An ischemic stroke happens when a blood clot or other blockage cuts off blood flow to part of the brain, starving brain cells of oxygen. About 87% of all strokes are ischemic, making it by far the most common type. The blockage can form inside the brain’s own blood vessels, or it can travel there from somewhere else in the body, usually the heart or the large arteries in the neck.
Plaque Buildup in the Arteries
The most well-understood cause of ischemic stroke is atherosclerosis, a slow process where cholesterol deposits build up inside artery walls. It starts when the inner lining of an artery becomes damaged, often from years of high blood pressure, smoking, or elevated cholesterol. Cholesterol particles seep into the damaged wall, and immune cells rush in to clean them up. Those immune cells absorb so much cholesterol they become bloated “foam cells,” forming the core of a growing fatty deposit called a plaque.
Over time, the body tries to wall off this plaque with a cap of tough connective tissue. But ongoing inflammation can thin and weaken that cap, making the plaque unstable. When the cap ruptures, the contents of the plaque are suddenly exposed to the bloodstream, triggering a blood clot that can grow large enough to seal off the artery entirely. Alternatively, part of the clot can break free and travel deeper into the brain’s smaller vessels, blocking blood flow further downstream. This process commonly occurs in the carotid arteries in the neck or in the large arteries at the base of the brain.
High cholesterol is a direct driver of this process. People with total cholesterol in the highest range (averaging around 290 mg/dL) have about 1.6 times the risk of ischemic stroke compared to those with the lowest levels. For strokes specifically caused by this plaque-rupture mechanism, the risk jumps to roughly three times higher.
Blood Clots From the Heart
In many ischemic strokes, the clot doesn’t form in the brain at all. It forms in the heart, then breaks loose and travels through the bloodstream until it lodges in a brain artery. This is called a cardioembolic stroke, and the most common heart condition behind it is atrial fibrillation (AFib), a type of irregular heartbeat.
During AFib, the upper chambers of the heart quiver instead of contracting fully. Blood pools in these chambers rather than flowing through efficiently, and stagnant blood is far more likely to clot. When a clot eventually breaks free, it has a direct path to the brain through the carotid arteries. AFib also causes long-term structural damage to the heart’s upper chambers, which further increases the tendency toward clot formation even during periods when the rhythm seems normal.
Other heart conditions can have the same effect. After a heart attack, damaged sections of the heart wall may not move properly, creating pockets where blood stagnates and clots. Heart failure creates a similar environment of sluggish blood flow combined with a heightened tendency to clot. About 25% of the general population has a patent foramen ovale (PFO), a small opening between the upper chambers of the heart that normally closes after birth. In some people, this opening persists and can allow a clot that formed in the veins to cross directly into the arterial system and reach the brain.
Small Vessel Disease Deep in the Brain
Not all ischemic strokes involve large arteries or clots from the heart. Lacunar strokes occur when tiny blood vessels deep inside the brain become blocked. These small vessels supply critical structures involved in movement, sensation, and coordination, so even a small blockage can cause noticeable symptoms.
High blood pressure is the single most important risk factor for lacunar strokes. Years of elevated pressure damages the walls of these small arteries, causing them to stiffen, narrow, and eventually close off. High cholesterol and clotting disorders also contribute. These strokes tend to be smaller than other types, but they can still cause lasting disability, and having one raises the risk of having more.
How High Blood Pressure Drives Risk
High blood pressure shows up as a risk factor in nearly every category of ischemic stroke, and the longer you have it, the worse the odds get. A large study published in the American Heart Association’s journal Stroke tracked how the duration of hypertension affected stroke risk over time. Compared to people with normal blood pressure, those who had been diagnosed with hypertension for five years or fewer had a 33% higher risk of ischemic stroke. For those with six to twenty years of hypertension, the risk was 55% higher. And for those living with it for more than two decades, the risk climbed to 80% higher.
This pattern reflects the cumulative damage that elevated pressure does to blood vessel walls throughout the body. It accelerates atherosclerosis in large arteries, weakens the tiny vessels deep in the brain, and promotes structural changes in the heart that encourage clot formation. Managing blood pressure is the single most impactful thing most people can do to reduce stroke risk.
Artery Dissection in Younger Adults
Ischemic stroke in people under 50 often has a different cause entirely. Carotid artery dissection, a tear in the inner wall of the carotid artery in the neck, is the leading cause of ischemic stroke in young and middle-aged adults. The tear creates a pocket where blood collects between the layers of the artery wall, narrowing the vessel and triggering clot formation.
Overall, carotid dissection accounts for only about 2% of all strokes, but it’s responsible for 10% to 25% of ischemic strokes in people under 50. It can happen after even minor trauma to the neck, such as a sports injury, a car accident, or vigorous chiropractic manipulation. In some cases, it occurs spontaneously, particularly in people with underlying connective tissue conditions that make artery walls more fragile.
Blood Disorders and Clotting Conditions
Some people are at higher risk of ischemic stroke because of conditions that make their blood more likely to clot or more difficult to push through small vessels. Sickle cell disease is one of the most significant. In sickle cell disease, red blood cells become rigid and crescent-shaped instead of round and flexible. These misshapen cells can get stuck in blood vessels supplying the brain, blocking flow and causing a stroke. Children with sickle cell disease are at particularly high risk.
Inherited clotting disorders (sometimes grouped under the term thrombophilia) increase the tendency to form blood clots throughout the body, including in the brain’s blood vessels. Certain autoimmune conditions can have a similar effect by creating antibodies that promote clotting. These causes are relatively uncommon but are especially important to identify in younger stroke patients who lack the typical risk factors.
Strokes With No Clear Cause
Even after a full workup, roughly 17% of ischemic strokes have no identifiable source, a category sometimes called cryptogenic stroke. This doesn’t mean there’s no cause. It means the standard tests didn’t catch it.
The most common hidden culprit is undetected atrial fibrillation. AFib can come and go in brief episodes lasting only minutes, easily missed on a standard heart monitor. When researchers extended heart monitoring well beyond the typical 24-hour window, they found previously unrecognized AFib in 10% to 20% of cryptogenic stroke patients. A PFO is another frequent finding, present in 25% to 58% of these patients depending on the study. Other suspected causes include non-narrowing plaques in the carotid arteries that don’t show up as severe on standard imaging, dysfunction of the heart’s upper chambers even without AFib, and in some cases, undiagnosed cancer that increases the blood’s tendency to clot.
Why Speed of Treatment Matters
Because an ischemic stroke is caused by a blockage, treatment centers on restoring blood flow as fast as possible. Brain tissue dies quickly without oxygen, so the time between symptom onset and treatment directly determines how much damage occurs.
Clot-dissolving medication can be given intravenously within 4.5 hours of symptom onset. For strokes caused by a large clot in one of the brain’s major arteries, a procedure to physically remove the clot can be performed within 6 hours, and in carefully selected patients, up to 24 hours after symptoms begin. For clots in the artery at the back of the brain (the basilar artery), removal is recommended within 24 hours. These time windows are strict, which is why recognizing stroke symptoms, such as sudden face drooping, arm weakness, or speech difficulty, and calling emergency services immediately is critical.