The Feasibility of a Prototype Thyroidoscope for Gasless Transoral Endoscopic Thyroidectomy: A Preclinical Cadaver Study

Introduction

The transoral endoscopic thyroidectomy vestibular approach (TOETVA) has recently grown in popularity due to its many advantages. These include the fact that it can be completed with only a mucosal incision, as a skin incision is unnecessary; the area of dissection is comparatively narrow; and the approach through a midline oral incision facilitates total thyroidectomy. However, during transoral thyroid surgery, an unnatural space is created in the anterior neck, and CO₂ injected to maintain the working space may lead to gas-related complications, such as subcutaneous emphysema, pneumothorax, pneumomediastinum, and CO₂ embolism. CO₂ is inert, colorless, inexpensive, and readily available and dissolves in the blood; it is thus widely used during endoscopic surgeries. However, the complications of its use can include hypercapnia, metabolic acidosis, and cardiorespiratory compromise. In transoral thyroid surgery, subcutaneous emphysema usually improves spontaneously, while CO₂ embolism during laparoscopic and endoscopic procedures is a rare but potentially fatal complication. ¹ In the largest study of the transoral approach, Anuwong found that among the >600 evaluated patients, only 10 developed subcutaneous emphysema; none had severe gas-related complications (CO₂ embolism, pneumothorax, or mediastinal emphysema). ² However, Fu et al. reported that 2 of 81 patients developed intraoperative CO₂ embolism, necessitating conversion to open thyroidectomy. ³ Kim et al. reported the development of CO₂ embolism during flap elevation before transoral robotic thyroidectomy; the patient was instead operated on using a retroauricular approach. ⁴ Chen et al. reported that subcutaneous emphysema occurred in 31 (28.8%) of 80 patients, with 8.8% remaining in the post-anesthesia care unit for 2 hours, which was a longer duration than after other forms of surgery (2.2%). ⁵ Wilhelm and Metzig reported one case of mediastinal emphysema among eight patients (21%) who showed improvement after conservative treatment. ⁶

To address these limitations, we developed a retractable device, named the Thyroidoscope, which could maintain the surgical working space without CO₂ insufflation. We conducted a preclinical cadaver study to evaluate its use in gasless TOETVA.

Materials and Methods

A prototype Thyroidoscope for gasless transoral endoscopic thyroid surgery

The thyroidoscope was designed by SG W., JO P., and SC S. and produced by SG W. It consists of a blade, handle, body, and two spreaders (Fig. 1A). The blade was designed to support the roof of the working space during surgery. A width of 12 mm was chosen for the blade to prevent inadvertent injury of the mental nerve. Two spreaders attached to the blade provided additional working space in the lower neck by unfolding them (Fig. 1B). The body of the Thyroidoscope was designed in conical shape to secure sufficient space between the mandible and the skin of the chin during surgery, which allows easy access and a sufficient range of endoscope motions. The handle is designed to be securely gripped by the operator during insertion and removal of the Thyroidoscope. The end of the handle can be connected to a common laryngoscope holder (Karl Storz GmbH, Tuttlingen, Germany) (Fig. 1C, D).

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FIG. 1.

A prototype of our Thyroidoscope for gasless transoral endoscopic thyroid surgery. (A) The Thyroidoscope consists of a blade, handle, body, and two spreaders. (B) Two spreaders can be folded (left) and unfolded (right). By unfolding them, additional space in the lower neck is secured. (C) The surgeon securely holds the handle during insertion or removal of the Thyroidoscope. (D, E) The Thyroidoscope is connected to a common laryngoscope holder (Karl Storz GmbH, Tuttlingen, Germany).

