Strokes happen when blood flow to part of the brain is cut off, either by a blockage or by bleeding from a ruptured blood vessel. About 65% of all strokes worldwide are caused by blockages (ischemic strokes), while roughly 29% involve bleeding within the brain and 6% involve bleeding around it. The causes range from chronic conditions like high blood pressure to acute triggers like intense anger, and understanding them is the first step toward reducing your risk.
Two Types, Two Different Causes
In an ischemic stroke, a blood clot or fatty deposit blocks an artery supplying the brain. Without oxygen, brain cells begin dying within minutes. The damage cascades quickly: excess calcium floods neurons, inflammation sets in, and cells that can’t withstand the oxygen deprivation start breaking down irreversibly.
In a hemorrhagic stroke, a weakened blood vessel bursts and bleeds into or around the brain. The leaked blood compresses surrounding tissue and can trigger dangerous spasms in nearby vessels. Bleeding within the brain itself accounts for about 80% of hemorrhagic strokes and is most commonly caused by long-term uncontrolled high blood pressure. The remaining cases typically involve bleeding in the space surrounding the brain, most often caused by a ruptured aneurysm (a balloon-like bulge in an artery wall).
High Blood Pressure: The Leading Cause
Uncontrolled hypertension is the single biggest driver of stroke across all types. The risk climbs measurably once systolic blood pressure (the top number) reaches 140 mmHg or diastolic (the bottom number) hits 90 mmHg. For hemorrhagic stroke specifically, the risk becomes significant at systolic readings of 160 mmHg or higher. In younger adults under 65, even readings at or above 130/80 mmHg are associated with increased stroke risk.
The 2024 American Heart Association guidelines for stroke prevention emphasize that more aggressive blood pressure control, targeting a systolic reading below 130 mmHg rather than the traditional 140, provides additional protection. Most people who need blood pressure medication end up requiring two or three drugs to reach their target, since a single medication gets only about 30% of patients to goal.
Heart Rhythm Problems
Atrial fibrillation, the most common serious heart rhythm disorder, is a major cause of ischemic stroke. When the upper chambers of the heart quiver instead of beating in a coordinated rhythm, blood pools and can form clots. Those clots can travel to the brain and block an artery. Atrial fibrillation is particularly dangerous because many people don’t realize they have it. The condition can come and go, producing no obvious symptoms between episodes while still raising clot risk.
Narrowed Neck Arteries
The carotid arteries, which run along each side of your neck and supply blood to the brain, are responsible for 10 to 20% of all ischemic strokes. The culprit is atherosclerosis: fatty plaques build up inside the artery wall over years. The most dangerous plaques aren’t necessarily the largest. “Vulnerable” plaques, those with a thin outer cap, a large fatty core, signs of internal bleeding, or active inflammation, are the ones most likely to rupture. When they do, the body forms a clot at the rupture site, and pieces of that clot break off and travel into the brain.
Even when plaques don’t rupture, severe narrowing can reduce blood flow enough that the brain struggles to clear tiny clots that would otherwise be harmless. Reduced flow and clot formation often work together, compounding the danger.
Diabetes and Blood Sugar
Diabetes roughly doubles the risk of ischemic stroke and raises hemorrhagic stroke risk by about 56%, according to a large pooled analysis of epidemiologic studies. Chronically elevated blood sugar damages blood vessels in several ways at once. It impairs the inner lining of arteries, reducing their ability to relax and regulate blood flow. It stiffens artery walls prematurely. And it promotes the kind of low-grade, persistent inflammation that accelerates plaque buildup.
In people with type 1 diabetes, structural changes in the carotid arteries, specifically thickening of the vessel wall, appear earlier than expected and represent an early sign of atherosclerosis. Type 2 diabetes produces similar vascular stiffening. Both types of diabetes are considered independent, modifiable risk factors for stroke.
Cholesterol and Lipids
High levels of LDL cholesterol (often called “bad” cholesterol) contribute to the plaque buildup that narrows arteries throughout the body, including those feeding the brain. Statin medications, which lower LDL, reduce first-stroke risk by roughly 19 to 22% in people at high cardiovascular risk. Omega-3 fatty acid supplements, despite their popularity, have not been shown to reduce first-stroke risk in clinical trials.
Smoking
Smoking damages the lining of blood vessels, promotes clot formation, and accelerates atherosclerosis. The good news is that quitting reverses some of the damage relatively quickly. Cardiovascular risk drops within five years of stopping, though former smokers still carry somewhat higher risk compared to people who never smoked. The sooner you quit, the more benefit you gain.
Strokes in Younger Adults Are Rising
Stroke is no longer just a concern for older adults. Since 2015, the rate of ischemic stroke in young adults has been climbing. A 2024 analysis spanning three decades found dramatic increases in the risk factors driving these strokes. The contribution of high body mass index to stroke burden in young people rose by 202% between 1990 and 2021. High fasting blood sugar increased its contribution by 76%, air pollution by 66%, high LDL cholesterol by 39%, and high blood pressure by 35%.
Behavioral factors also shifted significantly. Low physical activity increased its contribution by 48%, high alcohol use by 40%, and poor diet (specifically low whole-grain intake) by 34%. On top of these traditional risk factors, researchers point to newer contributors: recreational drug use, academic and work pressures, and rising psychological stress.
Acute Triggers That Spark a Stroke
Beyond the slow-building risk factors, certain events can trigger a stroke within minutes. The INTERSTROKE study, one of the largest studies of stroke triggers, found that about 9% of stroke patients had experienced intense anger or emotional upset in the hour before their stroke. Another 5% had engaged in heavy physical exertion.
Anger and emotional distress were linked to both ischemic and hemorrhagic strokes, likely because surges of stress hormones cause blood vessels to constrict and can provoke abnormal heart rhythms. Heavy physical exertion was specifically linked to hemorrhagic stroke, probably because it spikes blood pressure acutely, which can rupture an already weakened vessel. These triggers are most dangerous in people who already have underlying risk factors like hypertension or a known aneurysm.
Genetic and Structural Causes
Some strokes trace back to inherited conditions or structural abnormalities in blood vessels. CADASIL is a rare genetic disorder caused by a mutation in the Notch3 gene that thickens the walls of small blood vessels in the brain, progressively blocking blood flow. It causes repeated strokes, often beginning in midlife, that can eventually lead to dementia. A diagnosis is confirmed through genetic testing or a skin biopsy that reveals characteristic changes in small blood vessels. There is no cure, but managing blood pressure, cholesterol, and blood sugar can help reduce stroke frequency.
Arteriovenous malformations (AVMs) are tangles of abnormal blood vessels where arteries connect directly to veins without the usual network of tiny capillaries in between. These fragile connections can rupture and bleed. AVMs are present from birth, and while many never cause problems, they are a recognized cause of hemorrhagic stroke, particularly in younger people. Aneurysms, which are weak spots in artery walls that balloon outward, carry a similar rupture risk and are the most common cause of bleeding around the brain.
Multiple Risk Factors Multiply the Danger
Stroke rarely has a single cause. High blood pressure, diabetes, high cholesterol, smoking, obesity, and physical inactivity tend to cluster together, and each one amplifies the others. Someone with both diabetes and hypertension faces a far higher risk than the sum of each condition alone. The practical takeaway is that addressing even one risk factor, whether that means lowering your blood pressure, losing weight, or quitting smoking, creates a ripple effect that reduces your overall stroke risk.