side, activating the left auditory
pathway, activating the left facial nerve
and then eventually, activating the
left supedius muscle. And so, when you're
testing, actually you can test for both the
ipsilateral and contralateral reflexes,
depending upon where you're giving the
sound and where you're looking for the
reflex. So again, to review some of the
structures that are involved in the
reflex. You've got, with the sound
traveling through the external ear, then
through the middle ear, then reaching the
cochlea and from the cochlea, the sound
travels through the eighth nerve, to the
brain stem, and then in normals, eventually
that sound needs to travel all the way
to the part of the brain that finally
interpret that sound that's in the
temporal lobe. But in the process, if the
sound is loud,
it might result in triggering this
acoustic reflex somewhere in this
lower brain stem area.
So, and particularly, it reaches this place,
this lower brain stem structure known as
a superior olivary complex, which is
above the cochlear nucleus. When we talk
about the central auditory pathway, I'll
quickly review these structures, but it's
in the superior olivary complex where
the seventh nerve originates, the seventh
or the facial nerve originates.
So that's your facial nerve, while this
eighth would be your auditory, your auditory
vestibular nerve. So if the sound is loud
enough, a signal is sent by, from the
superior olivary complex via the
seventh cranial nerve or the facial nerve,
which activates the stapedius muscle. So
when the stapedius muscle contracts, it
results in stiffening the ossicular chain
and reducing the sound of that event
that'ss going into the inner ear, so that's
how
it serves as a protective function.
So here is another illustration of the
ipsilateral pathway. Again we've got the
the sound, it should be transduced,
sorry about that.
Sound needs to travel through the cochlea
and to the brainstem structures, and then
if it's loud enough, a message is sent
through the seventh cranial nerve to
activate the stapedius muscle, that kind
of stiffens the stapes and ossicular chain.
So this is the ipsilateral pathway. So if
this was the right ear, the stimulus
that we use for acoustical reflex testing
is given to the right ear and you
expect to see the reflex on the same
side. In the contralateral pathway, the
sound travels from the, the cochlea of
the right side but, and goes to the brain
stem, and then crosses over to stimulate
the left facial nerve, and then
stimulating the left stapes. So now you
see the reflex, reflexive contraction of
the stapedius muscle on the left side.
How the equipment does that, and most
diagnostic imittance meters is able to do
that is pretty much, they bounce the
sound off, just like the way we do in
tympanometry but, but now we are looking for
a sudden reduction in the sound going
through the middle ear. In other words,
you're looking for more sound to be
reflected back when this reflex is
being activated. And when you're testing
acoustic reflexes, you would test at the
middle ear pressure where you're seeing
maximum compliance, in other words you'll
be testing at the setting where, where
the tympanometric peak pressure is.
And for most normal hearing individuals,
you expect to see this bilateral
acoustical reflex when pure tones are
about 70 decibels sensation level, 70 to
200 decibels sensation level, in other words
above the threshold. On average, you expect to
see around 85 dB sensation level for
whatever frequency you're using to test
this acoustical reflex. And since it,
acoustic reflex involves a number of
structures and nerves, if you see a
normal acoustical reflex at an expected
loudness level, so in this case, for average
suppose we are 85 dB sensation level, you
would you're actually testing all
these important structures. So when you're
doing
acoustic reflex testing, again as I said,
you, you maintain the external ear canal,
its pressure, at the tympanometric peak
pressure, and then you give tones of
different frequencies, and then you try
to estimate at what's the lowest
intensity level where you're seeing a
substantial change in the admittance of
the middle ear. And what we consider as
significant change is if, it's there, the
compliance or the admittance changes by
.2 ml. In other words, you're seeing, you
expect to see a reduction, you expect to
see a reduction in the compliance when
the threshold is reached. And pretty much
like pure tone audiometry, something that
we're going to be talking about and
you've probably observed already for
your, your lab assignment, you're trying
to change the intensity of this probe
tone until you see this minimum change in
compliance of .2 ml. And you do
that for different frequencies, so you're
assessing, directly, an objective measure
of how well those different frequencies
travel through by trying, by seeing how
this acoustical reflex. And different types
of auditory pathologies affect the
acoustic thresholds differently. If there
is a problem in the external ear or in the middle ear
or on the middle ear, will be called as a
conductive hearing loss. The acoustic
refelx threshold might be either
elevated, in other words the sound needs
to be much louder than what we expect
for normals before you can see the
reflex, or it might be completely absent, so
even at the maximum intensity you test, you
don't see those acoustical reflexes. If the
problem is originating from the cochlea
or the auditory nerve, a type of pathology
that we call as a sensorineural
hearing loss. This acoustical reflex
threshold is either, again, elevated
or actually can be decreased, in other
words, you are seeing this reflex at a
lower sensation level than expected
or it can be absent. Particularly, if the
problem is in the cochea, it results
in this condition known as recruitment,
where this person might actually have a
hearing loss, in other words, he can't hear soft
sounds but loud sounds, even moderately
loud sounds, sound excessively loud for
them.
And that's what we call as
recruitment, something that we're going
to be talking about later, but in those
individuals, you might actually see this
acoustical reflex at a lower sensation
level then we expect. So on average, it's,
expect to see it 85 dB above the
threshold for that frequency, the
pure tone threshold for that frequency. But
in these individuals, we actually might
see it at a lower level, and that might
tell you indirectly that it's, that this
person has recruitment, and then you'll be
able to differential diagnosis that it's
a cochlear condition. On the other hand,
if the problem is because of somewhere
in auditory nerve, then you typically
won't see this acoustic reflex. What's
interesting is in some patients who have
facial nerve paralysis, one of the
complaints that they'll have is that
these patients hear, their voice sounds
to booming to them, and the reason for
that is because the acoustical
reflex is, an important nerve in acoustic
reflex is the stapedius muscle
which is innervated by the facial nerve.
So these individuals actually might have
normal hearing, because the eighth nerve
and the cochlea and everything is good, but
the seventh nerve, which is a facial
nerve, if that is paralyzed, as in the
case with facial paralysis, these people
wouldn't have an acoustic reflex, so
they don't have a protective function
which actually reduces their own voice.
And since they don't have that, their
voice sounds more booming to them. So the
acoustic reflex it provides us a very
good objective means of checking a
number of those auditory structures and
especially in people who are difficult
to test,
like, like children or
difficult to test adults. So if you see
an acoustical reflex at an unexpected dB
level or dB sensation level, that kind of
indirectly, you're assessing all those
important structures. But one caveat
in this, using acoustic reflex solely
for diagnostic purposes, is that there is
a small proportion of even normal
hearing individuals with no known
pathology that we don't see these
acoustical reflexes for some reason. It's
still unknown.
So about five percent of normal
hearing individuals, you don't see acoustic
reflexes, even though their pure tone
thresholds, and speech thresholds, and
everything is within normal limits.
So even though the presence of
acoustic reflex helps us validate our
test results; the absence solely cannot
be used to diagnose a hearing loss.