Angioarchitecture related to endovascular therapies 
Before contemplating therapy of an AVM, the angiography must be scrutinised for the following points: the nature and number of the feeding arteries, the presence or absence of flow-related aneurysms, the number of separate compartments of the malformation, any arterial or venous ectasias near to or within the malformation, and the nature of the venous drainage. On the arterial side, flow-related aneurysms are typically present at branching points of the major feeding arteries. They classically resolve following treatment of the AVM and are due to vascular remodelling following increased shear stress [27]. Although not a contraindication for endovascular treatment, they present a danger to the neurointerventionalist, because flow-directed catheters are prone to entering the aneurysm rather than the distal vessels. Concerning the arterial side of the AVM, both the number and the nature of the feeding arteries need to be assessed as they determine whether endovascular approaches will make sense. A large number of only slightly dilated feeders will make an endovascular therapy more challenging than those with a single large feeder [28]. Concerning the nature of the feeding artery, there are two basic types of feeding arteries. Direct arterial feeders end in the AVM. Indirect arterial feeders supply the normal cortex and also supply the AVM “en passage” via small vessels that arise from the normal artery. While direct feeders are safe targets for an endovascular therapy (Fig. 6), en passage feeders may carry the risk of inadvertent arterial glue migration to distal healthy vessels (Fig. 7). In this regard, the “security margin” of the catheter position has to be briefly discussed. Liquid embolic agents may cause reflux at the end of the injection. Depending on the agent, the microcatheter, the injection technique and the skills of the operator, this reflux may be as far as 1 cm proximal to the tip of the catheter. A safe deposition of liquid embolic agent is therefore only possible if the catheter tip is distal enough to be beyond any vessel that supplies normal brain tissue. In the case of en passage feeders, this may not be the case, especially if the catheter is only hooked into the feeding artery and will jump backwards because of the jet effect when injecting a liquid embolic agent. Concerning the angioarchitecture of the nidus, intranidal arterial aneurysms and venous varices that indicate weak points need to be recognised as well as the number of compartments and their nature (nidal vs. fistulous). Finally, on the venous side of the AVM, the number of draining veins per compartment (the more the better for endovascular treatment if venous migration should occur), possible drainage into the deep venous system (higher risk of haemorrhage, more difficult surgical treatment) and stenosis, which restrict venous outflow, have to be identified to fully determine the risk of a specific AVM. At the present time, this information can only be obtained by conventional digital subtraction angiography, which in our practice still precedes any treatment decision in AVMs.
Fig. 6 Single-feeder AVMs are easier to embolise with a higher chance of a complete cure compared with multi-feeder AVMs. In this single compartment AVM, the microcatheter is brought to an intranidal position where a histoacryl deposition was able to completely occlude the AVM
Fig. 7 Whereas the feeder type in Fig. 6 was of the terminal or “direct” type, the feeder type of this AVM is of the “indirect” or “en passage” type. These “en passage” feeders may carry the risk of inadvertent arterial glue migration to distal healthy vessels and in our opinion speak strongly to contraindicate an endovascular treatment approach