This retrospective review showed that EVOH is a safe embolic agent and has a valuable role in the treatment of various peripheral vascular pathologies including, but not limited to type II endoleaks, AVMs, VMs, postoperative peripheral PSAs, and arteriovenous (AV) fistulas.
Traditionally, administration of EVOH is considered technically complex and difficult to control. A steep learning curve of the embolization technique can result in an incomplete or nontarget embolization. This has likely limited the adoption of this embolic agent in the peripheral applications. Over a short period of use of EVOH at the current institution, high TS rates of EVOH embolization were achieved. The experience also suggested that EVOH can be safely used in the periphery.
Well-known within the literature, the delivery of Onyx involves a uniquely meticulous process (Guimaraes & Wooster, 2011; Kilani, 2015). Unlike n-butyl cyanoacrylate glue (NBCA), this nonadhesive agent does not attach to endothelium or microcatheters and has been shown to induce a very mild regional inflammatory reaction (Murayama et al., 1998; Duffner et al., 2002). Upon contact with the blood, slow solidification of EVOH in a centripetal fashion endows it with a foam like consistency. These properties allow slow, progressive and controlled deployment of the EVOH at the desired location leading to complete occlusion of desired targets of various sizes and shapes. Being a non-absorbable embolic agent, an irreversible target occlusion is achieved for permanent treatment of peripheral pathologies. However, permanent occlusion also raises the fear of healthy tissue necrosis in the event of nontarget embolization. As per authors’ experience, methodical techniques, adequate training and appropriate choice of EVOH viscosity play an important role for safe embolization and avoidance of complications. The EVOH with higher viscosity (Onyx 34) was chosen when the microcatheter tip was near the target and controlled injection was needed to prevent distal embolization. The lower viscosity EVOH (Onyx 18) was used when the target remained distant from the microcatheter tip or reaching the target proved challenging secondary to vessel tortuosity or increased vessel length. Lower viscosities of EVOH are safe for embolization of end arteries (Guimaraes & Wooster, 2011; Kilani, 2015). The size and flow rate of the target vessel was also considered in choosing the EVOH concentration.
Early literature reported higher rates (80%) of recanalization when only afferent arteries of the type 2 endoleaks (EL-2) were embolized with coils, without embolization of aneurysmal sacs, using transarterial (TA) approach. This retrospective study reported statistically significant lower rates (8%) of recanalization when aneurysmal sacs were embolized with coils using the translumbar approach (Baum et al., 2002). Recanalization through the interstices of coils could explain the incidence of recanalization with either approach. Another large (n = 84) retrospective study reported no statistically significant difference in the clinical success (i.e. absence of endoleak or aneurysm enlargement) rates of TA or TL approaches. In this study the TA approach included embolization of entire aneurysmal sac along with its afferent artery. The clinical success rates were 78% (18/23) when aneurysmal sac and afferent arteries were embolized with coils using TA approach, and 72% (45/62) when aneurysmal sac was embolized with coils/ NBCA using TL approach. (Stavropoulos et al., 2009) The study reported a complication i.e. nontarget embolization which could be related to the need for a rapid injection of NBCA to prevent its adhesion to the vessel or microcatheter tip. Another recent retrospective study showed no significant difference in aneurysm sac growth, persistent EL-2 or complications between TA and TL approaches. The study also reported technical success rate in 89% cases and absence of endoleak recurrence in 70% cases. The embolic agents used were NBCA, NBCA plus coils, or coils only (Yang et al., 2016). Few other studies reported good technical and clinical outcomes when EL-2 were embolized with EVOH or EVOH with other agents (Massis et al., 2012; Khaja et al., 2013; Marcelin et al., 2017; Abularrage et al., 2012). As per the current institutional experience, EVOH alone was sufficient to effectively occlude the afferent vessels as well as aneurysmal sac in 14/18 i.e. 78% of EL-2 cases. The 12 of total 18 EL-2 cases had prior endovascular interventions. Out of these twelve persistent EL-2 cases, EVOH alone was suffiecient in nine (75%) cases. In the remaining cases of new EL-2 (n = 1) and persistant EL-2 (n = 3), coils were used along with EVOH to occlude the aneurysmal sac. The approach for the embolization of EL-2 were either TA (89%) and TL (11%) with standard procedural steps as described before. (Bryce et al., 2018) The immediate post-embolization TS rate and CS rates were 100%. These results continue to support the use of EVOH in treatment of persistent EL-2 which are refractory to other embolic agents. The high TS rates are due to an ability of EVOH to penetrate and occlude afferent/efferent vessels and aneurysmal sacs of various sizes and shapes. Also the results such as absence of nontarget embolization or complications were similar to the results previously reported (Khaja et al., 2013; Marcelin et al., 2017; Abularrage et al., 2012).
