The external branch of the superior laryngeal nerve (EBSLN) is an important nerve tissue in thyroid surgery (1, 2), similar to the recurrent laryngeal nerve (RLN) and the parathyroid gland, and is responsible for motor innervation of the cricothyroid muscle (CTM), which acts to elongate and thin the true vocal folds during phonation, thereby elevating the pitch of the voice. Dysfunction of the CTM secondary to EBSLN injury results in hoarseness, a weak voice, vocal fatigue, or reduced vocal frequency; these consequences may be permanent and negatively affect patient quality of life, especially for professional voice users (3, 4).

Considering its high injury rate (up to 58%) and varying exposure rate (5, 6), routine identification of the EBSLN prior to surgical manipulation is necessary for a complication-free thyroidectomy (7, 8). The EBSLN descends posterior to the sternothyroid muscle towards the cricothyroid muscles, along and cross the STA. As described by Cernea classification, types 1, 2A and 2B means more than 1 cm above the upper pole of thyroid, within 1 cm of the upper pole, or below the upper pole respectively. While there are techniques to identify the RLN during thyroid surgery, there is no clear consensus or standard technique for the identification of the EBSLN. Due to its close anatomical relationship with the superior thyroid artery, its fine structure, and its highly variable anatomy (9), its susceptibility to injury and the difficulty in exposing the EBSLN during thyroidectomy are well recognized (10). Inadequate exposure of the upper polar region is the major factor affecting EBSLN identification and iatrogenic injury, which is mainly caused by occlusion of the sternalthyroid muscle, which is also assessed as a landmark for the nerve entry point (NEP) of the EBSLN on the inferior constrictor muscle (11, 12).

The aim of the present study was to validate a video-assisted method for visually identifying and preserving the EBSLN accurately and feasibly by expanding the scope and eliminating the dead angle of observation.

The fine size, variable course and label of “the neglected nerve” make the EBSLN at potential risk during thyroidectomy (20, 21). Injury to the EBSLN can occur by transection, traction, ligation, thermal damage, and disrupted blood supply (22), and identification though proper and sufficient exposure is important to avoid this (8). The laryngeal head of the sternothyroid muscle is assessed as a landmark for the course of the EBSLN on the inferior constrictor, but it also obstructs the surgeon's observation of the superior pole of the thyroid region and affects the exposure of the EBSLN (11, 23).

Some surgeons used to cut off part of the laryngeal head of the sternothyroid muscle to better expose the EBSLN (24). This process not only increases muscle damage but also increases the chance of unintentional EBSLN injury. To avoid this, a strap intermuscular approach has been described for exposing and protecting the EBSLN (25). Considering that the main factor affecting the exposure of the upper polar region is the angle of view, not the sternal thyroid muscle itself, we tried to change the visual angle and afford sufficient visualization of the superior thyroid region by means of endoscopic assistance, without sacrificing the integrity of the sternothyroid muscle or an extra expanded separation area.

Compared to COS, the new video-assisted method does not require amputation of the sternothyroid muscle, provides a better visual field to expose the EBSLN, and makes it more easily protected intraoperatively. Additionally, the new video-assisted method does not increase the time from cutting the skin to exposing the upper thyroid region. This method not only uses the magnifying effect of the endoscope to expand the observation field but also reduces surgical injury through precise and directional operation.

Currently, IONM has gained extensive acceptance as an adjunct to the gold standard of EBSLN identification (26). For the nerves that cannot be identified accurately and those that run inside the pharyngeal constrictor muscle (Friedman's classification type 3 (27), IONM plays an important role in EBSLN protection (2, 28). Moreover, visual exposure and protection can confirm the anatomical integrity of nerves, but functional integrity cannot be confirmed. Before the end of the operation, while anatomical integrity is confirmed, neural monitoring is required to exclude the elimination of thermal damage and strain injury and verify neural function. Electrostimulation causes simultaneous contraction of the cricothyroid muscle in both the straight and oblique abdomen as the standard of neurological integrity (3, 14, 29).

Nevertheless, IONM does not completely eliminate the risk of nerve injury caused by unintentional injuries (30, 31), and the surgeon's experience and full understanding of anatomy and possible variations are more important than the aid of high-tech instruments during thyroid surgery (32). Therefore, our strategy mainly depends on visual confirmation of the nerve during surgery, and electrical nerve stimulation is used for confirmation after exposure and functional examination before the end of the surgery.

