Clinical guidelines on intraoperative neuromonitoring during thyroid and parathyroid surgery

Introduction

Recurrent laryngeal nerve (RLN) injury is one of the most severe complications of thyroid surgery. Hoarseness due to unilateral RLN injury and breathing disorders and even asphyxia due to bilateral RLN injury can impose serious impacts on the patient’s life. It is estimated that the incidence of RLN injury during thyroid surgeries ranges 0.3-18.9% (1-5), making RLN protection a great concern among thyroid surgeons. Intraoperative neuromonitoring (IONM) combines both the functional and anatomic techniques and has the following features: intra-operative navigation and rapid identification of the RLN distribution; predicting nerve variation and protecting of the functional integrity of RLN; clarifying the mechanism and lowering the incidence of RLN injury; and easy to perform. It is a helpful adjunct for complicated surgeries (6,7).

With an attempt to help the surgeons thoroughly understand IONM and carry out this technique in a standardized and reasonable manner, the Chinese Thyroid Association established the Clinical Guidelines on IONM during Thyroid and Parathyroid Surgeries (China Edition).

Basic principles of IONM

Application of IONM in thyroid surgeries was initially proposed by Shedd in 1966 and by Flisberg in 1970. Based on the principle of electrophysiology, the motor nerve is stimulated by electricity during the surgery, and then the nerve impulses are formed and transferred to the dominant muscles to produce myoelectric signals, forming waves and alerts in electromyography (EMG), which will help the surgeons to judge the functional integrity of nerves (8,9) (Figure 1).

Figure 1 Basic principles of IONM. IONM, intraoperative neuromonitoring.

Clinical significance and technical advantages of IONM

For doctors who are transforming from generalist to specialist, junior doctors, and surgeons who are facing a complicated thyroid surgery, IONM undoubtedly is a good aid (10-12).

Facilitates the identification and locating of RLN

Before RLN exposure, the nerve can be accurately located at its distribution area using the cross method, which enables the doctor to rapidly determine the anatomic range of RLN as well as its rare anatomic variations such as non-recurrent RLN (13).

Facilitates the exposure and dissection of RLN

During the dissection of RLN, continuous monitoring together with naked-eye observation enables the differentiation between the monitored nerve and its surrounding non-nerve tissues and the accurate tracing of the nerve and its functional branches. Also, the intra-operative navigation is also helpful for the complete resection of lesions.

Facilitates the judgment of the functional integrity and injury mechanisms of nerves

Electrophysiological monitoring provides quantitative indicators for judging the functional integrity of nerves, can accurately locate the “injury point” of a nerve (14) (Figure 2), and help the operator to analyze the injury mechanism and timely recognize and resolve the injury during surgery. Thus, it can markedly reduce the incidence of RLN injury.

Figure 2 Locating nerve “injury site” by IONM. IONM, intraoperative neuromonitoring.

Indications of IONM (15-18)

IONM is a preferred option in the following patients, while its application in the other patients should be upon the doctor’s recommendation: (I) the mass is located at the dorsal side of the thyroid, with suspected recent capsular hemorrhage or thyroid cancer; (II) in patients with hyperthyroidism, pre-operative ultrasound indicates large gland and rich blood supply; (III) in patients with malignant thyroid tumor, neck lymph node dissection is required, especially in patients with swollen central lymph nodes; (IV) patients receiving a second thyroid surgery but with disordered anatomic structures and severe adhesions; (V) patients with retrosternal goiter and/or large thyroid mass, along with suspected RLN dislocation; (VI) pre-operative imaging indicated the presence of transposition of viscera or subclavian artery variation, along with suspected non-recurrent RLN; (VII) patients with unilateral vocal cord paralysis, with the contralateral lobe requiring surgical treatment; (IX) patients requiring total thyroidectomy, in particular an endoscopic surgery; (X) patients requiring a surgery to repair RLN injury; (XI) patients requiring a parathyroid surgery; and (XII) patients with special requirements on sound and tone and thus requiring IONM.

Notably, (I) if intra-operative exploration shows that the thyroid cancer has completely infiltrated the RLN, nerve preservation will unavoidably result in residual tumor; thus, the invaded nerve must be resected to thoroughly remove the tumor. Under such circumstances, the post-operative hoarseness can not be avoided even after the application of IONM; (II) in patients with pre-operative vocal palsy, IONM can be applied to search for the injury site, and nerve repair can be completed with the assistance of IONM; however, it is difficult to completely restore the nerve function.

Since it is impossible to predict all the complex cases before a thyroid or parathyroid surgery, the indications of IONM may be widened if condition allows. The same recommendation has also proposed in international literature: since it is difficult to predict RLN variation before surgery, IONM may be routinely applied if condition allows.

Essential monitoring devices of IONM

The IONM devices can be divided into recording side (recording electrode and its ground electrode) and stimulation side (stimulator probe and its loop electrodes) as well as EMG monitor, interface-connector box, anti-jamming silence detector, and patient simulators (19) (Figure 3).

Figure 3 Basic monitoring equipment setup. ET, endotracheal tube; REC, recording electrodes; GND, ground electrodes; EMG, electromyography.

Needle-like electrode and intubation surface electrode are two typical recording electrodes, with the latter being routinely recommended (20).

The stimulation probes can be divided into monopolar and bipolar types, with the monopolar Prass probe with ball tip is routinely recommended (21).

Standardized procedures of IONM

Non-standard application of IONM techniques will cause significant monitoring errors. The main aim of standardized IONM procedures is to guide and improve the quality of IONM techniques and avoid any adverse effect due to improper monitoring operations (22-24). The standard procedure of IONM is shown in Table 1.

Table 1 The standard procedures of IONM
Full table

Figure 4 Monitoring setup of non-recurrent laryngeal nerve.

Figure 5 Basic parameters of the EMG wave of IONM. IONM, intraoperative neuromonitoring.

Causes and solutions of common IONM errors

The incidence of IONM error ranges 3.8-23.0% (27). IONM errors can bring great psychological pressure to the operator, delay the surgical process, and even cause the operator make wrong decision. Therefore, it is important to master the causes and solutions of common IONM errors, so as to ensure the surgery is performed in a safe and smooth manner. Table 2 lists the causes and solutions of common IONM errors, and the troubleshooting process (28) is displayed in Figure 6.

Table 2 Causes and solutions of common IONM errors
Full table

Figure 6 Algorithm for troubleshooting common IONM errors. IONM, intraoperative neuromonitoring.

The standard application of IONM can help 85% of beginners to smoothly locate the laryngeal nerves, provide quantitative indicators of nerve function for surgeons, assist the surgeons to deal with the complicate anatomic structures and skillfully keep away from the dangerous areas. Also, by using both eyes and ears (by listening to the alerts), the operators can accurately resect the tumor. Thus, IONM has become an effect adjunct for the golden standard of naked-eye protection. With “simple, effective, and practicable” as the basic principles, the Clinical Guidelines on IONM during Thyroid and Parathyroid Surgeries (China Edition) established by Chinese Thyroid Association elucidates the standardized operation procedures and decision-making steps of IONM. Before the application of IONM, the surgeons need to fully know neuromonitoring systems and thoroughly understand the Guidelines and receive corresponding training and verification in a standardized monitoring base. With the assistance of IONM, an experienced surgeon will be able to further lower the incidence of RLN injury and improve the surgical safety and completeness, which can also be a new trend in laryngeal nerve protection during a thyroid surgery (29).

Acknowledgements

None.

Footnote

Conflicts of Interest: The authors have no conflicts of interest to declare.

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