An ischemic stroke occurs when a blockage, often a blood clot, reduces blood supply to a part of the brain. This prevents brain tissue from getting the oxygen and nutrients it needs to function. The brain requires a constant supply of oxygen, and when that flow is interrupted, brain cells can begin to die within minutes. A stroke is a medical emergency where rapid treatment is necessary to prevent lasting brain damage or death.
The Ischemic Core
At the center of a stroke’s impact is the ischemic core, the region where the blockage has caused the most severe reduction in blood flow. Blood flow within the core often falls below 20% of the normal level.
This severe lack of oxygen and nutrients triggers a rapid energy failure at the cellular level, leading to cell death within minutes. The damage within the ischemic core is considered irreversible because the tissue has infarcted, or died. The primary goal of stroke treatment is to prevent this damage from spreading to the surrounding tissue.
The Penumbra
Surrounding the ischemic core is a region of at-risk brain tissue called the penumbra. This area suffers from reduced blood flow but not as severely as the core. The penumbra is a transitional zone between the dead tissue and healthy brain tissue.
Cells in this zone receive just enough blood to stay alive, often from smaller, collateral blood vessels. While this blood flow is too low for neurons to function, which causes stroke symptoms, it is enough to prevent immediate cell death.
This tissue is salvageable if blood flow can be restored promptly. Without treatment, the penumbra will progressively die and become part of the expanding ischemic core, making the brain damage more extensive.
Identifying Damage with Brain Imaging
Advanced medical imaging allows physicians to visualize both the ischemic core and the penumbra. These techniques provide functional information about the brain tissue, not just its structure. Magnetic resonance imaging (MRI) is one of the most powerful tools for this purpose.
A specific type of MRI, Diffusion-Weighted Imaging (DWI), is highly sensitive in detecting the ischemic core by measuring the restricted movement of water molecules in dying cells.
To visualize the penumbra, doctors use Perfusion Imaging with either a CT or MRI scanner. This technique tracks a contrast agent’s flow through the brain, showing the entire area of reduced blood flow. By comparing the DWI image (core) with the perfusion image (core + penumbra), doctors can identify the “mismatch” area, which represents the salvageable penumbra.
The Therapeutic Window for Treatment
The salvageable penumbra creates a “therapeutic window,” a period to administer treatments that restore blood flow. This window highlights the concept of “time is brain,” as every minute that passes leads to more brain cell loss when the penumbra becomes part of the core. The goal of acute stroke therapy is to intervene within this window to rescue the penumbra.
Two primary treatments are used to reopen the blocked artery. The first is thrombolysis, which uses clot-busting drugs like tissue plasminogen activator (tPA). Administered intravenously, tPA dissolves the clot and is most effective within a few hours of the first symptoms.
The second treatment is mechanical thrombectomy, often used for large clots in major arteries. A catheter is guided to the blockage, where a device physically removes the clot. This procedure can be performed up to 24 hours after a stroke in select patients who have a significant area of salvageable penumbra identified by imaging.
Restoring blood flow allows oxygen to reach the penumbra’s stunned cells, halting their death. A patient with a large penumbra and a small core has a greater potential for a good recovery if the blockage is cleared quickly, which is why recognizing stroke symptoms and seeking immediate care is so important.