Marc Christensen and Dr. Jonathan Wisco, Department of Physiology and Developmental Biology

Introduction

Selective reinnervation of the posterior cricoarytenoid muscle (PCA), accomplished by anastomosis of a transected recurrent laryngeal nerve (RLN) with the phrenic nerve, is the best treatment for vocal cord paralysis and RLN compromise. However, anatomical variation of the RLN in human patients contributes to the likelihood of post-operative complications such as laryngeal synkinesis. The purpose of this study, Anatomical Variation of the Porcine Recurrent Laryngeal Nerve, is to establish a clear and detailed understanding of RLN branching patterns in pigs, the animal model most similar to humans in regards to laryngeal anatomy, to determine nerve branches that are best suited for anastomosis with the phrenic nerve, and serve as a basis for improving PCA reinnervation techniques in future preclinical studies.

Methods

Fifteen adult pig larynx specimens were obtained from Circle V Meats in Spanish Fork, Utah and embalmed in ten percent formalin solution inside a refrigerator at four degrees Celsius after a postmortem time of forty-five minutes. One specimen was damaged bilaterally during dissection and was not included in the study. The other fourteen specimens were dissected on both right and left sides by identifying the RLN trunk in the tracheoesophageal groove, and then following it superiorly using blunt dissection. A total of twenty eight recurrent laryngeal nerves were dissected, photographed, and drawn for subsequent categorical analysis.

Results

Dissection of 28 recurrent laryngeal nerves revealed three common branching patterns differing in the location of primary branch bifurcation and in total number of branches. In patterns A and B the innervation of the PCA by a single, large, primary middle branch is conserved.

Pattern A occurred in 44% of the dissected specimens and exhibited the following characteristics: the most proximal branch to diverge from the RLN trunk was a medial primary branch, this branch bifurcated into two secondary branches both of which clearly went on to innervate the PCA muscle, the second branch to arise from the RLN trunk was a lateral primary branch, this branch split into various small terminal branches, and all of these terminal branches ran away from the PCA to innervate the laryngeal adductor muscle.

Pattern B occurred in 32% of dissected specimens and exhibited the following characteristics: the medial primary branch innervated the PCA, half of the terminal branches of the middle primary branch innervated the PCA, and the other half of the terminal branches of the middle primary branch ran away from the PCA and targeted adductors along with the lateral primary branch.

Pattern C occurred in 24% of dissected specimens and exhibited the following characteristics: two primary branches, one medial and one lateral arising from the RLN trunk concurrently, the medial of these two branches further diverges into superior, middle, and inferior secondary branches as it projects toward the PCA, the most superior of these branches appeared to run supero-laterally and possibly innervates vocal fold adductors, and the middle and inferior secondary branches clearly innervate the PCA.

Conclusion

The primary branching of the RLN of pigs can be categorized into three main patterns. For both A and B branching patterns of the recurrent laryngeal nerve, the primary medial branch innervates the CPA, along with all of the medial branch’s terminal ends, and is ideal for anastomosis with the phrenic nerve. In pattern C, the innervation of the superior secondary nerve branch could not be observed via blunt dissection, and the ideal RLN branch for PCA reinnervation is unclear. Further testing, such as using Sihler’s stain or in vivo nerve stimulation, is needed to determine the ideal RLN branch for anastomosis with the phrenic nerve.

Discussion

Human RLN branching has already been well studied, and laryngeal synkinesis is a well-known complication of PCA reinnervation surgeries. We suggest that one possible cause for surgical complications is due to variation in RLN branching patterns and corresponding muscular innervation of these branches amongst patients. Once identified in humans, RLN branching patterns could be used to improve surgical techniques for the reinnervation of the PCA by identifying specific branches most favorable to anastomosis with the phrenic nerve. However, further testing is needed to validate that anastomosis of the identified nerve branches with the phrenic nerve increases the success of PCA reinnervation in pig models, and to identify the corresponding RLN branches in humans that can be used to the same effect.