Utility of continuous vagal neuromonitoring in thyroid and parathyroid gland surgery: a retrospective study of 500 cases

Introduction

Background

Recurrent laryngeal nerve (RLN) injury leading to vocal cord paralysis (VCP) is a serious complication of thyroid and parathyroid surgery, with highly variable prevalences in literature. Unilateral injury induces voice changes such as hoarseness and dysphonia, while bilateral injury can cause airway obstruction with an urgent possible need for tracheostomy (1). The different anatomical variants of the nerve (described in up to 23% of cases) make it especially vulnerable during surgery, and given that 5–7% of the world’s population suffers from some form of thyroid pathology, it is imperative to improve its protection. Due to the discrepancy between subjective voice assessment and laryngoscopic findings, the International Neural Monitoring Study Group recognizes the need to combine preoperative laryngeal data, macroscopic surgical data, and intraoperative electrophysiological data to optimize surgical safety of the recurrent nerve (2).

Rationale and knowledge gap

Neuromonitoring is used as a complement to visual identification of the nerve, but in terms of reducing its injury, the different published studies and meta-analyses show contradictory results (3-7). The intermittent technique is the most used option today, even though it does not provide data on nerve functionality or integrity between stimulations. The continuous variant does provide this knowledge in real time, theoretically decreasing the VCP by identifying neural damage as it develops. This technique helps predict the postoperative nerve functional outcome, making it possible to reverse the surgical maneuvers involved in the event of an abnormal amplitude or latency variation and thus verifying the functional recovery of the nerve in the intraoperative electroneurogram (ENG). Thanks to its high negative predictive value (NPV), an intact monitoring signal at the end of surgery is associated with adequate vocal cord (VC) functionality, but in the absence of complete intraoperative recovery, the technique allows the surgical plan to be modified, relegating the contralateral lobectomy to a second procedure once the affected VC has been able to recover (8-10).

Currently, more than 90% of surgeons routinely use nerve monitoring (2). Some European and American associations recommend it for all thyroid and parathyroid interventions, while others only use it in complex procedures such as reinterventions, neoplasia, giant goiters, or intrathoracic goiters (2,11). However, the adoption of the continuous variant has been modest, and there is still a significant lack of standardization of stimulation techniques and phases during surgery, so it is not considered the gold standard in preventing RLN injury (3,4,12,13).

Objectives

This study aimed to quantify the usefulness of continuous intraoperative neuromonitoring (CIONM) in thyroid and parathyroid surgery to evaluate the convenience of its clinical application and its continuity in our hospital department, since it represents a significant increase in surgical time and economic cost. If the results of this study demonstrate the superiority of the technique over visual identification, they could serve as a basis for its establishment in other centers in our field.

Given the hypothesis that CIONM during thyroid and parathyroid gland surgery complements visual identification in preventing RLN injury, as well as decreasing its severity by detecting imminent neural damage, this study aims to: quantify whether the use of the technique predicts or reduces the incidence or severity of RLN injury, identify independent risk factors that would especially benefit from its use, and determine the diagnostic predictive values of the technique. We present this article in accordance with the STROBE reporting checklist (available at https://gs.amegroups.com/article/view/10.21037/gs-2025-53/rc).

Methods

A retrospective, analytical, observational cohort study was conducted, in which the sample studied and compared with historical controls (individuals whose data were collected retrospectively from prior studies or medical records, representing a similar population under comparable conditions) were patients who underwent thyroid and parathyroid surgery and who underwent CIONM in the General Surgery Department of the Alicante University General Hospital Dr. Balmis. This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by the Ethics Committee for Medicine Research (CEIm) of the Alicante Department of Health - General Hospital (Ref. CEIm: PI2021-166) and by the Ethics and Integrity Committee in Research (COIR) of the Miguel Hernández University of Elche, Spain (Ref. COIR: ADH.SPU.APA.ABC.23). Individual consent for this retrospective analysis was waived.

