*3.2.2 Transvenous embolization*

Although this technique recalls the venous outflow occlusion, one of the most fearful situations for a neuroendovascular specialist during AVM embolization, there is increasing use of transvenous embolization in selective cases [45–47]. Deeply located hemorrhagic AVMs with small tortuous feeders or en passage feeders may have complex anatomy precluding the use of transarterial embolization. Transvenous embolization have theoretical advantages of better penetration of the AVM nidus, less risk of ischemic events due to arterial occlusion, and relatively easier navigation through enlarged and usually straighter veins. Significantly high complete obliteration rates between 80 and 100% [47, 48] were reported in the literature. A transjugular venous access with a triaxial system including a sturdy guiding system of an 8F sheath and distal access catheter is crucial for safe and efficient practice of transvenous embolization. Alternatively, a 6F transjugular access with a 6F distal access catheter can be used. Detachable tip microcatheters or balloon microcatheters in feasible anatomy may prevent reflux and occlusion of the venous drainage. Some centers intentionally let an approximately 3 cm reflux after a longer injection and leave the microcatheter inside the system after cutting the microcatheter shaft at the skin incision site. Mendes et al. [47] did not report any thromboembolism associated with transvenous embolization cases with transjugular access where all microcatheters were left intentionally within the venous system. Arterial access and arterial flow arrest with transient balloon occlusion or simultaneous transarterial embolization should be used if technically feasible. Although there is no established standard practice of transvenous embolization, theoretical criteria to select cases include small lesions with a nidus of <2 cm, hemorrhagic AVMs, patients who are not good surgical candidates, arterial feeders that are not amenable to transarterial embolization like en passage feeders, lenticulostriate arteries or choroidal arteries, and AVMs preferably with a single drainage vein [46, 48]. Nonadhesive ETOH should be used for transvenous embolization [49]. Systemic hypotension during and after transvenous embolization is also crucial for decreasing the arterial pressure that must be overcome for better nidal penetration. Another technical challenge for transvenous embolization arises from frequently encountered venous anatomical variations and fragile structure of the cerebral veins that may lead to disastrous consequences in case of vessel injury. One must pay extreme caution for safe transvenous navigation of the microcatheter without leading microwire. Aneurysms, ectasia, or stenosis in the draining vein may preclude transvenous embolization due to high risk of vessel injury during navigation.
