Reassessing optimal laryngeal nerve protection strategies in neck endocrine surgery: intermittent versus continuous neuromonitoring

Injury to the recurrent laryngeal nerve (RLN) is one of the most feared complications of neck endocrine surgeries. Dysphonia resulting from unilateral RLN injury can significantly impair quality of life and lead to long-term psychosocial, emotional, and occupational disturbances (1). In addition, dysphagia and impaired laryngeal protective reflexes can predispose the individual to laryngeal aspiration and potential respiratory infection. Bilateral RLN injury can result in severe and life-threatening airway compromise, often necessitating tracheostomy (2,3).

Intraoperative neuromonitoring (IONM) of the laryngeal nerves has developed as a technological adjunct to careful nerve dissection, designed to help identify the anatomical location of the RLN and to alert surgeons to impending nerve injury by providing electromyographic (EMG) data on vocal fold function. IONM can be performed intermittently or continuously. In intermittent IONM (IIONM), a handheld probe is used to manually stimulate the RLN during a surgical case, with the frequency of stimulation being determined by the operating surgeon (4,5). By contrast, methods of continuous IONM (CIONM) generate continuous EMG signals in the vocal folds throughout the entirety of a case and provide real-time feedback about nerve functional integrity (6,7). The stimulation probe used during IIONM can map out the location of the RLN prior to the nerve being definitively visualised. By contrast, CIONM cannot be used for neural mapping, but by virtue of its real-time feedback, can potentially prevent nerve injury from occurring. IIONM and CIONM are both useful for determining whether a staged surgical approach is necessary during total thyroidectomy in the setting of a unilateral loss of EMG signal. The methods are complementary and are best used together.

Although use of IONM in surgical circles has increased significantly over the past two decades, there remains some controversy over its utility in prevention of nerve injury (8,9). In part, this controversy stems from inappropriate outcome measures being applied during clinical trials comparing IIONM to nerve visualisation alone. As mentioned above, IIONM has limited ability to prevent nerve injury by virtue of the intermittent nature of the EMG information provided. As such, reporting the incidence of vocal fold paralysis as the primary outcome measure for studies on IIONM is questionable and more appropriate outcome measures could include prevention of bilateral nerve injury through staged surgery and decreased duration of surgery through quicker nerve identification. Prevention of nerve injury may be a more appropriate outcome when reporting results of CIONM; however, a continuous monitoring technique is only useful if the operating surgeon responds to the real-time EMG data being displayed. Another possible reason for the controversy over IONM utility relates to the type of nerve injury. Some nerve injuries would be unable to be prevented by any form of monitoring due to the suddenness of their occurrence (e.g., nerve transection or cauterization); however, the cause of a given nerve injury is rarely reported in series assessing the utility of IONM (10). Finally, the power of any prospective trial designed to evaluate the utility of IONM would be so large that it is virtually impossible to conduct.

The recent prospective trial by Barczyński et al. entitled “Clinical validation of NerveTrend versus NerveAssure^TM Mode of Intraoperative Neuromonitoring in Prevention of Recurrent Laryngeal Nerve Injury during Thyroid Surgery: A Randomized Controlled Trial prospective, single-center trial” compared a method of IIONM, NerveTrend^TM, to a method of CIONM, NerveAssure^TM (11). In this study, 264 patients undergoing bilateral thyroid surgery (528 nerves at risk) were randomly assigned to either the NerveTrend arm or the NerveAssure arm, with 264 nerves-at-risk in each arm. The primary endpoint was the prevalence of RLN injury on postoperative day 1, as assessed by videolaryngoscopy. In the NerveTrend^TM group, 3 of 264 nerves (1.14%) had postoperative evidence of RLN injury, compared with 1 of 264 (0.38%) in the NerveAssure cohort. This difference was not statistically significant. Staged thyroidectomy was not required in any patient in either group. The authors concluded that NerveTrend^TM was not inferior to NerveAssure in terms of early RLN injury risk.

