Results From DEFUSE 3

Introduction

Collateral blood vessels can provide crucial blood flow for patients with large vessel occlusions.^1,2 The main collateral pathways in the brain include circle of Willis arteries and the leptomeningeal collaterals, which are anastomotic connections between middle, anterior, and posterior cerebral artery branches. Prior research has shown that acute ischemic stroke patients with good leptomeningeal collaterals have a superior response to intravenous thrombolysis, endovascular thrombectomy (EVT), smaller final ischemic core volume, and improved neurological outcome.^3–5 Poor collaterals have been reported to predispose to hemorrhagic complications and death following EVT, although some studies have also shown that patients with poor collaterals can have a favorable outcome.^6–8 Collaterals vary widely between patients for demographic, genetic, and metabolic reasons.^9–11

The majority of research on collaterals has focused on stroke patients receiving intravenous thrombolytics within 4.5 hours from last known well or undergoing EVT within 6 hours from last known well. To evaluate the importance of collaterals in the late therapeutic window after stroke (>6 hours from last known well), we performed a prespecified analysis of the DEFUSE 3 (Endovascular Therapy Following Imaging Evaluation for Ischemic Stroke) randomized controlled trial. DEFUSE 3 provided evidence that ischemic stroke patients with occlusion of the cervical or intracranial internal carotid artery or the proximal middle cerebral artery, and salvageable brain tissue on perfusion imaging, benefit from EVT as opposed to medical therapy alone, 6 to 16 hours after last known well.¹² The National Institutes of Health (NIH) funded DEFUSE 3 through the StrokeNet, which contributed 38 hospitals who enrolled 182 patients before the trial was stopped when an interim analysis showed that the efficacy boundary had been exceeded. Our hypothesis is that good collaterals on baseline single phase computed tomography angiography (CTA) will correlate with better neurological outcome and less ischemic core growth and will beneficially moderate the effect of EVT in the late window. Although multiphase CTA has been shown to accurately measure collateral status,¹³ the DEFUSE 3 cohort has single phase CTA, which has also been validated as a reliable modality for collateral measurement and is widely used clinically.

Methods

Patient Selection and Outcomes

DEFUSE 3 was approved by the StrokeNet central institutional review board and the Food and Drug Administration for an investigational device exemption (IDE G150028). The data that support the findings of this study are available from the corresponding author on reasonable request. Enrolled patients or their surrogates provided written informed consent. To standardize our cohort, we only included DEFUSE 3 patients who had a baseline CTA. The primary outcome is functional independence (defined as modified Rankin Scale [mRS] score of ≤2) at day 90. The secondary outcome is the ordinal score on the mRS (range, 0 [no symptoms] to 6 [death]) at day 90. We performed analyses of neuroimaging outcomes (growth of the ischemic core between the baseline and 24-hour follow-up imaging and absolute volume of the ischemic core on the 24-hour follow-up imaging). Further details about the blinded determination of the 90-day mRS and volumetric neuroimaging outcomes have been published.¹⁴ Additional outcomes include stroke-related death at 90 days and successful recanalization after EVT, defined as a centrally adjudicated postprocedural conventional angiography grade of 2b or 3 on the modified Thrombolysis in Cerebral Infarction (TICI). We evaluated the outcomes of symptomatic intracranial hemorrhage at 36 hours from symptom onset (defined as a ≥4 point worsening of immediate predeterioration NIH Stroke Scale neurological status versus postdeterioration and associated with brain hemorrhage) and the incidence of early neurological deterioration before discharge (defined as ≥4 point worsening of the immediate predeterioration NIH Stroke Scale neurological status versus postdeterioration and not attributed to sedation). We also stratified patients by the outcome of reperfused/recanalized, defined as a >90% reduction in the region of perfusion delay (Tmax of >6 seconds) between baseline and 24 hours and complete recanalization on the 24-hour CT or magnetic resonance angiogram.

