Study on Microsurgical Anatomy of External Branch of Superior Laryngeal Nerve Related to Carotid Endarterectomy

Objective: To explore the methods of protecting the external branch of the superior laryngeal nerve during carotid endarterectomy through microsurgical anatomic study of the external branch of the superior laryngeal nerve in cadaveric specimens. Methods: A total of 30 cadaveric specimens (60 sides) were dissected to measure the thickness of the external branch of the superior laryngeal nerve. A triangular area was exposed, bounded by the lower border of the digastric muscle superiorly, the medial edge of the sternocleidomastoid muscle laterally, and the upper border of the superior thyroid artery inferiorly. The probability of the occurrence of the external branch of the superior laryngeal nerve in this area was observed and recorded. The distance among the midpoint of the external branch of the superior laryngeal nerve in this area with the tip of the mastoid process and the angle of the mandible as well as the bifurcation of the common carotid artery was measured and recorded. Results: Among 30 specimens of cadaveric heads (60 sides) examined 53 external branches of the superior laryngeal nerve were observed while 7 were absent. Of the 53 branches observed, 5 were located outside the anatomic triangle region mentioned above, while the remaining 48 branches were located within the anatomic triangle region with a probability of similar to 80%. The thickness of the midpoint of the external branches of the superior laryngeal nerve within the anatomic triangle region was 0.93 mm (0.72-1.15 mm [ +/- 0.83 SD]), located 0.34 cm [-1.62-2.43 cm ( +/- 0.96 SD)] posterior to the angle of the mandible, 1.28 cm (-1.33 to 3.42 cm ( +/- 0.93 SD)] inferiorly; 2.84 cm (0.51-5.14 cm +/- 1.09 SD) anterior to the tip of the mastoid process, 4.51 cm (2.82-6.39 cm +/- 0.76 SD) inferiorly; 1.64 cm [0.57-3.78 cm ( +/- 0.89 SD)] superior to the bifurcation of the carotid artery. Conclusions: During carotid endarterectomy procedure, using the cervical anatomic triangle region, as well as the angle of the mandible, the tip of the mastoid process, and the bifurcation of the carotid artery as anatomic landmarks, is of significant clinical importance for protecting the external branches of the superior laryngeal nerve.