Stent-assisted coiling is a safe and effective method for treating wide-necked aneurysms^1), but ischemic stroke related to stent thrombosis is a major potential drawback. All thromboses occur within one year after the initial procedure, very late stent thrombosis (VLST), which is defined as stent thrombosis occurring one year after implantation^2), following sole stent-assisted coiling has not been previously reported. Here, we report the first case of VLST, which occurred 14 months after a typical sole stent-assisted coiling procedure.

This case highlights the possible mechanism of VLST after single stent-assisted coiling and raises concerns about the duration of prophylactic antiplatelet therapy (APT).

Stent-assisted coiling is increasingly being used to treat aneurysms^6,11), but, although this is an effective and relatively safe technique^1), it has inherent problems such as thromboembolic complications and in-stent stenosis^10,18). The timing of stent thrombosis can be classified as acute (<24 h post procedure), early or subacute (24 h to 30 days post procedure), late (31 days to 1 year post procedure), or very late (>1 year post procedure)²⁾. In-stent thrombosis within 1 year after stenting has been reported to occur at a rate of around 3% after stent-assisted coiling^10,14,15,19). However, no report has previously described VLST following implantation of a sole stent, which is self-expanding intracranial stent designed for coiling assistance. The mechanisms of VLST previously proposed are; progressive in-stent stenosis, premature discontinuation of dual APT, delayed atypical response to adjacent polyglycolic acid-loaded "bioactive" coils, spontaneous occlusion of the parent artery, stenting across major arterial side branches, stent mal-apposition, stent fracture, stent strut penetration of the necrotic core, disruption of vulnerable plaque near the stent, radiation therapy, and hypersensitivity vasculitis^4,7). In our case, no in-stent stenosis or stenosis around the stented vessel was noted by 1-year follow-up angiography. Furthermore, we used only bare-metal coils. Thus, premature discontinuation of antiplatelet medication was the likely cause of VLST in our case. The prevention of thromboembolic complications after stenting is currently achieved with prophylactic dual APT^12), but the regimens and durations of prophylactic APT are controversial^9,21). It is recognized intracranial stents are thrombogenic until stent endothelialization occurs, and that the duration of APT should be based on the completion of endothelialization. It has been reported by some that endothelialization is complete within 6 months after BMS implantation^5,8,16), but no human data is available regarding the rate of stent endothelialization after stent-assisted coiling. In the interventional cardiology field, current guidelines recommend dual APT for at least 1 month for BMSs, but up to 1 year for drug-eluting stents, which cause delayed endothelialization²²⁾. However, in the neuroendovascular field, the optimal duration of prophylactic APT following stent-assisted coiling remains undefined, and clinical practices are highly discrepant; some authors have recommended only 3 or 6 months of antiplatelet therapy after stent placement for coiling^17,20). These recommendations seem to be based on the facts that the target vessel for stent placement is usually nonatherosclerotic and that stents being used for coiling assistance are of bare-metal and have self-expandable properties, and thus, do not delay endothelialization over stent struts or cause endothelial injury¹⁶⁾.

However, a much longer duration of APT may be required when stent endothelialization is only partially achieved. The occurrence of VLST may mean that formation of a complete neointimal layer incorporating stent struts has failed despite the passage of considerable time after stent implantation. Furthermore, it has been established that exposed stent struts are prone to thrombosis. Our patient had a large aneurysm with a circumferential wide neck, which suggests a large portion of the stent might have unopposed the parent vessel. In addition, it is likely that stent struts facing a circumferential wide aneurysmal neck are endothelialized at a far slower rate than that of a similar sized stents placed across a smaller aneurysmal neck. Another possible pathophysiology of incomplete endothelialization involves mechanical factors of closed-cell stents. These stents can be kinked or flattened in tortuous curves, which can result in incomplete stent apposition, causing incomplete endothelialization^3,13). Thus, the ultra-wide neck involving the parent artery circumferentially and the closed-cell stent deployed in the tortuous carotid siphon probably contributed to the incomplete stent endothelialization in our case. Although we present only a single case of VLST after sole stent-assisted coiling, our observation suggest that APT is needed for much longer period after sole stent-assisted coiling for a large aneurysm with an ultra-large circumferential wide neck. A study is required to determine optimal APT regimens according to stent type, number, and aneurysm size, shape after stent-assisted coiling.

We present a case of a VLST after sole stent-assisted coiling. Although VLST is extremely rare, it is a serious event with potentially devastating consequences. This case demonstrates that long-term APT and patient strict adherence are required to possibly prevent this complication when a large portion of the stent is anticipated to be unopposed to the vessel wall.