Removal of anterior clinoid process for basilar tip aneurysm: Clinical and cadaveric analysis
Sato, Shuzo
The difficulty in the operation for basilar tip aneurysm is the restriction in surgical working space. To resolve this problem, aggressive skull base surgery has been reported, but these techniques are not prevalent. Pterional and subtemporal approaches are commonly used for basilar tip aneurysms. In an attempt to increase the surgical working space during the pterional approach, the anterior clinoid process and the roof of the optic nerve were removed extradurally to increase the mobilization of the intracranial internal carotid artery and optic nerve. The effects of removing the anterior clinoid process and microanatomy in the perioptic area were analyzed by cadaveric procedures in 10 cases (20 sides). With this procedure, the internal carotid artery can be retracted medially with a spatula 6.1 0.8 mm (mean SD). The length and the area of dural fold in the bone defect region in the optic canal roof are 2.1 mm and 13.6 mm . In 10 clinical cases, this procedure allowed enough space to approach the basilar tip aneurysm without disturbing the internal carotid artery blood flow. The clinical outcome was satisfactory. [Neurol Res 2001; 23: 298-303]
Keywords: Anterior clinoid process; basilar tip aneurysm; internal carotid artery; optic canal; optic roof surgical approch
INTRODUCTION
Among recent remarkable developments in reported interventional techniques is the successful treatment of basilar tip aneurysm?1,2, but the long-term follow-up of this treatment is still unknown. At this time, most patients with basilar tip aneurysms are treated surgically. Surgery has the advantage of preventing post-operative vasospasms by removing the subarachnoid clot3. The pterional4-6 and subtemporal approach 7-10 are commonly used for basilar tip aneurysm surgery. These approaches are frequently used for various brain lesions and are the basic approaches in neurosurgical treatment. Both approaches have some advantages for exposing the basilar tip aneurysm, but the surgical working space is restricted. To solve this problem, we developed the extradural removal of the anterior clinoid process (ACP) and the roof of the optic canal during the standard pterional approach by which we obtained a better’ surgical working space. For the surgery around the perioptic canal, detailed microanatomy is important. As the dural fold formed the edge of the optic canal and there are no bony structures in the optic canal roof, special care should be taken for drilling the optic canal roof. This paper provides the technical procedure for removing the ACP and the roof of the optic canal and the microanatomy, the width and area of the dural fold in the optic canal roof and the extent of internal carotid artery (ICA) mobilization, were evaluated in the cadaveric dissection. These anatomical data may provide important information for understanding the variety of the dural fold in this region and also be useful for designing operative strategies.
OPERATIVE TECHNIQUE
In 10 clinical cases, prior to the surgery, spinal drainage was performed and cerebrospinal fluid was drained if the intracranial pressure was over 15 cm in water. The patient was placed in the supine position with the head rotated approximately 35. A skin incision was made as with the usual pterional approach. Four burr holes were made in the frontal bone immediately anteromedial to the junction of the three bony structures: the orbital ridge, the zygomatic process of the frontal bone, and the temporal line. A free bone flap was removed. The lateral part of the sphenoid wing was removed with ronguers. Using a high-speed drill under a microscope, the lateral aspect of the sphenoid wing was drilled to the superior orbital fissure. The dura mater was opened, and the sylvian fissure was opened in the conventional way using an operative microscope (Figure 1). After exposure of the intracranial portion of the ICA, extradural removal of the ACP, the roof of the optic canal and the medial portion of the sphenoid wing was performed. The extent of removed bone area is indicated as lower than solid line (Figure 1). As it is difficult to make an exact orientation without identifying the intradural structures, the intradural confirmation of the ICA and ACP is helpful in order to remove the extradural medial portion of the sphenoid wing and ACP. Partial opening of cavernous sinus is useful for the mobilization of both intradural and cavernous ICA (Figure 2). Bleeding from the cavernous sinus is easily controlled by elevating head position and packing of hemostatic substances. Care should be taken not to pack hemostatic substances firmly to avoid postoperative disturbances of ocular motility. Following the removal of the ACP and the roof of the optic nerve, the ICA can be mobilized without disturbing ICA flow by the spatula (Figure 3). The upper portion of the basilar artery, bilateral superior cerebellar arteries, posterior cerebellar arteries and oculomotor nerves were seen through the retracted ICA, and this allowed enough space to clip the upper basilar aneurysm (Figure 4). Retracting the ICA without removing the ACP and the roof of the optic nerve causes changes in the caliber of the ICA, which in turn causes the reduction of ICA flow and increases the risk of infarction of the ICA territory. There were no complications resulting from the removal of the ACP and the roof of the optical canal, and the clinical results were satisfactory.
CADAVER EXAMINATION AND ANATOMICAL FINDINGS
Perioptic microanatomy were evaluated in 10 cadaver cases (20 sides). The distance between the edge of dural fold and bony structure in the roof of the optic canal, the extent of bony defect area, the width of optic canal, were analyzed in the cadaver examination. The distance between the edge of dural fold and bony roof of the optic canal (Figure 5) is 2.1 0.6 mm (mean SD), ranging from 1.2 to 3.4 mm (Table 1). The width of optic canal (Figure 5) is 9.8 1.7 mm (mean SD), ranging from 6.6 to 12.7 mm (Table 1). The width of optic nerve is 5.90.7mm (mean+SD), ranging from 4.8 to 7.1 mm (Table 1). The total area of the bony defect in the roof of the optic canal (Figure 5) is 13.6 +/- 3.3 mm^sup 2^ (mean +/- SD), ranging from 7.0 to 21.8 mm^sup 2^ (Table 1). The extent of ICA mobilization by spatula was analyzed in the cadaveric procedure. ICA was mobilized from 6.1 +/- 0.8 mm (mean +/- SD) to the medial side, ranging from 4.5 to 7.5 mm (Table 1).
