Transoral Parathyroidectomy

Parathyroid surgery was initially described in 1925 when Felix Mandl, a young surgeon in Vienna, was able to successfully remove a parathyroid tumor from a patient with osteitis fibrosa cystica. ^[1]  In the decades that followed, few significant technical changes occurred in parathyroid surgery; however, the procedure is now most commonly performed to treat primary hyperparathyroidism.

Traditionally, the standard technique was four-gland exploration with removal of any abnormal or enlarged gland(s). As parathyroid imaging techniques advanced, minimally invasive parathyroidectomy became more commonplace and provided an equally high cure rate, a lower complication rate, a shorter hospital stay, and a large reduction in hospital costs. ^[2]  This approach uses a small anterior neck incision and targets a single parathyroid gland on the basis of imaging, as well as perioperative adjuncts (eg, gamma probe and intraoperative parathyroid hormone assay).  

The traditional surgical approach drastically changed in 1996 when Gagner ^[3]  described an endoscopic approach to subtotal parathyroidectomy with dissection in a subplatysmal plane, using four 5-mm trocars placed strategically in the neck. Despite a procedure duration of 5 hours and resultant subcutaneous emphysema, the door had been opened to investigate new approaches to an old surgical procedure.

In the following years, multiple minimally invasive procedures were developed—primarily for thyroidectomy—in an effort to avoid an anterior neck skin incision. Many procedures were described, including robotic ^{[4, 5, 6, 7]} and endoscopic ^{[8, 9, 10, 11]} approaches from the breast, axilla, or both breast and axilla; however, all necessitated varying degrees of tissue damage to obtain adequate access, as well as skin incisions elsewhere on the body.

In an effort to avoid skin incisions entirely, Witzel et al first proposed the idea of a transoral thyroidectomy in 2008 ^[12]  after successfully performing the procedure via a sublingual approach in living pigs and human cadavers.

In 2010, Karakas et al ^[13] performed both hemithyroidectomies and parathyroidectomies in pigs and human cadavers. The following year, they published the first description of a transoral parathyroidectomy in two patients. ^[14]  Although the procedure was successful, one of the two patients suffered from perioperative dysphagia, tongue paresthesia, and hypoglossal nerve palsy, all of which resolved by the 6-week follow-up appointment.

Subsequently, in 2014, the same group published a case series describing their experience with five transoral endoscopic parathyroidectomies. ^[15]  Two of the five patients required conversion to an open technique, three developed hematomas and dysphagia, one had persistent dysgeusia and transient hypoglossal nerve palsy, and one developed transient recurrent laryngeal nerve palsy. The procedure was therefore deemed a feasible option but not a safe or viable one. ^[15]

The idea of endoscopic transoral parathyroidectomy was revisited in a 2016 article that described a transoral vestibular approach, which avoided the floor of the mouth and the associated complications. ^[16]  Sasanakietkul et al performed 12 parathyroidectomies via three incisions in the oral vestibule with promising results. In this series, there was one transient recurrent laryngeal nerve injury (symptom resolution within 1 month), no need for conversion to open surgery, and no other complications (eg, infection or neurovascular injury).

The conclusion that transoral parathyroidectomy is both a safe and a feasible surgical option was further supported by a subsequent series from Russell et al. ^[17]  Using a similar transoral vestibular approach, the authors performed 12 thyroidectomies and two parathyroidectomies using either an endoscopic or a robotic technique, with no surgical complications in either parathyroidectomy patient and no permanent complications in the thyroidectomy group.

The endoscopic versions of these procedures are sometimes referred to collectively as TOET/PVA (transoral endoscopic thyroidectomy/parathyroidectomy via the vestibular approach) or separately as TOETVA (transoral endoscopic thyroidectomy via the vestibular approach) and TOEPVA (transoral endoscopic parathyroidectomy via the vestibular approach). ^[18]  By 2020, more than 2000 TOET/PVA procedures had been performed, including more than 400 in North America.

