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UF Health University of Florida
Veterinary Diagnostic Imaging College of Veterinary Medicine
LA Radiology Overview

Case reveal

Nuclear scintigraphy: 

  • 200 mCi of Tc99m-Sestamibi were administered intravenously. Images were initiated at 10 min, 2.5 hours, and 5.5 hours and 24 hours after injection. 
  • At the 10 minute and 2.5-hour time points, increased radiopharmaceutical uptake is identified within the thyroid glands and salivary glands, as expected. Additionally, a focal, round and well-defined area of moderate increase in radiopharmaceutical uptake is seen immediately to the left of midline at the level of the thoracic inlet. 
  • This region is again seen at the 5.5-hour timepoint; however, the uptake is mild and ill-defined.  
  • 24-hour post radiopharmaceutical injection, no abnormal radiopharmaceutical uptake remains, and increased radiopharmaceutical uptake is no longer identified in the region of the thyroid or salivary glands.

Conclusions:

  • Moderate ectopic parathyroid activity at the level of the thoracic inlet. Given the clinically suspected primary hyperparathyroidism, an ectopic parathyroid adenoma or adenocarcinoma is most likely. 

Follow-up:

  • An ultrasound of the thoracic inlet confirmed the presence of a small mass with a homogeneous echogenicity and containing a large lobular and thin rimmed cyst-like region filled with anechoic fluid. 
  • Ultrasound-guided fine needle aspirated of the mass were inconclusive due to low cellularity. 
  • Percutaneous ultrasound-guided ethanol ablation was elected and performed without immediate complications. 
  • iCa levels post ablation (normal 1.45–1.75 mmol/l): 
    • Day 2: ranged between 1.33-1.58 mmol/l
    • Day 3: ranged between 1.39-1.64 mmol/l
    • Day 4: ranged between 1.46-1.67 mmol/l
    • Day 5: 1.63 mmol/l
    • Day 6: 1.57 mmol/l
  • PTH level post ablation (normal 1.3-15 pmol/L)
    • Day 1: <0.5 pmol/L
    • Day 3: <0.5 pmol/L
  • An non-pathologic arrhythmia was diagnosed on presentation and resolved following the alcohol ablation, supporting the previously high calcium as a contributing cause of the arrhythmia.
  • At discharge, the patient was eating well and had normal vital signs.

A little bit more…

  • Primary hyperparathyroidism is uncommon horses and most often will result in weight loss. Other clinical signs reported include tremors, facial swelling, inappetence and shifting lameness.1-4 It most often is caused by the presence of an adenoma or a sequela of parathyroid gland hyperplasia. Multiple parathyroid lesions may result in an overproduction of PTH. Consequently, plasma calcium concentrations are increased due to increased absorption from the gastrointestinal tract, decreased renal excretion and increased mobilization of calcium from bone. A lack of negative feedback causes calcium and PTH levels to progressively increase and result in n the described clinical signs.1-2 
  • In addition to normal thyroid glands and associated parathyroid glands (usually external) in the cervical region, additional (e.g. ectopic) parathyroid tissue may be identified anywhere between the bifurcation of the carotid trunk to the ventral aspect of the cranial third of the neck.2 Krook et al. described that the majority of horses have an additional pair of parathyroid glands at the level of the thoracic inlet, unlike humans and dogs.1&5
  • Technetium (99mTc) sestamibi scintigraphy is helpful for the identification of ectopic parathyroid tissue as the cause of primary hyperthyroidism in horses. It may also be used to confirm an enlarged parathyroid gland as the site of increased metabolic activity and help target therapy if multiple abnormal parathyroid glands are identified.1-2 It is important to note that the absence of radiopharmaceutical uptake does not exclude the presence of a functional parathyroid lesion.1&6 For lesions located in the mid to cranial cervical region, computed tomography may be favored; however, patient size limits screening for more caudally located lesions.2
  • Complications associated with percutaneous ultrasound-guided ethanol injection and parathyroidectomy are not well described in horses. In accordance with prior reports, percutaneous ultrasound-guided ethanol injection of functional parathyroid tissue resulted in a fast decline in hypercalcemia and elevated serum PTH levels in the current case.2 Infiltration of ethanol along the needle tract and into the surrounding tissues, resulting in damage to recurrent laryngeal nerves and major blood vessels, have been described in dogs and should be considered in equine patients based on the location of the lesion.2 
  • In horses undergoing parathyroidectomy, acute kidney injury and hypocalcemic tetany have been reported.3 Following parathyroidectomy or ablation in dogs, hypocalcemia is the most common sequela, reported in approximately 40% of cases.7 In horses with osteopenia, general anesthesia for parathyroidectomy poses risk of catastrophic pathologic fracture during recovery and should be taken into consideration in the treatment selection process.2-3

References:

  • Darby, S., Porter, E., Beatty, S. S., Dark, M. J., Smith, A., Toribio, R. E., & Gomez, D. E. (2020). Primary hyperparathyroidism in a quarter horse mare associated with a chief cell adenoma. Journal of Equine Veterinary Science, 95, 103302. 
  • Colmer, Sarah F., et al. “Treatment of primary hyperparathyroidism in a miniature horse using chemical ablation of abnormal parathyroid tissue localized by 3‐phase computed tomography.” Journal of veterinary internal medicine36.2 (2022): 798-804.
  • Tomlinson, J. E., Johnson, A. L., Ross, M. W., Engiles, J. B., Levine, D. G., Wisner, W. A., & Sweeney, R. W. (2014). Successful detection and removal of a functional parathyroid adenoma in a pony using technetium Tc 99m sestamibi scintigraphy. Journal of veterinary internal medicine, 28(2), 687. 
  • Peauroi JR, Fisher DJ, Mohr FC, et al. Primary hyperparathyroidism caused by a functional parathyroid adenoma in a horse. J Am Vet Med. 1998; 212(12): 1915-1918.
  • Krook, Lennart, and John E. Lowe. “Nutritional secondary hyperparathyroidism in the horse: with a description of the normal equine parathyroid gland.” Pathologia veterinaria1.1_suppl (1964): 1-98.
  • Wong D, Sponseller B, Miles K, Butt T, Kersh K, Myers R. Failure of technetium Tc 99m sestamibi scanning to detect abnormal parathyroid tissue in a horse and a mule with primary hyperparathyroidism. J Vet Intern Med. 2004; 19: 589-593. 
  • Guttin T, Knox VW IV, Diroff JS. Outcomes for dogs with primary hyperparathyroidism following treatment with percutaneous ultrasound-guided ethanol ablation of presumed functional parathyroid nodules: 27 cases (2008-2011). J Am Vet Med. 2015; 247(7): 771-777.