A subanalysis from the SWIFT PRIME trial has found that time to randomisation was significantly longer in magnetic resonance imaging (MRI)-selected patients in the study, however, this time delay did not seem to impact the clinical response to endovascular therapy. In fact, the benefits of endovascular therapy in the MRI-selected subgroup were comparable to those seen in the computed tomography perfusion (CTP) subgroup.
Nicolas Menjot de Champfleur (Hôpital Gui de Chauliac, Montpellier, France) et al report their findings in the journal Stroke.
The SWIFT PRIME trial (Solitaire FR with the intention for thrombectomy as primary endovascular treatment for acute ischemic stroke) compared t-PA alone with t-PA plus endovascular therapy in patients with large vessel intracranial occlusions selected primarily with CT-based approaches and reported (presented at the 2015 International Stroke Conference and published in the New England Journal of Medicine) substantially improved outcomes in the endovascular arm of the study.
Even though SWIFT PRIME and some of the similar stroke trials carried out at the same time used both MRI and CT to select patients, there are no randomised controlled trials comparing MRI to CT for selection of candidates for either intravenous t-PA or endovascular therapy. The investigators in this SWIFT PRIME sub-analysis, however, noted that there are many centres that use MRI as the routine screening modality in the acute stroke populations. The SWIFT PRIME protocol allowed individual centres to use either CT or MRI to select patients. The aim of this sub-analysis, then, was to compare the clinical and imaging outcomes in SWIFT PRIME patients who were selected by diffusion/perfusion MRI versus CTP.
The authors included 173 patients with acute stroke in this substudy. MRI-based selection was performed in 34 patients (19.7%) and CTP-based selection in 139 patients (80.3%). Median age was 71 years (64–77) in the MRI group and 68 years (59–75) in the CTP group (p=0.078).
At baseline, National Institutes of Health Stroke Scale (NIHSS) score was 17 in both groups (MRI group: 17 [13–21] and CTP group: 17 [13–19]; p=0.46). The baseline ASPECTS (Alberta Stroke Program Early CT Score) score was lower in the MRI group: 8 (7–9) versus 9 (8–10) in the CTP group (p<0.001). In terms of radiological assessment, baseline ischaemic core volumes were not significantly different between the MRI and the CTP groups (p=0.40). The baseline volume of hypoperfused territory was smaller in the MRI versus CT groups: 97mL (66–110) versus 133mL (75–161; p=0.01). The target mismatch profile was observed in 19 out of 20 patients (95.0%) in the MRI group and 105 out of 126 patients (83.3%) in the CTP group (p=0.31).
As it relates to timing observed within the two groups, de Champfleur and colleagues found that all patients were treated with t-PA within 4.5 hours of stroke onset. Time from emergency room arrival to randomisation was 68.5 minutes (43–112) in the MRI group and 67 minutes (48–95) in the CTP group (p=0.61). Patients were transferred to study site from an outside hospital in 58.8% (20 of 34) in the MRI group versus 34.8% (48 of 138) in the CTP group (p=0.004). Consequently, time from stroke onset to randomisation was longer in the MRI group: 235.5 minutes (194–268) versus 179 minutes (129–261) in the CTP group (p=0.003).
Interestingly, despite the time difference between the two groups from stroke onset to randomisation, the mRS results did not differ. The rate of functional independence was the same in the MRI and CTP groups (p=1.0). The secondary radiological efficacy outcomes including revascularisation, 27-hour infarct volume, and infarct growth also did not differ (respectively p=0.37, p=0.43, and p=0.28).
Additionally, when the investigators compared the outcomes for SWIFT PRIME’s primary and secondary efficacy analyses they found similar results in both the CTP and MRI-selected subgroups. The primary efficacy analysis (distribution of mRS score at 90 days) demonstrated a statistically significant benefit in both the MRI (p=0.022) and CTP groups (p=0.014) favouring thrombectomy plus intravenous t-PA over the intravenous t-PA alone. They report that among MRI-selected patients, mRS score 0 to 2 at 90 days occurred in 63% of the thrombectomy group versus 33% of the t-PA alone group (absolute risk reduction 30%; p=0.17). Among CTP-selected patients, mRS score 0 to 2 at 90 days occurred in 60% of the thrombectomy group versus 40% of the t-PA alone group (absolute risk reduction 20%; p=0.025). In the MRI group, there was a trend toward lower absolute infarct growth (17mL versus 50mL; p=0.089) in the stent retriever group compared with t-PA alone that was similar in magnitude to the reduction observed in the CTP group (14 versus 27mL; p=0.047). Overall, successful reperfusion at 27 hours was more common in the endovascular subgroups, irrespective of selection modality (MRI, p<0.001 and CT, p<0.001), the study showed.
“Despite the fact that MRI-selected patients in SWIFT PRIME were slightly older and treated longer after symptom onset, there were no significant differences in either clinical or imaging outcomes compared with the CTP-selected patients. The longer time from symptom onset to randomisation in the MRI-selected group occurred primarily because of transfer delays because a larger percentage of the MRI patients were transferred to the study sites from outside hospitals. The time between arrival at the study site and randomisation were nearly identical for both the MRI and CTP groups,” de Champfleur et al write.
They add that while the persistent benefit of thrombectomy, even in patients with longer times from symptom onset to randomisation in the MRI-selected group, suggests that MRI may be a favourable modality for evaluating patients who present at extended time windows, this hypothesis requires assessment in large randomised studies.