Mediastinal Parathyroids
Thomas W. Shields
The two pairs of parathyroid glands derive from the endoderm of the third and fourth pharyngeal pouches bilaterally (Fig. 163-1). Hamilton and Mossman9 have presented evidence that the fifth pharyngeal pouch may fuse with the fourth pouch to form the so-called caudal pharyngeal complex and that parathyroid IV arises from the dorsal portion of this complex. The possible importance of this observation is discussed in this chapter.
Number of Parathyroid Glands
In an autopsy study of 527 specimens, Gilmour and Martin9 noted that the normal number of four glands was identified in 80%, whereas a lesser or greater number of glands was found in the remaining 20%. In 73 specimens (14%), three glands were found in 69 and only two glands in 6 specimens. Because the weight of the individually identified glands in these specimens fell within the norm, however, Gilmour and Martin surmised that the decrease in number resulted from not identifying the missing gland or glands rather than from a true abnormality in the normal number. In a subsequent report, however, Gilmour7 noted that because of studies by Rössle,15 Böttiger and Wernstedt,2 and Gilmour6 of 16 embryos in which thorough serial microscopic sections were made and that did not identify four glands in all specimens, it may be that less than four glands are present in some individuals.
The presence of supernumerary glands, however, was definitely established in 6% of the specimens: five glands in 31 (5.8%) and six glands in 2 specimens (0.3%). A greater number of glands have been even more rarely identified. Scholz and associates18 at the Mayo Clinic noted that when a supernumerary gland or glands was present in association with a normal number of glands in the neck, 14 of 15 were found in the mediastinum. Russell and associates16 reported that two-thirds of the supernumerary glands in their selected series of 15 patients were located in the mediastinum: three were associated with the aorta or great vessels and seven were within the mediastinal portion of the thymus. By extrapolating the data presented by Russell and associates17 of 2,770 operations for hyperparathyroidism, the incidence of a mediastinal location of one of the normal four glands is only 1%. This extrapolation essentially agrees with the data of Wang,21 who reported that 2% of people may have one parathyroid gland in the mediastinal portion of the thymus within 3 to 4 cm of the sternal notch.
Mediastinal Locations
The explanation for a mediastinal position of a gland is that, embryologically, the parathyroids III remain associated with or even imbedded in the cephalic tip of the thymic lobes as they migrate caudally. Their descent usually stops in the neck adjacent to the lower pole of the thyroid. However, Gilmour7 found 19 of 792 parathyroids III (2.4%) ≥3 cm below the lower poles of the thyroid gland: 1.4% at 3 cm, 0.9% at 4 cm, and 0.1% at 6 cm. The more distal of such glands or a supernumerary gland may be carried down into the prevascular (anterior) compartment of the mediastinum along with the thymus.
Retropharyngeal (Low Cervical) Area
The originally lower parathyroids IV become cephalad in position relative to the original upper parathyroids III. Because parathyroids IV arise in close association with the postbranchial bodies that emerge with the thyroid primordium, these glands (parathyroids IV) remain in contact with the upper posterior aspects of the thyroid lobes. When parathyroids IV assume an ectopic position, they remain in the visceral compartment of the neck. With caudad descent, an ectopic gland may come to lie in the posterior aspect of the superior portion of the visceral compartment of the mediastinum adjacent or dorsal to the trachea or even the esophagus and dorsal to the recurrent nerves. In a dissection of 160 postmortem subjects, Wang21 found 1% of parathyroid glands IV behind the lower pharynx or esophagus in the visceral compartment of the neck near the thoracic inlet. Gilmour7 found 46 parathyroids IV (5.8%) in a more posterior plane than that of the lower pole of the thyroid and in the same plane as the esophagus and 9 (1.1%) opposite the lower pole of thyroid behind the esophagus, which agrees with Wang’s findings. When a parathyroid gland IV descends below the thoracic inlet, it lies in the retrotracheal portion of the visceral compartment. According to Nathaniels and associates,14 such glands are often attached by a large vascular pedicle that ascends to the inferior thyroid vessels in the neck.
In reviewing a series of 112 patients who underwent reoperation for primary hyperparathyroidism, however, Wang22 found the missing gland in the retrotracheal space in 34 patients (38%) (Fig. 164-1). Most of these obviously represented an enlarged parathyroid IV gland that was originally located higher but that had descended into this abnormal location. Thompson and
associates20 recorded a similar incidence of enlarged parathyroids IV in this location (Fig. 164-2).
associates20 recorded a similar incidence of enlarged parathyroids IV in this location (Fig. 164-2).
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
