Even though endovascular coiling is now the cornerstone of intracranial aneurysm treatment, there are still limitations and unmet needs. The limitations include both the inability to achieve total occlusion in all saccular aneurysm treatments and aneurysm recurrence. The latter is still a significant problem for wide-necked, large, and giant aneurysms and frequently requires repeated sessions of treatment. Some aneurysms, such as dissecting aneurysms, fusiform aneurysms and aneurysms with branch incorporation in the neck region are still not satisfactorily treated with endovascular coiling, even with complicated combinations using the assistance of balloons and stents. In my opinion, liquid embolic agents have proved disappointing. With this background, another treatment strategy to target at the parent vessel, rather than the aneurysm was perceived as being important, which led to the development of flow-diverting stents.
How does the device work?
Flow diverting stents are flexible metallic stents with sub-millimetre pores. They act by uncoupling blood flow between the parent vessel and aneurysm sac, which reduces the aneurysm velocity and shear stress. These haemodynamic changes induce stasis within the aneurysm sac, gradual thrombosis and organisation of the aneurysm sac, and eventually endothelialisation of aneurysm neck completing the process of vessel healing and disease cure. Experimental studies have showed that a flow-diverting stent with metallic coverage of 35% and 18 pores/mm3 performed better than those with higher porosity and lower pore density.
The Pipeline Embolization Device (PED from eV3/Covidien) is one of the two commercially available flow-diverting stents. The PED is a mesh tube of woven wire, made of 25% platinum and 75% cobalt–nickel alloy, designed to provide 30–35% metal coverage of the inner surface of the target vessel, with a pore size of 0.02–0.05 mm2 at a nominal diameter. The PED is attached to a pusher wire, compatible with a 3F (0.027 inch internal diameter) microcatheter (Renegrade Hi-Flo from Boston Scientific; Mass Transit from Cordis; Marksman from eV3/Covidien) which has a platinum coil tip that extends 15mm beyond the distal edge of the device. The PED is delivered via a 0.027-inch inner diameter microcatheter that requires a 6F guide catheter support. Once positioned across the aneurysmal segment, the delivery wire is held while the distal one-third to one-half of the PED is carefully unsheathed. Once the unsheathed segment begins to expand, the distal end is released by clockwise rotation of the delivery wire. The proximal segment of the PED can then be deployed, mainly through the application of forward pressure on the delivery wire.
At what stage is it currently?
Current case series have suggested that PED is a safe and effective treatment. Over 90% total obliteration rate at 12 months have been consistently confirmed among studies. A recent meta-analysis suggested that in unruptured intracranial aneurysms unfavourable to treatment by coiling, the estimated risk of major complication and death is 5.3% and 1.3% respectively. In our Hong Kong territory-wide registry, an 85% obliteration rate at 18 months was achieved in 178 aneurysms treated with a 4% major morbidity and mortality. Fusiform aneurysms, including vertebral artery dissecting aneurysms, respond favourably to PED treatment.
However, uncertainty still exists in predicting the risk of, and preventing delayed rupture and thrombotic occlusion. From the results of the two most promising case series from Argentina and Australia, dual antiplatelet treatment for six months or more is recommended to reduce thromboembolic risk. For vertebrobasilar territories, there are case reports of acute parent artery thrombosis at 12 months after switching from dual to single antiplatelet treatment and delayed parent artery thrombosis at 23 months. It is thus recommended that dual antiplatelet treatment is continued for at least one to two years for fusiform aneurysms involving long segments, especially in the posterior circulation.
Pistocchi S et al. (Stroke 2012) have recently reported a 30 patient series using flow-diverting stents (including PED) for intracranial aneurysms beyond the Circle of Willis, including 16 patients with anterior communicating artery aneurysms and four patients with distal anterior cerebral artery aneurysms. Reversible neurological deficits occurred in 7% and permanent neurological complications occurred in 4%. An impressive 83% total occlusion rate was attained at a mean angiographic follow up of 13 months. When a flow-diverting stent is applied across the origin of a merging arterial branch, TIMI 2 flow was found in 24% and TIMI 0 flow was found in 38%. All cases of flow restriction were clinically silent, representing that they were an epiphenomenon of adequate collateral flow.
Promising case reports have also been recently published for treatment of acutely ruptured internal carotid artery blister aneurysms, in which surgical or conventional endovascular treatment remains challenging.
George Kwok-chu Wong is a senior member of the Hong Kong Society of Interventional and Therapeutic Neuroradiology. He is professor, Department of Neurosurgery, The Chinese University of Hong Kong, Hong Kong, China, and has reported no financial disclosures.