Pushing the Clock: The Expanding Time Window for IV Thrombolysis

In the mid-1990s, stroke treatment entered a new era with the approval of intravenous tPA within 3 hours of onset. Over the next three decades, progressive advances in imaging, patient selection, and trial design stretched the therapeutic window far beyond what once seemed possible. Here's how we got from 3 hours to >24 hours—and what it means for clinical practice today.

Updated to reflect the 2026 AHA/ASA Guidelines for Early Management of Acute Ischemic Stroke.

3 Hours: The NINDS tPA Trial (1995)

The original NINDS trial established that IV alteplase given within 3 hours of stroke onset improved the odds of a favorable outcome (mRS 0–1 at 90 days) by 12% absolute (NNT ≈ 8) despite a 6.4% risk of symptomatic ICH.

4.5 Hours: ECASS III (2008)

ECASS III extended the window to 4.5 hours. Patients saw an 8% absolute improvement in functional outcome (mRS 0–1), with ICH risk rising to 7.9%. Originally excluded groups (age >80, NIHSS >25, prior stroke + diabetes) are now variably included based on subsequent evidence.

Unknown Onset: WAKE-UP (2018)

WAKE-UP used DWI/FLAIR mismatch on MRI to estimate biological onset within 4.5h in patients with unknown clock time.

• Primary outcome: mRS 0–1 in 53.3% vs 41.8% (P = 0.02; NNT ≈ 9)
• sICH: 2.0% vs 0.4%
• Requires DWI lesion <1/3 MCA territory and no marked FLAIR signal change

2026 Guideline: Class 2a (B-R) — IVT can be beneficial within 4.5h of symptom recognition if MRI criteria met.

4.5–9 Hours: EXTEND (2019)

The EXTEND trial used perfusion imaging (CTP or MRI PWI-DWI) to identify mismatch up to 9h from onset.

• Primary outcome: mRS 0–1 in 35.4% vs 29.5% (aRR 1.44; P = 0.04)
• sICH: 6.2% vs 0.9%

2026 Guideline: Class 2a (B-R) — IVT may be reasonable in 4.5–9h window OR wake-up stroke within 9h of sleep midpoint with salvageable penumbra on perfusion imaging.

Clinical Pearl: Meta-analysis of EPITHET, ECASS-4, and EXTEND (n=414) confirmed benefit (OR 1.86 for mRS 0–1) despite increased sICH (OR 9.7).

4.5–24 Hours with LVO: TIMELESS, TRACE-III & CHABLIS-T II

TIMELESS (2024)

TIMELESS studied tenecteplase 4.5–24h in LVO patients with perfusion mismatch, with or without EVT.

• Primary outcome: Ordinal mRS — Neutral (aOR 1.13, P = 0.45)
• sICH: 3.2% vs 2.3% (no significant increase)
• Interpretation: In setting of rapid EVT, IVT did not add benefit

TRACE-III (2024)

TRACE-III tested tenecteplase in Chinese LVO patients 4.5–24h, where majority did not receive EVT.

• Primary outcome: mRS 0–1 in 33.0% vs 24.2% (RR 1.37; P = 0.03)
• sICH: 3.0% vs 0.8% (increased)
• Key finding: Benefit present when EVT not available or delayed

CHABLIS-T II (2025)

CHABLIS-T II randomized 224 patients with CTP-based selection in 4.5–24h window; 54.9% underwent EVT.

• Improved recanalization (aOR 2.5; P = 0.002)
• No significant difference in functional outcomes (mRS 0–1: 33% vs 30%) or sICH

2026 Guideline: Class 2b (B-R) — In LVO patients 4.5–24h with salvageable penumbra who cannot receive or will have delayed EVT, IVT may be beneficial when directed by thrombolytic experts. For patients receiving rapid EVT, late-window IVT does not add benefit.

4.5–24 Hours Posterior Circulation: EXPECTS (2025)

The EXPECTS trial evaluated alteplase in posterior circulation strokes 4.5–24h from onset (NIHSS ≥1, PC-ASPECTS ≥7), excluding planned thrombectomy.

• mRS 0–1: 74% vs 61% (improved)
• sICH: 1.7% vs 0.9% (no significant increase)

Clinical Pearl: Safe and effective for posterior circulation stroke when EVT is not planned.

4.5–24 Hours Without LVO: OPTION & HOPE

OPTION (2026)

The OPTION trial assessed tenecteplase 4.5–24h in non-LVO patients with salvageable tissue on CTP (n=566).

• Primary outcome: mRS 0–1 in 43.6% vs 34.2% (RR 1.28; P = 0.02; NNT = 11)
• sICH: 2.8% vs 0% (P = 0.004) — significantly increased
• Reperfusion at 24h: 37.7% vs 28.8% (P = 0.03)
• Mortality: 5.0% vs 3.2% (NS)

HOPE (2025)

The HOPE trial studied tenecteplase 4.5–24h in patients without LVO but with perfusion mismatch (n=372, though 63% had LVO).

• mRS 0–1: 40% vs 26% (improved)
• sICH: 3.8% vs 0.5% (increased)

Clinical Pearl: OPTION and HOPE are the first RCTs showing late-window thrombolysis benefits in non-LVO strokes using tenecteplase and perfusion imaging. Both show improved outcomes but at the cost of increased sICH — careful patient selection is essential.

Conclusion: From Clock to Core

The 2026 guidelines formalize the paradigm shift from clock-based to tissue-based patient selection. Physiologic imaging has extended thrombolysis from a fixed time approach to patient-specific selection—enabling safe and effective treatment well beyond traditional windows in appropriately selected patients.

Extended Window Trials (>4.5h): Summary Table

*TIMELESS reported mRS 0–2
CTP Criteria: OPTION: Core <50 mL, mismatch ≥10 mL, ratio ≥1.2 | HOPE, TRACE-III, TIMELESS, CHABLIS-T II: Core <70 mL, mismatch ≥10 mL, ratio ≥1.2