We're joined by Dr. Karen Shackelford from BoardVitals as we tackle a neuro question this week.
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A patient has a decreased level of consciousness and they're testing the gag reflex. The elevation of the soft palate is symmetric when he touched the left side. But when he touched the right side, there's no response.
Which of the following is true relating to this patient's condition?
(A) The glossopharyngeal nerve carries efferent fibers that mediate the gag reflex.
(B) The ideology of that absent reflex is a lesion of the right glossopharyngeal nerve.
(C) The cause of the patient's absent reflex is a lesion of the left vagus nerve.
(D) The reflex is mediated through the dorsal motor nucleus of the vagus.
(E) Both the glossopharyngeal nerve and the vagus nerve are damaged on the right side.
[Related episode: USMLE and COMLEX Prep: Glossopharyngeal Nerve Anatomy]
The correct answer here is B. The motor limb is mediated by the vagus nerve. Sensory, however, is mediated by the glossopharyngeal nerve. The vagus nerve also carries some sensory fibers through the recurrent laryngeal.
If the gag reflex is intact, the soft palate will rise symmetrically regardless of which side is touched. If both the glossopharyngeal and the vagus are damaged on one side, there is no response when touching the contralateral side. The soft palate will rise unilaterally on the side that's intact.
Also, there won't be any response whenever you're testing the affected side of the lesion if both nerves are damaged.
The vagus nerve is the only nerve damaged and there's a lesion on the single side of it. And the soft palate rises and pull to the intact side.
Regardless of the pathway, this is something worth remembering. If the glossopharyngeal is only damaged on a single side, there's not going to be a response on either side when you test the reflex on the affected side. This is because you're not receiving the sensory impulse.
Whenever you test the intact side, the palate will rise on both sides because the motor fibers of the vagus nerve are still intact.
The afferent fibers of the glossopharyngeal nerve mediate the sensory component of the gag reflex. Hence, answer choice A is wrong.
For C, if the left vagus nerve or the motor nerve was damaged on the left side resulting in an elevation of the soft palate on the right no matter which side was tested.
In other words, this is the lateral lesion of vagus nerve. There's also the elevation of the soft palate to the contralateral side regardless of which side you're testing.
For option D, this is also wrong because the reflex is mediated through the nucleus ambiguus.
For E, if that were the case, then there would be no response at all when testing the right side or the side of the lesion. There would also be no response when testing the left side because the motor portion is damaged on the side of the lesion. Hence, there'd be an asymmetric elevation of the soft palate on the contralateral side.
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