Safety of continuous vagal neuromonitoring in thyroid surgery from gastrointestinal point of view

Continuous intraoperative neuromonitoring (c-IONM) via the vagal nerve has been recently introduced to the surgical armamentarium for thyroid surgery in order to prevent the recurrent laryngeal nerve (RLN) injury (1-4). Automatic periodic stimulation of the vagal nerve with the electric current at frequency of 0.2–2.0 Hz allows for almost real time monitoring of the functional integrity of the entire RLN during thyroid operation. When compared to the current standard of visual identification of the RLN even with use of intermittent IONM (i-IONM) allowing for improved anatomical identification of the nerve this novel concept of functional preservation is considered to be a quantum leap in nerve protection with a potential of preventing nerve injury (1). As shown in many previous studies the majority of the RLN injuries are non-anatomical and traction related injuries caused by surgical manipulation during thyroid dissection (5,6). The impending neural injury can be recognized by the c-IONM system intraoperatively and the surgeon can be alerted as soon as a series of severe combined events (sCE) occur in intraoperative electromyography (EMG) of the vocalis muscles during the case. Most commonly the sCE is defined as decrease in amplitude of more than 50% from baseline with a simultaneous increase in amplitude of the EMG signal of more than 10% when compared to the initial baseline (3,4). What is extremely important is the fact that sCEs usually are reversible in the majority of cases and that loss of signal—an EMG event which is hardly reversible and highly predictive of neural injury—is most commonly preceded by the occurrence of sCEs (3,4). Hence, once the surgeon is alerted by the c-IONM system the surgical maneuvers should be stopped for a while until the recovery of the EMG parameters is observed (1). In this way surgical manipulation and dissection during thyroidectomy can be done in a much safer way for the RLN implying not only anatomical but also functional nerve preservation. Schneider et al. published recently encouraging data comparing prevalence of RLN injury with c-IONM versus i-IONM in a large cohort of 1,526 patients, 788 of whom (1,314 nerves at risk) underwent thyroid surgery using c-IONM and 738 (965 nerves at risk) had i-IONM. Operation with c-IONM resulted in fewer permanent vocal fold palsies compared with i-IONM after thyroid surgery in patients with benign disease (0.0% vs. 0.4%, P=0.019) (2).

Despite growing body of evidence based on prospective cohort studies, including large cohorts of patients that c-IONM with a repetitive vagal nerve stimulation is safe (1-4,7), there are three anecdotal reports of local and cardiac complications during thyroid operations with c-IONM in a very few patients (8-10). Hence, some surgeons expressed their concern that this technology designed to prevent harm to the RLN during thyroid dissection may carry its own risk and should not be recommended to the low-volume thyroid surgeons (8). However, it has been repeatedly shown that gentle vagal nerve handling and meticulous RLN dissection makes no harm to the nerve (2,7). In addition, the electrical current of 1–2 mA used during thyroid operation with c-IONM has no potential to activate thin demyelinated C-fibers responsible for the majority of the autonomic effects (11). The stimulation with 1 mA is supramaximal for only the efferent motor A fibers and myelinated autonomic B fibers. Moreover, low-level vagal nerve stimulation at frequencies less than 30 Hz (most commonly frequency used for c-IONM is between 0.5 and 2.0 Hz) has not been associated with subsequent adverse vagal effects. Furthermore, the 1 mA stimulation current used for c-IONM is not believed to initiate any adverse vagal effect leading to central (headache, numbness), cardiac (arrhythmias, bradycardia), pulmonary (bronchospasm), or gastrointestinal (nausea, vomiting) symptoms (1).

Thus, it is of utmost importance to realize that surgeons willing to utilize the c-IONM technology safe should be aware of the standardized approach to the neural monitoring of the RLNs and strictly adhere to some basic rules outlined by the International Neural Monitoring Study Group in the field (12). To make this procedure even safer we certainly need more studies focused on safety issues. Hence, Xiaoli et al. should be congratulated for undertaking in their most recent research the changes in gastric acid secretion and gastrin release during thyroidectomy with c-IONM as the primary outcome measure (13). The vagus nerves are very well-known to play a pivotal role in the regulation of gastric acid secretion and Gastrin release (14). Xiaoli et al. examined 58 patients undergoing thyroid surgery with c-IONM and compared gastric acid and serum gastrin at five time points during the operation: (I) before skin incision; (II) after baseline calibration of c-IONM probe; (III) at 20 min from baseline; (IV) before probe removal; and (V) after extubation. Gastric pH was assessed by transnasal placement of a sensor in the stomach by the anesthesiologist. Gastrin hormone level was determined with a commercially available radioimmunoassay kit. Non-significant differences in mean gastric pH values were observed at all time points of the operation. Comparisons of pH monitoring parameters revealed no significant differences if stratified to age, gender, side of c-IONM (left vs. right), sequence of c-IONM, or duration of c-IONM. Gastrin values were normal in sequential determinations and did not significantly differ at any time points. Hence, authors concluded that c-IONM performed via repeated vagal nerve stimulation during total thyroidectomy in healthy patients did not influence gastrin secretion and gastric pH (13).

