Section: USMLE

43) A 22-year-old woman presents to her physician with amenorrhea, weight loss, anxiety, tremor, heat intolerance, and palpitations. Laboratory examination is consistent with hyperthyroidism, and the physician prescribes propylthiouracil. The patient's response to propylthiouracil is disappointing, and the symptoms recur, then worsen. Subtotal thyroidectomy is successfully performed, but following the surgery, the woman is extremely hoarse, and can barely speak above a whisper. This hoarseness is most probably related to damage to a branch which of the following cranial nerves?

1. Facial
2. Glossopharyngeal
3. Hypoglossal
4. Trigeminal
5. Vagus

The recurrent laryngeal nerves are branches of the vagus (CN X) and supply all intrinsic muscles of the larynx except the cricothyroid. The right recurrent laryngeal nerve recurs around the right subclavian artery. The left recurrent laryngeal nerve recurs in the thorax around the arch of the aorta and ligamentum arteriosum. Both nerves ascend to the larynx by passing between the trachea and esophagus, near the thyroid gland. The recurrent laryngeal nerves are therefore particularly vulnerable during thyroid surgery, and damage may cause extreme hoarseness.

The facial nerve innervates the muscles of facial expression, the stapedius muscle, and the lacrimal, submandibular and sublingual glands. It also mediates taste sensation from the anterior two thirds of the tongue.

The glossopharyngeal nerve innervates the stylopharyngeus muscle and the parotid gland. Visceral afferents supply the carotid sinus baroreceptors and carotid body chemoreceptors, and mediate taste from the posterior one third of the tongue. Somatosensory fibers supply pain, temperature, and touch information from the posterior one third of the tongue, upper pharynx, middle ear, and eustachian tube.

The hypoglossal nerve innervates the intrinsic muscles of the tongue, the genioglossus, hypoglossus, and styloglossus muscles.

The trigeminal nerve receives sensory information from the face and innervates the muscles of mastication.