Kat Dakay, DO
The importance of collateral vessels in maintaining the penumbra during acute ischemic stroke is well-known, but how do we best measure them?
Robust collateral vessels are associated with better perfusion of the ischemic penumbra and a better outcome after endovascular therapy in large vessel occlusion as compared to poor collateral vessels (Bang, Saver et al. 2011). Patients with poor collaterals may be at a higher risk for hemorrhagic transformation following recanalization (Bang, Saver et al. 2011). In some cases, collateral vessels may influence the decision whether or not to intervene on a large vessel occlusion.
Cerebral angiography offers a detailed, dynamic view of the cerebral collaterals but is an invasive procedure, often occurring after the decision to intervene is already made. CT angiography is probably the most widely used method of measuring collateral vessels and can occur in conjunction with the non-contrast CT performed during an acute stroke activation.
However, as the authors of this paper suggest, MRI may be an underutilized tool that also offers some clues into a patient’s collateral status.
In this article by Nave and colleagues, the authors studied patients with a known M1 occlusion who had an MRI brain within 6 hours of symptom onset (Nave, Kufner et al. 2018). This was done by querying a prospective registry at three separate sites within Stuttgart and Berlin. They studied the MRI FLAIR sequence for hyperintense vessels (FHV), defined as serpentine or circular hyperintensities on at least two consecutive slices. The amount of FLAIR hyperintense vessels on MRI were quantified by an FHV-ASPECTS score, which split the MCA territory into seven subdivisions, graded on the presence or absence of FHV (see Figure 1 from the author’s study, below). The total number of hyperintense regions subtracted from seven is the FHV- ASPECTS. Similar to the ASPECTS score for CT brain, the presence of FHV means a point should be subtracted from seven. Thus, the highest FHV-ASPECTS score is seven, indicating a lack of hyperintense vessels; conversely, the lowest score, zero, correlates to having an abundance of hyperintense vessels throughout the MCA territory. Contrary to the ASPECTS score for CT brain, wherein a high ASPECTS score is favorable, a low FHV-ASPECTS is favorable. A low FHV-ASPECTS means that there are hyperintense vessels in several regions perfused by the middle cerebral artery, whereas a high FHV-ASPECTS indicates a lack of hyperintense vessels. The median score for the study group of 116 patients was 2.
Figure: Examples of fluid-attenuated inversion recovery hyperintense vessel–Alberta Stroke Program Early CT Score (FHV-ASPECTS) ratings.
The authors dichotomized patients into two groups: low FHV ASPECTS of less than or equal to two, and high FHV ASPECTS of three or greater. They discovered that patients with a lower FHV-ASPECTS score were more likely to have a good collateral score on angiography (83% vs. 57%; p value of 0.025), as graded by the ASITN (American Society of Interventional and Therapeutic Neuroradiology). Additionally, the median NIHSS on discharge was lower in the low FHV-ASPECTS group as compared to the high FHV-ASPECTS group (4 versus 8; p value 0.032). Clinical outcome was also different between the groups, with the low-FHV ASPECTS patients having a higher likelihood of a good mRS of 0-2 both at discharge (43% vs. 23%; p value 0.032) and at 3 months (51% vs. 24%; 0.005) as compared to the high-FHV ASPECTS group.
What does the FHV sign mean for acute ischemic stroke?
The authors acknowledge that there have been other studies whose findings contradict their results — in that, the presence of FHV were indicative of a poorer outcome. In one of these studies, the study group included both patients with and without large arterial occlusion; the patients with FLAIR hyperintense arteries were more likely to have an arterial occlusion, which would be a potential confounding factor associated with a more severe stroke (Ebinger, Kufner et al. 2012). Additionally, in their study, patients with FHA (FLAIR hyperintense arteries) had significantly higher perfusion deficits, indicating more penumbra. Thus, it could also be rationalized that patients with FHA may have more to lose in terms of ischemic tissue. However, the conflicting results of studies make it challenging to know how to reliably interpret these vessels.
The authors also caution that while their findings seem to suggest a correlation between FHV and outcome, “treatment decisions based on these associations are not yet justified.”
Additionally, a significant proportion of patients (24%) in the high-FHV ASPECTS group did have a good outcome at three months — a number that should be placed in the context of the potential morbidity and mortality of a large vessel occlusion.
However, in light of the increasing use of emergent MRI in acute stroke cases, it can be said that FHV are a topic worthy of further study, especially in the context of the subpopulation of strokes due to large vessel occlusion. Further studies in this specific subpopulation would be helpful to replicate these findings, especially given the prior conflicting studies.
References
Bang, O. Y., J. L. Saver, S. J. Kim, G. M. Kim, C. S. Chung, B. Ovbiagele, K. H. Lee and D. S. Liebeskind (2011). “Collateral flow predicts response to endovascular therapy for acute ischemic stroke.” Stroke 42(3): 693-699.
Ebinger, M., A. Kufner, I. Galinovic, P. Brunecker, U. Malzahn, C. H. Nolte, M. Endres and J. B. Fiebach (2012). “Fluid-attenuated inversion recovery images and stroke outcome after thrombolysis.” Stroke 43(2): 539-542.
Nave, A. H., A. Kufner, P. Bucke, E. Siebert, S. Kliesch, U. Grittner, H. Bazner, T. Liebig, M. Endres, J. B. Fiebach, C. H. Nolte, M. Ebinger and H. Henkes (2018). “Hyperintense Vessels, Collateralization, and Functional Outcome in Patients With Stroke Receiving Endovascular Treatment.” Stroke 49(3): 675-681.
