Intraoperative Neuromonitoring of Recurrent Laryngeal Nerve During Thyroidectomy with Adhesive Skin Electrodes

Intraoperative Nerve Monitoring in Thyroid Surgery: A Comprehensive Review of Technical Principles, Anesthetic Considerations, and Clinical Applications

Background: Intraoperative nerve monitoring (IONM) is increasingly recognized as an essential technique in thyroid surgery to preserve the integrity of the recurrent laryngeal nerve (RLN) and prevent postoperative complications. Although widely adopted, several technical and anesthetic factors can significantly affect the reliability and interpretation of electromyographic (EMG) signals. Methods: This narrative review summarizes the principles and methodologies of IONM in thyroid surgery, focusing on the mechanisms of RLN injury, the clinical benefits of IONM, and its limitations. Particular emphasis is placed on the anesthesiologic considerations, including the effects of neuromuscular blocking agents and anesthetic maintenance methods for EMG signal quality. Recent advances in alternative IONM techniques are also discussed. Results: IONM facilitates early detection of RLN injury and improves surgical outcomes. However, signal loss and technical errors occur in up to 23% of cases. Appropriate anesthetic management, such as the judicious use of neuromuscular blocking agents and the use of reversal agents like sugammadex, can significantly improve IONM accuracy. Alternative approaches, such as transcutaneous or thyroid cartilage electrode-based monitoring, show promise in overcoming current limitations. Conclusions: IONM is a valuable tool in modern thyroid surgery, aiding in the prevention of RLN injury. Anesthesiologists play a crucial role in optimizing IONM quality by managing factors that affect EMG signals. Ongoing research into novel monitoring techniques is expected to further enhance patient safety and surgical precision.

Feasibility of Cricothyroid Muscle Needle Electrode for Monitored Thyroidectomy

OBJECTIVES

Preservation of the recurrent laryngeal nerve (RLN) and external branch of the superior laryngeal nerve (EBSLN) is essential to maintain voice function after thyroidectomy. Intraoperative neuromonitoring (IONM) assists in nerve preservation. This study compares three IONM modalities, endotracheal surface electrode (ETSE), thyroid cartilage needle electrodes (TCNE), and cricothyroid muscle needle electrode (CTNE), in their monitoring of laryngeal muscle electromyographic (EMG) amplitude.

METHODS

An observational prospective cohort study included 29 patients (21F:8M) undergoing thyroidectomy for Bethesda III-VI nodules or benign nodules >4 cm. EMG amplitudes were recorded via ETSE, TCNE, and CTNE following stimulation of the EBSLN, RLN, and vagus nerve (VN).

RESULTS

A total of 105 nerves at risk were assessed, including 35 EBSLNs, 35 VNs, and 35 RLNs. The recorded amplitudes at S1 for the ETSE, TCNE, and CTNE were 45.6 ± 60.7 μV, 403.7 ± 865.4 μV, and 3049.4 ± 2749.8 μV. The CTNE demonstrate significantly higher S1 amplitudes and positive signal rates compared with both the TCNE and ETSE (p < 0.001). For V1, the recorded amplitudes for the ETSE, TCNE, and CTNE were 453.7 ± 406.7 μV, 1363.9 ± 1063.6 μV, and 700 ± 375.1 μV. For R1, the recorded amplitudes for the ETSE, TCNE, and CTNE were 567.1 ± 556 μV, 1544.6 ± 1269.6 μV, and 720.4 ± 363.1 μV. With a threshold of 100 μV to determine positive signal, CTNE detected all EBSLN, VN, and RLN signals, whereas TC missed 13 cases (37.1%) of EBSLN.

CONCLUSION

The CTNE showed a higher positive signal rate than ETSE and TCNE in detection of EBSLN, whereas it showed similar performance with TCNE in detection of VN and RLN. CTNE is a viable option for IONM during thyroidectomy.

LEVEL OF EVIDENCE

2 Laryngoscope, 135:1843-1849, 2025.

Application of Adhesive Skin Electrode for Intraoperative Neuromonitoring During Enucleation of a Cervical Vagal Schwannoma

BACKGROUND

No reports on applying adhesive skin electrodes for intraoperative nerve monitoring (IONM) of the vagus nerve during schwannoma enucleation have been reported in the literature.

