A stroke happens when blood flow to part of the brain is cut off or when a blood vessel in the brain bursts. In either case, brain cells begin dying within minutes, losing roughly 1.9 million neurons for every minute the stroke goes untreated. The causes range from blood clots and plaque buildup in arteries to uncontrolled high blood pressure and heart rhythm problems.
Two Types, Two Mechanisms
About 87% of strokes are ischemic, meaning a clot blocks blood flow to the brain. The remaining strokes are hemorrhagic, caused by a blood vessel that ruptures and bleeds into or around the brain. The causes behind each type are different, so understanding which kind of stroke occurred shapes everything that follows.
How Blood Clots Cause Ischemic Strokes
An ischemic stroke begins when a clot forms in or travels to one of the arteries supplying the brain. The clot itself is a meshwork of platelets and fibrin, a protein that acts like scaffolding. An enzyme called thrombin triggers both the activation of platelets and the conversion of a soluble blood protein into sticky fibrin strands. Those strands bind to platelets, creating a dense plug that can seal off an artery entirely.
Clots reach the brain through two main routes. In some cases, a clot forms directly inside a narrowed brain artery. In others, a clot forms somewhere else in the body, breaks loose, and travels through the bloodstream until it lodges in a smaller vessel in the brain. This second type, called an embolism, is especially common in people with certain heart conditions.
Plaque Buildup in the Arteries
Atherosclerosis, the gradual buildup of fatty deposits inside artery walls, is one of the most common underlying causes of stroke. The carotid arteries, two large blood vessels on either side of your neck that supply the brain, are particularly vulnerable. Over years, a mixture of fat, cholesterol, calcium, and other substances accumulates into plaques that narrow these arteries and restrict blood flow.
The danger isn’t just the narrowing itself. A plaque can become unstable and rupture, sending fragments or small clots downstream into the brain’s smaller arteries where they get stuck. Even a piece of plaque no larger than a grain of sand can block a vessel that feeds a critical area of brain tissue.
High Blood Pressure
Chronic high blood pressure is the single largest modifiable risk factor for both ischemic and hemorrhagic stroke. Current guidelines define stage 1 hypertension as a systolic reading of 130 to 139 or a diastolic reading of 80 to 89. Stage 2 hypertension starts at 140/90 or higher. The relationship between blood pressure and stroke risk is straightforward: for every 20-point increase in systolic pressure, the risk of cardiovascular events roughly doubles.
High blood pressure damages arteries in two ways. It accelerates the atherosclerosis process described above, and it weakens the walls of smaller blood vessels in the brain over time. Those weakened vessels can eventually rupture, causing a hemorrhagic stroke. This is why uncontrolled hypertension is the leading cause of bleeding inside the brain.
Atrial Fibrillation and Heart-Related Causes
Atrial fibrillation, an irregular and often rapid heart rhythm, increases ischemic stroke risk fivefold. When the upper chambers of the heart quiver instead of contracting effectively, blood pools and moves sluggishly, creating ideal conditions for clots to form. Those clots can then travel directly to the brain.
A less well-known heart-related cause involves a small hole between the upper chambers of the heart called a patent foramen ovale, or PFO. Everyone has this opening before birth, but in about one in four adults, it never fully closes. Normally, clots that form in veins get filtered out by the lungs before reaching the brain. A PFO creates a shortcut that lets a clot bypass the lungs and enter the brain’s arterial circulation directly. The average PFO is about 10 mm wide, more than large enough for a clot to pass through and block the middle cerebral artery, which is only about 3 mm in diameter. PFOs are found in 40% to 50% of younger patients whose strokes have no other obvious explanation.
Hemorrhagic Stroke Causes
When a blood vessel in the brain ruptures, the resulting bleed damages tissue both by depriving downstream areas of blood and by the pressure of pooling blood compressing surrounding brain structures. The most common cause is chronic high blood pressure weakening small arteries deep inside the brain.
Another major cause is a brain aneurysm, a bulge in a weakened section of a blood vessel wall. Aneurysms can exist for years without symptoms, but when one bursts, it causes a subarachnoid hemorrhage, bleeding into the space between the brain and its protective covering. This is the most common cause of that particular type of brain bleed. A less common cause is an arteriovenous malformation, an abnormal tangle of blood vessels in the brain that can rupture under pressure.
Lifestyle Risk Factors
Smoking significantly raises stroke risk by damaging blood vessel walls, promoting plaque formation, and making blood more likely to clot. The good news is that the damage reverses quickly after quitting. Within the first two years, stroke risk drops by about 22% compared to continuing smokers. By two to four years after quitting, stroke risk nearly matches that of someone who never smoked, with almost 90% of the full benefit of cessation achieved in that window.
Other lifestyle factors that contribute to stroke include physical inactivity, heavy alcohol use, obesity, and diets high in sodium and saturated fat. These factors often work together, each one worsening the others. Excess weight raises blood pressure. Inactivity promotes plaque buildup. Heavy drinking can trigger atrial fibrillation. Addressing even one of these reduces overall risk, but the greatest protection comes from managing several at once.
Diabetes and High Cholesterol
Diabetes damages blood vessels throughout the body, including those in the brain. Consistently elevated blood sugar accelerates atherosclerosis and makes blood vessels stiffer and more prone to injury. People with type 2 diabetes face roughly double the stroke risk of those without it, partly because diabetes rarely exists in isolation. It tends to travel alongside high blood pressure, high cholesterol, and obesity.
High LDL cholesterol contributes directly to plaque formation in the arteries. The cholesterol deposits that narrow the carotid arteries are largely composed of LDL particles that have worked their way into the artery wall and triggered an inflammatory response. Over time, this creates the unstable plaques that can rupture and send clots to the brain.
Genetic and Hereditary Causes
Most strokes result from a combination of lifestyle and aging, but some people carry a genetic predisposition. CADASIL is the best-known hereditary stroke condition. It’s caused by a mutation in the Notch3 gene, which leads to thickening of the walls of small blood vessels in the brain. Over time, this progressively blocks blood flow and causes multiple small strokes, often progressing to dementia. CADASIL can be inherited from a parent or, less commonly, arise from a spontaneous mutation. It’s confirmed through a blood test that looks for the specific genetic change.
Beyond rare single-gene conditions, family history of stroke increases your own risk even when no specific mutation is identified. Shared genetics influence blood pressure tendencies, cholesterol metabolism, clotting patterns, and the structural integrity of blood vessel walls.
Warning Signs: Transient Ischemic Attacks
A transient ischemic attack, often called a mini-stroke, produces stroke-like symptoms that resolve on their own, usually within minutes to an hour. TIAs happen when a clot temporarily blocks blood flow to the brain before dissolving or dislodging. They cause no permanent damage, but they are a serious warning. About 1 in 100 people who have a TIA will experience a full stroke within 90 days, and the highest risk period is the first few days.
The causes of TIAs are identical to those of ischemic strokes: arterial plaque, blood clots from atrial fibrillation, and the other mechanisms described above. The difference is simply that the blockage clears before brain tissue dies. A TIA is an opportunity to identify and treat the underlying cause before a disabling stroke occurs.
Why Speed Matters
Regardless of the cause, every stroke is a medical emergency. Research published in the journal Stroke quantified the damage: during a large vessel ischemic stroke, the brain loses 1.9 million neurons, 13.8 billion synapses, and 7.5 miles of nerve fibers every minute treatment is delayed. That rate of destruction is why recognizing stroke symptoms and calling emergency services immediately can mean the difference between full recovery and permanent disability.