39 Basilar Artery Apex Aneurysms
Abstract
Basilar apex (BA) is the most common location for posterior circulation aneurysms. BA location and aneurysms size are predictors of hemorrhage. The anatomical location of BA aneurysms in the interpeduncular fossa between the cerebral peduncles, under the floor of the third ventricle and surrounded by perforators, makes their surgical approach difficult. Symptomatic patients present with subarachnoid hemorrhage, brainstem, or third ventricle compression. Digital substraction angiography with 3D reconstruction is the gold standard for BA aneurysms visualization and characterization. Due to the high-risk natural history, conservative management is reserved for extreme cases. Nowadays, endovascular is commonly the first option in treatment for posterior circulation aneurysms, but because of the high recurrence rate, other endovascular techniques are necessary. Microsurgical clip ligation is still a preferred alternative for certain aneurysms including small rupture aneurysms and aneurysms causing mass effect. Appropriate clinical and radiological follow-up is mandatory in all intracranial aneurysms.
Introduction
Posterior circulation aneurysms represent about 20% of all intracranial aneurysms. The basilar artery (BA) apex is the most common location of aneurysm formation in the posterior circulation. They lie in the interpeduncular cistern and can grow undetected until they either rupture or cause symptoms due to compression of the brainstem. Data from the International Study of Unruptured Intracranial Aneurysms (ISUIA) revealed that basilar apex location in addition to aneurysm size was a predictor of hemorrhage. Unfortunately, in addition to their high-risk natural history, these lesions are also associated with a high surgical risk due to the complexity of the anatomy of the interpeduncular cistern, including the presence of small thalamoperforators emanating from the basilar tip. Endovascular treatment of these aneurysms can also be associated with significant procedural risk and overall higher recanalization rate. Select basilar apex aneurysms (BAAs) can present challenging anatomical characteristics that make them less than optimal for endovascular management. These challenges are primarily related to neck morphology. Excellent technical skill as well as a deep understanding of the three-dimensional anatomy of the region is required for an optimal outcome and low morbidity.
Major controversies in decision making addressed in this chapter include:
Hemodynamic factors involved in aneurysm formation and rupture.
Whether or not treatment is indicated.
Open vascular versus endovascular treatment for ruptured and unruptured BAA.
Risk factors and influence on treatment decision.
Flow-diverting and neck reconstruction endovascular technology and its risks/limitations.
Whether to Treat
The question whether to treat or not is a complex one. Before deciding on treatment, the natural history must be weighed against the risk of treatment. According to the data from the ISUIA and ISUIA 2, posterior circulation aneurysms have the highest risk of rupture of any aneurysm location with the 5-year cumulative rupture rates ranging from 2.5% for asymptomatic aneurysms less than 7 mm in size with no prior history of subarachnoid hemorrhage (SAH) up to 50% for giant aneurysms (≥ 25 mm). In a multivariate analysis of the data, in addition to aneurysm size, basilar tip location was the highest predictor of hemorrhage ( 1 , 2 in algorithm ). Morphological risk factors, genetics, and smoking push this risk up further. With the advances in technology and techniques, the safety and efficacy of treatment have improved. While the neurovascular community has been consumed with a debate regarding the relative merits of clipping and coiling, the growing number of options and tools has allowed for safer treatment in experienced hand and at high volume centers. Proper patient and aneurysm morphology selection is critical to success. In the vast majority of unruptured, the high-risk natural history in addition to the availability of various treatment modalities and options warrants serious consideration for treatment. Giant BAAs, specifically those presenting with brainstem compression, are notoriously difficult to treat. Flow diversion and BA occlusion (with or without bypass) may be viable options for these daunting lesions. For ruptured aneurysms, temporizing the patient with even partial coiling is a tactic that should be carefully considered.
Anatomical Considerations
The BA is formed by the convergence of both vertebral arteries. It ascends anterior to the brainstem and terminates in the interpeduncular cistern where it bifurcates into the posterior cerebral arteries (PCAs). Understanding the 3D anatomy of the interpeduncular cistern and the neurovascular structures that it contains is paramount to good treatment outcomes. Anterior to the BA is the clivus and the posterior clinoid. It is limited superiorly by the mammillary bodies, the perforated substance and the floor of the third ventricle. Laterally, it is bound by the mesial edge of the temporal lobe and the tentorial incisura. The cerebral peduncles are posterior to it. The length of the BA is variable and its course can be either straight or very tortuous. The BA can extend as high as the mammillary bodies or can be low lying at the level of the pontomesencephalic junction. The importance of the position of the basilar apex lies in the fact that it determines the surgical approach if BAAs were to be microsurgically treated. The position also influences the angle between the aneurysm and PCAs which can influence stenting strategies.
