Curr Treat Options Cardio Med (2016) 18:32 DOI 10.1007/s11936-016-0457-7

Vascular Disease (I Weinberg, Section Editor)

The Evolution of Mechanical Thrombectomy for Acute Stroke Feras Akbik, MD, PhD1 Joshua A. Hirsch, MD2 Pedro Telles Cougo-Pinto, MD, MSc3 Ronil V. Chandra, MBBS FRANZCR4 Claus Z. Simonsen, MD5 Thabele Leslie-Mazwi, MD2,* Address 1 Department of Neurology, Massachusetts General Hospital, Boston, MA, USA *,2 Neuroendovascular Service, Massachusetts General Hospital, Boston, MA, 02114, USA Email: [email protected] 3 Department of Neurosciences and Behavior Sciences, Ribeirão Preto Medical School, Ribeirão Preto, SP, Brazil 4 Interventional Neuroradiology, Monash Health, Monash University, Melbourne, Australia 5 Department of Neurology, Aarhus University Hospital, Aarhus, Denmark

* Springer Science+Business Media New York 2016

This article is part of the Topical Collection on Vascular Disease Keywords Acute stroke I Ischemic stroke I Large vessel occlusion I Proximal occlusion I Endovascular therapy I Intra-arterial therapy

Opinion statement The natural history of an acute ischemic stroke from a large vessel occlusion (LVO) is poor and has long challenged stroke therapy. Recently, endovascular therapy has demonstrated superiority to medical management in appropriately selected patients. This advance has revolutionized acute care for LVO and mandates a reevaluation of the entire chain of stroke care delivery, including patient selection, intervention, and post-procedural care. Since endovascular therapy is a therapy specifically targeting LVO, its application should be restricted to those patients only. Clinical and radiologic parameters need to be considered in patient selection. Data supports that all patients over the age of 18 years presenting with a National Institutes of Health Stroke Scale (NIHSS) of 6 or greater within 6 hours of symptom onset should be considered for emergent endovascular therapy. Radiologically, those with a LVO of the internal carotid artery (ICA) or middle cerebral artery (MCA) M1 portion, intermediate or good collaterals and without large established infarct should be considered endovascular candidates. Selection beyond these parameters remains an open

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question and is being actively evaluated. In all cases, revascularization should be attempted with a new generation device (stentriever or direct aspiration), as these techniques are most likely to deliver adequate reperfusion. Post-revascularization, patients are closely monitored in an intensive care setting followed by discharge to rehabilitation, if required, or directly home. Patients should be evaluated in delayed fashion to assess recovery (typically at 3 months post-treatment). Ultimately, the poor natural history of ischemic stroke from LVO and the potential significant benefit from endovascular therapy over medical management alone necessitate a national response to ensure we identify and treat all eligible patients as rapidly and effectively as possible.

Introduction Nearly one third of acute ischemic strokes are caused by an occlusion of a large, proximal cerebral vessel. The natural history of this disease is poor, with limited recovery and high mortality rates [1]. The recent demonstration of efficacy and superiority of endovascular therapy (ET) using mechanical thrombectomy has revolutionized acute stroke therapy [2, 3••, 4••, 5••, 6•]. Here, we review the recent history and technical advances in endovascular therapy for patients with large vessel occlusions (LVOs). The reperfusion hypothesis has driven modern therapy for acute ischemic stroke with the goal of using either mechanical or pharmacologic mechanisms to recanalize occluded vessels [7, 8]. Efficacy of ET was first demonstrated in the PROACT and PROACT-II trials, where intra-arterial delivery of urokinase led to modest but increased rates of recanalization as compared to heparin alone [9, 10]. ET was complicated by increased rates of symptomatic intracranial hemorrhage, but nevertheless improved outcomes at 90 days. After intravenous tissue plasminogen activator (IV tPA) emerged as first-line therapy for stroke in 1996, ET evolved towards the use of mechanical thrombectomy devices. In uncontrolled studies, these devices demonstrated higher rates of recanalization than previously achieved with intraarterial or intravenous thrombolytics [11, 12].