Collateral Assessment

The main predictor of outcome was leptomeningeal collateral status determined on the baseline CTA. To ensure reliable contrast transit into collaterals, we only included patients with CTAs that had contrast opacification of the major dural venous sinuses. The collateral assessment was performed by experienced neuroradiologists (Drs Heit and Marks) in the DEFUSE 3 core laboratory, who were blinded to patient treatment arm and outcome. All disagreement on collateral rating was resolved by consensus. Collaterals were rated with the binary modified Tan collateral scale as previously described.^15,16 On CTA axial maximum intensity projection images, good collaterals filled >50% of the vascular territory distal to the occluded middle cerebral artery and poor collaterals filled ≤50%. (Figure I in the online-only Data Supplement.) Although the predictive ability of a binary collateral scale is comparable to multicategory ordinal scales,¹⁷ for a confirmatory analysis we graded collaterals using the ordinal scale developed by Maas et al,¹⁸ which is scored: 1, absent; 2, less than the contralateral normal side; 3, equal to the contralateral normal side; 4, greater than the contralateral normal side; and 5, exuberant. Based on cut points proposed by Maas et al,¹⁸ we also made the scale binary, with poor collaterals defined as Maas 1 to 2 and good collaterals as Maas 3 to 5. The secondary predictor of outcome was the treatment arm assignment in DEFUSE 3 (EVT versus medical).

Statistical Methods

We compared demographics, clinical variables, and neuroimaging data between patients with good and poor collaterals using the χ² and Wilcoxon rank-sum tests.

To test the association between collateral status and neurological outcome in DEFUSE 3, we fit logistic regression models to the primary outcome and ordinal logistic regression models to the secondary outcome, with the primary predictor of collateral status. We fit 2 models, (1) adjusted for treatment arm in DEFUSE 3 and (2) fully adjusted for treatment arm and additional covariates that were independently associated with the primary outcome: patient age, NIH Stroke Scale, and serum glucose at enrollment, and time from last known normal to randomization. For ordinal logistic regression models, we verified that the proportional odds assumption was met (P>0.05). To test the hypothesis that patients with good collaterals would respond better to the treatment effect of late window EVT, we included an interaction term (treatment arm×collateral status) in the model, derived odds ratios for treatment effect by collateral status, and assessed if they were significantly different. We defined an α value of <0.05 as statistical significance and report 2-sided results. Statistical analysis was done using SAS 9.4.

Results

For the 182 patients enrolled in DEFUSE 3, 133 (73%) had a baseline CTA of which 130 (71%) were included in our final cohort. Three CTAs (2%) were excluded secondary to inadequate opacification of the dural venous sinuses. Of the 130 patients in our final cohort, 97 (75%) had good collaterals and 33 (25%) had poor collaterals on the binary Tan scale. The hypoperfusion intensity ratio (HIR) on CT perfusion has been previously shown to correlate well with collateral status on conventional angiography.¹⁹ The HIR in our cohort was significantly lower in patients with good collaterals, which is expected and provides validation of the collateral grading (median [IQR] HIR, good versus poor collaterals, 0.28 [0.15–0.49] versus 0.44 [0.26–0.58]; P<0.001). On the binary Maas scale, 61 patients (47%) had good collaterals and 70 (53%) had poor collaterals. The demographics of the entire cohort are shown in Table 1 with stratification by good versus poor collaterals on the Tan scale (Table I in the online-only Data Supplement for stratification by binary Maas scale). In patients with poor collaterals, there was a significantly higher proportion of Hispanic ethnicity (21% versus 4% for patients with good collaterals, P=0.006). Additional baseline clinical characteristics were not predictive of collateral status (Table 1).

