The brain requires a constant supply of blood, oxygen, and nutrients. In an ischemic stroke, this supply is interrupted, depriving brain tissue. A distinct region, the “penumbra,” describes brain tissue at risk of damage but not yet irreversibly injured. It is a potentially salvageable area that, with timely intervention, may recover function.
The Penumbra in an Ischemic Stroke
An ischemic stroke occurs when a blood clot blocks an artery, cutting off blood flow to a brain region. This blockage creates two distinct areas. The “infarct core” has severely restricted blood flow (below 10-12 mL/100g/min), leading to irreversible damage. Surrounding the core is the “penumbra,” where blood flow is reduced (20-40 mL/100g/min) but not absent. Neurons in the penumbra are functionally impaired and electrically silent, but remain structurally intact for a limited time.
The penumbra’s vulnerability stems from the ischemic cascade. Reduced blood flow compromises ATP production, leading to an energy deficit. This energy failure disrupts ion balance across neuronal membranes, causing depolarization and uncontrolled glutamate release.
Excessive glutamate overstimulates receptors, allowing calcium influx into neurons. This activates destructive enzymes and leads to oxidative stress, contributing to cellular injury and cell death.
Identifying the Penumbra with Medical Imaging
Medical imaging assesses brain damage and identifies salvageable penumbral tissue. Computed Tomography Perfusion (CTP) scans provide insights into cerebral blood flow (CBF), cerebral blood volume (CBV), and mean transit time (MTT). The infarct core shows reduced CBF and CBV. The penumbra is characterized by prolonged MTT and relatively preserved CBV, indicating delayed but present blood flow.
Magnetic Resonance Imaging (MRI) techniques, Diffusion-Weighted Imaging (DWI) and Perfusion-Weighted Imaging (PWI), are also used. DWI identifies the infarct core by detecting restricted water movement in damaged cells. PWI highlights areas of reduced blood perfusion. The “perfusion-diffusion mismatch” is the difference in volume between the smaller DWI lesion (core) and the larger PWI lesion (hypoperfused tissue). This mismatch approximates the ischemic penumbra, representing potentially salvageable brain tissue.
Medical Interventions to Save the Penumbra
Interventions for ischemic stroke restore blood flow to the penumbra. Thrombolysis, using drugs like tissue plasminogen activator (tPA), is a primary treatment. tPA is administered intravenously to dissolve the blood clot and re-establish blood flow. For effectiveness, tPA is administered within a narrow time window, up to 4.5 hours from symptom onset.
For large vessel occlusions, mechanical thrombectomy re-establishes blood flow. This procedure physically removes the blood clot using a catheter guided to the blockage site. Devices like the Penumbra System aspirate or extract the clot. Mechanical thrombectomy can be performed within a broader time window, up to 6 hours, and in selected cases with favorable imaging, even up to 24 hours after symptom onset.
The concept of “time is brain” highlights the urgency of these interventions. Every minute blood flow is interrupted, more penumbral brain cells risk transitioning into the irreversibly damaged infarct core. Rapid penumbra identification through imaging and subsequent reperfusion therapies minimize long-term disability from ischemic stroke. The goal is to salvage penumbral tissue, improving neurological recovery.