Orbitocranial Zygomatic Approach for Upper Basilar Artery Aneurysms

Antonio Bernardo and Philip E. Stieg

Abstract

Aneurysms arising from the basilar apex are perhaps the most technically challenging aneurysms to treat microsurgically. Endovascular techniques have an important role to play in the treatment of basilar artery aneurysms. However, giant or wide-necked aneurysms and aneurysms that involve the posterior cerebral artery at its neck are not usually amenable to endovascular treatment, and direct microsurgical repair remains the appropriate treatment for patients with these lesions. Conventional approaches used to treat basilar apex aneurysms include the pterional and subtemporal approaches. The anatomy of the basilar artery and the aneurysm and its relationships to bone influence the approach to use. The relationship between the aneurysm and upper clivus (posterior clinoid process) is crucial. Aneurysms located within 5 mm of the dorsum sellae are considered typical. If they occur higher or lower than that point, they are considered high lying or low lying, respectively. High-lying basilar apex aneurysms are partially obscured by the anterior clinoid process and by the anterior petroclinoid ligament. Low-lying aneurysms are covered by the anterior and posterior clinoid processes, the dorsum sellae, and the anterior petroclinoid ligament. The orbitozygomatic osteotomy supplements the exposure achieved by the pterional craniotomy. With this approach, the basilar apex and the four vessels arising at the apex can be identified through the carotico-oculomotor triangle. We routinely use intraoperative indocyanine green videoangiography to verify obliteration of the aneurysm and patency of the posterior cerebral, superior cerebellar, and perforating arteries after clip application. Postoperative care includes neurological intensive care monitoring. A new neurological deficit after surgery is usually investigated with a computed tomography (CT) scan to rule out a hemorrhage or hydrocephalus.

Keywords: basilar apex, basilar artery aneurysm, microneurosurgery, orbitozygomatic osteotomy, anterior clinoid, upper clivus, carotico-oculomotor triangle, clipping, videoangiography

12.1 Introduction

Aneurysms arising from the basilar apex are perhaps the most technically challenging aneurysms to treat microsurgically because the exposure is deep, visibility is poor, and maneuverability is limited. Furthermore, the margin for error is slim because of the rich network of critical perforating arteries in the vicinity that irrigate the brainstem and thalamus and because of the limited proximal and distal control of the aneurysm.

12.2 Patient Selection

Endovascular techniques have an important role to play in the treatment of basilar artery aneurysms, and new methods and devices are almost continuously being brought forward. However, giant or wide-necked aneurysms and aneurysms that involve the posterior cerebral artery at its neck are not usually amenable to endovascular treatment, and direct microsurgical repair remains the appropriate treatment for patients with these lesions ( ▶ Fig. 12.1a, b).

Fig. 12.1 Cerebral angiography. (a) Anteroposterior and (b) lateral views of a large basilar apex aneurysm.

Conventional approaches used to treat basilar apex aneurysms include the pterional and subtemporal approaches. The pterional approach has the advantages of ability to attack the aneurysm from its neck, with a good perspective on the interpeduncular cistern, and the ability to create different surgical corridors. These corridors include the opticocarotid window and the carotid oculomotor window. The window between the precommunicating segment of the anterior cerebral and sphenoidal segment of the middle cerebral artery and optic tract can be used as well. Complexities arise when these lesions are hidden by surrounding anatomical structures. In these instances, conventional approaches often do not provide enough working area.

In the pterional approach, both the subtemporal and the transsylvian routes eventually permit visualization of the clival region through the same deep windows: the carotico-oculomotor window and, occasionally, the opticocarotid window. The ventral brainstem exposure through the pterional approach is limited by the anterior and posterior clinoid process and the petroclinoid folds in the caudal direction and by the orbital rims and zygomatic arch in the rostral direction ( ▶ Fig. 12.2).

Fig. 12.2 The basilar apex exposure through the surgical corridors of the pterional approach is limited by the anterior and posterior clinoid process and the petroclinoid folds in the caudal direction and by the orbital rims and zygomatic arch in the rostral direction.

The subtemporal approach offers less arachnoid dissection initially and a better view of the posterior aspect of the aneurysm and any associated perforating arteries. The required temporal lobe retraction is better tolerated when the aneurysm is unruptured and, if ruptured, in cases where acute posthemorrhage hydrocephalus can be treated at the time of craniotomy. However, in obese patients and patients with ruptured aneurysms, the subtemporal approach can be associated with hemorrhagic infarction of the temporal lobe due to retraction and/or damage to the bridging veins.

The anatomy of the basilar artery and the aneurysm and its relationships to bone influence the approach to use. The basilar bifurcation and neck of the aneurysm can be as rostral as the mammillary bodies and the floor of the third ventricle, or there can be a “low” bifurcation below the pontomesencephalic junction. The relationship between the aneurysm and upper clivus (posterior clinoid process) is crucial. Aneurysms located within 5 mm of the dorsum sellae are considered typical. If they occur higher or lower than that point, they are considered high lying or low lying, respectively ( ▶ Fig. 12.3a, b).

Fig. 12.3 Artist’s depiction of the relationship between the aneurysm and upper clivus. (a) Aneurysms located lower than the dorsum sellae are considered low lying. (b) Aneurysms that occur higher than the dorsum sellae are considered high lying.

High-lying basilar apex aneurysms are partially obscured by the anterior clinoid process and by the anterior petroclinoid ligament. Low-lying aneurysms are covered by the anterior and posterior clinoid processes, the dorsum sellae, and the anterior petroclinoid ligament ( ▶ Fig. 12.2). These aneurysms may require skull-base surgical techniques for clipping. The main principle is maximization of bone resection. This allows the surgeon to work within a wide corridor, which facilitates the use of surgical instruments and minimizes retraction of the brain.

The orbitozygomatic osteotomy supplements the exposure achieved by the pterional craniotomy. It consists of removal of the superior orbital rim, orbital roof, and zygoma with the inferior mobilization of the temporalis muscle ( ▶ Fig. 12.4). In cases of high-riding lesions, the increased angle (10 degrees) over which this region can be viewed after an orbitozygomatic craniotomy becomes crucial ( ▶ Fig. 12.5). This technique alone offers a wide angle of exposure of the basilar apex and interpeduncular and prepontine cisterns, reduces the need for brain retraction, and gives a direct line of view to the undersurface of the medial frontal lobe.

Fig. 12.4 The orbitozygomatic osteotomy consists of removal of the superior orbital rim, orbital roof, and zygoma with the inferior mobilization of the temporalis muscle.

Fig. 12.5 In cases of high-riding lesions, the orbitozygomatic craniotomy increases the angle (10 degrees) over which this region can be viewed by removal of the superior orbital rim and the orbital roof. This technique offers a wide angle of exposure of the basilar apex, reduces the need for brain retraction, and gives a direct line of view to the undersurface of the medial frontal lobe.

Only gold members can continue reading. Log In or Register to continue