Chapter 16

The Pterional Approach to an Aneursym at the Basilar Bifurcation

Over the past few years, the pterional, or frontolateral, approach to the interpeduncular cistern has gained increasing acceptance.1–6

Surgical Approach

To visualize any targeted lesion in this cistern between the upper clivus and the midbrain, the surgeon must open a pathway between the sphenoid wing and the base of the sylvian fissure. Failure to drill the sphenoid wing and insufficient opening of the sylvian fissure under good magnification have been common causes of a surgeon’s disappointment in using the pterional approach. Figure 16.1a illustrates the relationship of the aneurysm to cerebral structures as seen through a right pterional approach. Figure 16.1b,c shows the opening of the sylvian fissure. Figure 16.1d emphasizes the lysis of the arachnoid and adhesions between the optic nerve and the base of the frontal lobe.

In Fig. 16.2, we see the pathway taken caudally. The wide separation of the right frontal and temporal lobes with visualization of the middle cerebral artery is evident. The exposure is developed between the internal carotid artery and the oculomotor nerve; the posterior communicating artery is followed (Fig. 16.2a,b). Occasionally, the exposure is between the optic nerve and the internal carotid artery (rare in my experience). The rostral pons, the superior cerebellar artery, the origin of the oculomotor nerve, and the junction of the posterior communicating artery with the P1 and P2 portions of the posterior cerebral artery (PCA) are evident (Fig. 16.2c,d).

At times, the posterior clinoid process is in the visual field. In such uncommon cases, I use a small high-speed diamond drill to remove this projection. Figure 16.3a shows the aneurysm at the tip of the basilar artery coming into view between the retracted right internal carotid artery and the oculomotor nerve. Often, the posterior communicating artery must be severed between small malleable clips (avoiding perforators) to gain an adequate view of the aneurysm (Fig. 16.3b). After the aneurysm is separated from the posterior thalamic perforators, the neck is clipped (Fig. 16.3c,d). A large aneurysm, such as that seen in the patient under discussion, often requires a long clip to be placed across the equator of the dome to collapse the aneurysm before clipping the neck. Otherwise, the clip on the neck will slip and occlude the P1 arteries.

Samson and colleagues7 and Wright and Wilson⁸ have emphasized the importance of appreciating the relationship of the posterior clinoid process to the neck of the aneurysm on the lateral arteriogram. They concluded that the fronto-lateral approach is too difficult if the neck of the aneurysm lies below the level of the posterior clinoid process. With wide opening of the sylvian fissure, however, and excellent brain relaxation, I believe it is still usually possible to deal with these low-lying basilar tip aneurysms.

Yasargil and colleagues⁶ preferred the frontolateral to the subtemporal route for the following reasons:

• There is less retraction pressure on the temporal lobe.

• The anatomy of the interpeduncular cistern is better seen (both P1 arteries and perforators) with a more frontal view of the aneurysm.

• The oculomotor and trochlear nerves are less disturbed.

• The surgeon can better treat additional aneurysms on the anterior circle of Willis at the same time.

Some surgeons have used a modified approach that combines the pterional exposure with the subtemporal, usually resulting in posterior retraction of the temporal lobe rather than a true transsylvian exposure.9,10 The supraorbitalpterional approach of Al-Mefty¹¹ provides further low-basal exposure of these lesions.

Early in my neurosurgical career, I operated on aneurysms at the basilar bifurcation through the standard sub-temporal approach described by Drake.¹² After observing the techniques of Yasargil and colleagues⁶ (who first used the pterional approach for those aneurysms), I changed to using the pterional approach because many patients had multiple aneurysms that could not all be clipped through the subtemporal approach during the same surgery. As I gained experience, I found myself more comfortable with the pterional approach and now rarely use the subtemporal approach to aneurysms in the interpeduncular fossa. In the patient described in this case, I would use the right pterional approach, operating when the patient is in good condition after the phase of vasospasm has passed.

