Question
A 27-year-old man comes to the physician because of a 1-day history of right-sided facial weakness and sound intolerance. Three days ago, he hit the right side of his head in a motor vehicle collision. He neither lost consciousness nor sought medical attention. Physical examination shows drooping of the mouth and ptosis on the right side. Sensation over the face is not impaired. Impedance audiometry shows an absence of the acoustic reflex in the right ear. Which of the following muscles is most likely paralyzed in this patient ?
| A. |
Stylopharyngeus
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| B. |
Cricothyroid
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| C. |
Anterior belly of the digastric
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| D. |
Stylohyoid
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Show Answer
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Correct Answer � D
Explanation
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Answer D) Stylohyoid
Asymmetric facial weakness and hyperacusis with an absent acoustic reflex indicate palsy of the facial nerve, which innervates structures derived from the 2nd branchial arch. An undisplaced temporal bone fracture can result in delayed-onset facial nerve palsy due to edema that leads to nerve compression.
Stylohyoid
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The stylohyoid muscle, which elevates the tongue and moves the hyoid bone backward during swallowing, develops from the 2nd branchial arch. Therefore, facial nerve palsy causes paralysis of this muscle.
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Other muscles that are derived from the 2nd branchial arch and thus innervated by the facial nerve include the facial muscles, platysma, posterior belly of the digastric, and stapedius.
Stapedius involvement is demonstrated in this case by hyperacusis with absent acoustic reflex.
Stylopharyngeus
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The stylopharyngeus muscle, which elevates the larynx and pharynx during swallowing, is derived from the 3rd branchial arch and innervated by the glossopharyngeal nerve.
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A facial nerve palsy would not cause paralysis of this muscle.
Cricothyroid
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The cricothyroid muscle, which tenses and elongates the vocal cords, is derived from the 4th branchial arch and is innervated by the external portion of the superior laryngeal nerve.
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A facial nerve palsy would not cause paralysis of this muscle.
Anterior belly of the digastric
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The anterior belly of the digastric muscle, which assists the posterior belly of the digastric muscle in jaw opening, is derived from the 1st branchial arch and is innervated by the mylohyoid nerve.
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A facial nerve palsy would not cause paralysis of this muscle.
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