Assistant Professor SUNY Upstate Medical University Syracuse, New York, United States
Abstract Body : The musculature of the larynx plays a pivotal role in vocalization and glottic airway protective mechanisms. The vagus nerve provides innervation to this musculature via two main branches, the superior and recurrent laryngeal nerves. The superior laryngeal nerve bifurcates into the internal and external branches. The external branch of the superior laryngeal nerve (eSLN) provides the sole efferent innervation of the cricothyroid muscle, whose function is to control vocal cord tension and thyroid cartilage tilt. The anatomical trajectory of the eSLN places it at risk during neck surgeries such as thyroidectomy, anterior approaches to the cervical spine, and carotid endarterectomy. Injury, usually iatrogenic in nature, may cause subtle voice changes including: a weak, breathy voice and, reduction on the pitch range (resulting in monotonous voice). In regard to thyroid surgery, studies have shown post-operative rates of eSLN injury to range from 0-58%. This wide range suggests variation in surgical technique and the need for careful dissection. eSLN preservation can be attempted via dissection and visual identification. Variable topography and difficulty in leveraging stable landmarks has hindered the ability to routinely identify eSLN trajectory. Several classification systems in the past have utilized the superior thyroid artery and the superior pole of the thyroid as landmarks for eSLN identification. However, these landmarks are usually not fixed/ stable since physiologic variability and pathologic conditions alter superior pole positioning. The present study provides topographic information/ measurements of the eSLN. Twenty specimens were dissected from 10 human formaldehyde fixed cadavers (7 women and 3 men). Measurements included point of entry distance from midline, posterior border of thyroid cartilage, tendinous arch of the inferior pharyngeal constrictor and cricothyroid, below the thyroid prominence, and relationship to the superior thyroid artery. In addition to examining the variability within each of the previously established classifications, we propose the posterior/ lateral border of the thyroid cartilage as a new landmark for eSLN identification.
