Transnasal Odontoid Resection: Is there an Anatomic Explanation for Differing Swallowing Outcomes?

Introduction: Swallowing dysfunction is common following transoral (TO) odontoidectomy. Preliminary experience with newer endoscopic endonasal (TN) approaches suggests that dysphagia may be reduced with this alternative. However, the reasons for this are unclear. We hypothesized that the TN approach results in less disruption of the pharyngeal nerve plexus (PP) and anatomic structures associated with swallowing.

Objective: This study aims to investigate the histologic and gross surgical anatomic relationship between the PP innervation of the upper aerodigestive tract, and the surgical approaches used (TN and TO). Also, this study aims to review the TN literature to evaluate swallowing outcomes following this approach.

Methods: Seven cadaveric specimens were used to examine the histology of PP (n= 3) and gross anatomy (n= 4) with the TO and TN surgical approaches. Particular attention was given to identifying the location of cranial nerves (CN) IX, X, the sympathetic chain, and their contributions to the PP. S-100 staining was performed to assess for the presence of neural tissue in proximity to the midline and fiber density counts were performed within 1 cm of midline. The relationship between the PP, clivus, and upper cervical spine (C1–C3) was defined.

Results: Histologic analysis revealed the presence of PP fibers in the midline and a significant reduction in paramedian fiber density from C2 to the lower clivus (p < 0.001). None of these paramedian fibers, however, could be visualized with gross inspection or layer-by-layer dissection. Laterally based primary PP nerves were identified by tracing their origins from CN IX, X, and the sympathetic chain at the skull base and following them to the pharyngeal musculature. A total of 15 studies presenting 52 patients undergoing TN odontoidectomy were reported. Of these, only 48 patients had been swallowing preoperatively. When looking only at this population, 83% (40/48) were swallowing by day 3 and 92% (44/48) were swallowing by day 7.

Conclusion: Despite the midline approach, both TO and TN approaches may injure a portion of the PNP. By limiting the TN incision to above the palatal plane, the surgeon avoids the high-density neural plexus found in the oropharyngeal wall and limits injury to oropharyngeal musculature involved in swallowing. This may explain the decreased incidence of postoperative dysphagia seen in TN approaches. However, further clinical investigation is warranted.