The inclusion criteria were patients with a surgical indication for hemithyroidectomy, total thyroidectomy (TT), with or without lymph node dissection, selective or subtotal parathyroidectomy, and reinterventions. Subjects with neurological pathology affecting cord motility and under 18 years of age were excluded. With a recurrence rate estimated according to similar studies in recent literature of 8.8% for temporary VCP and 1.9% for permanent VCP (14), the sample size considered by the EPIDAT 4.2 program was 166 individuals. This estimate is much lower than the one reviewed in the present study (500 patients). Of them, 477 underwent the continuous technique, one underwent intermittent technique and 22 underwent visual identification alone (in some cases, due to the unavailability of neurophysiology equipment, the monitoring technique was omitted for the intervention of certain parathyroid adenomas). For the analysis of nerves at risk, the distribution included 334 patients at risk of unilateral RLN injury due to hemithyroidectomy, parathyroidectomy, or reinterventions, and 164 patients at risk of bilateral RLN injury due to TT or subtotal parathyroidectomy. Therefore, the total number of nerves at risk was 662. Demographic, explanatory (patient comorbidities, peri- and postoperative information and neuromonitoring information), and outcome (nerve paralysis) variables were collected.

Reinforced flexible metallic endotracheal tubes were used (Lo-Contour oral/nasal tracheal tube, cuffed, reinforced, Murphy eye, Shiley™, COVIDIEN™), along with the Xltek Protektor 16 IOM System. The procedures adhered to international recommendations, utilizing certified stimulation and recording electrodes. For intermittent stimulation, a straight monopolar stimulation probe was employed (Friendship Medical Electronics^®). For repetitive nerve stimulation to monitor neuromuscular blockade, either needle or subdermal adhesive electrodes (Technomed^®) were placed on the median nerve, with motor recordings taken from the abductor pollicis brevis, or the posterior tibial nerve was stimulated at the foot, with recordings taken from the abductor hallucis. For continuous vagal nerve stimulation, the Delta electrode (INOMED^®) was used, and for recording motor responses from the RLN, the adhesive endotracheal tube electrode (INOMED^®) was utilized. At the beginning of the procedure, the vagus nerve was exposed for placement of the continuous stimulation electrode. Concomitant with glandular dissection, the mapping technique was used to locate the nerve, facilitating thyroid release. Any severe combined event (CE) (>50% decrease in amplitude together with >10% increase in latency) was established as an alarm signal. After completion of the surgical maneuvers, the integrity of the entire central circuit was tested, and if the ENG data suggested nerve paralysis, the procedure was restricted to unilateral lobectomy, considering, in necessary cases, a staged thyroidectomy when the affected VC recovered. Postoperative laryngeal morbidity was evaluated in the immediate postoperative period, at one month and at three or six months after the intervention. Based on the percentage decrease in response relative to the initial baseline control, mild to moderate conduction blocks were defined as those with amplitude reductions greater than 25–50% but not exceeding 80%. In contrast, severe conduction blocks were characterized by amplitude reductions exceeding 80% of the baseline value. In accordance with international standards, loss of signal (LOS) was defined as any ENG change with responses of very low amplitude (<100 µV).

Preoperative laryngeal examination was not routinely performed, only in cases of recent dysphonia perceived by the patient. If clinical suspicion of paresis or VCP arose during the medical evaluation, or if ENG at the end of the procedure indicated early VC dysfunction, a flexible laryngoscopic examination was requested before hospital discharge, performed by the Pulmonology Department of our hospital. Patients were reviewed at the end of the first postoperative month and after completing 3–6 months of specialized speech therapy in cases of paralysis. On each occasion, an individualized anamnesis was conducted to document perceived voice changes during this period, and progress was objectively assessed through successive laryngoscopies.

Statistical analysis

A descriptive analysis of the population was conducted in relation to clinicopathological variables. Quantitative parameters were expressed as mean ± standard deviation (after verifying their normal distribution using the Kolmogorov-Smirnov test) or as median ± range in cases where they were non-parametric. Qualitative variables were expressed as absolute frequencies, relative frequencies, and percentages.

