The Pipeline embolization device (PED, ev3 Endovascular, Plymouth, MN, USA) and Silk flow-diverting stent (Balt Extrusion, Montmorency, France) is a widely utilized flow diverter in the treatment of intracranial aneurysms, particularly those with unfavorable configurations.
It is a braided, platinum and nickel-cobalt chromium alloy, wire mesh cylindrical implanted device.
An aneurysm treated with a flow diverter is expected to involute over time, contrary to the immediate obliteration expected by surgical clipping or coiling. Yet, which aneurysms will respond to PED therapy and the time frame to expect full obliteration remains unclear 1).
It works by causing progressive flow redirection leading to thrombosis within the aneurysm.
Close to one-fifth of aneurysms, however, fail to occlude after PED placement. It offers an acceptable alternative for the treatment of difficult aneurysms according to their morphologies, including giant, wide-necked, fusiform, complex, and blister types.
PED was initially approved for adults with large or giant wide-necked (≥4 mm or no discernible neck) internal carotid artery aneurysms from the petrous to the superior hypophyseal segments. Studies have shown a superior aneurysm occlusion rate of 85 % at 6 months for the PED and mortality ranging from 2.6 to 4 %. There appears to be a knowledge gap in terms of the duration of dual antiplatelet therapy and efficacy of assessing platelet inhibition. However, increasing operator experience and favorable longer-term outcome data have led to the exploration of PED for a wide array of off-label uses. Given the paucity of good-quality studies comparing PED with other endovascular/surgical treatment options, several multicenter randomized trials are currently underway to answer these important questions 3).
Pipeline embolization device (PED) can be utilized in the treatment of distal anterior circulation aneurysms with difficult anatomy for conventional surgical or endovascular techniques. Larger-scale studies with long-term follow-up are needed to further elucidate the durability of PED treatment and its effect on perforator-rich vascular segments 4).
Use of the Pipeline embolization device (PED) in the posterior circulation is of some controversy. Publications have described adverse outcomes associated with the PED for vertebral artery and/or basilar artery pathology. In a case series publication of Albuquerque et al. stated that patient selection is essential for safe and effective PED treatment of posterior circulation aneurysms. The PED is equally effective in achieving aneurysm obliteration with an acceptable risk profile as it is in the anterior circulation. Dolichoectatic aneurysms were not included in this treatment cohort. PED may be a preferable alternative to open surgical treatment of posterior circulation aneurysms 5).
The main concern with the use of the pipeline embolization device (PED) in treating cerebral aneurysms is the risk of hemorrhagic and thromboembolic complications, including several cases of branch artery occlusion and delayed occlusion of the stented parent vessel shortly after antiplatelet medications were discontinued, highlighting the potential need for long-term antiplatelet therapy, and disastrous bleeding complications in unruptured aneurysm.
In addition, these microcell stents are difficult to use in distal aneurysms located over the ICA bifurcation and basilar tip because of the stiffness of the device, and perforating vessel occlusion is more likely to occur due to the characteristics of the stent. Before the era of flow-diverting microcell stents, large cell intracranial stents like the Neuroform stent (Boston Scientific/Target Therapeutic, Fremont, CA, USA) and Enterprise stent without coiling were used to provide flow-diverting effects for complex intracranial aneurysms.
Aneurysm treatment with the Pipeline Embolization Device is associated with the lowest complication rates when used to treat small ICA aneurysms. Procedure-related morbidity and mortality are higher in the treatment of posterior circulation and giant aneurysms 6).
In-Pipeline stenosis (IPS) is a common, early, and mostly benign complication. Patients with internal carotid artery aneurysms are more likely to develop IPS. Aspirin plays a key role in preventing IPS 7).
Aneurysm clips placed on canine parent arteries bearing a Pipeline flow diverter were unable to reliably stop blood flow. Application of aneurysm clips can cause mild damage to the device and neointima, which might translate into thromboembolic risks. If possible, vascular control should be sought beyond the terminal ends of the implanted device 8).
Acute embolism following use of the PED for treatment of intracranial aneurysms is more common than hypothesized. The only identifiable risk factor for embolism appears to be greater aneurysm size, perhaps indicating significant disturbed flow across the aneurysm neck with ingress and egress through the PED struts. The strength of antiplatelet therapy, as measured by residual platelet aggregation, did not appear to be associated with cases of procedural embolism. Further work is needed to determine the implications of these findings and whether anticoagulation regimens can be altered to lower the rate of complications following PED deployment 9).
Significant heterogeneity in dual antiplatelet therapy regimens following Pipeline Embolization Device (PED) placement and associated costs, exists at major academic neurovascular centers. The most commonly used first line dual antiplatelet regimen consists of aspirin and clopidogrel. Two major alternate protocols involving ticagrelor and prasugrel, are administered to clopidogrel hypo-responders. The optimal dual antiplatelet regimen for patients with cerebrovascular conditions has not been established, given limited prospective data within the neurointerventional literature 10).
Ravindran et al., from the Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts, USA and The National Neuroscience Institute, King Fahad Medical City, Riyadh, Saudi Arabia, present a case of a 63-year-old female with a persistent aneurysm in the communicating segment of the internal carotid artery treated with a second PED, 14 months after the deployment of a 1st PED, who subsequently developed a fatal intraparenchymal hemorrhage 3 weeks post-implantation. Histopathologic analysis at autopsy displayed evidence of endothelialization along the 2nd PED at this time, as well as neo-intimal growth between both devices. Patency of the vessel lumen with no intra-luminal thrombus, but thrombus showing early organization (endothelial cell ingrowth) was observed within the aneurysm dome.This case represents the earliest demonstration of intimal cell growth along the PED 11).
Bowers et al. report the microsurgical rescue and removal of a Pipeline stent embolization of a giant internal carotid artery aneurysm. After the initial placement of a Pipeline Embolization Device (PED), it migrated proximally to the cavernous carotid with the distal end free in the middle of the aneurysm, resulting in only partial aneurysm neck coverage. The patient underwent microsurgical rescue with trapping, bypass, and opening of the aneurysm with PED removal. The vessel remained patent in the proximal segment previously covered by the Pipeline stent. Microsurgical rescue for definitive aneurysm treatment with PED removal can be safe and effective for aneurysms unsuccessfully treated with PED 12).
A 40-year-old woman who had left facial pain and orbit discomfort. Angiography showed a giant fusiform aneurysm located in the cavernous segment of the left internal carotid artery. A PED was successfully deployed across the aneurysm. The procedure and post-procedural course were uneventful. After 3 months, angiography showed complete obliteration of the aneurysm with good patency of the branching vessels originating from the deployed segment. The patient's symptoms improved completely without complications 13).