Unique Features of Aneurysms by Location

Introduction

The rupture of intracranial aneurysms is associated with severe morbidity and mortality. Prehospital mortality estimates range from 10% to 15% . The rate of rerupture if untreated is 40% over 4 weeks with much of that risk early on; rerupture carries an 80% mortality underscoring the importance of early treatment for ruptured aneurysms. In patients who survive the initial hemorrhage long enough to undergo treatment, there is an additional 30% of 1-month mortality , and more than 50% of long-term survivors have difficulty in activities of daily living . The incidence of aneurysmal rupture is about 10 per 100,000 persons per year, with an average prevalence of aneurysms (ruptured or nonruptured) of 2–5% . With such devastating consequences, accurate assessment of the natural history of incidentally found, unruptured aneurysms and preventative treatment is necessary. Aneurysm characteristics and behavior can vary drastically according to location, thus affecting their management (see Table 90.1 ).

The overall frequency of intracranial aneurysms ranges from 2% to 5% . Their site distribution is nonrandom, with unruptured aneurysms occurring in the internal carotid artery (ICA, 28%), anterior cerebral artery (ACA, 27%), middle cerebral artery (MCA, 36%), and posterior circulation (8%). ICA aneurysms include cavernous, ophthalmic, anterior choroidal, and posterior communicating artery (PCOM) aneurysms. ACA aneurysms are predominantly composed of anterior communicating artery (ACOM) aneurysms. Posterior circulation aneurysms include those of the posterior cerebral artery (PCA), vertebrobasilar system, and cerebellar arteries. The site distribution of aneurysms that present with subarachnoid hemorrhage (SAH) is discordant (see Table 90.2 ). This evidence suggests a differential susceptibility for aneurysm formation and rupture according to location. Indeed, several prospective studies document differential risk of rupture based on location and aneurysm size . Particularly, they ascribe an extremely small risk of rupture to small anterior circulation aneurysms, which paradoxically, make up the majority of ruptured aneurysms . An increasing number of intracranial aneurysms are incidentally found with noninvasive advanced imaging techniques during the routine workup of other pathology and increasing resolution allows for the routine detection of increasingly smaller aneurysms .

Aneurysm pathogenesis is intimately related to the interplay of structural and hemodynamic effects. Most aneurysms arise at the intersection of two vessels with a subset arising at nonbifurcation points along vessels with significant curvature; in both situations the configuration gives rise to altered flow dynamics. Endothelial cells respond to alterations in shear stress and flow patterns with subsequent effects on the structure of the vascular cell wall composition. As an aneurysm develops, its geometry (i.e., size, shape, and relative configuration to the parent vessel) instantaneously dictates hemodynamic forces. The resulting wall shear stress is again sensed by the endothelium, and the tensile pressure is sensed by vascular mural cells; both factors are then transmitted into biological signals to the vessel wall affecting subsequent remodeling . In many instances, vessel wall remodeling is able to achieve a new equilibrium with hemodynamic stresses, and aneurysmal stability is achieved; in other instances the vessel wall remodeling is insufficient and hemodynamic forces overcome wall strength resulting in rupture. As hemodynamic forces are a fundamental driver of aneurysm formation and growth, the local vascular configuration (i.e., branching pattern and angles, flow speed, and wall thickness) is likely related to regional differences in aneurysm behavior.

It is reasonable to postulate that there may be location-specific differences in gene expression independent of hemodynamic properties, although this has not specifically been tested. The cerebral vascular development is a sequential and highly orchestrated process where different segments arise at different times, rendering the opportunity for segment-specific patterning or even mutagenesis to occur. Indeed, this may underlie the phenomenon of mirror aneurysms seen in 12% of patients most commonly in the MCA bifurcation. The distinct evolution of different segments of the cerebral vasculature also provides an opportunity for divergence of base components of the vessels. These events may give rise to a “positional identity” and differential susceptibility of aneurysmal formation to cells comprising the components of the vascular wall in different locations. Indeed, aneurysm location is a major factor in current models of future rupture risk.

Aneurysm location can also affect presentation. Mass effect or thromboembolic events into a specific vascular distribution can occur. Additionally, rupture with associated hematoma formation can also imbue symptoms referable to specific location. The risks of associated aneurysm treatment also vary with location due to accessibility and local perforators both from an open surgical standpoint and from and endovascular perspective.

