Morphometrical and Radiological Predictive Feasibility in Clipping of the Superior Hypophyseal Segment Aneurysms from Contralateral Subfrontal and Supraorbital Approach: Pilot Study

Background Superior hypophyseal segment aneurysms are typically medially (superior/inferior) projecting aneurysms. Microsurgical clipping of the aneurysms from the ipsilateral side is often encountered with incomplete visualization of the aneurysm anatomy thereby necessitating a blind clipping approach. Visualization of this segment from the contralateral side has been described to show the anatomy much better; however, selection process of such cases has to be streamlined. The literature does not describe the synchronicity of the morphometric and radiological parameters used for such approaches. The objective of our pilot study was to elucidate if any radiological features could assist the surgeon in decision making and obtain morphometrics of the contralateral approach.

Methods Ten sides of normal magnetic resonance imaging of the brain were analyzed for radiological parameters and similar extrapolation of the later was done on fixed injected cadaveric heads. Clipping angle was defined by the angle made between the inferior tangent of contralateral optic nerve and superior tangent of the ipsilateral or, both of which were incorporating the contralateral hypophyseal segment of the internal carotid artery (ICA). In addition, we measured interoptic triangle area, retraction feasible after mobilization of the contralateral optic nerve by drilling the superior optic canal and releasing the falciform ligament. Measurements were made using digital calipers.

Results In the radiological assessment, means of the interoptic distance, chiasm–sphenoidale distance, area of triangle, clipping angle right/left side, and optic/carotid distance from midline ratio right/left were 28.12 (25.5–30) mm, 4.9 (3.3–7.6) mm, 69.13 (48.96–96.14) mm², 22.32/21.54 degrees, and 0.66/0.62, respectively. Clipping angle relationship with the area of triangle and optic carotid ratio was computed using line chart analysis as well as linear chart plot regression analysis. Increased clipping angle was noted with increasing area of interoptic triangle (Fig. 1). No prominent relationship was noted between clipping angle and optic carotid ratio. Cadaveric morphometric analysis revealed the interoptic distance, chiasm-sphenoidale distance, and area of triangle to be 18.59 mm, 3.9 mm, and 36.25 mm², respectively. Clipping angle pre- and postmobilization of the optic nerve as 9.5 and 17.5 degrees, respectively. Drilling of the medial roof of optic canal along with the tuberculum sellae and limbus sphenoidale increased the chiasmatic-sphenoidale distance from 3.9 to 6.7 mm. Hypophyseal segment ICA exposed in anteroposterior dimension and circumferential exposure before and after optic nerve mobilization was 4.53 versus 9.3 and 4.06 versus 5.78 mm, respectively. Supraorbital approach was followed with subfrontal approach which did not increase the surgical corridor of clipping; however, there was a subjective ease of skull base bone drilling with the subfrontal craniotomy.

Conclusion Preliminary data suggest a relationship of the area of triangle with the potential angle of clipping measured radiologically. Adequate mobilization of the optic nerve is feasible from the contralateral approach to improve the surgical access of the contralateral hypophyseal segment. Drilling of the tuberculum sellae/limbus sphenoidale may increase the aneurysm clip landing zone.

Fig. 1