The relationship between the presence of RLN palsy (whether temporary or permanent) and the use of intraoperative neural monitoring (IONM) was analyzed using contingency tables. The Chi-squared test was applied for qualitative variables with two categories, and the Student’s t-test or its non-parametric equivalent (Mann-Whitney U test) was used for quantitative variables with two categories. For categorical variables with more than three categories, the analysis of variance (ANOVA) test or its non-parametric equivalent, the Kruskal-Wallis test, was employed. Univariate analysis from each parameter was done first, and variables with P<0.05 and those that were significant in many other investigations were selected for multivariate analysis, to estimate the magnitudes of the associations between the presence of RLN palsy and the use of IONM, considering potential confounding or effect-modifying variables. Relative risks (RRs) were calculated, along with their 95% confidence intervals (CIs). The statistical analysis was conducted using the IBM SPSS^® statistical software version 29.0. The methodologies applied to ensure the robustness of our analyses included, first, Type I error control in multivariate analysis through an omnibus test (model Chi-squared), where we reported a P<0.05, confirming that the logistic model significantly explains the dependent variable. To prevent Type I error inflation in subsequent analyses, we applied Yates’ correction for continuity in 2×2 contingency tables with low frequencies, thereby preserving precision and reinforcing the validity of our findings.

Results

The mean age of the studied patients was 57.52±14.36 years. Of the patients, 73.8% were women, while 26.2% were men. Of 477 patients with CIONM and 662 nerves at risk, 44 mild-moderate conduction blocks and 48 severe blocks or LOS were observed, which translated postoperatively into 27.6% of nerves at risk with transient VCP and 1.1% with permanent paralysis (Table 1). A total of 2.9% of the injured nerves (three subjects) suffered bilateral paralysis with an urgent need for tracheostomy (Table 2). The percentage of paresis was higher in patients with mild-moderate blocks (40.9% vs. 29.6% of LOS; P<0.001); the opposite was observed for severe blocks with 58.3% of paralysis. A total of 72.2% of temporary lesions were resolved in the first six months of follow-up, the majority being nerve paresis, in contrast to 85.7% of paralysis cases that took more than six months to normalize the voice (P=0.007).

The surgical plan was modified in 20 patients (Table 3) (31.3% of the total number of severe blockages and 9.1% of mild-moderate blockages; P<0.001). In most cases, hemithyroidectomy was performed without planning totalization, either due to a benign diagnosis or papillary carcinoma with free margins. In six cases, subtotal thyroidectomy was performed due to a high suspicion of malignancy, and in two other cases, the scheduled contralateral lymphadenectomy was suspended, and the pending procedure was performed between four and eight months after the nerve injury. Permanent paralysis showed ENG incidents in 72.2% of cases, and the decision was made to change the initial strategy in 21.4% of them; temporary lesions had an incidence of 82.1%, but only 15.8% of patients changed their surgical plan (P<0.001).

Patients with a diagnosis of neoplasia had a total of 10.8% of mild-moderate blockages and 14.9% of severe blockages (Table 4; P=0.03). Similarly, 1.7% of patients with thyroid cancer suffered from permanent VCP, while this percentage was 30.5% in temporary paralysis (Table 5; P=0.004). Likewise, 12.6% of patients with intrathoracic thyroid extension had mild-moderate blockages and 16.5% had severe blockages (P=0.003), thus being 1.9% of permanent paralysis and 34.3% of temporary paralysis (P<0.001). Of 29 patients with lymphadenectomy performed, 58.6% had some type of conduction block (P<0.001). Permanent lesions were present in 8.3% of lymphadenectomies and temporary lesions in 41.7% of them (P<0.001).

