Embolic Agents For The Treatment Of Avms

Overview

The aim of endovascular therapy is to obliterate the AVM or reduce its size with little or no increased medical or neurological risk to the patient. As with any intervention, the overall goal is to enhance the patient's outcome.

In 1960 Luessenhop and Spence (1) performed the first embolization of an AVM by injecting Silastic spheres through surgical exposure of the cervical carotid artery. Forty years of advances brought new embolic agents to the scene,

From: Minimally Invasive Neurosurgery, edited by: M.R. Proctor and P.M. Black © Humana Press Inc., Totowa, NJ

including polyvinyl alcohol (PVA), N-butyl cyanoacrylate (NBCA), Onyx (Micro Therapeutics, Irvine, CA), and neuracryl M (Provasis Therapeutics, El Cajon, CA); novel microcatheters and microwires; and endovascular operative techniques altering the approach to this entity.

The endovascular treatment strategy for an AVM is significantly influenced by the overall treatment plan and is tailored to the patient and the malformation. Endovascular embolization strategies can be considered within the following categories of goals and issues: (1) embolization as a preoperative tool, (2) embolization as a pre-Gamma-Knife radiosurgery tool, (3) embolization alone as a curative modality, (4) embolization for palliation of symptoms, and (5) embolization of associated aneurysms.

Embolization reduces the amount of blood loss that is associated with the resection of an AVM by decreasing the vascularity of the nidus (2). In a study by Jafar et al. (2), the amount of blood loss during the resection of large AVMs embolized with NBCA (an adhesive liquid polymer) was similar to that for nonembolized small AVMs. Many authors have noted that NBCA embolization increases the ease of the operation (2,3). For large AVMs, embolization can serve the purpose of gradually decreasing flow through the AVM, hence decreasing the risk of hemorrhage associated with "normal perfusion pressure breakthrough" (4,5). Normal perfusion pressure breakthrough can occur when embolization or resection of a high-flow AVM directs blood from the AVM to surrounding vascular beds that have been chronically hypoperfused. Chronic hypoperfusion leads to a loss of the autoregulatory ability of the vascular beds. When blood flow is redirected into these vascular territories, hemorrhage can occur as a result of the normal perfusion pressure breakthrough phenomenon, even at a patient's "normal" systemic blood pressure. Embolized AVM vessels can also serve as a roadmap during surgical resection by helping the surgeon to define the anatomy of the feeding pedicles and the nidus.

Despite these advantages, AVM embolization carries considerable risk. Only patients with AVMs who will significantly benefit from embolization before surgery should undergo this treatment. For example, a small AVM in the right frontal lobe with limited feeders that are easily accessible by surgery probably does not warrant embolization, unless the embolization may be curative (see Case Illustration section on p. 204). In presurgical planning of AVM emboliza-tion, the authors embolize those pedicles felt to be the most technically challenging for surgery. If lenticulostriate and other deep feeding vessels appear to be easy to embolize but difficult to access surgically, we attempt these feeders first. Alternatively, if the deep feeders appear too difficult to access, the larger, more superficial feeders are embolized first. After the occlusion of superficial feeders, deep feeders may recruit additional blood supply and become enlarged. The enlarged vessels may be more accessible during a second embolization session several weeks later. It is extremely important to coordinate with the operating surgeon as to the goals and timing of treatment.

For the patient in whom the management plan involves radiosurgery, embolization strategies have been used to reduce the volume of an AVM and to eliminate associated pedicle and proximal aneurysms when possible. The size of an AVM nidus influences the success of radiosurgery (6). Therefore, reducing the size of an AVM with embolization may increase the success rate of radiosurgery and help avoid use of the higher radiation doses needed to treat a larger AVM. Circumferential embolization of the AVM, rather than fragmentation of the nidus, is the preferred strategy here since a fragmented nidus may result in multiple targets for radiation dosing, which could make radiosurgery planning difficult, thereby negating the benefits of embolization. Some limited evidence exists to support the embolization of large AVMs to a size that is amenable to radiosurgery (7,8). The long-term results of this strategy have not yet been adequately documented. The authors' results with this strategy in select AVMs is encouraging thus far (unpublished data).

Endovascular treatment alone is rarely curative (8). In general, this strategy works for some small AVMs with limited arterial feeders and draining veins. Rarely, a medium-size AVM can be embolized in one or multiple sessions and a cure can be achieved (see Case Illustration section on p. 204).

Some high-grade AVMs may be too dangerous to treat by any method, and pure observation may be indicated. Embolization solely for palliation of symptoms is rarely indicated. An AVM may be responsible for debilitating symptoms such as severe headaches or ischemia related to steal phenomena. In these situations, partial embolization may help reduce flow through the AVM and lead to an improvement in the patient's symptoms. The embolization of dural feeders may be particularly helpful for amelioration of headaches. Steal symptoms can be reduced by embolizing large "incurable" AVMs (9). Adhesive embolic agents (such as NBCA) should be used for these types of indications. It has been suggested that partial embolization of an AVM may yield a worse prognosis than no treatment, so this strategy should be practiced with caution (8,10,11).

Embolization techniques can be used to treat associated aneurysms. Owing to the risk of rupture of proximal aneurysms during AVM embolization or excision, treatment of these lesions should precede AVM treatment when possible. If the aneurysm appears to be appropriate for coiling, it can potentially be occluded during the first embolization session before the AVM nidus is approached. If the aneurysm appears to be better suited for surgery, surgical clipping can be performed before AVM embolization. Aneurysms on feeding pedicles of the AVM (flow-related aneurysms) can be occluded primarily using detachable coils. Alternatively, an aneurysm that is close to the AVM nidus can often be occluded by embolization of the pedicle harboring that aneurysm while glue is injected from the microcatheter (the glue refluxes from the nidus into the pedicle and aneurysm). This occlusion can result from infiltration of glue into the aneurysm sac or secondarily as the feeding pedicle becomes occluded. Pedicle aneurysms can be embolized with detachable coils before glue is injected into the pedicle. If the pedicle aneurysm is distal enough from the AVM, it can be treated with the same glue injection used to embolize the nidus. Attention should be paid to intranidal aneurysms during embolization. If a hemorrhage has occurred, an intranidal aneurysm should be suspected as a rupture site. When possible, the pedicle feeding the nidus harboring the aneurysm should be treated first.

Tools for Embolization

Embolic Agents

Embolic agents can be classified as solid or liquid (Table 1). Each agent requires a significant learning curve. Earlier agents with low safety or high recanalization profiles have been abandoned for newer and safer agents. Our discussion will focus not only on the most commonly used agents, NBCA and PVA particles, but also on promising new agents, Onyx and neuracryl M. PVA

Before refinements were made in microcatheter technology, particulate agents such as PVA were the mainstay of preoperative embolic therapy for AVMs. First reported by Porstmann et al. (12), PVA particles are currently used only for those cases in which the AVM will be resected within several days or to slow the flow in a high-flow pedicle before NBCA embolization is performed. Slowing the flow in a high-flow pedicle with PVA may allow more NBCA to penetrate the AVM nidus, rather than escaping through the draining vein. As AVM obliteration caused by PVA embolization is the result of thrombosis, embolization with PVA particles results in slower occlusion of the nidus (compared with NBCA), and because the thrombus is broken down over the course of time, a relatively high rate of recanalization is associated with PVA embolization (13). Concerns regarding recanalization and recruitment of new feeders have made particle embolization fall out of favor at most centers except for the previously mentioned circumstances. In addition, PVA particles are not radiopaque.

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