The Cranio-orbital Zygomatic (COZ) Approach to Basilar Apex Aneurysms

• Maximize maneuverability and ergonomics for optimal dissection and clip placement with minimal brain retraction.13

The cranio-orbital zygomatic (COZ) approach achieves these tenets by providing a shorter and wider operative distance, early proximal and distal control, direct entry to the interpeduncular cistern, and multiple angles for viewing and dissecting, as well as minimizing brain retraction. An approach angle that is flat with the middle cranial fossa combined with the short, wide corridor enables the surgeon to place a hand in the middle cranial fossa for improved control (Fig. 16.4). This approach provides clear visualization of the thalamoperforators, the neck and dome of the aneurysm, the anterior circulation, and both PCAs. In addition, the clip can be placed without compromising visualization or control.

Case Presentation and Description of the Cranio-orbital Zygomatic Approach

In this chapter’s presented case, cerebral angiography with three-dimensional reconstruction delineates a broad-based aneurysm at the basilar apex (see the figures with the case presentation), and so this patient is considered high risk for endovascular treatment. We recommend that the patient undergo definitive clipping of the aneurysm through a right-sided COZ approach with removal of the posterior clinoid process and a transcavernous technique.

For this approach, the patient’s head is placed in the Mayfield headrest (with all pins placed behind the ears if possible), slightly extended, and turned 30 degrees away from the operative side. A scalp incision is made 1 cm anterior to the tragus and continued behind the hairline to the midline. This incision can be extended toward the contra-lateral temporal line for additional exposure, and should be superficial to the pericranium and temporalis fascia. The subcutaneous flap is dissected sharply from the pericranium, and dissection is carried anteriorly to the prominence of the superior orbital rim. Further dissection in this plane is done posterior to the skin incision. A bicoronal incision is then made in the underlying pericranium, from temporal line to temporal line, based on the supraorbital blood supply. The large vascularized flap is then elevated, and the supraorbital arterial supply is preserved along the bilateral superior temporal lines. The supraorbital nerve should be identified along the medial third of the superior orbital notch. It may exit either through a notch or a true foramen. If through a notch, the nerve can be safely dissected free. If through a foramen, an osteotomy must be done around the foramen and the nerve dissected free with its bony attachment. The periorbita is detached from the superior and lateral walls of the orbit and the scalp flap is reflected anteriorly.

To protect the frontotemporal branches of the facial nerve, the fat pad containing these branches is elevated with the superficial and deep layers of temporalis fascia. At a point 1.5 cm posterior to the lateral orbital rim, a 2-cm incision in the superficial and deep temporal fascia is made parallel with the zygomatic arch. In subperiosteal fashion, these layers are reflected with the fat pad and skin flap. The zygomatic arch is also dissected along its length in subperiosteal fashion. It is then sectioned obliquely at the malar eminence and the root of the zygoma. The temporalis muscle is incised posterior to the superficial temporal artery, elevated with subperiosteal dissection, and reflected downward.14

Bur holes are placed at the base of the zygoma and at the keyhole, which should expose both the dura and periorbita. An osteotomy is made on the lateral orbital rim and extended to the keyhole. The next cut extends from the zygomatic bur hole along the base of the temporal fossa to the sphenoid ridge. The sphenoid ridge is then removed with the high-speed drill, and this cut is connected to the keyhole. An osteotomy is then made from the temporal bur hole to the frontal bur hole and extended into the orbital roof. Next, an osteotomy is done from the keyhole through the orbital roof, extending medially. The skull flap can then be elevated, and the remaining portion of the roof and lateral orbit are removed in one piece with additional osteotomies. The first cut is in the roof of the orbit and extends posteriorly along but not into the ethmoid sinuses. The second cut is made at the base of the lateral wall of the orbit along the inferior orbital fissure. These cuts are then connected across the lesser sphenoid wing and the flap is elevated¹⁵ (Fig. 16.5). The osteotomies can be tailored to individual aspects of the patient’s anatomy and based on the surgeon’s experience. They can be done as described above, or the orbitozygomatic osteotomy can be taken as a single piece or as two pieces (orbit and zygoma).

