32 Fusiform Aneurysms of the Anterior Circulation
Abstract
The estimated prevalence of intracranial aneurysms (IA) in the general population ranges between 2 and 4%. Although fusiform aneurysms are more commonly found in the vertebrobasilar circulation, these challenging lesions can occur in the anterior circulation with a prevalence ranging from 0.1 to 0.3%. Fusiform aneurysms are complex lesions that involve more than 50% of the arterial circumference and are typically characterized by a lack of discernible neck. In general, this subset of lesions is associated with worse outcomes, higher rates of complications, and death. In this chapter, we discuss their anatomical features and explore pathophysiological mechanisms as well as current evidence in surgical and endovascular options. Microsurgery remains an adequate treatment option and some of the vascular reconstructions include trapping, wrapping, bypass, and excision and induction of aneurysm thrombosis by proximal clipping. Endovascular options for fusiform aneurysms are typically associated with the use of stents or flow diverters with or without the use of adjuvant coiling. Overall, these procedures have demonstrated a safe and effective profile favoring this option over microsurgery. However, in some instances, a combined approach can be done. Although there is no consensus for the optimal management of fusiform aneurysms in the anterior circulation, the decision is made on a case-by-case basis assessing the patient′s hemorrhagic risk over an estimated life span in contrast to neurosurgeon′s perceptions of potential complications, particularly major neurological morbidity and loss of functional independence.
Introduction
Intracranial aneurysms are estimated to occur in approximately 2.8% of the general population and giant aneurysms (≥25 mm in largest diameter) represent an infrequent subset of lesions representing only 3 to 5% of intracranial aneurysms. These lesions are often also categorized into saccular and fusiform based on their morphology and appearance on imaging studies. Saccular aneurysm implies that a discernible neck is present, which typically occurs following a localized defect in the arterial wall.
In contrast, fusiform aneurysms are complex lesions involving more than 50% of the arterial circumference and typically characterized by no discernible neck, which has important treatment implications. Although fusiform aneurysms are more commonly found in the vertebrobasilar circulation, these challenging lesions also occur in the anterior circulation with an estimated prevalence ranging from 0.1 to 0.3%. Within the anterior circulation, the majority of fusiform aneurysms occur in the internal carotid artery (ICA). Fusiform aneurysms involving the anterior cerebral arteries (ACAs) and middle cerebral arteries (MCAs) are rarely seen. Fusiform aneurysms in the supraclinoid segment of the ICA have a higher rate of rupture (~40–50% over 5 years) compared to aneurysms located in the cavernous segment (10% over the same period). The former lesions are also associated with worse outcomes, higher rates of complications, recanalization, and death.
In the past, some authors have suggested that fusiform aneurysms may have an atherosclerotic component in their etiology. However, other pathogenic factors unrelated to atherosclerosis have been previously demonstrated to increase the risk of aneurysm enlargement over time and bleeding. Age of the patient is a very good indicator of lesion pathogenesis. Young patients often develop these aneurysms in the context of vessel dissection or underlying vasculopathy, while older patients (older than 45 years) are more likely to develop these lesions in association with vessel atherosclerosis.
Major controversies in decision making addressed in this chapter include:
Imaging surveillance versus treatment.
Microsurgical treatment versus endovascular techniques.
Long-term durability of current treatment strategies.
State-of-the-art endovascular devices.
Whether to Treat
Compared to saccular aneurysms, the natural history and risk of rupture for fusiform aneurysms in the anterior circulation remains a topic only marginally understood and an area that would benefit greatly from further studies. As with other intracranial aneurysms, factors to aid in the treatment algorithm include (1) aneurysm size, (2) recent growth on imaging studies, (3) previous subarachnoid hemorrhage (SAH), and (4) patient preference. Smaller asymptomatic lesions can be safely monitored with serial imaging and rarely demonstrate further growth ( 1 in algorithm ). The decision to treat should always take in consideration the estimated risks associated with treatment weighted against the natural history.
Anatomical Considerations
Intracranial aneurysms are more likely to occur in certain segments of the artery, which has generally been based on regional differences in blood flow. Similarly, fusiform aneurysms tend to occur between areas of vessel bifurcation in both proximal and distal segments of major intracranial arteries. The majority of fusiform aneurysms in the anterior circulation are found in the cavernous segment of the ICA (~42%), followed by the remaining ICA (23–39%), MCA bifurcation (32–41%), and rarely ACA (0.2–1.0%). Fusiform aneurysms involving the ACA are usually restricted to the A1 segment and similarly, these lesions are more likely to occur at the M1 segment when the MCA is involved.
