Background Collaterals may affect revascularization, ischemic severity, and clinical outcomes in acute stroke owing to internal carotid artery (ICA) occlusion.

Objective To examine the hypothesis that morphology of occlusive thrombus and collateral flow patterns may influence the outcome of ICA occlusions after mechanical thrombectomy.

Methods Pooled analyses of ICA occlusions in the MERCI and Multi-MERCI trials employed central angiography review readings to categorize lesions as I, L, or T clots and functional lesions based on collateral flow patterns. Demographic variables, procedural details, and clinical outcomes were compared across ICA lesion types.

Results A total of 72 subjects (mean age 67 years (SD 16), 51% female, median National Institutes of Health Stroke Scale 20 (range 8–35)) were included, with 90-day modified Rankin score ≤2 in 28% and 51% mortality. Clots were categorized as an I lesion in 9/72 (12.5%), L lesion in 12/72 (16.7%), and T lesion in 51/72 (70.8%). Based on collateral flow patterns, cases were categorized as having a functional I lesion in 7/72 (9.7%), functional L in 38/72 (52.8%), and functional T in only 27/72 (37.5%). Multivariate analyses showed that a functional T lesion, with insufficient collateral flow to ipsilateral anterior cerebral arteries via the contralateral ICA, was a strong predictor of both revascularization success and subsequent clinical outcomes.

Conclusions Collateral flow patterns distinguish the nature and impact of ICA occlusions on expected revascularization and subsequent clinical outcomes in acute ischemic stroke. The nomenclature of terminal ICA occlusions introduced here (carotid I's, L's, and T's) may enhance future endovascular trials targeting such proximal occlusions.

Trial registration number NCT00318071 (http://clinicaltrials.gov). MERCI was not registered because enrollment began before July 1, 2005.