The maximum time for endovascular therapy (EVT) for acute ischemic stroke has dramatically expanded, moving from a narrow window to a broader, individualized approach. Acute ischemic stroke occurs when a blood clot blocks a blood vessel in the brain, leading to a sudden loss of blood flow. EVT, also known as mechanical thrombectomy, is a specialized procedure where a catheter is threaded through the blood vessels to physically remove the clot and restore blood flow. The success of the treatment is highly dependent on speed, a concept often summarized by the phrase “time is brain.” Determining when the stroke began is the single most influential factor in patient eligibility and outcome.
Understanding Last Known Normal
The starting point for the stroke treatment clock is the Last Known Normal (LKN) time. LKN is defined as the last moment the patient was observed without any signs of stroke symptoms and was at their baseline state of health. This precise time is the baseline clinicians use to calculate the time elapsed since the brain tissue first became deprived of oxygen.
Accurately determining the LKN is a fundamental challenge in stroke care because the onset of symptoms is often unwitnessed. A significant portion of stroke cases are “wake-up strokes,” where the patient wakes up with symptoms. In these cases, the LKN is the time they went to sleep the night before, which may be many hours earlier. The LKN serves as the critical reference point for all subsequent decisions regarding time-dependent therapies, including the maximum time window for mechanical thrombectomy.
The Initial 6-Hour Treatment Window
The first and most time-sensitive period for Endovascular Therapy is the initial 6-hour window from the Last Known Normal. Treatment within this timeframe is strongly recommended for patients who have an acute ischemic stroke caused by a Large Vessel Occlusion (LVO) in the anterior circulation. An LVO refers to a blockage in a major artery, such as the internal carotid artery or the first segment of the middle cerebral artery.
Patients presenting within this early window are typically eligible for immediate intervention if they meet basic imaging criteria, often assessed with a non-contrast CT scan and a CT angiogram. Treatment during these first six hours offers the highest probability of a positive outcome because the core area of irreversibly damaged brain tissue is generally at its smallest. For these urgent cases, advanced imaging studies, like perfusion scans, are often bypassed to avoid any delay, reinforcing the principle that faster reperfusion is associated with better patient recovery.
Extending Eligibility Up to 24 Hours
For select patients, Endovascular Therapy eligibility can be extended significantly, up to 24 hours from the Last Known Normal time. This maximum time window is not standard for all patients but relies entirely on favorable findings from advanced cerebral imaging. The ability to treat up to 24 hours is based on landmark clinical trials, specifically the DAWN and DEFUSE 3 studies, which proved the benefit of treating patients with specific tissue profiles.
This extended eligibility depends on identifying the “penumbra,” which is the area of brain tissue at risk of damage but is not yet irreversibly injured. Clinicians look for a “mismatch” between the small, already-damaged core infarct and a large surrounding area of salvageable tissue. Advanced imaging techniques like CT perfusion or MRI are used to map this mismatch, confirming the presence of substantial brain tissue that can still be saved by removing the clot. The presence of this favorable imaging profile dictates the maximum time for treatment in the extended window.
Patient Selection Criteria Beyond Time Limits
While the 24-hour time window represents the maximum time for EVT, time is only one of several factors determining a patient’s eligibility. The physical location and size of the clot must be in a major vessel, constituting a Large Vessel Occlusion, which is a prerequisite for mechanical clot removal. The severity of the patient’s symptoms, measured by the National Institutes of Health Stroke Scale (NIHSS), must also meet a certain threshold, indicating a disabling stroke.
Furthermore, the size of the irreversibly damaged area, or the core infarct volume, is a significant determinant. If the imaging reveals that a large portion of the brain tissue is already dead, the potential benefit of EVT is substantially diminished, regardless of the time elapsed. The criteria established by the DAWN trial, for instance, use a combination of the patient’s age, stroke severity (NIHSS score), and the core infarct volume to select patients who will most likely benefit from the procedure. These non-time-based criteria ensure that therapy is focused on patients whose brain tissue still has a realistic chance of recovery.