OBJECTIVE: The authors describe a trans-cavernous sinus approach with a pterygoid point craniotomy for the treatment of low basilar artery (BA) terminal aneurysms and compare this intradural procedure with the epidural procedure described by Dolenc et al.
METHODS: A standard pterygoid point intradural trans-lateral fissure approach coupled with transcavernous sinus exposure allows for lower exposure of the terminal basilar artery behind the saddle. The technical steps included in the procedure are as follows: 1) abrasion of the anterior bed process; 2) access to the medial aspect of the cavernous sinus arteriolar nerve; 3) filling of the venous duct of the cavernous sinus between the carotid artery and pituitary gland to open this area; 4) abrasion of the posterior bed process to expose the dorsal portion of the saddle including the cavernous sinus; and 5) excision of the exposed dura to obtain additional exposure of the circumferential pool. We illustrate the role of this approach in surgical treatment with eight cases of terminal basilar artery aneurysms.
CONCLUSION: Using the standard pterygoid approach, we found that the terminal basilar artery aneurysm was not only too low relative to the posterior bed prominence to be adequately exposed, but there was insufficient space to temporarily clip the proximal end of the terminal basilar artery aneurysm. Additional transcavernous sinus exposure solves these technical challenges and makes it possible to complete aneurysm clamping in every case.
The pterygoid point trans-lateral fissure approach is the standard procedure for treating terminal basilar artery aneurysms.15 However, aneurysms located below the level of the posterior bed process may not be reached using this route.13,15 In these cases, grinding away the posterior bed process allows for lower visualization.15 Dolenc et al4 described a modification of the lateral fissure approach to allow for very close visualization of the distal basilar artery. Using their approach, the posterior bed process and a portion of the dorsum of the saddle were revealed and abraded through the cavernous sinus. The antecedent step of the transcavernous sinus procedure is the abrasion of the anterior bed process, which Dolenc describes as an epidural modification of the standard pterygoid craniotomy. However, the same transcavernous sinus exposure can be achieved by using a fully intradural procedure after a standard pterygopontine craniotomy. This intradural technique will be described next, discussing its advantages over the partial epidural approach described by Dolenc et al. We illustrate the usefulness of this approach with eight cases of low basilar artery terminal aneurysms.
Clinical Data and Methods
Patient population
We describe eight cases of low basilar artery terminal aneurysms. In each case, the location of the aneurysm was too low to allow clamping through a standard pterygoid approach, necessitating additional visualization. This was achieved by using transcavernous sinus abrasion of the posterior bed process and ipsilateral saddleback. 5 patients had basilar artery bifurcation aneurysms and 3 patients had basilar/superior cerebral artery aneurysms.1 Angiographic presentation of the aneurysms: Table 1 shows the clinical features. 4 patients presented with subarachnoid hemorrhage (cases 1, 3, 5, and 6) and 3 basilar artery aneurysms were found accidentally during angiography of another suspected aneurysm (cases 2, 5, and 6). Three basilar aneurysms were discovered accidentally during angiography of another suspected aneurysm (cases 2, 4, and 8). The last case presented with dizziness and radiological findings of multiple aneurysms originating from the junction of the basilar and right superior cerebral arteries. The most obvious one was massive in size and partially thrombosed (case 7). Two patients had giant supratentorial aneurysms (cases 6 and 7). The top of the aneurysm was upward and extended posteriorly toward the ipsilateral posterior cerebral artery.
All cases started with a pterygoid point to expose the lesion via a lateral fissure approach. When adequate exposure was not possible for treatment, more space was obtained by trans-cavernous sinus abrasion of the posterior bed process and the ipsilateral saddle dorsum. In two patients (cases 2 and 3), we tried a more lateral approach before using the transcavernous sinus approach; however, it still did not help to reveal the lesion. The lateral approach in case 2 required dissection of the pontine vein in the temporal lobe and submural dissection of the hook gyrus, along with postero-lateral retraction of the temporal lobe.
There are multiple reasons for the need to expose these aneurysms more closely. In cases 1, 2, and 6, more space is needed to temporarily occlude the basilar artery in close proximity. Cases 1, 3, 5, and 8 were poorly exposed, and the anatomic relationship between the aneurysm and the adjacent vessels could not be distinguished. In cases 4 and 7, greater space was required for clamping of the aneurysm.
Surgical technique
Many authors have long described the components of the surgical technique used. These include pterygopoint craniotomy6,14, intradural abrasion of the anterior eminence bed9,10, access to the cavernous sinus through the top of the medial cavernous sinus of the arteriovenous nerve4,8,11, abrasion of the posterior eminence bed, and exposure of the dorsal saddle.4 Details regarding the techniques used in these cases are described below.