METHODS

A 45-year-old patient was referred to our institution with a 2.2 cm mass in the left lateral neck, suspected to be a vagal nerve schwannoma. A pair of surface electrodes were attached to the lateral margin of the thyroid cartilage. After exposing the tumor, the authors selected the incision site avoiding the expected course of the vagus nerve fibers after mapping on the surface of the tumor.

RESULTS

Postoperative vocal cord function was intact, without aspiration or dysphagia.

CONCLUSIONS

The authors report the successful use of skin electrodes for IONM of the vagus nerve during cervical vagal schwannoma removal, suggesting the feasibility of applying these electrodes for IONM of neck surgeries other than those associated with the thyroid or parathyroid glands.

Modern Surgical Techniques of Thyroidectomy and Advances in the Prevention and Treatment of Perioperative Complications

Thyroid cancer is the most common cancer of the endocrine system, and, in recent years, there has been a phenomenon of overdiagnosis followed by subsequent overtreatment. This results in an increasing number of thyroidectomy complications being faced in clinical practice. In this paper, we present the current state of knowledge and the latest findings in the fields of modern surgical techniques, thermal ablation, the identification and assessment of parathyroid function, recurrent laryngeal nerve monitoring and treatment and perioperative bleeding. We reviewed 485 papers, from which we selected 125 papers that are the most relevant. The main merit of this article is its comprehensive view of the subject under discussion-both general, concerning the selection of the appropriate method of surgery, and particular, concerning the selection of the appropriate method of prevention or treatment of selected perioperative complications.

Feasibility of facial nerve monitoring using adhesive surface electrodes during parotidectomy: a comparative study with needle electrodes

PURPOSE

Electrophysiological monitoring of the facial nerve during parotidectomy has been reported as an adjunctive method to prevent facial nerve injury. Classically, a needle electrode is used to obtain electromyographic (EMG) signals from facial muscles during facial nerve monitoring (FNM) of parotid surgery, likewise adhesive surface electrodes. This study aimed to investigate the feasibility of performing FNM with surface electrodes during parotid surgery and to compare EMG values with needle electrodes.

METHODS

Thirty patients who underwent parotidectomy under FNM using adhesive surface and needle electrodes were included. Two pairs of adhesive surface electrodes and needle electrodes were used for FNM during parotid surgery. Mean amplitudes were collected after electrical facial nerve stimulation at 1 mA after specimen removal.

RESULTS

The mean amplitude of the adhesive surface electrodes was 226.50 ± 118.44 μV (orbicularis oculi muscle) and 469.6 ± 306.06 μV (orbicularis oris muscle), respectively. The mean amplitude of the needle electrodes was 449.85 ± 248.10 μV (orbicularis oculi muscle) and 654.66 ± 395.71 μV (orbicularis oris muscle), respectively. The mean amplitude of the orbicularis oris muscle was significantly greater than that of the orbicularis oculi. The amplitude values measured in the orbicularis oculi muscle showed significant differences between the needle and skin electrodes.

CONCLUSIONS

Facial nerve monitoring (FNM) using adhesive surface electrodes is feasible in parotid surgery. Although the mean amplitude value of the surface electrode was relatively lower than that of the needle electrode, the surface electrode is considered a feasible and safe EMG recording device for FNM in parotid surgery.

Anatomical and functional identification of the external branch of the superior laryngeal nerve: classification based on morphology and electrophysiological monitoring

BACKGROUND

Motor function of the external branch of superior laryngeal nerve (EBSLN) is vital for voice quality. We studied the rate of EBSLN identification and integrity in the era of intraoperative neuromonitoring (IONM).

METHODS

Anatomic and functional identification of 515 EBSLNs-at-risk was performed under the guidance of IONM that motor integrity was electrophysiologically checked. The functional integrity was assessed with crico-thyroid muscle (CTM) twitches and/or recordable waveform amplitude. We tried to establish the systematic classification of EBSLN identification and integrity.

RESULTS

Visual, electrophysiological and total identification rates were 64.3%, 31.6% and 95.9%, respectively. We could identify 4.1% of EBSLNs neither anatomically nor electrophysiologically. We recorded CTM twitches alone or both CTM twitches and wave amplitude in 203(39.4%) and 291(56.5%) branches respectively. Identification features of EBSLNs were systematically classified under three main types: Visualized-monitored (1), non-visualized-monitored (2), unidentified (3), and electrophysiological integrity of EBSLNs under two subtypes: CTM twitches alone (a) and CTM twitches and wave amplitude (b).

