Large vessel occlusions of milder severity show better collaterals and reduced harm from thrombectomy transfer delays

Background: Patients with large vessel occlusion (LVO) stroke of milder baseline clinical severity are often missed by pre-hospital severity-based triage tools. We examined whether these patients had differences in 1) markers of collateral circulation and 2) the relative harm of transfer delays on outcome, compared to patients with clinically more severe baseline deficits. Methods: Registry data from two large Australian thrombectomy centers was used to identify all directly presenting and transferred LVO patients undergoing endovascular thrombectomy, divided into those with milder baseline deficits (NIHSS <10) and severe baseline deficits (NIHSS ≥10). Baseline CT-perfusion markers of collateral circulation and the association between transfer status and functional outcome (using the 90-day modified Rankin Scale) were compared. Results: Of 1210 included LVO patients, 273 (22.6%) had milder severity. Milder LVO patients showed smaller median ischemic core volumes (12.6 [IQR 0.0-17.9] vs 27.5 [IQR 6.5-37.1] mL, p<0.001)), higher median perfusion mismatch ratio (148.3 [IQR 5.0-500.0] vs 66.2 [IQR 3.6-17.2], p<0.001) and lower median hypoperfusion intensity ratio (0.25 [IQR 0.18-0.38] vs 0.40 [IQR 0.22-0.57], p<0.001). Transferred patients had a lower odd of excellent outcome than primary presenters, when deficits were severe deficits (aOR 0.759 [95% CI 0.576-0.999]) but not when deficits were milder (aOR 1.357 [95% CI 0.764-2.409], pinteraction=0.122). Conclusions: Patients with LVO stroke of milder clinical severity have significantly better imaging markers of collateral circulation compared to high-severity patients. Despite high rates of thrombolytics, secondary transfer to thrombectomy-centre was only associated with poorer 90-day functional outcomes in higher-severity patients. Failure of pre-hospital screening tools to detect lower-severity LVO patients for immediate pre-hospital bypass to a thrombectomy center may not deleteriously affect their outcome. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement No external funding was received ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: Data collection was approved by the Melbourne Health Human Research Ethics Committee I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes Data is available on reasonable request from a suitably qualified investigator