Parathyroid Cyst
Parathyroid Cyst: An Uncommon Cause of a Palpable Neck Mass and Hypercalcemia
PARATHYROID CYSTS ARE rare lesions arising in the neck and anterior mediastinum. Goris reported the first case in 1905, and since then, fewer than 250 cases have been published in the literature. Parathyroid cysts constitute only 0.6% of all thyroid and parathyroid lesions, but should be considered in the differential diagnosis of anterior neck masses, particularly in the presence of hypercalcemia.
Report of a Case
A 66-year-old man was referred to endocrinology for evaluation of a neck mass. For four years he had been followed by his primary care physician for hypercalcemia attributed to primary hyperparathyroidism with serum calcium concentrations as high as 3.1 mmol per liter (12.5 mg per dl; normal, 2.1-2.6 mmol per liter). The patient denied fatigue, polyuria, or history of peptic ulcers. During doppler ultrasonography of his carotid arteries, a complex 1.5 X 1.5 cm cystic mass was incidentally discovered in the left lower pole of the thyroid. Subsequent technetium 99 scanning showed homogeneously decreased tracer accumulation in the left lobe without evidence of focal hot or cold areas. He was then referred to us. On physical examination, he appeared well and was normotensive. A 1-cm flat nodule was palpable at the lateral aspect of the lower pole of his thyroid. Laboratory studies yielded the following results: calcium, 3.0 mmol per liter; phosphorous, 1.07 jxmol per liter (3.3 mg per dl; normal, 0.81-1.65 jxmol per liter); creatinine, 159 mmol per liter (1.8 mg per dl; normal, 44-115 mmol per liter); intact parathyroid hormone (hPTH), 112 ng per liter (normal, 10-65); thyroid-stimulating hormone (TSH), 1.93 mlU/ml (normal, 0.23-4.00). A 24-hour urine calcium totaled 11.8 mmol (471 mg; normal 0.0-7.5 mmol). Fine needle aspiration (FNA) of the cyst was performed under ultrasound guidance, and 8 ml of amber, blood-streaked fluid was removed. The fluid’s intact hPTH level was 163,000 ng per liter. Cytological examination revealed hemo-siderin-laden macrophages and a few, benign-appearing, follicular cells that did not stain for thyroglobulin. Two days after the FNA, the patient complained of fatigue and polyuria. The serum calcium level had risen to 3.3 mmol per liter, but fell to 3.0 mmol per liter over 72 hours with aggressive oral fluid intake and furosemide. One month after the FNA, his mass was surgically removed and found to be a parathyroid cyst arising from a degenerating parathyroid adenoma. Both his serum calcium and creatinine returned to normal, and his fatigue resolved.
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Discussion
Prior to the advent of fine needle aspiration, nearly all parathyroid cysts were diagnosed surgically. Most patients presented with either a mass or with symptoms arising from compression caused by a mass. In 1953, Crile reported the first case diagnosed preoperatively using FNA. Since then, the literature has focused on two broad categories of parathyroid cysts: functioning and nonfunctioning, the former being associated with clinical hyperparathyroidism. The majority of patients, nearly 91%, have nonfunctioning cysts. In this group, women outnumber men 3 to 1, and the mean age of affected individuals is 43.3 years. In contrast, men are more likely to have functioning cysts, outnumbering women 1.33 to 1, and tend to be older with a mean age of 51.9 years. The location of the cysts differs between the two groups. Nonfunctioning cysts are typically found in the inferior parathyroids and occur 60% of the time on the left side. Functioning cysts are much less predictable and have been located anywhere from the angle of the mandible to the mediastinum.
