8 Direct Aspiration Thrombectomy for Acute Stroke: Evolution of Technique and Evidence
Thrombectomy techniques for achieving recanalization of an occluded large vessel occlusion have undergone rapid advances since the first approved device. The first techniques were modestly effective and entailed long procedure times. Newer approaches over the last several years have resulted in very high rates of successful recanalization and increasingly faster procedure times. This chapter reviews the mechanics and rationale for the evolution of thrombectomy approaches and provides a summary of the latest evidence in support of the latest generation thrombectomy technique, direct aspiration.
Under mounting pressure to have evidence in support of thrombectomy over iv-tPA, five randomized controlled trials 1 , 2 , 3 , 4 , 5 were launched shortly after the release of the negative trials of 2013. An impressive collective effort and rapid enrollment culminated in the halting of the trials in 2015 due to overwhelming statistical efficacy of thrombectomy over medical management. That year marked the largest improvement in therapeutic options for acute ischemic stroke care since the NINDs trial in 1995 and armed us for the first time with a mechanical thrombectomy device, the stent retriever (SR), with level 1A evidence supporting its use. We now detail the origins and technical nuances of the next-generation thrombectomy approach: direct aspiration.
8.2 Direct Aspiration
Acute stroke thrombectomy approaches have evolved rapidly. Spurred primarily by advances in catheter technology as well as the thrombectomy device itself, we are now able to achieve higher recanalization rates than ever before. We review the key technological advances and design modifications that have allowed for navigation around the ophthalmic turn for more distal delivery of larger-bore catheters providing more aspiration force directly applied at the thrombus interface.
8.2.1 Direct Aspiration Origins: the Penumbra 054 Aspiration Catheter
The Penumbra aspiration system introduced in 2008 involved maceration of the thrombus with a separator which was repeatedly introduced and withdrawn from the thrombus under direct aspiration to prevent showering of fragments. 6 While its market competitor at the time, the Merci system, relied primarily on delivery of a microcatheter (the 18 L; Stryker) to the site of occlusion, the Penumbra aspiration system relied on the delivery of a much larger bore catheter to the thrombus (up to effectively a 5 F device). The introduction of highly flexible lubricious polymers with good hoop strength allowed for safe placement of large intermediate class catheters directly into the large intracranial vessels. The development of large-bore flexible catheters was essential to the function of the Penumbra system.
The original iteration of the Penumbra reperfusion catheter system included several different sized catheters (internal diameter 0.026”, 0.032”, and 0.041”) and accompanying separators to maximize clot interaction and force of aspiration in vessels of differing diameters (internal carotid artery terminus, M1, M2, M3) to address both proximal and distal thrombi. 7 The largest catheter at the time of first launch had a lumen diameter of 0.041 inch, yet it tracked suboptimally around the carotid siphon and required a median 45 minutes to achieve acceptable recanalization min. 8 In 2009, the Reperfusion Catheter 054 became available, which dramatically improved the aspiration efficiency to a median 20 minutes 8 due to its much larger tapered lumen. As the aspiration force is proportional to the square of the diameter of the catheter, the 054 catheter provided an estimated 4 × aspiration force over the next smaller catheter, the 041. 8
Although a larger catheter lumen provides higher suction and more rapid removal of thrombus, a drawback of its larger size was that the 054 catheter often required the use of a coaxial technique to facilitate navigation to the middle cerebral artery. When navigated over a 0.014 inch microwire alone, a significant ledge would get held up at the origin of the ophthalmic artery. To overcome this obstacle to the target lesion, access with the 054 catheter could be optimized with a coaxial technique (Fig. 8‑1). The smaller 032 and 026 reperfusion catheters could be delivered simply over either a 0.014 or 0.016 inch wire, and the larger 054 delivered over those. One of the major advantages of the Penumbra aspiration system was that once the catheter system was delivered to the target vessel, separator clot maceration could be performed without having to re-access (additional ‘passes’), as was the case for the Merci device. 9
Despite these advances in catheter technology, navigating past the carotid siphon was still a relative challenge during thrombectomy cases. In patients with very acute angulation in the ophthalmic segment, adjunctive techniques could be performed to obtain the necessary distal access. One approach used the Merci Retriever System (Concentric Medical) as an adjunct to improve the trackability of the 054 reperfusion catheter by altering the angle with which the catheter engages the ophthalmic segment and M1 origin. By deploying an appropriately-sized Merci Retriever, such as a V.2.0 or V.2.5 soft, in the mid M1 segment through either the 032 catheter or an 18 L microcatheter and then applying gentle traction on the Merci Retriever, the course of the wire straightens, approximating the inner curve of the vasculature, pulling the catheter complex away from the ledge of the vessel origins (“grappling hook” technique), 10 , 11 an approach now used routinely using SRs and intermediate catheters. The 054 catheter can then be more readily advanced into the target vessel. Once the 054 reperfusion catheter is in place, the retriever is resheathed into the 18 L microcatheter and then removed prior to separator placement and aspiration.
