Microsurgery of Basilar Apex Aneurysms

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CHAPTER 373 Microsurgery of Basilar Apex Aneurysms

Bernard R. Bendok, Daniel L. Surdell, Arun K. Sherma, H. Hunt Batjer

Basilar Apex Project

The surgical management of basilar apex aneurysms remains one of the most challenging areas of neurosurgery. The technical demands of treating these aneurysms has inspired several generations of surgeons to push the limits of technical achievement. Advances in neuroanesthesia, cerebral protection paradigms, and critical care management have enhanced overall outcomes. Although endovascular techniques present an alternative to open surgery for select aneurysms of the basilar apex, open surgical approaches remain necessary in many cases.

Basilar Apex Aneurysm Surgery: Clinical Experience

Dr. Charles Drake shared his insight into basilar apex aneurysm surgery throughout his career in multiple publications and lectures.¹^–¹⁴ In an update on his experience in 1990, he described 545 patients who had been treated, with good outcomes achieved in 475 patients (87%).¹³ During the past 40 years, other experienced surgeons have also contributed to this challenging area of neurosurgery.¹⁴^–²⁵ This list includes well-trained young individuals with intense dedication to neurovascular surgery and sufficient experience who also can achieve successful results when treating basilar apex aneurysms.¹⁵

In a review of their experience with 303 basilar apex aneurysms, Samson and colleagues ¹⁶ demonstrated a statistical correlation between poor outcome and various factors: poor admission grade (Hunt-Hess grades IV and V), patient age older than 65 years, computed topography (CT) demonstration of thick basal cistern clot, aneurysm diameter larger than 20 mm, and symptoms attributable to brainstem compression. In a review of all series published between 1980 and 1989, Wascher and Spetzler¹⁷ found, for a total of 957 patients, that the rate of good outcome was 82.4% and that the mortality rate was 5.1%. These series included results for a heterogeneous group of aneurysm sizes, patient ages, and clinical presentations. In a report focusing on the management outcomes of 179 unruptured posterior circulation aneurysms, of which 99 were bifurcation aneurysms, Rice and colleagues¹⁸ found a 4.2% combined morbidity and mortality rate. This variability in morbidity and mortality based on clinical factors and aneurysmal morphology must be considered when comparing clip ligation with endovascular options for specific patients. Lozier and associates reported their retrospective review of the perioperative and long-term clinical outcomes in a cohort of prospectively enrolled patients.¹⁹ They identified 98 consecutively treated basilar apex aneurysms. Eighty-four of the 98 aneurysms were directly clipped. Fifty patients presented with subarachnoid hemorrhage (SAH), and 19 aneurysms were giant. The surgical morbidity rates were 19% for the entire group and 8.8% for the unruptured and nongiant subgroup. The most common complication was perforator injury. Sixty-seven percent of patients were independent at discharge and 79% at the 3-month follow-up. Ninety-three percent of the subgroup of patients with nongiant and unruptured aneurysms were independent at discharge and 100% at the 3-month follow-up. In a multivariate analysis, unruptured giant aneurysm status was found to confer a tremendous risk for poor outcome in this series.¹⁹ An important issue to consider when assessing the efficacy of a treatment modality for aneurysms is the rate of complete aneurysm occlusion that can be achieved. In a series reported by Samson and colleagues,¹⁶ postoperative angiography was performed in 246 patients. Residual aneurysm was identified in 6% (i.e., complete aneurysm occlusion in 94%). This rate of complete aneurysm occlusion is superior to the current results achieved with endovascular treatment.²⁶^–³⁰

Microsurgical Anatomy of the Interpeduncular Cistern

The technical challenges of clipping basilar apex aneurysms are related to the complex anatomy in and around the interpeduncular cistern and the depth of dissection through narrow corridors that is required to safely secure these lesions. The subarachnoid space within the interpeduncular cistern is enclosed by the clivus and posterior clinoid process anteriorly, the medial aspects of the temporal lobes and tentorial edges laterally, the cerebral peduncles posteriorly, and the mamillary bodies and posterior perforated substance superiorly. The terminal basilar artery has a normal diameter of 2.7 to 4.3 mm and lies 15 to 17 mm posterior to the posterior aspect of the internal carotid arteries (ICAs).^25,^31,³² This proximity to the ICA provided a basis for seeking a transsylvian approach to basilar apex aneurysms. A point proximal to the bifurcation of the basilar artery gives rise to bilateral superior cerebellar arteries (SCAs), which may be duplicated. The dentate nuclei are irrigated by these vessels.²⁰

