Role of DWI-FLAIR mismatch

In wake-up strokes—where symptom onset is unknown—a DWI-FLAIR mismatch serves as a tissue clock:

• DWI lights up within minutes due to cytotoxic edema

• FLAIR remains dark for ~6 hours (requiring vasogenic edema to develop).

A positive mismatch implies the stroke occurred ≤6 hours prior.

Per the WAKE-UP trial, these patients safely benefit from IV alteplase with better functional outcomes versus placebo, effectively extending the thrombolysis window to 4.5–9 hours from "last seen well"—replacing the strict time clock with biology.