Thyroidectomy

Thyroidectomy

EMBRYOLOGY

During the fourth week of fetal development, an endodermal thickening develops in the floor of the primitive pharynx at the junctional area between the first and second pharyngeal pouches called the foramen cecum. The endodermal thickening, which represents the primitive thyroid tissue, penetrates the underlying mesenchymal tissue and begins to descend anterior to the hyoid bone and the laryngeal cartilages to reach its final adult position in front of the trachea. The thyroid diverticulum becomes bilobed and develops into two lateral lobes and a median isthmus. The thyroid gland is temporarily attached to the lumen of the foregut at the foramen cecum by the thyroglossal duct, which eventually solidifies and disappears. The distal portion of the thyroglossal duct gives rise to the pyramidal lobe and the levator superioris thyroideae in adults. Ectopic thyroid tissue can be found at any point along its embryologic descent.

ANATOMY

The thyroid gland weighs about 25 g and is composed of two lobes attached by the isthmus. The isthmus is related posteriorly to the second, third, and fourth tracheal rings; knowledge of this relation is pertinent during the performance of tracheostomy. A variable-sized pyramidal lobe arises from the isthmus and is directed upward usually to the left, although it may be absent in 50% of individuals. The gland lies on the anterolateral aspect of the cervical trachea and extends from the level of the thyroid cartilage to the fifth or sixth tracheal ring. Anteriorly it is covered by the pretracheal fascia, strap muscles, and, more laterally, the sternocleidomastoid muscle. The blood supply to the thyroid gland is derived from (1) the superior thyroid artery, which arises from the external carotid artery and descends to the upper pole of the gland; (2) the inferior thyroid artery, a branch of the thyrocervical trunk that arises from the subclavian artery; and (3) the inconsistently present thyroidea ima artery, arising from the innominate artery, right subclavian artery, or aortic arch. Venous drainage is from the (1) superior thyroid vein, which drains directly into the internal jugular vein, (2) middle thyroid vein, which drains into the internal jugular vein and is the first vessel encountered during thyroidectomy, and (3) inferior thyroid vein, which leaves the lower border of the gland to join the left innominate vein.

Several important structures are in close relation to the gland and are of surgical relevance. The external laryngeal nerve, a branch of the superior laryngeal nerve, accompanies the superior thyroid pedicle and travels medially to supply the cricothyroid muscle. If this nerve is severed, it alters the voice pitch, which is particularly important to singers. The recurrent laryngeal nerve is a branch of the vagus, and embryologically it arises in close relation to the fourth aortic arch. Because of the descent of the fourth aortic arch vessels (the subclavian artery on the right and the aortic arch on the left), the recurrent nerves first are drawn caudally into the mediastinum and then course upward toward their final destination, the vocal cords. The nerve usually lies in the tracheoesophageal groove near the terminal branches of the inferior thyroid artery. The recurrent laryngeal nerve is usually found in Simon’s triangle, which is formed by the inferior thyroid artery superiorly, the common carotid artery laterally, and the esophagus medially. The surgeon also needs to be aware of the presence of a nonrecurrent laryngeal nerve. The posterior aspect of each thyroid lobe is related to the parathyroid glands and is at risk of injury during thyroidectomy.

SPECIAL PREPARATION

Apart from ordering routine investigations and reviewing special investigations that may have been performed, such as thyroid function tests, ultrasonography of the thyroid, isotope scan, and fine-needle aspiration, cytology must be reviewed. Serum calcium level is obtained because hyperparathyroidism may coexist.

Indirect laryngoscopy is performed preoperatively to evaluate the mobility of the vocal cords and detect unsuspected vocal cord paralysis (if paralysis is present, it is essential not to damage the recurrent laryngeal nerve supplying the normal vocal cords). Patients who are thyrotoxic should be rendered euthyroid. This can be achieved medically by the use of carbimazole. If the patient has evidence of sympathetic overdrive such as tachycardia, a beta-blocker such as propranolol is added but must be continued postoperatively for 8 to 10 days. There is no need to use iodides, because there is no convincing evidence that it reduces vascularity of the thyroid gland.

Operative Procedure

POSITION

The patient is placed in a supine position initially with the neck extended by placing a ring beneath the head and a sandbag roll beneath the shoulder. The patient is placed in a reverse Trendelenburg position. The neck and the upper thorax are prepped, and a head towel is placed. To maintain sterility on the lateral aspect of the neck, folded towels are placed. Next, four towels are placed and secured to the skin with either silk sutures or towel clips. The surgeon stands on the side opposite to the lobe that is being resected.

INCISION

The incision is marked 2 to 3 cm above the sternal notch along a skin crease using an indelible pen. A very low incision can lead to a keloid scar and fixes the skin onto bony prominences. The lateral ends of the incision are curved to follow Langer lines and must be symmetrical. The incision should extend the same distance on each side of midline and usually continues beyond the anterior border of the sternocleidomastoid muscle. Shorter incisions not only provide inadequate exposure but may contract and thus be cosmetically unappealing. To avoid excessive dermal bleeding, the incision can be infiltrated with 1% lidocaine containing epinephrine. With a no. 15 scalpel the skin and subcutaneous tissues are sharply divided and the platysma identified. The platysma is then divided with electrocautery.

EXPOSURE AND OPERATIVE TECHNIQUE

Once the platysma is divided, the assistant lifts the skin and the platysma upward with double skin hooks to allow for the creation of a subplatysmal flap. Maintaining the dissection close to the platysma ensures that cervical fascia is not included in the flap. The superior flap extends upward to the thyroid notch and the lower flap extends downward to the sternal notch. This procedure should be blood free, because the superficial veins lie beneath the cervical fascia. To retract the skin flaps, either Weitlaner or Gelpi retractors can be used.