In light of these promising findings, three large clinical trials compared mechanical thrombectomy to intravenous thrombolysis [13–15]. These trials were neutral for benefit of ET over IV tPA. However, the interpretation of these results was confounded by several limitations [16], including the use of first and second generation devices, inclusion of patients with no documented LVO [17], enrollment of patients with large established infarct cores [13, 14], and significant delays to reperfusion [18]. The failure to detect a benefit of ET galvanized a reevaluation of mechanical thrombectomy with a new generation of clinical trials [19], summarized in Table 1. These trials were designed to use modern thrombectomy devices, strict patient selection criteria, and expedited intervention times. In this setting, ET demonstrated significant superiority over medical management of LVO, providing the first prospective clinical trial data in the era of intravenous thrombolytics that ET with mechanical thrombectomy improves patient outcomes [2, 3••, 4••, 5••, 6•]. These data justify the provision of emergent ET for patients with LVO [20]. Indeed, there has already been a meaningful increase in the number of projected ET cases [21, 22]. Here, we review a systematic approach to these patients, emphasizing the optimized delivery of ET.

Selection for endovascular therapy The shortcomings of the first generation mechanical thrombectomy clinical trials demonstrated that patient selection is critical to the appropriate use of ET [17]. A range of predictors of response to ET have been identified, but no prospective data justify rigid treatment thresholds [23]. Consequently, a variety of clinical practice patterns emerged [24].

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Table 1. Comparison of current randomized clinical trials evaluating endovascular therapy for stroke from large vessel occlusion Study

Baseline NIHSS median (IQR)

Imaging selection (beyond CT ASPECTS and CTA)

Median time onset to groin access (min)

Use of stentrievers (%)

mTICI 2b/3 (%)

Functional outcome at 90 days (% ET cohort vs % medical management cohort)

MR CLEAN

17 (14–21) 17 (14–20) 16 (13–20) 17 (13–20) 17 (13–20)

None

260

82

59

32 vs 19

None

269

100

66

44 vs 28

Multiphase CTA RAPID CTP

211

86

88

53 vs 29

224

100

72

60 vs 35

RAPID CTP

210

100

86

71 vs 40

REVASCAT ESCAPE SWIFT PRIME EXTEND IA

IQR interquartile range, NIHSS National Institutes of Health Stroke Score, ET endovascular therapy, CT computed tomography, CTA computed tomography angiogram, CTP computed tomography perfusion, ASPECTS Alberta stroke program early CT score, mTICI modified thrombolysis in cerebral infarction, MR CLEAN multicenter randomized clinical trial of endovascular treatment for acute ischemic stroke in the Netherlands, REVASCAT randomized trial of revascularization with solitaire FR device versus best medical therapy in the treatment of acute stroke due to anterior circulation large vessel occlusion presenting within 8 h of symptom onset, ESCAPE endovascular treatment for small core and anterior circulation proximal occlusion with emphasis on minimizing CT to recanalization times, SWIFT-PRIME solitaire with the intention for thrombectomy as primary endovascular treatment, EXTEND-IA extending the time for thrombolysis in emergency neurological deficits—intraarterial

The most recent clinical trials have helped clarify some of these issues. Trials required patients meet the following criteria: 1. Presence of a demonstrated LVO, and therefore, an appropriate target lesion for thrombectomy.

2. Presence of a significant neurologic deficit to justify the risk of intervention. The recent trials generally enrolled patients with NIH stroke scale (NIHSS) scores greater than 6. 3. Presence of a small established infarct, either visualized directly or inferred from perfusion imaging or collateral assessment. 4. Treatment initiation within a defined time window from onset. Beyond these core criteria, recent trials used a range of inclusion criteria that have not been directly compared [2, 3••, 4••, 5••, 6•]. Further investigation will be required to settle ongoing debates regarding selection criteria for ET in a variety of subgroups. Recent trial data have also challenged a previous understanding of infarct cores. A pivotal biomarker for response to ET, infarct cores can be measured with a variety of methods, including CT hypodensity, MRI diffusion weighted imaging (DWI), CT perfusion, or collaterals on CT angiography [25]. Patients with small infarct cores, significant clinical deficits, and a LVO have large amounts of