The posterior cerebr alarteries (PCAs) originate at the basilar bifurcation. They are usually 2 to 3 mm in diameter. The size of the segment of the PCA from the basilar bifurcation to the junction with the posterior communicating artery (i.e., P1 segment) depends on the extent to which the posterior communicating artery contributes to blood flow in the distal PCA. A fetal PCA implies that the P1 is a vestigial band, with all PCA blood flow originating from the carotid artery.

Visualization and preservation of the thalamoperforating arteries is an essential technical nuance of basilar apex aneurysm surgery. These critical perforators arise from the posterior aspect of the basilar trunk, the proximal P1 segments, and the posterior communicating arteries. The cranial nerve most intimately associated with this region is the oculomotor nerve that traverses the space between the PCA and the SCA within the interpeduncular cistern. The membrane of Liliequist ²¹ forms an anterior “curtain” for the interpeduncular cistern. This membrane is a thick layer of arachnoid that anchors from the mamillary bodies superiorly and extends anteriorly and inferiorly before folding posteriorly to form the roof of the prepontine cistern. The basilar apex can be located above, below, or at the level of the dorsum sellae.

Surgical Strategies

Basilar apex aneurysms account for about half of posterior circulation aneurysms. Posteriorly located perforators must be protected, or disabling neurological deficits will result. Optimal surgical results and outcomes require excellent technical skill, superb knowledge of operative anatomy, and familiarity with operative nuances accumulated by Drake, Yasargil, and a generation of surgeons who followed.

The two pure approaches for basilar apex aneurysms are the subtemporal approach and the transsylvian approach. In our practice, we employ both approaches but are increasingly relying on a new modification or hybrid approach.²² We emphasize the importance of tailoring the operation to the patient’s particular anatomy. Considering the key assets and liabilities of each approach allows a more rational design of operative strategies for each patient. The pure transsylvian approach has several assets:

• Neurosurgeons are familiar with this approach because it is used for more common aneurysms and tumors.

• Proximal control is straightforward.

• Exposure of both P1 segments for temporary trapping is uncomplicated.

• Wide exposure is possible.

The transsylvian exposure also has liabilities:

• Exposure of posteriorly located perforators is difficult.

• Inspection of distal aspect of clip blade is difficult.

• Technical features make treatment of directly anteriorly or directly posteriorly projecting aneurysms very difficult.

The subtemporal approach offers the surgeon many assets:

• Proximal control is easy

• The lateral view facilitates dissection of the perforators.

• Tentorial division allows exposure of the upper one third of the clivus for low-lying bifurcations.

• Fenestrated clips can be placed with excellent visualization of the thalamoperforators.

• Exposure and clipping of anteriorly or posteriorly directed aneurysms is more feasible than with the transsylvian approach.

The subtemporal exposure also has liabilities:

• The field is narrow.

• Access to the proximal contralateral P1 for temporary trapping is poor.

• The temporal lobe can be injured in fresh SAH in poor-grade or obese patients.

• Cranial nerve III palsy often occurs postoperatively (usually transient).

• Intraoperative bleeding can be difficult to control.

Pure Transsylvian Approach

The transsylvian exposure provides excellent visualization for aneurysms with necks at the level between the middle depth of the sella turcica and a line 1 cm superior to the posterior clinoid process (Fig. 373-1). Aneurysms with necks lying inferior to the midsellar level are better approached through the subtemporal corridor or a “half-and-half” conversion with tentorial division.^23,²⁵ Extremely high aneurysms are difficult to approach but are probably best tackled through a transsylvian approach above the carotid bifurcation. The orbitozygomatic osteotomy is helpful when tackling high aneurysms because the surgeon’s line of sight can angle more superiorly. Several maneuvers, including drilling the posterior clinoid for low-lying lesions, have been described to tackle anatomic problems encountered during transsylvian exposure.^3,³³ A transcavernous approach has also been described.³⁴

FIGURE 373-1 The pterional exposure provides excellent visualization of the neck of basilar bifurcation aneurysms that originate between the midsellar depth and a line 1 cm superior to the posterior clinoid process.

