Risk of Branch Occlusion and Ischemic Complications with the Pipeline Embolization Device in the Treatment of Posterior Circulation Aneurysms

Treatment of Posterior Circulation Aneurysms Using the PED

Posterior circulation aneurysms are often associated with a higher incidence of morbidity and mortality compared with their anterior circulation counterparts; this is mainly related to higher rates of aneurysm rupture and neurovascular compression caused by large dolichoectatic aneurysms.^10,11 Flow diversion using the PED has gained popularity as an off-label treatment option.^{3⇓⇓⇓⇓⇓–9}

Flow diverters are designed to seal aneurysms from the circulation by diverting blood flow away from the aneurysm, allowing intra-aneurysm thrombus formation followed by neointimal growth across the neck of the aneurysm.¹ Intimal growth over the luminal surface of a flow-diversion device is expected to be seen as a tissue layer consisting of smooth muscle cells covered by endothelium (endothelization), while thrombus organization is expected to be visualized in the form of smooth-muscle cell invasion and connective tissue formation within the clot.¹

Despite widespread reluctance to use the PED for treatment of posterior circulation aneurysms, a handful of studies have attempted to evaluate the safety and efficacy in this high-risk group of aneurysms. The incidence of thromboembolic complications in these studies ranged between 0% and 22.5%^{3⇓⇓⇓⇓⇓–9} and was particularly high in fusiform aneurysms. A possible explanation for this finding was the higher likelihood of branch coverage when fusiform aneurysms were treated with the PED. Moreover, fusiform aneurysms can often be associated with intramural thrombosis, and placement of the PED is particularly hazardous because critical perforators may only be supplied through tenuous channels crossing the thrombus.⁹ However, in this study, most ischemic complications were either temporary or asymptomatic, and only 8.5% of procedures were affected by permanent symptomatic ischemic complications.

Branch Coverage and Occlusion following PED Placement

Initial evaluation of the PED by Kallmes et al^12,13 showed that despite placement of multiple overlapping flow-diversion devices in the rabbit aorta, lumbar branch vessels remained patent at follow-up. In an assessment of the patency of anterior circulation branches following PED placement, Rangel-Castilla et al¹⁴ reported a 15.8% branch occlusion following coverage. The occlusion rate was lowest in the anterior choroidal artery (0%) and almost equal in the ophthalmic artery (10.5%) and posterior communicating artery (10.7%). Both cases of anterior cerebral artery coverage ended with occlusion on last follow-up. Despite branch obliteration, the authors did not identify any clinical sequelae.¹⁴

Similar results were reported by separate studies on the fates of the ophthalmic artery,^{15⇓⇓–18} posterior communicating artery,¹⁹ and anterior choroidal artery.^19,20 Brinjikji et al²¹ reported a 45% incidence of posterior communicating artery occlusion or diminished flow at last follow-up, but none of the patients showed clinical symptoms related to vessel obliteration. Most of these patients demonstrated diminished blood flow immediately following PED placement; this finding was significantly associated with a higher occlusion incidence at follow-up. Gawlitza et al²² reported using flow diverters to treat aneurysms in the middle cerebral artery bifurcation and anterior communicating artery complex. Of the covered branches, 10.5% (2/19) had completely occluded on last follow-up. Temporary symptomatic ischemic events in perforator territories occurred in 17.6% of cases, which were reversible in all cases within 24 hours. Follow-up MR imaging disclosed asymptomatic lacunar defects corresponding to covered perforating artery territories in 29.4%. The lack of symptoms was attributed to the supply of respective cortical territories by leptomeningeal collaterals in all cases.²²

The high incidence of occlusion of the ophthalmic artery and posterior communicating artery following flow-diverter coverage is attributed to the rich collateral supply from the external carotid artery and PCA, respectively. These collateral vessels might increase the tendency for proximal occlusion if a flow diverter causes some diminution of inflow and the distal anastomosis takes over the end-organ arterial supply.^14,15 This collateral supply also explains the absence of clinical symptoms. On the contrary, terminal arteries without significant collateral supply, such as the anterior choroidal artery or middle cerebral artery perforators, are more likely to remain patent after coverage by a flow diverter. The pressure gradient across these arteries is more than that across vessels with a rich collateral supply, thus increasing the threshold for branch occlusion by a flow diverter.^14,20

Other considerations regarding the use of the PED for treatment of aneurysms near large branches include the branch that is incorporated into the neck of an aneurysm. In these cases, the use of the PED is controversial and often does not result in aneurysm occlusion. This outcome can be seen with flow diversion of posterior communicating artery aneurysms and may be related to retrograde aneurysm filling.^18,21

In this series, the fate of major posterior circulation branches has been assessed for the first time in a large and diverse group of patients. Similar to the anterior circulation branches, major branching arteries including the PICA, AICA, and superior cerebellar artery had a low incidence of branch occlusion after coverage with the PED. However, following occlusion of these branches, the risk of ischemic complications might be high. In our series, all 3 cases in which branch occlusion was associated with ischemic complications in the same territory involved the AICA. The relatively high incidence of VA and PCA occlusion was thought to result from a rich collateral arterial supply and was not significantly associated with ischemic complications. Like the study of Puffer et al,¹⁵ the number of PEDs deployed was not found to have any significant relation to branch occlusion incidence. Moreover, there was no significant correlation between branch occlusion and thromboembolic events.

Limitations

The primary limitations are the retrospective study design and variability in the management and follow-up protocols of patients across centers. The inclusion of multiple institutions, however, improves the generalizability of the findings. Indications for using the PED for posterior circulation aneurysms were at the discretion of the participating institution. Immediate branch flow following PED placement was not assessed. The time of branch occlusion and the status of perforators, which is difficult to precisely detect on DSA, was also not assessed. Lack of consistent follow-up brain images could result in missed silent strokes. Moreover, MRA was used to assess the branch occlusion in a subset of patients, along with the limitations related to this imaging technique. The lack of significant association between branch coverage and end points of interest may be influenced by type II error.