Various tools are at the disposal of the interventional neuroradiologist and they are deployed according to the needs of the case. Below are brief desriptions of the platforms which are most frequently used. Dural and pial fistulas may be suitable for curative treatment by endovascular means. It is the treatment of choice for rare congenital vascular malformations of the vein of Galen in neonates.
In our practice currently embolisation is most frequently utilised to reduce blood flow through larger AVMs as a prelude to microsurgical removal or radiosurgical treatment rather than in an attempt to completely obliterate the malformation. Several treatments may be undertaken to gradually reduce blood flow rather than risking sudden large changes in blood flow that might create stress on the walls of blood vessels or produce swelling in adjacent brain.
In recent years endovascular embolisation of the AVM nidus are used less frequently than previously ahead of radiosurgical treatment as the material left within the malformation may make targeting of the lesion difficult and this adversely affect the prospects for obliteration of the malformation.
Endovascular Treatment of Aneurysms Assosciated with AVM
The same coils used to treat aneurysms elswhere can be deployed to secure aneurysms that develop on arteries feeding an arteriovenous malformation (flow aneurysms), those within the nidus or on veins draining blood from it. Where an aneurysm might be identified as a bleeding point on an AVM it can be selected for treatment by itself with a reduction in the further risk of bleeding. With very large AVMs which would otherwise be untreatable this may represent a very useful strategy to protect the patinet while exposing them to reasonable levels of procedural risk. Coils are sometimes utlilised to occlude veins draining fistulas formed with dural lining of the brain or spine.
Liquid Embolic Agents
NBCA glue is a variety of liquid embolic agent. These agents have the advantage of being easy to inject through the tiny catheters used for treatment. Adding iodine contrast or tantalum powder renders them visible on x-ray without adversely affecting its adhesive qualities. Liquid embolic agents can effectively penetrate deep into the nidus of an AVM which coils cannot. Different concentrations may be used to tailor how quickly the glue solidifies to the situation.
Onyx is the trade name for ethylene vinyl alcohol copolymer, a non-adhesive liquid embolic agent. It is combined with tantalum powder to make it visible on x-ray and offers additional control and predictability as it solidifies compared to NBCA. Again it is available in a variety of concentrations. The catheters through which it is administered are first flushed with a chemical called dimethylsufoxide (DMSO). The patient and those around them may be aware of a characteristic odour that lingers for 24 hours or so after treatment, the result of DMSO being expelled throught the skin and mucus membranes.
New liquid embolic agents are emerging with different physical characteristics intended to increased their ease of use and safety. Examples in clinical use at present include Precipitating Hydrophobic Injectable Liquid or PHIL and SQUID, another ethylene vinyl copolymer dissolved in DMSO. These agents have similar physical characteristics to Onyx and subtly different handling characteristics.
Your treating neuroradiolgists will discuss the choice of materials for embolisation with you and any asssociated risks. Reactions to the materials themselves are rare. Occasionally catheters delivering the agent to the AVM can become entrapped within the embolic cast and need to be left behind in part.
There are risks to embolisation of a vascular malformation which your treating specialist will discuss with you ahead of the procedure. The treatment is planned after a multidisciplinary discussion between neurosurgeons and interventional neuroradiologists. Not all AVMs undergoing surgery require embolisation but it may reduce blood loss and prevent post-operative bleeding and brain swelling for the larger lesions. There are the general risks of any endovascular intervention such as injury to the blood vessels through which the catheters are passed, allergic reaction to materials used during treatment and stroke. Unintended deposition of embolic material into the veins draining a malformation without completely closing off blood flow from feeding arteries may precipitate swelling of the malformation and cause bleeding. The risks are affected by the size and architecture of the malformation and your doctors will endeavour to estimate these for you as part of the consent process.