Strokes happen when blood flow to part of the brain gets cut off, either by a blockage or by a burst blood vessel. Brain cells start dying within minutes once they lose their oxygen supply. The specific causes range from common conditions like high blood pressure and irregular heart rhythms to less obvious factors like inherited blood vessel disorders and small holes in the heart.
Two Types, Two Different Mechanisms
Most strokes fall into one of two categories based on what physically happens inside the brain. Ischemic strokes, which account for the vast majority of cases, occur when an artery supplying the brain gets clogged. A clot or piece of debris lodges in a blood vessel, blocks flow, and the brain tissue downstream begins to die from oxygen starvation.
Hemorrhagic strokes work differently. A blood vessel in or around the brain ruptures and bleeds. The bleeding itself damages surrounding tissue, but the real danger comes from the pressure that builds as blood pools in a space not designed to hold it. That pressure compresses and irritates nearby brain cells, causing damage that can spread well beyond the initial bleed site.
High Blood Pressure: The Leading Cause
Chronically elevated blood pressure damages artery walls throughout the body, but the small, delicate vessels in the brain are especially vulnerable. The longer you live with high blood pressure, the greater the danger. A large study tracked by the American Heart Association found that people with hypertension for up to five years were 31% more likely to have a stroke than people with normal blood pressure. Those who had it for six to 20 years faced a 50% higher risk, and people who lived with high blood pressure for more than two decades had a 67% greater risk.
Current guidelines define stage 1 hypertension as a systolic reading (the top number) between 130 and 139, or a diastolic reading (the bottom number) between 80 and 89. Stage 2 starts at 140/90. Evidence consistently shows that getting systolic pressure below 130 reduces stroke risk compared with less aggressive targets.
Irregular Heart Rhythm and Blood Clots
Atrial fibrillation, a condition where the upper chambers of the heart quiver instead of contracting normally, is one of the most dangerous stroke risk factors. The mechanism is straightforward: when the heart’s upper chambers don’t squeeze properly, blood pools and stagnates instead of moving through efficiently. This pooling happens especially in a small pouch called the left atrial appendage.
Stagnant blood activates the body’s clotting system. The blood in that pouch becomes measurably more clot-prone than blood circulating elsewhere in the body, with denser clot structures that are harder for the body to break down naturally. The lining of the blood vessels in the area also stops producing as much of a protective molecule that normally prevents clots from forming. Once a clot forms and breaks free, it can travel directly to the brain and block an artery there, causing an ischemic stroke.
Plaque Buildup in the Neck Arteries
The carotid arteries, two large blood vessels running up either side of your neck, are the brain’s main blood supply. Over time, fatty deposits called plaque can accumulate along their inner walls, narrowing them. The stroke risk here isn’t just about the narrowing itself. Pieces of plaque, or the small clots that form on plaque surfaces, can break loose and travel into the brain, plugging a smaller artery upstream.
Sometimes this process causes only a temporary blockage, known as a transient ischemic attack or “mini-stroke.” A TIA resolves on its own, usually within minutes, but it’s a serious warning. People who experience a TIA have a measurably elevated risk of a full stroke in the following weeks and months. In one nationwide study, roughly 1% to 2% of people who had a first-time TIA went on to have a stroke within 90 days, with higher-risk individuals facing about double the rate of lower-risk ones.
Diabetes and Blood Sugar Damage
Persistently high blood sugar damages blood vessels from the inside out. It injures the thin inner lining of arteries, making them stiffer, more prone to plaque buildup, and less able to regulate blood flow. Adults with diabetes are nearly twice as likely to have a stroke as adults without it. Diabetes also tends to travel with other stroke risk factors, including high blood pressure, high cholesterol, and obesity, compounding the danger.
Smoking and Stroke Risk
Smoking roughly doubles the overall risk of stroke. The effect is even more dramatic for specific subtypes: smokers face nearly five times the risk of subarachnoid hemorrhage, a particularly dangerous type of bleeding stroke that occurs around the brain’s surface. Smoking accelerates plaque formation, makes blood more likely to clot, and damages the walls of blood vessels throughout the body.
The encouraging news is that quitting reverses much of this damage relatively quickly. Within two to four years of stopping, a former smoker’s overall stroke risk drops back to roughly the same level as someone who never smoked. For ischemic strokes specifically, about 80% of the benefit of quitting shows up within just the first two years. Even the elevated risk of subarachnoid hemorrhage returns to near-normal levels after about five years of not smoking.
Cholesterol and Artery Disease
High LDL cholesterol, the type that contributes to plaque in artery walls, is a well-established stroke cause. The connection is most direct for large-vessel strokes, where plaque narrows or blocks major arteries. Multiple large analyses have found that statin medications, which lower LDL cholesterol, reduce the risk of a first stroke by roughly 19% to 22% in people at high cardiovascular risk.
Genetic and Inherited Causes
Some people are genetically predisposed to stroke, particularly those who have strokes at a young age without the usual risk factors. One well-studied inherited condition, known as CADASIL, involves mutations in a gene that controls blood vessel structure in the brain. It causes progressive damage to small brain vessels, leading to recurrent small strokes often starting in middle age.
Sickle cell disease is another inherited condition that significantly raises stroke risk. The misshapen red blood cells characteristic of the disease tend to clump together and block small blood vessels, including those in the brain. Beyond these single-gene conditions, researchers have identified several genetic variants that modestly increase stroke susceptibility in the broader population, including variants linked to cardioembolic stroke and large-vessel disease. Having a parent or sibling who had a stroke raises your own risk, though the exact degree depends on shared genetics, shared lifestyle habits, or both.
A Hole in the Heart: Patent Foramen Ovale
About one in four adults has a patent foramen ovale, or PFO, a small opening between the heart’s upper chambers that normally closes shortly after birth but never fully sealed. Most people with a PFO never know it exists and never have problems. But in people who’ve had a stroke with no identifiable cause (called a cryptogenic stroke), 40% to 50% turn out to have a PFO.
The concern is that a clot forming in a leg vein or elsewhere could pass through this opening, bypass the lungs where it would normally get filtered out, and travel directly to the brain. The risk is higher when the PFO is accompanied by a floppy wall of tissue between the chambers, called an atrial septal aneurysm. People with both a PFO and this extra finding have a substantially elevated risk of stroke and recurrent stroke. Clinical trials have shown that closing the PFO with a small implanted device reduces the chance of another stroke by about 62% compared with medication alone, with the greatest benefit in those with larger openings.
Less Common but Real Triggers
Several other conditions can cause strokes that don’t fit neatly into the usual categories. Blood clotting disorders, whether inherited or acquired, make the blood too prone to forming clots. Arterial dissections, where the inner wall of a neck artery tears and allows blood to collect inside the vessel wall, are a notable cause of stroke in younger adults, sometimes triggered by minor neck trauma or even vigorous exercise. Infections, autoimmune conditions that inflame blood vessels, and certain drug use (particularly stimulants like cocaine and methamphetamine, which spike blood pressure) can all trigger strokes in people who would otherwise seem low-risk.
Pregnancy and the weeks immediately after delivery also carry elevated stroke risk, driven by hormonal changes that shift the blood toward a more clot-prone state, combined with the cardiovascular stress of pregnancy itself.