The next iteration of the Penumbra aspiration catheter family (Max series) was introduced in 2012 and also intended to be used with separators of varying size. The Max series catheters included larger inner diameters at the distal end as well as the proximal end to increase the aspiration power. The larger proximal lumen reduces resistance to low and therefore increases aspiration force at the catheter tip. Improvements in polymer and braid and ring reinforcement provide more catheter tip flexibility and an increased number of transition zones to improve trackability while maintaining hoop strength. The newly introduced intermediate catheters were named 5Max, 4Max, and 3Max. An increased number of transition zones in the catheter design and manufacturing allowed these catheters to be delivered primarily over either a 0.014 or 0.016 inch microwire, even past the ophthalmic origin.
8.2.2 Direct Aspiration Origins: the Distal Access Catheter
In 2004, the Merci Retriever became the first mechanical thrombectomy device cleared for human use in the United States by the FDA. 12 The Merci device primarily works by engaging the thrombus with a “corkscrew” distal wire and suture tip deployed from within the clot, then removing the thrombus en bloc to achieve recanalization. The device was employed using a balloon-guide catheter that was positioned at the carotid bifurcation or internal carotid artery to cause temporary flow reversal, allowing the Merci to be retrieved into the guide catheter while mitigating the possibility of emboli showering to distal territories. However, clot retrieval into the guide catheter still required a long distance to be traveled while maintaining purchase on the thrombus, most commonly from the M1 segment of the middle cerebral artery to the proximal cervical internal carotid artery. The vector force applied while pulling on the thrombus was suboptimal (downwards along the long axis of the cervical carotid artery, not horizontally along the axis of the middle cerebral artery). This caused considerable torqueing, stretching, and distortion of the parent vessel and presented a biomechanical disadvantage to thrombus removal, as well as traction on the vasculature, resulting in pain for the patient. To avoid the inevitable movement induced, many operators chose to perform thrombectomy under general anesthesia. In addition, the Merci technique required a long distance to be traveled while remaining engaged with the thrombus.
A landmark advancement came in 2010 with the approval of the Outreach Distal Access Catheter (DAC; Concentric Medical), which would have repercussions for the application of the Merci device and also for future iterations of thrombectomy approaches. The DAC was designed for the purpose of buttressing access for the Merci thrombectomy device, affording stable access to the target vessel. Use of the DAC optimized the vectors at play during pulling of the device. With further understanding of clot fragmentation and distal embolization, the DAC was used as an intermediate aspiration device which aided in preventing showering of distal emboli during clot retrieval, increased the aspiration power applied directly to the thrombus. 10 , 13 The development of large-bore flexible catheters that could be delivered into the intracranial circulation represented a major advance in thrombectomy technology and also in intermediate catheter technology. 13 , 14 The DAC has a flexible distal shaft with increased proximal shaft strength and axial load-bearing characteristics as well as good hoop strength, allowing it to be delivered to the intracranial circulation around the ophthalmic bend when navigated over a coaxial catheter system. A major drawback to the Merci Retrieval System was that it necessitated navigating past the ophthalmic bend with every pass, decreasing the efficiency of the system and adding to procedure times.