Surgical procedure to secure the working space for transoral endoscopic thyroid surgery using the Thyroidoscope

The cadaver was placed in the supine position with the neck extended by a pillow placed under the back. The head of the cadaver was fixed using cotton plaster to prevent movement during the operation. Draping was done so as to expose the upper lip and suprasternal notch. The operator was positioned above the patient's head and the monitor was near the patient's leg. After a 2-cm curvilinear incision was made in the midline vestibule using monopolar electrocautery, 40–50 mL of normal saline was injected into the subplatysmal layer of the anterior neck to create a working space, which was then widened using blunt instruments (Kelly clamp and vascular tunneler). After the working space was dissected, the Thyroidoscope was inserted into the subplatysmal layer of the anterior cervical area. Then a 10-mm 30° endoscope was inserted through the Thyroidoscope, to determine whether the working space was appropriate and of sufficient size (Fig. 2A). Two small vertical incisions were made in the vestibule near the premolars bilaterally, and a 5-mm cannula was inserted through each lateral incision site. The endoscopic instruments were then positioned through each lateral cannula. The working space was widened sufficiently by unfolding two spreaders, extending from the sternal notch inferiorly to both sternocleidomastoid muscles laterally, using an L-hooked electrocautery and ultrasonic device (Fig. 2B). Transoral endoscopic thyroidectomy was then performed stepwise as described in our previous report. ⁷ At the completion of the surgery, the cadaver was explored and dissected through a traditional Kocher incision to confirm the preservation of important anatomical structures (mental nerve, submandibular gland, facial artery and vein, carotid artery and jugular vein, recurrent laryngeal nerve, parathyroid gland, tracheal and laryngeal structures, and the esophagus) and to identify any remnant thyroid tissue.

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FIG. 2.

Securing the working space for transoral endoscopic thyroid surgery using the Thyroidoscope without CO₂. (A) A 10-mm 30° endoscope was inserted through the Thyroidoscope and the surgeon created the working space. (B) Sufficient working space was secured without CO₂ insufflation. (C) The right lobe of the thyroid gland was retracted medially, revealing the retrothyroidal area. (D) The right recurrent laryngeal nerve and the right superior parathyroid gland were identified and preserved.

Results

Discussion

Our newly developed retractable device, Thyroidoscope, allows transoral thyroid surgery without CO₂ insufflation. CO₂ is widely used in laparoscopic abdominal surgery, but gas-related complications, such as subcutaneous emphysema, pneumothorax, pneumomediastinum, and potentially fatal CO₂ embolism, while rare, may develop.^1,3,4 During TOETVA, the initial CO₂ pressure is typically set to 5–6 mmHg at the beginning of surgery and lowered to 3–4 mmHg during the procedure. Close cooperation from an anesthesiologist and continuous monitoring of end-tidal CO₂, which should not exceed 40–45 mmHg, are essential during transoral thyroid surgery using CO₂. Nonetheless, the surgeon must pay great attention to prevent complications associated with CO₂ use. Our Thyroidoscope is similar to a laryngoscope and can be fixed to the laryngoscope holder without any further adaptors. The initial design of the Thyroidoscope was simply a handle and right-angled blade, but vertical motion was limited due to the narrow inner diameter of the scope. To secure a sufficient range of motions, the body of the Thyroidoscope was redesigned in conical shape, which allows easy access and a sufficient range of endoscope motions. Also, two spreaders were attached to provide an additional working space in the lower neck. Of note, there was neither a blind zone—an area that cannot be visualized, nor a dead zone—an area where the instrument cannot reach to.

When CO₂ is continuously injected into a narrow working space, as the pressure changes, the surrounding tissues and the roof of the working space are repeatedly moved as if they are pulsating. This may be dangerous during surgeries involving major structures, such as the recurrent laryngeal nerve or vessels. However, there was no such pulsating movement during gasless TOETVA, resulting in an improved surgical technique. In addition, since the working space in TOETVA is a closed cavity, frequent suction and removal of smoke generated from the energy device will typically result in a collapse of the working space, due to the high negative pressure and the adherence of blood to the endoscope, thus blurring the surgical field. However, in gasless TOETVA, the working space is an open cavity that remains open even if smoke is drawn by a continuous application of negative pressure.

Nonetheless, gasless TOETVA also has a few minor drawbacks. Because the blade of the retractor is located on the roof of the working space, the height of that space is slightly lower, and the blade may interfere with the endoscopic view of the retrothyroidal area. We evaluated the continuity of the mental nerve after surgery. However, as this was a preclinical cadaver study, we could not confirm if any functional deficit due to compression or traction injury occurred.

In conclusion, our newly developed Thyroidoscope provides sufficient working space to perform transoral thyroid surgery without the need for CO₂ insufflation. Gasless TOETVA with the thyroidoscope seems to be feasible and safe. However, its drawbacks must be overcome before it can be used in patients.