Placement of a Stent within a Flow Diverter Improves Aneurysm Occlusion Rates

DISCUSSION

The combined use of a stent and flow diverter has been thought to decrease the success rate of flow diverters.¹¹ However, this possibility refers to previously placed stents, and the consequences of placing a stent within a flow diverter have not been studied. Although there are 4 reports in the literature mentioning a total of 17 stents placed within flow diverters for better wall apposition, the safety or efficacy of this technique is unknown.^5⇓⇓–8

We wanted to evaluate the safety and utility of stent placement within a freshly deployed flow diverter by comparing the aneurysms treated by flow diverters only (Mtg) with those treated by flow diverters followed by stents during the same procedure (Stg). We included only aneurysms treated by a single type of flow diverter without adjunctive intrasaccular treatment so that the study population would be uniform as to the efficacy of the device used (Surpass device)—that is, we wanted to eliminate the variables of device type and adjunctive treatments. Both groups of patients were also similar for all variables (age, sex, history of a prior thromboembolic event, smoking, hypertension, diabetes mellitus, hyperlipidemia, malignancy, and platelet inhibition level) except for the size of the aneurysm. The mean size of aneurysms was 14.8 mm in the Stg and significantly higher than the Mtg (9.1 mm). Because a cutoff point of 13 mm was suggested for lower occlusion and higher complication rates¹² in flow diversion, we would actually expect a lower occlusion and a higher complication rate for the Stg. This was not the case for our cohort. Despite a larger aneurysm size in the Stg, the occlusion rates were higher. Additionally, there was no difference in the occurrence of significant adverse events between the 2 groups.

An earlier occlusion was noted in the Stg with statistical significance at 9–12 months. As the aneurysms in the Mtg progressed to occlusion after 1 year, the difference between occlusion rates of the groups decreased, yet there was still a trend in the Stg toward better occlusion rates at or after 1 year.

These findings suggest that stent placement within flow diverters does not significantly increase the risk of flow diversion and is associated with increased efficacy. Indeed, adverse events without clinical symptoms, namely fish mouthing and/or in-stent stenosis, were only observed in the Mtg, hinting at enhancement of safety with stent placement, which is an outcome that remains to be definitely demonstrated.

Braided stents and especially flow diverters have a lower radial force compared with laser-cut stents.^5⇓–7 Consequently, flow diverters may not appose the arterial wall, especially in arterial segments with tight bends.¹¹ Hence, stents have been used only rarely for the apposition or anchoring of flow diverters in several patients as noted above.^5⇓–7 The latest clinical and preclinical data suggest that incomplete wall apposition causes inhomogeneous and delayed endothelial coverage of stent struts and aneurysm neck,^13,14 and suboptimal apposition at the aneurysm neck causes persistent filling of the aneurysm.¹⁵ Treatment failure rates (Raymond-Roy grade III) in malapposed flow diverters were 41.2% and significantly higher than in fully apposed devices (9.6%) in a study with 213 patients.¹⁶ In our study, there were several reasons for stent placement within the flow diverter and not all stents were deployed for better apposition. Nevertheless, the better angiographic outcome in the Stg was presumably due to better wall apposition regardless of the presence of an angiographically visible overt malapposition. Incomplete apposition is not only a handicap in the periprocedural period, it also leads to delayed stent occlusions in the coronary arteries¹⁷ and after flow diversion,¹⁸ which may occur as late as 3 years after device placement.^19⇓–21 Improvement of apposition by stents may help maintain flow-diverter patency in the long term.

Several strategies exist to enhance flow-diverter apposition to the arterial wall. Among these, the most commonly used ones are loading the device during delivery and recrossing it with the delivery microcatheter after deployment. Angioplasty is another technique that is adopted by some authors. However, angioplasty may paradoxically worsen device apposition, and this may be detected only by advanced imaging methods such as optical coherence tomography.¹⁵ Balloon angioplasty can also result in thrombus formation inside the flow diverter¹⁵ or, in case of a short landing zone, foreshortening and device prolapse into the aneurysm.²²

It may be argued that placement of a stent inside the flow diverter will increase the metal coverage and consequently result in both an enhanced flow diversion and also an increased rate of perforator injury in vulnerable segments like the M1 segment of the MCA or the posterior circulation. Wall coverage of the current intracranial stents is very low, in the range of 10%–15% at most, and the cumulative coverage of overlapping devices is much less than the numeric sum of the coverage of telescoped devices.²³ The minimal increase of coverage by the intracranial stent is likely neither to cause an increase in the rate of perforator injury nor to result in an appreciable increase in flow diversion.

After propensity score matching, there was no statistically significant difference in the occlusion rates of cerebral aneurysms between SAC and either of the FD groups (Mtg and Stg). However, there was a clear trend in both the Stg and Mtg for an increased rate of total occlusion compared with the patients with stent-assisted coiling. This finding is concordant with those previously reported in the literature using similar methodology for comparison of SAC and flow diversion.^24,25 We believe that the absence of a statistically significant difference results largely from the low number of patients remaining in the study when propensity score matching was performed. Thus, our findings can only suggest that increased aneurysmal occlusion is the result of the use of a flow diverter rather than placement of a regular stent for SAC when flow diversion is compared with SAC. Although per our results, a statistical difference in favor of Stg compared with Mtg exists during early follow-up, larger cohorts are needed to validate this finding. It is conceivable that such larger cohorts may reveal differences in mid- or late-term follow-up periods as well.

One other unexpected result was the diminution of the gap between the occlusion rates of large-versus-small aneurysms as previously reported in the literature.^12,26 In our whole cohort of patients treated with flow diversion in this study, there was no difference in the occlusion rates of aneurysms of ≥13 mm in diameter versus those aneurysms <13 mm in diameter. Because 18 of the 26 aneurysms were treated with additional stent placement in the ≥13 mm subgroup (reaching 94% occlusion at 1 year in those that were stented), it is possible that the absence of a significant difference arises from the higher obliteration rates in the Stg. Nevertheless, the absence of such significance should be appraised with caution in studies like ours that have a limited number of patients.

There are 2 major disadvantages of stent placement within the flow diverter. The first one is the risk of flow-diverter migration during recrossing of the device or during the microcatheter exchange maneuver through the flow diverter. The second is cost. Fortunately, there is a matching intracranial stent for each flow diverter in terms of a compatible delivery microcatheter for all of the current flow diverters except for the Surpass flow diverter, for which a technique to place a stent without an intracranial exchange maneuver has been described.²⁷ The stent is certainly an additional cost of the procedure. However, placement of a stent results in a comparable increase in cost with regard to the other adjunctive methods used to increase the efficiency of flow diverters such as intrasaccular flow diverters,⁴ Medina²⁸ embolization device, regular coils,²⁹ or telescopic placement of another flow diverter. Because of the concern related to cost, we suggest that stent placement within a flow diverter should be selective and limited to bailout situations such as those listed in the previous section of our article and to those aneurysms that are expected to have a lower rate of occlusion by placement of a single flow diverter (eg, large and giant aneurysms).

The main limitations of this study include its retrospective nature and relatively small sample size. None of the stents we deployed inside flow diverter have been approved for such use, and this technique is an off-label use of these stents. Finally, only 1 type of flow diverter was examined in this study, and other types of flow diverters need to be studied to verify the proposed relation of device apposition and aneurysm occlusion.