2.1 Patient selection
We performed a retrospective analysis of consecutive patients presenting with BAO treated by EVT in Beijing Tiantan Hospital from January 2012 to July 2018. IVT with tissue plasminogen activator (tPA) was acceptable before EVT, consistent with current guidelines.9 The protocol was approved by the Institutional Review Board of Beijing Tiantan Hospital.
Plain computed tomography (CT) of the brain was performed first, followed by either computed tomography angiography (CTA) or magnetic resonance imaging (MRI) with magnetic resonance angiography (MRA) to confirm the presence of acute BAO and to exclude both large brainstem infarction and intracranial bleeding. Imaging modality selection relied on physician preferences and the logistics of scheduling the procedures. We retrospectively analyzed clinical and radiologic data of these patients, including age; sex; stroke risk factors including hypertension, diabetes mellitus, hypercholesterolemia, and current smoking; initial stroke severity as expressed by the National Institutes of Health Stroke Scale (NIHSS) score; initial imaging modality; prior use of IVT; type of EVT devices; onset to puncture time; procedure time; onset to recanalization time; collateral status; and etiology of stroke according to the Trial of Org 10172 in Acute Stroke Treatment (TOAST) classification.10 The modality of treatment was a joint decision between the neurointerventionalist and the stroke team taking care of the patient.
2.2 Endovascular treatment
All EVT procedures were performed by a neurointerventionalist with more than 50 cases of experience in neurovascular intervention in MT for AIS. Cerebral angiography and EVT were performed under general anesthesia or conscious sedation after evaluation by a dedicated anesthesiology team. The thrombectomy technique was chosen at the interventionalist’s discretion, using either a stent retriever or aspiration first, with a possible switch to another strategy in case of recanalization failure [modified Thrombolysis in Cerebral Infarction (mTICI) grade 0-2a] with the first approach. If underlying intracranial atherosclerotic stenosis (ICAS) was revealed, intracranial angioplasty or stenting was performed when suitable. Patients undergoing angioplasty and/or stenting received a loading dose of aspirin (300 mg) and clopidogrel (300 mg) orally or via the nasogastric tube immediately after the intervention if they were antiplatelet-naive. Intraoperative bridging with intravenous IIb/IIIa inhibitor was also allowed at the operator’s discretion. Dual antiplatelet therapy was maintained for at least 3 months after the procedure, followed by aspirin or clopidogrel monotherapy lifelong.
2.3 Image interpretation
Pretreatment neurovascular images, including posterior circulation Acute Stroke Prognosis Early CT Score (pc-ASPECTS), Pons-Midbrain Index based on diffusion-weighted imaging, and two previous collateral scales [posterior communicating artery and American Society of Interventional and Therapeutic Neuroradiology/Society of Interventional Radiology (ASITN/SIR)]11,12 described in the setting of BAO, were retrospectively interpreted by two independent trained neurointerventionalists blinded to clinical data. A third neurointeventionalist with 20 years’ experience was involved to resolve any disputes. All collateral scales mentioned above were evaluated by digital subtraction angiography.
2.4 Outcome measurement
Primary effectiveness outcome measures were functional independence and favorable outcome at 90 days after the procedure. Functional independence was defined as an mRS score of ≤ 2. Favorable outcome was defined as an mRS score of ≤ 3, in accordance with the BASICS definition.1 The mRS was assessed by blinded trained interviewers over the phone using a standardized interview protocol. Other effectiveness outcome measures included the rates of successful recanalization (mTICI 2b-3) and complete recanalization (mTICI = 3).13
Safety outcome measures were 90-day mortality and rates of intracerebral hemorrhage and symptomatic intracerebral hemorrhage (SICH) within 24 hours. SICH was defined on the basis of the European Cooperative Acute Stroke Study III (ECASS III) criteria.14
2.5 Statistical analysis
Study data were collected on standard forms, evaluated for completeness, and double keyed into an EpiData statistics data document. Baseline and outcome data were described using means (standard deviations) and/or medians (25th and 75th percentiles) for continuous variables. Frequencies and/or proportions were used for categorical variables. Pearson’s chi-square test or Fisher’s exact test was used to compare the differences in frequencies and/or proportions. All tests were two-tailed, and statistical significance was determined at an α level of 0.05. All statistical analyses were performed with the statistical software packages R (http://www.R-project.org, The R Foundation) and Empowerstats (http://www.empowerstats.com, X&Y Solutions, Inc., Boston, MA).
Data availability statement: All data generated or analyzed during this study are included in this published article [and its supplementary information files].