the next question is from Board vitals the USMLE step one bank and comlex level
one bank of questions now board vitals is a comprehensive medical board review
and we're proud to be partnering with them at MEC ram comm so the question is
a patient has no direct or consensual corneal reflex on the right the lesion
is most likely at the level of the right temporal branch of the facial nerve be
the left zygomatic branch of the facial nerve see the right nasal ciliary branch
of the trigeminal nerve D the left facial motor nucleus or E finally the
right trigeminal nucleus so let's go over this and try to come up with what
they're trying to test you here okay so what I've done is and this looks a
little complicated but let me explain it we've got the cornea on the right here
at the top and we've got the cornea at the left here at the top and everything
in medicine is always flipped around so I'd like to present it this way when
you're looking at the human body things on the right are actually on the left
side of the screen and things on the left or on the right side of course so
what we're doing here is we're looking at this reflex so let's take this
through specifically the different types now you've got to understand what the
difference is between direct and consensual since you have two eyes the
kortnee reflex is where you actually touch the cornea with a very very light
tip of a q-tip or a piece of cotton and when you touch it on the for instance on
the right side if the right eyelid closes that's called a direct response
if the left closes that's called a consensual response so let's go through
a typical procedure here where we actually take the cotton swab and touch
it to the right cornea what would happen here is we would touch the cornea and we
would get a response to that and that would be picked up by the sensory nerve
here in yellow now the reason why I have this in yellow is it corresponds to the
answer choices which happens to be the nasal ciliary branch of the trigeminal
one nerve and that is the this little dot here represents the the nucleus of
that nerve so it gets picked up and it goes into
this body here which is the trigeminal nucleus okay and then what happens here
is very important not only does it continue back in the arc to the eye and
to the avicularia oculi muscles on the ipsilateral side it also gets
transmitted this is very important it gets transmitted to the other side why
would it do this well if something is getting into one eye and you need that
eye too close you're probably going to have to have the other eye close as well
to make sure that something doesn't get into that and you're not waiting for a
response to hit that cornea on that side so this is kind of a defense mechanism
where both eyes are going to close if just one cornea is stimulated so these
are inter neurons now what are these inter neurons connect to the internet
from the trigeminal nucleus on the right connects to bilateral facial motor
nuclei on the right and on the left both both of these nuclei have bodies which
come out of it which are nerves which are part of both the temporal and the
zygomatic branches of the facial nerve which is a motor nerve that's why I have
this here this M so we have sensory up top and we've got motor on the bottom so
let's go through this we have a stimulation of the cornea which
stimulates the nasal ciliary branch of v1 or the trigeminal one branch it goes
in hits the trigeminal nucleus it then splits to the contralateral inter neuron
and goes to the contralateral facial motor nucleus and to the ipsilateral
motor nucleus and at that point it then goes straight down on both sides hitting
both o bicular s muscles on the right and on the left and of course the same
thing happens on the other side if I were to do it on the left you would have
both eyes closed for the same reason so in other words if we're looking
specifically at the bicular as R which remember has no direct or consensual
that means that we're stimulating the right side and it's not contracting and
we're stimulating the left side and it's not contracting so already you can see
here that because there's a crossover you can see there's a dichotomy that's
been set up if both eyes
act the same regardless of which side you're putting it on then there has to
be something wrong in the sensory area the sensory area meaning that there's a
problem here or here or here or here if on the other hand one eye always seems
to work or one eye always doesn't seem to work that's got to be in the motor
section so the reason why I have these different colors is because a is a
problem with the temporal branch of the facial nerve and so I have enter choice
a in red and here you can see the a is right temporal branch of the facial
nerve let's go down through the other ones B is going to be the zygomatic
branch of the facial nerve but it is on the Left C is the right nasal ciliary
branch of the one I have that in yellow here and D in green is a lesion in the
left facial motor nucleus finally E is the lesion in the trigeminal nucleus on
the right hand side so let's go through these and we'll work on them backwards
let's first take option E if there was a lesion here at the right-sided
trigeminal nucleus that would mean that if there was a stimulation of the right
cornea basically what would happen is it would stop right here and you would get
no response in either eye when you stimulated the right cornea however if
you stimulated the left cornea you would get a response in both eyes since it's
not going through the lesion here again you have a situation where either both
eyes are working or both eyes are not working and that would tell you that
it's a sensory issue which we know because the trigeminal nucleus is
predominantly a sensory nuclei so E is wrong because that's not consistent with
what we're actually seeing in the patient which is no direct and no
consensual response only on the right hand side let's go to Dee Dee's issue is
that there is a lesion in the left facial motor nucleus what would happen
if we stimulated the right cornea things would go through beautifully on the
right eye but it would finally get over on the crossing contralateral
interneuron come to this one and you would have nothing on the left-hand side
okay similarly if we were to go to the left cornea this would come down and go
across without a problem but again you would have the same problem on the left
hand side so in other words one eye is always working and one eye is always not
working and so when you have that kind of a dichotomy you have a motor problem
and in this case it's the facial motor nucleus let's go to C C would be a
problem with the right nasal ciliary branch of v1 if we stimulated the cornea
on the right nothing would happen on either side because the lesion is here
if we stimulated the cornea on the left everything would work fine again we have
a situation where everything is either working fine or not working at all that
would be a sensory and in this situation it would be consistent with that however
these cannot be the answer choices because again we're supposed to be
looking for something where the right doesn't work on either direct or
consensual stimulation so let's go to B B would be a situation here where this
facial nerve either the temporal or the zygomatic and specifically B was the
zygomatic is not working that obviously is not consistent with what the problem
is in the in the stem but again what would happen if we stimulated the right
cornea it would go through here and all the way through and you would have
contraction of the right eye and the abit Galeras oculi however if when it
would cross over and go here there would be no output from the facial nerve and
so once again regardless of whether would be consensual or direct the left o
bicular s or I would not contract finally we get to the answer a which is
the correct answer as you probably have imagined by now and
that is this the right facial nerve specifically the right facial temporal
branch of the facial nerve is not working and that would be consistent
with no direct in other words we stimulate the same side cornea and it
goes through but it cannot go through and so there is no response or
consensual if we go to the left cornea and we stimulate it
it goes down it will stimulate the left side but when it crosses over on the
inter neuron to come over here to this side it will not stimulate the o bicular
on the right and so the correct answer here is a there is no direct and no
consensual response if you have a lesion in the right facial nerve specifically
in this case the temporal branch thank you for joining us
you