Background: The parapharyngeal space (PPS) is a potential space from the skull base to the greater cornu of the hyoid bone. The PPS can be divided into the 3 segments: upper PPS (UPS) from skull base to lateral pterygoid muscle, middle PPS (MPS) from lateral pterygoid muscle to mandibular insertion of medial pterygoid muscle, and lower PPS (LPS) from inferior border of the MPS to the hyoid bone. PPS lesions are challenging to achieve gross total resection, especially in the UPS where critical structures (parapharyngeal ICA [ppICA]) may be hard to access. Current approaches (transcervical) provide inadequate visualization and incomplete access. We investigate two novel minimally-invasive endoscopic approaches (preauricular and intermaxillary-mandibular [IMM]) compared to the transcervical approach in accessing the ppICA, particularly in the UPS.
Methods: Endoscopic dissections were performed in 3 freshly-injected cadaver heads. Open transcervical, endoscopic preauricular, and endoscopic IMM approaches were performed. The advantages and disadvantages based on the anatomic nuances of each approach are discussed. The width of surgical field measured by the most lateral points of the surgical field, is recorded and represents the post-styloid PPS access extent that each approach provides. The length of corridor, a measurement from incision entrance to the ppICA entering the skull base is measured using CT scans with navigation to show proximity of each approach. Quantitative measurements for width of surgical field, length of corridor, and maximal angle access are averaged across cadavers and analyzed via bivariate statistical analysis.
Results: All three approaches can access the upper PPS. However, unlike the open transcervical approach, both the preauricular and IMM approaches could completely mobilize the ppICA as it enters the skull base. The transcervical approach (fig. 1) allows a large inferior corridor, but limited visualization of the UPS due to the posterior border of the mandibular ramus. The endoscopic preauricular approach (fig. 2) allows a small superior and direct corridor to the UPS, but with limited access due to the posterior border of the mandibular ramus/condyle anteriorly, styloid process posteriorly, and facial nerve inferiorly. Subcondylar mandibulotomy improves visualization and access. Endoscopic IMM approach (fig. 3) provides greatest skull base visualization including the ppICA entering the carotid canal but limited visualization of the LPS. Advantages and disadvantages of each approach are described in Table 1.
Conclusion: Access to the upper PPS for ppICA lesions is difficult due to critical structures and the bony confines of the mandible, maxilla, and skull base. Lesions on or involving the ppICA require wide access to the ppICA including complete mobilization for optimal and safe surgical intervention. The novel endoscopic preauricular and IMM approaches are feasible techniques for access and ppICA mobilization. Continued cadaveric studies and utilization of current technological resources are needed for further mastery of the complex parapharyngeal space.
Figure 1: Right transcervical approach corridor, asterisk corresponds to navigation probe
Figure 2: Right preauricular approach corridor. Asterisk corresponds to navigation probe
Figure 3: Left intermaxillary-mandibular approach. Approach provides direct visualization of skull base and carotid canal. Asterisk corresponds to navigation probe.