M2 segment thrombectomy is not associated with increased complication risk compared to M1 segment: A meta-analysis of recent literature.

Introduction: Recent clinical comparisons of M1 and M2 segment endovascular thrombectomy have reached incongruous results in rates of complication and functional outcomes. This study aims to clarify the controversy surrounding this rapidly advancing technique through literature review and meta-analysis.

Methods: A Pubmed search was performed (January 2015-September 2019) using the following keywords: "M2 AND ("stroke" OR "occlusion") AND ("thrombectomy" OR "endovascular")". Safety and clinical outcomes were compared between segments via weighted Student's t-test, Chi-square and odds ratio while study heterogeneity was analyzed using Cochran Q and I tests.

Results: Pubmed identified 208 articles and eleven studies were included after full-text analysis, comprising 2,548 M1 and 758 M2 mechanical thrombectomy segment cases. Baseline National Institutes of Health Stroke Scale scores were comparatively lower in patients experiencing an M2 occlusion (16 ± 1.25 vs 13.6 ± 0.96, p < 0.01). Patients who underwent M2 mechanical thrombectomy were more likely to experience both good clinical outcomes (modified Rankin Scale 0-2) (48.6% vs 43.5% respectively, OR 1.24; CI 1.05-1.47, p = 0.01) and excellent clinical outcomes (modified Rankin Scale 0-1) (34.7% vs. 26.5%%, OR 1.6; CI 1.28-1.99, p < 0.01) at 90 days compared to M1 mechanical thrombectomy. Neither recanalization rates (75.3% vs 72.8%, OR 0.92, CI 0.75-1.13, p = 0.44) nor symptomatic intracranial hemorrhage rates (5.6% vs 4.9%, OR 0.92; CI 0.61-1.39, p= 0.7) were significantly different between M1 and M2 cohorts. Mortality was less frequent in the M2 cohort compared to M1 (16.3% vs 20.7%, OR 0.73; CI 0.57-0.94, p = 0.01). M1 and M2 cohorts did not differ in symptom onset-to-puncture (238.1 ± 46.7 vs 239.8 ± 43.9 min respectively, p=0.488) nor symptom onset-to recanalization times (318.7 ± 46.6 vs 317.7 ± 71.1 min respectively, p = 0.772), though mean operative duration was shorter in the M2 cohort (61.8 ± 25.5 vs 54.6 ± 24 min, p < 0.01).

Conclusions: Patients who underwent M2 mechanical thrombectomy had a higher prevalence of good and excellent clinical outcomes compared to the M1 mechanical thrombectomy cohorts. Additionally, our data suggest lower mortality rates in the M2 cohort and symptomatic intracranial hemorrhage rates that are similar to the M1 cohort. Therefore, M2 segment thrombectomy likely does not pose a significantly elevated operative risk and may have a positive impact on patient outcomes.