Anatomy - Exstirpation of a solitary parathyroid adenoma

Located between the anterior border of the M. sternocleidomastoideus, the mandible, and the jugular fossa, the anterior neck triangle in the area of the hyoid bone includes the supra- and infrahyoid musculature, vessels, nerves, and the thyroid gland. For thyroidectomy, among the infrahyoid muscles, possibly the medially located

• M. sternohyoideus (sternum → hyoid bone) and below it the
• M. sternothyroideus (sternum → thyroid cartilage of the larynx) as well as further laterally the
• M. omohyoideus (scapula → intermediate tendon → hyoid bone)

are of importance, as they partially cover the thyroid gland and must be pushed aside intraoperatively.

Blood Vessels
The common carotid artery divides at the upper edge of the thyroid cartilage at the level of its carotid sinus (receptors for blood pressure and chemoreceptors for blood gases) into its internal and external branches and runs previously immediately lateral in the carotid sheath next to the trachea and esophagus cranially. Here it touches as a large vessel the right and left thyroid lobes. From the sigmoid sinus of the skull emerges the internal jugular vein, which collects blood from the skull and neck, pulling caudally first adjoining the internal carotid artery in the carotid sheath, then running further laterally and enclosing the common carotid artery as well as the vagus nerve (X) laterally.

Nerves
In addition to the vagus nerve and its upper branch to the larynx (N. laryngeus superior → M. cricothyroideus anterior, as well as mucosa of the upper larynx half), located in the area of the thyroid gland laterally, descending from above, is the ansa cervicalis profunda (superior and inferior roots, from C1-C3) for the innervation of the three mentioned muscles of the anterior neck triangle and the N. transversus colli (from C2/3, skin innervation, platysma).

Fascial Layers
In the anterior neck triangle, several fascial layers (all belonging to the cervical fascia) are located under the skin, which exhibit certain peculiarities:

• The superficial lamina superficialis surrounds all neck structures except for the platysma and separately envelops the M. sternocleidomastoideus and dorsally the M. trapezius (N. accessorius XI),
• the middle lamina pretrachealis surrounds the infrahyoid musculature, and the
• deep lamina prevertebralis runs between the esophagus and the spine outside the surgical area.

The thyroid gland/parathyroid glands and the trachea possess, just like the lateral vessel-nerve bundle (A. carotis, V. jugularis interna, and N. vagus), in addition their own organ fascias. Due to the arrangement of the cervical fascias, compartments enclosed by them and spaces in between (spatia) arise, which extend into the mediastinum and represent potential infection pathways.

The unpaired thyroid gland (Glandula thyroidea) belongs, like the 4 parathyroid glands (Gld. parathyroidea), to the endocrine organs. It lies in the anterior neck region caudal and lateral to the thyroid cartilage and is composed of a left and a right lobe as well as an isthmus connecting both lobes. The lobes cover the lateral upper rings of the trachea, as well as the cricoid and thyroid cartilage from the front, the isthmus is located centrally at the level of the 2nd/3rd tracheal ring. The organ has its own connective tissue sheath (Capsula fibrosa) and is covered ventrolaterally by the infrahyoid musculature (M. sternohyoideus, M. sternothyroideus and M. omohyoideus) as well as the pretracheal lamina. Due to its embryonic development and its descent (via the ductus thyroglossus) from the base of the tongue (Foramen caecum) to its later, described location, cysts or ducts to the Foramen caecum, as well as functional glandular parts (e.g., as Lobus pyramidalis), can remain along this path.

Blood supply
Arteries from the A. carotis externa (→ A. thyroidea superior) and from the A. subclavia via the Truncus thyrocervicalis (→ A. thyroidea inferior) supply the thyroid gland with anterior and lateral/posterior smaller branches. Inconsistently, but certainly to be noted, the A. thyroidea ima comes directly from the aortic arch or the Truncus brachiocephalicus from below to the gland. The blood drainage, which has systemic significance in this endocrine organ, occurs via similarly named vessels (V. thyroidea superior und media → V. jugularis interna, V. thyroidea inferior → V. brachiocephalica).

Lymphatic vessels and nerves
The lymphatic drainage from the thyroid gland occurs paratracheally, as well as to deep cervical lymph nodes. Particular attention should be paid intraoperatively to the N. laryngeus recurrens (from the N. vagus X, running on the right around the A. subclavia, on the left around the aortic arch). It ascends in the groove between the trachea and esophagus cranially to the larynx, to supply there, in addition to the sensory mucosal innervation of the lower half of the larynx, all laryngeal muscles except for the M. cricothyroideus anterior (“Antikus”).
The 4 parathyroid glands (Gld. parathyroidea) are small oval glands on the posterior surface of the thyroid lobes, which are supplied via the A. thyroidea inferior.

In thyroidectomy, due to the close spatial proximity, there is a risk of injuring structures such as the recurrent laryngeal nerve and the parathyroid glands on the posterior side of the organ, but also parts of the sympathetic trunk or vessels and nerves near the common carotid artery.

The paired, lentil-sized parathyroid glands are located between the Capsula fibrosa and Capsula externa on the posterior surface of both thyroid lobes, occasionally they are also embedded in the thyroid tissue.
The upper parathyroid glands are usually located at the level of the lower edge of the Cartilago cricoidea, cranial to the intersection of the A. thyreoidea inferior with the Nervus recurrens, dorsal to the Nervus recurrens.
The normal location of the lower parathyroid glands is at the level of the 3rd – 4th tracheal cartilage, at or caudal to the intersection of the A. thyreoidea inferior with the Nervus recurrens, ventral to the Nervus recurrens.

The number and location of the epithelial bodies vary considerably; in 95% of cases, there are 4 or more. Due to the long developmental migration, embryologically induced positional anomalies are variable and relatively common. They can be found above and below the thyroid in the neck connective tissue. Displacements cranially up to the level of the mandibular angle (carotid bifurcation) and especially caudally into the Lig. thyreothymicum or with the thymus into the anterior mediastinum occur. In intrathyroidal location, they are mostly found in the lower third.

The A. thyreoidea inferior is the supplying blood vessel for the parathyroid glands in over 80%. The rest is supplied via the A. thyreoidea superior as well as from branches that arise from the anastomoses between the two thyroid arteries.

The function of the parathyroid gland consists mainly in the formation and secretion of parathyroid hormone (PTH), a polypeptide consisting of 76 amino acids.

Parathyroid hormone regulates calcium metabolism by increasing renal reabsorption in the distal tubule. The release of PTH is mainly determined by the level of the serum calcium level. The hydroxylation of 25-cholecalciferol to 1,25-dihydroxycholecalciferol, the active vitamin D3 metabolite, which takes place in the kidney, is controlled by PTH and regulates the intestinal absorption of calcium and phosphate.

The largest portion of serum calcium is bound to albumin (80%), but only the free portion, the ionized calcium, is of physiological significance. The binding of calcium to albumin is pH-dependent and is enhanced by alkalosis.

PTH, through the control of bone calcium metabolism, also has a direct influence on the serum levels of phosphate and magnesium.