Aneurysms of the Basilar Apex

General

Despite being the most common aneurysm of the posterior circulation, these lesions are distinctly uncommon in the general patient population, a rarity that means relatively few surgeons will develop a large-volume basilar aneurysm experience, especially in the early years of their practice. Furthermore, because the morbidity and mortality associated with the surgical treatment of these lesions are undisputedly high, they are increasingly referred for endovascular treatment as the initial option, which further reduces the volume of surgical experience and ultimately the availability of surgical expertise.

Due to this relative paucity of surgical volume, rather than experiment with a broad variety of approaches, as one might do with aneurysms in “easier,” more common locations, basilar apex aneurysm surgeons typically select a single operative approach that fulfills their needs on most occasions, and they subsequently develop minor modifications to more or less meet the exigencies of special situations. As a result, the literature is replete with numerically small surgical series compiled by individual surgeons who document excellent clinical outcomes, each dedicated to one of a variety of the recognized operative approaches. These generally range from subfrontal through transsylvian and pretemporal to subtemporal, and are often further distinguished as orbitozygomatic, transzygomatic, extreme lateral, transcavernous, and transtentorial.

A natural but unfortunate end product of this surgical selection process is the evangelical advocacy of one subspecies of “customized” operative approach, usually by the “customizer” himself. Although individual enthusiasm for personal innovation is understandable, the broad range of exposures currently in use by excellent technical surgeons is practical proof that there is no single “best” way to manage aneurysms located at the apex of the basilar artery. Young aneurysm surgeons should acknowledge this fact of life, focus their efforts on becoming facile with one standard operative approach to the extent that they recognize its limitations and are cognizant of modifications that will extend its application. For the purposes of this chapter, that standard exposure is the routine “pterional” or frontotemporal, transsylvian exposure.

Rationale for Use of the Transsylvian Approach

The initial, and by far the largest, successful surgical series detailing treatment of these aneurysms was that of Dr. Charles Drake, who employed the subtemporal exposure almost exclusively. Although the results of Dr. Drake’s extensive personal experience remain awe inspiring, most cerebrovascular surgeons were unable to even approach his level of success using this exposure, in part due to Drake’s own technical expertise and in part due to the unfamiliarity of the approach.

With the popularization of microsurgical techniques, the transsylvian exposure rapidly became the most frequently used operative approach to all common aneurysms of the anterior circulation, and very quickly Professor M. G. Yasargil pioneered its extension to lesions of the distal basilar artery during his revolutionary surgical experience at the University of Zurich. Yasargil noted that the basilar apex lay only some 12 to 15 mm deep to the carotid cistern, and that in most situations wide opening of the interpeduncular cistern provided excellent access to the distal basilar artery and the origins of both the superior cerebellar arteries and the posterior cerebral arteries.

An additional advantage of this operative exposure proved to be its unparalleled simultaneous visualization of all four terminal branches of the basilar artery; the corresponding drawback of this approach is the surgeon’s inability to view the dorsal aspect of the basilar bifurcation, and the critically important perforating arteries located there, without first mobilizing aneurysms arising from the basilar bifurcation. Refinement of Yasargil’s original approach has reduced, but not eliminated, this drawback.

Positioning

Laterality

Because the basilar apex generally lies in the midsagittal plane, it can usually be adequately exposed from either side. Most right-handed surgeons, when operating on right-handed patients, choose to employ a right frontotemporal approach through the right sylvian fissure. Even some outstanding left-handed surgeons prefer right-sided approaches when operating on left-dominant individuals because of the lower neurological morbidity associated with manipulation of the nondominant brain and its blood supply.

This right transsylvian exposure allows the surgeon to retract the internal carotid artery anteriorly with the nondominant or “sucker” hand while displacing the temporal tip posteriorly with a self-retaining retractor. This means the surgeon’s dominant hand is free for dissection, and that only intermittent retraction/stenosis/occlusion of the carotid artery occurs.

When using a left-sided approach, the surgeon’s dominant hand must simultaneously retract the carotid and manipulate the dissecting instruments, a combination that is somewhat more cumbersome and usually provides less adequate visualization than that provided by the contralateral approach. Some surgeons advocate using a self-retaining retractor blade routinely to displace the internal carotid artery (on either side), and on rare occasions we’ve found this maneuver necessary to obtain adequate visualization; however, if this can be avoided, the risk of inadvertent middle cerebral artery (MCA) ischemia or iatrogenic carotid injury is definitely reduced. Left-sided exposures are indicated when the basilar apex is displaced to the left, when the presence of multiple aneurysms dictates a left-sided approach, when some unique feature of the anatomy (arachnoid cyst) or of the collateral circulation (fetal posterior cerebral artery on the right, small posterior communicating artery [PCOMM] on the left) suggest a left-sided approach to be preferable, or in the presence of a left third cranial nerve palsy.