After the flaps are created, the key to this operation is locating the correct plane of dissection. Next, the investing fascia is opened in the midline between the anterior jugular veins; this opening extends from the thyroid cartilage superiorly to the suprasternal notch inferiorly. At the lower part there is usually a transverse cervical vein that needs to be clamped, divided, and ligated with 3-0 silk sutures. The strap muscles (the sternohyoid, and deep to that, the sternothyroid) are carefully dissected to allow their retraction laterally. At times these muscles may need to be transected to gain better access to the thyroid gland; when necessary this should be done at the level of the thyroid cartilage to preserve their innervation from the ansa hypoglossi nerve. If there is local invasion by a thyroid neoplasm, the thyroid lobe is resected en bloc with its overlying strap muscles.

Dissecting close to the strap muscles minimizes bleeding. The correct plane of dissection is entered when the vessels overlying the thyroid gland become prominent. The loose areolar tissue overlying the thyroid gland is divided with electrocautery. Blunt dissection on the lateral aspect of the thyroid gland must be avoided, as it invariably leads to bleeding because here the vessels are friable and tear easily. After the anterior surface of the thyroid has been thoroughly exposed, the entire gland is carefully explored and palpated.

The strap muscles are firmly retracted with a small loop retractor while the thyroid gland is drawn medially. To permit further medial rotation of the thyroid gland, the next step involves division of the middle thyroid vein after ligating it in continuity with 3-0 silk sutures. The silk tie on the distal aspect of the vein can be left long to aid in further subcapsular dissection. The capsule along the lateral border of the thyroid gland is dissected with fine Halsted mosquito hemostats and divided with bipolar electrocautery. Further mobilization of the thyroid gland to improve exposure of the posterior surface can only be obtained by dissecting the superior pole. With a retractor, the upper portion of the strap muscles is drawn cephalad. Concurrently, the thyroid gland is firmly retracted downward. With use of a Kittner dissector and a Jackson right-angle clamp, the upper pole is separated from the cricothyroid muscle, to which the external branch of the superior laryngeal nerve is adherent. The lateral portion of the upper pole is also freed. The terminal branches of the superior thyroid artery and vein are identified, ligated in continuity with 2-0 silk sutures, and divided. If necessary, two ligatures are placed proximally, because once divided the superior thyroid artery tends to retract. Do not mass-ligate the superior pole, because it will endanger the external laryngeal nerve.

After division of the superior thyroid pole, the thyroid gland can be easily rotated medially. Attention is now directed toward identifying the parathyroid glands and the recurrent laryngeal nerve. Parathyroid glands are small, yellowish brown, and soft and pliable, in contrast to lymph nodes or thyroid nodules, which are firm. Furthermore, a single small artery can be seen entering the gland; the artery radiates out over the capsule in a fernlike pattern. The upper parathyroid gland is invariably found behind the upper third of the thyroid gland adjacent to the cricothyroid junction.

Next, the carotid artery is identified and retracted laterally. At approximately the junction of the middle and lower thirds of the thyroid gland, the tortuous inferior thyroid artery is identified, which helps locate the recurrent laryngeal nerve. Often a triangle (Simon’s triangle) is formed by the common carotid artery laterally, inferior thyroid artery superiorly, and esophagus medially, and the recurrent laryngeal nerve can be seen coursing upward in this triangle to enter the larynx. The nerve usually appears as a white cord with fine red vasa vasorum coursing over its surface. Dissection should proceed directly over the recurrent laryngeal nerve to expose it along its course to the larynx. If the recurrent laryngeal nerve remains difficult to identify, the possibility of a nonrecurrent laryngeal nerve should be considered.

The inferior parathyroid gland should be identified next; it is usually found adjacent to the terminal branch of the inferior thyroid artery on the posterior surface of the thyroid. Once the integrity of the recurrent laryngeal nerve and the parathyroid glands has been ensured, the terminal branches of the inferior thyroid artery close to the thyroid capsule are ligated. Ligation of the main trunk of the inferior thyroid artery should be avoided, because it compromises the blood supply of the parathyroid glands.

Dissection now proceeds around the lower pole of the thyroid. The pole is mobilized by careful dissection with a combination of Kittner dissector, Halsted mosquitoes, or Jackson right-angle clamp, and all blood vessels entering are divided and ligated using 3-0 silk sutures. These maneuvers facilitate further medial rotation of the thyroid gland. For total lobectomy, the thyroid lobe and the isthmus are dissected off the anterolateral wall of the trachea while the recurrent laryngeal nerve is kept under direct vision. A straight Crile clamp is placed between the isthmus and the opposite lobe, and the specimen is sharply excised. The thyroid tissue within the clamp is oversewn with a running interlocking 3-0 silk suture for hemostasis.

If a total thyroidectomy is being performed, the remaining lobe is removed in a similar fashion, with division of the middle thyroid vein, identification of the recurrent laryngeal nerve and parathyroid glands, and ligation and division of the superior pole and branches of the inferior thyroid vessels.

CLOSURE

The operative area is irrigated, and hemostasis is achieved. To control troublesome oozing, instead of using electrocautery, topical hemostatic agents such as Gelfoam, thrombin, or Surgicel are used because they do not endanger adjacent vital structures. A Valsalva maneuver is performed on the patient by the anesthesiologist, and the operative area is again carefully checked for hemostasis; if achieved, there is no need for a drain.

To help with wound closure, the neck is brought into neutral position by removal of the shoulder roll. The deep cervical fascia in the midline is sutured with 3-0 absorbable sutures. For a good cosmetic result, the platysma must be carefully reapproximated with interrupted 3-0 absorbable sutures. The skin can be closed with subcuticular 4-0 nonabsorbable monofilament or absorbable sutures.