Timing Windows for Mechanical Thrombectomy
Mechanical thrombectomy should be performed within 6 hours of symptom onset for most patients with large vessel occlusion, but can be extended to 24 hours in carefully selected patients who meet specific imaging criteria from the DAWN or DEFUSE 3 trials.
Standard Time Window (0-6 Hours)
For patients with acute ischemic stroke due to large vessel occlusion (LVO) in the anterior circulation (ICA or proximal MCA-M1), mechanical thrombectomy is strongly recommended when treatment can be initiated (groin puncture) within 6 hours of symptom onset 1. This represents the standard treatment window where benefit is most robust and does not require advanced perfusion imaging for patient selection.
Extended Time Window (6-24 Hours)
6-16 Hours (DEFUSE 3 Criteria)
For patients presenting 6 to 16 hours after last known normal with anterior circulation LVO, mechanical thrombectomy is recommended (Class I, Level A) if they meet DEFUSE 3 eligibility criteria 1. These criteria include:
- Perfusion-core mismatch on CT perfusion or MRI
- Maximum core size thresholds
- Advanced imaging (CTP, DW-MRI, or MRI perfusion) is mandatory for patient selection in this window
16-24 Hours (DAWN Criteria)
For patients presenting 16 to 24 hours after last known normal with anterior circulation LVO, mechanical thrombectomy is reasonable (Class IIa, Level B-R) if they meet DAWN eligibility criteria 1. DAWN used clinical-imaging mismatch combining:
- NIHSS score
- Imaging findings on CTP or DW-MRI showing limited core volume
- Specific age-stratified criteria
Critical Implementation Points
Strict Adherence to Trial Criteria
The 2018 AHA/ASA guidelines explicitly state that only the eligibility criteria from DAWN or DEFUSE 3 should be used for patient selection beyond 6 hours 1. The guidelines emphasize: "Although future RCTs may demonstrate that additional eligibility criteria can be used to select patients who benefit from mechanical thrombectomy, at this time, the DAWN or DEFUSE 3 eligibility should be strictly adhered to in clinical practice" 1.
Imaging Requirements by Time Window
- 0-6 hours: Noncontrast CT and CTA sufficient; advanced perfusion imaging should not delay treatment 1
- 6-24 hours: CTP, DW-MRI, or MRI perfusion is mandatory (Class I recommendation) to identify appropriate candidates 1
Large-Core Stroke Considerations
The 2025 AHA science advisory on large-core strokes 2 notes that historically, patients with ASPECTS <6 within 6 hours or core volumes >50-70 mL in extended windows were excluded from trials. Recent evidence suggests benefit may extend to larger cores, but this represents evolving practice.
Special Populations and Vessel Locations
M2/M3 Occlusions
For M2 or M3 segment MCA occlusions, mechanical thrombectomy may be reasonable within 6 hours (Class IIb), but evidence is less robust than for proximal occlusions 1. Recent real-world data 3 suggests comparable outcomes in late versus early windows for M2 occlusions, though this remains outside formal guideline recommendations.
Posterior Circulation
For basilar artery, vertebral artery, or posterior cerebral artery occlusions, mechanical thrombectomy may be reasonable within 6 hours (Class IIb), though benefits are uncertain 1. Emerging data 4 suggests similar outcomes in early versus late windows for basilar occlusions, but guideline-level recommendations remain limited to 6 hours.
Common Pitfalls to Avoid
- Do not use non-validated imaging criteria beyond 6 hours—stick strictly to DAWN or DEFUSE 3 protocols
- Do not delay treatment within 6 hours to obtain perfusion imaging; noncontrast CT and CTA are sufficient 1
- Do not assume all late-window patients are candidates—advanced imaging showing appropriate mismatch is mandatory beyond 6 hours
- Do not confuse "last known well" with "symptom discovery"—timing is from when patient was last known to be neurologically normal
Technical Goal
Regardless of time window, the procedural goal should be achieving modified TICI 2b/3 reperfusion to maximize functional outcomes 1.