The Mediastinal Parathyroid-peeping through the Window

INTRODUCTION

Parathyroid adenomas are the most common cause of primary hyperparathyroidism (PHPT). In 6–16% cases these are ectopically located, and of these, 20% may be located in the mediastinum^1,2 The common etiology for ectopic locations of parathyroid is due to comigration along with other tissues with the same embryological origin.^3,4 These glands may also be supernumerary in up to 22% of cases with other parathyroids in their normal locations and are a major cause of failed parathyroid operations.⁴ Mediastinal parathyroid adenoma (MPA) was first reported by Churchill in 1932, after operating on Captain Charles E Martel, who had previously undergone six unsuccessful cervical explorations before a sternum-splitting procedure to remove an MPA was performed. Mediastinal parathyroid adenomas are located completely below the level of the clavicles and result from aberrant migration of the parathyroid gland during embryological development.⁵ Anterior and postero-superior mediastinum are common sites for MPA, whereas aortopulmonary window (APW), pericardium, within the vagus nerve sheath and right dome of the diaphragm are rare locations.³ Surgery for APW parathyroid was first reported by Cohn and Silen in 1982.⁶ Parathyroid adenomas located above the innominate vein can be approached via a cervical incision; however, those in the APW require a thoracic approach.⁴ Preoperative localization of ectopic parathyroid glands is essential as it may change the operative approach.³ We present a rare case of PHPT with an APW parathyroid adenoma which was surgically managed.

CASE DESCRIPTION

A 32-year-old man presented with a 7-year history of low-back-ache and recurrent renal calculi and had previously undergone a left open pyelolithotomy for the same. He also complained of fatigue, increased irritability, getting angry easily, and insomnia. His biochemical evaluation is detailed in Table 1.

His previous evaluation at another center had not shown any enlarged parathyroid and was advised to follow-up. The current ultrasound thyroid done after confirming the biochemical diagnosis of primary hyperparathyroidism showed small subcentimetric lesions inferior to both lobes of thyroid—suspicious for parathyroid lesions, also seen on the CT neck and thorax. Tc99SestaMIBI scan detected an MPA (Figs 1 to 3), which on CT neck and thorax was anatomically localized to an ~ 2.1 × 2 cm in the APW (Fig. 2).

MANAGEMENT

He underwent a left posterolateral thoracotomy for the excision of MPA with cervical exploration and excision of the left superior parathyroid and left inferior parathyroid biopsy. Intraoperatively, there was a 2 × 1 cm brownish-colored adenoma in the APW (Fig. 4). In the neck, all four parathyroids appeared enlarged (Fig. 5) probably secondary to chronic kidney disease the largest being the left superior parathyroid gland—weighing 0.123 gm.

Postoperatively he had mild symptoms of hypocalcemia, and negative signs. His calcium normalized on the 1st post-op day from 14.67 to 9.60 mg%, and his PTH level decreased from 1052.8 to 10 pg/mL. The histopathology of the mediastinal parathyroid (Fig. 6) was consistent with parathyroid adenoma, and the left superior and left inferior parathyroid glands were hypercellular (Fig. 7).

DISCUSSION

Aortopulmonary window parathyroid adenoma was first reported by Cohn and Silen in 1982.⁶ It is typically located in the middle mediastinum, under the concavity of the aortic arch, between the aorta and the left pulmonary artery. The embryologic development of the parathyroid glands has been described in five stages by Norris, and the location of an ectopic gland in the APW is postulated to be an aberrantly situated superior parathyroid gland because of close contact with the pericardium in the 3 mm embryo and future right pulmonary artery.⁴ It can also be a supernumerary gland. These adenomas are not in relation to the thymus, and derive blood supply from the mediastinum—bronchial arteries/internal mammary artery, as originally discovered on preoperative arteriograms. Ultrasound has the least sensitivity in identifying lesions in the thymus and in the retroesophageal location because of the anatomical location. MIBI and ultrasound are complementary to detect ectopic parathyroids.³ With the evolution of nuclear scintigraphy, high-resolution CT and MRI, accurate localization and surgical planning have improved. Newer modalities like positron emission tomography usingC-methionine (MET–PET) offer promising diagnostic results.^7–9

The aortic arch can be used as a landmark to decide on the surgical approach with lesions above the arch approaching transcervically, with those below the arch approaching trans-thoracically.⁵ The preferred surgical approach is video-assisted thoracoscopic surgery (VATS) or thoracotomy.⁸ A thoracoscopic left lateral approach is ideal, which offers an excellent view of the phrenic and vagus nerves, besides facilitating division of the ligamentum arteriosum for access to deep-seated, often small, adenomas.⁹ The ligamentum arteriosum was divided in our case to ensure complete excision of the adenoma.

CONCLUSION

Aortopulmonary window parathyroid adenomas are commonly supernumerary. Structural imaging is paramount when MPA is suspected. MIBI and ultrasound are complementary and can together facilitate a more focused approach.³ The surgical approach is different from thymus-related mediastinal parathyroids, for which the cervical approach is adequate.¹⁰ This location within the aortopulmonary window will require VATS/thoracotomy, with the requirement of opening the pericardium or division of the ligamentum arteriosum for access.

REFERENCES

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