The standard pterygoid point craniotomy6, 14 was used as originally described. The dura is elevated at the orbital apex, with the large wing of the pterygoid bone far medial to the lateral aspect of the supraorbital fissure, and the orbital bone is osteotomized in that position. The dura is incised to make a dural flap that is based on and drawn out from the orbital bone. The lateral fissure is opened in a lateral to medial fashion and the carotid and crossed pools are opened. This allows gentle retraction of the frontal lobe to obtain a view of the carotid region and Liliequist’s membrane through the lateral fissure. The Liliequist membrane is opened and the location of the aneurysm is determined. If closer exposure is required, a transcavernous sinus approach is used. The intended path of exposure through the cavernous sinus runs parallel to the medial aspect of the tricuspid ventricle. It is not always necessary to make an effort to retract the temporal lobe laterally to achieve the above access angle. It is also possible to preserve the parietal temporal drainage vein when possible (6-8 cases).
The first step is the intradural abrasion of the anterior bed. This both identifies the internal carotid artery3 in the cavernous sinus that crosses the proximal dural ring and reveals the entire top of the cavernous sinus. The dura covering the optic canal and anterior bed process is peeled away and the medial dura, which was elevated during the craniotomy, is retained to prevent traffic between the subarachnoid space and the epidural space. The optic canal is opened nearly 5 mm at first, then the lateral wall of the optic canal is opened and the optic nerve is freed from the attachment to the tip of the anterior bed process using a small spatula. From the side, the dura at the top of the optic canal is opened from posterior to anterior with a curved arachnoid knife to the base of the optic canal. Operating posteriorly along the site from the base of the optic canal to the dura adhering to the internal carotid artery, the dura lateral to the internal carotid artery is cut off, leaving only a small portion of the dural ring surrounding the internal carotid artery.3 A view of the medial aspect of the dural ring of the internal carotid artery and the adjacent actinic nerve is obtained. The adjacent septal or pterygoid airspace may be accessed during the operation. Failure to block traffic with these sinuses at the end of the operation may result in a postoperative nasal leak of cerebrospinal fluid10.
The top of the sphenoid sinus is opened just medial to the actinic nerve, and the dural canal of the actinic nerve can be directly visualized by sharp dissection of the dura at the dura that penetrates to the internal carotid artery. However, in two cases the nerve could be seen through the wall of the canal without opening it. The anterior opening of the cavernous sinus was just medial to the actinic nerve and posterior to the internal carotid artery. After entering the cavernous sinus, the hemorrhage is stopped by alternating filling with a gelatin sponge impregnated with thrombin and oxidized cellulose. Persistent venous hemorrhage can be relieved by elevating the patient’s head.8 The incision is widened posteriorly and continued to caulk the hemorrhage until the posterior bed prominence, and then the dura is freed as medially as possible from the dorsal anterior saddle. This step allows the freed dural flap to be draped medially, opening a triangle at the top of the cavernous sinus. After opening the top of the cavernous sinus, the filling hemostat is slowly removed and directly replaced with a smaller and more precise hemostat. After this work is done, a space is left at the connection between the lateral aspect of the tricuspid ventricle and the lateral aspect of the intradural internal carotid artery, which is located behind the saddle, medial to the pituitary gland, and below the saddle node, adjacent to the pterygoid bone. It is also possible to obtain a further reveal of each side of the cavernous sinus. The wall of the cavernous sinus is retracted from the side with a suture passing through the dura alongside the motor nerve (cases 5 and 8), or by placing an automatic retractor inside the open cavernous sinus. The lateral exposure can be increased by retracting the internal carotid artery within the dura.
Grinding away the exposed portion of the dorsum of the saddle and freeing the overlying dura completes the overall exposure. The bone of the dorsal saddle is removed, including from the excavated cavernous sinus. During grinding, the internal carotid artery within the dura is distracted to both enlarge the operative field and protect the vessel from damage by the rotating drill handle. This was done in cases 1, 4, 6, and 8. It bleeds at the base of the grinding and needs to be plugged. Because too much pressure can compress the abducens nerve in the Dorello canal, careful manipulation must be done below. Grinding the dorsum of the saddle may also cause opening of the pterygoid sinus, which should be plugged to prevent nasal leakage of cerebrospinal fluid.
Results
The exposure of the medial aspect of the basilar artery is extended by grinding away the posterior bed process and part of the saddle dorsum via the cavernous sinus, enlarging the operative field and providing sufficient space to clip the aneurysm and temporarily place an aneurysm clip proximal to the basilar artery in cases 1, 2, and 6. It is also possible to see the base of the pterygoid saddle near the midline. An example is shown for case 8 before and after transcavernous sinus visualization. Pre- and postoperative angiographic images of this patient are shown. Figure 4 Illustrative reproduction based on the photographic basis of the visualization.
Angiography was performed postoperatively in all cases. All aneurysms were completely resected except for case 3. This case was subsequently treated with a residual aneurysm neck via an endovascular manipulation section. No deterioration of the residual aneurysm neck occurred during the 5-year follow-up.
The neurological outcome was good in all cases except for oculomotor function. 6 patients developed temporary ipsilateral actinic nerve palsy. In case 1, a delayed onset of incomplete palsy occurred 24 hours after surgery. All cases had complete recovery of the articulating nerve within 3 months postoperatively, and the two cases with the slowest recovery had complete postoperative articulating nerve palsy.
