Objective Stroke patients with good collateral circulation achieve the best recovery after mechanical thrombectomy (MT) but strict imaging selection may result in untreated patients that could benefit from MT. We assessed whether the extent of collaterals had modifying effects on the amount of ischemic tissue saved from infarction with MT over best medical treatment (BMT).

Methods This was a single center cohort of consecutive patients (n=339) with proximal occlusions in the carotid territory. Patients were categorized according to a four point category scale on CT angiography as having good (scores 2–3) or poor (scores 0–1) collaterals. The primary outcome measure was the interaction between collaterals and MT on infarct growth. The secondary outcome assessed the treatment effect of MT over BMT on functional status in relation to collateral status. Safety outcomes were mortality and symptomatic intracranial hemorrhage.

Results Collaterals had a modifying effect of MT on infarct growth (P=0.004), with a greater reduction in 96 patients with poor collaterals (38.8 mL) than in 243 patients with good collaterals (1.9 mL). There was also a significant (P<0.001) interaction between the effect of MT and functional outcome in relation to collateral status, with more benefits of MT in patients with poor collaterals. MT was associated with lower mortality than BMT in patients with poor collaterals only.

Conclusion Compared with BMT, the use of MT in the early time window in large vessel stroke results in a more substantial limitation of infarct growth in patients with poor collaterals.