Among mild-moderate blockades, TT predominated (43.2%), while in severe blockades, hemithyroidectomy was the most frequent intervention (39.6%; P<0.001). However, thyroidectomy was the most frequent intervention for permanent paralysis (35.8%) and hemithyroidectomy for temporary paralysis (40.3%; P<0.001). A total of 58.3% of severe blockades and 78.5% of permanent paralysis exceeded two hours of surgical time, while in temporary paralysis, there was a more balanced relationship between the first and second hour (P<0.001). The traction mechanism was the most frequent among severe (47.9%) and mild-moderate (68.2%) blocks, as well as in temporary paralysis (46.4% of the total), but was the second most frequent in permanent blocks in 28.6% of cases (P<0.001). The nerve was macroscopically intact in 83.3% of severe blocks and in 95.5% of mild-moderate blocks (P<0.001), although in the permanent paralysis group, it was only seen anatomically intact in 42.8% of patients (in contrast to 96.8% of anatomical integrity in temporary injuries; P<0.001).

Multivariate logistic regression (Table 6) showed that the female gender (adjusted RR 0.3; 95% CI: 0.11–0.85) and traction or heat as a prevention factor for VCP compared to direct nerve shear (adjusted RR 0.12; 95% CI: 0.03–0.51) were protective factors against RLN injury. On the other hand, surgical lymphadenectomy and intervention duration greater than 120 minutes were independent risk factors for RLN injury (adjusted RR 5.15; 95% CI: 1.62–16.33 and adjusted RR 4.88; 95% CI: 1.49–15.99, respectively).

After performing the analysis of the diagnostic predictive capacity of CIONM for temporary and permanent VCP (Tables 7,8), continuous neuromonitoring in our sample predicted temporary paralysis with a sensitivity of 72% and a specificity of 97%, with a positive predictive value (PPV) of 86% and an NPV of 93%, with an overall diagnostic accuracy of 91.8%. Regarding permanent injury, we obtained a sensitivity of 75%, specificity of 97%, PPV of 21%, and NPV of 99%, with a diagnostic accuracy of 96.8% (Table 9). To calculate predictive values, we eliminated ten cases of permanent VCP by planned RLN section due to tumor infiltration, so they were not considered complications of the study. We also eliminated two patients with baseline ENG abnormalities who continued showing them at the end of the intervention and nerve monitoring, so the total number of patients used to calculate these predictive values was 465 instead of 477.

Discussion

Updated studies on RLN IONM published in high-impact journals show mean VCP rates of 7.95% (range, 2.3–20% between them), stratifying into temporary VCP (7.17%; range, 2.7–17.8%) and permanent VCP (1.39%; range, 0.03–3%). Cozzi et al. (15), Davey et al. (4), and Barczyński et al. (16) found no advantage of neuromonitoring over visual inspection, while Wong, Malik, and Schneider et al. (4,14) argue that it independently contributed to the prevention of both temporary and permanent postoperative VCP in a broad spectrum of thyroid pathologies and interventions.

However, there are arguments in favor of CIONM contributing to a shorter recovery time from injury and that the technique allows for intraoperative decisions and modifications that are beneficial to the patient. In fact, according to the theories developed in the middle of the last century by Seddon and Sunderland (17,18), the degree of neurological damage will depend on the damaged nervous tissue and the duration of the causal agent itself. Since the continuous technique is intended to reduce the duration of the iatrogenic maneuver by making it possible to minimize axonal damage, it is likely that recovery time from an injury will be shortened (even thermal lesions could benefit from the technique by reducing the prolongation of axonal damage if the use of the responsible energy instrument is stopped) (19). These findings show that it is a mistake to rely solely on the macroscopic integrity of the nerve (referring to the absence of visible lesions with preserved anatomical continuity of the RLN) to predict its postoperative functionality, demonstrating the importance of CIONM compared to intermittent or even exclusive visual inspection, which is still practiced in many hospitals in our country (however, without being able to conclude in this study its superiority compared to the intermittent mode or visual inspection). CIONM involves small intraoperative changes that theoretically decrease the number of incidents, enabling lower rates of LOS and VCP. Sinclair et al. (20) explain in their work that surgeons significantly modified the thyroidectomy technique used, thanks to the continuous feedback obtained from the CIONM. The study by Pei et al. (21) shows that staged thyroidectomy can be effective in preventing bilateral RLN lesions, especially for patients who already have unilateral VC paresis. In our sample, CIONM contributed to the identification of CE, being able to reverse the causal maneuvers and establish neuroprotection if necessary. Nevertheless, CIONM was not 100% effective, since 4 patients remained on VCP after 12 months of follow-up. They included a patient with a giant intrathoracic goiter, two total thyroidectomies with lymphadenectomy for neoplasia, and an inadvertent nerve transection during a hemithyroidectomy for multinodular goiter. Possible reasons for the greater severity of their injuries included technical factors, such as the fact that immediate incision injury is difficult to prevent with CIONM, and anatomical or procedure-related factors, such as distortion of structures adjacent to the giant goiters, nerve elongation during intrathoracic component removal, and prolonged surgical time due to the greater complexity of the procedure.

