Introduction

Thyroid and parathyroid surgeries are mostly indicated for benign thyroid goitre, hyperthyroidism, thyroid cancer and hyperparathyroidism. Total or partial thyroidectomy requires accurate identification of the recurrent laryngeal nerve (RLN) and the external branch of the superior laryngeal nerve (EBSLN), as well as meticulous technical skill and attention to detail by the surgeon. However, intraoperative RLN or EBSLN injuries are complications that affect vocal fold functionality and voice quality. Intraoperative RNL injury can lead to transient or permanent vocal cord palsy (VCP), in which postoperative dysphonia, dysphagia, and aspiration symptoms affect the patient’s quality of life1,2.

The incidence of intraoperative nerve palsy is relatively low among the studies and varies according to the study design. The transient RNL palsy incidence varies from 2.3 to 11.9%, and the incidence of permanent RLN palsy varies from 0.6 to 5.1%3,4,5,6. Morton et al.7 reported postoperative rates of EBSLN injury ranging between 0% and 58%.

Intraoperative neural monitoring (IONM) has become a valuable complementary tool for the surgical treatment of thyroid and parathyroid diseases in worldwide surgical services. IONM improves the visual and functional identification of the RLN and EBSLN and reduces and predicts postoperative VCP8,9,10. The high IONM sensitivity and specificity allowed to predict postoperative injuries and reduce the incidence of transient VCP (0.8–2.9%) and permanent VCP (0.2–1.0%)10.

IONM is a valuable complementary tool recommended by international guidelines for all thyroidectomies and parathyroidectomies following planned procedures11,12,13. However, the level of guideline adherence among endocrine surgeons in Spain is unknown.

The aim of this study was to determine the knowledge, experience, patterns of use, and indications for IONM among endocrine surgeons in Spain during thyroidectomies and parathyroidectomies.

Methods

Study design

A cross-sectional study was conducted to describe the endocrine surgery practices regarding using IONM in Spain. In 2021, we sent a web-based survey (Survio s.r.o., Brno, Czech Republic) about endocrine surgery practices to the 719 endocrine surgeons’ members of the Spanish Association of Surgery via email. The questionnaire design and scientific content ad hoc for this study were reviewed and approved by the scientific committee, the coordinator of the endocrine surgery section of the Spanish Association of Surgeons, the study director, and the authors of this manuscript. We assessed the procedure performance with the first 24 completed surveys as a pilot test.

The anonymous survey consisted of 35 questions (six dichotomous, ten Likert scale, three dropdown, nine multiple choice questions, and seven free-text questions) grouped into four main areas: (i) general demographic data of the surgeon, center and surgical unit (12 questions); (ii) preoperative-postoperative laryngoscopy examinations (six questions); (iii) general data of IONM of the EBSLN and RLN usage in thyroid and parathyroid surgery (10 questions); and (iv) indications for IONM of the EBSLN and RLN during thyroid and parathyroid surgeries (seven questions).

Data collection

The following data were collected. In group (i): gender, age, years of professional practice, population served by their centre per 1000 habitants, type of centre, number of beds per centre, availability of endocrine surgery unit, percentage of endocrine surgery activity, annual volume of operated benign and malignant thyroid pathology excluding incidental carcinomas, and annual volume of benign and malignant parathyroid surgical procedures. In group (ii): regularity of preoperative laryngeal exam in thyroid and parathyroid surgery, method used for laryngeal exam, reason(s) of preoperative laryngeal examination only in selected cases, regularity of postoperative laryngoscopy performance, and reason(s) of postoperative laryngeal examination only for selected patients. In group (iii): availability of IONM equipment, monitoring method used, type of IONM used (intermittent, continuous or both), acquaintance with the modality of continuous IONM (c-IONM), frequency of identification of the RLN during thyroidectomy, frequency of identification of the EBSLN during thyroidectomy, frequency of stimulation of the EBSLN, reason(s) to resort to EBSLN monitoring, identification of the cricothyroid muscle twitch during EBSLN stimulation; and usage of EBSLN stimulation as a complementary tool to thyroidectomy in their hospital. And in group (iv): frequency of IONM usage in thyroid surgery, reason(s) to perform IONM during thyroidectomy in selected cases, reason(s) for not performing IONM during thyroidectomy, frequency of IONM usage in parathyroid surgery, situations in which the IONM is performed in parathyroid pathology, reason(s) for not performing IONM during parathyroid surgery, and situations in which the participant considers useful the c-IONM.

Statistical analysis

The sample size of survey respondents was calculated using an estimated 80% IONM availability, 90% confidence interval, 7.5% precision, and an estimated 1% missing data. Thus, the survey research required a minimum of 77 respondents. A descriptive analysis of the quantitative and qualitative variables was performed by Survio s.r.o. and Rey Juan Carlos University Hospital (Madrid, Spain) using the IBM SPSS Statistics statistical method package, version 27.0. For categorical variables, we calculated the percentage of participant’ responses.

