Patient selection

Reconstructive strategies utilizing stents for the treatment of intracranial dissecting aneurysms and dissections consist of stent assisted coiling (SAC) and SAT. Both methods promote thrombosis of the aneurysm while preserving or even improving blood flow in the parent vessel. In most circumstances, SAC is preferential because coiling decreases blood flow impingement into the fragile vessel wall of the lesion and further reduces wall stress of the pseudoaneurysm, which is beneficial for thrombosis within the aneurysmal or dissection sac, thereby facilitating long term healing of the lesions and decreasing the possibility of bleeding compared with SAT.18 However, dissecting aneurysms of the basilar trunk may be fusiform or dolichoectatic, which often involve the whole circumference of the BA and lack a defined neck. On the other hand, some of these lesions involve vital perforating branches, the occlusion of which can have a catastrophic outcome. Therefore, sometimes SAC is not feasible for these lesions due to the morphological features because the coils may protrude into the parent artery or nearby large perforators, thus resulting in ischemic complications.

Conservative management using anticoagulation and antiplatelet therapy for these lesions presenting with ischemic stroke has been advocated by some authors. However, conservative management has been followed by disappointing results in some patients.1 ,11 For patients presenting with progressive ischemic symptoms despite adequate conservative treatment or brainstem mass effect, SAT is a potential alternative therapeutic option to heal the lesions or improve blood flow in the parent vessel and thus prevent infarct progression.

Reports suggest that stent struts play a role in diminishing intra-aneurysmal blood flow and promote thrombosis in the aneurysmal sac.19 ,20 It is still not known whether placement of a stent alone can effectively prevent rupture of aneurysms, especially for ruptured ones. Although we cannot confirm that SAT cannot effectively prevent rebleeding, we would suggest that using SAT for the management of hemorrhagic lesions requires great caution considering the high rebleed rate after SAT reported by some authors.10 ,11

In conclusion, SAT for the management of these lesions should be tailored to individual patients after giving cautious consideration to the clinical manifestations and angiogram results. In this series, there were seven patients presenting with recurrent progressive ischemic stroke (case Nos 2, 3, 4, 5, 8, 9, and 11) and three patients with progressive mass effect (case Nos 1, 6, and 11). Case No 7 presented with progressive headache within the previous 1 month before SAT, which was considered a sign of progression of the lesion. SAT was performed with the aim of preventing progression of the lesions in these cases. The remaining case No 10, who was incidentally diagnosed and clinically asymptomatic, developed severe depression after knowing the diagnosis and the potential risks, and strongly requested procedural intervention. Unfortunately, ischemic complications occurred. Hence we feel that the lesson we have learnt from this case is that treatment for such lesions should considered as a last resort only in instances where patients present with subarachnoid hemorrhage, progressive ischemic complications, mass effect, etc. The use of invasive interventions for such lesions needs cautious consideration, especially for those lesions that are asymptomatic.

Theoretical mechanism

There are theoretical and experimental data supporting the use of the SAT technique for the management of these lesions. Firstly, computational fluid dynamic and other experimental studies elucidated that stents alter blood flow within the aneurysmal or dissection sac and decrease blood flow impingement on the fragile wall by providing a mechanical hindrance to the inflow patterns, and postulated that sufficient flow remodeling and hemodynamic stress reduction within the aneurysm after placement of the stent would create favorable conditions for thrombus formation.19 ,20 Secondly, stents serve as a scaffold for subsequent neoendothelialization of the aneurysm neck or the injured parent vessel, consequently facilitating long term healing of the lesions.21–23 The inherent pathology of dissecting aneurysms and dissections may also account for their susceptibility to SAT. The deployed stent could tag down the intimal leaflet which is the inflow route of the dissection pseudolumen and support or repair the intimal flap of these lesions, thereby further reducing inflow to promote thrombosis. A previous report of real time observation of intimal collapse after stent placement via intravascular ultrasonography could explain this effect.24

Dissections and dissecting aneurysms involving the BA can cause ischemic symptoms or stroke as a result of stenosis or occlusion of the parent vessel, or by vital perforating or circumflex vessels. For these patients, SAT is a potential treatment option which may assist with ischemic stroke, as verified in previous reports.7 ,11 ,12 For patients presenting with ischemic symptoms caused by luminal narrowing and flow restriction of the parent vessel, SAT can improve blood flow in the parent vessel and thus prevent ischemic stroke. For patients presenting with progressive perforator ischemic symptoms, how SAT assists with these patients is not known because the pathophysiology of these lesion is not clearly defined. We postulate that SAT could prevent progression of these lesions and remodel blood flow in the parent artery, thus facilitating improvement in blood flow in the perforators to assists with progressive perforator ischemic symptoms.

