T HE direct surgical approach to an aneurysm on

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1 J Neurosurg 66: , 1987 Aneurysms of the basilar artery trunk KENICHIRO SUGITA, M.D., SHIGEAKI KOBAYASHI, M.D., TOSHIKI TAKEMAE, M.D., TSUYOSHI TADA, M.D., AND YUICHIRO TANAKA, M.D. Department of Neurosurgery, Shinshu University School of Medicine, Matsumoto, Japan u,, Ten cases of basilar artery trunk aneurysms are reported; nine were operated on by a middle subtemporal approach and one by a pterional transsylvian technique. The middle subtemporal approach facilitated clipping of aneurysms located as deep as 25 mm below the posterior clinoid process as seen on the lateral views of the angiogram, such that the junction of the vertebral arteries was visible in most cases. The authors emphasize the usefulness of direct retraction of the trigeminal nerve, the pons, and the aneurysm itself; occasionally, drilling off the pyrimidal edge is also valuable. Surgical results were satisfactory in seven cases and poor in one; two patients died. KEY WORDS 9 basilar artery subarachnoid hemorrhage 9 operative technique T HE direct surgical approach to an aneurysm on the basilar artery trunk is one of the most difficult operations to perform. The aneurysms are located in a deep, narrow region surrounded by critical structures, namely the ports and cranial nerves. Four routes of approach to the lesion have been reported in the literature: pterional, subtemporal, transoral, and suboccipital. We have operated on l 0 cases with basilar trunk aneurysms, using a pterional transsylvian approach in one case and a subtemporal technique in nine cases. A discussion of the surgical technique in these patients is presented. Illustrative Case Report This 39-year-old woman suffered a subarachnoid hemorrhage (SAH) in The vertebral angiogram showed a single aneurysm located at the junction of the right vertebral artery and the posterior inferior cerebellar artery. The aneurysm was clipped at another hospital. Mild paresis of the right lower cranial nerves and slight right hemiparesis continued after surgery. A second severe SAH caused her admission in August, Examination. Angiograms revealed that the previously clipped vertebral artery aneurysm had disappeared but that three new aneurysms arose from the trunk of the basilar artery. The largest aneurysm protruded laterally to the left, and seemed to have ruptured; a second, smaller one pointed to the right at the same level as the largest; and a third one, the smallest, was located on the proximal side of the basilar artery trunk close to the junction of the vertebral arteries. The largest aneurysm enlarged further during the 1-month interval between the second SAH and surgery. First Operation. In September, 1985, a fight middle subtemporal approach was performed with the patient in the lateral position. After lumbar cerebrospinal fluid (CSF) drainage had been instituted, the patient's head was elevated above the heart level. A U-shaped skin incision was made anteriorly close to the auricle and posteriorly 5 cm behind the auricle in the right middle temporal area. A bone flap was made, 6 cm wide and 5 cm long, and the lateral bone edge was removed as much as possible. The brain was slack. The temporal lobe was retracted gently and slowly while the lumbar CSF drain was concomitantly opened. The vein of Labb6 and all the veins on the subtemporal surface except for one small vein were preserved. 5 The tentorial incisura was cut posterior to the site where the trochlear nerve ran infratentorially. The tentorium was opened as wide as possible; the incision extended close to the transverse sinus laterally and to the pyramidal edge anteriorly. Bleeding from the petrosal and cavernous sinuses was controlled by packing with Oxycel. The arachnoid membrane of the middle portion of the pons around the fight trigeminal nerve was carefully dissected. Initially, exposure of the basilar artery trunk on the side proximal to the aneurysm was attempted. Medial retraction of the trigeminal nerve facilitated exposure of the proximal side of the basilar artery trunk, providing a space for temporary clipping of the proximal trunk, if necessary. Exposure of the distal portion of 500 J. Neurosurg. / Volume 66/April, 1987