Results

The web-based enquiry received a total of 269 visits from 719 endocrine surgeons between May 13, 2021, and December 19, 2021. The surveys were completed in only 38.3% (103/269) of visits, but the first 24 out of 103 completed surveys were discarded because they were used as initial pilot survey test. Finally, 79 surveys of endocrine surgeons geographically distributed throughout the Spanish territory were analysed (Fig. 1).

Figure 1
figure 1

(A) Geographical distribution of enquired endocrine surgeons in Spain; (B) Population attending the enquired endocrine surgeons’ centres. (), number of hospitals.

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Demographic data, centre and surgical unit

Seventy-nine respondents had a median (IQR) age of 52 (15) years and a median (IQR) surgical experience of 13 (15) years. Most developed their activity in public/university hospitals. Fifty-three percent of the respondents were men.

Sixty-eight (86.1%) respondents reported having an endocrine surgery unit in their department, and 11 (13.9%) declared that they did not have a specific unit. More than half of the respondents reported a high rate of endocrine surgical activity; 34 (43%) dedicated more than 75% of their total activity to endocrine surgery, and 12 (15.2%) dedicated full-time. Almost half of the respondents, 37 (46.8%), declared performing more than 50 benign thyroid procedures per year, and three (3.8%) more than 50 benign parathyroid procedures per year (Table 1).

Table 1 General demographic data of the surgeons, centre and surgical units (N = 79).
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Preoperative and postoperative laryngoscopy exams

Only 26 (32%) respondents routinely performed preoperative laryngeal exams in thyroid surgery and 52 (65.8%) only in selected cases, predominantly using outpatient fibreoptic laryngoscopy (94.1%). Preoperative laryngeal exams during parathyroid surgery were performed mostly for selected cases (67.1% of respondents) (Table 2). The main reasons for performing preoperative laryngeal exams in the selected cases were presenting preoperative voice changes (70.6%) or thyroid reoperation (70.6%).

Table 2 Preoperative and postoperative laryngoscopy examinations (n = 79).
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Seventeen (21.5%) respondents routinely performed postoperative laryngeal exams, and 59 (74.7%) only for selected cases, mainly when the patients presented dysphonia 57 (96.6%) responders.

IONM of RLN and EBSLN examinations in thyroid and parathyroid surgeries

Seven (8.9%) of total 79 respondents stated that they did not have IONM equipment in their endocrine unit or department (Table 3). Fifty-four (75%) of the 72 respondents who had IONM equipment stated to perform intermittent IONM (i-IONM). Although most respondents, 65 (90.3%), declared to be familiar with the c-IONM, only seven (9.7%) performed the c-IONM. Another seven (9.7%) respondents declared to be unfamiliar with the c-IONM.

Table 3 IONM of the RLN and EBSLN examinations during thyroid and parathyroid surgeries.
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All the respondents visually identified the RLN during thyroidectomy, and a total of 43 respondents affirmed identifying the EBSLN occasionally [40 (55.6%)] or never [3 (4.2%)]. Only eight (11.1%) respondents affirmed stimulating the EBSLN in all the cases, and 56 (77.8%) specified doing so when they operated a voice professional. However, 27 (37.5%) respondents declared not identifying the cricothyroid muscle twitch during EBSLN stimulation.

Indications for IONM of the EBSLN and RLN during thyroid and parathyroid surgeries

Regarding the performance of IONM in thyroidectomies, 62 (78.5%) out of 79 respondents reported always performing IONM, and 13 (16.5%) reported IONM only in selected cases, mainly in complex cases such as large or endothoracic goitres, reinterventions, and preoperative VCP, 12 (92.3%); in central neck dissection, 11 (84.6%); and in thyroid cancer, 10 (76.9%). However, four (5.1%) respondents affirmed that they did not use IONM equipment. Regarding the utility of IONM in thyroid surgery, they cited the lack of evidence to prevent RLN injuries or added value to their clinical experience in their responses (Table 4).

Table 4 Indications for IONM of the EBSLN and RLN examinations during thyroid surgeries.
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Regarding the performance of IONM in parathyroid surgery, half of the 79 respondents, 39 (49,4%), declared always performing IONM, and 32 (40.5%) only in selected cases, mainly in reintervention [66 (93.0%)], preoperative recurrent laryngeal nerve paralysis [60 (84.5%)], parathyroid cancer [54 (76.1%)], and the coexistence of surgical thyroid pathology [53 (74.6%)]. However, eight (10.1%) respondents affirmed that they did not perform this never. Twenty-one of the 79 respondents answered the question “If you do not perform intraoperative neural monitoring in parathyroid surgery, state the reason”: six (28.6%) responses expressed the non-availability of IONM equipment in their centre, eight (38.1%) the high cost, seven (33.3%) the lack of evidence regarding the prevention of recurrent laryngeal nerve injuries, six (28.6%) the lack of added value to their clinical practice and two (9.5%) the lack of experience or training for its use (Table 5).

