Welcome to this presentation on the vestibular system. The focus on this presentation is to inform you
about a vestibular test, the caloric nystagmus test. I also want you to understand horizontal conjugate gaze,
how that’s defined and the circuits that are involved in making sure the eyes do move together.
It’s always very, very important to dive down into clinical considerations. So we’ll be able to explore three
different clinical pathologies that are associated with the vestibular system. First is a test of the vestibular system.
This is the caloric nystagmus test. This is considered a variant of the vestibulo-ocular reflex. This test
involves, at the beginning, irrigation of an ear with cold water. For this slide, we’re going to irrigate
the right ear with cold water. Then we’re going to see what happens to the movement of the eyes in response
to this cold water irrigation. In response to irrigation of the right ear with cold water, the eyes will slowly
move right as if the head is moving in the opposite direction to the left. The nystagmus component is
the rapid eye movement. The rapid eye movement or nystagmus will occur then to the left. So we get slow
movement of the eyes to the right and then there’s rapid eye movement back to the left. Again, this is what occurs
in response to cold water irrigation of the right ear. Now, let’s do the same thing to the right ear but let’s irrigate
with warm water. In response to irrigation of the right ear with warm water, the eyes will slowly move
left as if the head is moving to the right. The nystagmus component, that rapid eye movement then will occur
to the right, to the same side as the ear being irrigated with the warm water. Now, there is a very simple
mnemonic that will help you remember the movement of nystagmus with respect to cold water irrigation
versus warm water irrigation and that is COWS. Cold water will have the nystagmus movement to the
opposite side, whereas warm water irrigation will have the nystagmus component occurring to the same side.
That’s the normal response. If you see an abnormal response to cold or warm ear irrigation, that will indicate
vestibular lesion. Next, I want you to understand control of horizontal gaze. Horizontal gaze is the
horizontal movement of both eyes in conjunction with one another. To help guide you through this
control mechanism and circuitry, we’re going to look at gaze to the right. First thing to keep in mind
is there is a gaze center in the frontal cortex. This will activate neurons that reside in the contralateral
paramedian pontine reticular nucleus. This is PPRN. So, gaze to the right in this direction and then
the contralateral PPRN becomes activated. Activation then will stimulate the abducens nucleus on the
right side. So, we see that occurring in this circuit right in through here and you see the stimulation then
of the abducens nucleus. In addition, the left oculomotor neurons are stimulated. We see that on this
particular side right in through here. What will then happen is activation of the nerve cells that reside in
the abducens nucleus will then send action potentials through the abducent nerve fibers to then cause
contraction of the lateral rectus muscle which is shown here. That will move the eye to the right.
It will abduct it. Then activation of the neurons from the oculomotor nucleus will cause action potentials
to travel along the nerve fibers of the oculomotor nerve. As a result of that doing that, the medial rectus
will contract and cause abduction of the left eye. As a result, the left eye moves to the right by action
of the medial rectus, the right eye moves to the right by action of the lateral rectus. That is the control
of horizontal gaze. If there’s a lesion in this pathway, then the eyes will not move in conjunction
with one another. So one eye might be fixed on the object of interest and the other eye is not in
alignment with that. So you can have double vision or diplopia as a result of a lesion in this circuitry.