Parathyroidectomy

Parathyroidectomy

EMBRYOLOGY

The inferior parathyroid glands develop from the dorsal bud of the third pharyngeal pouch, whereas the superior parathyroid glands are derived from the fourth pharyngeal pouch. These glands attach to the dorsal surface of the thyroid gland as it migrates caudally. Parathyroid glands derived from the third pharyngeal pouch come to lie in a more inferior position because they descend along with the thymus gland (which is derived from the ventral bud of the third pharyngeal pouch). Failure of the formation of the parathyroid gland and thymus from the third pharyngeal pouch results in the DiGeorge syndrome. Variations in embryologic migration can result in ectopic parathyroid tissue. Knowledge of these sites is essential during the performance of parathyroidectomy.

ANATOMY

The number of parathyroid glands may vary from two to six, but in 90% of patients there are four (two on each side). Each gland is oval, approximately 6 mm long, and frequently covered with adipose tissue. These glands are related to the posterior surface of the thyroid gland and often lie within its fascial covering. The two superior parathyroid glands are more constant in location and can be found at about the level of the cricoid cartilage. The two inferior parathyroid glands are more variable in location, although they frequently lie along the lower part of the thyroid gland. The blood supply to these glands is from an upper and a lower parathyroid artery that arise from the inferior thyroid artery. On rare occasions, the upper parathyroid artery can arise from the superior thyroid artery.

SPECIAL PREOPERATIVE PREPARATION

Before exploration, the basic laboratory tests that should be reviewed are the total ionized serum calcium, phosphate, alkaline phosphatase, and protein levels. Elevated levels of parathyroid hormone should be confirmed. Preoperative localization studies are not required, although high-resolution ultrasonography or nuclear medicine scintigraphy, including thallium/technetium subtraction scan and sestamibi scan, may be obtained and may be helpful. Preoperative laryngoscopy is performed routinely to rule out occult vocal cord paralysis.

Operative Procedure

POSITION

The patient is usually placed in a supine position, and the neck is extended either by placing the head on a ring or by placing a rolled towel beneath the shoulders. The neck and the upper thorax are scrubbed, prepped, and draped in the usual manner. The patient is placed in reverse Trendelenburg position.

INCISION

A curvilinear transverse skin-crease incision is marked with an indelible pen approximately two fingers-breadth above the clavicle. The skin at the proposed site of incision is infiltrated with 1% lidocaine with epinephrine. The skin incision is made and deepened with electrocautery until the platysma is identified.

EXPOSURE AND OPERATIVE TECHNIQUE

The platysma is divided with electrocautery, and subplatysmal flaps are developed superiorly and inferiorly. The midline is identified and the cervical fascia incised. The strap muscles are dissected off the thyroid gland and retracted laterally. Beginning on either the right or the left side or the side indicated by preoperative imaging, the thyroid lobe is elevated and rotated medially. The areolar tissue is carefully cleared from around the lobe, and the inferior thyroid artery and the recurrent laryngeal nerves are identified. It is important to expose the nerve from its entrance into the operative field to the level of the larynx. As a rule, it is safer to expose vital structures so that they are identified and preserved rather than avoiding their exposure and iatrogenically damaging them. It is essential to avoid unnecessary bleeding in the operative field because it can make the identification of parathyroid glands difficult. The parathyroid glands characteristically are yellowish brown and vary in shape and dimension. The thyroid tissue is usually red, lymph nodes are paler and pinker, and thymic tissue is pale grayish yellow. The parathyroid gland is also identified by observing a single small artery entering its hilus and radiating out over the capsule. The usual number of parathyroid glands is four, although five parathyroid glands can be found in 4% of cases, three in 5% of cases, and two in less than 1%. The anatomic location of the parathyroid glands is reasonably constant. In approximately 80% of the cases, the glands are situated symmetrically on opposite sides of the neck. The superior parathyroid glands are invariably found behind the upper pole of the thyroid gland or at the cricothyroid junction. The inferior parathyroid glands can be variable in location due to embryologic mobility but are generally found in the immediate vicinity of the lower pole of the thyroid. The surgeon should identify all parathyroid glands before removing any of them, however; some surgeons perform preoperative localization and explore and resect only the enlarged parathyroid gland(s).

Figure 2-1 Location of the inferior parathyroid adenoma adjacent to the inferior thyroid artery and recurrent laryngeal nerve. The superior parathyroid gland is invariably found behind the upper pole of the thyroid gland or at the cricothyroid junction

If an inferior parathyroid gland cannot be found after an extensive search in the operative area, the thymus should be gently elevated from the mediastinum. If the parathyroid gland still cannot be located, in addition to examining the thyrothymic ligament, the surgeon should examine the tracheoesophageal groove, retroesophageal space, and carotid sheath to locate an ectopic parathyroid gland. The key anatomic structures are the inferior thyroid artery and its branches, which will often guide the surgeon to the parathyroid glands. If three parathyroid glands have been identified and one is enlarged and the other two are of normal size, an extensive search is undertaken. If the fourth gland cannot be found, the procedure may be terminated because the chance of leaving behind a second hyperfunctioning parathyroid gland is approximately 5% to 10%. If, on the other hand, two or more enlarged parathyroid glands have been identified and the fourth gland cannot be found, a diligent search is necessary because the likelihood that the remaining fourth gland is enlarged is greater than 50%.

If the retroesophageal and retropharyngeal spaces and the carotid sheath have been explored and the fourth gland still has not been identified, an ipsilateral thyroid lobectomy should be considered if the diagnosis of parathyroid hyperplasia is considered. At the initial exploration, one should not proceed with a sternotomy but should terminate the cervical exploration and then proceed to confirm the diagnosis by performing localizing studies before undertaking a mediastinal dissection. Once the surgeon has identified the parathyroid glands, the extent of resection is based on the operative findings. When an adenoma is found and normal glands are identified, only the adenoma needs to be removed to cure hyperparathyroidism. Biopsy of all three remaining glands is unnecessary and undesirable because it can lead to ischemic injury.

If four hyperplastic glands are identified, the surgeon has the option of performing a subtotal parathyroidectomy, in which approximately 35 to 50 mg of one parathyroid gland is left in situ. Alternatively, a total parathyroidectomy with immediate transplantation of the parathyroid tissue in either the sternocleidomastoid or the forearm muscles may be performed. The remaining parathyroid gland, whether in the neck or in the forearm, should be marked with hemoclips for subsequent identification if hyperparathyroidism recurs. If four normal parathyroid glands are found, the surgeon has to diligently search for an adenoma of a supernumerary gland elsewhere in the neck, as described earlier, or within the thyrothymic ligament or thymus itself.

If at the time of surgery the enlarged parathyroid gland feels hard and appears grayish in color and is firmly fixed to surrounding tissue, the possibility of parathyroid carcinoma should be entertained. In this situation it is often necessary to perform an ipsilateral thyroid lobectomy and resection of the parathyroid tumor and the adjacent invading structures. If there are enlarged lymph nodes, a neck dissection may also be needed.

CLOSURE

The wound is irrigated and hemostasis is secured. Routine drainage is usually unnecessary. The platysma is carefully reapproximated with interrupted 3-0 absorbable sutures. The dermis is closed with interrupted 3-0 absorbable sutures and the skin is approximated with 4-0 subcuticular nonabsorbable monofilament or absorbable sutures. Steri-Strips are applied longitudinally, which can act as a dressing as well.