Abstract
Thrombectomy is a technique that has completely changed the management of acute stroke and current devices have shown that they can achieve upwards of 90% successful recanalization in selected cohorts. However, despite the effectiveness of these devices, there are a proportion of patients who still fail to achieve reperfusion of the affected vascular territory and an even larger portion of patients who have poor functional outcomes in spite of successful recanalization. There are no guidelines on how to treat these patients when such failures occur. In an effort to understand the underpinnings of how failed thrombectomy occurs, we extensively reviewed the current literature in clot properties, vascular access problems, stroke pathogenic mechanisms, embolic complications, failed procedures and pre-procedural imaging. A short summary of each of these contentious areas are provided and the current state of the art. Together these elements give a cohesive overview of the mechanisms of failed thrombectomy as well as the controversies facing the field. New techniques and devices can then be developed to minimize such factors during stroke thrombectomy.
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Introduction
In recent years, the rapid advancement of thrombectomy has revolutionized acute stroke care, establishing it as first-line therapy for acute ischemic stroke (AIS) with large vessel occlusion [1,2,3,4]; however, up to 20% of AIS patients treated with mechanical thrombectomy do not have successful recanalization and only around 50% eventually achieve good functional outcomes [5,6,7,8,9,10]. With each failed thrombectomy attempt, reperfusion to the brain is delayed, and each additional pass attempted augments the friction coefficient between the thrombus and the vessel wall with cumulative risks for procedural complications, such as dissection and distal embolization into unaffected territories [7, 9]. This article reviews the factors responsible for failure of thrombectomy that have been described in the recent literature.
Clot Properties
Thrombus composition is a key factor in determining its susceptibility to mechanical thrombectomy methods and the chance of successful recanalization. The precise composition of a thrombus is highly dependent on its source and etiology, but generally consists of fibrin and red blood cells with an additional minor content of white blood cells (WBC) (Fig. 1; [11, 12]).
Clots are predominantly composed of red blood cells (a) and fibrin (b)
The proportion of fibrin relative to the red blood cells in a thrombus determines its physical properties and how it responds to thrombectomy. Hypodense, fibrin-rich thrombi demonstrate decreased revascularization rates regardless of the technique employed [13, 14]. Laboratory flow model experiments have echoed similar findings found in clinical practice, with fibrin-rich thrombi that comprise <20% red blood cells having much higher static friction properties and are hence more difficult to mobilize and remove [15]. A mature clot rich in fibrin is also firmer and less deformable when interacting with the struts of a stent retriever, leading the clot compaction which in turn increases friction between the clot and the vessel wall. This leads to each attempt being less effective than the previous one and less likely to remove the clot [15, 16]. Newer generation stent retrievers with increased radial force or which are designed to scoop or capture the clot within its struts rather than penetrate the thrombus, may be more effective in extracting firmer clots rich in fibrin. These newer devices also tend to have a distal net to capture embolized particles.
The amount of time a thrombus has had to mature often determines how “sticky” it is and how difficult it is to be extracted [17]. This is reinforced by the water-hammer effect of systemic blood pressure that squashes and compacts the thrombus making it denser and more resistant to extraction. Finally, as it becomes more compressed, the fibrin-rich clot tends to remain in the same position during the thrombectomy attempt. This is due to the struts of the stent retriever having a greater tendency to “slip” over the dense clot as it is withdrawn, or the clot does not fit securely into the mouth of an aspiration catheter due to its firm characteristics. In contrast, clots rich in red blood cells are soft and easier to remove but tend to be friable and prone to fragmentation, which may lead to distal embolization complications.
