Peggy Nguyen, MD
Although the benefit of intra-arterial thrombolysis has been demonstrated in recent multiple randomized studies, identification of certain thresholds or markers such as imaging is still being evaluated. The presence of good collaterals has been associated with smaller infarct volumes and better outcomes after IV tPA and IAT. Because MR CLEAN did not stratify by collateral status, the study population is ideal for assessing whether the effect of IAT was modified by collateral status on baseline CTA.

In this post-hoc analysis of the MR CLEAN study, collaterals were graded on baseline CTA using a 4-point scale where 0 was absent, 1 for poor (>0% and ≀50% filling of the occluded territory), 2 for moderate (> 50% and < 100% filling of the occluded territory), and 3 for good collaterals (100% filling of the occluded territory). There was an independent and significant modification of the treatment effect on the primary outcome of shift in direction of better outcome on the mRS. In addition, there was a shift in the distribution on the mRS in favor of intervention across all collateral grades except those with grade 0; the strongest shift was in patients with grade 3 collaterals. Patients with grade 3 collaterals had a 29.5% absolute increase in the chance of becoming functionally independent at day 90 (mRS 0-2), whereas in patients with grade 0 collaterals, none of the patients achieved functional independence at day 90. 

On the one hand, the presence of good collaterals likely allows maintenance of perfusion to the ischemic penumbra, which may portend better outcomes, even beyond the 6 hour window; but the opposite hypothesis, that we should not take patients with grade 0 collaterals is not necessarily true. The use of collateral imaging and a standardized, simplified method of collateral grading may be a viable method of stratifying patients for IAT to maximize the benefit. As the authors point out, it may be that patients with poor collaterals would still benefit, but the window of therapeutic advantage is much narrower. Absent of collateral flow, these patients are entirely dependent on thrombolysis. Further studies are needed.