2 Aneurysms of the basilar artery trunk the basilar artery was far easier than the proximal part. After both portions of the basilar artery had been identified, the arachnoid membrane around the aneurysms was dissected and part of an aneurysm was visualized with slight retraction of the pons. The proximal side of the aneurysmal neck under the trigeminal nerve could not be seen, even with lateral retraction of the nerve. The dorsomedial portion of the petrous ridge (2 x 1 1 cm in area) and the lateral edge of Meckel's cave were then drilled off to make it possible to shift the trigeminal nerve further laterally. Two aneurysms and the basilar artery were widely exposed. The diameter of the basilar artery was pathologically large, and the diameter of the aneurysmal neck was twice that of the parent artery. A small semiconvex aneurysm was found on the basilar artery trunk on the same side as the approach (the right), and a large aneurysm pointed mainly to the left. The semiconvex aneurysm was located close to the neck of the largest aneurysm, and the walls of both lesions were strongly adherent, making the procedure difficult: isolating both aneurysms required delicate dissection and much time. The neck portion of the largest aneurysm was rather easily isolated from the surrounding structures. The ports was directly retracted with a tapered spatula 2 mm in width, held with a self-retaining retractor attached to the Sugita multipurpose head frame. 6 Retraction of the pons was indispensable because the basilar artery and some of the aneurysms were hidden in the basilar sulcus. While the neck of the aneurysm and the distal basilar artery were retracted with two tapered spatulas and hypotension was induced at about 70 mm Hg systolic pressure for 5 minutes, a straight clip, 21 mm in length, was placed on the lesion. However, the clip slipped a little from the neck onto the body of the aneurysm when it was released from the applier. Several attempts to work the clip toward the parent artery were unsuccessful because a large portion of the neck and body was located behind the artery, which limited the direction of application. The clip was shifted a little deeper because its blades were open due to the thick wall of the aneurysm. A second bayonet clip of the same length as the first was added on the side of the aneurysm to reinforce the closing pressure. Finally, the small aneurysm on the distal basilar artery was obliterated with a miniclip, 5 mm in length, and wrapped with two pieces of Bemsheet and fibrin glue to prevent the clip from slipping. The third small aneurysm on the proximal artery could not be visualized. Comments on First Operation. Had a left subtemporal approach been chosen, it would have been impossible to clip the small aneurysm because it was hidden by the large one. However, with that technique the neck of the large aneurysm could have been snugly clipped, although isolating it would have been more difficult because the ruptured dome was pointed toward the approach side. The choice of the side from which to approach a large aneurysm is a difficult problem. The direct-retraction method applied on the parent artery and two aneurysms was a very helpful maneuver; without the retraction, twice the time would have been needed and clipping would have been more difficult. First Postoperative Course. The initial postoperative course was uneventful and without additional neurological deficits. Postoperative angiography revealed that the small aneurysm and the body of the largest aneurysm had disappeared; a small portion of the neck of the large aneurysm remained, as was expected from the operative findings (Fig. 1). Follow-up angiography 8 months after the operation showed a newly developed large aneurysm protruding from the neck portion of the previously clipped largest aneurysm and pointing downward to the left side, with enlargement of the basilar artery. Angiographic analysis of the basilar artery trunk indicated that this was a dissecting aneurysm. The patient had suffered neither SAH nor new neurological deficits in the meantime. Second Operation. In June, 1986, an operation was performed via a left middle subtemporal route with the same steps as in the previous operation. The arachnoid membrane over the pons was very thick. The anterior portion of the pons felt pathologically hard because in that area it was thin, and the large hard aneurysm was located behind it. The lesion was growing into the basilar sulcus and the thinned anterior portion of the pons had covered it. The reason for the unusual hardness of the pons and the topographical relationships were not understood until the entire aneurysm had been isolated from the pons. Initially, the distal portion of the parent artery was approached at the site where the four tips of the two clips applied in the previous operation were identified under the thick arachnoid. Next, the proximal side of the aneurysm was exposed lateral to the trigeminal nerve, which was retracted medially. Arrhythmia appeared whenever the nerve was retracted excessively. The vertebral arterial junction and a small semiconvex