(From Apuzzo MLF, ed. Brain Surgery: Complications, Avoidance and Management. New York: Churchill Livingstone; 1993.)

Operative Technique

Yasargil ²⁰ pioneered the transsylvian approach for basilar apex aneurysms. The advantages of this approach include its familiarity to neurosurgeons, the proximity of the basilar artery to the carotid artery, and the wide exposure of the interpeduncular cistern that can be achieved by opening the membrane of Liliequist. We typically approach midline basilar apex aneurysms from the right side. A left-sided approach is valid when the patient has a right hemiparesis or left third nerve palsy. Some anatomic variants also warrant a left-sided approach. For example, an aneurysm that tilts to the left side makes it difficult to dissect the left PCA off the aneurysm from the right. Occasionally, the entire basilar artery is displaced to the left, making a right-sided transsylvian approach difficult.

Positioning

Precise positioning is critical to optimize exposure of the basilar apex. Excellent positioning can be achieved with four steps after the Mayfield-Kees skull fixation device is placed with two pins on the contralateral side of the frontal bone and with a single pin placed superior to the mastoid process (Fig. 373-2). First, the head is elevated above the long access of the body to maximize venous drainage. Second, the head is rotated away from the operative side by 20 degrees. Third, flexion of the neck brings the chin toward the contralateral shoulder and ensures the plane of the floor of the anterior cranial fossa is perpendicular to the long axis of the body. Fourth, the head is extended until the maxillary eminence is well above the orbital rim.

FIGURE 373-2 Positioning for a pterional approach. A, The Mayfield-Kees skull fixation device is placed with two pins anteriorly across the midline and the single pin posteriorly to facilitate a flat, nonobstructive surgical field. The head is elevated slightly from the long axis of the body. B, Rotation of the head to the contralateral side by 20 degrees minimizes subsequent temporal lobe encroachment on the operative field. C, Flexion of the neck brings the chin toward the contralateral shoulder and ensures that the plane of the floor of the anterior cranial fossa is perpendicular to the long axis of the body. This maneuver allows maximal surgical comfort and flexibility and prevents encroachment of the surgeon’s arms and microscope into the ipsilateral shoulder. D, Extension of the head is then achieved in a somewhat exaggerated degree compared with anterior circulation surgery to minimize subsequent diencephalic retraction. The maxillary eminence is elevated well above the superior orbital ridge.

(From Apuzzo MLF, ed. Brain Surgery: Complications, Avoidance and Management. New York: Churchill Livingstone; 1993.)

Scalp Incision

The incision is begun at the zygoma and carried superiorly in a straight line for about 10 cm above the superior temporal line and then curved gently forward toward the midline (Fig. 373-3). Care is taken to ensure that the incision is within 1 cm of the tragus to avoid damage to the frontalis branch of the facial nerve. We preserve at least one branch of the superficial temporal artery. In addition to maximizing scalp healing potential, preservation of the vessel may prove useful if a revascularization procedure is needed. We retract the scalp flap with fishhooks and perform an interfascial dissection of the temporalis muscle. Using a knife, we incise the temporalis fascia from the area of the zygoma to just 1 cm below the muscle attachment to the temporal bone and then to the area of the keyhole. The knife is used to prevent shrinkage of the temporalis fascia and to facilitate complete closure. We use cautery to cut the muscle through the opening made with the knife and then peel the muscle off the temporal squama and retract it over the scalp flap with fishhooks (Fig. 373-4).

FIGURE 373-3 Scalp incision for a pterional approach.

(From Apuzzo MLF, ed. Brain Surgery: Complications, Avoidance and Management. New York: Churchill Livingstone; 1993.)

