Is the External Branch of the Superior Laryngeal Nerve Dispensable in Thyroid Surgery?

There is growing evidence that the external branch of the superior laryngeal nerve (eSLN) participates in thyroarytenoid (TA) contraction, but little data quantify its role in vocal cord adduction. Injury to the eSLN, such as in thyroid surgery, is difficult to diagnose and likely underappreciated. It is the authors' belief that eSLN injury contributes to aspiration by depriving its contribution to the laryngeal plexus. The goal of this study was to measure the glottic closing force (GCF) from eSLN stimulation in a porcine model.

The recurrent laryngeal nerve (RLN) and eSLN were identified bilaterally in four porcine necks. Bilateral RLNs and eSLNs were stimulated simultaneously to obtain a control GCF using a pressure transducer placed in the glottis. Subsequently, bilateral eSLNs were stimulated and the GCF measured to quantify its percent contribution to the control value.

Stimulation of the RLNs and the eSLNs each led to TA muscle contraction and a measureable GCF in all four porcine necks. The control GCF was 1000.1 mmHg, while the eSLN mediated CGF was 800 mmHg. The percentage GCF attributable to the eSLN was thus 800/1000 = 80%.

Reflex glottic closure is one of the most important mechanisms for the prevention of aspiration during deglutition. The biomechanical quantification of glottic closure can be shown as the GCF. This study has shown that the eSLN contributes in a significant way to the GCF in a porcine model, a finding that has not been quantified to the best of the authors' knowledge. Therefore, greater focus should be placed on preserving this nerve in thyroid surgery.