Corneal Reflex Explained Clearly – Exam Practice Question


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 you

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