For the treatment of new EL-2, the TA approach was preferred when access and embolization of aneurysmal sac via afferent vessels were possible. Once in the aneurysmal sac, whenever required based on the sac size, we first used coils to fill the aneurysmal sacs. Then EVOH was injected under fluoroscopic guidance until the proximal portions of efferent vessels were completely occluded. Then proximal portions of afferent vessels were occluded with EVOH while slowly withdrawing the microcatheter. When accessing an aneurysmal sac via afferent vessel using TA approach was not possible, the TL approach was used to directly enter the aneurysmal sac and EVOH was injected until aneurysmal sac and proximal portions of afferent and efferent vessel were completely occluded. In the cases of recurrent EL-2 (n = 12), the aneurysmal sacs were already partially occluded with prior embolic material, hence in most of these cases (75%) EVOH alone was sufficient to achieve complete occlusion. The preference of approach and technique of embolization were similar to the management of a new EL-2 as described before. As per this experience, EVOH was an ideal embolic agent when the target cannot be directly catheterized, for example if the route was tortuous, small in caliber or distal to negotiate with a microcatheter. In complex cases of EL-2 when access to culprit afferent vessel by TA approach or access to aneurysmal sac via TL approach were not possible, then low viscosity EVOH (Onyx 18) was injected in the proximal aspects of afferent vessels. Other endovascular approaches such as transcaval (Giles et al., 2015) and perigraft (Coppi et al., 2014), though not used in our study, can be useful.
Uterine AVMs are rare but potentially life threatening. Similar to a prior case series, this retrospective review showed 100% TS and CS rates of EVOH embolization of three high flow AVMs in women of reproductive age group (22 to 40 years). At the current institution, the use of low viscosity EVOH (Onyx 18) allowed selective embolization of the niduses of uterine AVMs that were difficult to reach with the microcatheters. None of the patients underwent subsequent hysterectomy. There is no data reporting the most appropriate embolic agent to treat uterine AVMs. The available data indicates that resorbable agents and coils are ineffective. (Barral et al., 2017)
EVOH is considered ideal for embolization of AVMs due to it's ability to penetrate and conform to the shapes of tortuous afferent arteries and variable nidus sizes. Being a non-adhesive agent, it allows precise positioning and control of the tip of the delivery microcatheters during the EVOH injections. The Interventionalist can interrupt the injection, reanalyze the EVOH cast and reinject at the new location to occlude a large AVM without filling the draining veins (Regine et al., 2015). A prior study has reported reflux of EVOH within the afferent artery during embolization of high flow AVM and nontarget embolization. This was attributed to complex angio-architecture, short arterial feeders close to parent arteries and poor radiopacity of earlier generation of EVOH. During the final stages of EVOH injection in the afferent arteries, the pressure reaches critical threshold and EVOH can reflux into the afferent artery. However with continuous fluoroscopic monitoring, interventionalists can modulate the volume and rate of injection to effectively occlude the afferent artery and avoid the reflux. Also, the use of higher viscosity EVOH to embolize the afferent arteries of high flow AVMs can cause an instant onsite polymerization and occlusion. This can avoid the further passage of EVOH through fistulous component and venous reflux (Cantasdemir et al., 2012).
