In an SR thrombectomy for a typical case of acute LAO, a large bore (8 F or 9 F) balloon guide catheter (BGC) is preferred over a conventional guide catheter, as a BGC improves the recanalization rate and clinical outcome [14-16]. Inflation of the BGC arrests the downstream blood flow and thus prevents distal embolization of captured clots during the retrieval of the clots engaged by the SR [16,17]. A coaxial technique is invariably easier than an exchange technique for BGC placement. For the coaxial technique, a 5 F 125-cm length angiocatheter is introduced inside an 8 F or 9 F BGC. The relevant artery is catheterized using the 5 F angiocatheter and the external carotid artery navigated using a 0.035-inch wire (a stiff wire, if needed). The 5 F angiocatheter is then advanced over the 0.035-inch wire, followed by the BGC. In rare instances, a triple coaxial technique, a 5 F long (125-cm length) angiocatheter within a 100-115-cm length 6F catheter (Envoy or intermediate catheter group) within a 9 F BGC, is needed to overcome the challenges of very tortuous anatomy or ledge effects at the origin of the common carotid artery [14].

Another option for the guiding system is a combination of a 6 F or 7 F Shuttle guide catheter plus an intermediate 5 F or 6 F catheter. When this combination is used, the intermediate catheter must be advanced as close as possible to the target occlusion site. It would be better to go well above than below the ophthalmic curve of the internal carotid artery (ICA), which would likely arrest downstream blood flow during the retrieval of the SR. One advantage of this combination is that sequential or simultaneous thrombectomies are possible using an SR and an intermediate catheter.

For cases in which the relevant ICA is very tortuous, determining the optimal combination has been controversial. The combination of BGC 9 F+6 F intermediate catheter or the 8 F BGC+5 F intermediate catheter coaxial system both work well and the operator can proceed according to his or her preference. Depending on the amount of clots and the tortuosity of the vascular tree, it is also possible to apply a triple coaxial combination of an 8 F shuttle+8 F BGC+5 F intermediate catheter [14].

A 4-mm diameter SR is compatible with a microcatheter with a 0.0195-inch or 0.021-inch inner diameter. Thus, any type of microcatheter (Excelsior 1018, Stryker, Fremont, CA, USA; Rebar-18, Medtronic, Irvine, CA, USA; Prowler plus, Coddman, Ratham, MA, USA) with a ≥0.0195-inch inner diameter can be used at the operator’s discretion. However, when using a 5-mm or 6-mm diameter SR, a microcatheter with a 0.027-inch inner diameter is required. The microcatheter is navigated beyond the occluded point of the relevant intracranial artery using a 0.014-inch microwire. Thereafter, a small amount of contrast material is infused to confirm that the microcatheter has been correctly positioned in the main branch of the occluded parent artery. A manual microcatheter angiogram should be avoided because it may be potentially dangerous in the segment that has been closed both proximally and distally due to clots. An appropriate-sized SR is then introduced and deployed to span the entire length of the occluded clot. In the earlier phase of SR implementation, the usual “push-and-pull technique” was recommended. Recently, however, the “push and fluff technique” has proven superior for the successful removal of clots [14,18]. After positioning the distal tip marker of the SR, instead of unsheathing the microcatheter, the SR is deployed by pushing the delivery wire; this process is referred to as the “push and fluff” technique. This technique leads to a better wall apposition of the SR and cell size/configuration, which in turn promotes better engagement of the clot with the SR and increases the likelihood of first-pass recanalization [14,18]. Pushing of the delivery wire should be done only after confirming that distal marker of the SR is well expanded in order to avoid vessel injury by moving the SR. A control angiogram is generally obtained immediately after deploying the SR to confirm the accuracy of the SR position and restoration of the blood flow beyond the initial occlusion site. An additional control angiogram is typically obtained after waiting 3-5 minutes, at the operator’s discretion.

The BGC balloon is inflated, which appears cylindrical. During the simultaneous retrieval of the SR and the microcatheter, the BGC needs to be continuously suctioned with a 20-50-mL syringe to prevent any distal embolization. After retrieving the SR, the BGC still needs to be continuously suctioned until clear blood without any clot debris is aspirated. Finally, the BGC is deflated and a routine angiogram is obtained [14].

