Ischemic Stroke | What Happens When Blood Flow to the Brain is Blocked?

An ischemic stroke occurs when the blood supply to part of the brain is interrupted or reduced, preventing brain tissue from getting oxygen and nutrients. Brain cells begin to die in minutes. This condition is broadly categorized into two main types: thrombotic and embolic strokes. A cerebral thrombosis, or a thrombotic stroke, happens when a blood clot, known as a thrombus, forms in one of the arteries that supply blood to the brain. This clot usually forms in arteries damaged by atherosclerosis, a condition where fatty deposits cause the arteries to narrow and harden over time. The clot blocks blood flow, leading to a stroke. On the other hand, a cerebral embolism, or an embolic stroke, occurs when a blood clot forms elsewhere in the body—often in the heart or large arteries of the upper chest and neck—and travels through the bloodstream to lodge in a narrower brain artery. This wandering clot is called an embolus. Both types result in the same critical outcome: a sudden deprivation of blood and oxygen to a specific area of the brain, causing neuronal cell death and subsequent neurological deficits. Understanding the origin of the clot is crucial for determining the appropriate secondary prevention strategies to avoid recurrence.

The brain, despite accounting for only about 2% of the body's weight, consumes approximately 20% of the oxygen and glucose carried in the blood. This high metabolic demand means it is exceptionally vulnerable to disruptions in blood supply. Neurons require a constant, uninterrupted flow of oxygenated blood to function and survive. When this supply is blocked during an ischemic stroke, the affected brain cells are starved of oxygen and essential nutrients. Within minutes, a cascade of harmful cellular events, known as the ischemic cascade, is triggered. This process leads to energy failure within the cells, the release of toxic chemicals, and ultimately, irreversible cell death, or infarction. The specific functions that are lost—such as speech, movement, or memory—depend directly on which part of the brain has been damaged. Therefore, the vascular network of the brain is a critical infrastructure, and any occlusion within it constitutes a medical emergency where every second counts to restore flow and salvage brain tissue.

The symptoms of an ischemic stroke appear suddenly and can vary widely depending on the affected brain region. The most common and recognizable signs are encapsulated by the FAST acronym: Face drooping (one side of the face may droop or feel numb), Arm weakness (one arm may feel weak or numb, and when raised, it may drift downwards), Speech difficulty (speech may be slurred, or the person may be unable to speak or understand speech), and Time to call emergency services. Other sudden symptoms include confusion, trouble seeing in one or both eyes, difficulty walking, dizziness, or a severe headache with no known cause. These signs are neurological deficits corresponding to the functional area of the brain that is suffering from ischemia.

The concept "time is brain" is central to stroke treatment. During an ischemic stroke, millions of neurons die every minute that blood flow is not restored. The area of dead cells, the infarct core, is typically surrounded by a region of potentially salvageable tissue called the penumbra. This tissue is at high risk but can be saved if blood flow is restored quickly. Medical treatments, such as thrombolytic therapy (using drugs to dissolve the clot) or mechanical thrombectomy (physically removing the clot), are most effective when administered within a narrow time window, typically within a few hours of symptom onset. Delaying treatment increases the extent of irreversible brain damage, leading to greater disability and a lower chance of recovery.

Several risk factors increase the likelihood of an ischemic stroke, many of which are manageable. The most significant is hypertension (high blood pressure), as it can damage and narrow the arteries. Other major medical risk factors include atrial fibrillation (an irregular heartbeat that can cause clots to form in the heart), high cholesterol (which contributes to atherosclerosis), and diabetes mellitus (which damages blood vessels throughout the body). Lifestyle factors also play a critical role. These include tobacco smoking, which accelerates clot formation and atherosclerosis; a sedentary lifestyle; and a diet high in saturated fats and sodium. Age is also a primary non-modifiable risk factor, with the risk doubling each decade after age 55. A family history of stroke can also increase an individual's risk. Identifying and managing these risk factors is the most effective way to prevent a stroke from occurring.