Two patients developed postoperative complications. In case 3, a long aneurysm clip damaged the arteriolar nerve, causing paralysis of the arteriolar nerve in the contralateral eye. The articular nerve function did not fully recover. Case 6 presented with an early postoperative cerebrospinal fluid nasal leak. Studies showed that the traffic was through the saddleback, and a hole was found in the saddleback at the level of the base of the pterygoid saddle during the transsphenoidal surgery. We patched the hole with broad fascia, but the leak reoccurred 6 months later. At this point we refilled the hole, placed a ventriculoperitoneal shunt to treat the progressively worsening hydrocephalus, and the leak was successfully stopped.
Discussion
Initially, the inferior temporal approach was used to achieve good results in the surgical treatment of terminal basilar artery aneurysms.5 Subsequently, the pterygopoint approach proposed by Yasargil15 was used to achieve satisfactory results. The advantages of the pterygopoint approach over the infratemporal approach include a better view of the main vessels and the sides of the penetrating vessels. This is because the approach is more anterior, with less traction on the temporal lobe and less damage to the motor nerve. Moreover, this is an extension of the approach used to treat the more common anterior circulation aneurysms. It is more familiar to most surgeons. However, certain terminal basilar artery aneurysms are difficult to visualize using this approach.
Basilar artery aneurysms below the level of the posterior bed process may not be reached through the standard wing point trans-lateral fissure approach. Angiography has shown14-19 that all distal basilar artery lesions are below the level of the posterior bed process.7,13 Two of the 13,15 basilar artery bifurcation aneurysms and one of the 3 supratentorial aneurysms in the surgical cases presented by Samson and colleagues were located too low to use the pterygoid approach. The angiographic findings in those cases were consistent with those of the present case.15 Of the 50 cases of basilar artery bifurcation aneurysms in Yasargil, two lesions could not be initially visualized because of posterior bed protrusion obstruction.
Those basilar artery aneurysms hidden behind the posterior bed prominence or saddle required additional exposure when using a pterygoid approach.Yasargil successfully clinched two hidden aneurysms by high-speed abrasion of the posterior bed prominence.15 Dolenc et al4 described a transcavernous sinus approach with even further expanded exposure below the posterior bed prominence. The technique reported in the modern literature is a modification of Dolenc’s surgical steps and allows the same exposure to be achieved. Using the transcavernous sinus technique, the basilar artery can be exposed below the level of the base of the butterfly saddle in the proximal midline of the dorsal saddle. It also expands the operative field into a pyramidal shape, with the top at the anterior bed protuberance and the bottom at the dorsal aspect of the saddle after abrasion. Exposure of the basilar artery below the level of the base of the pterygoid saddle requires an inferior temporal5 or skull base approach1.
The beginning step of the transcavernous sinus approach is the abrasion of the anterior bed process. In the case described above, we used an intradural approach rather than the epidural procedure used by other authors.1,2,4,16 The choice of intradural grinding of the anterior bed process is influenced by a number of factors. These factors include the possibility of intraoperative decisions regarding the need for further exposure, the ease and safety of the intradural procedure, and keeping the dura closed and airtight at the end of the procedure. The following is a discussion of these factors. The use of an intradural approach allows for the location of the basilar artery to be identified. If it is found that a transcavernous sinus approach is not required, abrasion of the anterior bed process can be avoided. If additional exposure is required, the amount of bone that can be abraded from the anterior bed process using the intradural approach is much less than that of the epidural approach. Moreover, because it can be performed under direct vision with important anatomic structures indicated, intradural grinding of the anterior bed prominence is easy to manage. These important structures include the intradural internal carotid artery, optic nerve, and anterior bed prominence. Because we have not found a higher complication rate using the intradural procedure, the dura is theoretically less obvious in protecting the intradural structures during posterior epidural bed process grinding16. Finally, the intradural approach to abrade the anterior bed prominence makes it easy to close the dura, which is densely impermeable and open only on the convex side of the brain. After the epidural procedure, Day and colleagues noted that a fascial graft to repair the dural gap may be essential.
Transient motoneural dysfunction occurred in 6 of our 8 patients during transconjunctival sinus surgery. dolenc et al4 reported 7 of 11 patients, while 3 of 10 patients by Day and colleagues1 occurred. The incidence did not exceed 506 in cases operated on using the standard pterygoid point approach,13,15 and the higher incidence in transconjunctival sinus surgery cases may be related to the increased manipulation of this nerve during removal of the top of the actinic canal.
Conclusion
Pterygoid point trans-lateral fissure trans-cavernous sinus access up to the end of the basilar artery may increase exposure at the level of the posterior bed process and its underlying vessels, and may also extend exposure at the level of the base of the pterygoid saddle. This increased exposure is most useful in the management of low basilar artery terminal aneurysms and those that are higher but require simultaneous temporary clamping of the proximal basilar artery. Aneurysms that were thought to be unreachable using the wing-point approach can be clamped with this technique. This technique makes it easier to expand the operative field compared to the standard pterygoid approach. It is unnecessary and unhelpful to sacrifice the bridging veins of the temporal lobe for the sake of visualization. Temporary postoperative ipsilateral arteriovenous nerve palsy is common in these cases.