Intraoperative neuromonitoring versus optical magnification in the prevention of recurrent laryngeal nerve injury in thyroid surgery: a prospective randomized study

Background Although a standardized technique in thyroid surgery, still a certain percentage of both early and late complications is reported and undoubtedly the recurrent laryngeal nerve (RLN) paralysis remains the most fearful. Thus, different technological innovations have been introduced over the last decades with the aim to guarantee major accuracy and decrease the risk of severe complications. Intraoperative neuromonitoring (IONM) and optical magnification (OM) facilitate dissection and increase the surgeon’s precision. The aim of our study is to compare these two techniques in terms of complications rate especially in the incidence of RLN paralysis during thyroid surgery. Materials and Methods In our prospective randomized longitudinal study, from October 2018 to February 2020 total thyroidectomy was performed in a population of 100 consecutive patients that was divided into 2 groups of 50 patients. In the first group (OM - Optical Magnification), only surgical binocular loupes (2.5x-4.5x) were used as an aid in the RLN identification and dissection, while in the second group (IONM – Intraoperative Neuromonitoring) was adopted only the NIM in intermittent modality. The preoperative assessment included either indirect laryngoscopy or fibrolaringoscopy and all the patients had normal vocal cord motility. Written informed consent was obtained and thyroid surgery was performed by the same experienced surgeons of the team. Exclusion criteria were previous thyroid surgery, lobectomy, neck irradiation, concomitant parathyroidectomy, lymph node dissection, minimally invasive procedures such as MIVAT, TOETVA. The follow-up period was 6 months. No cases of mortality were reported in our series. Results The two groups were homogeneous in distribution of age, sex and type of thyroid disease at the admission. No statistically significant differences in the presence of hyperfunction or thyroiditis was shown. In the IONM group, the most relevant data was the presence of 2 cases of bilateral RLN paralysis that needed immediate airway management and intensive care unit transfer of the patients. However, the two transients bilateral RLN paralysis in the IONM Group (4%) versus none in the OM Group (0%) were not statistically significant (p>0.05). Furthermore, statistically relevant data was found regarding the duration of the operation, transient hypocalcemia and the length of hospitalization (p<0.05). The duration of the operation seemed to be significantly shorter in the OM Group (median 80 vs 100 minutes, p<0.05). With regard in the length of hospital staying, there was a statistically significant difference in favor of the OM Group (median 2 vs 4 days, p<0.05). Nevertheless, the OM group reported a 4-fold higher risk developing transient hypocalcemia than the IONM group (OR 3.78, Adj OR 4.11, p=0.01). In addition, the multivariate analysis adjusted by group and gender documented a relevant difference regarding the transient postoperative dysphonia with the males having a 5-fold higher risk developing transient dysphonia than the females (Adj OR 5.19, 95% IC 0.99-27.18, p=0.05). A collateral data of our study was the finding of occult carcinomas in relation to the histological report and cytological examination for each group (90% and 80% of incidence rate in OM and IONM group respectively) and the presence of overall histological carcinomas in the OM group was statistically significant (p<0,05). Finally, no significant difference was found regarding definitive hypoparathyroidism and in the follow-up at six months after surgery all patients were found with preserved vocal cord motility and no cases of hypocalcemia or hypoparathyroidism were detected. Conclusion To our knowledge, this is the first study in the literature that directly compares the use of IONM with OM alone in the prevention of RLN injuries. The risk of recurrent complications in both methods remains comparable and the 2 cases of RLN paralysis in the IONM group of our series are not statistically significant. While the OM is advantageous for the accurate identification, isolation and dissection of the RLN, the IONM allows intraoperative assessment of the nerve integrity and functionality and can add greater confidence to the surgeon, especially if less experienced. In our opinion the two techniques can be considered a valid alternative in thyroid surgery, at least in highly specialized endocrine-surgical centers, and their simultaneous application provides better outcomes. A new multicenter study with a larger cohort of patients and using the IONM in continuous modality could certainly provide us further information.