CC-BY-NC 3.0 THYROIDECTOMY Eugenio Panieri & Johan Fagan Thyroidectomy is a very common opera- Superior Laryngeal n (Internal) tion. The most frequent indications for sur- gery are uncertainty about the nature of a Superior thyroid artery thyroid mass, or treatment of a large goitre Superior Laryngeal n (External) causing compressive symptoms, thyroid cancer, or thyrotoxicosis refractory to med- Common carotid artery ical management. Internal jugular vein Surgical Anatomy Thyroid Isthmus (R) thyroid lobe A detailed knowledge of thyroid anatomy is a prerequisite for thyroid surgery. Particu- Inferior thyroid vein lar attention needs to be paid to identifying Trachea and preserving the recurrent laryngeal Figure 2: Anatomy of thyroid gland nerve (RLN), the external branch of the su- perior laryngeal nerve (SLN) and the para- The pyramidal lobe is a superior extension thyroid glands. near the midline and is present in up to 70% of cases (Figure 1). Thyroid gland The thyroid is encased by a fine capsule of The gland consists of two lateral lobes pretracheal fascia which is part of the joined anteriorly by the isthmus which typ- middle layer of deep cervical fascia. The fas- ically overlies the 2nd and 3rd tracheal rings cial layers fuse to form Berry’s (Figures 1, 2). ligament which posteromedially firmly at- taches the thyroid to the trachea. The gland encircles the anterior and lateral aspects of the cervical trachea and is applied to the surface of the larynx. Lateral to the gland are the carotid sheath (common ca- rotid artery, internal jugular vein and va- gus/Xn) and the sternocleidomastoid (SCM) muscle (Figures 1, 2). Anterior to the thyroid are the infrahyoid strap muscles (sternohyoid and sternothyroid) (Figure 3). The deep/medial anatomical relations are the thyroid (caudad to attachment of ster- Figure 1: Anatomy of thyroid gland, pyramidal lobe nothyroid muscle to oblique line) and cri- and relations to carotid sheath and vasculature coid cartilages, trachea, inferior constrictor http://openbooks.uct.ac.za/ENTatlas 22-1 Open Access Atlas of Otolaryngology, Head & Neck Operative Surgery and cricothyroid muscles, oesophagus, su- the RLN, the inferior thyroid artery, Berry's perior and inferior thyroid arteries, and ligament and the superior RLNs (Figures 3, 4, 5). Ext Carotid artery Sup thyroid art Inf constrictor Cricothyroid Parathyroids Inf thyroid art Figure 3: The superficial relations of the thyroid are RLNs the infrahyoid strap muscles (sternohyoid and sterno- thyroid) and SCM Oesophagus Thyrocervical trunk Figure 5: Posterior view of the thyroid gland demon- strating the deep / medial anatomical relations, the RLNs and the superior and inferior thyroid arteries parathyroid gland. The tubercle usually projects lateral to the RLN. Elevating the tu- bercle allows the RLN to be readily located. Less commonly the RLN courses lateral to an enlarged tubercle; this places the nerve at Figure 4: Structures deep to thyroid gland: Note the risk of injury. The superior parathyroid oblique line to which the sternothyroid muscle inserts gland, also derived from the 4th branchial and which defines the anterosuperior limit of the thy- cleft, is commonly located close to and roid (Wikipedia) cephalad to the tubercle. The thyroid gland weighs 15-25g. The thy- roid lobes are normally cone-shaped and measure approximately 5cm in length and TZ 2-3cms in width in both transverse and an- teroposterior dimensions. Superior parathyroid RLN The Tubercle of Zukerkandl is a pyramidal enlargement of the lateral edge of the thy- roid lobe that stems from the fusion of the lateral and medial thyroid anlages (Figure Figure 6: Tubercle of Zuckerkandl (TZ) and its rela- 6). It is recognisable in up to 75% of thy- tionship to the superior parathyroid gland and RLN roids. It is anatomically closely related to http://openbooks.uct.ac.za/ENTatlas 22-2 Eugenio Panieri & Johan Fagan Blood supply pole of the thyroid (Figure 5). It provides blood supply to the thyroid, upper oesoph- The arterial supply is based on the superior agus and trachea, and is the sole arterial thyroid (STA) and inferior thyroid (ITA) supply to all the parathyroid glands, both arteries. Occasionally the thyroidea ima ar- superior and inferior. The relationship of tery is encountered inferiorly but is seldom the ITA and RLN is reviewed later. of surgical relevance. It arises from the innominate artery or aortic arch and as- Venous drainage is quite variable and oc- cends along the front of the trachea. curs via a capsular network of thin-walled, freely intercommunicating veins which The superior thyroid artery (STA) is the drain through the superior thyroid veins first branch of the external carotid artery (adjacent to the STA), the inferior thyroid (Figures 2, 5, 7). It courses over the external veins (exit the inferior pole), and the mid- surface of the inferior constrictor muscle of dle thyroid vein(s), which course laterally the pharynx, entering the gland posterome- to drain directly into the internal jugular dially just below the highest point of the up- vein (Figure 1). The