40 Posterior Cerebral Artery Aneurysms
Abstract
Posterior cerebral artery (PCA) aneurysms represent less than 2% of all intracranial aneurysms. The fetal morphology of the PCA makes it the most anatomically and morphologically variable cerebral artery. The PCA is divided into four anatomical segments and three surgical approaches. Symptomatic patients can present with subarachnoid hemorrhage (SAH), contralateral hemiparesis, and/or third nerve palsy. Computed tomography (CT) and CT angiography are the initial imaging evaluation. Digital subtraction angiography is the gold standard for aneurysm visualization and characterization. When selecting the modality of treatment of PCA aneurysms, factors such as patient′s age, clinical condition, aneurysm size and morphology, location on the PCA, and presence of SAH should be taken into consideration. Nowadays, most of the posterior circulation aneurysms are treated endovascularly; however, microsurgical clipping remains an excellent alternative treatment, and in certain cases it is the ideal treatment. Simple and small proximal PCA aneurysms can be coiled or clipped. Complex, wide-neck proximal aneurysms associated with a fetal PCA are more challenging for clip ligation and often require parent vessel occlusion and bypass reconstruction; therefore, balloon-assisted or stent-assisted coiling is a good alternative. Distal PCA aneurysms can be clipped through a supracerebellar infratentorial approach or with simple coiling; however, balloon- or stent-assisted coiling is not recommended. The outcome is variable and depends on initial clinical presentation, patency of PCA, and brainstem perforators. Appropriate clinical and radiological follow-up is mandatory in all intracranial aneurysms.
Introduction
Posterior cerebral artery (PCA) aneurysms are rare, representing 0.7 to 2.3% of all intracranial aneurysms. Few centers worldwide have clinically relevant experience with a large number of patients with PCA aneurysms. Most PCA aneurysms occur on the proximal portion of the artery, and patients with these aneurysms usually present with subarachnoid hemorrhage (SAH). Management of PCA aneurysms must preserve circumflex arteries as well as direct perforating arteries. Exposure of the various segments of the PCA requires a flexible assortment of surgical and endovascular skills.
Major controversies in decision making addressed in this chapter include:
Whether treatment is indicated.
Open versus endovascular treatment for ruptured and unruptured PCA aneurysms.
Endovascular parent vessel occlusion (PVO) as a treatment option, and the risk of visual deficit.
When an advanced surgical technique (bypass) should be considered.
Whether to Treat
Posterior circulation aneurysms, including PCA aneurysms, are considered high-risk aneurysms. As with other intracranial aneurysms, the risk of rupture of a PCA aneurysm increases with the size and history of SAH from a previous aneurysm. Of the 4,060 patients with unruptured intracranial aneurysms in the prospective cohort of the International Study of Unruptured Intracranial Aneurysms (ISUIA) trial, 345 (8.5%) had aneurysms of the posterior communicating artery (PCoA). Of these, 246 (71.3%) did not receive treatment, 84 (24.3%) underwent open surgical repair (i.e., clipping), and 15 (4.3%) underwent coiling. In patients with posterior circulation aneurysms (including PCA, vertebrobasilar, and PCoA aneurysms) who did not undergo surgery or endovascular repair, the 5-year cumulative rupture rate for aneurysms less than 7 mm was 2.5% in patients with no previous SAH and 3.4% in those with previous SAH from other aneurysms: for aneurysms sized 7 to 12 mm, 14.5%; for aneurysms sized 13 to 24 mm, 18.4%; and for aneurysms larger than 25 mm, 50%. Risk of rupture was higher in posterior circulation aneurysms than in internal carotid artery (ICA), anterior communicating or anterior cerebral artery, or middle cerebral artery aneurysms. A prospective Japanese study found that for posterior circulation (basilar tip and basilar) aneurysms, the rate of rupture per aneurysm per year was 0.23 (95% confidence interval [CI]: 0.03–1.61) for aneurysms sized 3 to 4 mm, 0.46 (0.06–3.27) for those sized 5 to 6 mm, 0.97 (0.24–3.89) for those sized 7 to 9 mm, 6.94 (3.74–12.9) for those sized 10 to 24 mm, and 117.82 (16.60–836.43) for aneurysms 25 mm or larger.
Unless the patient is moribund or very elderly, all ruptured posterior circulation aneurysms, including PCA aneurysms, should be treated ( 1 in algorithm ). Endovascular treatment should be considered as the first-line treatment. Because of the relatively high risk of rupture compared with that of anterior circulation aneurysms, the majority of unruptured PCA aneurysms should also be treated ( 2 in algorithm ). For unruptured aneurysms, their size and morphology as well as the life expectancy of the patient are the primary determinants of whether treatment should be undertaken.
Conservative Management
Patients with unruptured PCA aneurysms found incidentally should be counseled regarding treatment options. Age, family history, medical condition, aneurysm size, multiplicity of aneurysms, and the preferences of the patient are variables to consider. The option of conservative management should be offered to patients with small aneurysms (<3 mm), taking into consideration that a risk of rupture still exists for these small aneurysms ( 2 in algorithm ).