Rotation, Extension, and Tilt

If the head is rotated 45 to 50 degrees around the long axis of the body, the sphenoid ridge and underlying sylvian fissure will be approximately perpendicular to the horizontal meridian (floor, ceiling, horizon, etc.). This orientation will equalize retraction on the frontal and temporal lobes and maximize the light transmitted to the depths of the exposure. Less rotation (30 degrees) superimposes the optic nerve and carotid artery directly above the basilar apex, necessitating more retraction on these structures or mandating an approach between the nerve and artery; a degree of rotation greater than 50 degrees closes the sylvian fissure by bringing the temporal lobe anteriorly and transforms the oblique view of the distal basilar complex into a more lateral orientation, making visualization of the contralateral P1 segment more difficult.

Once the head is appropriately rotated, the neck should be extended so as to drop the vertex of the skull, elevating the maxillary eminence above the level of the orbital rim. This step is critical to maximizing the beneficial effects of gravity on the frontal and temporal lobes and minimizing the degree of brain retraction necessary to expose the interpeduncular cistern.

From this position, the nonoperative ear should be tilted slightly toward the ipsilateral shoulder, which will have the effect of bringing the floor of the frontal fossa perpendicular to the body’s long axis. This, in turn, will permit the surgeon to remain in the patient’s sagittal plane throughout the procedure, avoiding the necessity to orient toward the operative shoulder, which further complicates instrument exchanges with the scrub nurse and makes it very difficult for the assistant to access the wound if necessary.

Skin/Muscle Incision

This procedure can be done through any of several scalp incisions designed for frontotemporal exposure centered on/about the pterion (Fig. 12.1). It’s important the incision begin at the level of the zygoma and terminate in the midline at the hairline; a scalp flap placed too high will not provide adequate access to the skull base and will result in excessive brain retraction. The flap can be turned either as a combined musculocutaneous flap or in two layers, with the scalp flap being elevated in the interfascial plane; the latter alternative generally maximizes the inferior extent of the bony exposure. Using the two-layer approach, once the scalp flap is reflected anteriorly to expose the orbital rim, the attachments of the underlying temporalis muscle to the outer table of the skull are cut in a horseshoe-shaped incision, leaving a cuff along the superior temporal line to facilitate closure (Fig. 12.2). Importantly, the fascial incision should extend across the anterior aspect of the frontozygomatic process. The muscle flap is then reflected inferiorly across the zygoma to be held under tension with spring-loaded fishhook retractors.

Fig. 12.1 Skin incision for frontotemporal approach to basilar apex aneurysms.

Fig. 12.2 Muscle dissection and craniotomy for basilar apex approach.

Craniotomy/Craniectomy

A relatively large frontotemporal free bone flap (~8 cm × 4 cm) is removed, extending from the midpupillary line to the squamosal portion of the temporal bone and sufficiently superiorly to incorporate the anterior aspect of remaining muscle cuff. Then a generous subtemporal craniectomy is done (Fig. 12.3). The inner table of the craniotomy defect above the orbital roof is erased with the drill, and then an aggressive removal of the sphenoid ridge performed. The ridge should be completely flattened at least to the level of the orbitomeningeal artery, and further if the residual wing rides up high into the dural reflection over the sylvian fissure (Fig. 12.4). It’s neither necessary nor advisable to remove the anterior clinoid process.

Durotomy/Ventriculostomy

The dura mater should either be stellated into the four corners of the craniotomy defect (which is helpful not only for exposure but also for epidural hemostasis) or opened in a lengthy, curvilinear fashion along the inferior margin of the craniotomy defect. The initial 2 cm of the hemispheric portion of the sylvian fissure must be visible, and if the surgeon plans to use an intraoperative ventricular puncture, an additional portion of the frontal lobe must be accessible. Evacuation of cerebrospinal fluid (CSF), by ventriculostomy or lumbar puncture, is strongly recommended in almost all cases to diminish the degree and force of brain retraction required for adequate exposure of the interpeduncular cistern. Routinely, we insert a “Payne’s point” ventricular catheter and institute CSF drainage at this stage of the procedure (Fig. 12.5).

Fig. 12.3 Burr hole placement for craniotomy.

Fig. 12.4 Extradural drill resection of sphenoid ridge.

Steps of Exposure

1. Opening Sylvian fissure-basal cisterns: A routine retrograde opening of the entire sylvian fissure from the MCA bifurcation to the internal carotid artery (ICA) bifurcation is essential to maximizing the potential of this approach. Once the entire course of the M1 segment is exposed, a self-retaining retractor blade can be placed on the posterior aspect of the orbital cortex at the level of the ICA bifurcation and the arachnoid dissection extended anteriorly into the prechiasmatic cistern, freeing the gyrus rectus from its attachments to the optic nerve. When the A1 segment of the anterior cerebral artery is completely exposed, the carotid artery can be skeletonized of its arachnoid covering, and the origins of the PCOMM and anterior choroidal artery (ACHRD) are identified. The frontal lobe retractor is then advanced over the posterior aspect of the gyrus rectus to lie immediately superior to the ICA bifurcation, where it will stay for the remainder of the procedure. Before the exposure is carried deep to the carotid artery, the temporal pole should be mobilized.