According to the published works, a subgroup of patients with a high risk of RLN injury during surgery can be categorized (22-24), such as thyroidectomies for cancer, especially those associated with cervical lymphadenectomies (5.8% of our patients). Consistent with literature, our rate of permanent VCP was low (1%), with 64.3% of these occurring in the context of malignant tumor infiltration, where the rate of permanent VCP increased to 1.8% (P<0.05). Other high-risk parameters carried a permanent VCP higher than the general average: 1.8% for intrathoracic multinodular goiter and 1.5% for lymphadenectomy (P<0.001), respectively. Furthermore, given the existence of normal laryngoscopies in patients with tumor infiltration of the RLN, the use of IONM may be beneficial in all interventions and not only in those with evident preoperative risk, since it would reduce the habituation of the technique and also the injury does not always occur when deliberately risky maneuvers are performed.

Traction and compression cause 80% of RLN lesions and are probably the most preventable using CIONM due to their gradual development and reversibility. From the work of Dionigi et al. (5) with 281 intraoperative LOS, it was reported that only 14% were visually evident during the intervention, which emphasizes the importance of the information provided by the IONM and the lack of sensitivity of the visual identification of the RLN. Regarding the appearance of the RLN once injured, it was shown macroscopically intact in 83.3% of our severe blocks and 95.5% of mild-moderate ones (P<0.001).

It has been described that 73% of severe CEs are usually resolved if the causal maneuvers are stopped, while in the case of LOS, this only occurs in 17% of cases (25). When the scheduled TT is suspended due to a unilateral VCP, it is crucial to evaluate the need for a second surgical stage, considering in a multidisciplinary manner the pathology and the factors related to the patient to propose alternative treatment modalities (2). It is likely that thanks to IONM, a large part of the temporary injuries that we reported in our sample were less serious than expected as a result of modifying the causative maneuver and establishing intrafield preventive measures, as detailed in the works of Phelan et al. and Schneider et al. (8,25).

Multivariate logistic regression analysis

Schneider et al. showed that age over 60 years [odds ratio (OR) 3.24] was independently associated with temporary paralysis, whereas reintervention (OR 2.60) and male sex (OR 2.57) were independent risk factors for permanent injury (14). Leonard-Murali also reported that increasing age (OR 1.07), malignant pathology (OR 1.22), and central lymphadenectomy (OR 1.29) were associated with nerve injury, whereas the use of IONM (OR 0.83) was a protective factor against it (26). In our case, female sex was a preventive factor for VCP with an adjusted RR of 0.3 (95% CI: 0.11–0.85). Although the mechanism of injury does not prevent it, the adjusted RR shows that the incidence of VCP was lower when the injury was caused by traction or heat compared with other more severe injuries such as direct nerve transection or cutting (adjusted RR 0.12; 95% CI: 0.03–0.51). On the other hand, surgical lymphadenectomy and intervention duration greater than 120 minutes were independent risk factors for the development of VCP, with adjusted RR 5.15 (95% CI: 1.62–16.33) and adjusted RR 4.88 (1.49–15.99), respectively.

