Stroke occurs when a blood clot causes a blockage in a blood vessel (artery) within the brain. This type of stroke is called an ischemic stroke and carries a high risk of disability or death. Stroke must be treated very fast. Any delay of even 10 minutes can result in the difference between an independent and a disabled outcome, and in some cases between life and death. Endovascular therapy (EVT) or Thrombectomy is a procedure to remove the blood clot (thrombus) from a blood vessel to reopen it (recanalization). Patients are likely to benefit from a thrombectomy procedure when it is performed in a larger blood vessel. Currently it is not known if thrombectomy procedure will benefit the patients presenting with the stroke that has been caused by a blood clot in a medium sized blood vessel (medium vessel occlusion, MeVO). This trial will enrol patients diagnosed with acute stroke due to a clot in the medium sized vessel. The patients will be randomized within 12 hours of their symptom onset to either standard of care or standard of care plus thrombectomy procedure. The participation will last for 12 months Escape MeVO coordinating centre is located at the University of Calgary. There will be up to 75 sites. We will be recruiting a total of 530 patients.

ESCAPE-MeVO is a multicenter, prospective, randomized, open- label study with blinded endpoint evaluation (PROBE design). Participants will be randomized to routine best medical stroke care governed by current guidelines (control group) or to EVT plus best medical care. EVT will be performed with one of the Solitaire group of intracranial stent-retriever devices (Solitaire X, Medtronic) as the first line approach according to the manufacturers' specifications for use.

Patients with clinical symptoms of acute stroke, last seen normal within the last 12 hours, and with either NIHSS ≥ 5 or NIHSS 3-5 due to disabling symptoms in the judgement of the stroke team shall undergo imaging to identify the MeVO and assess the status of the affected brain parenchyma. As is being currently practiced across different stroke centers, imaging may follow different imaging paradigms:

• Option 1: NCCT + mCTA (first phase covers intracranial and extracranial vessels)
• Option 2: NCCT + mCTA/spCTA + either mCTA tissue level perfusion maps or CTP perfusion maps
• Option 3: DWI-MRI + MRA (of both the intracranial and extracranial vessels). Approach to the EVT procedure and technique will be at the discretion of the interventionist and team, with the exception that the first thrombectomy attempt is performed with one of the Solitaire group of intracranial stent-retriever devices (Solitaire X, Medtronic). Available, approved off-the-shelf, secondary devices may be used if reperfusion success is not achieved after use of the first device, at the discretion of the neuro-interventionalist

This study consists of one 90-day study period for each participant. Participants will be hospitalized for care after their acute stroke according to the current standard of care. Participants are required to return to clinic on Day 90 for end-of-study assessment.

Inclusion Criteria:

1. Acute ischemic stroke clinically eligible for immediate EVT

2. Age ≥18 years at the date of randomization

3. Time from onset (or last-seen-well) to randomization <12 hours

4. Disabling stroke defined as follows:

   1. baseline National Institutes of Health Stroke Scale (NIHSS) score >5 at the time of randomization
   2. NIHSS 3-5 with disabling deficit (eg. hemianopia, aphasia, loss of hand function) as determined by the attending physician in context of the patient's life situation

5. Confirmed symptomatic and endovascularly treatable MeVO based on neurovascular non-invasive imaging (mCTA or MRA), at one or more of the following locations: M2 or M3 segment, A2 or A3 segment, P2 or P3 segment1.

6. . Clinical deficit commensurate with MeVO occlusion location

7. . Signed informed consent, two-physician consent, or deferral of consent where approved

   Exclusion Criteria:

8. ASPECTS ≤ 5

9. The following depend on the imaging modality of the participating site:

9a. NCCT + mCTA

• Well demarcated hypodensity in the majority of the brain parenchyma supplied by the occluded vessel or absence of collaterals in the affected territory on the delayed phases of the mCTA OR 9b. NCCT + (m)CTA + CTP**

• Lack of core: penumbra mismatch (if the CTP is uninterpretable, e.g., due to motion artifacts, apply exclusion criteria from 9a. If single-phase CTA is performed, score collaterals accordingly. If NCCT + mCTA + CTP are all performed, the core: penumbra mismatch criteria are sufficient for exclusion) OR 9c. MRI

• Diffusion restriction in the majority of the brain parenchyma supplied by the occluded vessel

• if MR perfusion is performed: lack of core:penumbra mismatch 10) Any evidence of intracranial hemorrhage on qualifying imaging 11) Patients living in a nursing home or requiring daily nursing care or assistance with activities of daily living.

  12) Patient has a major co-morbid illness, such as severe dementia, advanced cancer, advanced heart failure etc. such that they are unlikely to be able to complete follow-up or they are unlikely to achieve the primary outcome due to the underlying illness (rather than the stroke or its treatment).

  13) Pregnancy: female with positive urine or serum beta human chorionic gonadotropin (β-hCG) test 14) Participation in another clinical therapeutic intervention trial

We plan to share IPD in due course after the study is completed and published.

36 months after study completion.

We will attempt to post the data in a public data archive.