forumdoctor
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A physician is performing a cranial nerve examination on a patient. While testing the gag reflex, it is
noted that when the right side of the pharyngeal mucosa is touched, the patient's uvula deviates
to the right. When the left side of the pharyngeal mucosa is touched, the patient does not gag.
Which of the following is the most likely location of his lesion?
A. Left glossopharyngeal nerve and left vagus nerve
B. Left glossopharyngeal nerve only
C. Left vagus nerve only
D. Right glossopharyngeal nerve and right vagus nerve
E. Right glossopharyngeal nerve only
F. Right vagus nerve only
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| new_n_lost
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C. Left vagus nerve only ???
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| Jackofknives
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Vagus nerve lesion is a lower motor lesion resulting in flaccid paralysis, causing atrophy of the ipsilateral levator veli palatini muscle, and thus resulting in uvula to deviate to the contra lateral side even without stimulating the gag reflex. This symptom is not mentioned in the question, therefore I’d assume the patient does not have this problem. Thus no vagus nerve lesion.
And because glossopharyngeus nerve innervates the ipsilateral side of the pharynx, the answer would be B.
That is, the a lesion in the afferent limb of the reflex pathway.
B
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| forumdoctor
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guys i picked answer B, but the explanation say A is the right answer, i went through the explanation, but still i am not able to understand the expalnation, after reading the explanation pls explain me in simple words, here is the explanation.
To evaluate hypoglossal nerve function, a neurologist asks her patient to protrude his tongue. On doing
so, his tongue deviates to the right side. This finding results from paralysis of which of the
following muscles? The correct answer is A. The gag reflex requires the glossopharyngeal nerve for the sensory
limb of the reflex (unilateral) and the vagus nerve for the motor limb of the reflex
(bilateral). A lesion of the left glossopharyngeal nerve will denervate the sensory receptors
on the left side of the pharynx. Thus when the left side is touched, the patient does not feel
it and does not gag. The gag reflex requires the vagus nerve for the motor limb of the reflex.
If the left vagus nerve is lesioned, the left side of the soft palate will not elevate during a
gag and the uvula will deviate to the right. In this case, the patient only feels the touch on
the right side and only elevates the right side of the palate. Thus there is a lesion of both
the left glossopharyngeal nerve and the left vagus nerve.
If the patient had a lesion of the left glossopharyngeal nerve only (choice B), there would
have been no gag when the left side is touched but there would be a normal gag, without
deviation of the uvula, when the right side was touched.
If the patient had a lesion of the left vagus nerve only (choice C), the patient would have
deviation of the uvula to the right when a gag was elicited, but touching either side of the
pharynx would elicit a gag.
If the patient had a lesion of the right glossopharyngeal nerve and the right vagus nerve
(choice D), touching the right side of the pharynx would not elicit a gag and touching the left
side of the pharynx would elicit a gag with the uvula deviating to the left.
If the patient had a lesion of the right glossopharyngeal nerve only (choice E), there would be
no gag when the right side is touched but there would be a normal gag, without deviation of the
uvula, when the left side was touched.
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| Jackofknives
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Aha, thx for posting this question mate, I just learned something new here. The efferent (motor) limb for gagging requires BILATERAL innervation of both vagus nerves.
They are testing 2 more concepts.
1. Glossopharyngeal nerve is the afferent limb. Meaning no CN9, no reflex.
2. What kind of response would take place if one one vagus nerve is working? Ok, so you have stimulated CN9 and this afferent limb in term will stimulate both vagus nerves, one of which is not working, hence only one vagus nerve will have an effect. As a result you won't get a gag reflex but something else. The working CN10 will in term activate the ipsilateral levator veli palatini muscle, causing elevation of the ispilateral soft palate, thus uvula will deviate to the side with the working vagus nerve, i.e. in other words uvula deviates to the side away from the lesion.
With these concepts in place, now let's take a look on the question.
(1) "when the right side of the pharyngeal mucosa is touched, the patient's uvula deviates to the right."
This means, CN9 right is working, CN10 left is not.
(2) "When the left side of the pharyngeal mucosa is touched, the patient does not gag."
This means CN9 is not working. (Note: I don't like how they phrase this part of the question, they should say NO RESPONSE IS ELICITATED.)
OK now, question is solved. CN9 and CN10 on the left side are damaged.
Heh I was thinking when I answered B, it can't be that easy heh. Lesson #2, listen to your subconcious.
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| forumdoctor
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jack i have a q, in normal condition the uvala is in what position, i mean it is deviated or not.
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| Jackofknives
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Normally uvala does not deviate. In case of vagus nerve lesion, there is a droop of the soft palate at the same side, thus uvula will deviate to the contralateral side.
Apart from that, was my explanation easy to understand?
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| forumdoctor
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yes, thanks jack, we nailed this q.
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| determine08
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wow!! awesome !! thx for the explanation. it was really helpful.
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| Jackofknives
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no problem, one very important notion is that deviation of uvula is always a sign of pathology.
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