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.