Brain Aneurysm Resource Center

Coil Embolization vs. Flow Diversion

By Michael Chen, MD

Helpful information between two commonly used brain aneurysm treatment options.

In 1995, treatment of brain aneurysms began a fundamental change in approach from predominantly open surgery to an endovascular, catheter and X-ray-based approach.  Open surgery with clip placement aimed to reoppose the walls of the normal arterial segment and effectively strangle the aneurysm, no longer allowing any further pulsatile arterial blood flow to enter the aneurysm dome and have contact with the weakened arterial wall.  Endovascular coil embolization aimed to use a platinum-based substrate of coils, which, combined with the thrombus that it induces, yields a physical barrier that protects the weakened arterial aneurysm wall from pulsatile arterial blood. 

Catheters are typically advanced, via a small incision at the crease of your groin in the femoral artery, under X-ray guidance, up the aorta, and within seconds are able to access the arteries in your neck.  From there, small catheters are advanced within your arteries into the head under high magnification X-ray guidance into select the dome of the aneurysm. 

Over time, large randomized studies such as the International Subarachnoid Aneurysm Trial (ISAT) demonstrated that overall, coiling led to better outcomes when compared to clipping.  It became increasingly obvious that recovery times were shorter as well.  As a result, significant investments in device innovation were made, and practitioners became increasingly familiar with the procedure.  Currently, the overall majority of aneurysms treated worldwide, including unruptured aneurysms, are now through endovascular means, with relatively high standards with regards to safety and efficacy.   

One important limitation to coil embolization is the long-term durability, particularly in larger or wide-neck aneurysm.  The coils themselves can only occupy a finite volume of space, much like large ice cubes vs. crushed iced cubes in a glass.  The larger the aneurysm, the greater the challenge of obtaining a dense construct and easier it is for there to be pockets, especially near the neck, of continued pulsatile arterial blood flow.  Very large aneurysms that were causing symptoms because of local adjacent pressure on important brain structures or nerves also presented a limitation to coiling.  Coiling these types of very large aneurysms would entail the use of a very large amount of coil mass that was not as elegant a solution.

​Approved by the FDA in 2011, flow diversion embolization became an approach that allowed for more of a traditionally “open surgical” type of result but still through an endovascular approach.  This approach consists of delivering a higher density stent-like device, with pores in the sub millimeter range, much like a screen door, within the segment of normal artery that the aneurysm arises from.  As such, no device needs to be placed within the actual sac of the aneurysm, which has obvious safety advantages.  However, the catheter used to deliver the device is almost twice as large as the catheters typically used for coil embolization and this can sometimes be difficult to navigate into the appropriate position.  Nevertheless, flow diverters allow for anatomic exclusion by uncoupling the hydrostatic pressure of pulsatile arterial blood within the normal parent artery and the blood that exists within the aneurysm dome, promoting blood stasis, and eventually thrombosis.  Durability, particularly in very large aneurysms is much more reliable.  Reducing mass effect on very large aneurysms can be more elegantly achieved than with coilembolization.  Concerns still exist with regards to the need for blood-thinning medications such as aspirin and clopidogrel.  Unusual complications have been reported but the overall outcomes have been rather impressive and reassuring.