Anatomy of Parathyroid Glands

Number of Parathyroid Glands

The presence of four parathyroid glands is most common in humans. In dissection studies of 428 human subjects by Gilmour, four parathyroid glands were found in 87% of all patients and three parathyroids in 6.3%.5 Akerstrom and colleagues reported comparable rates in an autopsy study of 503 cases.6 Four parathyroids were found in 84% and three parathyroids in 3% of all patients in this study. The occurrence of supernumerary parathyroid glands is a rare entity that nevertheless has important clinical consequences, particularly in patients with hyperparathyroidism resulting from multiple-gland disease. In a series of 2015 patients who were operated on for primary hyperparathyroidism, a hyperfunctioning supernumerary fifth parathyroid caused hypercalcemia in 15 patients (0.7%).7 Nine of these patients required reoperations to reveal the parathyroid tumor. The majority of these fifth gland tumors were located in the mediastinum, either in the thymus (n = 1) or related to the aortic arch (n = 3). Edis and Levitt8 reported a rate of persistent hyperparathyroidism of 10% resulting from an enlarged supernumerary parathyroid in patients with secondary hyperparathyroidism. In a series of 762 patients with primary hyperparathyroidism, Wang and coworkers documented 6 patients with persistent hyperparathyroidism caused by hyperfunctioning supernumerary glands (0.6%), all of which were located in or in close association with the thymus.9 In a dissection study of 428 cases, Gilmour found supernumerary parathyroid glands in 29 cases (6.7%).5 Five parathyroids were observed in 25 cases (5.8%), 6 parathyroids in 2 cases (0.05%), 8 parathyroids in 1 case, and 12 parathyroids in another case. Akerstrom and colleagues concluded in an autopsy study that most supernumerary glands were either rudimentary or divided.6 When supernumerary parathyroids weighing less than 5 mg were excluded, there were 24 cases of supernumerary glands (5%). These supernumerary parathyroids were most frequently found in the thymus or in relation to the thyrothymic ligament.

Location of Parathyroid Glands

The location of parathyroid glands varies widely as a result of differences in degree of migration during embryologic development. Superimposed on the various locations of parathyroid glands are the displacements of parathyroid glands that become enlarged in the process of ensuing hyperparathyroidism. Enlarged parathyroid glands tend to migrate in an areolar plane, which offers little resistance as a result of gravity and perhaps swallowing and lower intrathoracic pressures.10 In some patients, these migrations result in considerable displacement of parathyroid tumors. Awareness of the common "pathways" of migration is invaluable in parathyroid surgery.

Eighty percent of the upper parathyroid glands are found at the cricothyroid junction, about 1 cm cranial to the intersection of the recurrent laryngeal nerve and the inferior thyroid artery (Fig. 47-1).6 The upper parathyroids, which are tucked posteriorly to the upper pole of the thyroid, are usually covered by a fascial sheath connecting the thyroid to the pharynx. More anteriorly situated upper parathyroids are located on the surface of the thyroid, frequently underneath the capsule of the thyroid. The unique feature of these subcapsular parathyroids is the freedom of movement of the parathyroids within the capsule. This feature distinguishes

FIGURE 47-1. Locations of the upper parathyroid glands. The more common locations are indicated by darker shading. The numbers represent the percentages of glands found at the different locations. (From Akerstrom G, Malmaeus J, Bergstrom R. Surgical anatomy of human parathyroid glands. Surgery 1984;95:17.)

FIGURE 47-1. Locations of the upper parathyroid glands. The more common locations are indicated by darker shading. The numbers represent the percentages of glands found at the different locations. (From Akerstrom G, Malmaeus J, Bergstrom R. Surgical anatomy of human parathyroid glands. Surgery 1984;95:17.)

parathyroids from thyroid nodules, which cannot move freely. The occurrence of intrathyroidal parathyroids is rare and controversial. A subcapsular parathyroid is easily confused with a true intrathyroidal parathyroid, which is surrounded by thyroid tissue. Akerstrom and colleagues noted true upper intrathyroidal parathyroid glands in three cases (0.2%) among 503 autopsies.6 Wang considered the upper parathyroid gland the most likely to be intrathyroidal because of the close embryologic relationship of the primordium of the upper parathyroid gland with the lateral complex of the thyroid.11 However, Wheeler and coauthors12 reported eight intrathyroidal parathyroid tumors in 7 patients (3.5%) in a series of 200 patients undergoing exploration of the neck for hyperparathyroidism. Seven of these eight intrathyroidal parathyroids were considered to be lower parathyroid glands. The incidence of intrathyroidal parathyroids ranges from 0.5% to 3% in the literature.11'1317

Normal upper parathyroid glands are found in the retro-or paraesophageal space in 1% of all cases.18 This space is the site where enlarged upper parathyroids descend to the superoposterior mediastinum. The importance of this ectopic location of upper parathyroid glands is illustrated in Figure 47-2. In a series of 104 patients with persistent hyperparathyroidism, 34 parathyroid tumors were found in the superoposterior mediastinum.19

