Several imaging modalities were used to diagnose VBD, including digital subtraction angiography (DSA), computed tomography (CT) angiography (CTA), and magnetic resonance (MR) angiography (MRA). Of them, DSA is still considered to be the gold standard, while MRA is cost-effective and widely available for the diagnosis of VBD (6, 24). CTA can reveal the relationship between the vertebrobasilar trunk and bone structures in VBD (4, 25). On CT and MR images, intraluminal thrombus or calcification and brain parenchymal changes can be well-assessed (5). Transcranial Doppler is useful to study blood flow in VBD (26).

To date, there has been no consistent imaging standard to define and classify VBD (27). In 2004, using MRA, Ubogu and Zaidat defined VBD as a BA length >29.5 mm with a >10 mm lateral shift from the vertical line between the initial point and BA tip and an intracranial VA length >23.5 mm with a >10 mm shift of either VA from the line between the initial point of the intracranial VA and the initial point of the BA (28). In 2005, according to vertebrobasilar artery morphology on arteriography and cross-sectional imaging, Flemming et al. defined VBD as uniform dilatation of BA >1.5 times normal involving either the entire BA or VA or both, with any degree of tortuosity (29). When VBD can be associated with superimposed aneurysms, it is called the transitional type (29).

Ubogu and Zaidat (28) and Flemming et al. (29) did not classify or grade cases of VBD. An exact grading system to judge VBD severity is necessary. Using high-resolution CT, Smoker et al. proposed a scoring system for BA as follows: the height of BA bifurcation was scored as 1 (within the suprasellar cistern), 2 (at the level of the third ventricle floor) or 3 (indenting and elevating the third ventricle floor), and lateral displacement of the BA was scored as 1 (medial to lateral margin of clivus or dorsum sellae), 2 (lateral to lateral margin of clivus or dorsum sellae), or 3 (within the cerebellopontine angle cistern) (3).

In Smoker's et al. study, the diameter of normal BA averaged 3.17 mm (1.9–4.5 mm), 98% of normal BA courses in the midline or in a paramedian position, medial to the lateral margins of the clivus and dorsum sellae (3). Therefore, it was feasible to define VBD based on the following criteria: (a) the vertebrobasilar arteries were >4.5 mm in caliber at the mid pons level of the BA; (b) the vertebrobasilar arteries laterally reached/surpassed the lateral margin of the clivus or the BA tip reached/surpassed the level of the floor of the third ventricle (Figure 2) (3, 4). That is, VBD should include both the ectasia and lengthening of the vertebrobasilar arteries presented with the lateral, upward or lateral and upward movement of the vertebrobasilar arteries.

CTA can provide more information, including the maximum intensity projection and three-dimensional vessel reconstruction (4, 25). Based on CTA in cases of VBD, according to Smoker et al. the scoring system can be simplified to produce three types of VBD: (1) the mild type has 3 points (1 + 2 or 2 + 1), (Figure 3A); (2) the moderate type has 4–5 points (1 + 3, 2 + 2, 3 + 1, 2 + 3, or 3 + 2) (Figure 3B); and (3) the severe type has 6 points (3 + 3) (Figure 3C).

Occasionally, dolichoectasia can affect the anterior circulation, in which diffuse intracranial dolichoectasia is a new, distinct type (Figure 4) (30, 31). Posterior circulation has less sympathetic innervation than anterior circulation, resulting in less trophic support on the vessel wall, which can make the posterior circulation more prone to deformity when exposed to increases in blood flow and pressure (26).

More than 40% of VBD patients are asymptomatic, although they meet the angiographic diagnostic criteria of VBD (1). Currently, there is no good quality evidence that treating asymptomatic VBD with EVT is better than surgery or better than the natural history of the disease (23, 32). In Wolters's et al. systematic review of 5-year risks in patients who were asymptomatic when VBD was diagnosed, ischemic stroke was 4%, hemorrhagic stroke was 10%, case fatality was 12.5%, and there was no progressive brainstem compression or hydrocephalus (21). Therefore, asymptomatic mild VBD can be left alone; that is, left “untouched.”

When ischemic stroke occurs, antiplatelet treatment can be tried, associating clopidogrel and aspirin for 4 weeks and then long-term aspirin (6). However, antiplatelet or anticoagulant agents are often not as effective as expected in preventing recurrent ischemic events in VBD, which puts them at risk for intracranial hemorrhage (6, 33, 39).

Highly selective moderate and severe VBD patients can be EVT candidates, such as those VBD patients with large aneurysms and a greater bleeding risk or those with lower flow conditions and lower velocity, vorticity, and more oscillatory flow (36, 40, 41). EVT for VBD should reconstruct the vertebrobasilar artery to improve blood flow, target the aneurysm to prevent rupture or rebleeding, or reduce the occupying effect. However, overall, it should be managed on a “case by case” basis. Of most importance, the procedure-related morbidity of EVT should be less than the malignant natural history (11).

In VBD, FDSs can play two roles. First, FDSs can reconstruct dilated and tortuous vessels; second, they can cure either aneurysmal dilation or associated aneurysms (44, 45). Successful flow diversion should result in progressive aneurysm thrombosis while maintaining side branch patency and requires complete apposition of the implant of the FDS to healthy vessel segments (43).

FDSs will foreshorten significantly because segments affected by VBD are often larger than the largest diameter of the latest FDSs (23). Therefore, large FDSs, such as Surpass (5.3 × 50 mm) (Stryker Neurovascular, Fremont, CA, USA) and Silk (5.5 × 40 mm) (Balt Extrusion, Montmorency, France), are helpful (43). To avoid FDS prolapse, adjunctive coiling of most saccular segments of the VBD can be helpful and act as a scaffold to organize thrombi (51). When the FDS is successfully deployed and steady apposition to the vascular wall is secured, the coil may be left in situ or withdrawn to avoid occlusion of the perforating vessels (42).

