Aneurysms of the posterior circulation are among the most difficult lesions encountered by cerebrovascular surgeons. Their treatment requires in-depth preoperative and intraoperative planning, and their successful obliteration requires adherence to the basic tenets of aneurysm surgery. As with the treatment of all intracranial aneurysms, these basic tenets include but are not limited to vascular control, sharp dissection, and meticulous preservation of perforating vessels, selective intraoperative angiography, and intraoperative monitoring. When possible, we prefer to clip lesions involving the posterior circulation directly. However, giant aneurysms in this location may require trapping, excision, aneurysmorrhaphy, or vessel occlusion.

Preoperative Considerations

Treatment of aneurysms involving the posterior circulation involves a thorough review of all imaging studies before surgery. In the current health care market, most patients present with computed tomography (CT) of the brain as well as magnetic resonance (MR) images.¹ Traditionally, patients subsequently undergo digital subtraction angiography to help delineate their vascular lesion. Recently, however, patients have undergone surgery based solely on MR angiography or CT angiography. Recent advances in the imaging quality of CT angiograms have made this study particularly useful in treating lesions of the posterior circulation. ² In addition to providing the standard information about aneurysm size and morphology as well as location of branching vessels, CT angiography also provides information about surrounding bony structures that is crucial in preoperative planning and in understanding the anatomy of the lesion. In the treatment of aneurysms involving the posterior circulation, a thorough understanding of the anatomy is imperative for determining the appropriate operative approach.

Cerebral Protection

At our institution, all aneurysm patients receive intravenous barbiturates (thiopental) titrated to electroencephalographic burst suppression. Barbiturates are known to reduce cerebral injury from prolonged ischemia.³ Therefore, they are most effective when administered before periods of temporary ischemia, as in the case of temporary vessel occlusion⁴. In addition, we favor slight hypertension during periods of temporary vessel occlusion, as this serves to enhance cerebral protection as well.

A ventriculostomy is placed in patients presenting with significant subarachnoid hemorrhage when they enter the institute. Spinal fluid drainage facilitates brain relaxation and helps minimize brain retraction.

Surgical Approaches to Posterior Circulation Aneurysms

We employ one of four surgical approaches when treating aneurysms of the posterior circulation: the extended orbitozygomatic approach, transpetrosal approaches, retrosigmoid approach, and extended far-lateral approach. We choose an approach based on the relation of the lesion to the basilar artery and clivus. Lesions of the posterior circulation can be placed into one of three distinct conceptual zones based on the basilar artery (Fig. 10-1). The upper basilar zone comprises the upper two-fifths of the basilar artery. The midbasilar zone represents the middle fifth of the basilar artery. Finally, the vertebrobasilar zone represents the lower two-fifths of the basilar artery and includes all lesions involving the intradural segment of the vertebral artery.

FIGURE 10-1 Three conceptual zones of the basilar artery and approaches to lesions in these zones (with permission from Barrow Neurological Institute).

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