Battle between the Stent Retrievers and Aspiration Catheters in the treatment of Acute Ischemic Stroke
17 million strokes occur every year worldwide, and it is the second leading cause of death after coronary artery disease. Ischemic strokes, make up for 87% of all strokes, the rest being haemorrhagic.
Ischemic stroke results from a cerebral artery blockage that leads to a loss of oxygenation in downstream brain tissue. The impact of this leads to neuronal cell death and potentially irreversible neurological deficit. There is a critical therapeutic window whereby timely intervention and subsequent clot removal, may help reverse or offset such risk of injury.
A choice of treatment
The most effective treatment for strokes has long been debated. Alongside pharmaceutical intervention – using clot-busting drugs combined with the physical removal of a blood clot in mechanical thrombectomy, has proved to be an effective therapy.
Two main methods have emerged with two quite different methods of operation - the stent retriever and the aspiration catheter. The first uses a very small wire mesh to grip the clot and remove it, the second uses a suction system to drag the clot away.
The Stent Retriever
Stent retrievers are self-expandable stents that are deployed past the site of occlusion, used in conjunction with a thin, highly flexible microcatheter. Once at the blood clot, the stent retriever will expand and engulf the thrombus and seek to restore blood flow to the patients vessel.
Medtronic were the pioneers behind the first stent retriever, named the Solitaire. This breakthrough device received FDA approval in 2012 and quickly became the first choice device for many physicians after the success of the SWIFT trial, which compared the Solitaire with the Merci retriever. The Merci device was the first mechanical thrombectomy device to receive FDA approval back in 2005. Stryker soon followed suit with their Trevo device, which again demonstrated superior outcomes in comparison to the previous Merci device.
Numerous trials have since followed that have continued to demonstrate favourable outcomes for patients receiving endovascular treatment, compared to those receiving standard care. That being said, there can be several potentially difficult situations that can arise during stent retriever thrombectomy procedures. For example, If the parent cerebral artery is particularly tortuous or if the clot is located too distal to reach successfully. These challenges have paved the way for next generation thrombectomy devices from innovative start-ups like Rapid Medical and NeuroVasc Technologies.
Rapid Medical, an Israel based neurovascular start-up, have pioneered the development of the Tigertriever 13. A next generation stent retriever designed for patients with distal, medium vessel occlusions (DMVO). DMVO are said to account for 25-40% of acute ischemic strokes, and more than half of these occlusions fail to be recanalized with existing modes of treatment. The Tigertriever 13 is comprised of a fine wire mesh mounted on a flexible shaft that can be expanded by the physician. The wires of the mesh are radiopaque, which means the physician has full visibility under fluoroscopy and can control the device until it conforms to the vessel diameter. The Tigertriever 13 is also the smallest stent retriever currently available on the market, measuring at just 20.5mm in length it is perfectly suited to reaching the more distal areas of the blocked blood vessel.
The Aspiration Catheter
Aspiration thrombectomy had its origins from the peripheral vasculature. Here, a large-bore catheter is progressed to the proximal surface of the clot and manual suction is applied using a syringe. Initially, this technique was restricted to non-tortuous vessels due to the large, stiff catheters only available at the time.
Penumbra then paved the way for a new kind of aspiration thrombectomy, following the launch of the Penumbra System. The catheter had a much more flexible internal diameter which made it much more effective at advancing into the intracranial vessels.
Recently, a survey from interventional neuroradiologists reported that the most common first-line mechanical thrombectomy approach was the direct aspiration first-pass (ADAPT) technique. Although continued innovation in the space has resulted in faster procedure time and better outcomes, failure to recanalize with currently available aspiration catheters still occurs in one-third of patients. This is often due to thrombi being located in difficult to reach, highly tortuous, small diameter vessels.
As a result of these vascular size constraints, the design of an effective aspiration catheter can often mean there is a compromise between suction strength and ability to reach the distal vessels. A tapered design has tried to combat this, whereby the distal tip of the catheter is smaller than the proximal end. But this has been limited by the thickness of the required guide catheter and the minimum wall thickness of the aspiration catheter.
These limitations have led to an innovative breakthrough from MIVI Neuroscience, who have looked to combat these limitations with their R4Q aspiration catheter. This device eliminates the proximal three-quarters of the catheter shaft, replacing it with a stainless steel control wire which utilises the full internal area of the guide catheter to provide a higher aspired flow rate and suction force than the standard tubular catheter designs.
Two treatment methods - which comes out on top?
In light of the breakthrough of these two different modes of treatment, there has been an ongoing debate regarding the optimal first-line thrombectomy technique for large-vessel occlusion. Numerous studies have therefore looked to compare the two in a bid to identify the most successful treatment method.
A meta-analysis was performed which looked at 18 studies, including a total of 2893 patients. It was found that there was no significant different in the rate of final successful reperfusion or good functional outcome between the stent retriever thrombectomy and aspiration groups. The stent retriever thrombectomy group did achieve a statistically higher first rate pass however and resulted in lower use of a rescue device. Whilst the aspiration first approach did result in a statistically shorter groin to reperfusion time. Highlighting that final reperfusion rates and functional outcomes are comparable between the two, but stent retrievers were superior in achieving reperfusion as a stand a lone first line technique, albeit with a longer groin to reperfusion time.
More recent studies have looked to combine the two modes of treatments to achieve more favourable outcomes. Combining stent retrievers and large bore aspiration catheters helps trap the thrombus between the catheter tip and the stent retriever, while maintaining local aspiration. Other combined approaches include the Solumbra (Solitaire device + Penumbra aspiration catheter) technique, where the stent retriever is retracted into the aspiration catheter. These combination techniques have shown improved reperfusion rates and first pass effects, however there remains significant room for improvement in achieving early and complete reperfusion.