The Overwhelming Majority but not All Motor Fibers of the Bifid Recurrent Laryngeal Nerve are Located in the Anterior Extralaryngeal Branch

Background Few small studies reported that motor fibers are located exclusively in the anterior branch of the bifid recurrent laryngeal nerve (RLN). The aim of this study was to investigate the location of the motor fibers to the intrinsic muscles of the larynx among the bifid RLNs, and assess the prevalence of RLN injury with respect to nerve branching in a pragmatic trial. Methods This was a prospective cohort study of 1250 patients who underwent total thyroidectomy with intraoperative neural monitoring. The primary outcome was the position of the motor fibers in the bifid nerves. Adduction of the vocal folds was detected by the endotracheal tube electromyography and abduction by finger palpation of muscle contraction in the posterior cricoarytenoid. The secondary outcomes were the prevalence of the RLN branching and the prevalence of RLN injury in bifid versus non-bifid nerves. Results The bifid RLNs were identified in 613/2500 (24.5 %) nerves at risk, including 92 (7.4 %) patients with bilateral bifurcations. The motor fibers were present exclusively in the anterior branch in 605/613 (98.7 %) bifid nerves, and in both the RLN branches in 8/613 (1.3 %) bifid nerves. Prevalence of RLN injury was 5.2 versus 1.6 % for the bifid versus non-bifid nerves (p < 0.001), odds ratio 2.98 (95 % confidence interval 1.79-4.95; p < 0.001). Conclusions The motor fibers of the RLN are located in the anterior extralaryngeal branch in the vast majority of but not in all patients. In rare cases, the motor fibers for adduction or abduction are located in the posterior branch of the RLN. As the bifid nerves are more prone to injury than non-branched nerves, meticulous dissection is recommended to assure preservation of all the branches of the RLN during thyroidectomy.