3. Discussion
Undescended parathyroid glands are typically defined as those located at or above the carotid bifurcation.1 This classification of parathyroid glands only constitutes 2-7% of parathyroid adenomas, making it the least common form of parathyroid glands found in ectopic locations.2 While rare, undescended parathyroid adenoma should be considered when initial preoperative imaging fails to identify a target adenoma or after unsuccessful parathyroid surgery.
Pre-operative localization studies are critical in the identification of possible undescended parathyroid adenomas. Imaging modalities typically include high resolution neck ultrasound, SPECT, parathyroid scintigraphy, CT and/or magnetic resonance imaging (MRI). In a meta-analysis of 1276 patients, Wong et al. demonstrate a sensitivity of 86% [Confidence Interval: 0.81-9.90] for99mTc-sestamibi SPECT/CT in identifying ectopic parathyroid adenomas, which is superior to the sensitivity of SPECT and planar imaging modalities alone.3 The addition of anatomical imaging can further enhance the diagnostic localization of the candidate lesion. In a review of 656 patients at a single institution, Zerizer et al. found that combining99mTc-MIBI with anatomical scans (CT or MRI) significantly improved diagnostic accuracy (improving sensitivity and specificity to 100%).4 Anecdotally, the localization of parathyroid adenomas can be complicated by false positive signals generated by thyroid nodules as well as the salivary glands, as was the case for our undescended parathyroid. Additional imaging modalities, such as CT neck with contrast or MRI can be helpful to pinpoint the location and distinguish it from the neighboring structure as it did in this case. A recent review article demonstrated that MRI had both a sensitivity and specificity of up to 97% in the detection of parathyroid adenomas.5 Common MRI traits of adenomas can include elongated morphology, T2 fat saturation hyperintensity, and strong enhancement T1 post-contrast.5
Four-dimensional CT has shown promise for the detection of ectopic parathyroid glands, but has the drawback of a greater radiation exposure compared to the sestamibi SPECT modality.6Four-dimensional CT specifically used for localization of ectopic parathyroids has not been clearly defined in the literature, however, it demonstrates a higher sensitivity (82%) and specificity (92%) compared to other imaging modalities, suggesting its benefit as a useful adjunct to localizing ectopic parathyroid adenomas.7
Importantly, in pre-operative planning, it is imperative to consider the potential need to perform a second incision in order to conduct a four-gland exploration in the case that the PTH did not decrease by > 50% after the removal of the adenoma. The unique location of the surgical incision employed for this patient lends itself to potential surgical complications that are otherwise uncommon in standard parathyroidectomies. Specifically, surgeons must consider the increased risk of injury to the hypoglossal and marginal mandibular nerves, and injuries to the carotid artery, internal jugular vein, and vagus nerve if dissection of the gland is required from these structures. Furthermore, the use of a two-incision surgical approach necessitates the surgeon to counsel patients on potentially managing two scars post-surgery. As such, it is important to note that pre-operative discussions with patients regarding surgical risks may differ from that of a standard parathyroidectomy.