Aneurysms of the Basilar Artery
Basilar Apex Aneurysms
Basilar apex aneurysms should be treated by experienced aneurysm surgeons. Today, the majority of basilar apex aneurysms can be managed endovascularly with primary coiling or a stent-coil construct. Those aneurysms not amenable to coiling generally represent surgical challenges; they tend to have wider necks and often incorporate a large portion of the basilar apex as well as one or both P1 origins. The decision to treat a basilar apex aneurysm surgically is a serious one and should not be undertaken lightly. More so than at any other location, surgical treatment of a basilar apex aneurysm leaves no room for error. Occlusion of even the smallest perforator can result in devastating consequences to the patient. Although imperfect in terms of durability, endovascular options have generally resulted in dramatically improved neurological outcomes compared with open surgery, as most surgeons are not able to clip apex aneurysms reliably without incurring significant morbidity for their patients.
When surgery is planned, the choice of approach will depend on surgical experience, personal preference, and aneurysm orientation. Options include the subtemporal, pterional, and half-half procedures, which can be augmented by skull base dissections such as an orbitozygomatic (OZ) osteotomy. We have included several videos of basilar apex lesions because of their relative rarity, including examples of each of the above approaches . Adequate brain relaxation and a certain amount of intraoperative flexibility are important so that the surgeon is able to adapt to the specific anatomy encountered. Apex perforators must be preserved without exception, and temporary clipping of the distal basilar artery (BA), either below or immediately above the origins of the superior cerebellar arteries (SCAs), is an important tool to soften larger aneurysms that fill a tight working area. Note that the author has no experience with the so-called “transcavernous” approach to the basilar apex, as we have preferred to avoid opening the cavernous sinus in this setting.
In the setting of a serious subarachnoid hemorrhage (SAH), some form of cerebrospinal fluid (CSF) drainage is mandatory when approaching a basilar apex aneurysm. We generally prefer a ventricular drain, although a spinal drain is a reasonable alternative, assuming the fourth ventricle is not occluded by fresh clot. Early in our experience, when treating an unruptured BA lesion, we always used a lumbar drainage catheter. More recently, we have found that aggressive opening of the basal cisterns will generally provide adequate relaxation if one is patient at the beginning of the operation. When utilizing a true subtemporal approach, the use of a spinal drain remains an important surgical adjunct.
The Pterional Approach
The various approaches to the basilar apex carry advantages and disadvantages ( Fig. 7.1 ). The main advantage of the pterional approach is its familiarity to the aneurysm surgeon. Since it is used for the treatment of almost all anterior circulation aneurysms, the aneurysm surgeon should be extremely comfortable working through this familiar trajectory. We have found that aggressive drilling of the anterior clinoid process and opening of the falciform ligament allow the surgeon to mobilize the optic nerve, increasing the working angle to the basilar apex. It is striking how even a small amount of increased working room can dramatically change the situation from an almost impossible one to a manageable challenge.
By opening the Sylvian fissure widely, the surgeon can untether the attachments of the medial temporal lobe. Mobilization of the medial temporal lobe will enable the surgeon to work past the internal carotid artery (ICA) along the course of the posterior communicating artery (PCommA) and anterior choroidal artery (AChA) to reach and open the membrane of Lilliquiest. At this point, the surgeon will have to decide the best trajectory to the basilar apex. This will depend heavily on the local vascular anatomy. Most often, the space between the ICA and third nerve will provide the best exposure ( Figs. 7.2–7.4 ). In some cases, the anatomy affords a generous working angle between the optic nerve and ICA ( Fig. 7.5 ). Rarely, the surgeon can work over the top of a short supraclinoid ICA, but in such a circumstance, one must diligently protect all medial lenticulostriate perforators.
The main disadvantage of the pterional approach is the limited working angle to the basilar apex. In some cases, the exposure can be improved by dividing the PCommA judiciously between anterior thalamoperforating arteries. Because the PCommA represents an important potential source of collateral supply to the upper BA, we have avoided using this maneuver when the PCommA is large. It should be noted that low-lying aneurysms of the basilar apex will be hidden by the posterior clinoid process and can be almost impossible to treat using a pterional approach unless one is prepared to remove the posterior clinoid. In addition, the surgeon must work past several normal critical structures, and when focused intently on the deep anatomy within the interpeduncular cistern, it can be challenging to avoid their inadvertent damage. In particular, the literature bears testament to cases of carotid laceration, optic nerve injury, and third-nerve contusion during the treatment of basilar apex aneurysms.
When the M1 is short, it can be difficult to mobilize the medial temporal lobe adequately, and the anterior temporal artery can be easily stretched or even avulsed during deep retraction of the temporal pole. Finally, the pterional approach gives the surgeon a relatively anterior view of the basilar apex. For more narrow-necked lesions, this may be adequate, but when the aneurysm has a very broad base in the coronal plane, it can be difficult to fully visualize the apex perforators hidden behind the aneurysm dome. In such cases, Samson and Batjer have advocated reflecting the entire aneurysm from its bed, much as we do with ICA bifurcation aneurysms. Unfortunately, for all but the most experienced of aneurysm surgeons, the idea of reflecting a large basilar apex aneurysm out of its bed deep within the interpeduncular fossa is a daunting task with potentially devastating consequences.