19.2.1 Aneurysm Residual Due to Inappropriate Device Position

Inappropriate device position is a rather broad term that can include a number of actual problems with the device construct. However, it is very important to recognize that of all the different etiologies behind failed flow diverter treatment, residual aneurysms that are felt to be secondary to inappropriate device position should theoretically be the easiest to retreat. The main reason for this is simply that the flow diverter construct that had been previously created is not expected to be successful if it is not placed in a manner that maximizes its flow-diverting properties.

If the flow diverter from the original treatment was undersized with regard to its diameter, particularly at the proximal aspect of the construct, inadequate wall apposition of the flow diverter can result in an endoleak, with subsequent continued filling of the aneurysm. Other reasons for inadequate wall apposition include placement of the proximal end of the stent within a sharp curve, as can be seen with the anterior genu of the cavernous internal carotid artery, and placement of the stent within an irregular, atherosclerotic segment of artery. Finally, incomplete opening of an appropriately sized stent can result in an endoleak. Unfortunately, incomplete stent opening can also result in acute thrombus formation within the lumen of the parent artery. Largely due to this concerning risk, great efforts are made to recognize and manage incomplete stent opening at the time of initial treatment with balloon angioplasty or other manipulations. Therefore, it is unlikely to be the cause of a residual aneurysm as seen on later follow-up angiography. Occasionally, however, it is not immediately clear if a stent is not opened uniformly because of an issue with the stent itself, or if it is as a result of irregularity in the parent artery. Therefore, if there is any question regarding the adequacy of stent apposition, we generally perform a computed tomography angiography, such as Dyna CT (Siemens, Malvern, PA), in the angiography suite. This may help differentiate areas of the stent construct that may benefit from angioplasty, and those that are simply not fully expanded due to areas of stenotic artery.

While, in theory, inadequate wall apposition is something that can be detected during the initial treatment procedure, occasionally the stent configuration or location can change at some point of time after completion of the procedure. Complete migration of a flow diverter, in which the entire stent moves distally, is a rare complication that can be seen with undersizing of the device. Fortunately, this has become more of a theoretical concern, given that our understanding of flow diverters has improved. Nevertheless, it can occur, and certainly if the stent migrates past the aneurysm neck, it would result in an incompletely treated aneurysm, and possibly occlusion of the vessel that it migrates to.

Another more common cause for flow diverter malposition is contraction, or partial migration of the device. ³ , ⁴ Unlike complete migration in which the whole stent moves, contraction involves a portion of the stent contracting or foreshortening while the remainder of the stent stays in a relatively stable position. This may more likely occur in particularly tortuous arteries when a vector of force is directing the stent into the aneurysm, and in general is a result of deploying the flow diverter with an insufficient length and apposition on either side of the aneurysm. Unfortunately, it may be difficult to determine the exact amount of stent that should be deployed in the normal artery proximal and distal to the aneurysm to ensure that it does not eventually contract, as it likely varies on an individual basis. However, it may more likely occur with particularly tortuous arteries or those that require significant tension buildup to pass the stent into the desired location. For this reason, it may be reasonable to select longer stents for those aneurysms in which one anticipates potential issues with contraction.

Contraction may also be seen with telescoping flow-diverting stents. This is generally avoided by ensuring that a significant portion of the stent (30–50%) is overlapping, and by choosing a diameter for the inner stent that is at least as large as the outer one. In other words, deploying a smaller stent within a larger stent may greatly increase the chances of contraction and disconnection of a stent construct.

19.2.2 Aneurysm Residual Due to Device Failure

Unlike residual aneurysms that are secondary to inappropriate device positioning, a number of aneurysms may have device constructs that are perfectly positioned and sized, but yet still result in incomplete occlusion of the target aneurysm. These are aneurysms that fail not because of technical issues with the treatment but rather because of something inherent to the aneurysm, which for some reason may make them less likely to respond to treatment with flow diversion.

Aneurysm size and morphology certainly play a large role in the likelihood that the aneurysm goes on to complete occlusion. Decreased rates of complete occlusion are seen with fusiform or dissecting aneurysms. ⁵ , ⁶ , ⁷ Aneurysms that are associated with particularly sharp curves of the parent artery may have a jet stream of flow into the aneurysm that may not correct with flow diversion. Another group of aneurysms that are less likely to have successful treatment with a flow-diverting stent are those aneurysms that have already been previously treated, particularly those with a history of prior stent placement. ⁶ , ⁸ , ⁹ , ¹⁰ , ¹¹ There may be a number of reasons for this. Passage of the microcatheter through a preexisting stent can be problematic because if the microcatheter unknowingly passes through a cell of the existing stent, a subsequent point of restriction is created upon deployment of a flow diverter. Assuming the true lumen of the stent is maintained by the microcatheter, in the process of deployment the distal end of the flow diverter may catch on the prior stent leading to stretching of the device and less ideal results. ¹² In addition, placement of a flow-diverting stent within another stent may be associated with an increased risk of in-stent or stent-adjacent stenosis. ¹³

There is reasonable literature regarding the effect that prior treatments have on flow diverters. Heiferman et al reported their series of 25 aneurysms that had failed stent-assisted embolization. ¹¹ Following subsequent treatment with flow-diverting stents, complete occlusion was seen in only 38% of cases. Nelson et al reported in their series of 31 aneurysms that of the two aneurysms not initially occluded with a flow-diverting stent, one was treated with another stent. In this small series, the authors noted that prior stent placement makes apposition of a flow-diverting stent more challenging and may decrease neointimal formation and endothelialization required for aneurysm obliteration. ¹⁴ In a review of 63 aneurysms treated with flow diversion, Lylyk et al found that the only aneurysm remaining patent at 12-month follow-up had been previously treated with stent-assisted coiling. ¹⁵ The authors concluded that the prior stent likely decreased the apposition of the flow-diverting stent to the parent vessel, which created an endoleak and likely decreased endothelialization.

In a matched cohort analysis, 21 patients with previously stented recurrent aneurysms who later underwent Pipeline Embolization Device placement (group 1) were compared with 63 patients who had treatment with the Pipeline Embolization Device with no prior stent placement (group 2). ¹⁶ Pipeline Embolization Device treatment resulted in complete aneurysm occlusion in significantly fewer patients in groups 1 (55.6%) versus group 2 (80.4%). The rate of good clinical outcome at the latest follow-up in group 1 was 81.0 versus 93.2% in group 2 (p = 0.1), and complications were observed in 14.3% of patients in group 1 and 9.5% of patients in group 2 (p = 0.684). Intrinsically, it appears that the use of the Pipeline Embolization Device in the management of previously stented aneurysms is less effective than the use of this device in nonstented aneurysms. Prior stent placement may also worsen the safety and efficacy profile of this device. It is also reasonable to suggest that aneurysms that require further stent placement may be more complex aneurysms that may already have a higher risk of morbidity and lower occlusion rate. Nevertheless, recurrent stent placement following previously stented aneurysms is challenging and must be performed cautiously.