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Curr Treat Options Cardio Med (2016) 18:32 at risk tissue and are thought to benefit most from reperfusion. Conversely, those with large infarct cores are generally not thought to benefit from reperfusion, though whether they are potentially harmed remains unclear. Specifically, patients with a pretreatment ischemic core volume greater than 70 cm3 on DWI, or a non-contrast CT Alberta Stroke Program Early CT Score (ASPECTS) score less than 5, are generally unlikely to benefit from reperfusion [26, 27]. However, this may be modified by younger age and successful reperfusion [28]. While recent trial data suggest that ET is unlikely to lead to a modified Rankin Scale (mRS) of 0–2 in these patients, ET more than doubled the chance of improving these patients’ mRS. This improvement in functional outcome in this cohort produces larger numbers of patients surviving with chronic dependence, and thus, requires further determination as to whether individual and societal benefits justify the risks and costs of expanding inclusion criteria for ET.

Time The ultimate goal of reperfusion is to rescue salvageable tissue. Time from onset is a predictor of the fate of tissue at risk and has been used as a criterion in all reperfusion studies. Reperfusion should occur as fast as possible as there is a time-dependency to the benefit of reperfusion, whether mechanical or pharmacologic [29, 30]. In anterior circulation stroke, the best evidence supports a therapeutic window of 6 h from symptom onset [4••, 31]. Nevertheless, time is only one predictor of at risk tissue, and in select patients, good clinical outcomes can be achieved beyond 6 h. Time dependency is likely collateral dependent, as penumbral decay is inversely correlated to the quality of collateral vascular supply. Consistent with this, poor collateralization predicts large core infarcts [32]. Patients with good collaterals may therefore have more resilient penumbral tissue. Several radiographic and functional observations support this hypothesis. For instance, in a cohort of patients presenting with LVO, over 75 % maintained perfusion-diffusion mismatch 9 h post stroke onset [33]. In a separate cohort, diffusion-perfusion mismatch was maintained in a subset of patients up to 24 hours after stroke onset [34]. These radiographic findings have functional implications, as the presence of good collateral circulation portends a favorable prognosis. In a retrospective review of IMS I and II, the dependence of functional outcome on time was significantly degraded in patients with good collaterals [35]. Critically, select patients have been treated at 18 hours or even later with equivalent functional outcomes to those treated within 6 hours [36, 37]. Together, these data demonstrate that time is simply a surrogate for tissue at risk. In most patients, 6 hours remains the appropriate window for therapy. However, in select patients with demonstrable perfusion-diffusion mismatch beyond 6 hours, ET is likely to rescue tissue at risk. In the absence of prospective data to support this possibility, this approach has yet to gain widespread acceptance, but is currently

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being actively researched. Therefore, the time dependence of benefit requires all operators to establish rapid and robust reperfusion as soon as possible.

Imaging The time sacrificed to conduct neuroimaging can only be justified if it is performed quickly, reliably, and reproducibly guides clinical decisions. There is no uniform approach to the choice of neuroimaging modalities to select patients for ET, summarized in Table 2. The most recent trials all used CT heavily, sometimes exclusively, with confirmation of a vessel occlusion being common to all, and variable means of infarct core evaluation (CT ASPECTS, collateral evaluation, automated perfusion imaging) [2, 3••, 4••, 5••, 6•]. The variable use of imaging modalities reflects the limitations of each approach. The ideal imaging modality of choice would be fast with high interrater reliability. For instance, MRI provides the most accurate measurement of infarct core, but it is limited by availability, speed, and patient eligibility [38, 39]. For select patients presenting outside of set therapeutic windows, such as patients with wake-up strokes or late presentations, MRI will likely play a critical role in selecting patients who would benefit from ET [40]. Its role in the hyperacute setting remains a matter of debate, but retrospective data suggests it is at least comparable to CT, and may be better [41]. While the recent studies confirm the efficacy of CT selection in the first 6 hours from onset, accurately measuring core infarct with CT techniques remains challenging. In the hyperacute setting, CT-based modalities for assessing core are all rapid and accessible, but are limited by inter-rater variability. ASPECTS is fast, simple and acquired from primary CT data, but has only moderate interrater reliability when dichotomized at a score of 7 [42]. CT perfusion allows the size of the core to be inferred but is complicated by the reliance on limited, often expensive, vendor software or the use of variable perfusion thresholds and institutional protocols (making comparisons difficult at a broader level). More recently, multiphase CTA has been validated as a novel modality to utilize adequacy of collateral circulation to infer the size of the infarct, with high interrater reproducibility and minimal processing time [43]. The safety and efficacy of this imaging approach were validated in hyperacute patient selection in the ESCAPE trial [4••], making it an appealing option. Nevertheless, there is no standard practice paradigm at this time. Finally, it is important to recognize different goals in clinical practice and

