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A total of 1000 RLNs from 574 patients were assessed in this review.
Nearly one-quarter (23%) of the RLNs deviated from their anticipated anatomical course,
30% of RLNs were fixed, splayed, or entrapped at the level of the capsule of the thyroid.
Entrapment at the ligament of Berry was observed in 41% of nerves.
Traction injury was the most common cause of RLN injury (63%), especially at the site of the ligament of Berry.
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Clinical ThyroidologyVol. 34, No. 1 Thyroid SurgeryFree Access
Anatomical Variations of the Recurrent Laryngeal Nerve During Thyroid Surgery and the Dangers of Nerve Injury
Iatrogenic injury of the recurrent laryngeal nerve (RLN) is a serious complication during thyroid surgery. Reports from the literature show that the rates of RLN injury during thyroidectomy range from 0.3 to 8% (1,2). Generally, the complication is unilateral and transient, but it can be bilateral and/or permanent. Anatomic variations of the RLN can potentially compromise the safety of the nerve during surgery. The current gold standard is visualization of the RLN before beginning dissection of the thyroid, but an anatomically intact nerve does not substantiate a functional nerve. In recent years, intraoperative neuromonitoring (IONM) and continuous intraoperative nerve monitoring (CONM) have been a useful adjunct in RLN identification and are helpful in assessing RLN electrophysiologic function during surgery. The objective of this study (3) was to provide a global assessment of RLN anatomic variation and to identify potential correlations between RLN anatomy and electrophysiologic responses using IONM, with the aim of minimizing the rate of RLN injury during thyroidectomy.
This was a large multi-institutional prospective survey of patients undergoing thyroidectomy with or without central or lateral neck dissections following the standards set forth by International Neural Monitoring Study Group guidelines across 17 institutions on five continents between March 2015 and November 2017. A total of 1000 RLNs from 574 patients were assessed. All patients underwent preoperative and postoperative laryngoscopy. Individuals with postoperative vocal-cord paralysis and paresis (VCP) were followed using laryngoscopy for 12 months postoperatively. The International RLN Anatomic Classification System was used to assist in the classification of RLN anatomic variations. Patients with extensive lymph node metastases and cases with technical IONM failure or failure to visualize the RLN were excluded from the study.
Nearly one-quarter (23%) of the RLNs deviated from their anticipated anatomical course, and 30% of RLNs were fixed, splayed, or entrapped at the level of the capsule of the thyroid. Entrapment at the ligament of Berry was observed in 41% of nerves. Loss of signal during surgery was more likely when there was abnormal nerve trajectory, fixed splayed or entrapped nerves, or cancer invasion or in cases that included lateral lymph node dissection. Although the right RLN had loss of signal more often than the left, laterality was not statistically significant (P = 0.12). Using IONM loss of signal as a test of postoperative VCP, the positive predictive value was 74.3% and the negative predictive value was 99.8%. Traction injury was the most common cause of RLN injury (63%), especially at the site of the ligament of Berry.
Anatomical variations of the RLN could not be predicted preoperatively, but nearly one quarter of RLNs followed an abnormal trajectory. Anatomical variations were associated with higher rates of RLN injury. The use of IONM was a helpful risk-minimization tool. Traction was the most common cause of injury, and although most VCP resolved within 1 year, the recovery rate was less than previously reported.
Intraoperative verification of functional and anatomical RLN integrity is needed for safe thyroid surgery. Furthermore, there is significant value in identifying anatomical RLN variants and possible risk factors in order to maximize preservation of nerve function. This study sought to further understand anatomical RLN variants and to intraoperatively assess nerve injury and electrophysiologic responses using IONM during thyroid surgery. Even though there are several studies in the literature that assessed anatomic RLN variation, there are very few with such a large and diverse sample size that prospectively investigated anatomical data (4,5). This study supports findings from other studies, including traction injuries as a common cause of vocal-cord paralysis (6–8).
This work (3) is an important addition to the literature: not only is it a large multicenter international study, but it also included intraoperative electrophysiologic responses using IONM and long-term follow-up to assess functional outcomes of VCP. As surgeons, we often are limited by individual or institutional experience and are not always able to pinpoint the root cause of inadvertent nerve injury. Such a broad study more comprehensively illustrates the startling diversity of anatomic RLN variants. This heterogeneity is perhaps not surprising given the anatomic variation observed in other major nerves, such as the facial nerve in parotidectomy (9) or the nerve to the vastus lateralis in microvascular surgery (10). The rates of variation, loss of signal, and eventual recovery can help surgeons better inform patients about the underlying risks of an operation.
The study is particularly noteworthy in its assessment of the RLN position at the ligament of Berry, reporting rates at which the nerve is fixed, splayed, or entrapped. This area is of high interest to surgeons, who must balance nerve preservation, nerve traction, and safe removal of the maximal amount of thyroid tissue. Paradoxically, cancer is not often found at this junction, yet leaving too much thyroid tissue behind may lead to higher levels of thyroglobulin and the potential need for radioactive iodine. The study’s findings will likely increase focus on the variation observed and better enlighten surgeons about their approach to this sensitive area.