Although NerveTrend^TM has occasionally been marketed as a form of CIONM, it is actually a form of IIONM as it relies on surgeon driven stimulations of the recurrent and/or vagus nerves. Initial stimulation data (amplitude and latency) are used as a ‘baseline’ and all subsequent surgeon-driven data points are compared back to these initial baseline values, with warning signals provided should the amplitude or latency decline by certain thresholds below the baseline. Prior literature has suggested that NerveTrend^TM, when compared to conventional IIONM, may be associated with lower RLN injury rates and reduced need for staged completion surgery. However, this current study was the first to compare NerveTrend^TM to CIONM. The main strengths of this study were its randomized, prospective design with direct head-to-head comparison between the two modalities. All patients were examined with laryngoscopy postoperatively on day 1 to assess vocal fold function and the same two experienced surgeons performed all operations.

While the trial is commendable, several limitations temper how broadly its conclusions may be adopted. First, event rates were extremely low in both groups with only 4 nerve injuries combined, reflective of the considerable experience of the two operating surgeons. However, there were trends to increased severe combined events (sCEs) and RLN injuries in the IIONM (NerveTrend^TM) arm compared to the CIONM (NerveAssure) arm. Specifically, sCEs occurred in 7.2% of the experimental group versus 3.41% in the control group (P value =0.052), and RLN injuries occurred in 3/246 versus 1/264 nerves, respectively. These differences did not reach statistical significance likely due to sample sizes being too small to detect differences in outcomes, however a modest but clinically meaningful superiority of one technique over the other cannot be excluded.

Second, this was a single-center trial conducted by surgeons highly experienced in neuromonitoring. As such, the frequency of intraoperative stimulation of laryngeal nerves in the NerveTrend^TM group was likely high and possibly higher than that of a less-experienced surgeon. Data on frequency of nerve stimulation intraoperatively was not provided, thus limiting the generalizability of reported results and raising the question of whether the non-inferiority observed may be most reflective of operator technique and expertise rather than a true difference. Also, the trial was unable to be blinded and knowledge of which IONM method was in use may have subtly influenced surgical behaviors (e.g., more cautious dissection in one arm, higher manual stimulation rate in the NerveTrend IIONM arm). Perhaps an interesting outcome to investigate in the future would be the frequency of surgeon-driven intermittent stimulations during NerveTrend monitoring needed to lower rates of sCEs.

Third, the primary outcome measure for this study was the prevalence of RLN injury on postoperative day 1 as denoted by an absence of vocal fold motion on videolaryngoscopy. There was no mention of the use of laryngeal videostroboscopy to assess for more subtle forms of vocal fold hypomobility that could denote incomplete or resolving nerve injury. The absence of this data makes correlating intraoperative sCE rates with postoperative function impossible and reduces the credibility of the calculated positive predictive values for the two IONM datasets. Such data would have been a useful inclusion.

Despite these caveats, the trial’s main conclusion—that NerveTrend may approximate NerveAssure in safety for early RLN injury—is novel and warrants consideration of a multi-center randomized trial powered to detect small differences sCEs and vocal fold motion impairment. It is reassuring that the negative predictive value for both forms of IONM was 100%. Consideration could also be given to stratification by surgical complexity and surgeon experience, and inclusion of cost-analysis data. Overall, one must acknowledge that IONM—whether continuous or intermittent—remains an adjunct, not a substitute, for meticulous surgical technique, knowledge of anatomy, and surgical judgment.

Acknowledgments

None.

Footnote

Provenance and Peer Review: This article was commissioned by the editorial office, Gland Surgery. The article has undergone external peer review.

Peer Review File: Available at https://gs.amegroups.com/article/view/10.21037/gs-2025-aw-485/prf

Funding: None.

Conflicts of Interest: The author has completed the ICMJE uniform disclosure form (available at https://gs.amegroups.com/article/view/10.21037/gs-2025-aw-485/coif). C.S. received a $200 USD Honorarium in 2025 to be a Keynote speaker at the Asia-Pacific Thyroid Conference in Korea; and is named as an inventor on a patent held by Mount Sinai Innovation Partners filed on 12/23/2016, and entitled “Improved Method and System for Assessing Laryngeal and Vagus Nerve Integrity in Patients Under General Anesthesia”. The author has no other conflicts of interest to declare.

Ethical Statement: The author is accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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