The baseline Alberta Stroke Program Early CT Score did not differ between the collateral groups (median [IQR], good versus poor collaterals, 8 [7–9] versus 8 [7–9], P=0.975), and there was no difference in the time from last known normal to baseline imaging (Table 1). In patients randomized to EVT (n=65), collateral status did not determine successful recanalization (n [%], mTICI 2b/3, good versus poor collaterals, 35/49 [71%] versus 14/16 [88%], P=0.318). In the whole cohort, there was no difference in the rates of recanalization, reperfusion, or symptomatic intracerebral hemorrhage between patients with good versus poor collaterals (Table 2; Table II in the online-only Data Supplement). On the baseline neuroimaging, patients with good collaterals had smaller core ischemic regions but also smaller perfusion lesion volumes (Table 1; Figure II in the online-only Data Supplement). As a result, the baseline mismatch volume was not different. Patients with good collaterals had less growth of the ischemic core between the baseline and 24-hour follow-up imaging, (median [IQR], good versus poor collaterals, 26.9 mL [13.1–54.5] versus 43.1 mL [13.6–131.3], P=0.031) and a smaller absolute ischemic core volume at 24 hours (Table 2). After dividing the cohort by reperfusion/recanalization, it was evident that patients with poor collaterals have a significantly larger ischemic core volume and growth on the 24-hour follow-up imaging if not reperfused/recanalized (Figure 1; Figure III in the online-only Data Supplement).

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Despite the expected imaging findings based on collateral status, there was no association between collateral status and neurological outcome (Figure 2). There was no difference in the number of patients who achieved functional independence (n [%], good versus poor collaterals, 29/97 [30%] versus 13/33 [39%], P=0.314]. After adjustment for treatment arm, collateral status still did not predict functional independence (odds ratio [95% CI], 0.61 [0.26–1.45]). The difference in the treatment effect of EVT between patients with good versus poor collaterals was not significant (P=0.845 for the difference between odds ratios). When we ran the same analysis with the binary Maas scale, there was no difference in rate of functional independence (Table II in the online-only Data Supplement) or the EVT treatment effect between good versus poor collaterals (P=0.528).

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In the ordinal logistic regression fit to the individual values of the 90-day mRS and adjusted for treatment arm, good collaterals were not predictive of mRS (odds ratio [95% CI], 1.05 [0.52–2.11]). In the fully adjusted model, collaterals remained unassociated with the neurological outcome (Table 3). Identical results were seen with the binary Maas scale, both when adjusted for treatment arm and in the fully adjusted model. Finally, there was no difference in the rate of early neurological deterioration (n [%], good versus poor collaterals, 12/97 [12%] versus 1/33 [3%], P=0.182) or stroke-related death (n [%], good versus poor collaterals, 18/97 [19%] versus 8/33 [24%], P=0.481).

Discussion

We did not find that collaterals had the expected effect on acute ischemic stroke patients with anterior circulation large vessel occlusion and target mismatch in the late therapeutic window. For example, we failed to find an association between collaterals and several baseline variables that have previously been associated with collateral status, including patient age, sex, premorbid hypertension, baseline systolic blood pressure, creatinine, time from stroke onset to baseline imaging, NIH Stroke Scale, or Alberta Stroke Program Early CT Score.^9,11,20 These baseline variables were correlated with collateral status before any study procedures, reducing the role of possible confounding. We did find that patients with Hispanic ethnicity were more likely to have poor collaterals, which has not been reported in prior studies and requires validation in a larger cohort. We also did not find that collaterals influenced the rate of early neurological deterioration, successful recanalization after EVT, functional independence at 90 days, death, or symptomatic intracranial hemorrhage. Our findings are contrary to prior studies that showed good collaterals were associated with better neurological and radiographic outcomes for acute ischemic stroke patients in earlier time windows.⁷

The central paradox of this study is that good collateral status did not affect neurological outcome but did reduce the size and growth of the ischemic core, a reliable biomarker of neurological outcome.²¹ There are several possible explanations. The collateral circulation ultimately fails in the majority of stroke patients with large vessel occlusion who are not recanalized.²² The ischemic core of the DEFUSE 3 patients with good collaterals, particularly those who were not recanalized, may have continued to grow after the 24-hour follow-up imaging and, ultimately, be comparable to patients with poor collaterals.²³ An additional imaging study at 72 or 96 hours after randomization would help answer this question and also provide valuable information on collateral evolution over time, which is dependent on dynamic factors, such as intravascular volume, adrenergic tone, blood pressure, brain edema, core temperature, and medications being administered.²⁰ Until we better understand collateral evolution, we will not fully appreciate their effect on neurological outcome or comprehend how to therapeutically modify that effect.