If one FDS is not sufficient, a second stent can be telescoped into the first FDS. In this situation, efforts should be made to match the overlapping portion of the FDSs with the most severely ectatic segment. The LEO stent (Balt Extrusion, Montmorency, France) had a larger size, such as 5.5 × 75 mm. Therefore, flow diversion using the “diverter-in-stent” technique was helpful. In a report by Cohen et al. after the longest LEO stent was deployed, multiple LEO and Silk stents overlapped (52). After the FDS is deployed, the self-expanding traditional stent can be used to telescope the FDS to cover the proximal and distal vessels, which is a good option to protect the relatively normal vessels (43).

In addition, the use of an FDS may be technically difficult due to the stiffness of the stent and the tortuous path in the VBD. The FDS can be replaced by an LEO stent, with 14% metal coverage, or an LVIS Blue stent (MicroVention, Tustin, California, USA), with 22–28% metal coverage, alone or with coiling assistance (40, 42, 53).

In VBD, it is feasible to perform EVT targeting associated aneurysms. Saccular aneurysms with a definite neck can be coiled with/without stenting. EVT for fusiform aneurysms remains challenging (40). For large fusiform aneurysms in VBD, conventional self-expandable stents assisted with coiling have difficulty completely curing these aneurysms, especially those with branches arising from the aneurysmal wall; however, the technique is still useful for target embolization, which targets weak points or blebs ruptured by coiling assisted by conventional stents to await subsequent treatment (54).

In stent-assisted coiling, large and long stents can be helpful, especially for tandem aneurysms, such as the largest-size LEO stents (Balt Extrusion, Montmorency, France), Solitaire stents (Medtronic, Irvine, California, USA) and Enterprise stents (Codman Neurovascular, Raynham, MA, USA) (Figure 8). A low-profile Neuroform Atlas stent (Stryker Neurovascular, Fremont, California, USA) can be helpful for passing through highly tortuous vessels (Figure 9).

VBD often involves bilateral VAs. During stenting reconstruction, the non-dominant VA should be occluded immediately after stent deployment to reduce the blood flow into the space between the stents and the vascular wall, which can further improve the hemodynamics of the VBD (55). The occlusion should be made distal to the posterior inferior cerebellar artery, and any branches originating from the VA in these cases must be preserved, especially those of the anterior spinal artery (56).

Currently, parent artery occlusion (PAO) has gradually been replaced by a constructive approach, and PAO is still a useful technique. VBD can be treated by bilateral VA occlusions. In 2003, Omahen and Findlay successfully treated a 48-year-old man with a giant aneurysm in VBD by bilateral microsurgical VA occlusions (57). Therefore, flow reversion and intra-aneurysmal thrombosis after bilateral VA occlusions can be therapeutic options for VBD.

Currently, VA occlusion can be performed safely with EVT, including detachable balloons or coils or with a combination of the two (58, 59). However, two complications after PAO must be considered, thrombotic events and potential hemodynamic insufficiency. Therefore, a balloon occlusion test (BOT) was necessary. Tolerance of bilateral VA occlusion is currently defined as both a lack of new neurological deficits during BOT and angiographic evidence of at least one large posterior communicating artery, as well as evidence of collateral circulation (58). If a patient with VBD could tolerate BOT, bilateral VA occlusion may be performed. To reduce complications, staged occlusion might also be helpful. In addition, both anticoagulant and antiplatelet therapy should be used after the procedure (59).

Currently, it seems certain that the use of EVT for treating ruptured aneurysms is effective. Only coiling of saccular aneurysms in VBD can result in a good prognosis due to providing the least hemodynamic disturbance. In our previous report, a secular aneurysm on the BA trunk with VBD achieved a good outcome after simple coiling with/without stenting assistance (47). However, for large fusiform and complex aneurysms in VBD, complex EVT, such as FDS deployment, can result in an uncertain and varied prognosis (49).

In a previous small series by Siddiqui et al. 7 large or giant fusiform aneurysms (most of which conformed to VBD) were treated with FDS deployment; 57.1% of the patients had a poor prognosis, and the overall infarct rate was 71.4% (61). In a large series by Bhogal et al. 56 patients with posterior circulation non-saccular aneurysms accepted FDS deployment, in which 58.5% of patients had dolichoectatic and transitional aneurysms. After FDS deployment, in the symptomatic group, 72.7% of patients presented with an mRS score of ≤2, and 54.5% of patients had an mRS score of ≤2 at the last follow-up (23).

Regarding stenting reconstruction for VBD, the reported outcomes of either conventional stents or FDSs vary. He et al. reported that 19 consecutive patients with VBD were treated with 1–3 large LEO stents, and 84.2% of patients had satisfying angiographic reconstruction; the mean mRS score was 3.00 ± 1.15 before treatment and improved to 1.89 ± 1.33 after treatment (42). However, in other reports, the outcomes were disappointing (40, 62). In Wu et al. 9 patients with VBD were implanted with multiple LEO and/or Solitaire stents with or without coil assistance, and only 41.7% of VBD patients demonstrated morphological improvement (40). In Wang's et al. report, 22 symptomatic VBD patients underwent EVT, and only 41% achieved a satisfactory clinical and/or digital subtraction angiography outcome after endovascular treatment (62). Therefore, further studies are needed to investigate this matter.