Multiple aneurysms are present in about 20% of cases, which can make determination of the ruptured aneurysm difficult. Multiple aneurysms are present in about 20% of cases that necessitates accurately distinguishing the ruptured aneurysm versus nonruptured aneurysm(s), so that the at risk, ruptured aneurysm can be treated. Traditionally, the identification of the aneurysm that ruptured in these is imprecise and relies on assessment of the pattern of SAH (which varies by location in classical aneurysms) and evaluation of the relative risk of rupture for each aneurysm taking into account the aneurysms location, size, and morphology. This method is likely to result in misidentification of the ruptured aneurysm and inadequate hemorrhage source protection in a subset of cases. More recently, advanced imaging of vessel wall inflammation using new MRI sequences has been shown to accurately distinguish ruptured and nonruptured aneurysms in patients with SAH and multiple aneurysms .

Factors independent of location can precipitate aneurysm formation and subsequent rupture. Hypertension, smoking, female, increasing age, dysmorphic morphology, and size have all been associated with increasing risk of rupture . There is a congenital predisposition to aneurysm formation in polycystic kidney disease, and first-degree relatives of patients with SAH or unruptured aneurysms .

Internal Carotid Artery Aneurysms

ICA aneurysms account for about 30% of all unruptured intracranial aneurysms , and 35% of ruptured aneurysms in clinical series . Cavernous segment, ophthalmic, anterior choroidal, and PCOM aneurysms are typically included in this category. However, these numbers may shift in accordance with newer imaging that allows for the detection of smaller aneurysms and aneurysms in more difficult locations such as the ICA .

Cavernous segment aneurysms comprise 2–9% of all aneurysms and follow a relatively benign course . The risk of SAH is exceedingly low due to their extradural location within the cavernous sinus (0.2–0.4% yearly), although rupture can cause the formation of carotid–cavernous fistulae. Additional symptoms may stem from local mass effect on the cranial nerves (CNs) of the cavernous sinus (CN IV, VI, II, and V ₁ ) or ischemic stroke; 61% present with CN palsies, 23% with retro-orbital pain, and 10% with trigeminal pain . As the risk of SAH is low these aneurysms are prone to becoming quite large prior to becoming symptomatic adding to treatment morbidity and mortality. Newer treatment strategies with flow diverters may prove to improve treatment outcomes.

It is important to properly distinguish cavernous segment aneurysms from other intradural paraclinoid aneurysms, as intradural aneurysms carry a higher risk of devastating SAH. The optic strut is typically used as the defining landmark with those aneurysms distal to this landmark being considered intradural, and those proximal considered intradural. In 86% of cases, the ophthalmic artery arises intradurally, it arises extradurally in 6.7%, and arises from between the two dural rings in the remaining instances . Clinical application of this can be difficult, and it may be more prudent to consider aneurysms at or near the optic strut to be “junctional,” with the possibility of being intradural . It should also be noted that some larger cavernous segment artery aneurysms which originate extradurally, may extend upward with a portion of their dome traversing the dural barrier giving them the ability to rupture into the subarachnoid space .

Blister aneurysms are classically described as nonbranch point aneurysms arising from the dorsal and anterior wall of the ICA. They represent a minority of ICA aneurysms (0.9–6.5%) most often presenting ruptured (likely due to the difficulty in discerning an aneurysm that small on routine imaging). Believed to arise from a dissection, the arterial wall is extremely fragile and repeated imaging often demonstrates progression to a more saccular shape over a span of days. Re-hemorrhage is the norm, with surgical treatment rendering high morbidity and mortality . Flow diverters or even single traditional stents may improve outcomes .

Unruptured PCOM aneurysms can be associated with a CN III palsy depending on their orientation. The third nerve begins its course traveling between the PCA and subclavian artery (SCA) before traveling along the PCOM, hence aneurysms of the basilar, PCA, and SCA can also compress the nerve . Third nerve compression from an aneurysm typically involves the pupil on clinical examination, whereas a pupil-sparing third nerve palsy is more likely to result from microvascular ischemia. Most PCOM aneurysms causing isolated CN III palsy are 4 mm or larger, but smaller aneurysms have also been reported . Retro-orbital pain is a common symptom, preceding CN compromise by 10 days. Full or partial recovery can be achieved with treatment, and is correlated with the duration of CN palsy. It often proceeds with initial improvement in the levator palpebrae, and then the medial rectus muscle . Notably, decompression of the nerve by open resection of the aneurysm is not always necessary for return of CN function, as complete recovery has also been seen with endovascular treatment that presumably does not engender a rapid decrease in the aneurysmal mass effect but would be expected to decrease pulsatility .

SAH from ICA aneurysms typically fills the basal cisterns with the classic starburst appearance and can be somewhat asymmetric in density toward the side of the ruptured aneurysm; 20% of intracerebral hemorrhage (ICH) associated with aneurysmal rupture are from ICA aneurysms . While aneurysmal subdural hemorrhage (SDH) is more rare (0.9–5.8%), ruptured PCOM aneurysm is an associated risk factor for SDH . The occurrence of associated SDH predicts a worse outcome .