The carotid-oculomotor window in exposure of upper basilar artery aneurysms: a cadaveric morphometric study

The carotid-oculomotor window remains the traditional deep window in the exposure of aneurysms of the upper basilar artery. Although several techniques have been described to expand this window, few morphometric studies document either the degree of its expansion or its contribution to the exposure of the basilar artery. We review the microsurgical anatomy of the carotid-oculomotor window, describe expansion techniques, and analyze morphometrically the contribution of each step (i.e., extradural anterior clinoidectomy, mobilization of the internal carotid artery [ICA], and posterior clinoidectomy) to the expansion of the window and/or exposure of the artery.

Ten formalin-fixed, alcohol-preserved, cadaver heads injected with pigmented silicone were prepared for bilateral dissection. The vertebrobasilar system was injected with pigmented silicone mixed with barium (1:1), rendering it radiopaque. After completing a frontotemporal-orbitozygomatic craniotomy, we performed dissection in two stages: Stage I consisted of a conventional transsylvian exposure of the upper basilar artery through the carotid-oculomotor window; and Stage II added anterior clinoidectomy, ICA mobilization, and posterior clinoidectomy. A clip was applied to the lowest accessible point of the basilar trunk at each stage. Measurements obtained during each stage included: 1). the transverse carotid-oculomotor distance, that is, anteriorly between the oculomotor foramen and ICA, and posteriorly between the oculomotor nerve and ICA; and 2). the exposed length of the basilar artery, as seen under the microscope and on angiograms.

Measurements were obtained before and after the addition of anterior clinoidectomy, mobilization of the ICA, and posterior clinoidectomy. Increases in expansion of the window and exposure of the upper basilar artery were documented as percentages of the control values. The anterior carotid-oculomotor distance averaged 7.1 mm (range, 5-10 mm) and 10.1 mm (range, 7-15 mm) before and after the additional surgical steps to expand the window, respectively. The posterior carotid-oculomotor distance averaged 12.7 mm (range, 9-18 mm) and 16.1 mm (range, 11-22 mm) before and after the additional surgical steps to expand the window, respectively. The exposed length of the basilar artery from the bifurcation to the clip was 4.2 mm (range, 1-13 mm) before expansion and 7 mm (range, 3-15 mm) after expansion.

Anterior clinoidectomy and ICA mobilization increased the carotid-oculomotor space 44% anteriorly and 28% posteriorly. Posterior clinoidectomy increased the exposed length of the basilar artery by 69%. Superficial wide field exposure, expansion of the carotid-oculomotor window, and increased exposure of the upper basilar artery facilitate both visualization of the aneurysm for clip application and the use of proximal vascular control as an adjunct to basilar aneurysm surgery.