The content of WBCs in the thrombus has also been shown to be associated with the ease of recanalization and the length of the procedure [18]. Histological studies have shown that vessel injury leads to platelet attraction, which catalyzes further platelet clumping. The WBCs then migrate to the edges of the platelet clump and fibrin strands form, which serve to trap RBCs giving the thrombus its shape and form [19, 20]. The proportion of WBCs and the degree of migration into the clot are therefore a surrogate marker of the maturity of the thrombus. In more mature clots, the WBCs have had more time to invade the thrombus and this is associated with an increased degree of organization of the thrombus, which make them more resilient and difficult to remove. This organization is closely associated with the stability of the clot and the friction to the vessel wall. More mature clots are therefore more difficult to remove [21]. While there are different types of WBCs, neutrophils specifically catalyze the thrombotic pathway by formation of neutrophil extracellular traps (NETs) [22]. These NETs are created by decondensed deoxyribonucleic acid (DNA) fibers released by neutrophils into the extracellular matrix. The DNA fibers act as supports for red blood cells (RBCs) and platelets to bind to and propagate the clotting cascade [23]. A potential diagnostic use was put forth when an association between NETs and cardioembolic etiology was found [24]. The proportion and location of WBCs and NETs can also determine the underlying etiology of the thrombus; in cardioembolic thrombi a higher proportion of WBCs and NETs are found in the clot, which may be due to the longer time it had to mature in the heart chamber before embolization.
There are other distinct histological features cited in the literature, which may have the potential to determine if clots are resistant to mechanical thrombectomy. Partial endothelialization at the edges of the thrombus is thought to represent early organization and maturation as the body attempts to grow endothelium over the thrombus [25]. It is thought to be associated with mature thrombi that are more difficult to remove. Furthermore, thrombi of atypical origin, such as calcified plaques or neoplastic thrombi can be more resistant to removal [26, 27]. Several cases have been described with failed thrombectomy where the plaque was heavily calcified. After multiple stent retriever attempts were unsuccessful, intracranial balloon angioplasty was required to improve the blood flow. In another case report, a stent retriever was trapped in a dense calcification and could not be withdrawn and finally the stent retriever had to be re-sheathed within the microcatheter [13]. Finally, other plasma constituents, such as von Willebrand factor may contribute to clot resistance. Von Willebrand factor is a large multimeric plasma glycoprotein that links platelets in conjunction with fibrin. A novel study showed that the level of von Willebrand factor correlates with more organized and firmer clots [28, 29]. Despite this level of understanding of how clot composition interacts during thrombectomy, the histological make-up of a clot remains unknown prior to the procedure, and hence all thrombi are currently approached in the same manner. Methods to determine the thrombus type before the procedure are a crucial gap that needs to be addressed and such methods are presently being developed for an in vivo examination of the clot before the first retrieval attempt.
The volume of the thrombus or thrombus burden has a strong association with the location of the occlusion. The thrombus burden tends to be largest in the internal carotid artery and smallest in the distal middle cerebral artery [30]. Even though a larger thrombus burden would have a commensurately larger surface area for thrombus-vessel interaction and therefore increased friction, the literature on thrombus size and successful thrombectomy recanalization has not been able to show such a linear relationship. Higher clot burden scores were associated with better recanalization rates but a longer clot was shown to have poorer functional outcomes in the THERAPY trial [31,32,33,34,35,36,37,38]. Nonetheless, the optimal thrombectomy device is dependent on the thrombus burden. For example, an intermediate catheter that is too small will lead to shearing of the edges of the clot when it is withdrawn into the guide catheter. An AIS that has been proven to be resistant to one thrombectomy method may respond well to a different approach. Traditionally after stent retrievers or aspiration catheters have failed three passes, it is thought to be useful to switch to another method [39]. Currently it appears better to use techniques that optimize all factors, such as the PROTECT-plus technique [40]. In the future, it may be possible to individualize the thrombectomy devices and techniques with respect to different clot locations, clot composition and patient factors.
Access Problems and Stroke Etiology
A frequent reason for failed thrombectomy or abandonment of the procedure is the problem of difficult vascular access. There were two recent large series that looked at factors associated with failed thrombectomy (defined as TICI [thrombolysis in cerebral infarction] 0/1). In the first study, 72 out of 648 consecutive stroke patients had failed thrombectomy. In one fifth of these patients the occlusion could not be reached and in another fifth the thrombus was not passed [41]. In the second series, 63 out of 592 patients had failed thrombectomy, with older age and M2 occlusions being factors associated with failure [42]. In both series, major equally frequent reasons for failed thrombectomy were difficult anatomical access and resistant occlusions [41, 42].
If the patient has calcified or non-patent femoral arteries, an ectatic aortic arch or tortuous neck vessels, it can be difficult to gain access even to the intracranial circulation. An ectatic aorta can be difficult to navigate and other variations such as a bovine arch can render the procedure even more challenging, with the risk of the guiding catheter prolapsing into the aorta during the procedure and the surgeon having to start again [43, 44]. Tortuous, looped or kinked cervical vessels can also prevent smooth navigation as well as take up too much of the catheter length rendering it too short to reach the occlusion [45, 46]. In the light of such challenges, which can often be determined on a preprocedural computed tomography (CT) angiogram, occlusions in certain vessel are sometimes much more easily accessible via an alternative access site; a common example is basilar occlusion for which a radial artery approach can be a superior access point. This will reduce complications and diminish wastage by avoiding changing catheters or wires that are not suitable, as well as cutting down the length of the procedure, ultimately leading to earlier reperfusion of the brain. When the aortic arch or the carotid artery is seen to be challenging in an anterior circulation thrombectomy, a direct common carotid puncture can sometimes help with access as well as to simplify and speed up the procedure; however, there is a shortage of tailored material and risks for carotid wall hematoma formation as well as for postprocedural embolic complications. In addition, there is equipoise regarding closure of the puncture site where both manual compression as well as closure devices and open surgery have been advocated. Because of the risk of a neck hematoma in cases of insufficient closure, most surgeons prefer to treat these patients while they are under general anesthesia to have the airways protected.
In the intracranial circulation, the geometric anatomy of the blood vessel can be a factor leading to the failure of the stent to remove the thrombus. An increase in vessel diameter can cause the clot to fall out of the stent as it is being pulled, e. g. if the stent does not expand to accommodate the wider vessel. A vessel shaped in a hairpin loop will increase the friction of the clot as it is being withdrawn by dragging it against the vessel wall [47, 48]. Similarly, with a hairpin loop or an S‑configuration vessel, avulsion injuries of small adjacent blood vessels are possible during withdrawal of a stent retriever (Fig. 2). This occurs because the pulling force is perpendicular to the direction of the blood vessel. Using an intermediate catheter will improve the vector of force and reduce the risk of avulsion injuries to the vessel or other complications (Fig. 3).
Flow model demonstrating the importance of an intermediate catheter in tortuous anatomy. a Stent retriever without an intermediate catheter in a hairpin turn. b On withdrawal, the friction causes traction and deformation of the vessel without clot removal. c Stent retriever with an intermediate catheter in a hairpin turn. d The change in the direction of force allows withdrawal of the clot without deformation of the vessel (courtesy of Neuravi Inc, Galway, Ireland
Digital subtraction angiography in the lateral views (a, c) and anteroposterior view (b, d). An S‑shaped turn of the vessel and the tortuous path taken by the guidewire. On stent retriever thrombectomy, the high friction between the stent retriever and the wall caused an inadvertent detachment of the stent retriever (arrow in c and d)
An underlying stenotic lesion due to an intracranial atherosclerotic plaque is a possible reason for immediate reocclusion postthrombectomy that results in failure of the thrombectomy procedure [49]. This is more prevalent in Asians and African-Americans compared to the Caucasian population, although typical cardiovascular risk factors such as hypertension and diabetes mellitus remain important for intracranial atherosclerosis in any population [50,51,52]. The location of the occlusion can occasionally suggest an underlying intracranial atherosclerotic stenosis, such as a mid-basilar occlusion with flow in the basilar termination or mid-M1 occlusion with flow in the MCA bifurcation (Fig. 4; [53]). When performing thrombectomy for patients with underlying atherosclerotic lesions, there is often a tendency to require other adjunct devices in addition to the stent retriever, culminating in longer procedure times. This is in contrast to simple cardioembolic occlusions causing AIS, where a stent retriever alone is successful in up to 83.3% of the patients [54]. Other underlying vessel wall pathologies, such as vasculitis or early moyamoya disease can also render the thrombectomy devices ineffective and are rarer causes for failure of thrombectomy [55, 56]. In such cases, without vessel wall magnetic resonance imaging (MRI), it is often difficult to confirm the presence of such a pathology. Therefore, a high degree of clinical suspicion is important to prevent futile repetitive procedures.
Postthrombectomy attempt, there is an underlying mid-basilar atherosclerotic stenosis seen on the final diagnostic angiogram. a Anteroposterior view, b lateral view
In stroke patients with failed mechanical thrombectomy (MT) attempts, approximately 60% of the occlusions can be passed. In such cases, rescue treatment might be considered to improve recanalization and clinical outcome [41]. In these situations, a permanent intracranial artery stent deployment might be a potential solution. Intracranial stenting was first performed in AIS with off-label use of balloon-mounted cardiac stents [57,58,59,60]. Recently, a Korean study demonstrated more favorable outcomes in a group of patients with failed thrombectomy but were rescued with permanent stenting as compared to the non-stented group of patients with failed thrombectomy [5]. A meta-analysis was recently performed on rescue stenting in failed thrombectomy and 8 studies comprising 160 patients were included in which the rate of mTICI 2b-3 was 71%. This led to good outcomes in 43% of the patients and death in 21%. Symptomatic intracerebral hemorrhage was seen in only 12% of the cohort although glycoprotein IIb/IIIa inhibitors were used in 89% of the cases and almost all the patients had antiplatelet therapy after the procedure [61]. The current iteration of intracranial self-expanding stents is based on nitinol and the stents are designed to provide a balance of sufficient outward radial force at body temperatures to open up occluded vessels, yet minimizing the incidence of negative remodeling and in-stent restenosis that have been reported in studies using balloon-mounted stents [62, 63].
Other rescue modalities have also been studied. An interesting study from the North American Solitaire Stent-Retriever Acute Stroke registry looked at failed thrombectomy and intra-arterial tissue plasminogen activator (IA tPA) rescue therapy. There was no significant increase in recanalization, functional outcome, mortality or SICH with IA tPA; however, a sub-group analysis limited to M1 occlusions only and an onset-to-groin puncture ≤8 h, resulted in significantly higher successful revascularization rates with IA tPA rescue therapy (77.8% versus 38.9%; P = 0.02) [64]. A similar study used a low dose of the glycoprotein IIb/IIIa inhibitor tirofiban as rescue treatment in patients with failed thrombectomy. In 154 patients tirofiban was not associated with increased numbers of symptomatic intracranial hemorrhage (SICH) or mortality. A subset of patients with large artery atherosclerosis were associated with a mortality benefit when treated with tirofiban [65].
Distal Emboli and Futile Revascularization
When a thrombectomy procedure is performed, one of the complications that can lead to a worse functional outcome despite good recanalization is embolization from the original occlusive thrombus distally to the same or previously unaffected cerebral territories (Fig. 5; [66, 67]). Embolic complications have been reported with different thrombectomy devices [68,69,70,71]. Many various thrombectomy techniques were used in the MR CLEAN study which reported embolization to new territories in 8.6% of patients, which resulted in 5.6% developing new neurological deficits [1, 72]. In stent retriever studies, 5–22% had distal emboli, with 0–7% of these in new vascular territories [4, 7, 73,74,75,76]. Older thrombectomy devices, such as the MERCI (Concentric medical, Mountain View, CA, USA) device or the initial iteration of the Penumbra system with the separator (Penumbra Inc., Alameda, CA, USA) tended to macerate the clot more with more fragments being generated causing distal emboli. They were shown to have worse outcomes compared to intra-arterial tPA thrombolysis, and therefore are no longer used [72, 77,78,79]. Direct aspiration techniques have been developed with recent iterations having larger lumens and improved trackability. The rates of embolization for these are, however, not yet well described in vivo.
Digital subtraction angiography in the anteroposterior and lateral views. After direct aspiration thrombectomy, there are distal emboli in the M3 vessels (arrow)
Emboli can be generated during thrombectomy in several different ways: The water-hammer effect of the blood flow can shear off parts of the thrombus. Crossing a clot, especially with larger catheters, such as a 3Max (Penumbra Inc., Alameda, CA, USA), can also potentially cause distal embolization into same arterial territory. Another way emboli are generated is via fragmentation of the thrombus by friction between the vessel wall and the thrombus while withdrawing it. The thrombus can be sheared off the sides of the guide catheter if the lumen is too small as the thrombus is pulled into it. As the thrombus is withdrawn into a larger vessel from a smaller one, there can be a temporary loss of apposition between the stent retriever and the thrombus. Finally, during deployment of stent retrievers, pieces of the thrombus can be forced between the struts of the mesh comprising the thrombectomy device [67, 80].
Factors found to be associated with emboli during thrombectomy include posterior circulation occlusions, the use of conscious sedation and the thrombus length [81]. Several studies have shown that the use of a balloon guide catheter during a thrombectomy to create a state of flow arrest reduces the risk of emboli which results in better functional outcomes [76, 82,83,84]. Posterior circulation occlusions are described to have significantly more distal emboli. This is likely in part due to the lack of flow arrest, as balloon guide catheters are mostly not used for such procedures [85]; however, an aplastic or hypoplastic vertebral artery contralateral to the cannulated vertebral artery for thrombectomy was in a recent study associated with complete recanalization meaning fewer distal emboli. It was suggested that contralateral flow modulation or ipsilateral balloon guide catheter use in patients with bilaterally patent vertebral arteries should be considered and further studied in the future [86].
The incidence of thromboembolic complications into other previously unaffected vessels also increases with the length of the clot. In in vitro experiments the rate of thromboembolic events was 0% with 10-mm length thrombi but increased to 7.4% with 20-mm thrombi and 14.8% with 40-mm thrombi [87]. Finally, procedures using conscious sedation have been reported to have a higher incidence of distal emboli than those using general anesthesia. This may be attributed to movement of the patient during thrombectomy and its attendant difficulties [88]. The actual incidence of emboli during a thrombectomy procedure could be much more than previously thought [77, 89] as during a thrombectomy, a large portion of the thrombus fragments that break off are very small and not visible on standard digital subtracted angiography, as seen in in vitro experiments. These tiny emboli can obstruct collateral flow to the salvageable ischemic penumbra, induce inflammatory injury at the capillary circulation level or even cause infarcts in a previously unaffected vascular territory [73, 90,91,92]. The presence of these fragmented clots has been shown to be associated with worse clinical outcomes [90, 93].
The number of attempts with each device has a strong association with the functional outcome. In a series of 330 patients treated with stent retrievers, recanalization was 46.8% with the first retrieval but only 22.7% with the fifth attempt. The number of passes was an independent negative predictor of good clinical outcome (OR 0.65; 95% CI, 0.435–0.970; P = 0.035) and therefore patients with 1–3 attempts had a significantly better clinical outcome [94]. Further attempts had good recanalization rates, but the rate of favorable clinical outcome did not improve. Another Korean series with 467 patients treated with stent retrievers noted that recanalization rates became lower commensurate with the number of passes [95]. In their multivariable analysis, 1–4 passes were significantly associated with better outcomes compared to patients without recanalization; however, 5 or more passes was not associated with better outcomes. The recanalization rate for ≥5 passes of the stentretriver (SR) was 5.5% and the authors concluded that it was futile for both recanalizations as well as for good functional outcome. Finally, in sub-studies from the ARISE-II dataset, the first-pass effect, i. e. removal of the thrombus with TICI 2b-3 recanalization with only one attempt, was more important for good clinical outcome, SICH and mortality than a final equivalent recanalization [96, 97].
Imaging to Predict Thrombectomy Failure
Preprocedural imaging, whether CT-angiography (CTA) or MR-angiography (MRA), is crucial for the operator to quickly plan the thrombectomy strategy. It can predetermine the best site for vascular access as well as anticipate any problems the surgeon is likely to face. Ideally, it can also determine which are the ideal tools and technique to use and prognosticate the chance of a successful thrombectomy.
The hyperdense middle cerebral artery (MCA) sign scan be seen on a non-contrasted CT scan and is strongly associated with a thrombus containing a higher RBC content (Fig. 6; [98]). Despite this predilection towards RBCs having higher Hounsfield units, fibrin, RBCs and WBCs all affect the density of a clot on a CT-scan [13]. The IV tPA (intravenous tissue plasminogen activator) has a higher rate of recanalization with RBC-rich clots compared as to fibrin-rich clots [99], whereas the literature available on CT thrombus density and endovascular thrombectomy is still uncertain. The older thrombectomy devices do not appear to perform better with certain types of clots, although stent retrievers appear to show greater success with increased thrombus density [32, 33, 100, 101]. The RBC-rich thrombi can also be identified using MRI scans, where the iron in the RBC of the thrombus induces a blooming artifact on Gradient echo (GRE) and susceptibility weighted imaging (SWI) sequences [102, 103].
Axial non-contrast computed tomography (CT) of the brain. Dense middle cerebral artery sign (arrow)
Larger and more proximally located thrombi have been shown to have lower recanalization rates with intravenous thrombolysis and are therefore associated with worse clinical outcomes and higher complication rates [104, 105]. Conversely, thrombi which are longer have been shown to have better recanalization rates with aspiration thrombectomy when compared to IV tPA. It is therefore important to identify and know the thrombus burden as it affects the therapeutic decision making. A method of measuring the thrombus length is by measuring the filling defect on CTA. On a single phase CTA, the arterial filling defect depends on imaging acquisition timing after the contrast bolus and the presence of collateral circulation which will allow the contrast to reach past the occlusion and showcase the distal end of the thrombus [106, 107]. In newer techniques, such as the dynamic CTA or multiphasic CTA, this is less of a problem as the delayed scans will allow the contrast time to fill the entire gap and reflect the thrombus boundaries [108]. A less precisely validated alternative is the clot burden score which is a semiquantitative validated method of measuring the clot burden using prespecified vessel segments that fail to be opacified by contrast on a single phase CTA [109]. Longer and curved susceptibility vessel signs on MRI have also been associated with reduced effectiveness of reperfusion therapies [102, 103].
A good collateral circulation is associated with a better chance of recanalization with intravenous thrombolysis but may also potentially aid in removing the thrombus in mechanical thrombectomy [110]. Theoretically, a good collateral circulation allows the intravenous thrombolytics to act on both ends of the clot and using the increased surface area enhances the penetration of the clot thereby softening it. A good collateral circulation also reduces the water-hammer effect from the systolic blood pressure in the original vessel and in doing so reduces the pressure gradient across the clot allowing easier withdrawal of the clot, either by direct aspiration or by a stent retriever.
The perviousness of thrombus to contrast agents on the CT scan can be determined by the change in Hounsfield units (HU) precontrast and postcontrast administration. Fibrin-rich clots seem to have a characteristic of increased perviousness as compared to RBC-rich clots with a lower initial HU, but with a higher increase in HU after contrast is administered [111]. This has clinical implications as some recent literature has suggested that more pervious clots with an increase in HU of 23 or more after contrast administration was associated with smaller infarcts, superior recanalization, and better functional outcomes after IV thrombolysis [99, 108, 112]. This method can be used to predetermine the thrombus make-up and select devices more likely to successfully remove usually difficult fibrin-rich clots, although more research will need to be done in this aspect.
The clot gap on CTA has also been suggested to have the potential to determine the underlying pathology of the occlusion. An occlusion involving the MCA bifurcation in the sylvian fissure was defined as a branching type occlusion and was more likely to be a thrombus of embolic origin. Conversely a clot in the trunk of the artery with opacification of all distal MCA branches from the bifurcation was associated with an underlying intracranial atherosclerotic lesion, likely with a thrombus forming on it (Fig. 7). While truncal type occlusions comprise only 12% of all occlusions, they are associated with poorer recanalization rates of 18.2% with a tendency to immediately re-occlude after the stent retriever is withdrawn. They therefore have longer procedural times, worse clinical outcomes and more frequent additional device use. This is in comparison to branching type occlusions which have been reported to have a 4 in 5 chance of successful recanalization [113].
a Computed tomography(CT)-angiogram of the brain showing a “truncal” occlusion of the middle cerebral artery (MCA) (arrow), with branches of the MCA bifurcation visible. b The corresponding post-thrombectomy angiogram showing an underlying atherosclerotic lesion (arrow)
Artificial intelligence (AI) for analysis of imaging is now creeping into the field. A study of the clot characteristics on CT and CTA was done in 67 patients with proximal anterior circulation occlusions treated with IV alteplase. A total of 326 radiomics features were extracted from each thrombus and used to train a linear support vector machine classifier. When ROC curves were constructed, the combination of radiomics features was significantly better at predicting early recanalization with IV alteplase than any other single radiological feature from CT or CTA (area under the curve = 0.85) [114].
Finally, there are angiographic features of poorer outcomes with thrombectomy. While poor or failed recanalization is a well-known and instinctively understood marker of poor functional outcomes, there are several more subtle signs which are indicative of poorer prognosis despite good recanalization. Early venous shunting can be associated with already infarcted tissue especially involving the basal ganglia (Fig. 8; [115]). Similarly, leptomeningeal collaterals which persist longer than the venous phase are also associated with the area of infarction and poor functional outcomes (Fig. 9; [116]).
Early venous shunting seen on the computed tomography(CT)-angiogram showing the vein of Labbe (a, arrow). Despite TICI3 recanalization with thrombectomy the patient already had infarcts present (b)
Digital subtraction angiography, lateral view. Very late leptomeningeal cortical vessels seen long after the venous drainage phase (arrow). These are associated with infarcted tissue and worse outcomes
Conclusion
Failed thrombectomy in AIS is associated with poor outcomes, with a high incidence of functional dependency. This article describes the key problems and challenges that interventionists commonly face today. Future research efforts need to be focused on these areas to develop techniques and devices that make it possible to reach and remove even difficult clots in a difficult anatomy in a timely fashion.
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The authors gratefully acknowledge David Vale and Michael Gilvarry of Neuravi/Cerenovus Inc. for the use of figures and videos from their in vitro modeling.
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L.L.L. Yeo has received substantial grant funding from the National Medical Research Council (NMRC), Singapore, substantial grant funding from the Ministry of Health (MOH), Singapore and a moderate grant funding from I2R, A‑STAR, Singapore. P. Bhogal is consultant for Neurvana Medical and Phenox. T. Andersson is a consultant for Ablynx, Amnis Therapeutics, Medtronic, Neuravi/J&J, Rapid Medical and Stryker. A. Gopinathan, Y. Cunli and B. Tan declare no conflict of interest.
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Yeo, L.L.L., Bhogal, P., Gopinathan, A. et al. Why Does Mechanical Thrombectomy in Large Vessel Occlusion Sometimes Fail?. Clin Neuroradiol 29, 401–414 (2019). https://doi.org/10.1007/s00062-019-00777-1
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DOI: https://doi.org/10.1007/s00062-019-00777-1