In this series, the complex or large venous malformations (VM) were percutaneously embolized with a combination of liquid embolic agents; NBCA for the superficial component and EVOH for the deeper component of VM (Fig. 2). As described in the recent literature (Salaskar et al., 2020), the use of EVOH is advantageous in the embolization of deeper parts of complex VM for several reasons. Being a nonadhesive agent, EVOH could be precisely delivered in the deeper aspects of VMs. After intravascular precipitation, the deeper aspect of VM retained a soft sponge-like consistency. This facilitated surgical handling during resection of VM. The use of EVOH embolization has been previously shown to be superior for surgical resection of AVMs when compared with NBCA embolization (Akin et al., 2003). EVOH was shown to incur minimal intra or perivascular inflammatory reaction (Murayama et al., 1998; Duffner et al., 2002). Similarly absence of inflammation facilitated safe surgical resection of complex VMs. To prevent non target embolization, high viscosity EVOH was used when pre-embolization venography of VM revealed contrast entry into the central veins. The conventional alcohol sclerotherapy of VM has side effects such as tissue swelling, mucosal blistering, necrosis, and neuropathy (Cantasdemir et al., 2012). None of these side effects were observed with the use of EVOH.
Few case reports have described the use of EVOH in the treatment of renal PSAs and AV fistulas (Vanninen & Manninen, 2007; Carberry et al., 2013; Zeleňák et al., 2009). In this series, four cases of post-surgical renal PSAs were successfully embolized, including an urgent case in which successful cessation of life threatening bleeding was achieved by EVOH embolization of a post nephrectomy PSA (Fig. 4). In these cases, EVOH embolization involved filling the afferent & efferent arteries as well as the PSA sac. In order to prevent distal nontarget embolization, high viscosity EVOH was used.
EVOH embolization has also been used in the treatment of active gastrointestinal hemorrhage with good outcomes (Kolber et al., 2015; Lenhart et al., 2010). The results of one prior retrospective series demonstrated 100% TS rate without any complications in patients who underwent EVOH embolization for the treatment of persistent gastrointestinal bleeding despite endoscopic interventions (Lenhart et al., 2010). Unlike other embolic agents such as coils, EVOH polymerization does not depend on a functional coagulation cascade. This property is pivotal in controlling the active bleeding in patients with underlying coagulopathies (Müller-Wille et al., 2011; Carberry et al., 2013). In the management of iatrogenic & traumatic arterial ruptures, EVOH was preferred to occlude the damaged target vessel which could not be catheterized directly. Also the ability to deploy EVOH without exerting any radial pressure to the damaged vessel walls makes EVOH an ideal agent.
Previously reported DMSO related side effects such as local pain after rapid injection of Onyx (Vanninen & Manninen, 2007), foul breath, severe respiratory distress, pulmonary edema due to DMSO related oxygen desaturation (Pamuk et al., 2005; Murugesan et al., 2008), cardiovascular instability secondary to vasovagal reaction from irritation of nociceptive nerve fibers of intercostal arteries and/or aortic side branches by DMSO (Wildgruber et al., 2016) were not seen in this study group. As per the current experience, use of an appropriate amount of DMSO to adequately fill the lumen of the microcatheter can avoid these risks. The tantalum powder in the mixture of Onyx may cause streak artifacts on radiographic and CT images. This may hinder visualization of future recurrence or regional tissues (Jia et al., 2015). On MR images, EVOH appears hypointense and does not cause artifacts.
Prior study has reported longer fluoroscopy and procedure time with EVOH embolization when compared to those with NBCA embolization. This was attributed to the slow injection rate of EVOH, however in our experience these are operator dependent. The slow, controlled injection of EVOH is in fact desirable for precise and effective embolization (Velat et al., 2008).
The perceived high cost of EVOH embolization may limit its adoption. A vial of EVOH costs approximately $2000 USD. A vial of NBCA is approximately the same cost at the institute. The average cost of total EVOH used per case is estimated to be approximately $4000 USD at our institute. The use of EVOH can sometimes leads to reduction in the use of coils, thereby leads to cost savings. However, if multiple vials of EVOH or coils are required for the procedure, the cost advantage can be quickly lost. Therefore each case should be prudently planned to preserve resources.
The limitations of this study are its retrospective nature and small sample size. Ideally the safety and efficacy of EVOH embolization should continue to be evaluated by comparing it to standard embolic therapies in prospective studies.
