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The recent publication of the ARUBA trial, showing higher complication rates with treatment compared with the natural history over a short period of follow-up, puts even more pressure on the physician to achieve complete BAVM eradication without complication. These lesions are often treated by multimodality therapy with some combination of endovascular embolization, radiosurgery, and microsurgical resection; however, multimodality therapy involves the additive risk of procedural complication with each procedure. While surgical resection has long been accepted as monotherapy with good cure rates, staged pre-operative endovascular embolization has facilitated microsurgical resection with lower blood loss. Endovascular embolization is more often utilized in conjunction with surgical resection, and often the portions of the AVM and feeders that are completely embolized with Onyx or glue may not be surgically resected since they have been ‘‘internally obliterated.’’ We present a case where the AVM was preoperatively embolized with Onyx and subsequently partially surgically resected. Post-operative angiography showed complete obliteration or ‘‘cure’’ of the AVM with no filling of the nidus or early venous drainage. The patient presented 12 months later with seizures and imaging showed volume loss in the residual Onyx cast and recanalization of the AVM nidus. The patient subsequently underwent repeat resection with complete removal of the residual AVM and Onyx cast. To our knowledge this is the first published report of volume loss within the Onyx cast leading to recanalization of the AVM nidus. This suggests that extreme care should be taken with partial resection of the AVM nidus or with embolization for cure, as late recanalization may occur. Keywords Brain arteriovenous malformations, intracranial AVM, Onyx copolymer, therapeutic embolization Introduction Onyx embolization of brain arteriovenous malformations (BAVMs) was first described by Taki et al. in 1990.1 Since that time Onyx embolization has become a widely utilized tool in the multimodality treatment of BAVMs or in some cases as a stand-alone, curative treatment.2,3 Cure rates for those BAVMs that are completely embolized as the initial treatment have varied,4 and rates of angiographic stability for those BAVMs that are partially embolized have also been demonstrated.5 When employed in multimodality therapy, Onyx is often used for embolization of the deep feeding arteries of the AVM, making surgical excision safer and with less blood loss. The Onyx cast is easily visualized at surgery, and embolized feeding vessels coursing from the brain parenchyma can be cut with no bleeding. While the intention of surgery is generally to remove all of the embolic material and the remaining AVM, Onyx can be left in the parent artery permanently.6 It is widely thought that good nidal penetration with Onyx is permanent,7 and AVM recurrence occurs only through angiogenesis or recruitment of new channels which were not embolized. To our knowledge, the case below is the only report of resorption of part of the Onyx cast with nidal recanalization and recurrence. Case report A 9-year-old girl presented to our institution with an asymptomatic right occipital Spetzler-Martin Grade 2 AVM (Figure 1). She was initially monitored conservatively with intention to treat once she was older; however, she then transferred her care to another institution where she was treated with endovascular embolization with Onyx, followed by partial surgical resection of the nidus and Onyx cast. After embolization, immediate Cerebrovascular Center, Neurological Institute, Cleveland Clinic, Cleveland, OH, USA Corresponding author: Andrew M. Bauer, Cerebrovascular Center, 9500 Euclid Ave / S80 Cleveland, OH 44195 USA. Email: bauera3@ccf.org 352 Interventional Neuroradiology 21(3) Figure 1. A 9-year-old girl with a right occipital Spetzler–Martin grade II AVM. a and b: Axial T2 weighted MRI showing right occipital AVM, pre-treatment. c and d: A-P (c) and lateral (d) digital subtraction angiographic images via left vertebral injection, showing right occipital AVM, pre-treatment. post-operative angiography demonstrated no residual flow in the nidus and there was no early venous drainage (Figure 2). Post-operative angiography revealed a partial resection of the Onyx cast again with no flow in the nidus and no early venous drainage (Figure 3). Nine months later she presented at our institution with spells consistent with occipital lobe seizures but remained otherwise asymptomatic. Angiography at that time revealed recurrence of the AVM with flow in the nidus and early venous drainage (Figure 4). Careful examination of the Onyx cast showed overall volume loss of the Onyx and recanalization of previously embolized vessels (Figures 5 and 6). She subsequently underwent re-resection of the AVM at another institution and angiography revealed gross total resection. Discussion Brain AVM treatment has evolved greatly