Morphological and functional asymmetry of the human recurrent laryngeal nerve

The aim of the current study was to analyze some parameters that are involved in nerve conduction in the right and left recurrent laryngeal nerves (RLNs) in humans. We have studied two morphological parameters such as the length of the two nerves, and the total intraperineural area (TIPA) from necropsies, and one functional parameter such as the latency of the thyroarytenoid muscle from operated patients.

The morphological parameters of six white female adult cadavers were analyzed. The RLNs were totally removed on both sides with the vagus nerve from their origin to the entrance in the larynx at the cricothyroid joint. The lengths were measured with the help of a digitalized caliper. Selected sections were obtained from each third of the RLNs for histological analysis. The TIPA was estimated at each selected level using digitized images of nerve sections and a manual micrometer. All the measurements were carried out by specialists in histology. The intraoperative vagal nerve stimulations were conducted using the NIM3 Medtronic(®) monitoring system (Medtronic Xomed Inc., Jacksonville, FL, USA) in ten adult patients without laryngeal or nervous disease during thyroid gland surgery. The evocated laryngeal electromyography was performed with the Medtronic bipolar needle electrode directly inserted into the thyroarytenoid muscle. The direct vagal nerve stimulation was achieved with the Medtronic stimulation from 0.5 to 1 mA. The latency of the thyroarytenoid muscle was recorded on the NIM-response monitor.

The mean lengths of the left and right RLNs were, respectively, 136.6 and 75.0 mm with a mean difference of 61.6 mm (range 50-75 mm). The quantitative comparison of the TIPA between proximal segments of the left and right RLN showed no significant difference. The distal and proximal TIPA ratio demonstrated differences between the left and right RLN, respectively, 53.76 and 38.88 % without any statistical meaning. The comparison of the TIPA of the distal segments of the RLNs showed no significant difference. The intraoperative vagal nerve stimulation (0.5 and 1 mA) displayed a mean latency of the right and left thyroarytenoid muscle, respectively, of 3.55-3.68 and 5.90-5.98 ms with a mean difference of 2.35-2.30 ms (range 1.75-3.30 ms).

If length and latency asymmetry of the right and left RLNs in humans can be demonstrated, the synchronicity of the vocal folds requiring precise controlled variations within intrinsic laryngeal muscles needs further investigations.