In the present study, we achieved a 91% EBSLN exposure rate (61 in 67 nerves) based on visual exposure and confirmed that nerve function was good through the contraction of the cricothyroid muscle. This rate is satisfactory compared with those reported in previous studies with IONM (33–35).

We used IONM to further increase the exposure rate by 9% (6 in 67 nerves), and the total exposure rate of the EBSLN reached 100%. This result revealed that the EBSLN could be visually identified in the majority of cases, with or without IONM. For hospitals without neural monitoring equipment or the economic ability to purchase neural monitoring equipment, medical insurance limits expenses, and this method is conducive to the identification and isolation of the nerve, thereby minimizing iatrogenic injury and postoperative complications.

In the present study, we also calculated the distribution of Cernea groupings of the EBSLNs. Among the total of 127 EBSLNs identified in the two groups, we found a high prevalence of Cernea type 2 nerves (88.98%), with 49.61% being type 2B. These data are similar to those reported in previous studies (11, 36, 37). Moreover, notably, traction of the upper pole of the thyroid may change the position and classification of the EBSLN, increasing the proportion of type 2b. Due to the close proximity of the EBSLN to the STA, the high proportion of type 2 EBSLNs also indicates that it is meaningful to identify the EBSLN by routine exposure to reduce the risk of injury.

It was our experience that the STA was unsuitable as a landmark to find the EBSLN intraoperatively, and there were many variations both in the EBSLN and STA (38). Different from cadaveric anatomy, the position of the sternal thyroid muscle and superior posterior thyroid EBSLN and their relationship with the superior artery change during surgery, especially when the thyroid lobe is pulled. This also explains why the intraoperative exposure rate of the EBSLN is generally lower than that of cadaveric studies (39). Moreover, the possible small branches of the superior thyroid artery may bleed and affect the exposure.

According to a previous study (11), we used the insertion of the sternothyroid muscle as a reliable landmark to localize the NEP of the EBSLN. The NEP consistently lies close to the sternothyroid muscle insertion onto the oblique line and does not vary with the direction of pull on the lobe; there is not a bothersome ligament or vessel nearby. We look for the EBSLN in this area visually with the video assistance of a wide field of vision and magnification. In our study, the NEP was approximately 1.18 mm from the sternothyroid insertion and 5–15 mm from the anterior border of the muscle.

After finding the NEP, the nerve was readily traced superiorly to the intersection with the STA. With identification of the EBSLN, the superior thyroid vessels were adequately dissected for safe ligation and division. Our strategy greatly improved the exposure and ease of dissection of the EBSLN, thus reducing the chance of postoperative voice problems. For experienced surgeons with an understanding of local anatomy, this process can be accurate and fast.

The present study has some limitations. First, the sample size of a single institution was not sufficient. For the purpose of controlling postoperative sound function and operation time, we only included patients with unilateral lobectomy, and a large-scale multicenter study with a wider range of patients is needed to further compare the clinical effects. Second, we performed intraoperative visualization, IONM, voice assessments and laryngoscope evaluation to analyze the rates of exposure and injury but did not conduct CTM electroneuromyography (EMG), which is the most accurate tool for diagnosing abnormal EBSLN conductivity. For IONM, we only used cricothyroid muscle contraction to evaluate neural integrity. Due to the limitation of medical insurance costs, we cannot use the intratracheal electrode and cricothyroid electrode to quantitatively measure the EMG signal, which makes us unable to predict postoperative nerve function according to the change in signal strength. Third, the present study did not involve patients with large goiters, short necks or obesity. We also excluded patients with ETE of the upper pole; thus, the 100% exposure rate in the VAS group and 0% injury rate in this study may not accurately reflect the real status of clinical practice. We will conduct further research on these topics in follow-up studies.

In this study, we used a video-assisted method to fully expose the upper pole of the thyroid. Without increasing muscle injury or increasing the extent of surgery, a good visual exposure rate of the EBSLN was obtained, which was helpful for identifying and protecting the EBSLN during thyroidectomy. This method also represents a new strategy for exposing the EBSLN in patients when IONM cannot be applied. We showed that the EBSLN entry point was localized within 2 mm from the sternothyroid muscle insertion and 5–15 mm from the anterior border of the sternothyroid muscle. A detailed understanding of the anatomical variation in the EBSLN is helpful for finding the EBSLN in the cricothyroid space to avoid the chance of EBSLN injury. In the future, we intend to make the operation simpler and more feasible by using a smaller wireless lens that can be magnetically attached to the hook.