Another possibility is that we were unable to accurately measure collaterals. There is a low percentage of DEFUSE 3 patients with poor collaterals on the Tan scale (33/130, 25%), compared with 45% with poor collaterals in a recent meta-analysis of 2004 acute ischemic stroke patients who had EVT in the standard time window.⁷ This was expected because of the inclusion criteria of a target mismatch. When using the binary Maas scale, we found a higher percentage with poor collaterals (70/130, 53%), but only 2 of those 70 patients were in category 1 (absent collaterals), which is lower than previous studies in the 6-hour window.²⁴ However, the unexpected presence of patients with poor collaterals and a target mismatch in the late window introduces the possibility that we were not able to fully visualize their collaterals. On CTA, we infer collaterals from retrograde blood flow into the distal branches of the middle cerebral artery but cannot visualize the more numerous pial arteries and parenchymal arterioles that allow retrograde blood flow.^25,26 Some patients in our cohort may have poor collaterals on single phase CTA while still perfusing through pial arteries and parenchymal arterioles. Prior studies have shown that time-resolved multiphase CTA or tissue-specific measures of blood flow derived from perfusion imaging, such as the HIR, can be independent predictors of clinical outcome because they portray a more comprehensive picture of collateral flow.¹⁹ Although we did find that the HIR in our cohort showed the expected difference between patients with good versus poor collaterals, it is possible that with multiphase CTA we would have found delayed filling of collaterals in patients who appeared to have poor collaterals on single phase CTA.

Our study has additional limitations. Although this is a prespecified analysis, it is a selected cohort of patients from a clinical trial, which could create bias. To standardize our collateral assessment, we introduced a selection bias by only including patients with CTA as their baseline imaging, although that was >70% of patients enrolled in DEFUSE 3. Additional neuroimaging limitations include that we did not have full control of the CTA scan parameters, collateral assessment is inherently subjective, and we do not have more sensitive measures of collateral status, such as multiphase CTA or catheter angiography, for all patients in the study.^13,17,27 A separate analysis of the CT perfusion source data will examine the role of collaterals visualized during the venous phase of image acquisition, while we focused on single phase CTA because of its widespread use in research and clinical care. The small subgroup of patients with poor collaterals may have reduced the precision of our point estimates. We do not have data on patients who failed to qualify for DEFUSE 3, which would be an important comparison group to understand the clinical spectrum of collateral status in the late window. DEFUSE 3 also had a large number of wake-up strokes, which is expected in the late window because awake patients with large vessel occlusion are typically treated early. However, the wake-up strokes may have presented earlier from true stroke onset than 6 hours. Despite these limitations, the strengths of our study are notable and include its unique patient population, rigorous adjudication of neurological outcomes, use of 2 previously validated collateral scales with distinct methodology, high-quality neuroimaging, and serial measurements of ischemic core and ischemic penumbra with validated postprocessing software.

Conclusions

In DEFUSE 3, good collaterals on single phase CTA were associated with smaller ischemic core volume at baseline and reduced ischemic core growth but not with improved neurological outcome, success of endovascular therapy, hemorrhagic complications, or death. We also did not find an association between collaterals and many of the traditional demographic predictors of baseline collateral status. These unexpected findings require further study to confirm their validity and to better understand the role of collaterals for anterior circulation large vessel occlusion stroke patients in the late therapeutic window.

Acknowledgments

We thank the DEFUSE 3 (Endovascular Therapy Following Imaging Evaluation for Ischemic Stroke) Primary Investigators and the National Institute of Neurological Disorders and Stroke.

Footnotes