Letter: Flow Diversion for Middle Cerebral Artery Aneurysms: An International Cohort Study

To the Editor: We read with great interest the article by Diestro et al1 describing a multicenter experience of the use of flow-diverting stents in the treatment of middle cerebral artery (MCA) bifurcation aneurysms. As the field of endovascular neurosurgery quickly expands the limits of distal access, it is inevitable that MCA bifurcation aneurysms fall into the horizon of flow diversion. To date, no endovascular series, regardless of device, has shown equality or superiority to outcomes of surgical clipping in terms of complete occlusion, complications, and clinical outcomes. In this article, the authors have reported the outcomes of 54 MCA aneurysms treated with flow diversion in which complete aneurysm occlusion was achieved in 55.6% of cases, with a retreatment rate of 9.3% and periprocedural thromboembolic complications in 16.7%. Combined analysis of 3 large studies on flow diversion using the “Pipeline Embolization Device” has reported a complete aneurysm occlusion rate of 85.5% at 1-year follow-up.2 The results of this pooled data and other reports have vaulted the use of flow diversion as a mainstay therapy for large unruptured proximal carotid artery aneurysms. However, despite the growing experience with flow diversion, the use of flow-diverting stents for other locations such as the MCA bifurcation has not achieved the same impressive results as these earlier studies. Perhaps one of the most significant limiting factors has been anatomic considerations of the MCA bifurcation that make aneurysms in this location particularly difficult to treat with flow diversion. The presence of a large side branching vessel is known to decrease the effectiveness of flow diversion, such as in posterior communicating artery aneurysms associated with fetal origin posterior cerebral arteries.3 Needless to say, this configuration is nearly always encountered at the MCA bifurcation. Diestro et al1 reported that 82.6% of stent deployments jailed either the superior or inferior division of the MCA. Without adjunctive coiling, which was present in only 20.4% of cases in this series, it is unlikely that flow diversion would be effective at this location. Therefore, it is unsurprising that the complete aneurysm occlusion rate was dramatically lower than the 85.5% reported in large series. Even more concerning is the high complication rate with endovascular therapy at this location; thromboembolic events are higher in the current cohort and other studies4-6 than in surgical series of MCA aneurysms. We assume that the same rate of complications in surgical clipping cohorts would be considered unacceptable. Five-year outcome data from PUFS demonstrated progressive aneurysm occlusion rates from 86.8% at 1 year to 95.2% at 5 years.7 Therefore, it is reasonable to assume that some aneurysms reported in this study will continue to progress to greater occlusion over time. In fact, the 55.6% rate of complete occlusion reported may underestimate the efficacy of flow diversion at this location because it remains to be seen whether aneurysms at the MCA bifurcation progress more slowly toward occlusion than aneurysms at other locations. The fact that only 36 of the 54 aneurysms in this series had a median follow-up period of 12 months leaves this conclusion unresolved. One wonders why this critical patient group with a novel treatment for which there is no established track record in this aneurysm location are lost to follow-up or their follow-up is so limited on which no reasonable conclusion can be made. Furthermore, despite the growing experience of flow diversion, safer distal access systems, and novel adjunctive endovascular devices, modern cerebrovascular specialists should compare the results with open microneurosurgery. In addition to our older data on the treatment of unruptured MCA aneurysms, which showed significant superiority to endovascular treatment,8 surgical treatment for MCA aneurysms in more recent series has proved to be extremely efficacious and safe, with a complete occlusion rate of more than 98% and a treatment-related complication rate of 5.7%.9 Our recent experience based on unpublished data (manuscript in preparation) of the past 7 years shows a rate of thromboembolic/ischemic complications of less than 6% in a large series of unruptured and ruptured MCA aneurysms including even those rare patients who necessitated aneurysm trapping and bypass. Given the effectiveness of surgery, the authors aptly conclude that flow diversion for MCA aneurysms may be more appropriate as a salvage technique in those aneurysms requiring retreatment for which no other reasonable option is available. We do believe that in complex cases, surgical clipping, surgical revascularization,10 or combined flow diversion and open surgical techniques are more appropriate treatment strategies, especially at the current stage of endovascular technology, rather than treatment with flow diversion alone, which is riskier and clearly less efficacious. We recognize the authors' effort to compile the largest series to date on flow diversion for MCA bifurcation aneurysms and for their enthusiasm for pushing the limits of endovascular treatments; however, the heterogeneity of the data, very low average number of adequately followed aneurysms per institution (2.25), lack of significant long-term follow-up, and most importantly, the high complication rate represent major shortcomings of these results. This article in our view further demonstrates that the best and most efficacious treatment for unruptured MCA aneurysms remains surgical clipping or surgical revascularization in some selected more complex and recurrent situations.