Table 2. Imaging modalities in acute stroke care, and technique to accomplish imaging assessment goals Goal

CT

MRI

Presence of stroke Presence of LVO Volume of established infarct core Assessment of penumbral tissue volume

Tissue hypodensitya CTA ASPECTS CTP

DWI hyperintensity MRA (TOF or gadolinium enhanced) DWI volume MRP

ASPECTS Alberta stroke program early CT score, CT computed tomography, CTA computed tomography angiogram, CTP computed tomography perfusion, DWI diffusion weighted imaging, LVO large vessel occlusion, MRI magnetic resonance imaging, MR magnetic resonance perfusion, TOF time of flight a Often normal in early presentations

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research. In the clinical setting, selection criteria are geared towards maximizing sensitivity in order to extend ET to all potential patients that may benefit. Conversely, clinical trial selection criteria are geared towards optimizing both sensitivity and specificity to ensure that enrolled patients reflect a group with homogenous pathophysiology that allow trial goals to be achieved [23]. The probability that trial eligibility criteria excluded some patients who could have benefited from ET therefore complicates the direct translation of clinical trial data into clinical practice algorithms. Consequently, defining clinical practice algorithms requires the involvement of all stakeholders in reviewing existing data and using expert opinion in establishing local treatment criteria.

Posterior circulation Selection criteria in posterior circulation infarction are relatively undefined as compared to anterior circulation LVO and have not been evaluated in randomized, prospective fashion. The resilience of the posterior circulation to ischemia generally extends the therapeutic window, but treatment response remains time-dependent [44]. Given the eloquence of the pons, midbrain, and thalamus, stroke location is more predictive of patient outcome than stroke volume. Hyperacute imaging is complicated by the limitations of CT imaging in the posterior fossa. While perfusion imaging is currently being studied in this population, emergent MRI is likely the most helpful imaging study in posterior circulation ischemia [45]. The natural history of posterior circulation infarcts is poor, although aggressive therapy increases the odds of good outcomes [46]. The Basilar Artery International Cooperation Study (BASICS) study, a multicenter phase 3 trial, is currently enrolling patients to study the efficacy of ET in patients with basilar occlusions already receiving intravenous thrombolysis. In the interim, the poor natural history and recent anterior circulation data support ET in the absence of prospective trial data.

Intravenous thrombolytic exposure Even with thrombolytics, the natural history of a LVO is poor. Less than 50 % of treated patients recanalize [47]. In the most recent endovascular trials, the majority of patients were treated with intravenous tPA, with a range of 68– 100 %[3••, 5••]. While thrombolysis did not statistically modify patient outcome in any of these trials, the approach to intravenous thrombolytic treatment failure differed between trials 2, 3••, 4••, 5••, 6•]. REVASCAT and MR CLEAN required treatment failure, measured at 30 and 60 min after thrombolysis, respectively, before proceeding to ET. EXTEND-IA, ESCAPE, and SWIFT PRIME did not wait to assess for tPA failure and reported faster groin puncture times and higher revascularization rates. It is unclear if the increased revascularization rate reflects tPA response or accelerated intervention. Documenting treatment failure did not modify outcomes in appropriately selected patients. Given the potential for benefit from ET and the absence of apparent harm, all eligible

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patients should be treated with both intravenous tPA and ET without waiting for evidence of tPA failure, though active debate exists [48].

