Protecting the recurrent laryngeal nerve: NerveTrend vs. NerveAssure in thyroid surgery

Thyroid surgery is the most commonly performed endocrine surgery worldwide, with recurrent laryngeal nerve (RLN) injury being the most consequential complication of the surgery. Riddle emphasized the importance of visually identifying the RLN during thyroid surgery to minimize injury (1). Efforts have continued to better characterize injury patterns and develop improved strategies to safeguard the RLN. Whilst visualization of the anatomical integrity of the RLN provides a temporary reassurance to the surgeon, there is no means to detect the subtle mechanical injuries in the form of traction, compression or thermal injury. Any injury to the RLN, whether temporary or permanent has consequences, ranging from transient voice change to vocal cord dysfunction and long-term airway issues which are associated with poor quality of life for the patients (2).

Advent of intraoperative neuromonitoring (IONM) for RLN happened the 1970’s and use of it has become the standard practice in many centres at present. IONM has evolved from a supplementary tool to an increasingly essential adjunct, designed to provide real-time feedback and reduce nerve-related morbidity, although value in preventing nerve injury remains debated. IONM is especially useful in difficult situations like malignancy, re-operative surgery, anatomical aberrations and aids in nerve preservation for a low volume thyroid surgeon. It has a significant value, in identifying neuropraxic injuries with segmental loss of signal (LOS type I) which is a common scenario in traction of the gland resulting in injury at the ligament of Berry. Similar segmental injuries are noted when an adherent malignancy is shaved off the RLN (partial layer resection). This paves way to real time modification of technique or resort to a staged thyroidectomy, where the nerve may be compromised (3). Thermal injuries on the contrary demonstrate a similar injury but involves deeper neuronal layers with a bleaker recovery.

There is a fundamental distinction between the 2 modalities. NerveTrend is a conventional IONM platform, where the stimulation is intermittent or episodic applied usually at key steps in the conduct of thyroidectomy. The requirement for this study arose due to the previous experience in comparing NerveTrend^TM to traditional intermittent IONM (I-IONM), which demonstrated a tendency to reduce RLN injury and a significant reduction for staged surgery (4). Initially the vagus nerve is demonstrated at a level above the ipsilateral upper pole, to establish the integrity of the RLN. As surgery proceeds, the RLN is dissected and stimulated every 2–3 minutes. Following detachment of the ipsilateral gland off of the thyroid bed, a final reading is made and this is sequence is repeated on the contralateral side. The trends of the nerve signals alert the surgeon of impending nerve injury usually by a directional change in electromyographic (EMG; falling amplitude and/or rising latency) over time similar to the continuous IONM (C-IONM). An LOS is interpreted as a red flag sign, where the nerve is at risk or is in a state of non-recovery. The Nerve Trend approach is the most favoured by most surgeons, but it inherently suffers from “gaps” between stimulations, during which injury may already have occurred.

In contrast, NerveAssure^TM (C-IONM) captures dynamic, real time EMG activity by applying an Automatic Periodic Stimulation (APS) electrode on the vagus nerve to generate ongoing EMG “trend” data (amplitude/latency), which allows the surgeon to capture “evolving” nerve injuries, before irreversible injury sets in. The theoretical appeal of continuous monitoring has been evident, but high-level comparative data have been limited, until the recently reported randomized trial randomized controlled trial comparing NerveTrend and NerveAssure which provides important clinical validation and prompts reconsideration of the monitoring standard (5). The prospective single centre two-arm randomized trial involving 264 patients undergoing total thyroidectomy were randomized into (NerveAssure vs. NerveTrend with 132 patients each). The primary outcome reported was RLN injury on postoperative day 1 and showed 3 (1.14%) nerves at risk in the NerveTrend group compared 1 (0.38%) in the NerveAssure group with no statistical significant difference between the groups. The study concluded that NerveTrend was “not inferior” to NerveAssure regarding early RLN injury risk, and both modes had the potential to abolish the need for staged thyroidectomy (in the study setting).

Though the study has reported encouraging findings, several questions remain unanswered. Importantly, the outcomes reported are from a high-volume centre with very experienced surgeons, routinely using IONM. Despite this, there is an increased trend towards higher nerve injury with NerveTrend, implying superiority of NerveAssure. Perhaps the results may have been different if conducted by moderate or low volume thyroid surgeons. C-IONM therefore may be less attractive for high-volume thyroid units mainly because it adds workflow complexity without always delivering a proportional outcome gain when baseline RLN injury rates are already low. In large volume centres, it is used as a selective tool for cases (reoperations, malignancy with fibrosis, large goitres, invasive disease, difficult anatomy) rather than as default for every straightforward thyroidectomy, to balance safety benefits with list efficiency (6), but the current study performed the comparative study on the less complex pathologies with a low (mean volume about 2- to 2.5-fold higher) weight.