aneurysm close to the anterior inferior cerebellar artery were identified. A space for temporary proximal clipping was obtained, although temporary clipping was considered to be a last resort as the parent artery was probably dissecting in nature. The main aneurysm was covered with thick arachnoid membrane and thinned pons. The aneurysmal body was larger than was suspected from the angiogram, and only one-third of it was visible before retraction of the pons. The pons was isolated from the posterolateral portion of the aneurysmal body with great care. The edge of the pons compressed by the aneurysm had become so thin that it was impossible to isolate it without some damage. Two small perforating arteries were sacrificed in order to expose the entire aneurysmal body: one arose from the aneurysm and the other was tightly adherent to the aneurysmal body. Under induced hypotension of about 70 mm Hg systolic pressure, a Sugita No. 22 t_-shaped clip with a 10-mm blade length was placed on the distal side of the neck close to the clips applied at the previous operation. The residual neck of the proximal side was J. Neurosurg. / Volume 66/April,

3 K. Sugita, et al. FIG. 1. Illustrative case, first operation. Upper. Vertebral angiograms. On the lateral view (left), the aneurysm neck was located 20 mm below the posterior clinoid process. A small aneurysm is visualized (arrows, center). On the postoperative angiogram, the arrow shows the residual neck of the large aneurysm after clipping (right). A metric rule is shown. Lower: Operative sketches showing the right subtemporal approach and clipping of the aneurysm. successfully obliterated with a Sugita No. 42 angled ring clip with 7.5-mm blades. The parent artery was found to have a large diameter as the preoperative angiogram had shown. The parent artery and the small aneurysm were wrapped with three pieces of Bemsheet. Comments on Second Operation. The aneurysm was one of the most difficult in our series to treat. Two things may have contributed to the postoperative complications: sacrifice of two perforating arteries and mechanical damage to the pons due to excessive retraction, and wide isolation of the pons; however, these procedures were essential for successful clipping. It was strongly suspected that the vertebrobasilar artery in this case was pathological for the following reasons. The artery had four aneurysms and was dilated with angio- 502 J. Neurosurg. / Volume 66/April, 1987

4 Aneurysms of the basilar artery trunk FIG. 2. Illustrative case, second operation. Upper." Angiograms, anteroposterior views: preoperative study showing a small aneurysm (arrow, left), and postoperative views with findings suggestive of a dissecting parent artery (center and right). A metric rule is shown. Lower: Operative sketches showing the left subtemporal approach. A: Two-thirds of the large aneurysm is hidden by the thinned pons (dotted line). B: The pons is isolated and retracted. C: Two L-shaped angled ring clips are applied. graphic findings consistent with a dissecting aneurysm. Although the wall of the residual neck of the large aneurysm had appeared to be thick enough at the first operation, the aneurysm clipped at the second operation had developed from the residual portion within the short period of 1 year. There is a good possibility that another aneurysm will develop and the parent artery will dilate further in the future. Second Postoperative Course. Two days postoperatively, the patient was semicomatose with right hemi- plegia. She became fully conscious 1 week later and, at the time of discharge, motor function had improved and approached the preoperative state (Fig. 2). Operative Approach The aneurysms of the basilar artery trunk in nine cases were approached via a middle subtemporal route. In one case, managed at the beginning of the series, a pterional transsylvian approach was used for double aneurysms on the basilar artery trunk and superior J. Neurosurg. / Volume 66/April,

5 K. Sugita, et al. cerebellar artery as reported previously? Thus, we routinely chose a middle subtemporal approach for a basilar trunk aneurysm except for an aneurysm located distally close to the superior cerebellar artery, in which case a pterional transsylvian approach was used. In our experience, a middle subtemporal approach facilitates visualization of basilar artery trunk aneurysms located no more than 25 mm below the top of the posterior clinoid process as measured on the lateral view of the vertebral angiogram. Exposure can be obtained as deep as the junction of the vertebral arteries. Aneurysms located as far as 20 mm below the posterior clinoid process on the angiogram can be exposed from the medial