**4. Complications of the endovascular treatment**

Overall morbidity and mortality rates from the largest series of AVM embolization range from 0 to 22% and 0 to 3%, respectively [52–56]. Intracranial hemorrhage during or after the embolization can be seen in between 2 and 4.7%. Most common reason for hemorrhage is vessel perforation due to microcatheter or microwire manipulations. Venous outflow obstruction and normal perfusion pressure breakthrough are other mechanisms that may cause bleeding after embolization. Normal perfusion pressure breakthrough is believed to occur due to overdilated capillaries in the parenchyma surrounding the AVM in the setting of steal phenomenon or ischemia. After embolization or resection of the AVM, increased pressure or perfusion in the surrounding parenchyma may cause rupture of the maximally dilated capillaries [57]. Staged embolization is preferred in large AVMs in order to prevent normal perfusion breakthrough phenomenon [58]. If the surgical resection is not planned to immediately follow the embolization, a staged embolization in several sessions is performed. Although there is no established rule, less than 50% size reduction is aimed in every session in order to prevent postembolization hemorrhage. About 4–6 weeks can be waited in between every session for hemodynamic stabilization. If there is significant slowing of the venous drainage during any moment of embolization, either complete obliteration must be achieved by embolization or surgical resection must be performed urgently following embolization. Therefore, it is utmost important to evaluate the venous drainage pattern before and immediately after the embolization. In rare cases, perforation and bleeding may occur during superselective microcatheter injection in a small branch. If there is any concern for intraoperative rupture or postprocedural neurological deterioration happens, a brain computerized tomography must be obtained to rule out hemorrhage. In case of hematoma with mass effect or impending expansion risk, emergent craniotomy for hematoma evacuation helps minimize neurological deficit and may avoid mortal consequences [59].

Ischemic complications can occur due to thromboembolism from the embolic material or thrombus formation within or along the guiding catheter or microcatheter. Mechanisms or thromboembolism are reflux or nontargeted flow of the liquid embolic into a normal branch during embolization. Heparinization is routinely used during AVM embolizations. Other general complications related to angiographic procedure include groin hematoma, retroperitoneal hematoma, arteriovenous fistula, and contrast nephropathy with similar frequencies compared to other neurointerventional procedures.

Buffalo grading system [60] is used to stratify the risks associated with endovascular treatment with curative intent. Risk of the endovascular treatment increases with number of arterial feeders, smaller diameter of the feeders, and eloquent location. One point is given for AVMs with one or two arterial feeders, 2 points is given

#### *Endovascular Treatment of Cerebral Arteriovenous Malformations DOI: http://dx.doi.org/10.5772/intechopen.89185*

for three or four arterial feeders, and 3 points is assigned for four or more pedicles. If the diameter of the most feeders is less than 1 mm or AVM is in an eloquent location, an additional point is given. Starke et al. [61] proposed another scale to predict various factors increasing complication risk in AVM embolizations. This scale is very similar to Spetzler-Martin grading system for surgical morbidity-mortality and includes same factors including size (1 point for <3 cm and 2 points for >6 cm), eloquent location, and deep venous drainage. They proposed the need for more than one session of embolization as an additional risk factor with increased complication rate. However, Crowley et al. [56] did not find any significant difference in the complication and morbidity/mortality rates depending on the Spetzler-Martin grades. The same study did neither show a significant difference in complication rates between the ETOH versus NBCA cases.

Microcatheter retention may occur between 3 and 8% of cases [42]. Risk of microcatheter retention increases after longer injections with long reflux, in smaller branches and branches with significant curves and tortuosity. If the vessel accommodating the microcatheter has tortuosity, long refluxes must be avoided. Safe reflux for an efficient push and plug technique is usually between 15 and 20 mm. Smaller vessels with or without tortuosity require a smaller reflux of less than 10 mm [50]. Although ETOH is nonadhesive and initially microcatheter retention was expected to decrease with ETOH use, clinical practice and experience demonstrated the contrary. The risk increased due to longer injection times and the need for a significant reflux to form a plug for efficient ETOH injection. Loh and Duckwiler [40] found higher incidence of microcatheter retention and difficulty in removing the delivery microcatheter in the first trial comparing Onyx to NBCA, which led to FDA approval of the Onyx. ETOH is less thrombogenic than NBCA and does not cause immediate occlusion in case of an unintended flux.

Development of detachable tip microcatheters including Apollo (Medtronic, Irvine, CA, USA) and Sonic (Balt, Montmorency, France) has substantially decreased the incidences of microcatheter retention [62]. These microcatheters have variable lengths (1.5 or 3 cm) of detachable tip segment that allows better estimate of tolerable reflux thanks to markers, and detachment will occur if there is enough reflux to create a tension point. Sonic (Balt, Montmorency, France) microcatheter usually detaches after a long injection with substantial reflux; whereas Apollo (Ev3) microcatheter may not detach in many cases and can be retrieved as a whole without detachment.

Introduction of distal access catheters helped better navigation of very distal tortuous feeders and facilitated retrieval of microcatheters after a long injection. The use of distal access catheters is almost the standard of practice especially for cases with superficial lesions with distal and tortuous feeders. Distal location of a guide catheter eliminates the need for repeated navigation of a microcatheter through a tortuous supraclinoid internal carotid, anterior, middle cerebral artery, or basilar artery.

Although recanalization after complete obliteration of an AVM is extremely rare, there is still risk of recanalization that mandates angiographic follow-up after complete obliteration. Potts et al. [63] in their extensive review of multiple series of AVMs cured with embolization found 4.5% recurrence rate on follow-up angiography. However, there was not any case of recurrence with rupture or any adverse event during follow-up period. Mechanisms that may lead to recurrence include incomplete embolization due to nonvisualization of a component during initial embolization, mass effect from a hematoma, or recanalization of an initially thrombosed compartment.

*Vascular Malformations of the Central Nervous System*
