Embolization - Agents

Agents

Liquid embolic agents - Used for AVM, these agents are able to flow through complex vascular structures so the surgeon does not need to target his catheter to every single vessel. Onyx is an example for a liquid embolic agent.

• nbca - n-butyle-2-cyanoacrylate - This agent is a permanent rapidly acting liquid, similar to glues sold under trade names such as "SuperGlue," that will polymerize immediately upon contact with ions. It also undergoes an exothermic reaction which destroys the vessel wall. Since the polymerization is so rapid, it requires a skilled surgeon. During the procedure, the surgeon must flush the catheter before and after injecting the NBCA, or the agent will polymerize within the catheter. The catheter must also be withdrawn quickly or it will stick to the vessel. Oil can be mixed with NBCA to slow the rate of polymerization.

• ethiodol - Made from iodine and poppyseed oil, this is a highly viscous agent. It is usually used for chemoembolizations, especially for hepatomas, since these tumors absorb iodine. The half life is five days, so it only temporarily embolizes vessels.

Sclerosing agents - These will harden the endothelial lining of vessels. They require more time to react than the liquid embolic agents. Therefore, they cannot be used for large or high-flow vessels.

• ethanol - This permanent agent is very good for treating AVM. The alcohol does need some time to denature proteins of the endothelium and activate the coagulation system to cause a blood clot. Therefore, some surgeons will use a balloon occlusion catheter to stop the blood flow and allow time for ethanol to work. Ethanol is toxic to the system in large quantities and may cause compartment syndrome. In addition, the injections are painful.

• ethanolamine oleate - This permanent agent is used for sclerosing esophageal varices. It contains 2% benzyl alcohol, so it is less painful than ethanol. However it does cause hemolysis and renal failure in large doses.

• sotradecol - This agent is used for superficial lower extremity varicose veins. It has been around for a very long time and is a proven remedy. However, it does cause hyperpigmentation of the region in 30% of patients. It is less painful than ethanol.

Particulate embolic agents - These are only used for precapillary arterioles or small arteries. These are also very good for AVM deep within the body. The disadvantage is that they are not easily targeted in the vessel. None of these are radioopaque, so they are difficult to view with radiologic imaging unless they are soaked in contrast prior to injection.

• Gelfoam hemostasis - Temporarily occludes vessels for five weeks. Works by absorbing liquid and plugging the vessel. Composed of water-insoluble gelatin, so the particles may travel distally and occlude smaller capillaries. One way to localize the injection of gelfoam is to make a gelfoam sandwich. A coil is placed at a precise location, then gelfoam is injected and lodged into the coil.

• polyvinyl alcohol (PVA) - These are permanent agents. They are tiny balls 50-1200 um in size. The particles are not meant to mechanically occlude a vessel. Instead they cause an inflammatory reaction. Unfortunately, they have a tendency to clump together since the balls are not perfectly round. The clump can separate a few days later, failing as an embolic agent.

• acrylic gelatin microspheres - These are superior permanent particulate embolic agents. They are similar to PVA, but are perfectly round, so they do not clump together. The balls are fragile, so they may crack inside small catheters.

Mechanical occlusion devices - These fit in all vessels. They also have the advantage of accuracy of location; they are deployed exactly where the catheter ends.

• coils - These are used for AVF, aneurysms, or trauma. They are very good for fast-flowing vessels because they immediately clot the vessel. made from platinum or stainless steel. They induce clot due to the Dacron wool tails around the wire. The coil itself will not cause mechanical occlusion. Since it is made of metal, it is easily seen in radiographic images. The disadvantage is that large coils can disrupt the radiographic image. The coil may also lose its shape if the catheter is kinked. Also, there is a small risk of dislodging from the deployed location.

• detachable balloon - Treats AVF and aneurysms. These balloons are simply implanted in a target vessel, then filled with saline through a one-way valve. The blood stops and endothelium grows around the balloon until the vessel fibroses. Unfortunately, the balloon may be hypertonic relative to blood and hence rupture and fail, or it may be hypotonic and shrink, migrating to a new location.