By Maxim Mokin, Elad I Levy and Tareq Kass-Hout
Over the last decade, we have witnessed rapid advancement of endovascular techniques for acute ischaemic stroke treatment. Accumulating evidence from studies testing both intravenous and intra-arterial revascularisation approaches clearly demonstrates that improved outcomes are directly associated with our ability to achieve early successful recanalisation of the occluded vessels. Our initial experience with mechanical thrombectomy using the Merci retriever device and the Penumbra aspiration system showed higher recanalisation rates, when comparing the results to historical controls from the Prolyse in Acute Cerebral Thromboembolism (PROACT) II trial of intra-arterial pharmacological thrombolysis. Still, both thrombectomy approaches have been criticised for higher rates of complications (namely, intracranial haemorrhage) and high mortality rates as well as technical complexity; moreover, neither device was able to demonstrate 100% recanalisation rates in major trials.
The use of intracranial stents for acute stroke revascularisation was originally attempted as “the last resort” approach when stroke interventionists faced challenging cases refractory to pharmacological and mechanical intra-arterial manipulations.
Several centres began to use first balloon-mounted stents and later self-expandable stents and soon realised that stenting allowed robust restoration of blood flow. Instead of a rescue therapy, stenting was now considered a first-line treatment.
Closed-cell stents allow partial expansion and recapture (re-sheathing), creating a concept called “temporary endovascular bypass”. In this technique, a stent is temporarily deployed, achieving instant recanalisation. The theory behind this approach is that the radial force generated during stent expansion disrupts the thrombus and plasters it against the vessel wall, allowing robust restoration of blood flow to the ischaemic brain. Restored blood flow potentiates the endogenous thrombolytic system, which can be further enhanced by pharmacological agents, such as recombinant tissue plasminogen activator (rtPA) or glycoprotein IIb/IIIa inhibitors, administered locally via a microcatheter injection. Once sufficient blood flow is established, which can assessed with serial angiographic injection runs, the stent is recaptured. The fundamental difference of the “temporary endovascular bypass” approach from Penumbra aspiration thrombectomy or Merci mechanical thrombectomy is the ability to establish instant recanalisation. Therefore, during the time it takes for the endogenous thrombolytic system to lyse the clot, the affected brain territory receives full blood supply.
The obvious advantage of recapturing and removing the stent, rather than permanently deploying it, is avoiding the need for dual antiplatelet therapy, which is mandatory for patients in whom an intracranial stent is implanted. Patients with large strokes are at high risk for reperfusion haemorrhage, and the dual antiplatelet therapy would certainly elevate such risk. This issue can become of even more concern in patients with atrial fibrillation (which is a frequent cause of large vessel occlusion), given the requirement for long-term anticoagulation agents. Also, patients with permanently placed intracranial stents are at risk for in-stent restenosis.
The obvious concern for the temporary endovascular bypass concept is that complete thrombus dissolution might not be achieved when relying on the endogenous thrombolytic system alone. The use of additional pharmacological agents would carry risk for intracranial haemorrhage. Although clot disruption and augmented blood flow as a result of temporary stent placement are powerful facilitators of the process of thrombus lysis, larger thrombus load and time-dependent thrombus organisation in patients with stroke presenting after several hours of symptom onset would make the temporary endovascular bypass approach alone prone to recurrent thrombosis.
These limitations have been successfully overcome by combining the concept of temporary endovascular bypass with thrombus extraction using stent-retrieval technology. The stent-retriever is used not only to establish immediate blood flow but also as a capture device and is retracted into a guide catheter with the thrombus trapped within the stent walls. Efficacy, safety, and superiority of this approach in comparison to earlier-generation mechanical thrombectomy with the Merci device were recently demonstrated in the Solitaire FR With the Intention For Thrombectomy (SWIFT) trial. The trial was stopped nearly one year before the anticipated finish date because of clear evidence of superiority of the Solitaire device.
In our opinion, the concept of temporary endovascular bypass is fundamental to the success of the stentriever approach to acute stroke treatment. Establishing early blood flow is key in preserving brain from irreversible ischaemic damage. Undoubtedly, endovascular therapies for rapid recanalisation of occluded vessels have already demonstrated improved clinical efficacy and safety and will continue to rapidly evolve, leading to improved outcomes in patients with acute stroke.
Maxim Mokin and Tareq Kass-Hout are Stroke fellows, Department of Neurology, School of Medicine and Biomedical Sciences, University at Buffalo, State University of New York, Buffalo, New York, USA. Mokin reports no financial relationships. Kass-Hout has received an educational research grant from Genentech.
Elad I Levy is professor of Neurosurgery and Radiology, School of Medicine and Biomedical Sciences, University at Buffalo, State University of New York, Buffalo, New York, USA. He receives research grant support (principal investigator: Stent-Assisted Recanalization in acute Ischemic Stroke, SARIS), other research support (devices), and honoraria from Boston Scientific* and research support from Codman & Shurtlef and ev3/Covidien Vascular Therapies; has ownership interests in Intratech Medical and Mynx/Access Closure; serves as a consultant on the board of Scientific Advisors to Codman & Shurtlef; serves as a consultant per project and/or per hour for Codman & Shurtleff, ev3/Covidien Vascular Therapies, and TheraSyn Sensors; and receives fees for carotid stent training from Abbott Vascular and ev3/Covidien Vascular Therapies. He receives no consulting salary arrangements. All consulting is per project and/or per hour. (*Boston Scientific’s neurovascular business has been acquired by Stryker)