Procedural considerations Patient cooperation General anesthesia has previously been the standard approach to minimize patient motion, increase comfort, and minimize operational risk. A growing body of data now suggests worse patient outcomes with general anesthesia as compared to conscious sedation or monitored anesthesia care (we will refer to both as conscious sedation). In retrospective studies, conscious sedation is associated with a lower incidence of pneumonia, shorter ICU stays, lower final infarct burden, improved functional status, and lower mortality rates [49, 50]. Crucially, there was no increased incidence of procedural complications with conscious sedation. MR CLEAN recently demonstrated a similar benefit in a prospective cohort where general anesthesia was associated with delayed treatment initiation and decreased benefit from ET [51]. Furthermore, conscious sedation has numerous practical advantages, including eliminating induction and intubation delays, avoiding hemodynamic alterations during induction, and facilitating intra- and post-procedural neurological examination. When tolerated by the patient, conscious sedation should be the preferred approach. With the exception of posterior fossa strokes and large hemispheric strokes with oropharyngeal dysfunction, the majority of our patients tolerate conscious sedation. Emergent anesthesia support is nonetheless required given the potential need for rapid intubation in the setting of acute decompensation and for potential hemodynamic management. In the absence of national guidelines, selection criteria for procedural sedation should be developed on an institutional basis.

Routes of access Engineering advances have principally focused on optimizing thrombectomy, with far less attention paid to the base catheter. To date, transfemoral access is the standard approach. Nevertheless, carotid catheterization is frequently delayed, taking over 20 min in half of cases and over 30 min in a quarter of cases [52]. Longer delays to carotid catheterization independently predict decreased recanalization rates and worse clinical outcomes [52]. Multiple strategies exist to accelerate carotid catheterization. First, a pre-procedure CTA can be used to assess the vascular anatomy and define the most efficient approach. Second, radial and brachial access can be used to bypass tortuous aortic anatomy. These access sites can comfortably accommodate 6 French systems and are excellent choices for vertebral access targeting posterior circulation lesions [53]. Alternatively, ultrasound guided carotid micropuncture or direct carotid exposure bypasses the aortic arch and provides a short working distance for a 6 French sheath [54, 55]. The shorter working distance theoretically allows more efficient suction and traction on the thrombus, but this requires further modifications to existing equipment. Puncture sites should be in the lower cervical region to ensure cannulation of the vessel proximal to the carotid bifurcation. Similarly, short sheaths (such as 5 cm) should be used to avoid injuring the internal carotid artery. Securing the carotid arteriotomy remains challenging and often requires manual compression in addition to a closure device to achieve hemostasis. If a carotid

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Curr Treat Options Cardio Med (2016) 18:32 exposure has been performed, the arteriotomy can be secured with a purse-string suture.

Base catheter Although there is no standard base system to deliver thrombectomy devices, all options should be large bore to accommodate the appropriate devices. Balloon guide catheters are frequently used in the anterior circulation. These devices inflate a balloon proximal to the thrombus and obstruct anterograde carotid artery flow, theoretically minimizing the risk of distal embolization as the thrombus is retracted under continuous aspiration. In vitro models of mechanical reperfusion have demonstrated decreased distal embolization with the use of balloon guide catheters [56]. In retrospective clinical studies, balloon guided catheters use with first generation thrombectomy devices was associated with faster procedure times, decreased adjunctive therapy, and improved clinical outcomes [57]. Interestingly, there was no effect on distal embolization, although the detection threshold may confound this result. Despite encouraging data regarding the use of balloon guide catheters, there remains no prospective data to demonstrate superiority over a nonballoon guide catheter. As an alternate, a large bore guide catheter (6 or 7 French) is typically used in conjunction with an intermediate aspiration catheter. Continuous aspiration is applied to the thrombus close to the clot face during withdrawal to minimize the risk of distal embolization.

Thrombectomy devices Over the past decade, thrombectomy devices have rapidly evolved, with increasing efficacy. The latest generation of mechanical thrombectomy devices includes Bstentriever^ devices. The two devices approved for use in the USA are the Solitaire (ev3 Endovascular, Plymouth, Minnesota, USA) and Trevo (Stryker Neurovascular, Kalamazoo Michigan, USA) series. A variety of other devices are pending FDA approval in the USA. Stentrievers are named such because of the ability to deploy effectively a retrievable stent that intercalates the thrombus (Fig. 1). A microcatheter is advanced across the thrombus, and the stent retriever is unsheathed within the thrombus itself. The stent-like component expands and engages the thrombus, temporarily restoring cerebral blood flow as an Bendovascular bypass.^ After a period of several minutes, the stent retriever has intercalated the thrombus and is carefully pulled back into a guide catheter with the engaged thrombus. Operators are advised to pull gradually to avoid losing the thrombus during accelerations around vascular curves. More recently, mechanical thrombectomy with stentrievers has been augmented by concomitant aspiration of the thrombus, as discussed below (Fig. 2) [58]. The stentriever generation of thrombectomy devices increases recanalization efficiency, improves clinical outcomes, and maintains an equivalent or better safety profile as compared to first generation devices [59, 60]. The use of stentrievers in the most recent trials contributed to improved clinical outcomes with ET. Consequently, there is no longer a role for earlier generation devices in revascularization of acute LVOs. Although stentrievers are now well described, a novel direct aspiration technique has gained traction both as a stand-alone approach as well as a complimentary tool. Labeled BA Direct Aspiration first Pass Technique^ (ADAPT), this is a novel technique that has been made possible by engineering advances that permit

