Risks of Endovascular Treatment of AVMs

Introduction

Intracranial arteriovenous malformations (iAVMs) are challenging lesions to treat, and patients often require multimodal management to achieve optimal outcomes. Options for AVM management include conservative management, endovascular embolization, surgical removal (resection) with or without preoperative embolization, and radiosurgery with or without preprocedural embolization. Endovascular embolization is an important tool in this treatment arsenal because it improves the efficacy and safety of other treatments and can also be used for selective noncurative treatments. In well-selected cases, endovascular embolization may also serve as the primary treatment. Since the first report of iAVM embolization by Luessenhop and Spence in 1960, much progress has been made, with techniques evolving from the use of steel-methylmethacrylate spheres to detachable balloons to liquid embolic agents. Although advances in endovascular technology have made embolization much safer, it is not yet a risk-free procedure. In this chapter, we focus on the evaluation of patients and potential staging of endovascular procedures; preoperative, preradiosurgical, curative, targeted, and palliative uses of endovascular embolization; and various risks and mitigation strategies associated with iAVM embolization.

Preoperative Evaluation and Staging

The preoperative evaluation of the patient and determination of an accurate diagnosis are the first steps in understanding the risks of endovascular management. The preoperative assessment should include a proper understanding of the patient’s symptoms and earlier hemorrhagic events, if any, because these factors can guide targeted treatment. Noninvasive imaging to be assessed includes CT and MRI, as well as the associated CT angiography (CTA) and MR angiography (MRA). MRI is used to better define the anatomical location of the lesion because this location can guide the understanding of the angiographic anatomy and vice versa. In addition, MRI is better than CTA at showing the surrounding parenchyma and the presence of previous hemorrhages or microhemorrhages.

The embolization of AVMs can sometimes be staged; this staging can be used as part of a multimodal treatment plan or when embolization is sought as a standalone treatment. Usually, the decision for staging is made before embarking on treatment on the basis of the initial imaging characteristics. Alternatively, the decision to use a staged approach can be made after starting embolization due to the limitations of a single-session radiation dose or contrast load. As a general rule, we prefer multiple sessions when treating AVMs with a maximum size greater than 3 cm and/or numerous arterial pedicles. Although radiation and contrast dosing are important limitations during embolic procedures, the primary risk of “heroic” embolization efforts that significantly alter hemodynamic characteristics is normal perfusion pressure breakthrough. This concept, initially described by Spetzler et al. and more recently refined by Rangel-Castilla et al., suggests that a sudden increase in perfusion pressure can occur in a chronically hypoperfused capillary bed surrounding the AVM caused by the sudden decrease in arteriovenous shunting following resection. These patients may lack normal autoregulatory mechanisms to counter these changes, which results in cerebral edema or hemorrhage. Patients with large lesions may undergo staged embolization before planned resection.

Before starting a treatment course, the conceptual goals of treatment should be laid out clearly for the benefit of both the treatment team (neurosurgical, neurointerventional, and radiosurgical) and the patient. Although embolization can be used as a standalone treatment in selected patients (discussed later), the neurointerventionalist should avoid undertaking embolization without a clear goal in mind. Furthermore, when embolization is being performed as an adjunctive treatment, the specific targets should be discussed between the neurointerventionalist and the surgeon to maximize the value of the procedure. For these reasons, iAVMs should be treated at high-volume centers that offer comprehensive care (medical, surgical, interventional, and radiosurgical), and treatment teams should be familiar with one another to optimize outcomes (unpublished data).

Preoperative Embolization

Preoperative treatment is the most common indication for iAVM embolization. The goals and strategy for preoperative embolization should be as precise as the surgery itself. It should be kept in mind that the goal is to facilitate resection, making it safer and easier. The goal of embolization should not be to “go for broke” and possibly cure the iAVM. When preoperative embolization is pursued to facilitate resection, the neurointerventionalist can balance the risks and benefits of certain procedural steps. The aggregate risk of surgery with embolization should not exceed the risk of surgery performed without the embolization. That is, the potential benefit of embolization (how much it reduces the risk of surgery) should offset the risk of the embolization itself. This risk can be calculated either by a “dual-trained” neurosurgeon or between colleagues in neurosurgery and neurointerventional surgery. The result is generally a less aggressive approach to treatment than if a curative endovascular procedure were performed. Because the lesion is to be resected in total after preoperative embolization, the embolization need not be as durable as it would have to be for a curative procedure. In preoperative embolization, arterial inflow should be diminished in a stepwise and safe manner, with a focus on vessels that are less surgically accessible (usually the deep face of the AVM) depending on the surgical approach. The result of this approach is that, in many cases, embolization can be performed safely and improve surgical safety.