Cervical carotid artery atherosclerotic disease is responsible for 15-30% cases of acute stroke, of which the majority are due to an artery-to-artery embolism with occlusion or severe stenosis. If the cervical carotid artery or vertebral artery disease causes tandem cervical and intracranial artery occlusions, which is usually the case, it is unclear whether to first treat the intracranial artery occlusion or the cervical artery occlusion. However, in the SR thrombectomy era, neurointerventionalists seem to favor opening the cervical artery occlusion first, followed by a SR thrombectomy for the tandem intracranial artery occlusion, as the cervical artery must be sufficiently dilated when using an SR [14,19-25]. Another reason for treating cervical artery disease is that a tandem intracranial occlusion can spontaneously dissolve after sufficient dilatation of the cervical ICA [20,26]. Finally, timely recanalization of the cervical ICA not only improves the collateral flow to the ischemic penumbra, but also augments the regional perfusion pressure and delivers fresh blood to the intracranial occlusion site to facilitate endogenous thrombolysis, thereby leading to the possibility of delayed recanalization after finishing the procedure, even in the case of a failed intracranial thrombectomy [14,26-28].

The procedural details are as follows (Figure 1):

In the case of a large quantity of clots in the cervical artery concomitant with a tandem intracranial artery occlusion, it is not possible to remove all of the clots using just an SR. In that situation, it is more effective to first perform a suction thrombectomy using a BGC and/or large bore shuttle sheath before the SR thrombectomy. Since the BGC itself can be occluded by packed clots during the suction thrombectomy, a coaxial combination of an 8 F BGC within an 8 F shuttle sheath is also needed, which, in the case of occlusion, allows the BGC to be deflated and retrieved outside the shuttle sheath while maintaining the negative suction of the BGC. Thereafter, the shuttle sheath left in the cervical artery needs continuous suctioning until clear blood is aspirated (Figure 2). Following the suction thrombectomy, an SR thrombectomy can be performed to remove any remnant distal clots [14].

If the relevant parent artery is very tortuous or includes a stenotic segment proximal to the occlusion site, the probability of losing the SR-engaged clots increases. In addition, continuous suction of the BGC may not be sufficient to prevent distal embolization. In this case, the coaxial use of a 5 F (compatible with an 8 F BGC) or a 6 F intermediate (compatible with a 9 F BGC) may help increase the likelihood of complete clot retrieval. An intermediate catheter is coaxially introduced through the BGC and advanced as close as possible to the SR-engaged clots. In the event that it is difficult to advance the intermediate catheter close enough to the clot, SR is first deployed spanning the clot, which serves as an anchor during the intermediate catheter advancement (Figure 3). This combination of an SR and an intermediate catheter helps to prevent the engaged clots from being lost upon retraction of the SR back through the tortuous or stenotic segment of the relevant arterial tree. It can be also useful to facilitate the sequential or simultaneous performance of a combined technique of an SR and aspiration thrombectomy (Figure 4) [14].

In Asian populations, intracranial atherosclerotic stenosis (ICAS) is one of the major causes of acute stroke, with recent studies showing that ICAS is responsible for 15-20% of acute LAO strokes [29,31-34]. The majority of non-embolic occlusions are truncal-type occlusions (TTO), as revealed in angiograms during intra-arterial recanalization treatment, and most are due to ICAS thrombo-occlusion (Figure 5) [29,30]. TTOs tend to show refractoriness to an SR due to repeated re-occlusion [29-36]. The application of an SR can damage the atheromatous surface if an acute LAO is due to ICAS thrombo-occlusion. In this case, the use of an SR can result in further platelet activation, leading to repeated re-occlusion (Figure 6) [29-36]. To prevent repeated re-occlusion in the case of an LAO due to ICAS, inhibition of the platelet function can play a key role. Therefore, the first option is to administer a glycoprotein IIb/IIIa inhibitor, which inactivates platelets and thereby prevents repeated re-occlusion [29,31,36-38]. However, ICAS occlusions are occasionally refractory to glycoprotein IIb/IIIa inhibitors. For recanalization in such a case, permanent stenting with or without balloon angioplasty can be applied (Figure 6) [29,31,32].

An organized (hard, fibrin-rich) clot is more resilient and less sticky than fresh (soft, red blood cell-rich) clots, causing less engagement with an SR and leading to clot missing during SR retrieval, especially in the case of a tortuous arterial tree. Furthermore, because an organized clot may cause more tension in the SR-deployed segment of the parent artery, this can also likely induce an arterial spasm. Such effects then increase the probability of SR failure [29].

For successful recanalization of an LAO due to an organized clot, the first and simplest option is the intra-arterial administration of a vasodilator, which releases the tension of the SR deployed arterial segment, thereby increasing the vessel diameter to ease clot retrieval (Figure 7) [29]. The second option is the simultaneous utilization of an SR and an aspiration or intermediate catheter. After deploying the SR, an aspiration catheter or intermediate catheter is advanced as close to the clot as possible. An SR and suction thrombectomy can be performed sequentially or simultaneously (Figure 8) [29].