The current literature suggests that TOET/PVA is a safe and viable alternative for patients who are concerned about the aesthetics of an anterior neck incision. ^{[19, 20, 21]} Additional study (eg, of complications, expense, instrument limitations, and overall safety) is warranted. ^[22]

The indications for parathyroidectomy in general and the workup to determine whether surgical intervention is appropriate are discussed elsewhere (for more information, see Parathyroidectomy). The indications for the transoral approach to parathyroidectomy are exclusively cosmetic in nature. Candidates would include patients with a history of hypertrophic scarring or keloids, as well as those with a particular interest in avoiding a cervical skin incision.

Transoral parathyroidectomy is contraindicated in patients who have a history of significant thyroiditis, have undergone previous neck surgery or irradiation, or have a suspected malignancy. In a description of this approach for thyroidectomy, Anuwong also included the presence of dental braces as a contraindication. ^[23]

Other relative contraindications are the presence of large thyroid nodules or a significantly enlarged thyroid gland. In one study, patients were excluded if a thyroid nodule larger than 6 cm was found on preoperative imaging ^[17] or if the thyroid gland was larger than 10 cm in diameter. ^[23] Although not explicitly mentioned in current reports, additional relative contraindications to consider would include obesity and inability to achieve adequate neck extension (eg, from previous cervical spine injury or fixation).

Given that this surgical technique is still at a relatively early stage of development, it is possible that additional contraindications may become apparent in the future. The contraindications for traditional parathyroidectomy still apply (see Parathyroidectomy).

Anatomy

Although the anatomy of a traditional open parathyroidectomy remains relevant, additional factors must be considered during a transoral approach. The method that has proved to be best tolerated and to cause the fewest complications employs vestibular incisions only (see the image below).

The midline incision does not place any neurovascular structures at risk. On the other hand, the lateral incisions (used for placement of 5-mm trocars), if placed incorrectly, may approach the mental foramen and the facial vessels as they cross the mandibular body.

An anatomic study by Cai et al evaluated the position of these structures in five human cadavers. ^[24]  The mental foramen—the exit point for the mental nerve and vessels—is found in the midpoint of the mandibular body, typically below the second premolar tooth. The mean distance from the mental foramen to the median mandibular point is approximately 3.5 cm, and the mean distance to the location where the facial vessels cross the mandibular body is 6.0 cm.

These landmarks are critical for determining where to make the oral vestibule incisions, all of which should remain medial to the mental foramen. Of note, the marginal mandibular branch of the facial nerve is unlikely to be injured during this procedure, given its superficial location. After the initial incisions, the development of submental and subplatysmal planes will ensure that no critical structures are at risk for injury during the approach.

With any parathyroid surgery, an understanding of parathyroid gland anatomy and embryology is critically important for ensuring that the procedure is completed safely and effectively. (For a more detailed discussion, see Parathyroid Gland Anatomy.)

Two case series describing transoral vestibular parathyroidectomy showed good outcomes. ^{[16, 17]} In the 14 described parathyroidectomies, the cure rate was 100%, with no postoperative infections or permanent complications; however, the follow-up in one study ^[16] was only 30 days, which makes it harder to determine whether the operation was in fact curative. As noted, this remains a relatively novel surgical technique, and available data are therefore quite limited.

It should be noted that this technique may be less than ideal for four-gland exploration or for excision of intrathyroidal parathyroid glands, given the longer operating times. ^[16]

Bhargav et al prospectively evaluated the feasibility and safety of transoral lower-vestibular endoscopic parathyroidectomy in 12 patients with hyperparathyroidism. ^[25] The mean operating time was 112 ± 15 minutes (range, 95-160). Postoperatively, there was no major morbidity, hypocalcemia, or recurrent laryngeal nerve palsy. Cure and diagnosis were confirmed by a fall of more than 50% fall in intraoperative parathyroid hormone levels and histopathology (all were benign solitary adenomas).