Strokes happen when blood flow to part of the brain is cut off, either by a blockage or by bleeding. More than 795,000 people in the United States have a stroke every year, and the causes range from common conditions like high blood pressure to rarer triggers like arterial tears and genetic disorders. Understanding what causes a stroke helps you recognize which risk factors apply to you.
Two Types, Two Mechanisms
About 87% of all strokes are ischemic, meaning a blood clot blocks an artery supplying the brain. The remaining strokes are hemorrhagic, caused by a blood vessel in or around the brain rupturing and bleeding into surrounding tissue. The causes behind each type are quite different, though some risk factors (like high blood pressure) contribute to both.
What Causes Ischemic Strokes
Ischemic strokes come down to two core problems: a clot that forms somewhere and travels to the brain, or a clot that builds up inside a brain artery itself.
Clots From the Heart
The heart is one of the most common sources. When the heart beats irregularly, as it does in atrial fibrillation, blood can pool in the heart’s chambers and form clots. Those clots can then travel up to the brain and lodge in a narrow artery. Atrial fibrillation alone increases ischemic stroke risk fivefold. Other heart conditions that raise the risk include recent heart attacks, damaged or artificial heart valves, infections of the heart lining, and a weakened, enlarged heart muscle. Collectively, clots originating from the heart account for 20% to 30% of all ischemic strokes.
Clots From Diseased Arteries
The second major source is atherosclerosis, the gradual buildup of fatty plaque inside artery walls. Plaques in the large arteries of the neck (the carotid and vertebral arteries) or in the aorta can crack open, and when they do, clot material or plaque debris breaks off and travels to the brain. This artery-to-artery process accounts for 15% to 20% of all ischemic strokes. In other cases, a plaque grows large enough to block the artery right where it sits, cutting off flow directly.
Small Vessel Disease
Deep inside the brain, tiny arteries supply critical structures involved in movement, sensation, and cognition. Years of high blood pressure can damage these small vessels, thickening and weakening their walls until they narrow or close off entirely. This type of small vessel disease causes another 20% to 30% of ischemic strokes. Because the affected arteries are so small, these strokes tend to be smaller too, but they can accumulate over time and lead to significant cognitive decline.
What Causes Hemorrhagic Strokes
Hemorrhagic strokes occur when a blood vessel bursts. The most common culprit is chronic high blood pressure, which weakens artery walls over years until they eventually give way. But structural problems in blood vessels also play a major role, especially in younger people.
Brain aneurysms, which are balloon-like bulges in artery walls, can rupture and cause bleeding on the brain’s surface (subarachnoid hemorrhage). Arteriovenous malformations (AVMs) are tangles of abnormal blood vessels where arteries connect directly to veins without the normal capillary network in between. These fragile, high-pressure tangles can bleed into the brain tissue itself. A related but lower-risk type, cavernous malformations, involves clusters of enlarged, thin-walled capillaries that can also leak or bleed.
Many AVMs and cavernous malformations are present from birth or develop from genetic mutations that occur early in life. About 20% of cavernous malformations run in families, passed down through specific gene mutations that weaken the junctions between cells lining blood vessels.
High Blood Pressure: The Dominant Risk Factor
No single factor drives more strokes than high blood pressure. It damages arteries throughout the body, accelerates plaque buildup, weakens small vessels in the brain, and increases the chance that an aneurysm or malformation will rupture. Blood pressure above 140/90 mm Hg is considered hypertensive, but even levels above 130/85 mm Hg are associated with elevated risk. Because high blood pressure rarely causes symptoms on its own, many people live with it for years without knowing, silently accumulating damage to the blood vessels that feed their brain.
Infections and Inflammation
Acute infections can act as a trigger for stroke, particularly in people who already have other risk factors. When the body fights an infection, it ramps up inflammation, and that inflammatory response shifts the blood toward a clot-friendly state. The lining of blood vessels becomes stickier and more prone to attracting clot-forming cells, while the body’s natural clot-dissolving systems are suppressed. In people with atrial fibrillation or other conditions that already predispose them to clots, an infection can be the tipping point that causes a clot to form and travel to the brain.