If the organized clot is still refractory after the simultaneous utilization of an SR and an intermediate catheter, permanent stenting can be considered as a final resort [29,32]. The organized clot may be refractory due to less engagement with the SR [29,39]. By the same token, if permanent stenting is conducted, the stented artery is likely more patent as the organized clot is less engaged inside the stent struts (Figure 9) [29,32].

Irrespective of the etiology of the refractoriness of an LAO, a rescue modality is needed for such a refractory case given that successful recanalization is the most powerful factor for a good outcome [6,31,40]. One possible modality is the intra-arterial infusion of thrombolytics (tissue plasminogen activators or urokinase) and/or anti-platelets (glycoprotein IIb/IIIa inhibitors), which can promote mTICI 2b-3 recanalization in some refractory LAOs [29,31,32,36-38]. For cases that are still refractory to the combination of an SR and an aspiration thrombectomy, as well as to intra-arterial infusion of thrombolytics and/or a glycoprotein IIb/IIIa inhibitor, the final rescue modality is permanent stenting, which has also been suggested both as a primary approach and a rescue tool for the recanalization of an acute LAO [29,31,32,41-47]. An SR-failed LAO is likely due to either ICAS (Figure 5) or an organized (hard) clot (Figure 9), as previously discussed. In the case of an ICAS occlusion, the problem is mainly due to repeated occlusion [29,31-36,48]. As mentioned above, a glycoprotein IIb/IIIa inhibitor can help prevent such reocclusion. Notwithstanding, ICAS occlusions are often refractory to glycoprotein IIb/IIIa inhibitors, probably because of underlying severe stenosis. In such cases, permanent stenting combined with a glycoprotein IIb/IIIa inhibitor can be very effective to recanalize [29,31,32,41-47]. Permanent stenting can also be the last resort after the failure of an SR and aspiration thrombectomy due to an organized (hard, fibrinrich) clot (Figure 10). In a recently published study on the safety and efficacy of permanent stenting following a failed mechanical thrombectomy, 83.3% of stented patients showed thrombolysis with a cerebral ischemia 2b or 3 recanalization rate. The stented patients had significantly more favorable outcomes (modified Rankin Scale score [mRS] of 0-2, 35.5%) and less cerebral herniation (11.8%) than did the non-stented patients (mRS, 7.1%; cerebral herniation, 42.9%), while there were no differences in the symptomatic intracranial hemorrhage and mortality rates between the patient groups (symptomatic hemorrhage and mortality, 11.8% and 23.9%, respectively, in the stented group vs. 14.3% and 39.4%, respectively, in the nonstented group) [32]. Among 17 stented cases, only 40% underwent balloon angioplasty. A Wingspan stent was used in 7 cases, while a Solitaire stent, which is also used for thrombectomies, was used in 10 cases [32]. These results suggest that most of the refractory occlusions were due to a soft-plaque ICAS or organized embolus, rather than a hard-plaque ICAS [32]. In other words, the radial force of the Solitaire stent, whose radial force is weaker than a Wingspan which is approved for intracranial stenting, was enough to open the occlusion, and angioplasty was not always required in the case of a soft-plaque or hard embolus occlusion. Meanwhile, platelet inactivation by a glycoprotein IIb/IIIa inhibitor was found to be essential to prevent reocclusion due to acute in-stent thrombosis [29]. While the major concern with permanent stenting is that it requires antiplatelet medication, thereby possibly increasing the risk of intracranial hemorrhage in an acute stroke setting, the results of previous studies suggest that the benefit of recanalization success by permanent stenting outweighs this drawback in patients with an SR-failed LAO that would be otherwise left non-recanalized [29,32,40-47].

In order to avoid the potentially increased rate of intracranial hemorrhage and severity due to anti-platelet medication in an acute stroke setting, it may be beneficial to obtain a flat-panel computed tomography scan in the angio-suite prior to permanent stenting and the administration of a glycoprotein IIb/IIIa inhibitor. If contrast agent enhancement is extensive in the affected brain, the EVT procedure should be stopped. Contrast agent enhancement of the affected brain indicates not only complete infarction, but also the breakdown of the blood-brain barrier and potentially increased hemorrhagic risk. On the other hand, if the contrast agent enhancement is small and not intense, administration of a glycoprotein IIb/IIIa inhibitor with permanent stenting can serve as a final resort (Figures 6E and 10H) [29].