Embolization Techniques and Applications

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Describe embolotherapy and some of its indications.

Embolotherapy is temporary or permanent vascular occlusion induced by the intravascular administration of materials via a percutaneous route. Embolization has various clinical applications, including control of bleeding; treatment of vascular malformations; and tumor or organ ablation for curative, palliative, or preoperative purposes.

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What materials are most commonly used for embolization?

Various embolic materials are commercially available. The most widely used embolic materials are absorbable gelatin sponge (Gelfoam), a temporary agent; metallic coils; and polyvinyl alcohol (PVA) particles. Specific clinical situations may warrant the use of other agents, such as absolute ethanol, synthetic microspheres, or liquid “glues.”

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What is Gelfoam, and how is it prepared and delivered?

Gelfoam is a reabsorbable gelatin that is most widely used in its sheet form. Wedges (1 to 2 mm) of Gelfoam are divided from the larger sheets. The Gelfoam can be injected as “torpedoes” through a catheter placed in a blood vessel, or the Gelfoam can be suspended in a contrast/saline slurry, which can be injected.

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How does Gelfoam work, and when is it used?

Gelfoam causes vascular occlusion by mechanically obstructing vessels, serving as a matrix for thrombus formation, and causing endothelial inflammation that incites further thrombus formation. Gelfoam is reabsorbed in 5 to 6 weeks, during which time vessel recanalization is anticipated. Clinical situations in which temporary vascular occlusion is preferred include pelvic arterial hemorrhage after trauma, priapism, peripartum hemorrhage, and some cases of upper gastrointestinal (GI) bleeding. The rationale for using Gelfoam in these situations is based on the belief that use of a temporary agent would minimize long-term ischemic effects on the end organ.

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What are metallic coils?

Metallic coils are made of either stainless steel or platinum. Dacron fibers are woven into some coils to promote thrombosis. Coils are available in a wide variety of shapes, sizes, and configurations. Special wires are used to push the coils through catheters that have been placed into the vessel intended to be embolized. Coils occlude vessels by causing mechanical obstruction, inducing clot formation, and provoking an inflammatory reaction.

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When are coils preferred?

One way to classify embolic materials is based on whether they cause permanent or temporary occlusion. Another way is based on the size (diameter) of the vessel where the occlusion occurs. Coils cause vascular occlusion in vessels that are 1 to 2 mm in diameter and larger. Coils are preferred in clinical situations in which permanent occlusion is intended in vessels of this size. This includes the treatment of arteriovenous fistulas (AVFs), GI bleeding, aneurysms, endoleaks after endovascular repair of abdominal aortic aneurysm (AAA), and traumatic vascular injuries in the proper clinical settings. Coils should not be used if occlusion is desired in vessels smaller than 1 mm, including situations in which the target of embolization is an organ. When embolizing the uterus or performing chemoembolization of the liver, coils are not used. PVA particles or ethanol can be used to cause microvascular thrombosis, depending on the specific clinical situation.

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What happens when coils are the wrong size?

When coils are undersized, they can remain mobile after they are pushed out of the catheter and continue to travel with the flow of blood until they embolize to a vessel that is smaller than the diameter of the coil. If this happens while a venous embolization is being performed, the coils travel back toward the right heart and often through the pulmonary artery before lodging in a small vessel. The lungs are also a likely final destination for coils that inadvertently pass through an AVF. When deployed in an artery, coils that are too small can migrate into distal branches or into another vascular distribution. This migration may result in nontarget embolization and decreased efficacy of the intended embolization. Coils that are too large can partially recoil into the parent vessel or cause the catheter to back out of the proper vessel. Retrievable coils are now available and allow optimization of positioning, configuration, and sizing before full deployment.

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When are PVA particles used?

PVA particles range in size from 50 to 2000 µm. Their mechanism of action is to obstruct vessels physically and incite extensive granulation tissue formation. PVA particles result in permanent occlusion. The size of the particle used is based on the location of desired thrombosis. The smaller the particles are, the more distal the embolization. Particle selection is often based on the experience of the operator. If the particles selected are too small, end-organ ischemia and necrosis may occur. If the particles are too large, collateral vessels may quickly reconstitute blood flow to the target organ, significantly decreasing the efficacy of the procedure. The smallest particles are reserved for tumor embolization or preoperative devascularization of other tissues because they can cause significant tissue ischemia via occlusion down to the capillary level.

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