Lauren Peruski, DO
Patients presenting with an acute ischemic stroke usually undergo non-contrasted CT (NCCT) scan of the brain, followed by a CT angiogram (CTA) of the head and neck if applicable. These studies, along with physical examination, would guide recommendations regarding thrombolysis and endovascular thrombectomy. In 2000, the Alberta Stroke Program Early Computed Tomography Score (ASPECTS) was introduced as a way to assess early ischemic changes on NCCT, providing a prediction of functional outcome and ischemic core volume. In 2018, the DAWN and DEFUSE 3 trials were published, showing an added benefit of CT perfusion (CTP) imaging in the assessment of certain acute ischemic strokes. Unfortunately, countless hospitals worldwide do not yet have CT perfusion technology available and continue to rely on standard CT scanning alone. We previously did not have data applying the ASPECTS model to cases beyond 6 hours from last known normal time. The authors of this paper hypothesized that NCCT ASPECTS and CTA-source image (CTA-SI) ASPECTS would correlate with automated CTP core volume estimates beyond 6 hours.
This study examined patients that were admitted to a comprehensive stroke center between the years 2016 and 2018. The inclusion criteria were as follows: (1) anterior circulation large vessel occlusion; (2) baseline workup including NCCT, CTA, CTP; (3) stroke onset > 6 hours. 158 patients met the inclusion criteria. A majority of the patients were white males, with a median age of 63 years. Hypertension, hyperlipidemia, diabetes, and tobacco use were common comorbidities. Median baseline NIHSS was 15, and median time from last known well to CTP imaging was 632 minutes. Most patients (71%) had an MCA occlusion.
Both NCCT and CTA-SI ASPECTS significantly predicted CTP core volume, with NCCT ASPECTS having a slightly better correlation than CTA-SI ASPECTS. The optimal threshold of each study was examined to predict an ischemic core ≤70 mL (the core value used in the DEFUSE 3 trial). The optimal cutoff for NCCT ASPECTS to predict an ischemic core ≤ 70 mL was ≥6 (positive predictive value of 0.93, and negative predictive value of 0.36). The optimal cutoff for CTA-SI ASPECTS to predict an ischemic core ≤70 mL was ≥5 (positive predictive value of 0.94, and negative predictive value of 0.31). Clearly combining these two modalities provides the most accurate prediction of CTP core ≤70 mL as compared with either study alone.
The authors also assessed which method (CTP criteria versus NCCT and CTA-SI ASPECTS) was more inclusive or exclusive for EVT patient selection. In the study population, eleven patients would have been excluded from EVT using the guideline of NCCT ASPECTS <6 as compared with seven patients who were excluded using CTP criteria. When combining the NCCT ASPECTS with CTA-SI ASPECTS, an additional seven patients would have been excluded, for a total of eighteen. This suggests that CTP criteria are more inclusive with respect to EVT patient selection.
In this unique study, examining NCCT and CTA-SI ASPECTS within the extended (6-24 hour) time window, there was a moderate correlation between these imaging methods and the current standard, CTP. Contrasted with prior studies showing CTA-SI ASPECTS being more accurate in early (<6 hour) time windows, this article found a better correlation with NCCT ASPECTS in the delayed window. The authors also reported a trend for CTP to be more inclusive for EVT selection compared with NCCT and CTA-SI ASPECTS, although this finding was not statistically significant. This information and these trends are important to document because CTP is not currently available at all centers. For those hospitals relying on NCCT and CTA data, further investigations could help in selecting candidates for EVT. Moving forward, additional research on this topic is needed to establish formal changes to the current acute stroke paradigm.
