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
Explanation:
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 theuvula, when the left side was touched. If the patient had a lesion of the right vagus nerve only (choice F), the patient would have deviation of the uvula to the left when a gag was elicited and touching either side of the pharynx would elicit a gag.
برچسبها:
معاینه اعصاب کرانیال سوال