Chronic infections matter too. Ongoing low-grade infections, including gum disease (periodontitis), promote sustained inflammation that damages artery walls over time, shifts cholesterol profiles in a direction that favors plaque growth, and repeatedly introduces bacteria into the bloodstream that can activate clotting. Severe infections like sepsis are especially dangerous, triggering widespread changes in blood vessel function and clotting that dramatically raise stroke risk in the short term.
Stroke Causes in Younger Adults
When strokes happen in people under 45, the causes often look different from those in older adults.
Arterial dissection is one of the most important. A tear in the inner lining of a carotid or vertebral artery (the major arteries in the neck) allows blood to seep into the artery wall, narrowing or blocking the vessel. This affects about 2.6 per 100,000 people per year, with an average age around 45, and accounts for roughly 24% of strokes in adults aged 18 to 44. Trauma is a frequent trigger, from car accidents to sports injuries to even vigorous chiropractic manipulation, though some dissections occur spontaneously, particularly in people with connective tissue disorders like Ehlers-Danlos syndrome. The outlook is generally favorable: about two-thirds of people who experience a cervical artery dissection recover well.
Patent foramen ovale (PFO) is another cause seen disproportionately in younger stroke patients. A PFO is a small opening between the heart’s upper chambers that normally closes after birth but remains open in roughly 15% to 35% of the population. In most people it causes no problems, but it can allow a blood clot from the veins to pass through the heart and travel to the brain, bypassing the lungs where clots are typically filtered out. PFO is commonly found in younger adults whose strokes have no other clear explanation.
Genetic Conditions That Cause Stroke
Some people carry inherited conditions that make stroke far more likely, regardless of typical risk factors like blood pressure or cholesterol. CADASIL is the most well-known. Caused by mutations in the NOTCH3 gene, it damages the smooth muscle cells that line small blood vessels in the brain, leading to progressive narrowing and blockages. People with CADASIL typically experience their first stroke in mid-adulthood and often have multiple strokes over their lifetime, along with migraines, mood changes, and gradual cognitive decline. Only one copy of the mutated gene is needed to cause the condition, so a parent with CADASIL has a 50% chance of passing it to each child.
What makes CADASIL particularly important to recognize is that it occurs independently of the usual stroke risk factors. Someone with normal blood pressure, normal cholesterol, and no heart disease can still have recurrent strokes if they carry this mutation.
Environmental Triggers
Where you live and what you breathe can also influence stroke risk. Extreme heat is a significant and growing concern. Research on heatwave exposure found that the odds of dying from an ischemic stroke increased by 17% to 50% during heat waves, depending on severity and duration. Adults over 85 were the most vulnerable. Ozone pollution, which rises during hot weather, compounds the danger. The combination of extreme heat and high ozone levels has a synergistic effect, meaning together they raise risk more than either would alone. An estimated 3.7% to 4.2% of ischemic stroke deaths during summer months can be attributed to the joint exposure of heat waves and ozone.
Fine particulate air pollution from vehicle exhaust, industrial emissions, and wildfire smoke promotes the same kind of systemic inflammation and blood vessel damage that chronic infections do. For people already carrying other risk factors, a day of high air pollution can serve as the acute trigger that tips the balance.
Other Contributing Factors
Diabetes damages blood vessels throughout the body and accelerates atherosclerosis, making both ischemic and hemorrhagic strokes more likely. Smoking narrows arteries, promotes clot formation, and roughly doubles stroke risk. Obesity and physical inactivity contribute indirectly by driving up blood pressure, blood sugar, and cholesterol. Heavy alcohol use raises blood pressure and can trigger atrial fibrillation. Cocaine and amphetamines cause sudden, severe spikes in blood pressure that can rupture weakened blood vessels in the brain, a cause seen frequently in younger adults presenting with hemorrhagic stroke.
Sleep apnea is an underrecognized contributor. The repeated drops in oxygen that occur during sleep promote inflammation, raise blood pressure, and increase the tendency for blood to clot. Many people with sleep apnea are unaware they have it, making it a hidden but treatable source of stroke risk.