Anatomy - Total thyroidectomy in bilateral nodular goiter - general and visceral surgery

Situated between the anterior margin of the sternocleidomastoid muscle, the mandibula and the jugular fossa, the anterior cervical triangle near the hyoid bone comprises the suprahyoid and subhyoid muscles, vessels, nerves and the thyroid. The only important subhyoid muscles in thyroid surgery are the medial

• sternohyoid muscle (sternum →hyoid bone) which covers the

• sternothyroid muscle (sternum →thyroid cartilage of the larynx) and more laterally the

• omohyoid muscle (scapula →intermediate tendon→hyoid bone)

because they partly cover the thyroid gland and must be retracted laterad in surgery.

Blood vessels

Before dividing into the internal and external carotid artery, its two main branches, at the superior margin of the thyroid cartilage at the level of its carotid sinus (pressoreceptors for the blood pressure and chemoreceptors for the blood gases), the carotid artery courses in the carotid sheath immediately lateral to the trachea and esophagus. Here, it touches the left and right thyroid lobe as a major blood vessel. The internal jugular vein arises from the sigmoid sinus in the skull, collects the blood from the head and neck, and while coursing caudad it first accompanies the internal carotid artery in the carotid sheath before pursuing a more lateral course, enclosing the lateral aspects of the common carotid artery and vagus nerve (CN X).

Nerves

The ansa cervicalis (superior and inferior roots, from C1-C3), which innervates these three above muscles of the anterior triangle of the neck, and the transverse nerve of the neck (from C2/3, innervation of skin and platysma) courses cephalocaudad lateral to the thyroid and next to the vagus nerve and its superior branch to the larynx (superior laryngeal nerve →anterior cricothyroid muscle and mucosa of the superior laryngeal half).

Fascial layers

The skin of the anterior triangle of the neck covers several fascial layers (all belonging to the cervical fascia) with distinctive features:

• The superficial lamina invests all structures of the neck, except for the platysma, and separately invests the sternocleidomastoid muscle as well as the posterior aspect of the trapezius muscle (accessory nerve XI),

• with the medial pretracheal lamina investing the infrahyoid muscles and

• the deep prevertebral lamina coursing outside the surgical field between the esophagus and spine.

Just like the lateral vascular and nerve pedicle (carotid artery, internal jugular vein and vagus nerve), the trachea and thyroid / parathyroids also have their own organ fascias. With their three-dimensional configuration, the fascias invest compartments interspersed with spaces which extend into the mediastinum and thus represent potential routes of infection.

The unpaired thyroid (glandula thyreoidea) as well as the 4 parathyroids (glandulae parathyroideae) belong to the endocrine organs; rest in the anterior cervical triangle inferior and lateral to the thyroid cartilage. Its left and right lobe are connected by the thyroid isthmus. The lobes cover the lateral superior rings of the trachea as well as the anterior aspect of both the cricoid and thyroid cartilage; the isthmus is situated at the level of the 2^nd/3^rdtracheal ring. The organ is invested by its own capsule of connective tissue (capsula fibrosa) and covered anterolaterally by the infrahyoid muscles (sternohyoid, sternothyroid and omohyoid) and the pretracheal lamina. Due to its embryology and descent (via the thyroglossal duct) from the base of the tongue (foramen caecum) to its later location, its path may leave cysts or ducts to the foramen caecum, as well functional glandular tissue (e.g., pyramidal lobe).

Blood supply

Arteries from the external carotid (→superior thyroid artery) and the subclavian artery via the thyrocervical trunk (→inferior thyroid artery) with their small anterior and lateral/posterior branches supply the thyroid gland with blood. The thyroidea ima artery, an inconsistent but noteworthy artery arising directly from the aortic arch or the brachiocephalic trunk, terminates in the thyroid from below. Venous drainage from this endocrine organ is via corresponding veins (superior and medial thyroid vein →internal jugular vein, inferior thyroid vein →brachiocephalic vein) and has systemic significance.

Lymphatics and nerves

Lymphatic drainage from the thyroid is paratracheal and to the deep cervical lymph nodes. Particular attention must be paid to the recurrent laryngeal nerve (from the vagus nerve, on right: looping around the subclavian artery; on left: looping around the aortic arch). It travels cephalad in the groove between trachea and esophagus to the larynx, where it supplies the sensory mucosal innervation of the inferior half of the larynx and all laryngeal muscles except the anterior cricothyroid (“anticus”).

The four parathyroids (glandulae parathyroideae), small oval glands on the posterior aspect of the thyroid lobes, are supplied by the inferior thyroid arteries.

Due to their proximity, thyroid surgery runs the risk of injuring not only structures such as the recurrent laryngeal nerve and the parathyroids on the posterior aspect of the organ, but also parts of the sympathetic trunk or vessels and nerves adjacent to the common carotid artery.

While the lentil-sized, paired parathyroids are usually found on the posterior aspect between the fibrous and external capsule of each lobe, sometimes they are located within the thyroid tissue itself. The superior parathyroids usually are located at the level of the inferior margin of the cricoid cartilage, while the inferior parathyroids are found at the level of the 3^rdor 4^thcartilaginous tracheal rings.

The number and location of the parathyroids vary greatly, and in 95 % of cases there are at least 4 parathyroid glands. They have also been found in the connective tissue of the neck superior or inferior to the thyroid gland. Displacement craniad up to the level of the hyoid and caudad down to the level of the upper mediastinum has been reported.

More than 80% of the parathyroid tissue is supplied by the inferior thyroid artery. The blood supply for the remainder comes from the superior thyroid artery and branches arising from the anastomoses between both thyroid arteries.

The main function of the parathyroids is the formation and secretion of parathyroid hormone (PTH), a polypeptide comprising 76 amino acids.

PTH controls the calcium metabolism by up-regulating renal re-absorption in the distal tubule. PTH release is primarily controlled by the serum calcium level. Renal hydroxylation of 25-cholecalciferol into1,25-dihydroxycholecalciferol, the active metabolite of vitamin D3, is controlled by PTH and regulates the intestinal absorption of calcium and phosphate.

Most of the serum calcium binds to albumin (80%), but only the free, i.e., ionized, calcium exhibits physiological significance. The calcium-albumin binding depends on the pH and is strengthened by alkalosis.

Since it controls the calcium metabolism, PTH impacts directly on the serum level of phosphate and magnesium.