The clinical utility of IONM is expressed in terms of its predictive values

Multiple previous studies agree that the NPV and specificity are usually high (over 85%), while the PPV and sensitivity are lower (under 75%) (2,27). Our results are comparable to those obtained in centers experienced in thyroid surgery with IONM, making it clear that severe conduction blocks are more aggressive and have a higher risk of developing permanent VCP (16.6% vs. 4.8% in mild-moderate blocks). Our high NPV reflects the low probability that a normal ENG at the end of surgery results, contradictorily, in postoperative VCP, increasing the safety of patient extubation.

Our data are similar to the different works reviewed (22,24,28,29). We could attribute some false negative values to poor intraoperative records and false positives to equipment-related problems, especially endotracheal tube displacement (11). For all these reasons, laryngoscopic confirmation of changes in cord mobility is still required to validate the results of CIONM.

In general, the technique exhibits statistical excellence, and we recommend its incorporation for the intraoperative prediction of VC injury, since in our country, CIONM is only used in 9.7% of hospitals, being the intermittent mode the most used globally (30,31). Still, regarding the possible clinical implications of these predictive values, due to the low PPV of the method, only one-third of patients would benefit from avoiding the risk of bilateral VCP. In contrast, the remaining two-thirds could have preserved cord mobility after surgery, subsequently requiring unnecessary interventions to complete the contralateral resection. Many articles with large patient cohorts show, as our work does, that CIONM does not improve the rates of permanent PCV, and indeed, the sensitivity of the technique in our case is not greater than that published in the literature. Therefore, we consider it important to emphasize that surgeons cannot rely entirely on this technology, even in the continuous variant, since knowledge of cervical anatomy and surgical skills are fundamental for endocrine surgery today.

Our study showed some limitations, such as the lack of routine monitoring of the external branch of the superior laryngeal nerve, since if its identification were protocolized, it could contribute to improving the quality of life and subjective satisfaction of patients. If there is no ENG signal after the first thyroid lobectomy, the resection of the contralateral lobe should be performed in a second stage; however, many patients who do not need a two-stage surgery could undergo a reintervention that would delay the resection of aggressive malignant tumors. On the other hand, this is a single-center study, and due to the low incidence of RLN injury in thyroid surgery, studies with a larger sample size are required to prove or refute the usefulness of the technique, with large-scale multicenter collaborations or meta-analysis methodology likely to represent the most fruitful means to achieve a consensus on this issue.

Conclusions

CIONM contributes to reducing the incidence and severity of RLN injury by facilitating its identification and enabling the application of intraoperative corrective measures, specifically benefiting in more complex surgical scenarios. In addition, the high specificity of the continuous ENG is very useful in the prognostic assessment of VCP based on the severity of the conduction block, with a high NPV that guarantees the absence of RLN paralysis when the final ENG is normal and allows the immediate start of rehabilitative phoniatric treatment on which the prognosis of nerve recovery depends if not normal. However, standardized guidelines and a meta-analysis methodology are needed to prove or refute the usefulness of the technique and demonstrate its economic profitability.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://gs.amegroups.com/article/view/10.21037/gs-2025-53/rc

Data Sharing Statement: Available at https://gs.amegroups.com/article/view/10.21037/gs-2025-53/dss

Peer Review File: Available at https://gs.amegroups.com/article/view/10.21037/gs-2025-53/prf

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://gs.amegroups.com/article/view/10.21037/gs-2025-53/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are 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. This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by the Ethics Committee for Medicine Research (CEIm) of the Alicante Department of Health - General Hospital (Ref. CEIm: PI2021-166) and by the Ethics and Integrity Committee in Research (COIR) of the Miguel Hernández University of Elche, Spain (Ref. COIR: ADH.SPU.APA.ABC.23). Individual consent for this retrospective analysis was waived.

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/.

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