Stent type and techniques

As mentioned above, flow remodeling and repair of the intimal flap are two important reasons for using SAT to manage dissecting lesions. Porosity is the key factor in creating hemodynamic changes.20 The stronger radial force of the stent is advantageous for tagging down of the intimal leaflet in these lesions. Thus the mesh design is strongly correlated with efficacy of SAT. The advantage of a balloon expanding stent with respect to porosity and radial force is clear. However, these stents are relatively inflexible and difficult to advance into the tortuous intracranial vasculature. Also they may exert a traumatic radial force in fragile vasculature during deployment, with risks of chronic reactive intimal proliferation or acute vessel injury, especially when inflating balloons across a freshly ruptured lesions. Therefore, we prefer the self-expandable nitinol intracranial stents to the balloon mounted coronary stents, not only because they are easier to navigate but also they exert a lower radial force against the fragile dissection wall, thereby avoiding the dangers associated with balloon inflation. To achieve a lower porosity, we prefer using a thick stent, the diameter of which is larger than the parent vessel, to provide relatively stronger radial force and less porosity. In order to induce neoendothelialization, we use bare metal stents rather than drug eluting stents. This is why 4.5 mm Enterprise and 6 mm Solitaire stents were so frequently used in this case series.

The overlapping stent in stent technique is based on the premise of a less porous mesh causing greater intra-aneurysmal hemodynamic changes, and also providing relatively stronger radial force. In addition, many basilar dissections and dissecting aneurysms are too long to be completely covered with only one stent, and complete coverage of lesions with stent is ideal when using SAT because we could not judge the location of the torn intimal leaflet in these lesion. In our series, the true overlapping stent in stent technique was used in five of the 11 cases. This small number of cases cannot definitively clarify whether the overlapping stent technique is superior to the single stent technique in producing favorable remodeling; however, as advocated by some authors,12 ,25 this technique may be useful in cases of remodeling failure with single stent treatment.

Treatment result and complications

Currently, appropriate treatment of basilar dissection and dissecting aneurysms is very challenging and there is no satisfactory management to obtain ideal result for these lesions.1 ,2 ,6 ,9–11 Only certain lesions responded to SAT and benefitted from this treatment in this series. For other cases, there was no change in the configuration of the lesions or hemodynamic remodeling in the control angiography during the procedure or even in the follow-up angiography. We found that there was contrast media retention in the preoperative angiogram in all three patients who had technical treatment success, which may be an indicator for using SAT. We postulate that contrast media retention may mean the existence of a torn intima, and blood flow through this defect into the pseudoaneurysm may have been responsible for contrast media retention. Contrast media retention may disappear with spontaneous healing of the torn intima which act as the inflow route of the dissection pseudolumen. So the effect of SAT for lesions without contrast media retention may be limited, because these lesions are often relatively stable. However, not all the lesions showing contrast media retention responded to SAT. In this series, no change in the configuration of the lesions was observed on follow-up angiogram in two cases presenting with postprocedural contrast media retention. From this small series, we cannot definitely explain why certain lesions responded to treatment while others did not. A study involving a larger population and longer follow-up is necessary to clarify this issue and evaluate the efficacy of this treatment modality.

SAT for these lesions carries a substantial risk of complications. Ischemic complications after treatment occurred in two patients. Previous reports indicated that levels of P2Y12, a platelet membrane protein receptor which plays a key role in platelet activation, vary in individuals and influence the effect of antiplatelet therapy.26 ,27 To reduce ischemic complications for these patients receiving reconstructive endovascular treatment, P2Y12 platelet function tests, to evaluate if there is P2Y12 resistance to antiplatelet therapy, and coagulogram tests, such as thrombelastography to monitor blood coagulation, may be helpful.27 Unsatisfactory results of thrombelastography could prompt a change in the anticoagulation and antiplatelet strategy to obtain a better effect.

There are many other issues to clarify for this treatment modality. Improvement or stable condition of initial symptoms was observed in eight cases, apart from those who encountered complications. However, was the improvement in initial symptoms in the above cases caused by stenting or the antiplatelet therapy after SAT, or was it a natural course of the disease? For example, although comparison of preprocedural and postprocedural angiograms could confirm whether SAT remodelled blood flow and improved perfusion of the brainstem perforators and bilateral posterior cerebral arteries in case No 4 (who suffered from locked-in syndrome resulting from spiral dissections of the BA), it is not clear whether this treatment had anything to do with the gradual clinical improvement. In most of these cases, follow-up time was short. Does SAT improve the prognosis of patients with basilar dissections and dissecting aneurysms in the long-term? Only certain lesions responded to SAT. Are there lessons to be leant in terms of patient selection, stent selection, and use of a single stent or overlapping stent in stent technique that may impact on the treatment result? If a flow diverter is available for treating these lesions using SAT, what is the result? Could SAT effectively prevent rebleed of ruptured lesions? A study with a larger population and longer follow-up is necessary for validation of the efficacy of this treatment modality.