What are metallic coils used for embolisation?
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.
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.
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.