CLASSIFICATION OF EMBOLIC AGENTS

When looking at all of the available agents for embolization, it is often helpful to classify these agents from a clinical perspective, allowing a particular class or type of embolic agent to be an option for a specific indication or procedure. However, this is easier said than done because it can be difficult to define the terms that are traditionally used for this type of classification system. In addition, there is a significant amount of potential overlap between agents in different categories.

Traditional Classification System

The most common way to classify embolic agents has been to define them as being either temporary or permanent.1 This is helpful when selecting an embolic agent because some applications of embolization, such as trauma, may only require the use of a temporary agent, whereas in other applications, a permanent agent may be more appropriate. When this system is used, the temporary category is small and consists of only gelfoam, collagen, thrombin, and new biodegradable microspheres that are being developed but are not commercially available at this time.

The remaining available agents are

considered to be permanent.

When using this classification system, however, the question needs to be asked regarding what one is referring to when using the terms temporary and permanent. This classification system refers to temporary and permanent as terms used to describe the biodegradability of the actual embolic agent. It does not refer to the occlusion caused by the embolic agent because if it does, the group assignment might change for several agents. This can be exemplified with PVA particles. These particles have been classically described as permanent because they are not biodegradable and can be found in embolized tissue years after embolization.20 However, the occlusion caused by PVA particles is not always permanent. Recanalization has been demonstrated, with proposed mechanisms including angiogenesis and

capillary regrowth caused by vascular proliferation inside organized thrombus and resorption of the thrombus in between PVA particles found in the vessel lumen after resolution of the initial inflammatory response.

in 1945. Later studies have demonstrated the temporary nature of the occlusion induced by gelfoam,

embolized has also been described.

Although this system can be useful in guiding interventionalists toward the use of appropriate agents, it becomes clear that overlap can exist between these categories. For example, coils are available in various sizes, making them appropriate to use for certain indications in both large and small vessels.

abnormality) or flow-directed (delivered by flow from a catheter position proximal to a vascular abnormality (Table 1.1).

Various mechanical agents are available to treat focal vascular abnormalities. The most common agents include coils and plugs. Detachable balloons, which are available in most of the world outside of the United States, can also be used as a mechanical agent to treat focal vascular abnormalities. In theory, they can be used to quickly occlude a vessel at a precise position and have the ability to be repeatedly repositioned as needed. However, they were recalled in the United States due to manufacturing and placement issues and have essentially been replaced with new, detachable coils.

Pushable and detachable coils are available, and both are manufactured in sizes that can be delivered through standard 4-Fr or 5-Fr angiographic catheters or microcatheters. Synthetic fibers are often attached to these coils to increase their thrombogenicity (Fig. 1.1). Pushable coils require a guidewire or dedicated coil pusher to advance the coil through and out of the delivery catheter to the site of the vascular abnormality. Detachable coils are attached to the coil pusher and are released either mechanically or electrically when they are appropriately positioned (Fig. 1.2). Typically, multiple coils are required for embolization, although the hydrogel-coated coils may decrease the number of coils required for occlusion.

available and include the Medusa Vascular Plug (EndoShape, Inc., Boulder, Colorado) and the MVP Micro Vascular Plug System (Reverse Medical Corporation, Irvine, California) (Fig. 1.4). Both of these plugs have received U.S. Food and Drug Administration (FDA) approval in 2013 for use in the peripheral vasculature. The MVP Micro Vascular Plug is a smaller system and is delivered through a microcatheter to occlude small vessels.

Irregularly shaped PVA particles were the initial particulate embolic agent and still remain the standard particulate agent used by most interventionalists (Fig. 1.5). There are disadvantages that are inherent to the use of particulate PVA, including size variability, particle aggregation, and microcatheter occlusion during delivery. This prompted the development of spherical embolic agents, allowing for significant growth in this area during the last two decades.34 Calibrated spherical agents are now available, including PVA-based microspheres, trisacryl gelatin microspheres, and Polyzene-F–based microspheres. These agents are used commonly for procedures such as uterine fibroid embolization and other tumor and organ­based indications. Drug-eluting microspheres (Fig. 1.6) and yttrium 90 microspheres have helped develop an entire subspecialty of interventional oncology. Resorbable microspheres represent a significant future advance in this area, potentially allowing for the creation of temporary, reproducible arterial occlusion with embolization.