FIGURE 373-4 The temporalis fascia and muscle are incised with the cutting cautery, taking care to avoid injury to the deep neurovascular bundle in the muscle. The resultant single-layer flap is dissected from the underlying bone and retained with fishhook retractors.

(From Apuzzo MLF, ed. Brain Surgery: Complications, Avoidance and Management. New York: Churchill Livingstone; 1993.)

Craniotomy

We perform a three- or four-hole craniotomy (Fig. 373-5). The fourth hole is reserved for older patients with presumed adherent dura. The first bur hole is placed just above the zygomatic arch. The second bur hole is superior to the first bur hole and just inferior to the superior temporal line. The third bur hole is placed at the anatomic key hole. The optional fourth bur hole is placed medial and anterior to the second bur hole. A power-driven craniotome is used to incise the bone. After elevating the bone flap, we drill enough bone to expose the temporal tip and the floor of the middle fossa all the way back to the zygomatic root. This degree of temporal exposure can be critical if conversion to a half-and-half or subtemporal approach is needed. We then perform an aggressive resection of the sphenoid ridge with rongeurs and a cutting bur on an air-driven drill (Fig. 373-6). Preferably, the bony exposure is carried to the lateral aspect of the superior orbital fissure. We remove the inner table of the frontal bone for a distance of 2 cm from the bur hole at the anatomic key hole. After adequate bony exposure, we obtain hemostasis with bone wax, oxidized cellulose, and tack-up sutures. This avoids run-down later in the case.

FIGURE 373-5 Pterional craniotomy. A critical aspect of the craniotomy is a generous anterior temporal craniectomy.

(From Apuzzo MLF, ed. Brain Surgery: Complications, Avoidance and Management. New York: Churchill Livingstone; 1993.)

FIGURE 373-6 Aggressive resection of the sphenoid ridge.

(From Apuzzo MLF, ed. Brain Surgery: Complications, Avoidance and Management. New York: Churchill Livingstone; 1993.)

We then incise the dura in a semilunar fashion, with the medial limb extending to the frontal bone 1 cm below the medial edge of the craniotomy. The lateral limb crosses the sylvian fissure and extends anteriorly to a point 1 cm below the sylvian fissure at the edge of the craniectomy. At this point, for ruptured aneurysms, we place a ventriculostomy catheter using anatomic points defined by Paine and coworkers ³⁵ if the brain appears to be tight. The point of entry is the vertex of an isosceles right triangle whose hypotenuse overlies the sylvian fissure and whose sides are 2.5 cm long (Fig. 373-7). A ventricular catheter is used to puncture the frontal lobe at this point. We suture the catheter posterior to the dura and allow continuous drainage during the microsurgical dissection.

FIGURE 373-7 Reliable access to the frontal horn of the lateral ventricle may be obtained during a pterional craniotomy employing Paine’s point. The landmarks for this point involve the creation of a 2.5-cm isosceles right triangle whose anterior limb abuts on the dura overlying the sphenoid ridge and whose posterior limb touches the sylvian fissure. The hypotenuse overlies the sylvian fissure. A Silastic brain catheter is used to enter the frontal cortex perpendicularly at the vertex of the triangle.

(From Apuzzo MLF, ed. Brain Surgery: Complications, Avoidance and Management. New York: Churchill Livingstone; 1993.)

Subarachnoid Exposure

Subarachnoid dissection is done under the microscope. During the initial subarachnoid dissection, two goals are kept in mind: (1) maximizing brain relaxation by generous arachnoidal opening, allowing for egress of cerebrospinal fluid (CSF), and (2) extensive dissection of the sphenoidal portion of the sylvian fissure. CSF released from the subarachnoid space results in further brain relaxation, even after spinal or ventricular drainage and particularly when SAH has produced loculated pockets of CSF. The principle of wide and complete sylvian fissure dissection is critical because, during the procedure, it is necessary to displace the frontal and temporal lobes independently. To avoid kinking a middle cerebral artery (MCA) branch during the approach, a lateral to medial dissection is performed to expose the M2 branches, MCA bifurcation, M1 trunk, carotid bifurcation and the contents of the carotid cistern.

