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.

Procedures	Note
Record vocal cord movement before surgery	Use of fiberoptic laryngoscope
Recommended anesthesia method for IONM	Intermediate- or short-acting muscle relaxant is recommended for preoperative anesthesia induction. The dosage of intermediate-acting muscle relaxant is smaller than that used for routine anesthesia induction. 1x ED95 intermediate-acting non-depolarizing muscle relaxant is recommended; avoid adding muscle relaxant before the termination of intraoperative monitoring
Body position and tube placement	Anesthesia intubation is performed after positioning (intubation under video laryngoscope is recommended)
Device connections and checking
The ground electrodes are routinely placed under the skin at the shoulders or xiphoid
Confirm the monitoring system has been effectively established
Check electrode impedance and differences in impedance values	Electrode impedance <5 kΩ, with deviations <1 kω
Check initial EMG	Initial fluctuations: about 10 μV
Set up event thresholds	Typically 100 μV
The current intensity of stimulator probe should be routinely set at 1-3 mA
The monitoring device should be placed far away from electro-surgical devices and be connected with anti-jamming silence detectors
The recording electrode positions should be confirmed during surgery	Directly detect, locate, and record the electrode depth at the antemedial laryngeal line using stimulator probe
IONM four-step method
Step 1: V1 signal	Obvious bipolar EMG signal is obtained at the ipsolateral vagus nerve at the plexas thyreoidea inferior level (point B), confirming the successful establishment of the monitoring system. If signal is absent at point B, detect the vagus nerve at the plexas thyroid superior level (point A); the presence of signal at point A confirms the presence of non-recurrent laryngeal nerve (25) (Figure 4)
Step 2: R1 signal	Before the exposure of RLN, its EMG signal is located using the Cross method by applying the probe vertical to trachea at its traveling area and then parallel to trachea
Step 3: R2 signal	Continuous monitoring is applied during the dissection of RLN and the signal change is compared in a real-time manner. After the RLN is exposed, the most proximal end of the exposed part is detected for EMG signal
Step 4: V2 signal	After complete hemostasis is achieved at the surgical field, the EMG signal of the vagus nerve is detected before closing the incision
Signal analysis
Basic EMG parameters	The biphasic waveform should be differentiated from the monophasic non-EMG artifacts
No obvious decrease in R2 and V2 signals	The basic EMG parameters include amplitude, latency and duration (26) (Figure 5); the RLN has intact function
Loss of R2 and V2 signals	If the RLN is injured during the surgical operation, detect the “injury site” and search for the injury cause*
Photo recording the exposed RLN during surgery	To confirm the RLN continuity (visual integrity)
Postoperative laryngoscopy

*, If no “injury site” is detected, it is important to determine whether there is a “real” loss of signal. (I) stimulate the nerve and then observe the contact between endotracheal tube electrode and vocal cord via a laryngoscope; (II) detect signals at vagus nerve and RLN again before closing the incision. IONM, intraoperative neuromonitoring; RLN, recurrent laryngeal nerve.

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.

Common IONM errors	Causes	Solutions
Electrode impedance is too high	The subcutaneous electrodes have not completely removed after its withdrawal from patients	Check whether the subcutaneous electrodes fall off and keep the electrode clean
Subcutaneous electrodes >10 kΩ	The impedance of the electrode itself is too high	Replace the electrodes and fix them with tape
Stimulator probe electrodes >25 kΩ	The electrode core does not connect well with the interface-connector box	Check the connections of the interface-connector box
	The interface-connector box does not connect well with the monitor
Recording electrodes	The recording electrode does not connect well with vocal cord	Adjust the tube depth and angle under fiberoptic laryngoscope
Single electrode impedance >5 kΩ	The surface electrode of endotracheal tube is displaced	Indwell the tube under conventional video laryngoscope
Impedance deviation>1 kΩ	Application of insulating lubricant before intubation	Avoid the application of insulating medium at the recording electrodes
Electrode impedance is zero	Two subcutaneous electrodes contact with each other	Re-indwell the subcutaneous electrodes, with the inter-electrode distance of >1 cm
Electrosurgical interference	The probe of anti-jamming detector is not connected	Circle the cable of the electrosurgical device, with the anti-jamming detector clipped on the twisted cable
After the establishment of standard monitoring system	Preoperative vocal cord palsy	Re-check the preoperative laryngoscopic records
Before thyroid surgery	The nerve detected by the operator is actually not a vagus nerve	Detect at 1 mA after confirming the exposure of vagus nerve
V1 signal is absent	The vagus nerve is injured during its exposure	Directly detect the carotid sheath at 3 mA to obtain the V1 signal
	non-recurrent laryngeal nerve is present	If the signal of vagus nerve is absent at the plexas thyreoidea inferior level, re-check it at the plexas thyreoidea superior level
	Anesthesia induction is not performed as recommended	Wail until the muscle relaxant wears off or use a proper dose of a muscle relaxant antagonist
	Improper type or dosage of muscle relaxant	Check the matching between the measured stimulation (as displayed on monitor) and the stimulation setup value
	The detection current is not high enough	Re-check the connections between the electrodes and the interface-connector box
		Check whether the fuse in the interface-connector box has been burned out
	The frequency of stimulus pulse is too low	Stimulus pulse frequency: 4 times/s by default
	The setup of event threshold is too high	Routinely 100 μV; avoid changing this parameter value casually
	The selected monitoring mode, channel, and volume are improper	Re-check the monitoring mode, channel, and volume setup
	The duration of detection for nerve is too short	Each detection should be maintained at least 1s
	The probe is damaged, with insulation layer falling off	Avoid reuse
	The shunt of the nerve detection area is too large	Clear liquids at the detection area
	The muscle for detecting neurological effects is detached from the recording electrode	Re-check whether the electrodes are off
		The depth of the surface electrodes of endotracheal tube can be detected and located at the laryngeal anteromedian line
EMG signal is present while no nerve is detected	Consecutive “sequence” EMG response cannot be explained	Light anesthesia, with spontaneous activity of laryngeal muscle;
	Artifacts occur in the non-neural traveling area	the recording nerve or muscle is tracted by other nerve or muscle
	The detection current is too large	The surface electrode of endotracheal tube is placed too deeply
		Direct detect the nerve trunk (1 mA is recommended)
		Adjust according to the anatomic structures and EMG signals during the surgery
V1 signal is good, confirming the establishment of standard monitoring system, whereas there is the decrease of signal by >50% or LOS during the dissection of RLN	Intraoperative anesthesia or muscle relaxation status changes	Avoid adding muscle relaxant before the termination of monitoring
	Nerve transection injury	Check the nerve continuity
	Nerve injury not visible to the naked eye	Locate the injury site and analyze the possible injury mechanisms: traction injury, heat injury, suction injury, and/or thread-cutting injury
	Monitoring system failure	Re-check the electrode connections to ensure a good circuit performance
	Recording electrode displacement due to changes in head position or body position during the surgery	Use simulators to re-check the monitor and interface-connector box (e.g., fuse)
		Re-check the laryngoscope and adjust the endotracheal tube

RLN, recurrent laryngeal nerve.

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).