side of the trigeminal nerve. Conversely, medial retraction of the trigeminal nerve was often necessary for aneurysms located approximately 25 mm below the posterior clinoid process (Fig. 3). The space along the lateral side of the trigeminal nerve, however, is narrower than on the medial side. One disadvantage of the subtemporal approach is the possibility of postoperative edema or hemorrhage in the temporal lobe. To minimize such complications, we took care to preserve the veins on the subtemporal surface as well as the vein of Labbf, and also to retract the temporal lobe intermittently. ~o Direct retraction of the trigeminal nerve, pons, and aneurysm was a useful procedure in the surgery of basilar trunk aneurysms. Each was retracted with a tapered spatula held by a self-retaining retractor attached to the multipurpose head frame. 6 The trigeminal nerve was retracted laterally in most cases and medially in some. Retraction of the nerve provided an empty space around the aneurysm. In our experience, the trigeminal nerve withstood mechanical manipulation best among the 12 cranial nerves. Retraction of the pons was another important method which was used in almost all cases. Most aneurysms pointing posteriorly could not be exposed without retracting or pushing the pons because they were located inside the basilar sulcus. Pushing a small portion of the pons around the aneurysm posteriorly with a tapered spatula made it easy to keep the lesion in sight without the unsteady and often excessive pressure of the surgeon's hand. The consistency of the pons, which could be felt by the surgeon's fingers and measured with our strain-gauge spatula, 1~ was almost the same as that of other brain tissues. In our experience, the pons had as strong a tolerance to retraction as other structures. Direct retraction of an aneurysm by a tapered spatula freed both of the surgeon's hands for dissecting, suctioning, and clipping. This was often safer than retracting an aneurysm with suction or a dissector in the surgeon's hand (Fig. 4). Drilling off the dorsomedial portion of the pyramidal bone and the lateral edge of Meckel's cave sometimes helped to displace the trigeminal nerve further and to visualize an aneurysm and the proximal portion of the basilar artery hidden behind the nerve. Another purpose of this bone removal was to visualize the lesion from the anterior side with minimal retraction of the pons (Fig. 4). Drilling off the pyramidal edge is similar to unroofing the optic canal to displace the optic nerve in the case of an ophthalmic artery aneurysm. Results Clipping of 10 basilar trunk aneurysms was successful in eight patients. In the remaining two cases, the aneurysms were wrapped because they were truly fusiform. Satisfactory surgical results were obtained in seven patients who observed no persistent neurological deficits. One patient with a poor result was a 49-yearold man who had suffered transient ischemic attacks; he had advanced arteriosclerosis in the carotid and vertebral arteries and a large fusiform aneurysm of the FIG. 3. Schematic illustration of an approachable distance from the posterior clinoid process. A (solid arrows): Route medial to the fifth cranial (Vth) nerve. B (open arrows): Route lateral to the fifth nerve. FIG. 4. Drawing to illustrate pons retraction and drilling off the pyramidal edge (cross-hatched areas) in the right subtemporal approach. Arrow indicates the direction of the approach. 504 J. Neurosurg. / Volume 66/April, 1987

6 Aneurysms of the basilar artery trunk basilar artery. A large intracerebral hematoma developed in the temporal lobe even though only partial wrapping was done with minimum retraction of the temporal lobe. One patient died on the 8th postoperative day due to infection combined with vasospasm. Another patient with a large fusiform aneurysm died due to aneurysm rupture 4 years after surgery. The details of these two deaths have been presented elsewhere. ~ Discussion Aneurysms of the posterior circulation are divided into five main groups according to their source: vertebral artery, basilar artery trunk, superior cerebellar artery, basilar artery bifurcation, and distal posterior cerebral artery. In addition, there are several unusual subgroups of aneurysms arising from the distal portion of the main branches, such as those of the posterior inferior cerebellar artery. A lesion is designated as a basilar artery trunk aneurysm when it is located on the portion of the artery between the junction of the vertebral arteries and the branching point of the superior cerebellar artery. An aneurysm immediately proximal to the superior cerebellar artery should be included in the superior cerebellar artery group. Reports of basilar artery trunk aneurysms from the viewpoint of surgical technique are rare, and the number of cases is small except for the large series reported by Drake.l Four routes of approach have been reported in the literature for basilar artery trunk aneurysms: pterional, 9 subtemporal, 1 transoral, 2"4's and lateral suboccipital. 