Quantitative T2'-Mapping in Acute Ischemic Stroke
Advanced neuroimaging has provided important insight into the spatiotemporal evolution of the ischemic stroke. Clinically, diffusion- and perfusion-weighted imaging have been shown to provide a reasonably accurate approximation of the ischemic core versus the surrounding penumbra. Despite this, exactly defining the perfusion-weighted imaging thresholds of the ischemic penumbra, which is the target of therapeutic interventions, has remained challenging. More accurate mapping may be achieved by measuring the cerebral metabolic rate of oxygen and the oxygen extraction fraction. Bauer et al sought to investigate whether T' may serve as an MRI-based surrogate marker of the oxygen extraction fraction (whereby 1/T2'=1/T2*−1/T2). For this proof-of-concept study, 11 patients screened for endovascular stroke therapy with MRI were included and T2' quantitated within the presumed ischemic core (apparent diffusion coefficient <550×10–6 mm2/s) and the hypoperfused tissue (using different time-to-peak delays over the corresponding, contralesional, normal hemisphere). Both absolute and relative (compared with the corresponding, normal hemisphere) T2' was reduced within the ischemic core. Furthermore, T2' was reduced in the hypoperfused tissue with no significant difference between different time-to-peak -ranges. Overall, T2' was significantly lower within the core versus surrounding hypoperfused tissue. As the authors point out, the results are not immediately consistent with expected tissue signatures (ie, previous positron emission tomography studies have shown relatively preserved cerebral metabolic rate of oxygen with increased oxygen extraction fraction in the ischemic penumbra). Accordingly, future research will need to clarify the exact nature of observed T2' tissue signatures. Yet, this study highlights the feasibility of obtaining complementary data on tissue metabolism in brain ischemia with clinically available MRI, which is expected to expand on our imaging armamentarium to identify tissue most likely to benefit from therapeutic interventions. See p 3280.
Use of Coumarin Anticoagulants and Cerebral Microbleeds in the General Population
One of the most feared complications associated with oral anticoagulant therapy is intracerebral hemorrhage. In the setting of pre-existing cerebral microbleeds (CMB), this risk may be substantially higher depending on the presumed pathomechanism. Akouda et al asked the clinically relevant question whether oral anticoagulation is associated with a higher prevalence and, probably more important, incidence of CMB. To this end, they studied patients from the population-based Rotterdam Study that had T2*-weighted gradient recalled echo sequences available for review. Of 3069 participants, a total of 427 (8.6%) participants had used oral coumarin anticoagulants at some time before the first MRI and 181 (5.9%) participants had used coumarin before the second MRI scan. The cumulative incidence of microbleeds was 6.9% during a mean follow-up of 3.9 years. Multiple logistic regression analyses adjusted for other predictors indicated a higher prevalence of infratentorial microbleeds (with or without lobar microbleeds dichotomized as present or absent) compared with never-users. Notably, the data suggested an increased CMB prevalence with greater international normalized ratio variability and higher CMB incidence with coumarin use; however, these associations were not significant. Although the data are highly suggestive, and possibly the result of insufficient statistical power, they require cautious interpretation. As pointed out, CMB prevalence was unexpectedly higher for infratentorial CMB for which reason it is possible that an unmeasured risk factor rather than coumarin use/international normalized ratio variability contributed to the results. Furthermore, it would be interesting to know the associations between coumarin use/international normalized ratio with the absolute CMB count and whether these change with the now available MRI sequences that are more sensitive for detecting CMB. Most importantly, are patients developing CMB also at greater risk for subsequent frank intracerebral hemorrhage, which is intuitively the case? Regardless of its limitations, this study highlights that coumarin use is associated with CMB and provides the rationale for studying whether tighter international normalized ratio control could reduce the risk for CMB and thus intracerebral hemorrhage. See p 3436.
Secular Trends in Ischemic Stroke Subtypes and Stroke Risk Factors
Given recent efforts at increased control of vascular risk factors, Bogiatzi et al sought to investigate whether the proportion of cardioembolic ischemic strokes and transient ischemic attacks increased over time within a local urgent transient ischemic attack clinic at a designated regional stroke hospital in London, Ontario (ie, by protocol all patients with stroke in the region are brought to this hospital). This was a retrospective cohort study of 3445 patients diagnosed with first-ever minor or moderate stroke/transient ischemic attack from 2002 to 2012. Stroke pathomechanism was classified according to the SubtyPes of ischemic stRoKe cLassification systEm (SPARKLE), which was previously developed by the authors using a random subpopulation chosen from the herein investigated population. Any cases for whom the stroke subtype was equivocal were reviewed by 2 physicians together to arrive at a consensus stroke subtype. Overall, there was a significant increase in cardioembolic stroke/transient ischemic attack, from 23% to 56% of cases, and a significant decrease in large artery strokes from 43% to 26%, small vessel from 12% to 7%, other explained from 9% to 4% during the study period. Although this may suggest that cardioembolic strokes represent a higher proportion of patients with stroke, it is important to note that there was no change in the proportion of patients presenting with proven atrial fibrillation over time. How can these observations be reconciled? In SPARKLE, each presumed stroke subtype is classified according to the categories evident, probable, and possible. So one of the obvious questions is whether more patients are classified as probable and possible (yet unproven) cardioembolic stroke cause. If so, further investigation on this issue may provide valuable insight into a potentially changing landscape of ischemic stroke causes. See p 3208.
- © 2014 American Heart Association, Inc.