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Fig. 1. Stentrievers. The photograph illustrates two different sizes of the Trevo (Stryker Neurovascular, Fremont CA) device with two different roles. The larger device (measuring 6 mm in diameter) is designed to engage and extract clot in the internal carotid artery terminus and proximal middle cerebral artery. The smaller device (measuring 3 mm in diameter) is designed to extract clot from smaller branches of the cerebral arterial tree. Clot is still adherent to portions of the 3 mm device in this image (red arrow).

intracranial delivery of large-caliber aspiration catheters to the thrombus interface [61, 62]. The Penumbra 5Max (Penumbra, Oakland, California, USA) was the original reperfusion catheter specifically intended for this purpose, but several additional options have become recently available. A large bore aspiration catheter is advanced over a microcatheter to the clot. Once at the thrombus, the microcatheter is removed and aspiration is applied to engage the thrombus and slowly retract it under continuous aspiration pressure. This is often performed with a pump system to ensure continuous suction and thrombus engagement. As the thrombus is withdrawn into the guide catheter, continuous aspiration through a separate sideport prevents the thrombus from dislodging at the guide catheter aperture. Smaller vessels can accommodate a 4Max or 3Max reperfusion catheter (Penumbra) to deliver sufficient suction pressure to the thrombus [58]. Successful

Fig. 2. Left internal carotid cerebral angiogram, anteroposterior views. a Imaging shows complete occlusion of the proximal left MCA (red arrow). b Following successful thrombectomy, full restoration of blood flow is seen with a normal appearance to the arterial tree.

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Curr Treat Options Cardio Med (2016) 18:32 thrombectomy with aspiration eliminates the need for stentrievers, thereby reducing procedural cost and potentially accelerating recanalization time. No direct prospective comparison exists yet between the use of stentrievers and ADAPT. Further studies are required to evaluate initial suggestions that ADAPT accelerates recanalization time, decreases costs, and improves angiographic outcome [61, 62]. Alternatively, hybrid approaches also exist, augmenting the efficacy of stentrievers with the suction provided by aspiration. For instance, an aspiration catheter can deliver a stentriever to the thrombus interface. The stentriever is then deployed with an aspiration catheter used to retrieve the thrombus and stentriever together, maintaining constant suction throughout withdrawal. Dual engagement of the thrombus via stentriever intercalation and continuous aspiration may decrease the risk of non-target embolization during thrombus withdrawal. Nevertheless, this approach cannot be recommended over the use of balloon guide catheters in the absence of rigorous data comparing the two approaches.

Special considerations Patients with tandem occlusions (concomitant cervical internal carotid artery and intracranial artery occlusion) were excluded from EXTEND-IA and SWIFT PRIME while MR CLEAN, REVASCAT, and ESCAPE included them in trial selection [2, 3••, 4••, 5••, 6•]. In the latter three trials, ET improved clinical outcomes for patients with tandem occlusions when compared to intravenous thrombolysis, and data therefore supports emergent ET in this patient population. Technically, tandem lesions require a two pronged approach, with a separate cervical and intracranial focus. In the setting of cervical dissection or atherosclerotic obstruction, intra-procedural re-occlusion of the cervical lesion is not uncommon [63– 65]. A common approach is thus to secure the carotid lesion rapidly with either balloon angioplasty or a cervical stent and then advance intracranially. We often place a cervical stent, but a variety of alternate approaches exist [66, 67]. Posterior circulation treatment requires only minor adaptations to technique with a smaller base system being the most notable difference. The caliber of most vertebral arteries cannot accommodate the large bore catheters that are used in the anterior circulation. The devices and maneuvers for thrombectomy are otherwise similar.

