Chapter: Invasive therapy
Article: 7 of 15
Update: Feb 07, 2021
Author(s): Müller-Wille, René
Embolization is a minimally invasive procedure for closing blood vessels or vessel malformations using intravascular application of special embolization materials. Depending on the lesion, embolization is performed via an arterial, venous or direct intravascular access. Traditionally, it is performed by means of transarterially inserted catheters.
After insertion of a guiding sheath into an access vessel (Seldinger technique, inserted in the groin for example), a guiding catheter is navigated through the vascular system in the direction of the vascular malformation. The catheters are maneuvered in the vascular system by means of specifically configured guidewires. The area of interest is initially imaged diagnostically using digital subtraction angiography (DSA). If the vessels are very small, an even smaller microcatheter can be advanced through the guiding catheter, which is then used for embolization (so-called coaxial technique).
This makes it possible to close large fast-flow vascular formations (arteriovenous malformations) practically anywhere in the body via a very small percutaneous access relatively gently and with few complications (minimally invasive therapy). It should be noted that several interventions are usually necessary and the vascular defect cannot be corrected in one session.
The various embolization materials used to close pathological vessels differ considerably in their principle of action, release mechanism and level of closure.
The choice of the right material requires a lot of experience and depends mainly on the morphology and type of vessel malformation as well as the hemodynamics. The different materials can also be combined. The following materials are used in the treatment of fast-flow vascular malformations and vascular tumors:
A coil is a very fine metal spiral of varying length and thickness, which is applied via a catheter. Coils consist of biocompatible materials such as platinum, which can remain permanently in the body. The occlusion of the blood vessel is not generated by the metal coil itself, but by local activation of blood coagulation by the coil. To accelerate local blood coagulation, some coils are additionally equipped with synthetic fibers or bioactive coatings. Apart from their size and form (helical, spiral, 3D shaped), the coils essentially differ in their release mechanism. So-called “pushable” coils are released by simply pushing them forward through the catheter lumen with a special wire (“coil pusher”) without them being retractable or replaceable. So-called “detachable” coils are introduced via the catheter lumen, but stay connected to a wire and can be intentionally separated from this wire, e.g., by a mechanical release mechanism. This enables very precise placement of the metal coil, as it can be pulled back via the connected wire and replaced if the initial position is unfavorable. Coils are usually selected 15–20% larger than the diameter of the target vessel to prevent the spiral from slipping or being washed away with the blood stream. Coils play a major role especially in the targeted treatment of pulmonary AV fistulas. Occasionally, coils are also used as a complementary embolization material in the treatment of complex arteriovenous malformations for flow modulation or to close communicating veins.
Vascular plugs are flexible metallic sealing bodies consisting of a self-expanding nitinol braid. Depending on type and diameter, the plug is connected to a wire and inserted through the lumen of a catheter or a guiding catheter. When the wire is turned counterclockwise, the plug is released from the wire or, if the plug is initially in an unfavorable position, it is pulled back and repositioned. To ensure secure fixation of the plug, the diameter of the plug is often at least 30% larger than that of the target vessel. The finely braided nitinol mesh leads to activation of blood coagulation and thus to closure of the blood vessel. The plug is suitable, for example, for closing large arteries in pulmonary AV fistulas. Even large dysplastic veins in the context of complex venous malformations can be closed with the plug. Another possible application of the plugs is the occlusion of dominant drainage veins or flow modulation in arteriovenous malformations.
Nowadays, there are also smaller plug-like closure devices such as the Micro Vascular Plug (MVP), which can also be applied via a microcatheter. They are suitable for closure of smaller arteries.
Ethylene vinyl alcohol (EVOH) copolymer is available as a mixture with dimethyl sulfoxide (DMSO) and micronized tantalum. The added tantalum ensures extremely good fluoroscopic visibility throughout the entire intervention. Upon injection through a microcatheter and contact with blood, DMSO diffuses out of the mixture, resulting in very slow, controlled precipitation of EVOH. The result is a viscous and coherent lava-like mass that can be modeled for many minutes. EVOH is mainly used in the treatment of complex arteriovenous malformations. By means of the so-called “plug-and-push” technique, the complete nidus can often be embolized from one catheter position. Since DMSO leads to irritation of the vessel wall, pain during the injection is not uncommon. It is therefore recommended that complex embolizations with EVOH be performed under general anesthesia.
The tissue adhesive n-butyl-2-cyanoacrylate (“glue”) consists of monomers which polymerize very quickly in combination with fluid containing electrolytes (e.g., blood). Embolization is purely flow-controlled and takes only a few seconds after injection via a catheter lumen until the adhesive is hard and polymerized. Polymerization can be slightly delayed by the addition of Lipiodol. The instant hardening is the main disadvantage of n-butyl-2-cyanoacrylate, as the adhesive can stick to the catheter tip like superglue. In addition, the depth of tissue penetration cannot be easily controlled. To prevent the catheter lumen from getting stuck with glue, it must be pre-rinsed with glucose solution. The handling of n-butyl-2-cyanoacrylate requires a great deal of experience and is not very predictable. N-butyl-2-cyanoacrylate is mainly used in the embolization of cerebral or spinal arteriovenous malformations.
In addition to sclerotherapy of venous malformations, pure alcohol is also used in the embolization of arteriovenous malformations. The vaso-occlusive and thus embolizing effect of pure ethanol is based on massive damage to the endothelium, with necrosis and subsequent thrombosis, and is locally effective. A major disadvantage is the lack of X-ray density as well as the low viscosity, which makes embolization with alcohol difficult to control without systemic contamination. The choice of the correct dosage for the treatment of high-flow malformations requires an extremely high level of experience, since the systemic (e.g., occlusion of the pulmonary tract) and local side effects (e.g., necrosis, nerve damage) can be considerable in some cases when alcohol is applied intra-arterially. Embolizations with pure ethanol are very painful and should therefore be performed under general anesthesia.
During and after embolization, adequate pain management must be administered, as embolization can lead to tissue inflammation and ischemia. Incorrect embolization can cause tissue damage (necrosis). Thrombosis and thromboembolism can also occur, in the worst case with pulmonary embolism. Catheter guidance with guidewires can cause damage (a tear or dissection) to the vessel wall, which may further lead to bleeding or occlusion from the vessel injury. In addition, the potential complications caused by iodinated X-ray contrast media must be taken into account (e.g., renal insufficiency, hyperthyroidism, contrast medium allergy). Complications at the puncture site include a small amount of bleeding or very rarely life-threatening large hematomas caused by bleeding. Regular monitoring of the puncture sites or the pressure dressing is therefore necessary in addition to bed rest.