CONCLUSION

Dedicated thyroid surgeon could visually identify EBSLNs. IONM contribution significantly increases the identification rate. Systematic classification of identification and electrophysiological integrity of EBSLNs may increase comprehensive knowledge about its motor function that is crucial for complication-free thyroidectomy.

Recent Developments of Intraoperative Neuromonitoring in Thyroidectomy

At present, intraoperative neuromonitorization (IONM) with surface electrode-based endotracheal tube (ETT) is a standard method in thyroidectomy and can be performed either intermittently IONM (I-IONM) or continuously IONM (C-IONM). Despite the valuable contribution of I-IONM to the thyroidectomy, it still has limitations regarding the recording electrodes and stimulation probe. New approaches for overcoming the limitations of I-IONM and developing the method are taking attention. Most of the technical issues of IONM with surface electrode-based ETT are related with inadequate contact of electrodes to the vocal cords. Nowadays, efficiency of various recording electrodes is under investigation. Recording electrodes such as needle electrodes applied to thyroarytenoid or posterior cricoarytenoid muscle (PCA), surface electrodes applied to the PCA, and needle or adhesive electrodes applied to the tracheal cartilage or skin, can make safe recordings similar to the ETT electrodes. Despite their invasiveness, needle electrodes record higher electromyography (EMG) amplitudes than tube electrodes do. Adhesive surface electrodes make safe EMG recordings, although amplitudes of these electrodes are usually lower than those of the tube electrodes. These different types of electrodes are less affected by tracheal manipulations and amplitude changes are lower compared to the tube electrodes.

During C-IONM, an additional stimulation probe is applied to the vagus nerve after dissecting the nerve circumferentially. Recently, without applying a probe, a new continuous monitorization method called laryngeal adductor reflex CIONM (LAR-CIONM) using sensorial, central, and motor components of LAR arch which is an automatic, primitive brainstem reflex protecting the tracheoesophageal tree from foreign body aspiration, has been implemented. Afferent track of LAR communicates laryngeal mucosa to the brainstem by internal branch of the superior laryngeal nerve and efferent track reaches larynx through recurrent laryngeal nerve. Total outcome of LAR activation is the closure of laryngeal entry by bilateral vocal cord adduction. In LAR-CIONM, a stimulus is given by an electrode from one side of surface electrode-based ETT and amplitude response of the LAR at the vocal cord is followed on the operation side. Recently, it has been reported that real-time EMG response can be obtained with stimulation probe cables applied to dissectors or energy devices during the dissection through I-IONM.

Optimization of electromyographic endotracheal tube electrode position by UEScope for monitored thyroidectomy

OBJECTIVE

Proper position of an electromyographic (EMG) endotracheal tube within the larynx plays a key role in functional electrophysiologic intraoperative neural monitoring (IONM) in thyroid surgery. The purpose of this study was to determine the feasibility of a portable video-assisted intubation device (UEScope) to verify the optimal placement of an EMG tube.

METHODS

A retrospective study enrolled 40 consecutive patients who underwent monitored thyroidectomies. After positioning the patient for surgery, an anesthesiologist performed tracheal intubation with UEScope and checked the position of the tube at the proper depth without rotation to the vocal cords. The main outcome measured was the proper EMG tube position, free from further adjustment. The secondary outcomes assessed were the percentage of available initial vagal stimulation (V1) signals.

RESULTS

All tracheal intubations were successful at first attempt. Proper EMG tube placement without position adjustment was found in 97.5% of the patients. Tube withdrawal was required in a male patient. All patients obtained detectable V1 signals; the lowest and median V1 amplitude was 485 and 767 μV as a reference value, respectively.

CONCLUSION

The UEScope is a valuable and reliable tool for placing an EMG tube and confirming its position during monitored thyroidectomy. In addition, further tube adjustment might be waived in most cases when the anesthesiologist placed the EMG tube after patient positioning for surgery. Routine use of video-assisted intubation devices is highly recommended.

LEVEL OF EVIDENCE

4.

Investigation of attachment location of adhesive skin electrodes for intraoperative neuromonitoring in thyroid surgery: Preclinical and clinical studies

BACKGROUND

Recently, adhesive skin electrodes have been reported to be useful for recording electromyographic signals from intrinsic laryngeal muscles for intraoperative neuromonitoring in thyroid surgery and have shown good results compared to existing recording methods. In this study, we investigated the optimal attachment location of adhesive skin electrodes for intraoperative neuromonitoring in both porcine models and human cases.