Anatomical Considerations
Embryologically, PCAs form in the fetal brain at a late stage. Several embryonic arteries located distally on the PCoA fuse to form the PCA. This fetal morphology, in which the PCA is primarily supplied by the PCoA, frequently persists, making the PCA the most morphologically variable cerebral artery. Different types of PCA morphology are defined by the contribution of the proximal PCA and the PCoA to the distal PCA. The transitional type receives equal contribution from the proximal PCA and the PCoA. The fetal type is mainly supplied by the PCoA, whereas the adult type is supplied by the proximal PCA. The morphological type of PCA is important when considering proximal control, collateral perfusion, and the potential need for revascularization in treating PCA aneurysms.
The PCA is divided into four segments: P1, extending from the origin of the PCA to the intersection with the PCoA; P2, from the PCoA to the posterior margin of the midbrain; P3, from the posterior margin of the midbrain to the anterior limit of the calcarine fissure; and P4, consisting of the remainder of the distal PCA to the calcarine cortex (▶ Fig. 40.1 ). The PCA can also be divided into three segments on the basis of the surgical approach: S1, anterior; S2, middle; and S3, posterior. The S1 segment extends from the basilar bifurcation to the most lateral aspect of PCA and is usually approached by pterional, orbitozygomatic (OZ), or subtemporal craniotomy. The S2 segment proceeds from the lateral-most aspect of the PCA to the collicular point and is treated through a subtemporal craniotomy or supracerebellar infratentorial approach. The S3 segment extends from the collicular point to the distal branches in the calcarine sulcus and is accessed through an occipital interhemispheric or a supracerebellar infratentorial approach.
In a Finnish study of 81 patients with 93 PCA aneurysms, the most frequent location was the P1 segment (n = 39), followed by the P1/P2 junction (n = 25). Of the 93 aneurysms, 69 (74%) were saccular and 24 (26%) were fusiform. Unruptured saccular aneurysms were the most common (n = 48, 52%). The diameter of the parent PCA was related to rupture status; the mean diameter was larger for unruptured than for ruptured aneurysms. An angiographically visible branch originating from the aneurysm base or the actual aneurysm was found in 59% of the saccular aneurysms. The aneurysm dome projection was typically upward for aneurysms in the P1 segment (67%), anterior or upward for the P1/P2 junction (80%), lateral for the P2 segment (67%), and posterior for the P3 segment (50%). Fifteen of the 81 patients had fetal configuration of the PCA; 10 patients had PCA aneurysms associated with an arteriovenous malformation, and all of these were considered to be flow related.
Workup
Clinical Evaluation
SAH is the most common clinical presentation of a PCA aneurysm. Patients with saccular proximal PCA aneurysms commonly present with contralateral hemiparesis and third nerve palsy. Other clinical presentations include visual disturbances secondary to direct compression or distal emboli, memory deficits, and seizures.
Imaging
A diagnostic cerebral angiography (DSA) with three-dimensional reconstruction of the aneurysm should be performed in all patients with SAH. The Alcock test, in which the dominant vertebral artery is injected while the ipsilateral common carotid artery is compressed, should be performed to evaluate for patency and characteristics of the PCoA.
Treatment
Choice of Treatment
Posterior circulation aneurysms, including PCA aneurysms, are more and more frequently being treated by endovascular methods (▶ Fig. 40.2 ) ( 3, 7 in algorithm ). Although this approach has become the rule in some centers, other centers continue to treat both ruptured and unruptured PCA aneurysms with microsurgical clipping ( 4, 6, 9 in algorithm ). The difficulty or ease of PCA aneurysm treatment is related to aneurysm size and configuration, location on the PCA (proximal vs. distal), presence of SAH, and experience of the surgeon or interventionist. Simple and small P1 segment and P1/P2 junction aneurysms can be coiled ( 3, 7 in algorithm ) or clip ligated ( 9 in algorithm ). Complex, wide-necked P1 segment and P1/P2 junction aneurysms with an associated fetal-type PCA could be more challenging for clip ligation and often may require PVO with extra-cranial-to-intracranial (EC-IC) bypass ( 4, 6 in algorithm ). For unruptured complex aneurysms located proximally on the PCA with an associated fetal-type PCA, a good alternative is balloon-assisted or stent-assisted coiling ( 5 in algorithm ). Aneurysms on the P2 segment or the distal PCA can undergo microsurgical clip ligation either through a subtemporal or a supracerebellar infratentorial approach (▶ Fig. 40.3 ) ( 9 in algorithm ). Primary coiling is another alternative; however, balloon-assisted or stent-assisted coiling is not recommended because of the relatively small size of the distal PCA. In more complex or fusiform distal PCA aneurysms with collateral pathways, PVO is a good option ( 10 in algorithm ).
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