Critical View of Safety: Anatomical Key to Avoid Injury to the Recurrent Laryngeal Nerve in Transoral Endoscopic Thyroidectomy

INTRODUCTION

Safe surgery is crucial, both for the patient and the surgeon, as it minimizes the risk of iatrogenic complications and the risk of any litigation later. The term “critical view of safety” (CVS) was coined, and its philosophy was propagated by Strasberg et al. as a standard and most effective method for conclusive identification of the cystic artery and duct, to prevent vasculobiliary injuries due to misidentification during laparoscopic cholecystectomy. It is not an operative step or technique, but the final view that is achieved after a complete dissection.

The risk of inadvertent trauma to the recurrent laryngeal nerve (RLN) is well known in open and endoscopic thyroid surgery. Thyroid surgeons are familiar with the common site for an iatrogenic injury to the RLN and strive to avoid the error traps in this location. CVS and the operative steps to achieve it to avoid injury to the RLN have been described recently for lateral route endoscopic thyroidectomy. We propose CVS for the same purpose, for transoral endoscopic thyroidectomy by vestibular approach (TOETVA). Our objective is to promote a standardized anatomical approach for visual identification of the RLN when intraoperative nerve monitoring (IONM) is unavailable.

METHOD

Operative steps are similar to standard technique: insertion of telescope and working instruments, creation of working space, identification and division of linea alba to expose the trachea and isthmus, division of isthmus, dissection of the sternothyroid– laryngeal triangle, visualization of the external branch of superior laryngeal nerve (EBSLN) and superior pole vessels after division of sternothyroid muscle, dissection of medial surface of the superior pole from cricothyroid muscle to expose the medial and lateral thyroid spaces followed by retracting the thyroid gland supero-laterally to expose the vessels. The EBSLN, which crosses the superior thyroid artery superiorly or inferiorly, is safeguarded, and the vessels are ligated with an ultrasonic device.

Creation of CVS and delineation of its boundaries (Figure 1) are done by following the five steps:

1. The crico-thyroid space of Reeves is dissected, and the superior pole is cleared as it forms the medial and superolateral walls of the CVS.
2. The lobe is gently retracted, and dissection with an ultrasonic energy device/bipolar energy device exposes its medial surface, which forms the roof of the CVS.
3. Careful dissection will reveal the superior parathyroid gland, which forms part of the lateral boundary of CVS.
4. Lateral to medial dissection under the surface of the lobe exposes Berry’s ligament, which forms the middle medial boundary of the CVS. The supero-medial boundary of CVS is the cricothyroid muscle, and the lower medial boundary is the trachea.
5. RLN can be seen forming the floor of the CVS and entering the trachea just posterior to Berry’s ligament; now, dissection can proceed caudally, safeguarding it by remaining anterior to the nerve.

Thyroid lobectomy: Inferior pole is dissected off by medial traction of the upper part of the lobe, which leads to visualization of the inferior parathyroid gland. Lateral traction leads to identification of the inferior thyroid vein just lateral to the trachea, which is then sealed off. The specimen is finally taken out, and the surgical cavity is inspected for any bleeding. Routine drainage of the thyroid bed is not done.

RESULTS

This retrospective study was conducted from January 2022 to October 2023 in the endocrine surgery unit of a tertiary teaching hospital in Central India. Institutional ethics committee approval was obtained before starting the study, and written informed consent was obtained from all patients.

A total of 36 patients underwent TOETVA without IONM. All patients (two males, 34 females with a median age of 32years; age range 22-47years) had benign nodules with a mean size of 4.2cm +/−1.8cm and underwent hemithyroidectomy. CVS allowed the visual identification of RLN in all patients. All patients underwent standard laryngoscopy after 48hours for assessment of vocal cord function, and none had RLN palsy.