Pathophysiology
The events leading to the development of fusiform aneurysms are often unknown; atherosclerosis has been postulated as a potential mechanism due to disruption of the internal elastic lamina (IEL). It has also been hypothesized that these lesions may arise from arterial microdissections with intramural hemorrhage between the intima and the media leading to progressive dilatation and tortuosity. As previously mentioned, dissection or nonatherosclerotic vasculopathy is more likely to occur in younger patients, and atherosclerosis occurs more often in older patients. In addition, turbulent flow within the aneurysm lumen has been shown to create nonphysiological transmural pressures and shear stress on the vessel wall, which may induce changes in smooth muscle cell homeostasis and loss of endothelial integrity. Fusiform aneurysms often have unique underlying pathological features on autopsy including calcified walls, onion skin pattern in the vessel wall, partial aneurysm thrombosis, and absence of aneurysm neck. Although uncommon, infection involving the vessel has been found to predispose the arterial wall to fusiform dilation, which has been correlated with medial fibrosis, loss of smooth muscle layer, destruction of the IEL, and intimal hyperplasia. Tumor cell infiltration of intracranial vessels via the vasa vasorum has also been associated with pseudoaneurysm formation and fusiform dilatations with partial destruction of the vessel wall, microvascular occlusion by the tumor, and direct invasion of the arterial wall. Rare cases of fusiform aneurysm formation have been reported in patients with X-linked lymphoproliferative (XLP) syndrome in which the immune system is unable to mount an adequate response to viral infections, particularly by the Epstein–Barr virus, which may result in diffuse necrotizing vasculitis affecting major intracranial arteries. Fibromuscular dysplasia may also facilitate aneurysm formation by causing various degrees of collagen hyperplasia, IEL rupture, and disorganization of the medial layer, which can result in dilatation of the artery.
Classification
From an angiographic and pathological standpoint, these lesions can be classified into the following:
Type 1: classic dissecting aneurysms. Angiographic features of a fusiform aneurysm with irregular wall and irregular stenotic portion near the proximal or distal end. Pathological features include widespread disruption of the IEL without intimal thickening, and the presence of a pseudolumen, which is filled with thrombus.
Type 2: segmental ectasia. Angiographic features of a fusiform aneurysm with a smooth contour and typically associated with other cerebrovascular diseases. Pathological features such as stretched or fragmented IEL, moderately thickened intima, and no evidence of thrombi.
Type 3: dolichoectatic dissecting aneurysm. Angiographic features of tortuous fusiform appearance with irregular contrast opacification caused by intraluminal thrombus. Pathological features include fragmentation of IEL combined with multiple dissections of thickened intima, and organized laminar thrombi.
Type 4: saccular aneurysm arising from arterial trunk. Angiographic features of saccular aneurysms unrelated to the branching zones. Pathological features of a mixed type such as IEL pattern resembling a type 1 lesion without a discernible pseudolumen or organized thrombus as well as absence of IEL at the dome of the aneurysm with distended fragile adventitia.
Workup
Clinical Evaluation
Fusiform aneurysms arising in the anterior circulation often present with symptoms of mass effect such as headaches, cranial neuropathy (especially visual symptoms) as well as transient ischemic attacks (TIAs) or stroke. SAH occurs less often and similar to saccular aneurysms; the majority of fusiform aneurysms (nearly 60%) are found incidentally. Visual deficits due to optic nerve compression are more frequently associated with fusiform aneurysm located in the ACA ( 2 in algorithm ). Skull base erosion with massive epistaxis has been reported with these lesions although the true incidence is unknown.
Imaging
Imaging of fusiform aneurysms is similar to its saccular counterpart and should evaluate the aneurysm wall, lumen, and flow. Magnetic resonance imaging (MRI) based techniques are clearly the best modality for wall imaging since these provide important information about wall thickness, the presence of intramural thrombus, and the extent of mass effect. Lumen imaging can be assessed with multislice helical computed tomography angiogram (CTA) and has become the primary modality for noninvasive imaging. Time-of-flight (TOF) magnetic resonance angiography (MRA) is a reasonable alternative in patients with severe renal disease or in patients requiring repeated imaging, with the caveat that MRA TOF can provide misleading information such as apparent vessel stenosis or occlusion. However, the “gold standard” modality remains digital subtraction angiography (DSA) because it provides real-time imaging of blood flow inside the parent vessel and aneurysm as well as accurate vessel measurements, which are essential for endovascular strategies. Catheter-based imaging also allows us to perform better assessment of collateral flow, very often useful for treatment of these lesions. Balloon test occlusion for the carotid artery or superselective into the MCA and ACA (with newer low-profile balloons) often provides valuable therapeutic guidance.
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