Table 5 Indications for IONM of the EBSLN and RLN examinations during parathyroid surgeries.
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Finally, respondents considered the c-IONM useful mainly for reoperative surgery [63 (80.8%)], for central neck dissection [62 (79.5%)], and for preoperative recurrent laryngeal nerve paralysis [59, (75.6%)] (Table 6).

Table 6 Indications for continuous IONM during thyroid and parathyroid surgeries.
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Discussion

To our knowledge, this is the first survey performed in Spain about IONM management during thyroidectomies and parathyroidectomies. Thirty-eight percent of surgeons completed the web-based survey, which aligns with other published IONM usage surveys14,15,16. Our survey outcomes reveal that 86% of enquired endocrine surgeons have an endocrine surgery unit in their department. Half of them dedicate more than 75% of their activity to thyroid and parathyroid surgery, mainly performing benign thyroid surgeries (> 50 cases/year). Among the surveyed surgeons, 91.1% had IONM equipment in their unit. Mostly apply the i-IONM mode. However, the survey also reveals a low adherence to international guidelines and recommendations that should be improved.

Preoperative and postoperative laryngeal examinations during thyroid and parathyroid surgery are mandatory for all patients for functional diagnosis of the RLN. Verification of postoperative glottic examination is the most accurate postoperative RLN outcome measure and serves as a quality control tool for intraoperative electromyography data from intraoperative neural monitoring11,12,13,17,18.

Our study reveals that only 32% of the respondents performed preoperative laryngeal exams before thyroidectomy on all patients, and 65.8% only on selected cases, predominantly in patients presenting voice changes or facing thyroid reoperation. The rates were even lower for parathyroidectomy (19% for all patients, 67.1% for selected patients, and 14% surgeons never performed preoperative laryngeal exams). Most respondents (74.7%) performed postoperative laryngeal exams only on selected cases, mostly those presenting voice changes. These results are below those observed in other surgeon surveys worldwide. In a worldwide survey of 1,094 surgeons, Feng et al.19 showed that 65.6% of non-American surgeons and 80.4% of American surgeons performed preoperative laryngeal exams on all patients, and 56.1% of non-American surgeons and 47.9% of American surgeons performed postoperative laryngeal exams on all patients. In another worldwide survey of 170 endocrine surgeons, Barczyński et al.14 reported that 61% of surgeons performed systematic preoperative laryngeal examinations and 49.1% of surgeons performed postoperative laryngeal examinations on all patients. Varadharajan et al.20 reported the results of a survey of 142 surgeons in the UK, which revealed that 58% of endocrine surgeons and 95% of ear, nose and throat (ENT) surgeons performed preoperative laryngeal assessments on all patients.

IONM is a valuable surgical adjunct tool recommended by international guidelines11,12,13 and expert consensus18,21 to apply in all thyroidectomies and parathyroidectomies following planned procedures because it allows better identification of RLN and ESBLN, aid for further dissection, identifies an impending neural injury facilitating the opportunity to modify surgical manoeuvres including staging the procedure in cases of planned total thyroidectomies to avoid a contralateral loss of signal, and gives prognostication of postoperative neural function.

To our knowledge, the IONM was introduced in Spain in 2007 by a few surgeons and its use was gradually accepted by colleagues and hospitals. Our study shows that 91.1% of the respondents have IONM equipment in their unit. Of them, 75% predominantly use the i-IONM model and 9.7% use the c-IONM model; however, the standardized use of these models has not yet been fully established.

All the respondents affirmed identifying RLN; 78.5% reported using IONM always for thyroidectomies, and 16.5% only for selected cases. These data are comparable to those reported by Feng et al.19 (65% of surgeons stated always using IONM and 18% in selected cases) and Ritter et al.15 (72% of surgeons stated always using IONM and 21% in selected cases); and contrast to those reported by Dionigi et al.16 in Italy (28% of surgeons stated always using IONM and 62% in selected cases). This is likely due to medico-legal concerns and increased confidence in the accuracy and benefits of IONM, while in Italy, the reimbursement does not encourage the use of new approaches in thyroid surgery16.

In our study, 60% of the respondents declared having difficulties identifying the EBSLN. Only 11% of the respondents stimulated EBSLN in all the cases, and 9.7% in selected cases, predominantly on voice professionals. These outcomes are low accordingly to those reported by Barczyński et al.14 (16% of surgeons declared always using IONM for EBSLN identification and monitoring, and 50% reported selective use) in an international survey on the identification and neural monitoring of the EBSLN.

