Introduction
Recurrent laryngeal nerve (RLN) injury remains one of the most feared complications in thyroid surgery, often resulting in hoarseness, aspiration, and even airway compromise. Intraoperative neuromonitoring (IONM) was initially developed to augment the visual identification of the nerve and minimize injury risk. It has since evolved into a standard adjunct tool, especially for high-risk or reoperative cases.
Traditional IONM involves the use of electromyography (EMG) endotracheal tubes with surface electrodes to detect RLN activity. However, clinical practice has revealed limitations including signal variability, loss of contact due to displacement, and technical artifacts caused by patient anatomy or surgical manipulation. As such, there has been a growing interest in alternative sensor modalities such as accelerometers, piezoelectric devices, needle electrodes, and skin surface electrodes. These modalities aim to overcome the technical limitations of EMG-based systems and expand the applicability of neuromonitoring to more diverse surgical contexts.
Recent advancements in sensor technology and neuromonitoring protocols have further optimized these approaches, offering higher signal fidelity and procedural adaptability [1-5]. This review provides a comprehensive evaluation of contemporary IONM modalities with a focus on technical principles, clinical outcomes, limitations, and future directions.
Electromyography Tube-Based Intraoperative Neuromonitoring
The use of an EMG endotracheal tube remains the most conventional and widely utilized technique for RLN monitoring during thyroid surgery. These tubes are embedded with electrodes that detect the electrical activity of the vocalis muscle, providing immediate feedback during dissection.
Its popularity stems from the real-time nature of the feedback and the ability to correlate anatomical and functional data. However, signal instability is a frequent limitation, often due to displacement during surgical retraction, poor contact with the vocal cords, or changes in neck positioning. Signal degradation can also occur due to laryngeal edema, secretions, or interference from electrosurgical instruments.
Studies have reported loss of signal (LOS) rates ranging between 5% and 15%, necessitating a stepwise troubleshooting algorithm to differentiate technical failures from true nerve injury. This includes checking electrode placement, reconnecting cables, and stimulating the vagus nerve bilaterally to assess global nerve integrity [6-8] (Table 1).
To address these limitations, recent innovations have focused on improving tube design with better electrode adhesion, adjustable depth markers, and integrated monitoring circuits. However, despite these efforts, patient-specific anatomical variation remains a challenge, prompting the exploration of alternative sensor modalities [9].
Supplementary Monitoring with Pressure and Accelerometer Sensors
Emerging neuromonitoring technologies have sought to address the limitations of EMG tubes by introducing sensor-based alternatives that offer higher signal reliability and broader anatomical flexibility. Among these, piezoelectric pressure sensors and accelerometer-based devices have gained traction in both experimental and clinical settings.
Piezoelectric pressure sensors convert the mechanical vibrations of vocal fold movement into electrical signals. These sensors are typically placed externally on the neck or integrated into flexible interfaces, reducing dependency on vocal fold contact and thereby minimizing signal loss due to tube misplacement or patient movement. In a multicenter study involving over 200 patients, pressure sensors demonstrated a comparable detection rate to EMG tubes, with fewer LOS events and improved response stability [10].
Accelerometers operate by detecting multidimensional laryngeal motion through vibration analysis. They can be applied externally to the skin or embedded into surgical retractors or collar-type devices. Unlike EMG-based monitoring, these systems are not affected by airway manipulation or mucosal edema. Accelerometer technology has been particularly advantageous in robotic or transoral thyroidectomy, where tube-based systems are often impractical or incompatible [10,11].
Despite their promise, further validation is re-quired in large-scale, randomized trials to confirm their sensitivity, specificity, and real-time response characteristics in a variety of surgical scenarios.
Emerging Alternatives: Needle and Surface Electrodes
To overcome the anatomical limitations and signal instability of EMG tubes, alternative electrode placements such as percutaneous needle and surface electrodes have been investigated. Needle electrodes inserted into the thyroid cartilage or cricothyroid membrane provide direct access to the vocalis muscle without requiring endotracheal tube adjustments. They produce high-fidelity signals with consistent amplitude and latency, even during prolonged surgeries or extensive dissection. [5] (Figure 1).
Surface electrodes, meanwhile, are non-invasive and adhere externally to the skin overlying the thyroid cartilage. Although they often produce lower amplitude signals than internal electrodes, advances in electrode design and adhesive technology have significantly improved their reliability and signal-tonoise ratio. Transcartilage electrodes, fixed to the external thyroid cartilage, offer a middle ground, delivering EMG-quality signals without mucosal contact, particularly useful in minimally invasive and reoperations (Figure 2).
One notable advantage of these alternatives is cost-effectiveness. Needle and surface electrodes are often reusable and less expensive than EMG tubes, expanding access to IONM in resource-limited settings or procedures requiring tracheostomy. However, anatomical factors such as cartilage ossification in elderly patients or thick neck tissue may affect placement and performance.
Overall, these emerging technologies are redefining how neuromonitoring can be integrated into personalized thyroid surgical strategies and may help standardize IONM use across broader clinical contexts.
Conclusion
The landscape of IONM in thyroid surgery is undergoing significant transformation. While EMG tubebased monitoring remains the mainstay, the emergence of advanced sensor technologies and external electrode alternatives marks a critical shift toward more reliable, flexible, and cost-effective methods for RLN protection.
Piezoelectric sensors and accelerometers reduce reliance on vocal fold contact, offering improved signal stability and suitability for minimally invasive or ro-botic surgeries. Meanwhile, needle and surface electrodes provide adaptable, economically favorable solutions, particularly in complex or resource-constrained environments.
To ensure widespread adoption, future directions should prioritize multicenter randomized trials, standardization of protocols, and integration with surgical navigation and robotic systems. These developments hold promise for enhancing patient safety, surgical efficiency, and overall outcomes in thyroidectomy.
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