The dissection is begun into the opercular insular fissure about 2 cm posterior to the sphenoid ridge. We carry this dissection medially to incorporate the carotid cistern. A T-shaped incision is made in a posterolateral direction toward the insertion of cranial nerve III into the cavernous sinus. We then return to the lateral sylvian fissure and dissect the arachnoid down to the carotid bifurcation, effectively liberating the entire segment of the MCA (i.e., M1 segment). When necessary, bridging veins are coagulated and cut to facilitate the separation of the frontal and temporal lobes. In effect, this exposure allows for generous displacement of the frontal lobe without placing undue traction on the carotid artery, the anterior cerebral artery, or the MCA.

We then turn our attention to the arachnoid, which tethers the gyrus rectus to the optic nerve. Dissecting the arachnoid further frees the frontal lobe and opens the prechiasmatic cistern. At this point, the temporal lobe is usually found to be displaced posteriorly, which can result in uncal tissue herniating medially over the incisura and obstructing the view of the interpeduncular cistern. Although retraction of this tissue is an option, we have occasionally performed a subpial uncal resection with good results. Post and colleagues ³⁶ reported their initial experience using uncal resection (10 × 10 × 15 mm volume) to optimize transsylvian access to the basilar apex without sacrifice of the pretemporal veins. Their initial clinical experience included eight patients—four of whom were treated for basilar apex aneurysms. They found that no patient developed venous infarction or new postoperative seizure. Cadaver analysis revealed that there was similar exposure to the upper basilar complex; however, the additional exposure from uncal resection did increase visualization of the ipsilateral PCAs and SCAs compared with the pretemporal approach. These investigators did report that the angle of view is limited to a more superior-lateral perspective when compared with the pretemporal approach. Therefore, the transuncal approach may be less suited for very large or giant basilar apex aneurysms when proximal control of the basilar trunk is obscured by the dome of the aneurysm.³⁶ For this approach, we routinely use frontal and temporal lobe retractors. When a significant amount of posterior temporal lobe retraction is needed with or without conversion to a half-and-half or extended lateral approach, it is wise to coagulate and cut the veins that drain the temporal lobe into the sphenoparietal sinus. Rupture of one of these veins later could obstruct the view during a critical part of the procedure.

After temporal and frontal lobe retraction is optimum, we turn our attention to the three corridors that allow access to the interpeduncular cistern from this vantage point: the space between the carotid and the optic nerve, the retrocarotid space, and the area superior to the carotid bifurcation. Ideally, we dissect through each of these corridors to maximize the routes of access to the interpeduncular cistern. Individual anatomic variations may make one of these spaces more or less accessible. A medially displaced carotid, for example, makes the retrocarotid space more relevant for access. With these three windows in mind, we then identify the posterior communicating artery and dissect along its inferior surface toward its junction with the PCA. This is the safest approach to the basilar apex because it avoids injury to the anterior thalamoperforating vessels and ensures that the dissection can lead to the inferior aspect of the P1-P2 junction (the P2 segment represents the PCA from the junction with the posterior communicating artery to where the artery passes the posterior midbrain). This prevents dissection superior to the P1 segment, which could result in perforator injury and an inadvertent and premature encounter with the aneurysm. After the P1-P2 junction is defined, we turn our attention to sharply opening the membrane of Liliequist inferiorly and medially. This dissection facilitates access to the basilar trunk and proximal carotid and allows removal of subarachnoid clot.

Our dissection then turns medially along the inferior edge of P1, across the front of the basilar apex toward the contralateral SCA and the inferior aspect of the contralateral P1. The inferior aspect is favored because perforators are spared and inadvertent aneurysmal rupture is avoided. At this point, we define an area on the basilar artery that can safely and comfortably accommodate a temporary clip. Occasionally, this area is below the SCA if the space between the PCAs and the SCAs is too narrow for a clip. After this is done, attention is turned to the superior aspect of the contralateral P1, where we dissect arachnoid adhesions and free perforators that may be adherent to the neck or dome. We then do the same for the ipsilateral P1.

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