DISCUSSION
Drake9 first described the successful treatment of four cases of basilar tip aneurysm using the subtemporal approach. The pterional approach for basilar tip aneurysm was first reported by Yasargil et al.4,11 who used the frontotemporosphenoidal approach. The pteriona 14-6,11 and subtemporal approaches7-10 are commonly used for basilar tip surgery. Both approaches are prevalent for various brain lesions, but the surgical field is restricted for basilar tip aneurysms. In an attempt to solve this problem, temporo-polar 12, orbitozygomatic13,14, transcavernous15, and transcallosal16 approaches and various combinations of these approaches 17-20 have been reported. An effort to remove the orbital rim or zygomatic arch has been reported 13,14. However performing these unfamiliar skull base approaches requires special experience. These approaches are, of course, useful but are not familiar to general neurosurgeons because of the small numbers of cases of basilar tip aneurysms (5% of all aneurysms). To avoid retracting the brain, the transcavernous approach has been developed15, but the surgeon who has no experience with cavernous surgery would have some hesitation about opening the cavernous sinus. Samson et al.5 reported that an aneurysm of the distal basilar, proximal posterior cerebral, and proximal superior cerebellar arteries can be exposed with reasonable facility utlizing the pterional approach, but either the opticocarotid or retrocarotid route is used and retraction of the carotid artery is required with either route. The major complications in basilar tip aneurysm surgery are brain compression, ICA injury and ischemia associated with the retraction of the ICA or occlusion of the perforating vessels when clipping the aneurysm21. To avoid ishcemic infarction of the perforating artery, an attempt has been reported to place ozycellulose between and aneurysm and its surrounding perforators to make a space for inserting the clip blades22, but once the performating vessels are interrupted, there is a significant risk of infarction21. During the retraction of the carotid artery, ischemic complications to the ICA territory or direct injury of the carotid artery can be expected, and to avoid ischemic accident, intermittent retraction of the ICA is recommended6. The extent of ICA mobilization in cadavers showed a relatively wide range, from 4.5-7.5 mm. The extent of ICA mobilization is dependent on the length of the supraclinoid ICA and the shape of the ACP. As the length of intracrania; portion of the ICA ranges from 14-25mm, this explains the wide variation of ICA mobilization in cadavers23. For basilar tip aneurysm surgery with a short intracranial ICA some special technique is required24. The cause of ICA mobilization restriction is the length of the intracranial ICA. To solve these problems, resection of the ACP and the roof of the optic canal were performed to increase the length of the ICA. With this procedure, the ICA can be retracted more medially or laterally. By cadaveric procedure, ICA was mobilized about 6.1 mm to the medical side, ranging from 4.5-7.5 mm. Also, the optic nerve can be mobilized medially by removing the roof of the optic nerve. Both the resection of the ACP and the unroofing of the optic canal can provide a larger surgical working space compared with the usual pterional appraoch. The removal of the ACP is useful in opthalmic aneurysm surgery18. We have been forced to remove the ACP intradurally because the operative findings were unexpected, but is is important to routinely remove the ACP and the roof of the optic canal extradurally before starting the operative procedure for a basilar tip aneurysm.
The anatomy of the optic canal in the dry skull has been reported, but in surgery around the perioptic area, bone is covered with dura. It is important to know the anatomical data of the optic canal with dural structures. The edge of the optic canal is formed by dural fold and has no bony structures. The exact anatomy of bony defect area in the optic canal roof is important for the operation around the optic canal. Without knowing the extent of bony structure defects in the optic canal roof, optic nerve may be injured during the removal of ACP or optic canal roof by using high speed drill. Dural fold is formed by dura of the intra-optic canal and intracranial dura. The length between the edge of dural fold and the bony structure is 2.1 +/- 0.6 mm and the bony defect area is 13.6 +/- 3.3 mm^sup 2^, but both the length and the area have the variation from 1.2-3.4 mm and from 7.0-21.8 mm^sup 2^. In the bony structure defects region, the mean distance is 2.1 mm and the mean area is 13.6 mm^sup 2^, but during the removal of the roof of the optic canal, we should keep in mind that the length and the area in the bony defects region have wide variation. In conclusion, both cadaveric and surgical investigations have confirmed that ACP and the roof of the optic canal can be safely removed with the information of the anatomical knowledge in the perioptic area and the removal of the ACP and the roof of the optic canal for basilar tip aneurysm surgery is useful.
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Shuzo Sato*, Minako Sato*, Taro Oizumi*, Masatoyo Nishizawa*, Mami Ishikawa^, George Inamasut^ and Takeshi Kawase^
*Center for Neurological Diseases, International University of Health and Welfare, Tochigi ^Department of Neurosurgery, Keio University, Tokyo, Japan
Correspondence and reprint requests to: Shuzo Sato, MD, Center for Neurological Diseases, International University of Health and Welfare, 537-3 Iguchi, Nishinasuno-machi, Nasu-gun, Tochigi-ken, Japan 329– 2763. [shu@iuhw.ac.jp] Accepted for publication August 2000.
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