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 from lack of oxygen. The specific cause depends on the type of stroke, but most fall into two categories: a blockage (ischemic stroke) or a bleed (hemorrhagic stroke). Ischemic strokes account for roughly 87% of all strokes.
How Blocked Blood Vessels Cause Ischemic Strokes
Ischemic strokes happen when something blocks an artery feeding the brain. The blockage is almost always a blood clot, but how that clot forms and where it comes from varies. The most common scenario involves atherosclerosis, a slow buildup of fatty deposits (plaque) inside artery walls. Over years, plaque narrows the artery and creates a rough surface where blood clots form easily. This can cause a stroke in several ways.
Sometimes a clot forms directly at the narrowed spot and grows large enough to shut off blood flow entirely. Other times, a piece of plaque or clot breaks loose, travels through the bloodstream, and lodges in a smaller artery deeper in the brain. This is called an embolism, and it’s one of the most common stroke mechanisms. In a third pattern, a severely narrowed artery doesn’t fully block but reduces blood flow so much that the most distant, vulnerable areas of the brain stop getting enough oxygen. These “borderzone” strokes tend to produce a distinct pattern of damage in the regions between two arterial territories.
How Heart Problems Send Clots to the Brain
Not all clots originate in the arteries near the brain. Many form in the heart itself and then travel upward. Atrial fibrillation (AFib), an irregular heart rhythm affecting millions of people, is the most significant cardiac cause. When the heart’s upper chambers quiver instead of contracting properly, blood pools and moves sluggishly, especially in a small pouch called the left atrial appendage. That stagnant blood is prone to clotting. The chamber walls also undergo structural changes over time, becoming stretched and damaged in ways that further promote clot formation. If a clot escapes the heart, it can reach the brain in seconds.
AFib-related strokes tend to be larger and more disabling than other ischemic strokes because the clots that form in the heart are often bigger than those produced by narrowed neck arteries. Other heart conditions that raise stroke risk include heart valve disease, recent heart attacks (which can damage the wall and create a surface for clots), and structural heart defects like a patent foramen ovale, a small hole between the heart’s upper chambers that can allow a clot from the venous system to cross into arterial circulation and reach the brain.
What Causes a Brain Bleed
Hemorrhagic strokes are less common but more deadly. They happen when a weakened blood vessel in the brain ruptures and spills blood into surrounding tissue. The most frequent cause is chronic high blood pressure. Years of elevated pressure damages small arteries deep inside the brain, making their walls fragile and prone to bursting.
Two structural abnormalities also cause hemorrhagic strokes. An aneurysm is a balloon-like bulge in an artery wall. Most people with brain aneurysms never know they have one, but if it ruptures, the sudden bleed is a medical emergency. Arteriovenous malformations (AVMs) are tangles of abnormal blood vessels, usually present from birth, where arteries connect directly to veins without the normal network of tiny capillaries in between. This puts extreme pressure on vessel walls, making them thin and weak. The risk of an AVM bleeding is about 2% to 3% per year. AVMs account for roughly 2% of all hemorrhagic strokes.
Risk Factors That Set the Stage
Most strokes don’t come out of nowhere. They’re the end result of risk factors that build over years. High blood pressure is the single biggest contributor, involved in nearly half of all strokes. It damages artery walls on both sides of the equation: promoting plaque buildup that leads to blockages and weakening small vessels until they burst.
Diabetes accelerates atherosclerosis and makes blood more prone to clotting. High cholesterol feeds plaque growth. Smoking damages the lining of blood vessels and makes blood stickier. Obesity, physical inactivity, and heavy alcohol use all compound these effects. Each of these factors is significant on its own, but they rarely exist in isolation. Someone with high blood pressure, diabetes, and a smoking habit faces a dramatically higher combined risk than any single factor would suggest.
Age is the strongest non-modifiable risk factor. Stroke risk roughly doubles every decade after age 55. Family history matters too, partly through shared genetics and partly through shared lifestyle patterns. Black Americans have nearly twice the stroke risk of white Americans, driven by higher rates of high blood pressure, diabetes, and sickle cell disease, along with disparities in healthcare access.
Strokes in Younger Adults
When strokes happen in people under 45, the usual suspects like atherosclerosis and AFib are less likely to be the cause. Instead, doctors look for conditions like clotting disorders (inherited tendencies for the blood to clot too easily), arterial dissections (a tear in the wall of a neck artery, sometimes triggered by trauma or even aggressive chiropractic manipulation), and heart defects that allow clots to cross from one side of the heart to the other.
Even after thorough testing, about 25% to 30% of strokes in younger adults have no identifiable cause. These are called cryptogenic strokes. Some may be linked to undetected brief episodes of AFib, undiagnosed clotting disorders, or structural heart defects too subtle to catch on initial imaging. Cryptogenic strokes can also occur in people with no known risk factors at all.
Environmental Triggers
Beyond the body’s internal risks, environmental factors play a measurable role. Air pollution raises stroke risk, particularly fine particulate matter that enters the bloodstream through the lungs and promotes inflammation and clotting. Extreme heat compounds these effects by straining the cardiovascular system while also worsening air quality. People with existing risk factors are especially vulnerable during heat waves and high-pollution days.
Warning Signs That a Stroke Is Coming
A transient ischemic attack (TIA), sometimes called a mini-stroke, is a temporary blockage that resolves on its own, usually within minutes. The symptoms are identical to a full stroke: sudden numbness, confusion, trouble speaking, vision changes, or loss of balance. The difference is that a TIA doesn’t cause permanent brain damage. But it’s a powerful warning. About 20% of people who have a TIA will have a full stroke within three months, and most of that risk is concentrated in the first few days.
For people at highest risk based on their age, blood pressure, symptoms, and how long the TIA lasted, the chance of a stroke within just 48 hours can be as high as 8%.
Recognizing a Stroke in Progress
The BE-FAST checklist captures the most recognizable stroke symptoms:
- Balance: sudden loss of coordination or trouble walking
- Eyes: sudden blurred or double vision, or vision loss in one or both eyes
- Face: one side of the face droops when trying to smile
- Arm: one arm drifts downward when both are raised
- Speech: slurred or garbled words, or inability to speak
- Time: call emergency services immediately
The original FAST checklist (face, arm, speech, time) misses about 14% of strokes. Adding the balance and eyes criteria cuts that miss rate to roughly 4%. Gait problems and visual disturbances are the most common symptoms in strokes that FAST alone fails to catch, appearing in about 70% of those missed cases. Every minute without treatment, roughly 1.9 million neurons die, which is why speed matters more than anything else once symptoms appear.