Decompression of the Optic Canal via an Endoscopic Endonasal versus a Transcranial Approach: A Quantitative Analysis

Introduction: Given the varying pathology that may compromise an optic nerve, it is critical that neurosurgeons have a detailed understanding of optic canal anatomy when determining a treatment paradigm for approaching this vital structure. Prior studies have described the bony dimensions of the optic canal and assessed the degree of decompression one can obtain from an endoscopic endonasal approach alone. However, a detailed quantitative analysis of the decompression obtainable via an endoscopic endonasal versus a transcranial route to the optic canal prior to violation of the other compartment has not been reported.

Methods: Ten fresh cadaveric heads were procured for this study, totaling twenty optic canals. Thin-cut (0.625 mm slice) CT scans were obtained. In 6 heads, endoscopic decompression of the optic canal was performed initially and another scan was performed, stopping prior to entering the intracranial compartment. Transcranial decompression then followed via a pterional craniotomy with extradural clinoidectomy and unroofing of the optic canal. A CT was again obtained to verify circumferential decompression. In 4 heads, the transcranial decompression preceded the endonasal route, stopping prior to entering the sphenoid sinus. Osirix software (Pixmeo, Geneva, Switzerland) was utilized to measure the canal surface area and angle of decompression obtained via each route. Finally, the area of surgical freedom for operating instruments was quantified via these two routes and additionally compared with the endoscopic transorbital corridor using Brainlab navigation software (Brainlab AG, Feldkirchen, Germany). A t-test was performed to assess for statistically significant difference in decompression and surgical freedom between approaches, as indicated by p < .05.

Results: The 20 optic canals had a mean surface area of 360.9 mm2. An endoscopic endonasal approach provided an average decompression of 114.8 +/− 18.8 degrees prior to entering the intracranial compartment, whereas the transcranial route afforded an average of 245.2 +/− 18.8 degrees of decompression without violating the sphenoid sinus. This difference was statistically significant (p < .001). Surgical freedom of operating instruments was found to be greatest via the transcranial approach (109.4 +/− 3.4 cm2), followed by transorbital (37.2 +/− 4.9 cm2), and least via the endonasal route (11.1 +/− 5.6 cm2). The differences between each of these approaches was statistically significant (p < .01).

Conclusion: This is the first study providing a quantitative comparison of the circumferential decompression obtainable via the endoscopic endonasal and transcranial approaches prior to violation of the other compartment. As expected, a significantly greater degree of decompression may be obtained via craniotomy than the endonasal route using these parameters. The degree of surgical freedom by approach was greatest via craniotomy, followed by transorbital, and least via the endonasal corridor. The location of compressive pathology within the canal and individual anatomical characteristics should dictate the selected approach in a given clinical scenario. A combination of procedures may be utilized for circumferential decompression when indicated.