The etiology of parathyroid cysts is uncertain, but four major theories suggest that they develop from embryological remnants of the third or fourth branchial pouch, coalescence of microcysts, simple retention of parathyroid secretions, or cystic degeneration in preexisting adenomas. No one theory completely explains their origin, and many factors probably contribute to their etiology. generic albendazole
There are histological differences between functioning and nonfunctioning cysts. Nonfunctioning cysts are simple cysts lined by a flattened-cuboid to low-columnar epithelium. Several types of parathyroid cells are found in their walls—chief cells, water-clear cells, and oxyphil cells—but all three are not necessarily present. The presence of smooth muscle in the wall of some parathyroid cysts suggests that these may arise from branchial pouch remnants.
Some functioning cysts lack an identifiable lining and are more properly termed pseudo-cysts. They may contain foci of hemorrhage or necrosis and brown, turbid or even bloody fluid with hemosiderin-laden macrophages. It is believed that these may arise from degenerating adenomas. Other functioning cysts are multilocular, complex lesions with thin walls and are found in close association with either an adenoma or hyperplastic gland.
The pathogenesis of hypercalcemia in patients with functioning cysts has not been fully elucidated. Nests of parathyroid cells within the cysts may secrete PTH directly into the circulation; alternatively, PTH from the cyst’s lumen may leak into the bloodstream. In one instance, FNA of a parathyroid cyst was temporally related to the development of hypercalcemic crisis. It was postulated that needle tracks opened holes in the cystic wall, which allowed fluid rich in PTH to leak into the circulation. This was the likely cause of worsening hypercalcemia in our patient following FNA. generic ampicillin
The correct diagnosis of a parathyroid cyst requires three steps: consideration of parathyroid cysts in the differential diagnosis of a cystic mass in the neck, thyroid or elsewhere in the mediastinum; fine needle aspiration; and analysis of cyst fluid for PTH. Although rare, parathyroid cysts are being detected more frequently due to widespread application of FNA, and it is particularly important to distinguish parathyroid from thyroid cysts. The former typically produce a clear, colorless fluid, whereas the latter contain fluid that is amber or brown. The presence of amber, brown, or even hemorrhagic fluid, however, does not exclude the diagnosis of parathyroid cyst.
To further differentiate between the two, aspirated fluid should be analyzed for PTH. Although intact hPTH assays are gaining wider use, Birnbaum and Van Herle suggest that the measurement of PTH using a mid-molecule assay may be the more appropriate test. They report a case of a woman with a parathyroid cyst whose intact hPTH level was deceptively low, although her mid-molecule hPTH was elevated. They observed that parathyroid cysts occasionally produce fragments, rather than intact PTH, and that exclusive reliance on intact hPTH assay could lead to a missed diagnosis.
Once the diagnosis has been made, management depends upon whether the cyst is functioning or nonfunctioning. The definitive treatment for both remains surgical removal. This can be avoided, however, in nonfunctioning cysts by performing FNA. Complete disappearance of nonfunctioning cysts after FNA is well documented in the literature. In one instance of a recurring nonfunctioning cyst, successful sclerosis was performed with injection of tetracycline, and surgery was avoided. Observation without intervention may be appropriate in small, nonsymptomatic cysts. With functioning cysts, the consensus is that surgical removal is warranted. Although our review of the literature failed to find any reports of multiple functioning cysts, we recommend surgical exploration of all four parathyroid glands. Care should be taken to remove each encountered cyst or adenoma, leaving as many of the remaining parathyroids intact as is possible.
In conclusion, parathyroid cysts are rare, but the diagnosis should be entertained in a patient with a cystic neck mass, especially one occurring in the lower poles of the thyroid or discovered in a patient with hypercalcemia. Diagnosis is best made by sending cyst fluid for PTH analysis using the mid-molecule hPTH assay, if available, or, if not, by measuring intact hPTH. Aspiration of functional cysts may worsen preexisting hypercalcemia, as in the patient we report, and may even lead to hypercalcemic crisis. Serum calcium levels should be monitored after aspiration of suspected, functioning parathyroid cysts. Nonfunctioning cysts may be cured by FNA with or without the use of a sclerosing agent, but functioning parathyroid cysts should be surgically removed.