33 Dissecting Intracranial Aneurysms of the Anterior Circulation

10.1055/b-0038-162162

33 Dissecting Intracranial Aneurysms of the Anterior Circulation

Stephen R. Lowe, Jan Vargas, Alejandro Spiotta, and Raymond D. Turner, IV

Abstract

Dissecting intracranial aneurysms are anatomically unique and thus are not easy to classify in the same way that saccular aneurysms have been in the neurosurgical literature. These lesions are dynamic and may present with both hemorrhagic (i.e., subarachnoid hemorrhage) and ischemic symptoms. These are complex anatomical lesions and generally require some form of neurosurgical intervention. Neurosurgical intervention may be both an open surgical procedure or endovascular vessel reconstruction, or vessel sacrifice. In cases where endovascular reconstruction is feasible, it is generally preferred. However, given the variability in location and morphology of these lesions, treatment must be individualized as much as possible. In this chapter, we present the relevant natural history, prognosis, anatomy, pathophysiology, workup, and management of dissecting intracranial aneurysms of the anterior circulation. We will also discuss blister-type aneurysms, a special subset of dissecting aneurysms with a unique physiology, natural history, and treatment algorithm

Introduction

Dissecting intracranial aneurysms (DIAs) represent a unique challenge to the cerebrovascular surgeon. These rare lesions must be addressed carefully and thoughtfully to ensure a safe and durable treatment for the patient. Their friable anatomy makes them technically complex lesions to treat, either by open or by endovascular techniques. More significantly, these are lesions that do not conform to the typical saccular morphology seen with aneurysms described in the large International Subarachnoid Aneurysm Trial (ISAT) and International Study of Unruptured Intracranial Aneurysms (ISUA) series. Due to this lack of high-quality randomized and observational data, and due to the relative paucity of reports in the literature regarding the natural history, prognosis, and treatment of these lesions, developing a well-validated treatment algorithm for these lesions is challenging. We aim to describe the classification, natural history, pathogenesis, and treatment considerations for DIA of the anterior circulation.

For the purposes of this chapter, we will consider dissecting pseudoaneurysms (i.e., those that arise either spontaneously or secondary to trauma or iatrogenic causes), which we will term DPA, separately from a unique group of dissecting aneurysms, which we will term blister-type aneurysms (BTAs). The abbreviation “DIA” will refer to DPAs and BTAs collectively.

Major controversies in decision making addressed in this chapter include:

Whether or not treatment is indicated.
Open versus endovascular management for DIAs.
Advanced strategies for open reconstruction of DIAs.
Advanced strategies for endovascular reconstruction of DIAs.

Whether to Treat

DIAs are uncommon lesions with an ill-defined incidence in the literature. While BTAs are reported to represent 0.3 to 2% of all intracranial aneurysms, DPAs of the anterior circulation are even more unusual, with less than 100 reports of spontaneous DPAs in the literature and less than 50 reports of DIA secondary to trauma reported in the literature. Unlike the more common saccular or “berry” aneurysm, where long-term rates of rupture are well defined, the natural history of DIAs is not well defined due to their infrequent presentation and lack of observational studies. The large majority of these lesions in the literature are described in the setting of subarachnoid hemorrhage (SAH), suggesting a malignant natural history ( 1 , 2, 3 in algorithm ). Additionally, many retrospective studies have shown these lesions to be dynamic in nature (particularly for BTAs), demonstrating rapid growth and rapid change in the conformation of the aneurysm, even in short intervals of follow-up. Rapid growth and change in these lesions is even observed after attempted treatment, particularly with BTAs. In the setting of SAH, patients with DIAs tend to have worse outcomes than those with a ruptured saccular aneurysm history ( 2 in algorithm ).

**Algorithm 33.1** Decision-making algorithm for dissecting intracranial aneurysms of the anterior circulation.

As noted earlier, the natural history of these lesions is not well documented. BTAs are almost always described in the ruptured setting, and short-interval follow-up vascular imaging suggests that these lesions are dynamic, demonstrating rapid conformational change suggestive of instability and a malignant natural history. DPAs were historically implicated as a rare cause of ischemic symptoms in young patients; however, recent reports suggest that they are more commonly associated with SAH. When presenting with SAH, DIAs have been reported to have a higher rate of rebleeding (44%) compared to saccular aneurysms (14%), and as such the prognosis is worse in these patients. As such, when a DIA is diagnosed in the setting of SAH, it should be treated aggressively and promptly ( 1 , 2, 3 in algorithm ).

DPAs with ischemic symptoms, on the other hand, can have a more benign course. Compared to dissecting aneurysms of the vertebral artery, DPAs of the internal carotid artery (ICA) tend to persist longer, but carry little risk of recurrent ischemic events. Patients with recurrent ischemic symptoms may warrant definitive treatment, but in the light of the good prognosis of these lesions, medical management to prevent thromboemboli is usually first-line treatment before subjecting a patient to invasive treatments ( 4, 12 in algorithm ). Despite the associated higher risk of treatment, the aggressive course of DIA seen in the literature suggests that these lesions should be treated aggressively when presenting with SAH. Treatment should be offered to all patients with evidence of a ruptured DIA.