34 SUPERIOR POSTERIOR MEDIASTINUM 21 ANTERIOR MEDIASTINUM 19 BEHIND UPPER POLE OFTHYROID 13 BEHIND CLAVICLE WITHIN THYMIC TONGUE 10 BEHIND LOWER POLE OFTHYROID 5 BEHIND ESOPHAGUS 1 BEHIND ANGLE OF THE JAW 1 INTRATHYROIDAL

The distribution of locations of the lower parathyroid glands varies more widely (Fig. 47-3). More than half of the lower parathyroids are located around the lower pole of the thyroid. Twenty-eight percent of the lower parathyroids are found in the thyrothymic ligament or within the thymus. A rare location of lower parathyroids is high in the neck at the carotid bifurcation, resulting from absence of embry-ologic migration. When lower parathyroid glands become enlarged, they tend to migrate into the anterior mediastinum. Figure 47-2 shows that one third of all missed parathyroid tumors were found in the thymus or in the anterior mediastinum.

Gross Features of Parathyroid Glands

It is essential in parathyroid surgery to distinguish between normal and hyperfunctioning parathyroid glands. Hyperfunctioning parathyroid glands are enlarged. Therefore, definition of the normal size of a parathyroid gland is crucial. The size of normal parathyroids varies considerably because parathyroids are easily molded as a result of their soft consistency. The shape of the parathyroid gland is dependent on its anatomic position. Parathyroids that are located in loose tissue have an oval, bean, or teardrop shape. When parathyroids lie underneath a capsule, their shape is flat with sharp edges. Although a particular shape of a parathyroid gland is not associated with hyperfunction, a spherical shape often indicates hyperactivity of the parathyroid.20

FIGURE 47-3. Locations of the lower parathyroid glands. The more common locations are indicated by darker shading. The numbers represent the percentages of glands found at the different locations. (From Akerstrom G, Malmaeus J, Bergstrom R. Surgical anatomy of human parathyroid glands. Surgery 1984;95:17.)

FIGURE 47-3. Locations of the lower parathyroid glands. The more common locations are indicated by darker shading. The numbers represent the percentages of glands found at the different locations. (From Akerstrom G, Malmaeus J, Bergstrom R. Surgical anatomy of human parathyroid glands. Surgery 1984;95:17.)

Because the shape of parathyroids is diverse, the parenchymal weight of parathyroid glands is the most reliable parameter for parathyroid function.21 The weight of parathyroid glands attains its maximum in men in the third decade of life.22 In women, there is a progressive rise until about the age of 50 years. The weights of parathyroid glands are lower in patients with chronic illnesses, except renal disease, and lower in women than in men.23 The weights of the lower parathyroids are greater than those of the upper parathyroids. The normal weight of a parathyroid gland remains uncertain. An upper limit of normal parenchymal weight of 38 mg for a single gland was reported by Gilmour and Martin.22 Dufour and Wilkerson found an upper normal limit of 49 mg for the parenchymal weight of a single gland.23 Akerstrom and colleagues found a maximal normal glandular weight of 59 mg in an autopsy study of 368 cases without evidence of hyperparathyroidism.24 Dufour and Wilkerson demonstrated that 95% of the individual glands weighed between 8.2 and 75.0 mg. It should be noted that an overlap of the weights of normal and abnormal parathyroid glands exists. In our series of 1080 patients with hyperparathyroidism, several patients became normocalcemic after removal of abnormal parathyroid glands weighing only 60 mg. Underestimation of the weight of parathyroid glands can easily occur when the parathyroid lies underneath the capsule of the thyroid and only part of the parathyroid can be examined. Therefore, the entire parathyroid should be exposed before assessing its weight. Alternatively, the weights of parathyroid glands, which are surrounded by an abundance of fat, can be overestimated.

The color of normal parathyroid glands ranges from yellowish brown to reddish brown. The color depends on the amount of fat, number of oxyphil cells, and degree of vascularity.25 Enlarged parathyroid glands display colors varying from dark brown to light yellow. In secondary or tertiary hyperparathyroidism, the enlarged parathyroids sometimes have a typical gray color. Parathyroid carcinomas can also have a grayish white surface.

Blood Supply of Parathyroid Glands

Preservation of the blood vessels supplying the parathyroid glands in parathyroid surgery is essential to prevent damage to normal parathyroid glands. Most parathyroid glands have a single artery (80%).26 The length of the parathyroid artery can vary from 1 to 40 mm. In general, the parathyroid glands derive their arterial branches from the inferior thyroid artery. However, 20% or more of the upper parathyroid glands are vascularized by the superior thyroid artery. Delattre and coworkers26 found in an autopsy study that 10% of the lower parathyroid glands were dependent on an anterior branch of the superior thyroid artery. In most of these cases, the inferior thyroid artery was absent, which is not unusual at the left side. Mediastinal parathyroids often have an artery that is a thymic branch of the internal mammary artery.4 The venous return of parathyroid glands runs almost parallel to the arterial vessels.

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