The top of the BA gives rise to the superior cerebellar arteries (SCAs) and to the PCA. The P1 segment of the PCA can be atretic with the blood flow to the PCA being supplied by the posterior communicating artery (PCoA). This anatomical variant is termed Fetal PCA. Perforators emanate from the basilar trunk, the basilar apex, the P1 segments of the PCA, as well as from PCoAs. The oculomotor cranial nerve emerges from the midbrain into the interpeduncular cistern and travels between the SCA and the PCA lateral to the PCoA until it pierces the dura and enters the cavernous sinus. The oculomotor nerve can be easily injured during microsurgical dissection when approaching a basilar tip aneurysm.
Most intracranial saccular aneurysms are known to form at bifurcations, and the basilar apex follows this pattern. As the blood flow from the BA bifurcates into the bilateral PCAs and SCAs, there is a decrease in the kinetic energy and blood flow velocity that occurs at the apex, leading to progressive weakening of the vascular wall and therefore predisposing to aneurysm formation. Wall shear stress (WSS) is defined as the tangential frictional force of the column of blood flowing against the vessel wall. Abnormal WSS, both low and high, predisposes to vessel inflammation and remodeling. High WSS has been associated in multiple studies with loss of the internal elastic lamina, thinning of the media, and bulge formation, all of which are precursors to aneurysm formation. Low WSS on the other hand induces a cascade of proinflammatory reactions leading to vessel wall degradation and endothelial dysfunction (apoptosis and proliferation) and aneurysm growth. High bifurcation angle has been shown to decrease WSS at the basilar apex, thus predisposing to aneurysm growth. In addition, a small BA diameter leads to higher blood flow velocity and increases hemodynamic stress at the apex level. A direct cause–effect relationship between bifurcation angle and aneurysm formation has not yet been proven and the question is whether the wide angle is a result of the growing aneurysm or is it truly a risk factor of formation. It is also important to note that the relationship of vascular geometry and hemodynamics is very complex and the interplay of the laws of physics that govern these variables is very difficult to narrow down to a model. Other variables that have not been as extensively studied might yet prove to be integral to the pathophysiology (e.g., intra-aneurysmal flow pattern and angle between aneurysm and parent vessel).
Workup
BAAs often present after they rupture with typical SAH symptoms and signs including “thunderclap” headaches, meningismus, and even coma and death. They can also present with neurological symptoms due to mass effect on the brainstem and/or the third nerve. Finally, they can be found incidentally during workup for other reasons.
Diagnosis is typically made with noninvasive vascular imaging first such as a magnetic resonance angiography (MRA) or computed tomography angiography (CTA; ▶ Fig. 39.1 ). A CTA could also help determine the location of the basilar apex vis-a-vis the clivus and therefore guide the surgical approach in case aneurysm clipping was decided. A conventional digital subtraction angiography (DSA) typically follows to better understand the 3D anatomy of the aneurysm and its relation to the parent vessel. Perforator anatomy when visible should be studied on DSA imaging.
Treatment
Conservative Management
Typically, due to the high-risk natural history of BAA, conservative management is reserved for extreme cases. Patient presenting with giant BAA should be counseled on all options and risks involved. All patient-specific and aneurysm-specific variables should be taken into account including patient′s age, life expectancy, comorbidities, modifiable risk factors, family history, aneurysm neck size, perforator anatomy, and presenting symptoms (incidental vs. brainstem compression). It is important to keep in mind the notoriously poor outcome of these lesions if left untreated. In a series of 31 patients with untreated giant BAA, Dr. Charles Drake reported a mortality rate of 68% at 2 years and about 80% at 5 years. Therefore, unless untreatable due to perforator anatomy or any other complex features, it is highly advised to offer treatment for these lesions ( 1 –6 in algorithm ). On the other hand, small aneurysms (< 3 mm) with no high-risk features could potentially be followed with serial imaging with plans to treat if there are signs of growth with the caveat that posterior circulation aneurysms ( 7 in algorithm ), independent of their size, have a higher risk of rupture compared to their anterior circulation counterpart.
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