Surgical and Endovascular Management of Ruptured Posterior Circulation Aneurysms

Introduction

Nearly 30,000 Americans suffer from aneurysmal subarachnoid hemorrhage (SAH) every year, and the overall mortality rate is 40% . About 5–15% of all intracranial aneurysms are located in the posterior circulation . The posterior circulation provides the blood supply to the medulla, pons, midbrain, cerebellum, occipital lobes, and posterior parietal and posteroinferior temporal watershed zones. A comprehensive review of the posterior circulation vasculature is beyond the scope of this chapter, but an understanding of the anatomy allows for proper pattern recognition of the characteristic aneurysmal locations and stereotypical presentations, thus facilitating successful management. This chapter will review the endovascular and surgical treatment of ruptured posterior circulation aneurysms.

Presentation and Diagnosis

The usual presentation of a ruptured posterior circulation aneurysm does not differ significantly from that of SAH caused by aneurysm rupture in other locations. Most frequently the patient presents with the “worst headache of my life.” Other associated symptoms include nausea/vomiting (77%), loss of consciousness (53%), and nuchal rigidity (35%) .

The initial diagnostic test of choice is noncontrast computed tomographic (CT) scan (Class I evidence) . Lumbar puncture is strongly recommended in cases in which the clinical suspicion for SAH is high but the CT scan result is negative. After confirmation of SAH, a noninvasive CT angiogram (CTA) of the head and neck can be rapidly performed to define the vascular anatomy (Class IIb evidence) . Axial, coronal, sagittal, and three-dimensional (3D) reconstructions are extremely helpful to define anatomic relationships, aneurysmal geometry, and direct therapeutic decisions. More rapid, less invasive techniques such as CTA and magnetic resonance angiography (MRA) have shown comparable sensitivities in detecting aneurysms, but selective catheter cerebral angiography still remains the gold standard for diagnosing cerebral aneurysms in the setting of SAH (Class I evidence) . It provides dynamic information on intracranial transit time and collateral circulation. It also has better spatial resolution to detect small aneurysms and adjacent perforating vessels. Rotational angiography allows for 3D reconstructions that are invaluable to the clinician in treatment planning. These advancements in diagnostic imaging have dramatically improved the understanding of the anatomy, thus facilitating successful treatment.

Natural History

The International Study of Unruptured Intracranial Aneurysms (ISUIA) demonstrated 5-year rupture rates of aneurysms based on their size and location . Posterior communicating and posterior circulation aneurysms, especially basilar tip aneurysms, demonstrated the highest risk of rupture. The higher risk natural history was also associated with higher treatment risk. Variables associated with poor surgical and endovascular outcomes in the treatment of unruptured aneurysms included aneurysm diameter >12 mm and its location in the posterior circulation, particularly the basilar tip .

Treatment Options

There is no consensus on the best technique, endovascular versus microsurgical, for securing all the different types of ruptured posterior circulation aneurysms. The treatment modality is generally selected based on the risk-benefit ratio—the likelihood of the most definitive obliteration of the aneurysm with the least risk to the patient. Patient-specific considerations such as clinical status, anatomy, aneurysm location, aneurysm projection, and aneurysm geometry are important determinants of treatment approach and operator experience.

After aneurysmal rupture, the main objective is to secure the aneurysm and minimize secondary injury caused by mass effect, edema, hydrocephalus, and ischemia. The surgical exposure of posterior circulation aneurysms is more challenging and has higher risk than that of anterior circulation aneurysms . Posterior circulation aneurysms require more involved skull base approaches necessitating dissection between cranial nerves, deep brainstem nuclei, and critical, tiny perforating brainstem vessels. These exposures frequently offer limited opportunity for proximal control of the aneurysm. For these reasons, at most centers, endovascular techniques are the preferred first treatment method for posterior circulation aneurysms. We will briefly review surgical and endovascular techniques in the following sections.

Surgical Approaches

The basilar bifurcation aneurysm is the most common aneurysm of the posterior circulation and accounts for 5–8% of all intracranial aneurysms ( Fig. 154.1 ). The two main surgical methods to expose the basilar bifurcation are the subtemporal approach and the transsylvian approach. The third option is a combination of the two (a half-and-half approach). Each method has its advantages, disadvantages, proponents, and opponents. Generally, the selection of approach is based on the aneurysm’s location in relation to the posterior clinoid. The transsylvian approach is better suited for high-riding aneurysms, whereas the subtemporal approach is more appropriate for aneurysms at or slightly below the posterior clinoid ( Fig. 154.2 ).