Anatomy of the Optic Canal: A Computed Tomography Study of Endoscopic Nerve Decompression

OBJECTIVES: Endoscopic optic nerve decompression has variable success rates. Our goal was to further delineate the radiographic anatomy of the optic canal to determine whether the variable success can be explained on anatomic principles. METHODS: The optic canal dimensions and the degree of optic canal exposure to the sphenoid sinus were measured on sinus computed tomography images of 96 patients. RESULTS: A total of 191 optic canals were analyzed (111 female subjects and 80 male subjects). The average medial canal wall length was 1.48 cm (range, 0.7 to 2.3 cm). The length in male subjects was 1.61 cm (range, 1.1 to 2.3 cm), as compared to 1.39 cm (range, 0.7 to 2.0 cm) in female subjects (p < 0.001). Onodi cells and pneumatized anterior clinoid processes were present on 14 and 16 images, respectively. The average degree of exposure of the optic canal to the sphenoid sinus in optic canals without Onodi cells or clinoid pneumatization was 99.3 degrees, and in optic canals with both Onodi cells and clinoid pneumatization it was 117.7 degrees. The potential area of canal exposed was 0.66 cm2, or 28% of the total surface area. CONCLUSIONS: A wide variation in medial canal wall length and exposure of the optic canal to the sphenoid sinus exists on computed tomography images. Variation in medial canal wall length and optic canal exposure may limit the surface area of nerve available for endoscopic optic nerve decompression.