The central neck compartment is bounded superiorly by the hyoid bone, laterally by the carotid arteries, anteriorly by the superficial layer of the deep cervical fascia, and posteriorly by the deep layer of the deep cervical fascia. Because the location of the thyroid gland is low in the neck near the thoracic inlet, the lymphatic drainage is contiguous with the anterior superior mediastinum that is accessible via a cervical approach. As a result, the inferior border of the central compartment is defined as the innominate artery on the right and the corresponding axial plane on the left (Fig. 2).
FIG. 2. Schematic anterior view of the neck indicating boundaries of the central neck compartment.
The most commonly involved central lymph nodes in thyroid carcinoma are the prelaryngeal (Delphian), pretracheal, and the right and left paratracheal nodes (Fig. 3). The paratracheal nodes may be anterior as well as posterior to the recurrent laryngeal nerves. These nodes may be found posterior to the common carotid on the right along the inferior thyroid artery because of the more ventral and medial location of the proximal carotid on the right compared with the left. The more posterior retropharyngeal and retroesophageal lymph nodes are less commonly involved. The vast majority of nodes within the central compartment exist inferior to the larynx; however, superior pole tumors may occasionally metastasize to paralaryngopharyngeal nodes deep to the sternohyoid and omohyoid muscles along the course of superior thyroid vasculature.
FIG. 3. Detailed anterior view of the central neck compartment indicating locations of lymph node basins relevant to central neck dissection for thyroid carcinoma.
Anatomic structures contained within the central compartment include the following: the larynx, hypopharynx, trachea and esophagus; endocrine glands (thyroid and parathyroid); cervical thymus; laryngeal nerves (superior and recurrent); vessels (inferior and superior thyroid arteries; inferior, middle, and superior thyroid veins).
The superior thyroid artery is a branch of the external carotid and provides blood supply to the superior pole of the thyroid and occasionally to the superior parathyroid glands via the posterior branch. The inferior thyroid artery is a branch of the thyrocervical trunk, courses posterior to the common carotid, and provides blood supply to the thyroid gland, as well as to the inferior and superior parathyroid glands via inferior and superior branches. The superior, middle, and inferior thyroid veins drain into the internal jugular vein and the inferior veins often directly into the brachiocephalic vein. The thymus lies just ventral to the brachiocephalic vein into which it drains directly.
The external branch of the superior laryngeal nerve comes from the vagus nerve coursing inferiorly along the superior thyroid vasculature to insert into the cricothyroid muscle, an extralaryngeal tensor of the vocal cords. This nerve is identifiable in most cases except when it courses deep into the inferior pharyngeal constrictor and may only be visible within the fascia of the cricothyroid muscle.
The recurrent laryngeal nerve comes from the vagus nerve in the superior mediastinum coursing around the arch of aorta on the left and around the subclavian artery on the right. From there it travels superiorly in the tracheoesophageal groove on the left and somewhat more lateral to medial on the right. As it ascends it can give off several regional sensory branches to the trachea and esophagus. The recurrent laryngeal nerve motor fibers continue superior and medially in close proximity to the ligament of Berry and then enter the larynx under the inferior pharyngeal constrictor muscle, providing innervation to intrinsic laryngeal muscles (lateral and posterior cricoarytenoid, thyroarytenoid, and interarytenoid).
The superior parathyroid glands primarily receive blood supply from the inferior thyroid artery via its superior branch, and occasionally from the posterior branch of the superior thyroid artery. The superior parathyroid glands descend from the fourth pharyngeal pouch and most commonly are found just posterior to the recurrent laryngeal nerves near the ligament of Berry. The inferior parathyroid glands descend from the third pharyngeal pouch and are more variable in location but usually are anterior to the recurrent laryngeal nerve near the inferior pole of the thyroid gland. The inferior parathyroid glands receive blood supply from the inferior thyroid artery and are often in close proximity to it.