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The Flow Diverter: A New Era of Aneurysm Treatment

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This blog post serves to simplify the following scholarly article:


Aneurysms are bulges in brain arteries formed from accumulating blood pressure on weakened vessel walls. This poses the risk of applying pressure on surrounding brain tissue or even bursting and bleeding out within the brain. For many years now, treating aneurysms could be described as trying to plug a leak from the inside.


That is, the “gold standard” for aneurysm treatment is called endovascular coiling. This treatment works by filling these sacs with coils of thin platinum in order to promote clotting of the aneurysm (stopping the flow of blood within the aneurysm sac) so that the weakened walls will stop expanding and eventually fill with coagulated (solid) blood. While this worked, these coils could shift and migrate down the vessel causing complications that require long term and expensive imaging and re-treatment, especially in cases such as wide-neck aneurysms where the aneurysm sac’s entrance is bigger than average.

To address these issues, Flow Diverters approach the problem through a different method entirely; slowing the parent artery’s blood pressure rather than plugging the affected sac. This advance in aneurysm treatment has made it possible to address cases once thought to be unsuited for intervention. 


Flow diverters make this possible by serving two vital purposes. First, flow diverters work by slowing the flow of blood within the artery walls. This reduces the stress that the weakened walls within the sac feel while also reducing the hindrance on the coagulation of blood within the sac. Secondly, the flow diverter itself physically acts as a supporting scaffold for new vessel lining. This is achieved through a balance in the mesh’s metal coverage/porosity. Essentially, there needs to be enough mesh to slow the blood flow within the artery but there needs to be enough porosity (holes/space between the mesh) to allow for blood flow into side arteries that the flow diverter may block. While this requires very precise designing and manufacturing, the Sump Effect helps facilitate this. The Sump Effect is simply that thin side vessels within the brain have low pressure, which acts as a vacuum/suction force that pulls blood through the low porosity mesh of the Flow Diverter. This is the same effect that allows wells to pull water up from deep underground! Collateral flow, or the interconnected vascular system of the brain, also aids in this process by allowing blood to reroute and continue supplying all regions of the brain with blood even if the Flow Diverter blocks out one entrance to a side artery.

The PITA (Pipeline Embolization Device for Intracranial Treatment of Aneurysms) and PUFS (Pipeline for Uncoilable or Failed Aneurysms) trials were critical clinical trials that helped prove the effectiveness and plausibility of a future where Flow Diverters could be used to help treat many wide-neck aneurysms that could not be treated with traditional platinum coil embolization. The PITA trial displayed the feasibility of using basic mesh stents (rudimentary Flow Diverters) to treat wide-neck aneurysms. Flaunting a 93% complete occlusion rate (blocking/closing off of the aneurysm sac) for all 31 patients in the study, this case shook the world of aneurysm treatment and marked a major success and proof-of-concept that justified further development of Flow Diverters. 


The PUFS trial on the other hand, could be considered the most important case in the officiation of Flow Diverters in American healthcare. This trial was made to gain approval of the FDA, requiring adhesion to extremely strict regulations of safety and effectiveness. This case studied the employment of Flow Diverters to what are classified as large ( >10 mm) wide-neck aneurysms in the carotid artery, an especially high risk region. At 6 months, the occlusion rate stood at 73.6%. This on its own, wasn't what made the case such a success. Rather, it was that 3 months later, the occlusion rate rose to 95.2%. This case not only proved the effectiveness of Flow Diverters to the FDA, but that the performance of Flow Diverters improve over time as opposed to platinum coils that can shift, migrate, and cause many other complications over time.


While Flow Diverters provide many benefits, they come with their own risks and tradeoffs. Being made of metal mesh, a foreign material to the human body, platelets in the blood would otherwise start to clot and stick to the diverter in a process called stent thrombosis (which poses the risk of stroke) if patients didn’t take the Dual Anti-platelet Therapy (DAPT) that is mandatory in cases where Flow Diverters are used. This reliance on DAPT is the biggest trade-off: without the platelets that coagulate and block off blood vessels when there is a cut or wound, bleeding doesn't stop and can pose a deadly risk. Additionally, unlike coils that provide a more immediate solution to the issue, Flow Diverters support a slower process of endothelialization (where new layers of tissue can grow on the diverter while also allowing the aneurysm sac to clot and close up. In the first few weeks/months when the sac has not yet fully clotted, it's vulnerable. If the sac was already unstable or ruptured, it would be at an even higher risk of bleeding out. This is a big reason why Flow Diverters are still being researched and often left as a Plan B for emergencies where coils aren’t feasible.


As Flow Diverters continue to progress, some are looking into the use of bio-resourceful materials that can dissolve away over time to reduce lasting side effects and interference with imaging. Additionally, employment of artificial intelligence and machine learning shows potential in anticipating whether or not Flow Diverters are the best choice for each individual patient's unique case. Other mechanical systems like CorPath (robotic-assistance for endovascular embolization) are also assisting surgeons in the delivery of Flow Diverters into the affected regions of the brain with sub-millimeter accuracy. Ultimately, these give us a glimpse into the range of all the innovation that is going into Flow Diverters as they continue to grow as a viable choice to help treat otherwise untreatable aneurysms.


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