from the time where surgical resection was the only treatment option, to multimodality therapy employing endovascular embolization, stereotactic radiosurgery, and surgical resection. The use of newer liquid embolic agents such as n-butyl-cyanoacrylate (n-BCA) (Cordis Neurovascular, Miami Lakes, Florida, USA) and ethylene vinyl alcohol copolymer (EVOH) (Onyx, eV3 Neurovascular, Irvine, California, USA) have shown great advantages over particle embolization in that the liquid embolics can be delivered directly to the nidus of the AVM and achieve deeper penetration. In addition, they seem to offer longer-term occlusion compared with particulate embolic agents. Onyx has the advantage of longer polymerization time, allowing for intermittent infusion, longer infusion times, and perhaps better control. As endovascular embolization techniques for BAVMs have improved, many reports have been published using embolization as a stand-alone technique for AVM cure.7–14 Although highly dependent on individual AVM and cerebrovascular anatomy, rates of complete angiographic occlusion have been reported between 15 and 50%.2,4,9,12,13,15,16 More commonly, Bauer et al. 353 Figure 2. A-P (a) and lateral (b) digital subtraction angiography left vertebral injection, post-embolization, shows complete obliteration of right occipital AVM. Figure 3. A-P (a) and lateral (b) digital subtraction angiographic images via left vertebral injection, post-surgical resection of right occipital AVM show no residual flow into the AVM. Figure 4. Digital subtraction angiography via left vertebral injection, A-P (a) and lateral (b) show flow into the previously embolized and partially resected right occipital AVM nidus. 354 Interventional Neuroradiology 21(3) Figure 5. Fluoroscopic images immediate post-embolization, A-P (a) and lateral (b), showing the well-formed Onyx cast. Images 1 day post-surgical resection of the AVM and partial resection of the Onyx cast, A-P (c) and lateral (d). Fluoroscopic images, A-P (e) and lateral (f) at 9-month follow-up showing overall loss of volume of the Onyx cast. The AVM nidus had recanalized at follow-up (see Figure 4). Onyx embolization is used as an adjunct to surgical or radiosurgical therapy. After embolization, surgical resection is hastened as the embolic material is easily visualized within the nidal and feeding vessels. Live AVM feeders are sacrificed with bipolar electrocautery or ligation with AVM clips. Some centers have reported the safe division of feeders filled with Onyx, leaving a small amount of Onyx behind as the intravascular equivalent of an AVM clip.17 In this report, we show a case where Onyx embolization followed by surgical resection showed complete angiographic resolution of the AVM with no filling nidus or early draining vein. As described above, some of the Onyx cast was retained at surgery, likely due to the proximity of eloquent structures. This AVM showed early recurrence at less than 1 year. There are two proposed mechanisms of AVM recurrence after embolization/resection. First, if there Bauer et al. 355 Figure 6. T2 axial MRI images on post-operative day 1 (a and b), showing post-surgical changes and Onyx within the residual AVM nidus. MRI imaging at 9-month follow-up (c and d) shows markedly decreased volume of Onyx with recanalization of the AVM nidus (c). is incomplete nidal penetration, channels that may not have filled initially may be recruited after embolization and flow appears in areas where it was not seen previously. Second, there may be some local angiogenic factors that contribute to early regrowth of abnormal vessels and communication within the nidus itself. Until now, Onyx embolization was thought to be permanent, such that the Onyx cast could be resected or left in place with no risk of recanalization or recurrence of the embolized portion of the AVM. In this case, the overall volume of the Onyx cast decreased from the initial post-resection angiogram to the 9-month follow-up angiogram with gross recurrence of flow within the AVM (Figures 5 and 6). This reabsorption or breakdown of the Onyx suggests a third mechanism of possible AVM recurrence after embolization. In our experience with the treatment of AVMs, this is the only case where we have seen this phenomenon. In our practice, we do not use Onyx embolization as a stand-alone treatment, and only occasionally as an adjunct to surgical therapy. It is our strong belief that if embolization is used in conjunction with surgery, attempts should be made to resect the entirety of the Onyx cast, placing AVM clips or using bipolar electrocautery on the AVM feeders. Although we do not advocate embolization as a stand-alone treatment for cure in BAVMs, if this method is used, these patients should be followed long term to mitigate the chance of recurrence and future hemorrhage. 356 Funding This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors. Conflict of interest The authors declare no conflict of interest. References 1. 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