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Dr. Thomas Yasuda
Interventional Neuroradiologist
Clinical Hospital of the University of Sao Paulo
Brazil
Thrombectomy for large vessels has become widespread and is part of the neurointerventionalist’s routine with its indications expanding every day. However, when compared to the treatment of medium vessel occlusion (MeVO) – which represents 25-40% of the ischemic strokes – current guidelines recommend IV thrombolysis, despite the fact that 50% of these cases have not been analyzed. This leaves us with the conclusion that mechanical thrombectomy (MT) is a new frontier to cross in the treatment of MeVO, and a selection of cases is favorable and necessary to establish the best technique.
The more distal the vessel, the smaller the lumen, the more tortuous the route to achieve the clot with a consequent increase in friction. These vessels are more susceptible to straightening and angle modifications, and are more prone to injuries. As an example, with more friction, perforators can suffer avulsion, especially with stent retrievers (SR) and those that are larger and longer promote more retraction and damage. It is also true that the smaller the vessel, the more easily it spasms.
So, an excellent device to use in these cases would have several criteria: the softness to decrease the damage to the vessel, it should be balanced to provide an optimal radial force to trap the embolus and, at the same time, not promote too much traction to avoid perforator avulsion or a subarachnoid hemorrhage (SAH). It’s not easy to imagine a device like this. Smaller devices in diameter and in length have recently been designed and are good devices for these vessels, like SRs with a diameter preferentially smaller than 4mm, fitting into a 017" microcatheter. An example is the Solitaire™ X 3mm (Medtronic) which was recently launched in the market, and which adapts easily into 1.5mm diameter vessels. But when used alone, an SR might not be so effective and frequently it has difficulties in grabbing properly the thrombus, leading to distal and collateral embolization.
Aspiration is also a great tool, but smaller vessels require smaller caliber catheters and despite being smaller embolus, fragmentations can easily occur.
A more recent technique can be accomplished – and maybe achieve better results. This combines aspiration and SR, a mini-SOLUMBRA technique, using a low-profile stent retriever placed in a 0.017” microcatheter and a 0.035 inch inner diameter distal aspiration catheter. It’s necessary to perform a blind exchange of the microcatheter 0.017” to the 0.035” aspiration catheter with the stent guidewire, the so-called blind exchange/mini-pinning (BEMP) technique.
A recent meta-analysis comparing SR, aspiration or both showed no difference, however there was a huge heterogeneity between studies to consider, still a trend to a better functional outcome and mortality rates using SR + aspiration was observed, despite the higher risk of subarachnoid hemorrhage.
Not yet proven but promising, cyclic aspiration can change this paradigm and offer better recanalization rates and, who knows, decrease complications of the MTs while being a powerful tool for MeVO as well.
The last but valuable point concerns intra-arterial thrombolysis. The endovascular treatment for stroke began here and the more distal the vessel, the smaller the clot, the more powerful intra-arterial thrombolysis can be. We should remember that PROATC II showed decades ago that this therapy is effective for M1 occlusions and that recently the CHOICE study, a phase II trial, showed that IV thrombolysis after LVO MT had better reperfusion rates.
There’s no answer yet about what’s the best technique for MeVO, but new devices with smaller profiles and better navigability will soon allow us to achieve adequate safety and efficacy for endovascular treatment of this new MeVO frontier.
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