An acute ischemic stroke occurs when a blood clot blocks a vessel, cutting off blood flow to a part of the brain. This sudden loss of circulation deprives brain cells of the oxygen and nutrients necessary for survival. Rapid action is paramount, as millions of neurons are lost every minute the brain is starved of blood flow. Prompt intervention is the most important factor in determining a patient’s long-term outcome.
The Role of Mechanical Thrombectomy in Stroke Care
The treatment for a large blood clot causing an ischemic stroke involves physically removing the obstruction. This procedure, known as mechanical thrombectomy, is a specialized, minimally invasive technique performed by neurointerventional surgeons. It is reserved for patients suffering from a large vessel occlusion, where a major artery in the brain is blocked. During the procedure, a catheter is threaded through an artery, usually in the groin, up to the site of the clot.
Specialized devices, such as stent retrievers or aspiration catheters, are deployed to capture and extract the clot, immediately restoring blood flow. This method is used when a clot is too large to be effectively dissolved by standard clot-busting medications, called intravenous thrombolytics. Mechanical removal is also the primary option for patients who cannot receive those medications due to contraindications, such as recent surgery or a high bleeding risk. The goal is to salvage as much brain tissue as possible from irreversible damage.
Defining the Expanded Treatment Window
Historically, the opportunity to perform a mechanical thrombectomy was limited to patients presenting within six hours of the onset of their stroke symptoms. This initial time limit was based on the understanding of how quickly brain tissue dies following the loss of blood supply. Recent scientific evidence has challenged this time-centric approach, resulting in an expanded treatment window extending up to 24 hours after a patient was last known to be well.
This extension is not a blanket recommendation, but applies only to a select group identified through advanced medical imaging. The shift moves away from a strict clock-based protocol to one focused on individual patient physiology. Some patients possess better collateral circulation—smaller vessels that reroute blood flow around the blockage—protecting brain tissue for a longer period. For these patients, the window of opportunity remains open long after the traditional six-hour mark.
This expansion allows a much larger population of stroke victims, including those who suffer a “wake-up stroke” where the exact time of onset is unknown, to be considered for intervention. Successful treatment up to 24 hours after symptom onset represents a major advancement, provided that imaging proves a significant amount of brain tissue is still viable.
Advanced Imaging Criteria for Late-Window Eligibility
The determination of which patients qualify for mechanical thrombectomy in the six-to-twenty-four-hour late window relies entirely on sophisticated brain imaging. Physicians use these advanced techniques to separate the already-dead core of the stroke from the surrounding tissue that is merely at risk, often referred to as the penumbra. The presence of a substantial penumbra indicates that there is still salvageable brain tissue that could recover if blood flow is restored.
Two primary imaging modalities are used to make this distinction: CT Perfusion (CTP) and Diffusion-Weighted Magnetic Resonance Imaging (DWI/MRI). CTP involves injecting a contrast dye to create detailed maps of blood flow, volume, and transit time across the brain, allowing for the quantitative measurement of the ischemic core and the tissue at risk. DWI/MRI provides highly sensitive images of the brain’s water molecules, which quickly reveal areas of irreversible damage.
Eligibility for late-window treatment is established by identifying a significant “mismatch” between the size of the core infarct and the size of the penumbra. For example, the criteria established by the DAWN trial recommend treatment for patients with a small core infarct but a large clinical deficit, suggesting a large area of brain is stunned but not yet dead. The DEFUSE 3 trial used perfusion imaging to define a favorable mismatch ratio, requiring a core infarct volume below a certain threshold and a penumbra volume above a minimum size. These specific, imaging-based protocols, rather than elapsed time, are now the gatekeepers for accessing mechanical thrombectomy in the expanded time frame.