37 Vertebral Artery Aneurysms
Abstract
Vertebral artery aneurysms are generally rare aneurysms of the posterior circulation that deserve special consideration. As with all posterior circulation aneurysms, they may present a higher rupture risk when compared to anterior circulation aneurysms. A careful workup, including high-quality vascular imaging, is a necessity when determining the best treatment options for the patient. Treatment options include both open microsurgical and endovascular techniques. However, given the confines of the operative corridor and the emergence of flow diversion, endovascular techniques may offer a specific treatment advantage. Long-term results with these newer endovascular therapies are generally lacking and a thorough discussion within a robust, multidisciplinary cerebrovascular service and the patient is beneficial in optimizing treatment results.
Introduction
Aneurysms of the vertebral artery (VA) are relatively rare, comprising 20 to 30% of aneurysms of the posterior circulation. This rarity, combined with their often unusual morphological characteristics, makes these lesions particularly challenging to treat. Patients with VA aneurysms frequently present with complaints such as ataxia, lower cranial nerve palsies, and decreased level of consciousness.
Major controversies in decision making addressed in this chapter include:
Whether or not treatment is indicated.
Open versus endovascular treatment for ruptured and unruptured VA aneurysms.
Management of VA aneurysms that present with intracerebral hematoma (ICH).
The role of flow diversion.
When should an advanced surgical technique (bypass) be considered?
Whether to Treat
Risk of rupture for VA aneurysms varies and is affected by both aneurysm size and a history of previous aneurysmal subarachnoid hemorrhage (SAH). Based on the International Study of Unruptured Intracranial Aneurysms (ISUIA) 1 and 2, the 5-year cumulative risk for rupture in aneurysms of the posterior circulation in patients without previous SAH is 2.5% for aneurysms less than 7 mm, 14.5% for those between 7 and 12 mm, 18.4% for those between 13 and 24 mm, and 50% for those greater than 24 mm ( 1 , 2 in algorithm ). While posterior circulation aneurysms in general were in a higher rupture risk category than anterior circulation aneurysms in the ISUIA study, other studies such as that performed by the Japanese Unruptured Aneurysm Investigator Group (UCAS) suggest that only a subset of these aneurysms (i.e., posterior communicating artery lesions) were actually more prone to rupture. Patients who have suffered previous SAH are also at significantly higher risk of aneurysm rupture than those who have not, with the ISUIA group finding an 11-fold higher risk of rupture for aneurysms less than 10 mm in size. Other risk factors, including morphological characteristics of the aneurysm (e.g., presence of a daughter sac) and the patient′s demographic characteristics (e.g., history of tobacco use, hypertension), also likely play a significant role in a specific patient′s risks of SAH.
Weighed against the risk of rupture in these lesions is the risk of treatment itself. Overall treatment morbidity and mortality at 1 year for patients in the clipping arm of ISUIA was 12.6% in those patients without previous SAH and 10.1% in those with previous SAH. Morbidity and mortality at 1 year for patients in the endovascular treatment arm was 9.8% in those patients without previous SAH and 7.1% in those with previous SAH. Patients with aneurysms of the posterior circulation, such as VA aneurysms, were at higher risk of postoperative morbidity/mortality than those with aneurysms of comparable size of the anterior circulation. The risks of surgical intervention, such as the risk of rupture, vary from patient to patient. Larger aneurysms, older age (particularly intervention in those older than 60 years), prerupture aneurysm-related symptomatology, and a history of ischemic stroke all placed patients at higher risk following intervention. Regardless of these risks, intervention is clearly indicated for almost all cases of ruptured VA aneurysms.
Conservative Management
With the increasing ubiquity of advanced neuroimaging, the management of incidentally identified VA aneurysms becomes increasingly important. Patients must be carefully counseled regarding the risks of both intervention and watchful waiting, and these recommendations must be tailored based on the aneurysm′s size, its morphology, and the patient′s specific risk profile. In patients who are treated nonoperatively, the need for follow-up and subsequent imaging to assess for aneurysm growth must be carefully considered.