middle thyroid vein is per pole where it usually is located superfi- surgically most relevant; it is encountered cial to the external branch of the SLN (Fig- early during thyroid mobilisation, and fail- ure 2). Its branches communicate with the ure to secure it causes bothersome bleeding. ITA and cross to the contralateral thyroid lobe via the thyroid isthmus. Lymphatic drainage parallels the venous drainage and occurs to the lateral deep cer- vical and pre- and paratracheal lymph nodes (Figure 8). Understanding the pattern of nodal drainage is particularly important in managing patients with thyroid cancer STA since the cervicocentral compartment is most commonly involved in metastatic thy- ITA roid cancer. Thyrocervical Subclavian a Figure 7: Superior thyroid artery (STA), subclavian artery, thyrocervical trunk and inferior thyroid artery (ITA) The inferior thyroid artery (ITA) is a branch of the thyrocervical trunk which originates from the subclavian artery (Fig- ures 5, 7). It courses superiorly along the surface of the anterior scalene muscle be- Figure 8: Posterior view of the course of the lymphatics fore it turns medially behind the carotid and RLNs sheath from where it reaches the inferior http://openbooks.uct.ac.za/ENTatlas 22-3 Open Access Atlas of Otolaryngology, Head & Neck Operative Surgery Recurrent Laryngeal Nerve (RLN) During thyroid surgery, identification and preservation of the RLN and all of its divi- sions is essential to minimise morbidity. The RLN innervates all the intrinsic mus- cles of the larynx except the cricothyroid muscle (SLN) and provides sensory inner- Xns vation to the larynx. Even minor neuro- praxia may cause dysphonia; irreversible in- RLNs jury confers permanent hoarseness. The re- Subclavian arteries ported incidence of RLN injury during thy- roidectomy is 0 - 28% and is the most com- Aortic arch mon reason for medicolegal claims follow- ing thyroidectomy. The RLNs originate from the Xn. After cir- Figure 9: Typical anatomical course of RLNs (Non-re- cling around the subclavian artery (right) current RLN in red) and aortic arch (left) the RLNs ascend supe- riorly and medially toward the trache- The RLN may be non-recurrent in approx- oesophageal groove (Figures 8, 9). The right imately 0.6% of patients i.e. does not pass RLN enters the root of the neck from a more around the subclavian artery, but branches lateral direction. Its course is less predicta- from the Xn higher in the neck, passing di- ble than that of the left RLN. The RLNs en- rectly to the larynx close to the superior thy- ter the larynx deep to the inferior constric- roid vessels (Figure 9). This aberration al- tor muscles and posterior to the cricothy- most always occurs on the right side and is roid joint. associated with a retroesophageal subcla- vian artery. Knowledge of the anatomical relationships of the RLN to the tracheoesophageal groove, ligament of Berry, and ITA is essen- tial. The course of the RLN with respect to the ITA is quite variable. Most commonly it crosses behind the branches of the artery, more predictably so on the left. However, the nerve may pass deep to, superficial to, or between the terminal branches of the ITA. Up to twenty anatomical variations have been described. In Figure 10 the RLN is seen to pass anterior to the artery. http://openbooks.uct.ac.za/ENTatlas 22-4 Eugenio Panieri & Johan Fagan Superior Laryngeal Nerve (SLN) The SLN is a branch of the Xn and has both an external and internal branch (Figures 2, 12). The internal branch is situated above and outside the normal field of dissection; it is sensory and enters the larynx through the thyrohyoid membrane. Figure 10: RLN passing over the inferior thyroid ar- The external branch innervates the crico- tery (right neck, thyroid reflected medially) thyroid muscle, a tensor of the vocal cord. Injury to the SLN causes hoarseness, de- The majority of RLNs are located within creased pitch and/or volume, and voice fa- 3mm of Berry’s ligament; rarely the nerve is tigue. These voice changes are more subtle embedded in it, and more commonly lies than those relating to a RLN injury, and are laterally to it. frequently underestimated and not re- ported. The external branch of the SLN is at Classically, the RLN is identified intra-op- risk because of its close proximity to the eratively in Simon’s triangle, which is STA (Figures 12, 13). Understanding its re- formed by the common carotid artery later- lationship to the upper pole of the thyroid ally, the oesophagus medially, and the ITA and the STA is crucial to preserving its in- superiorly (Figure 11). tegrity. XIIn SLN (internal) SLN (external) STA Figure 11: RLN crossing Simon’s triangle formed by Sup pole thy- oesophagus, inferior thyroid artery (ITA) and com- mon carotid artery (right neck, thyroid reflected me- roid dially) Figure 12: RLN crossing Simon’s triangle formed by oesophagus, inferior thyroid artery (ITA) and com- The Tubercle of Zukerkandl may also be mon carotid artery (right neck, thyroid reflected me- used as an anatomical landmark to identify dially) the nerve (Figure 6).