A stroke happens when blood flow to part of the brain is cut off or when a blood vessel in the brain bursts. In both cases, brain cells begin dying within minutes because they lose their oxygen supply. Globally, stroke was the third leading cause of death and disability in 2021, with nearly 12 million new cases that year alone. Understanding what triggers this process can help you recognize your own risk and take steps to lower it.
The Two Main Types of Stroke
About 85% of strokes are ischemic, meaning a blockage stops blood from reaching part of the brain. The remaining 15% are hemorrhagic, caused by a blood vessel that ruptures and bleeds into or around the brain. The causes, risk factors, and warning signs overlap significantly, but the underlying mechanics are different.
How Blockages Cause Ischemic Strokes
An ischemic stroke starts one of two ways. In a thrombotic stroke, a blood clot forms directly inside an artery that supplies the brain. This typically happens at a spot where fatty plaque has already narrowed the vessel wall over years. The clot grows until it chokes off blood flow entirely. A smaller version of this, called a lacunar infarct, occurs in the tiny vessels deep inside the brain and often results from long-term high blood pressure damaging those small arteries.
In an embolic stroke, the clot forms somewhere else in the body, usually the heart or a major artery in the neck, then breaks loose and travels through the bloodstream until it lodges in a narrower brain vessel. The result is the same: a sudden loss of blood flow to the tissue downstream.
Once blood flow stops, brain cells launch a destructive chain reaction. Energy production shuts down almost immediately, calcium floods into cells, and a signaling chemical called glutamate is released in toxic amounts. This cascade damages not just the cells at the center of the blockage but also the surrounding tissue, which is still receiving some blood but not enough. That vulnerable border zone is why speed of treatment matters so much. Restoring blood flow quickly can save tissue that hasn’t yet been permanently damaged.
How Brain Bleeds Cause Hemorrhagic Strokes
When a blood vessel in the brain ruptures, blood spills into the surrounding tissue or into the space between the brain and its protective covering. This pooling blood puts direct pressure on brain cells and also deprives downstream areas of their blood supply. There are two main patterns.
An intracerebral hemorrhage is bleeding within the brain tissue itself. The most common cause is chronic high blood pressure, which weakens small arteries over time until one gives way. This type of hemorrhagic stroke tends to strike deep brain structures and can be devastating.
A subarachnoid hemorrhage is bleeding into the fluid-filled space surrounding the brain. The most common cause is a ruptured brain aneurysm, a balloon-like bulge in an artery wall that may have been growing silently for years. Less commonly, a tangle of abnormally formed blood vessels called an arteriovenous malformation can rupture. Severe head injuries can also cause this type of bleeding.
High Blood Pressure: The Leading Risk Factor
High blood pressure is the single biggest contributor to stroke risk for both ischemic and hemorrhagic types. It damages artery walls through constant mechanical stress, accelerating plaque buildup and weakening vessel walls at the same time. Current guidelines define stage 1 hypertension as a systolic reading of 130 to 139 or a diastolic reading of 80 to 89. Stage 2 starts at 140/90.
Keeping systolic blood pressure below 130 reduces stroke risk compared to less aggressive targets. Large clinical trials have even shown additional benefit from pushing the target below 120 in high-risk individuals. Most people who need blood pressure medication end up requiring two or three drugs to hit their goal, since a single medication gets only about 30% of people to target on its own.
How Atrial Fibrillation Triggers Clots
Atrial fibrillation, an irregular and often rapid heart rhythm, is one of the most potent causes of embolic stroke. When the upper chambers of the heart quiver instead of contracting in a coordinated way, blood pools and stagnates. That stagnant blood is far more likely to clot. If a clot forms and escapes into the bloodstream, it can travel directly to the brain. People with atrial fibrillation face a stroke risk three to five times higher than people without it, even after accounting for other risk factors.
Smoking, Diabetes, and Cholesterol
Cigarette smoke exposes the bloodstream to more than 7,000 chemicals that collectively damage the inner lining of blood vessels. This damage triggers inflammation, promotes plaque formation, reduces the blood vessels’ ability to relax, and shifts the blood into a state that clots more easily. Every one of those effects pushes stroke risk upward. The damage is dose-dependent, meaning the more you smoke and the longer you smoke, the higher your risk climbs.
Diabetes accelerates the same destructive process. Chronically elevated blood sugar injures vessel walls, promotes inflammation, and speeds up atherosclerosis throughout the body, including the arteries feeding the brain.
High cholesterol, specifically high LDL cholesterol, fuels the fatty plaques that narrow arteries and set the stage for thrombotic and embolic strokes. Statin medications reduce the risk of a first stroke by roughly 19 to 22% in people who qualify for cholesterol-lowering treatment and have elevated cardiovascular risk.
Stroke in Younger Adults
When someone under 50 has a stroke, the cause is often different from the typical plaque-and-clot scenario. One surprisingly common finding is a patent foramen ovale (PFO), a small hole between the upper chambers of the heart that was supposed to close after birth but never did. In one large study of young adults with unexplained ischemic strokes, 37.5% had a PFO. The hole can allow a clot from the venous system to cross into the arterial side and reach the brain.
Other causes that disproportionately affect younger people include a tear in the wall of a neck artery (arterial dissection), which can happen after trauma or even sudden neck movements, and blood-clotting disorders that make clots form too easily. Autoimmune diseases, inflammatory bowel disease, chronic kidney disease, and certain blood cancers also raise stroke risk in younger populations. In fact, among young stroke patients with a PFO, these nontraditional risk factors were more strongly associated with stroke than classic ones like high blood pressure and smoking.
Warning Signs That Precede a Stroke
A transient ischemic attack, often called a “mini-stroke,” produces the same symptoms as a full stroke (sudden numbness, confusion, trouble speaking, vision loss, severe headache) but resolves within minutes to hours. A TIA is not harmless. Some people who have a TIA will go on to have a full stroke within three months, and half of those strokes happen within 48 hours. A TIA is a medical emergency and a clear signal that the underlying cause needs to be identified and treated immediately.
How Multiple Risk Factors Compound
Stroke rarely has a single isolated cause. High blood pressure weakens arteries while high cholesterol fills them with plaque. Smoking inflames vessel walls while diabetes impairs their ability to heal. Atrial fibrillation generates clots while narrowed arteries make it easier for those clots to get stuck. Each risk factor amplifies the others, which is why prevention guidelines focus on managing all of them simultaneously rather than treating any one in isolation. The most effective approach is controlling blood pressure, managing cholesterol, staying physically active, not smoking, and treating heart rhythm problems if they exist.