Anatomical Considerations
The VA is composed of four segments. From its origin, most commonly from the subclavian artery, V1 extends to the transverse foramen of the sixth cervical vertebra. V2 is composed of the segment from this transverse foramen to the transverse foramen of the axis. The portion of the artery from the transverse foramen of the axis to the point at which the artery pierces the dura comprises the V3 segment. V4, the only intradural portion, ends at the confluence of the left and right vertebral arteries where they become the basilar artery. Variations may be present and must be considered prior to any open surgical or endovascular interventions. These include the dominance of one artery over the other (approximately 40–50% of the population is left dominant) or codominance (approximately 25% of the population), duplication of a portion of the artery (fenestration), or duplication of an entire vessel segment.
The precise location of the VA aneurysm also has implications on management. Attention must be paid to the relationship of the aneurysm to the posterior inferior cerebellar artery (PICA), and the location of the aneurysm on the proximal versus distal artery can have implications on which approach will offer the optimal exposure.
Classification
Two specific subtypes of VA aneurysms warrant discussion: giant aneurysms and dissecting aneurysms. Giant aneurysms, classically defined as lesions greater than 25 mm in diameter, often present a unique challenge in the posterior circulation. The mass effect exerted by these lesions in the posterior circulation can result in significant neurological deficits, sometimes necessitating aneurysmectomy for optimal symptomatic relief. A large proportion of these aneurysms also demonstrate intraluminal thrombosis, further contributing to mass effect and making precise determination of aneurysm morphology more challenging on vessel imaging. The need for vessel bypass in some cases also necessitates precise knowledge of local vascular anatomy and collateral blood flow.
Dissecting VA aneurysms are a rare entity that can present in the form of SAH or brainstem ischemia. The precise cause of these lesions is not completely known, although disruption of the internal elastic lamina leading to vessel dilation and pseudoaneurysm formation is thought to play a role. Dissecting aneurysms exhibit both a high risk of rebleeding—more than 71% in some studies—and high operative morbidity. A variety of treatment strategies, including trapping, proximal occlusion, and reconstruction using endovascular techniques, have been used to treat these lesions, and the specific approach chosen must be tailored for each individual aneurysm.
Workup
Clinical Evaluation
Patients presenting with SAH must be evaluated and stabilized. These patients must be closely monitored for the development of complications of SAH such as seizures and hydrocephalus. Patients presenting with incidental lesions must have careful neurological evaluation for possible symptoms of their aneurysm, and their medical, social, and family history should be scrutinized to better stratify their risk for subsequent aneurysm rupture.
Imaging
Several forms of neuroimaging are available for the assessment of aneurysms. The initial evaluation of patients presenting with SAH is typically with noncontrast computed tomographic (CT) scanning. For patients referred to a tertiary care hospital, these have often been done at the outside referring facility. The CT scans must be carefully evaluated for both the pattern of hemorrhage, which may offer clues to the location of the aneurysm, and the presence of intraparenchymal hemorrhage, intraventricular hemorrhage, and early ventricular dilation. Noninvasive vessel imaging such as CT angiography and magnetic resonance angiography (MRA) can help in the identification of the aneurysm and may also help with the construction of a three-dimensional model of the lesion. The gold standard for cerebral vessel imaging remains diagnostic cerebral angiography, because it can offer insights into the aneurysm itself such as precise morphological characteristics and flow dynamics, as well as further clarification of the configuration of surrounding vasculature prior to any intervention.
Differential Diagnosis
Patients presenting with SAH should be carefully evaluated for other possible causes apart from aneurysmal bleeding. Hemorrhage in the perimesencephalic region in the absence of aneurysms is a known entity, and these patients with perimesencephalic hemorrhage must be carefully scrutinized, often with repeat angiography, to rule out the presence of an underlying vascular lesion. Other vascular lesions, such as cavernomas, and less common causes of bleeding, such as neoplastic processes, must also be considered until a definitive diagnosis can be reached.
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