The medial aspects of the sphenoid wing are removed with the drill, which unroofs the superior orbital fissure. The optic canal is located medial to the superior orbital fissure at the apex of the orbit. It is opened extradurally and the optic strut is drilled, allowing for an extradural removal of the anterior clinoid process (Fig. 16.6). The dura is then opened with a curvilinear incision and reflected anteriorly. The surgeon can gain additional working space by depressing the orbit with anterior dural traction and fixation to further flatten the orbital trajectory.^16,17

The sylvian fissure is widely dissected, allowing the temporal lobe to fall laterally and posteriorly. The tethering veins running from the sylvian fissure to the sphenoparietal sinus are dissected and preserved. The lateral and medial carotid cisterns, the lateral wall of the cavernous sinus, and the edge of the tentorial incisura are then exposed. The entrance of the third nerve into the cavernous sinus must be identified and will guide further dissection. Untethering the third nerve and opening the oculomotor foramen provides additional exposure and facilitates safe retraction of the nerve (Fig. 16.7). Dissection is done along the third nerve toward the brainstem, and the origins of the PCAs and superior cerebellar arteries are identified. At this point, proximal control of the distal basilar artery should be established by identifying a “landing zone” for the application of a temporary clip. The posterior communicating artery is dissected from its origin on the carotid to the junction of the PCA. Beneath the posterior communicating artery, the interpeduncular cistern is approached obliquely from an anterior-to-inferior direction, and the posterior communicating artery is followed back to the PCA and the aneurysm. The superior aspect of the aneurysm is further dissected anterior to the third nerve in the interpeduncular cistern (Fig. 16.8). The back wall of the aneurysm and the thalamoperforators may be dissected by working laterally and beneath the third nerve.

This exposure is typically sufficient for high-lying and normal basilar apex aneurysms; for low-lying aneurysms, additional exposure may be necessary. Skeletonizing and removing the posterior clinoid process may create further anterior and lateral exposure (Fig. 16.9). Exposing the proximal basilar artery for temporary clipping is possible at the level of the fifth nerve; cutting the tentorium behind the fourth nerve facilitates this by allowing for further inferior visualization^13,16 (Fig. 16.10).

These maneuvers provide a wide exposure of the interpeduncular cistern with multiple viewing angles, allowing the surgeon to view the top of the basilar artery, the posterior communicating artery, and both PCAs. Clips may be applied in a lateral-to-anterolateral arc (Fig. 16.11). If projecting dorsally, the aneurysm may be difficult to clip from this arc and a more lateral trajectory can be obtained by working through the ambient cistern. The COZ approach allows for this with minimal retraction of the temporal lobe.¹⁶

The COZ approach, as described here and with variations by others,13,14,16,18–20 provides distinct advantages in the clipping of basilar tip aneurysms as compared with the pterional and subtemporal approaches. The lateral sub-temporal approach to basilar apex aneurysms provides the advantage of exposure to the posterior aspect of the aneurysm and to the thalamoperforators, but exposure of the apex and the contralateral P1 is limited. In addition, temporal lobe retraction is necessary and this approach may not be feasible in the setting of subarachnoid hemorrhage and cerebral swelling. The pterional approach provides access to the apex and contralateral P1, but limits the exposure to the back wall of the aneurysm. The trajectory is long, and aneurysms placed both high and low are not well visualized because of the one-directional, envelope-shaped field. The COZ approach provides both superior and posterior access and increases the number of viewing angles, fundamentally providing both the transsylvian and sub-temporal avenues.^13,16 The wide exposure of the interpeduncular cistern allows access to the apex of the aneurysm, the carotid artery, and the posterior communicating and PCAs.^13,17 Temporary clipping distal or proximal to the superior cerebellar arteries is also possible (Fig. 16.8). The flat working area along the middle cranial fossa exposes the neck and the thalamoperforators. Untethering the third nerve and drilling the posterior clinoid process opens up additional working angles and access to a low-lying bifurcation.^16,18

The essential advantage of the skull-base approach is a shorter, wider corridor. This corridor provides the surgeon with greater visualization, maneuverability, control, and ergonomic advantages at the deepest and most delicate points of dissection. The additional working angles ensure safer clipping with better visualization of the thalamoperforators and the contralateral P1.^13,15–17 Advancements in endovascular surgery have resulted in fewer basilar tip aneurysms treated through clipping. As a result, the aneurysms approached surgically are often complicated and present unique challenges, such as a large size, wide neck, and difficult projection. The advantages of the COZ approach are most profound in the surgical management of these complicated aneurysms, when both anterior and lateral dissection is necessary for safe and successful clipping.