9 Selection of the route greatly influences surgical results because the aneurysm is deep and is surrounded by critical neural tissues. We prefer the middle subtemporal approach, except for aneurysms of the distal portion of the basilar artery which are approached most often by a pterional transsylvian route and in some cases by an anterior subtemporal route. In the pterional approach, the operative field is far narrower and deeper and contains a large obstacle in the form of the posterior clinoid process. Conversely, however, an advantage of this approach is to be able to visualize both sides of the parent artery; the approach should be chosen only for aneurysms located distally, close to the superior cerebellar artery. Kawase, et al, 3 recently proposed an extradural subtemporal approach with the anterior pyramidal edge drilled off. The transoral approach has several disadvantages, including a narrow field where only a small clip can be positioned in a perpendicular direction and a high possibility of postoperative infection, although several methods for guarding against such a complication have been reported. 2,8 The lateral suboccipital approach also has some disadvantages. The pons must be retracted more than with the subtemporal approach. The facial and acoustic nerves are located in the center of the approach in the same way that the trigeminal nerve is in the center of the subtemporal route; the seventh and eighth cranial nerves are more vulnerable than the fifth. A middle subtemporal approach was chosen rather than an anterior subtemporal technique because the petrous bone edge blocks the view in the latter approach. Care should be taken to preserve the trochlear nerve. The tentorium is cut posteriorly to where the trochlear nerve is visible in the crural cistern. During the main procedure, it is useful to cover the trochlear nerve with rubber sheets because it is too vulnerable to cover directly with standard cotton patties. Before directly dissecting the aneurysm, a small space for temporary clipping of the basilar artery in case of rupture should be made both proximally and distally. This is especially recommended in the case of a large aneurysm. It is noteworthy that the pons has a strong tolerance to retraction and the trigeminal nerve is the strongest cranial nerve with regard to mechanical manipulation. The aneurysm and parent artery can be exposed with direct retraction of these structures, with the proviso that a highly controllable retraction system be used. The surgeon can then use both his hands for dissecting and suctioning without the necessity of pushing obstacles aside. Perforating arteries from the basilar trunk should be preserved whenever possible. When an aneurysm is directed posteriorly, it is particularly difficult to identify the perforating artery on the contralateral side of the lesion. References 1. Drake CG: The treatment of aneurysms of the posterior circulation. Clin Neurosurg 26:96-144, Hayakawa T, Kamikawa K, Ohnishi T, et al: Prevention of postoperative complications after a transoral transclival approach to basilar aneurysms. Technical note. J Nenrosurg 54: , Kawase T, Toya S, Shiobara R, et al: Transpetrosal approach for aneurysms of the lower basilar artery. J Neurosnrg 63: , Sano K, Jinbo M, Saito I: [Vertebro-basilar aneurysms, with special reference to the transpharyngeal approach to the basilar artery aneurysm.] No To Shinkei 18: , 1966 (Jpn) 5. Sugita K: Microneurosurgical Atlas. Berlin: Springer-Verlag, 1985, pp Sugita K, Kobayashi S, Takemae T, et al: Direct retraction method in aneurysm surgery. Technical note. J Neurosurg 53: , Sugita K, Kobayashi S, Yokoo A: Preservation of large bridging veins during brain retraction. Technical note. J Neurosurg 57: , Yamaura A, Makino H, Isobe K, et al: Repair of cerebrospinal fluid fistula following transoral transclival approach to a basilar aneurysm. Technical note. J Neurosurg 50: , Ya~argil MG: Microneurosurgery. Stuttgart: Georg Thieme Verlag, 1984, Vol 2, pp Yokoh A, Sugita K, Kobayashi S: Intermittent versus continuous brain retraction. An experimental study. J Neurosurg 58: , 1983 Manuscript received August 8, Address reprint requests to: Kenichiro Sugita, M.D., Department of Neurosurgery, Shinshu University School of Medicine, Asahi 3-1-1, Matsumoto 390, Japan. J. Neurosurg. / Volume 66/April,