To date, there is no concern about safety of c-IONM during thyroid surgery from gastrointestinal point of view as any alimentary tract associated severe adverse effects of a repetitive stimulation of the vagal nerves have not been reported so far. From practical point of view, the clinical benefit of utilization of c-IONM during thyroid surgery seems to far overweight the minimal risk of the procedure related problems. Prevalence of permanent vocal fold palsy is lower with c-IONM and accuracy in predicting early postoperative vocal cord function is superior to the i-IONM technique.

Acknowledgments

Funding: None.

Footnote

Provenance and Peer Review: This article was commissioned and reviewed by Section Editor Dr. Xiaoli Liu (Deputy Director of Science and Education Department, Department of Thyroid Surgery, China-Japan Union Hospital of Jilin University, Changchun, China).

Conflicts of Interest: The author has completed the ICMJE uniform disclosure form (available at http://dx.doi.org/10.21037/aot.2017.07.01). The author has no 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/.

References

1. Schneider R, Randolph GW, Barczynski M, et al. Continuous intraoperative neural monitoring of the recurrent nerves in thyroid surgery: a quantum leap in technology. Gland Surg 2016;5:607-16. [Crossref] [PubMed]
2. Schneider R, Sekulla C, Machens A, et al. Postoperative vocal fold palsy in patients undergoing thyroid surgery with continuous or intermittent nerve monitoring. Br J Surg 2015;102:1380-7. [Crossref] [PubMed]
3. Phelan E, Schneider R, Lorenz K, et al. Continuous vagal IONM prevents recurrent laryngeal nerve paralysis by revealing initial EMG changes of impending neuropraxic injury: a prospective, multicenter study. Laryngoscope 2014;124:1498-505. [Crossref] [PubMed]
4. Schneider R, Randolph GW, Sekulla C, et al. Continuous intraoperative vagus nerve stimulation for identification of imminent recurrent laryngeal nerve injury. Head Neck 2013;35:1591-8. [Crossref] [PubMed]
5. Mattsson P, Hydman J, Svensson M. Recovery of laryngeal function after intraoperative injury to the recurrent laryngeal nerve. Gland Surg 2015;4:27-35. [PubMed]
6. Schneider R, Randolph G, Dionigi G, et al. Prospective study of vocal fold function after loss of the neuromonitoring signal in thyroid surgery: The International Neural Monitoring Study Group's POLT study. Laryngoscope 2016;126:1260-6. [Crossref] [PubMed]
7. Mangano A, Kim HY, Wu CW, et al. Continuous intraoperative neuromonitoring in thyroid surgery: Safety analysis of 400 consecutive electrode probe placements with standardized procedures. Head Neck 2016;38:E1568-74. [Crossref] [PubMed]
8. Terris DJ, Chaung K, Duke WS. Continuous vagal nerve monitoring is dangerous and should not routinely be done during thyroid surgery. World J Surg. 2015;39:2471-6. [Crossref] [PubMed]
9. Almquist M, Thier M, Salem F. Cardiac arrest with vagal stimulation during intraoperative nerve monitoring. Head Neck 2016;38:E2419-20. [Crossref] [PubMed]
10. Brauckhoff K, Vik R, Sandvik L, et al. Impact of EMG changes in continuous vagal nerve monitoring in high-risk endocrine neck surgery. World J Surg 2016;40:672-80. [Crossref] [PubMed]
11. Groves DA, Brown VJ. Vagal nerve stimulation: a review of its applications and potential mechanisms that mediate its clinical effects. Neurosci Biobehav Rev 2005;29:493-500. [Crossref] [PubMed]
12. Bacuzzi A, Dralle H, Randolph GW, et al. Safety of continuous intraoperative neuromonitoring (C-IONM) in thyroid surgery. World J Surg 2016;40:768-9. [Crossref] [PubMed]
13. Xiaoli L, Wu CW, Kim HY, et al. Gastric acid secretion and gastrin release during continuous vagal neuromonitoring in thyroid surgery. Langenbecks Arch Surg 2017;402:265-72. [Crossref] [PubMed]
14. Debas HT, Carvajal SH. Vagal regulation of acid secretion and gastrin release. Yale J Biol Med 1994;67:145-51. [PubMed]