A Novel Approach to Continuous Monitoring of the Recurrent Laryngeal Nerve during Thyroid Surgery

This article refers to:

Pre-Prototype Stimulating and Recording Endotracheal Tube for Continuous Monitoring of the Recurrent Laryngeal Nerve During Thyroid Surgery

Intraoperative neural monitoring (IONM) during thyroid surgery has gained widespread acceptance as an adjunct to the gold standard of visual nerve identification [1].

The technique of IONM has been standardized owing to research and guideline statements published in recent years by the International Neural Monitoring Study Group [1–4].

The major discrete domains of neural monitoring applications in thyroid surgery include: intraoperative neural identification and neural mapping, aid in dissection allowing clear cut differentiation between neural versus non-neural structures during neural dissection, real time intraoperative information regarding impending injury as well as mechanism and site of injury, and neural prognostication at the completion of surgery as it relates to postoperative glottic function [3].

Intermittent IONM (I-IONM) was shown in a randomized controlled trial to decrease the incidence of transient but not permanent recurrent laryngeal nerve (RLN) paresis compared with visualization alone, particularly in high-risk patients [5]. In addition, it has been proven to be effective in prevention of bilateral vocal fold palsy (VFP) in planned bilateral thyroid surgery in patients with true loss of signal following unilateral lobectomy (staged thyroidectomy) [3]. Unfortunately, the I-IONM format can reliably identify neural injury once it occurred but it has no potential for preventing impending neural injury. Many of the RLN injuries are dissociated in time as a result of accumulation of micro-injuries related to intraoperative manipulations and traction and the impending neural injury can only be identified based on real-time and continuous electrophysiological neural data analysis. Hence, the modern art of neural monitoring has been quite recently switched to the continuous IONM (C-IONM) format which overcomes the key methodological limitations inherent in I-IONM (nerve can be at risk of injury in between the stimulations). While the I-IONM helps to detect injury only after it occurs the c-IONM format has a potential to prevent neural injury. Nevertheless, both systems represent complimentary approaches and if utilized together may improve not only anatomical identification but also functional preservation of the RLN during thyroid surgery.

C-IONM technique generally consists of a multichannel EMG system, electromyographic (EMG) display, sensing endotracheal surface electrode, handheld stimulation probe, and is complemented by additional temporarily placed vagus electrode. The clinically important combined EMG events, indicative of impending RLN injury, prevents the majority of traction related injuries to the anatomically intact RLN enabling modification of the causative surgical behavior in the majority of cases (80%). These EMG changes can progress to loss of EMG signal with postoperative VFP if corrective action is not taken [6–8]. Thus, C-IONM may prevent not only bilateral but also unilateral RLN injury. As a further extension, C-IONM also helps to identify intraoperative functional nerve recovery with restitution of amplitude to ≥50% of initial baseline (and ≥250µV) indicating when planned bilateral thyroid surgery can be attempted safely with no risk of bilateral VFP [8].

On one hand, C-IONM appears to be the emerging technology which has the ability to overcome the major disadvantage of the I-IONM format. On the other hand, some surgeons reported that they found it challenging to approach and dissect the vagus nerve for C-IONM vagus electrode placement. Gentle 360° dissection of a small vagus nerve segment at positive stimulation point may not be easy in obese patients or with a high-volume of goiter or with a retrosternal extension of goiter. In addition, it is extremely important to avoid nerve devascularization during its mobilization which needs proper training and surgical skills. Hence, the commented study describing a pre-prototype of a trans-tracheal stimulation and recording endotracheal tube (SRET) for the stimulation of RLN in the neck is of interest in the field as it has a potential to provide continuous RLN monitoring while avoiding the need for the vagus nerve dissection [9]. This proof of concept was tested on 10 RLNs in 5 pigs as animal models with encouraging initial results. Authors found that SRET was capable to deliver a continuous trans-tracheal stimulation to the proximal end of the RLN resulting in ipsilateral vocal fold movements which were recorded by superficial electrodes on endotracheal tube as biphasic waveforms with latency and amplitude (evoked responses following stimulation of the ipsilateral RLN) [9].

The major strength of the commented study is novelty of the proposed format of SRET for both stimulation and recording of EMG for neural monitoring during thyroidectomy. However, there are several concerns which need to be recognized and solved in the near future. The developed SRET is actually a “pre-prototype”, not a “prototype”, which means that it is the earliest phase of the new product development process allowing to test the invented concept. The self-manufactured SRET by authors needs to optimized to become more user friendly before it can be termed “prototype” ready for approval as a medical tool. This pre-prototype of SRET has 1 channel only, whereas it is necessary to have at least 2 channels for bilateral thyroid surgery. Any intraoperative maneuvers like intraoperative rotation of the tube cannot be accepted as the desired format of a prototype tube for neural monitoring in thyroid surgery (as it complicates the protocol of neural monitoring increasing the operating time). Moreover, in the concept of SRET it is the proximal area of the RLN which is stimulated, adjacent to the tip of the SRET (where the vagus electrode is fixed) but it is not possible to precisely define if the integrity of the entire RLN can be evaluated in this way. This issue needs further validation in a well-designed experimental study to allow for suggesting that SRET can replace the current format of well-tested and easily reproducible and reliable C-IONM format (via the vagus nerve in which the entire RLN is tested automatically).

Hence, the next steps in future experimental research with SRET is to show more solid evidence based on prospective and preferably multi-center studies that this technology may advance and change the current practice of RLN management in thyroid surgery. This change of paradigm is likely to happen based on mutual feedback of surgical expertise supported by novel technology being under constant development and clinical evaluation.

Disclosure statement

No potential conflict of interest was reported by the author(s).