Angiographic goals The goal of ET is reperfusion of ischemic territory, not simply recanalization of the primary occlusion. A variety of scales grade angiographic outcome, although the modified thrombolysis in cerebral infarction (mTICI) score is commonly used [68]. Although limited by variable definitions in the literature, the mTICI score is simple and has good interobserver reliability [69–71]. It is now clear that patients have the best chance at recovery when angiographic reperfusion is mTICI 2b-3. The angiographic definition of success should therefore be at least mTICI 2b-3 reperfusion (Fig. 3), with the goal of achieving this within 30 min of the base catheter being established [23]. Emerging technologies should be judged against these benchmarks to determine efficacy.

Additional considerations The selection and treatment of LVO patients occur in a care continuum. This includes pre-hospital and post-acute considerations.

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Pre-hospital triage Acute stroke therapy is limited by poor public awareness of stroke symptoms and delayed time to presentation [72, 73]. While public health initiatives continue to improve public awareness, the delivery of emergent stroke care must now account for the role of ET. Thrombolysis is instructive, as a diffuse network has been established to rapidly deliver emergent pharmacotherapy to patients irrespective of locale. However, the nature of ET necessitates centralization and consolidation in centers with rapid access to comprehensive neuroimaging, endovascular operators, intensive care, and neurosurgical support. An integrated triage system is therefore required to emergently transfer patients to experienced, high-volume centers to receive appropriate interventions. One such model is to use decision aids to presumptively diagnose a LVO in the field. Emergency medical service personnel can be trained to use basic assessment scales which identify patients with anterior circulation LVO with high sensitivity and specificity [74]. Patients with a likely LVO should be transferred to a center capable of ET, as delays in transfer times limit eligibility for endovascular therapy [75, 76]. This may mean that primary stroke centers are bypassed for a comprehensive stroke center, but given the superiority of ET, this delay would likely be offset by the faster reperfusion time. Alternatively, a novel model using ambulance delivered telestroke care could remove the uncertainty in pre-hospital triage. In a German pilot study using ambulances equipped with an on-board neurologist and CT scanner, tPA delivery

Fig. 3. Right internal carotid artery, lateral view. Assessment of angiographic success requires evaluation of the parenchymal phase, and this is usually best accomplished in the lateral projection. In this image, a patient presenting with acute right middle cerebral artery (MCA) occlusion has been successfully recanalized. Patient orientation is with gaze to the left (red arrow shows the faint choroidal blush of the eye). In the parenchymal phase, the entire MCA territory fills with the exception of the wedge of tissue demarcated with dotted lines in the parietal and occipital region. This is due to a more distal occlusion. In assessing modified thrombolysis in cerebral ischemia (mTICI) grades, the entire vascular territory pre-procedure that was at risk is evaluated and any non-filling regions are subtracted from the total reperfused tissue. In this case, the wedge of missing tissue is less than 50 % of the total MCA volume, giving a final angiographic result of mTICI 2b. If complete reperfusion is achieved, the result is graded as mTICI 3. For improved patient outcomes, a goal angiographic result of mTICI 2b or 3 is the target.

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Curr Treat Options Cardio Med (2016) 18:32 was safe and significantly faster than traditional delivery models [77]. Ongoing trials are evaluating the safety of ambulance-based imaging and thrombolysis using remote telemedicine prior to arrival at the hospital [78, 79]. This model has the potential to dramatically decrease both thrombolysis and ET delays with prehospital diagnosis, thrombolysis, and triage of patients with a LVO.

The team approach BTime is brain,^ and the benefit of ET is time dependent [29]. All members of the ET team must therefore be efficiently mobilized to minimize reperfusion times [80]. The current comprehensive stroke center recommendation of door to groin puncture of less than 2 h still remains elusive for many major centers, highlighting a need to streamline interdisciplinary care [81–83]. For instance, over the last 10 years, procedure times have shortened while the delay between symptom onset and groin puncture has remained stagnant [84]. The complex and multidisciplinary approach to care necessitates an integrated delivery model to minimize delays. Further acceleration of reperfusion requires identification of delays in care delivery, and more specifically, the latencies to arrival to the emergency department, neuroimaging, decision to intervene, arrival to the angiography suite, anesthesia initiation, groin access, vessel catheterization, and reperfusion. Measuring the latency of each phase within each center will help identify areas for improvement and maximize the potential for rapid care and therefore greater likelihood of recovery [23, 85].

