Strokes happen when blood flow to part of the brain is cut off, either by a blockage or by bleeding from a ruptured vessel. Nearly 12 million new strokes occur worldwide each year, killing over 7 million people. The causes range from conditions you can control, like high blood pressure, to factors you can’t, like inherited disorders. Understanding what triggers a stroke is the first step toward reducing your risk.
Two Types of Stroke, Two Different Causes
Every stroke falls into one of two broad categories, and each has distinct causes. About 85% of strokes are ischemic, meaning a blood clot blocks an artery feeding the brain. The remaining 10% to 15% are hemorrhagic, caused by a blood vessel in or around the brain rupturing and bleeding into surrounding tissue. Hemorrhagic strokes are less common but far more deadly.
Ischemic strokes happen through two main mechanisms. In a thrombotic stroke, a clot forms directly inside a brain artery, usually at a spot where fatty plaque has already narrowed the vessel. In an embolic stroke, the clot forms somewhere else in the body, typically the heart, and travels through the bloodstream until it gets stuck in a narrower brain artery. The result is the same: brain cells downstream of the blockage are starved of oxygen and begin dying within minutes.
High Blood Pressure: The Biggest Risk Factor
Hypertension is the single most important modifiable cause of stroke. Persistently high pressure damages artery walls throughout the body, but the small arteries deep inside the brain are especially vulnerable. Over time, chronic high blood pressure causes the smooth muscle in these tiny arteries to break down, their walls to thicken and stiffen, and microscopic bulges called microaneurysms to form. These weakened spots can rupture, causing a hemorrhagic stroke, or the damaged vessel walls can promote clots, causing an ischemic stroke.
Blood pressure at or above 130/90 mmHg increases stroke risk, even below the traditional “high blood pressure” threshold of 140/90. People with readings in this pre-hypertension range still face elevated risk. The numbers on the protective side are equally striking: every 10 mmHg drop in systolic blood pressure (the top number) is associated with a 27% decrease in stroke risk. For people with diabetes, aggressive blood pressure control has reduced stroke incidence by as much as 41%.
Atrial Fibrillation and Heart-Related Causes
The heart is the most common origin point for clots that travel to the brain. When the heart’s upper chambers quiver instead of contracting normally, a condition called atrial fibrillation (AFib), blood pools and moves sluggishly in those chambers. This stagnant blood can form clots. If a clot breaks loose, it enters the bloodstream and can lodge in a brain artery within seconds.
People with AFib face a three- to five-fold higher stroke risk compared to people with normal heart rhythm. AFib is especially dangerous because it can come and go without obvious symptoms, meaning clots can form during episodes you never noticed. Other heart conditions that raise stroke risk include damaged heart valves, recent heart attacks (which can cause clots to form on injured heart tissue), and a hole between the heart’s upper chambers called a patent foramen ovale, or PFO. PFOs are particularly relevant in younger stroke patients, where a clot from the venous system can cross through the hole and reach the brain.
Plaque Buildup in the Carotid Arteries
The carotid arteries run up each side of your neck and are the brain’s main blood supply. Atherosclerosis, the gradual buildup of fatty plaque inside artery walls, can narrow these vessels or create unstable deposits that break apart and send debris into the brain. Carotid artery disease accounts for roughly 10% to 20% of all ischemic strokes.
The degree of narrowing matters, but it’s not the whole story. A severely narrowed carotid can reduce blood flow to the brain enough to cause a stroke on its own. More often, though, the danger comes from the plaque itself. Plaques with a large fatty core, a thin outer cap, internal bleeding, or surface ulceration are considered “vulnerable.” These unstable plaques can crack open, triggering a sudden clot that either blocks the carotid or breaks free and lodges in a smaller brain artery downstream. This embolic mechanism is the most frequent way carotid disease causes strokes.
Lifestyle Factors That Raise Your Risk
Smoking roughly doubles stroke risk. Tobacco damages blood vessel linings, accelerates plaque buildup, makes blood more prone to clotting, and raises blood pressure. The combined effect is significant: a large study of adults over 40 in northern China found that smokers had a 2.18 times higher odds of stroke compared to nonsmokers.
Physical inactivity is another major contributor. People who don’t exercise regularly have about 75% higher stroke risk compared to those who do. Exercise helps control blood pressure, blood sugar, cholesterol, and weight, all of which independently affect stroke risk. Obesity contributes through these same pathways, promoting inflammation, insulin resistance, and the kind of metabolic dysfunction that damages arteries over years.
Diabetes deserves special mention. Persistently high blood sugar damages the inner lining of blood vessels and accelerates atherosclerosis. People with diabetes are at heightened risk for both ischemic and hemorrhagic stroke, and their strokes tend to be more severe.
Strokes in Younger Adults
About 15% of all strokes occur in people between 15 and 49. The causes in this age group often differ from those in older adults. Arterial dissection, where the inner lining of an artery tears and blood collects in the vessel wall, is the most common artery-related cause of stroke in young people. It can happen after trauma, vigorous physical activity, or sometimes spontaneously.
Other causes more common in younger adults include blood vessel inflammation (vasculitis), moyamoya disease (a condition where arteries at the base of the brain progressively narrow), reversible blood vessel spasms, and clotting disorders that make blood too “sticky.” