Kat Dakay, DO
Mechanical thrombectomy has been recognized as the standard of care in acute ischemic stroke due to proximal large vessel occlusion. However, despite best efforts, it is not always successful: According to the authors, about 20-30% of clots are refractory to stent retriever thrombectomy. However, even if the vessel is eventually recanalized, the patient may still not necessarily have a favorable outcome, often termed “futile recanalization”; rates of futile recanalization vary widely depending on the definition used. Additionally, there are risks to a long and complex thrombectomy procedure in cases with refractory clots. In this article, the authors examine the number of stent retriever attempts, or passes, as a marker for futile recanalization.
In this multicenter, retrospective study [1], patients with a proximal anterior circulation large vessel occlusion treated with stent retriever thrombectomy were included. Additionally, patients needed to have an NIHSS of 4 or greater and be treated within 10 hours of last known well. The number of stent retriever passes required to achieve successful recanalization of TICI 2b or 3 was measured. A total of 467 patients were included in the study, with a median age of 67.3 years, median NIHSS of 15, and median ASPECTS of 8. The median number of stent retriever passes was 2, although rates ranged from 1 to 7.
First, the authors looked at the recanalization rate by number of passes, and found that the rate of recanalization on the first and second passes were relatively high, 45.3% and 27.6%, respectively. By the time the fifth pass is reached, 94.5% of patients who will ultimately achieve recanalization have already done so, and only an additional 3.9% will obtain it on the fifth pass. With each successive pass, the yield of recanalization becomes lower.
Second, patients were divided into two groups: those with a favorable outcome (defined as mRS 0-2), and those with an unfavorable outcome (mRS 3-6). Patients who had recanalization within four passes were more likely to have a favorable outcome, mRS 0-2, than those without recanalization. However, patients who had recanalization with five or more passes, even though the vessel was open, were no more likely to have a favorable outcome than those without recanalization. Thus, the authors conclude that in most cases, successful recanalization is going to occur within 4 stent retriever passes, if it is going to occur at all. They also suggest the consideration of switching to a rescue modality when the number of failed passes is approaching 5, with contact aspiration being a possible alternative.
This study deals with two topics — futile recanalization, or a poor outcome despite resolving the large vessel occlusion, and refractory clots, or clots which fail to recanalize despite best efforts with a stent retriever.
Many factors influence the efficacy of stent retriever thrombectomy in reopening an occlusion. First, vascular access itself must be gained, which can be influenced by vessel tortuosity or challenging aortic arch anatomy, as well as peripheral vascular disease. Secondly, the occlusion may be reached, but stent retriever may not be efficacious due to inability to pass and extract the thrombus, or due to vessel reocclusion. According to a retrospective study of reperfusion failure, the most common cause was stent retriever failure [2]. Larger thrombi, in particular, have been associated with a higher risk of stent retriever failure, whereas use of a balloon-guided catheter has been promising in terms of increasing the likelihood of recanalization [3]. Additionally, the clot may become more impacted and compressed with the increasing number of stent retriever attempts, leading to decreasing likelihood of success with each consecutive attempt [1]. Possible alternatives when stent retriever is unsuccessful include use of contact-aspiration or intra-arterial thrombolytics [4].
On the other hand, futile recanalization is equally challenging; the definitions are heterogenous, but generally indicate a poor clinical outcome despite angiographic success in clot retrieval. The literature currently cites rates of futile recanalization with modern stent retriever thrombectomy ranging from 28-49% depending on the criteria used. Futile recanalization is unsatisfactory from both the patient’s and the physician’s point of view; as Nie wrote in a recent literature review, “we expect not only to recanalize the occluded vessel but also to save the ischemic but still viable brain tissue.” It is likely a multifactorial phenomenon. Poor collateralization has been associated with futile recanalization [5], perhaps because the collateral vessels were not sufficient enough to sustain the ischemic penumbra while thrombectomy is being performed. Second, vessels can reocclude, either immediately or subacutely. Third, leukocyte and platelet aggregation of microcirculatory vessels, which has been studied in myocardial infarction though not widely studied in ischemic stroke, has been postulated by some authors to be at least partially responsible for futile recanalization [6, 7]. To that point, white matter disease, or leukoaraiosis, has been associated with futile recanalization [8].
This study prompts a compelling discussion about the factors that influence stent retriever failure, as well as the possible remedies, the pathophysiology behind futile recanalization, and the definition of futile recanalization itself. Though the modern devices are far superior in efficacy and patient outcomes to older methods, there is still room for refining the technology and the technical approach. It also demonstrates that our understanding of futile recanalization, while growing, is still far from complete. One challenge in the futile recanalization literature is parsing out non-modifiable biomarkers of comorbidity or disease severity associated with poor outcome, such as advanced age or high arrival NIHSS [9], with things we can change, such as improving time to recanalization or switching to a rescue therapy when appropriate. Another interesting aside is that in most of the literature, futile recanalization is defined as an mRS of 3 or greater at 90 days. However, given the natural history of large vessel occlusion, it remains open to debate whether an mRS of 3 should be included with a favorable outcome, such as was used in some of the decompressive hemicranectomy trials; one study indicated that patients with large vessel occlusion-related ischemic stroke and an mRS of 3 have a similar quality of life as patients with an mRS of 2, and are more similar in their overall function to an mRS of 2 rather than an mRS of 4 [10].
In conclusion, this study addresses the issue of stent retriever success rate and suggests that the more stent retriever passes that occur, the less likely the clot is to recanalize, and rescue therapy should be considered. It also poses the second, more challenging question: At what point are further attempts futile, and at what point to stop? The authors discuss when to consider switching therapies, but the latter issue remains a topic of debate. As they note, every case is unique in terms of individual anatomic factors, imaging biomarkers, and clot composition and morphology itself, making broad generalizations challenging. However, this study illustrates the technical challenges of stent retriever thrombectomy and provides some guidance for how to navigate them.
References:
- Baek JH KB, Heo JH, Nam HS, Kim YD, Park H, Bang OY, Yoo J, Kim DJ, Jeon P, Baik SK, Suh SH, Lee K-Y Kwak HS, Roh HG, Lee Y-J, Kim SH, Ryu CW, Ihn Y-K, Kim B, Jeon H-J Kim JW, Byun JS, Suh S, Park JJ, Lee WJ, Roh J, Shin B-S. Number of stent retriever passes associated with futile recanalization in stroke. Stroke. 2018;49:2088-2095
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- Baek JH, Yoo J, Song D, Kim YD, Nam HS, Kim BM, et al. Predictive value of thrombus volume for recanalization in stent retriever thrombectomy. Sci Rep. 2017;7:15938
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- Gilberti N, Gamba M, Premi E, Costa A, Vergani V, Delrio I, et al. Leukoaraiosis is a predictor of futile recanalization in acute ischemic stroke. J Neurol. 2017;264:448-452
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