ICU/stroke unit care Systolic blood pressure is elevated in the setting of persistent arterial occlusion and physiologically decreases in the setting of recanalization [86]. Given this observation, many operators reduce post-procedure blood pressure if treatment is successful. Patients should be subsequently admitted to a dedicated stroke or neurocritical care unit as the interdisciplinary care provided in these environments improves clinical outcomes [87]. Routine practice includes a 24-h noncontrast enhanced CT scan of the brain to assess for any interval hemorrhage and determine the infarct size.

Rehabilitation Secondary stroke prevention and maximal rehabilitation continue to be the standard of care post reperfusion. Most patients will require rehabilitation, and the patient’s disposition on discharge predicts final outcome [88, 89]. In patients who have similar comorbidities and neurologic deficits, those discharged to a skilled nursing facility were significantly less likely to achieve a good clinical outcome than those discharged to independent rehabilitation facilities. Regardless of disposition status, patients should be reevaluated at 90 days to assess the efficacy of intervention and progression of rehabilitation.

Accountability Concurrent to these dramatic advances in stroke therapy, health care delivery has been rapidly evolving. In the USA, in 2010, the Affordable Care Act called for additional quality and value to be extracted from the traditionally fee for service system [90]. In early 2015, the US Department of Health and Human Services set ambitious targets for achieving value-based paradigms of care,

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underscoring these impending changes [91, 92]. In April 2015, the US Congress passed the Medicare Access and CHIP Reauthorization Act of 2015 (MACRA). The MACRA advances the agenda of value-based paradigms of treatment by providing clearer pathways of how to get there through the use of alternative payment models. This has dramatic implications for the delivery of stroke care [93], which has already been a point of focus for the bundled care initiative [94]. No doubt this policy evolution will be a driving consideration in care delivery for ET in the future.

Conclusion Proximal artery occlusions lead to severe strokes that were previously largely refractory to therapy. ET is now firmly established as the standard of care for LVO. In the immediate phase, operators are striving to improve mTICI 3 rates, reduce non-target embolization, and accelerate treatment times [95]. Nevertheless, fast, safe, and efficacious ET requires an interdisciplinary approach to effectively transition patients through all phases of therapy. A systematic approach to each phase is required to deliver evidence-based care consistently and to identify opportunities for improvement. Insights gained from this approach will inform practice guidelines at a local and national level and focus the next generation of trials aimed at improving patient outcomes.

Compliance with Ethical Standards Conflict of Interest Feras Akbik, Pedro Telles Cougo-Pinto, Ronil V Chandra, Claus Z Simonsen, and Thabele Leslie-Mazwi declare no conflicts of interest. Joshua A Hirsch declares that he holds shares in Intratech Medical, unrelated to the work. Human and Animal Rights and Informed Consent This article does not contain any studies with human or animal subjects performed by any of the authors.

References and Recommended Reading Papers of particular interest, published recently, have been highlighted as • Of importance •• Of major importance 1.

2.

Lima FO, Furie KL, Silva GS, et al. Prognosis of untreated strokes due to anterior circulation proximal intracranial arterial occlusions detected by use of computed tomography angiography. JAMA Neurol. 2014;71:151–7. Berkhemer OA, Fransen PSS, Beumer D, et al. A randomized trial of intraarterial treatment for acute ischemic stroke. N Engl J Med. 2015;372:11–20.

3.••

Campbell BC, Mitchell PJ, Kleinig TJ, et al. Endovascular therapy for ischemic stroke with perfusion-imaging selection. N Engl J Med. 2015;372:1009–18. Randomized data comparing endovascular stroke therapy with medical management to medical management alone. This is one of the five landmark recent trials in press that definitively prove the benefit of endovascular therapy in the first 6 hours after stroke onset.

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The Evolution of Mechanical Thrombectomy for Acute Stroke.

The natural history of an acute ischemic stroke from a large vessel occlusion (LVO) is poor and has long challenged stroke therapy. Recently, endovasc...
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