Regarding parathyroidectomy, 49% of the respondents always used IONM, and 40% only for selected patients. These results were comparable to those obtained by Ho et al.22, in which 50% of ENT surgeons used IONM during parathyroidectomy.

In our study, the most frequent reasons for not using IONM were the unavailability of the device, the high cost, the absence of added value to clinical practice, the lack of evidence in preventing recurrent laryngeal nerve injury, the prolonged operative time, and the lack of experience or training.

We found that almost 10% of the enquired endocrine surgeons did not have an IONM device available in their unit/department, likely because of the high cost, among other reasons. We believe that this high rate can hamper the progress of setting standardized protocols.

The cost-effectiveness of IONM implementation concerns public and private health institutions. It is difficult to evaluate this topic considering different scenarios that may be different among countries. Wang et al.23 analysed the cost-effectiveness of neural monitoring in recurrent laryngeal nerve recovery course of thyroid surgery using simulation economic modelling. According to his model, IONM seems to be a clinically cost-effective intervention only for patients who would experience permanent VCP. However, this model did not consider the cost-effective impact of the quality-of-life factors of patients with VCP, such as voice therapy, laryngoscopy exams, and litigation, among many others, nor the effect of IONM on surgeon confidence during the surgical performance, and the possible postoperative litigation24,25.

In general, many experienced surgeons believe that IONM will not add value to their clinical practice. IONM allows to identify and map the RLN 26, although the superiority of IONM over visual nerve identification for reducing transient and permanent VCP is controversial3,5,6,27. The benefits of i-IONM and c-IONM compared to visual identification were reviewed by Scheneider et al.10 elsewhere. Briefly, i-IONM identifies the loss of signal types 1 and 2, prevents bilateral VCP on intact RLN anatomy, and leads to intraoperative recovery of RLN function. The c-IONM has the same functions, and in addition, it identifies intraoperative recovery of RLN function, minimizes traction-induced RLN injury and monitors the real-time function of the RLN10.

The sensitivity for predicting postoperative vocal cord function of i-IONM reaches 91.3%, and the specificity reaches 99.5%, leading to a positive predicted value of 37.8–80.5% and a negative predicted value of 97.3–99.8%10. In this context, the sensitivity of c-IONM reaches 100%, and the specificity reaches 99.7, leading to a positive predicted value of 47.6–88.2% and a negative predicted value of 99.8–100%. Thus, IONM may considerably reduce transient and permanent palsy10.

Some experienced surgeons believe that using IONM can prolong their surgery time, face possible technical problems related to the new equipment, and require prolonged learning time. Since the introduction of IONM technology, several studies have tried to overcome these myths and barriers. Certainly, IONM does not substitute experienced surgeons but provides all the advantages mentioned above and enhances confidence in trainees. It is not clear whether IONM reduces the length of surgery since it could depend not only on the surgeon’s skills but also on the type of operation, pathology, or patient characteristics28,29.

The most common technical problems for trainees are positioning the endotracheal tube, placing the nerve electrode and interpreting the intraoperative loss of signal when learning the technique28,29,30. Some studies agree that IONM training gradually improves the success of identifying the RLN, while technical problems decrease28,29,30. The learning curve for a technically acceptable level of performance may vary from 50 to 300 thyroid procedures30,31. Currently, efforts are aimed at developing IONM training programs32.

This study provides, for the first time, an approach to how endocrine surgeons manage the prevention of laryngeal nerve injury during thyroid and parathyroid surgery in Spain. The sample of respondents was higher than the estimated 90% confidence level and 7.5% precision and was widely distributed in the territory. Our findings show shortcomings that will require additional investigation to reach better outcomes for patient quality of life: (i) low adherence to guidelines in laryngoscopy procedures, (ii) poor management of c-IONM, (iii) lack of training procedures, and (iv) poor adherence to standardized protocols.

The limitations of this study are mainly the implicit bias associated with an anonymous webmail survey, the surgeon’s greater willingness to participate if they had previous experience using IONM, and the lack of questions providing more accurate information about the preoperative and postoperative protocols used. In addition, the responses evidenced in this survey reflect a moment in time of a constantly evolving tool.

Future research will aim to investigate more precisely the procedures used, the standardization of protocols and the promotion of training.

In conclusion, the usage of IONM for neck endocrine surgery is a reality in Spain, especially the intermittent mode. Its use is superior in thyroid than parathyroid surgery. Its standardized use has not yet been fully established, and the routine adherence to standardized guidelines should increase. Continuing to promote training from scientific societies, especially among young surgeons, could improve its dissemination and efficiency. This survey is a starting point for continuous improvement in thyroid and parathyroid surgery practices among Spanish endocrine surgeons.