METHODS

Attachment locations were divided vertically into upper, middle, and lower locations and horizontally into medial and lateral locations to determine the optimal location of placing adhesive skin electrodes preclinically in four porcine models. This study included a total of 78 patients who underwent thyroidectomy under intraoperative neuromonitoring with adhesive skin electrodes. Sixteen patients were monitored using both adhesive skin electrodes and an electromyographic endotracheal tube. Two pairs of skin electrodes were attached to the level of the thyroid cartilage lamina. Evoked electromyographic data, including data on mean amplitude and latency, obtained by stimulating the recurrent laryngeal nerve and vagus nerve, were collected.

RESULTS

Lateral attachment of adhesive skin electrodes showed significantly higher evoked amplitudes than medial attachment in both animal models and human patients. In cases where skin electrodes and an electromyographic endotracheal tube were used together, the electromyographic endotracheal tube showed a significantly higher amplitude than skin electrodes, and laterally attached skin electrodes showed a significantly higher amplitude than medially attached skin electrodes.

CONCLUSION

Intraoperative neuromonitoring using adhesive skin electrodes was feasible in both animal models and human patients. We suggest that it would be better to attach adhesive skin electrodes to the lateral side of the thyroid cartilage lamina. Lateral attachment closer to the cricoarytenoid joint may be better for measuring muscle movement around the cricoarytenoid joint.

The use of thyroid cartilage needle electrodes in intraoperative neuromonitoring during thyroidectomy: A case-control study

BACKGROUND

The most commonly used recording-side method in intraoperative neural monitoring (IONM) detects the stimulus with the endotracheal tube surface (ETS) electrodes placed in the endotracheal tube during thyroidectomy. The thyroid cartilage needle (TCN) electrode method is an alternative recording-side system in IONM. This study compared two recording-side techniques in IONM.

METHODS

Data were retrospectively analyzed from 885 patients who underwent thyroidectomy between January 2012 and December 2020, with 110 ETS and 775 TCN electrodes. Patients' demographics, diagnosis, surgery type, and amplitudes of all stimulation steps were compared. Costs per patient were calculated.

RESULTS

No significant differences were found in the demographic data between the two groups. All amplitudes were higher in the IONM system where TCN electrodes were used than that with ETS electrodes (all stimulation steps p < 0.001, except left-V2 p = 0.007). Further, TCN electrodes were 20 times cheaper than the ETS electrodes.

CONCLUSION

TCN electrodes are an inexpensive and efficient alternative to ETS electrodes in IONM.

New Developments in Anterior Laryngeal Recording Technique During Neuromonitored Thyroid and Parathyroid Surgery

A recurrent laryngeal nerve (RLN) injury resulting in vocal fold paralysis and dysphonia remains a major source of morbidity after thyroid and parathyroid surgeries. Intraoperative neural monitoring (IONM) is increasingly accepted as an adjunct to the standard practice of visual RLN identification. Endotracheal tube (ET) surface recording electrode systems are now widely used for IONM; however, the major limitation of the clinical use of ET-based surface electrodes is the need to maintain constant contact between the electrodes and vocal folds during surgery to obtain a high-quality recording. An ET that is malpositioned during intubation or displaced during surgical manipulation can cause a false decrease or loss of electromyography (EMG) signal. Since it may be difficult to distinguish from an EMG change caused by a true RLN injury, a false loss or decrease in EMG signal may contribute to inappropriate surgical decision making. Therefore, researchers have investigated alternative electrode systems that circumvent common causes of poor accuracy in ET-based neuromonitoring. Recent experimental and clinical studies have confirmed the hypothesis that needle or adhesive surface recording electrodes attached to the thyroid cartilage (transcartilage and percutaneous recording) or attached to the overlying neck skin (transcutaneous recording) can provide functionality similar to that of ET-based electrodes, and these recording methods enable access to the EMG response of the vocalis muscle that originates from the inner surface of the thyroid cartilage. Studies also indicate that, during surgical manipulation of the trachea, transcartilage, percutaneous, and transcutaneous anterior laryngeal (AL) recording electrodes could be more stable than ET-based surface electrodes and could be equally accurate in depicting RLN stress during IONM. These findings show that these AL electrodes have potential applications in future designs of recording electrodes and support the use of IONM as a high-quality quantitative tool in thyroid and parathyroid surgery. This article reviews the major recent developments of newly emerging transcartilage, percutaneous, and transcutaneous AL recording techniques used in IONM and evaluates their contribution to improved voice outcomes in modern thyroid surgery.

