A stroke happens when blood flow to part of the brain is cut off, either by a blockage or a bleed. Without oxygen, brain cells start dying within minutes. The specific cause depends on the type of stroke, but most cases trace back to a handful of well-understood conditions, with high blood pressure at the top of the list.
The Two Types of Stroke
About 87% of strokes are ischemic, meaning a blood clot blocks an artery feeding the brain. The remaining cases are hemorrhagic, where a blood vessel in or around the brain ruptures and bleeds. The causes, risk factors, and warning signs overlap significantly, but the underlying mechanics are different enough that understanding both matters.
How Blood Clots Block the Brain
In an ischemic stroke, the process usually starts with atherosclerosis, a slow buildup of cholesterol-containing fatty deposits (plaque) along the inner walls of arteries. This buildup can narrow the artery on its own, but the real danger comes when a piece of plaque breaks off or triggers a clot to form on its surface. The enzyme thrombin activates platelets in the blood and converts a soluble protein called fibrinogen into an insoluble mesh of fibrin. Platelets bind to that fibrin mesh, creating a clot that can grow large enough to block the artery entirely.
Sometimes the clot forms right where the plaque is, often in the carotid arteries on either side of the neck. Other times a clot forms elsewhere, frequently in the heart, and travels through the bloodstream until it lodges in a smaller artery in the brain. Either way, the tissue downstream loses its blood supply and begins to die.
In a healthy body, clots naturally shrink after forming, squeezing out liquid and partially restoring blood flow around the blockage. Research published in Arteriosclerosis, Thrombosis, and Vascular Biology found that this contraction process is impaired in people experiencing an ischemic stroke, which may worsen the blockage and increase the damage.
How Brain Bleeds Happen
Hemorrhagic strokes fall into two categories. An intracerebral hemorrhage is bleeding directly inside the brain, most often caused by years of high blood pressure weakening small arteries until one bursts. A subarachnoid hemorrhage is bleeding in the space between the brain and the thin tissue covering it, typically triggered when an aneurysm (a balloon-like bulge in a weakened artery wall) ruptures.
Arteriovenous malformations, which are tangled clusters of abnormal blood vessels, can also rupture and cause bleeding in the brain. These are present from birth and often go undetected until they cause symptoms. While hemorrhagic strokes are less common than ischemic ones, they tend to be more deadly.
High Blood Pressure: The Leading Cause
High blood pressure contributes to both types of stroke. It accelerates plaque buildup in arteries, making ischemic strokes more likely, and it weakens small blood vessels over time, setting the stage for hemorrhagic strokes. The 2025 AHA/ACC guidelines confirm that the relationship between blood pressure and stroke risk is graded and continuous: for every 20-point increase in the top number (systolic) or 10-point increase in the bottom number (diastolic), stroke risk roughly doubles. That escalation starts at surprisingly low levels, with cardiovascular event rates beginning to climb at systolic readings as low as 90 mm Hg in people without other risk factors.
Atrial Fibrillation and Heart-Related Causes
Atrial fibrillation, an irregular and often rapid heart rhythm, is one of the most significant stroke risk factors. When the upper chambers of the heart quiver instead of contracting properly, blood pools and moves sluggishly. That stagnant blood is prone to clotting. If a clot forms and escapes the heart, it can travel to the brain and cause an ischemic stroke. People with atrial fibrillation face roughly five times the stroke risk of those with a normal heart rhythm.
Other heart conditions that raise stroke risk include heart valve disease, heart failure, and a hole between the upper chambers of the heart (patent foramen ovale), which can allow small clots to bypass the lungs and reach the brain directly.
How Diabetes Damages Blood Vessels
Type 2 diabetes increases stroke risk through several overlapping mechanisms. Chronically high blood sugar promotes the same kind of fatty plaque buildup that drives atherosclerosis, while also making the blood more prone to clotting. Glucose triggers increased production of a substance called tissue factor, which activates the clotting process in local blood vessels.
At the level of smaller blood vessels, high blood sugar generates harmful molecules that damage cell walls, alter blood flow, and cause inflammation. These effects can impair the brain’s blood supply even before a full blockage occurs. People with diabetes also tend to have high blood pressure and abnormal cholesterol levels, compounding the risk further.
Smoking and Sedentary Behavior
Smoking damages the lining of blood vessels, accelerates plaque formation, and makes blood thicker and more likely to clot. Even secondhand smoke exposure increases stroke risk by 20 to 30%, according to CDC data. The effect is dose-dependent: the more you smoke and the longer you smoke, the higher the risk, though quitting at any point begins to reverse the damage.
Physical inactivity is now recognized as a standalone risk factor. The 2024 AHA/ASA stroke prevention guidelines specifically recommend screening for sedentary behavior, noting that prolonged sitting and low activity levels contribute to the cluster of conditions (obesity, high blood pressure, high blood sugar) that drive stroke risk up.
Risk Factors Specific to Women
Several conditions unique to or more common in women raise stroke risk in ways that were historically underappreciated. The 2024 prevention guidelines highlight pregnancy complications, including dangerously high blood pressure during pregnancy or in the six weeks after delivery, as a significant risk factor. Blood pressure readings of 160/110 or higher during this period carry a real risk of fatal brain hemorrhage.
Endometriosis, premature ovarian failure (before age 40), and early menopause (before age 45) are all associated with increased stroke risk. Transgender women taking estrogen for gender affirmation also face elevated risk, making monitoring of blood pressure and other cardiovascular factors especially important in this group.
Transient Ischemic Attacks as a Warning
A transient ischemic attack, sometimes called a mini-stroke, is caused by the same mechanism as an ischemic stroke: a clot or plaque fragment temporarily blocks blood flow to the brain. The difference is that the blockage clears on its own, usually within minutes, and causes no permanent damage. Symptoms like sudden numbness, confusion, or trouble speaking resolve completely.
A TIA is a medical emergency, not a false alarm. About one in three people who have a TIA will eventually have a full stroke, with roughly half of those strokes occurring within a year. TIAs most often happen in the hours or days before a major stroke, making immediate evaluation critical.
Rare and Genetic Causes
Most strokes are driven by the common risk factors above, but a small number result from genetic conditions or unusual causes. CADASIL is a rare inherited disorder caused by a mutation in the Notch3 gene. It thickens the walls of small blood vessels in the brain, progressively blocking blood flow. People with CADASIL typically experience multiple strokes starting in mid-adulthood, often progressing to dementia. It can be confirmed through genetic blood testing or a skin biopsy, since the blood vessel changes appear throughout the body.
Vasculitis, an inflammation of blood vessel walls caused by autoimmune disease or infection, can also trigger strokes by narrowing or weakening arteries. Blood clotting disorders like antiphospholipid syndrome and autoimmune conditions like lupus raise stroke risk enough that the 2024 guidelines recommend preventive blood-thinning treatment for people with these conditions, even if they have never had a stroke.