In patients with an incidentally discovered DPA with ischemic symptoms, conservative management is appropriate, unless the patient suffers recurrent ischemic events. DIA secondary to trauma should be given strong consideration for treatment in the unruptured setting given they likely have an aggressive natural history. The natural history of incidentally discovered BTAs is not well documented, but the malignant natural history of these lesions suggests that conservative management is not appropriate and these lesions must be treated aggressively despite clear risks of treatment.

Anatomical Considerations

Dissecting Pseudoaneurysms

The majority of dissections occur in the extracranial ICA, and most spontaneous DPA arise in the same location. DPAs that originate at the skull base, however, are more challenging to access and treat, both with open microsurgery and endovascular techniques. These lesions tend to have large, irregular domes with irregular and variable neck segments, and generally arise from nonbranching segments of their parent vessel.

DPAs secondary to trauma are generally seen arising from distal branches of the anterior cerebral artery. However, traumatic dissections can be seen in any location involved with a penetrating trauma or iatrogenic injury, including in association with malpositioned ventriculostomy catheters or intracranial pressure monitors. Traumatic DPA can also be seen in the ICA along the skull base secondary to blunt trauma and often in association with fractures of the skull base. Iatrogenic DPAs tend to be unique to each individual circumstance. Both of these types tend to demonstrate large, irregular aneurysms with ill-defined neck segments arising at nonbranching segments of their parent vessel.

Blister-Type Aneurysms

While occasionally described at other sites, such as the anterior or middle cerebral arteries, the BTA classically originates from a nonbranching segment of the supraclinoid ICA. They are typically “hemispheric” in appearance, with a thin-walled protruding dome generally seen arising from the dorsal or anteromedial wall of the supraclinoid ICA, although other morphologies can be seen (see section Pathophysiology and Classification). Unlike saccular aneurysms, these lesions do not typically have a well-defined neck.

Pathophysiology and Classification

Due to the paucity of literature regarding anterior circulation DIA, much of the proposed pathophysiology has been adapted from histopathological studies of vertebrobasilar DIA.

Dissecting Pseudoaneurysms

Primary DPAs arise from a dissection of the parent vessel, and as such the natural history of these lesions is linked to that of cerebral artery dissections (▶ Fig. 33.1 ). Most dissections of the anterior circulation present spontaneously. Connective tissue disorders, dissections of multiple vessels and redundancies of vessels, and a history of migraines and tobacco use have been identified as risk factors for the formation of DPAs after an intracranial dissection. A subset of these lesions will be secondary to trauma or to iatrogenic causes. Aneurysms secondary to trauma can be either penetrating or blunt, but those arising secondary to blunt trauma are extremely rate, accounting for approximately 0.5% of all intracranial aneurysms. The pathophysiological mechanism of the classic type of traumatic DIA arising from the anterior cerebral artery (ACA) is felt to be related to injury to the vessel arising from contact with the falx cerebri. Iatrogenic causes are generally a result of complications from surgical or endovascular manipulation of the intracranial vasculature.

**Fig. 33.1** Artist′s illustration demonstrating the pathophysiology of a dissecting aneurysm.

In cases of DPA that present with SAH, a dissection plane is generally seen confined to the subadventitia, whereas ischemic strokes are generally associated with dissection planes seen in subintimal layer. Hirao et al classified DPA of the ACA into types I, II, and III. Type I originates at the ICA and extends into the ACA and middle cerebral artery (MCA). Type II often occurs at the A1 segment of the ACA, and type III generally involves the distal ACA branches. There are no classification schemes described for DPA secondary to trauma or iatrogenic causes and the exact pathophysiology is not well delineated owing to the paucity of reports and lack of anatomical studies. Indeed, these aneurysms tend to be unique to the process that created them, and as such each aneurysm is slightly different.

Blister-Type Aneurysms

BTAs of the anterior circulation are manifested by a disruption of the normal internal elastic lamina and media with normal adventitia covering the defect. Sim et al noted an interesting corollary between the BTAs seen classically on the supraclinoid ICA and Mizutani type IV dissections of the vertebral artery, suggesting a focal dissecting process is responsible for the unique morphology of this lesion. While generally the causative factor is thought to be shear stress on the arterial wall caused by the unique flow dynamics of the supraclinoid ICA, other rare causes of BTA, such as Ehlers–Danlos syndrome and invasive Aspergillosis have also been described, generally in conjunction with BTAs in locations outside of the supraclinoid ICA.

There are no widely accepted classification schemes for BTAs of the supraclinoid ICA. Bojanowksi et al proposed a four-tiered classification scheme, with type I dissections representing a small bulge in the arterial wall without an appreciable neck segment (▶ Figs. 33.1 , ▶ 33.2 , and ▶ 33.3 ). Type II BTAs are larger, with a defined neck that is not greater in size than the diameter of the ICA. Type III has a neck segment that is longer in the longitudinal plane than the diameter of the ICA. Type IV represents circumferential disease of the carotid at the diseased segment, with or without a focal outpouching. The authors recommend simple clip reconstruction for types I and II, a multiclip reconstruction for type III lesions owing to their large size, and a clip-over-wrapping technique for type IV lesions. They note that these lesions may not be separate, but may in fact represent different stages of the same disease process. This survey included only 10 patients, which underscores the paucity of literature on the topic.