Informed Consent for Intraoperative Neural Monitoring in Thyroid and Parathyroid Surgery - Consensus Statement of the International Neural Monitoring Study Group

In the past decade, the use of intraoperative neural monitoring (IONM) in thyroid and parathyroid surgery has been widely accepted by surgeons as a useful technology for improving laryngeal nerve identification and voice outcomes, facilitating neurophysiological research, educating and training surgeons, and reducing surgical complications and malpractice litigation. Informing patients about IONM is not only good practice and helpful in promoting the efficient use of IONM resources but is indispensable for effective shared decision making between the patient and surgeon. The International Neural Monitoring Study Group (INMSG) feels complete discussion of IONM in the preoperative planning and patient consent process is important in all patients undergoing thyroid and parathyroid surgery. The purpose of this publication is to evaluate the impact of IONM on the informed consent process before thyroid and parathyroid surgery and to review the current INMSG consensus on evidence-based consent. The objective of this consensus statement, which outlines general and specific considerations as well as recommended criteria for informed consent for the use of IONM, is to assist surgeons and patients in the processes of informed consent and shared decision making before thyroid and parathyroid surgery.

Intraoperative Neuromonitoring Using a Single Transcartilage Needle Electrode During Thyroidectomy

OBJECTIVES

Application of transcartilage needle electrode for intraoperative neuromonitoring (IONM) during thyroidectomy has been considered an alternative method of electromyography (EMG) tube recording. However, needle electrodes must be inserted into both sides of the thyroid cartilage with exposure of the cartilage lamina. We sought to evaluate the feasibility of applying a single ipsilateral transcartilage needle electrode for IONM during unilateral hemithyroidectomy.

METHODS

Thirty-four patients underwent IONM during unilateral thyroidectomy. A dual disposable needle electrode was inserted obliquely into the inferior lower third of the ipsilateral lamina of the thyroid cartilage. Patients were classified as deep (≥ 5 mm) or superficial (< 5 mm) by the depth of the needle electrode inserted into the thyroid cartilage. Without using an EMG tube, IONM was done according to the standardized procedure using a single needle electrode only.

RESULTS

IONM was successful in all nerves at risk. Amplitude of the EMG signal was stable during the surgery, with no cases presenting loss of signal. Amplitude of the signal from vagal and recurrent laryngeal nerve stimulation was significantly lower when needle insertion was superficial (< 5 mm). None of the patients showed postoperative vocal cord paralysis, and complications related to needle electrode were not identified.

CONCLUSION

IONM using a single ipsilateral transcartilage needle electrode during unilateral hemithyroidectomy was feasible with no significant complications. It may serve as an alternative method of IONM with the advantage of minimal exposure of the thyroid cartilage lamina.

LEVEL OF EVIDENCE

4 Laryngoscope, 131:448-452, 2021.

The transcutaneous electromyography recording method for intraoperative neuromonitoring of recurrent laryngeal nerve during minimally invasive parathyroidectomy

Intraoperative neuromonitoring (IONM) facilitates recurrent laryngeal nerve (RLN) protection in thyroid and parathyroid surgeries. This study aimed to investigate a novel transcutaneous electromyography (EMG) recording method for IONM of the RLN during minimally invasive parathyroidectomy (MIP). Twenty patients with primary hyperparathyroidism undergoing MIP were enrolled. Two paired needle electrodes were percutaneously inserted into the bilateral laminas of thyroid cartilage for monitoring the vagus nerve and RLN. A standardized IONM procedure (V₁-R₁-R₂-V₂ signals) was strictly followed, and the RLN was routinely located and mapped. Pre- and postoperative laryngofiberoscopy was performed to confirm vocal cord function. The proposed technique was successfully used in all patients, and typical EMG signals were effectively detected. No significant change in EMG signals before and after tumor resection was noted, and a normal vocal cord movement was ensured in all patients with postoperative laryngofiberoscopy. IONM helped localize the position of the RLN and facilitated the safe resection of the parathyroid tumor during MIP. The novel transcutaneous EMG recording method proposed in this study was feasible, convenient, reliable, and inexpensive.