Location of motor fibers within branches of the recurrent laryngeal nerve with extralaryngeal terminal bifurcation; Functional identification by intraoperative neuromonitoring

BackgroundExtralaryngeal terminal bifurcation (ETB) of the recurrent laryngeal nerve (RLN) is an anatomic variation that threatens the safety of thyroid operation. Therefore, it is important to identify motor function in nerve branches to preserve appropriate motor activity. Intraoperative neuromonitoring (IONM) is an accepted procedure to identify motor function of the RLN.MethodsWe established the operative anatomy of RLNs with ETB in 47 patients. The main trunk, bifurcation point, and the branches were identified and exposed completely during thyroid operation. The location of motor fibers within nerve branches was investigated by identifying motor function via IONM. Wave amplitudes were recorded after electrophysiologic stimulation.ResultsA total of 61 RLNs had ETBs with anterior and posterior branches. Bifurcation occurred early along the pre-arterial (proximal) segment in 13% of bifid RLNs. IONM showed motor function in all anterior branches. IONM identified motor activity in 4 (18%) posterior branches of 22 right, 3 (8%) posterior branches of 39 left, and 7 (12%) posterior branches of all 61 RLNs with ETB. The rate of recorded wave amplitudes of motor function in seven posterior branches was between 14 and 78% of those of corresponding anterior branches.ConclusionIn the RLN, the anterior branch always and the posterior branch uncommonly contain motor fibers. Wave amplitude analysis showed that motor function in the posterior branch is weaker than that in the anterior branch. On the basis of the location of motor fibers in both branches, total exposure and preservation of anatomy and function of all branches of the RLN is mandatory for complication-free thyroid surgery. Electrophysiologic testing may be as an important adjunct to visualization of the nerve with anatomic variation.