Mechanical Thrombectomy Time Window for Acute Ischemic Stroke
Patients with acute ischemic stroke and large vessel occlusion (LVO) are candidates for mechanical thrombectomy up to 24 hours after last known well, provided they meet specific imaging criteria beyond 6 hours. 1
Time-Based Treatment Windows
Within 6 Hours of Symptom Onset
For patients who can be treated within 6 hours of stroke onset or last known well, mechanical thrombectomy is recommended if they meet these criteria 1:
- Age ≥18 years
- Pre-stroke mRS score 0-1
- Causative occlusion of internal carotid artery or MCA (M1)
- NIHSS score ≥6
- ASPECTS ≥6
- Treatment can be initiated (groin puncture) within 6 hours
In this early window, no advanced perfusion imaging is required—proceed directly to thrombectomy based on non-invasive angiography showing LVO.
6 to 16 Hours: DEFUSE 3 Criteria
Between 6-16 hours from last known well, thrombectomy is recommended for anterior circulation LVO when patients meet DEFUSE 3 eligibility criteria 2:
- Advanced imaging (CTP or DW-MRI) showing perfusion-core mismatch
- Maximum core size ≤70 mL
- Mismatch ratio demonstrating salvageable tissue
The DEFUSE 3 trial demonstrated 44.6% vs 16.7% achieved functional independence (mRS 0-2) with thrombectomy, representing a dramatic 28% absolute benefit 2.
16 to 24 Hours: DAWN Criteria
Between 16-24 hours from last known well, thrombectomy is reasonable for anterior circulation LVO when patients meet DAWN eligibility criteria 2:
- Clinical-imaging mismatch using combination of NIHSS and imaging findings on CTP or DW-MRI
- Specific age-stratified criteria matching clinical deficit severity to infarct core size
The DAWN trial showed 49% vs 13% achieved functional independence with thrombectomy—an unprecedented 36% absolute benefit and the largest treatment effect ever demonstrated in acute stroke trials 2.
Critical Implementation Points
The guidelines explicitly state that DAWN or DEFUSE 3 eligibility criteria must be strictly adhered to in clinical practice for the 6-24 hour window 2. These are the only two randomized controlled trials demonstrating benefit beyond 6 hours, and their specific imaging and clinical criteria should not be loosely interpreted.
Imaging Requirements by Time Window
- 0-6 hours: Non-invasive angiography (CTA) to confirm LVO; advanced perfusion imaging should NOT delay treatment 2
- 6-24 hours: CTP, DW-MRI, or MRI perfusion is REQUIRED to determine eligibility, applying strict DAWN or DEFUSE 3 criteria 2, 1
Common Pitfalls to Avoid
Do not observe patients after IV alteplase to assess clinical response before pursuing thrombectomy—this delays definitive treatment and worsens outcomes 1. Proceed directly to angiography if LVO is confirmed.
Do not use time alone as the determining factor beyond 6 hours—the paradigm has shifted from "time window" to "tissue window." 3 Patients with good collateral flow may have salvageable tissue well beyond traditional time limits, while others with poor collaterals may have completed infarction within hours.
Do not apply these extended time windows without proper advanced imaging—the 6-24 hour window expansion is predicated on rigorous patient selection using perfusion imaging to identify clinical-imaging mismatch 2.
Time-Critical Execution
Every hour delay reduces the odds of good functional outcome 2. In pooled data from early trials, each 1-hour delay significantly decreased treatment benefit, with statistical significance maintained through 7 hours 18 minutes for the 0-6 hour cohort 2. The technical goal remains achieving mTICI 2b/3 reperfusion as rapidly as possible within the therapeutic window 1, 2.
The 2024 data shows that window expansion to 24 hours was associated with increased procedure volumes and reduced in-hospital mortality nationally 4, confirming real-world effectiveness when proper patient selection is applied.