Secondly, there is no mention of the mean number of stimulations used in the randomized control trial (RCT) though it may not have an impact on the ultimate outcome of the study. As NerveTrend method is intermittent stimulation, there is no fixed number of stimulations as it depends on operative duration and how often risk events occur. The minimum number of stimulations required per side to define the reference amplitude and latency tends to be around 20 stimulations, with additional stimulations performed intermittently during dissection, often about every 3–5 minutes (7). Extra checks may be necessary during high-risk manoeuvres such as traction near Berry’s ligament but the number may vary with the surgeon’s experience and familiarity of the trends.

Finally, though the RCT showed a slightly superior benefit with C-IONM, will the broader adoption of continuous monitoring, prove to be cost-effective at scale? Neuromonitoring in thyroid surgery definitely carries an additional monetary cost (machine, special endotracheal tubes), which may delay it appearing as the standard in many centres worldwide, especially in low resource settings though may be high volume centres. Traditional visual identification is clearly the less costly approach, but the added quality-of-life value of IONM remains uncertain—particularly whether it meaningfully increases completion of safe bilateral thyroidectomy or reduces the need for tracheostomy and prolonged voice therapy (8). The additional time to dissect vagus and setup the electrode is minimal as 5 minutes in the hands of the expert, but may be longer in the not-so experienced thyroid surgeon (9). Although machine setting up, endotracheal tube adjustments may take variable amounts of operating time, resulting in added financial costs.

In summary, the trial represents an important study in endocrine surgery and if the results can be corroborated with larger multicentre studies, it may well catalyze a paradigm shift in how surgeons worldwide approach the delicate balance of safety and precision in thyroid surgery. The real-time monitoring that is available from NerveTrend, shifts the surgeon’s decision-making paradigm. Rather than where the injury is diagnosed after it occurs, continuous monitoring introduces the possibility of prevention in its truest sense, during periods of reversible stress. For patients, obviously this translates into reduced morbidity and better quality of life. From an ethical standpoint, the data strengthen the argument for standardizing IONM as part of modern thyroid surgery, particularly when the technique demonstrate measurable superiority, may soon represent the new standard of care.

Acknowledgments

None.

Footnote

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

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Conflicts of Interest: Both authors have completed the ICMJE uniform disclosure form (available at https://gs.amegroups.com/article/view/10.21037/gs-2025-aw-459/coif). The authors have no conflicts of interest to declare.

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References

1. Riddell VH. Injury to recurrent laryngeal nerves during thyroidectomy; a comparison between the results of identification and non-identification in 1022 nerves exposed to risk. Lancet 1956;271:638-41. [Crossref] [PubMed]
2. Lynch J, Parameswaran R. Management of unilateral recurrent laryngeal nerve injury after thyroid surgery: A review. Head Neck 2017;39:1470-8. [Crossref] [PubMed]
3. Schneider R, Randolph GW, Dionigi G, et al. International neural monitoring study group guideline 2018 part I: Staging bilateral thyroid surgery with monitoring loss of signal. Laryngoscope 2018;128:S1-S17. [Crossref] [PubMed]
4. Barczyński M, Konturek A. Clinical validation of NerveTrend versus conventional i-IONM mode of NIM Vital in prevention of recurrent laryngeal nerve events during bilateral thyroid surgery: A randomized controlled trial. Head Neck 2024;46:492-502. [Crossref] [PubMed]
5. Barczyński M, Dworak M, Krakowska K, et al. Clinical Validation of NerveTrend Versus NerveAssure Mode of Intraoperative Neuromonitoring in Prevention of Recurrent Laryngeal Nerve Injury During Thyroid Surgery: A Randomized Controlled Trial. Ann Surg 2025;282:709-16. [Crossref] [PubMed]
6. Caruso E, Pino A, Pontin A, et al. Limitations of Continuous Neural Monitoring in Thyroid Surgery. Sisli Etfal Hastan Tip Bul 2019;53:81-3. [Crossref] [PubMed]
7. Pino A, Frattini F, Sun H, et al. An Improved Recurrent Laryngeal Nerve-Monitoring Device: Technical Note for Nim Vital™. Surg Technol Int 2021;38:109-24. [Crossref] [PubMed]
8. Tae K. Cost-effectiveness of intraoperative neural monitoring in thyroid surgery: comment on "Analyzing cost-effectiveness of neural-monitoring in recurrent laryngeal nerve recovery course in thyroid surgery". Gland Surg 2019;8:304-6. [Crossref] [PubMed]
9. Patel A, Ally M, Venkatachalam V, et al. The learning curve and safety of continuous intraoperative vagus nerve monitoring in thyroid surgery. Ann R Coll Surg Engl 2022;104:618-23. [Crossref] [PubMed]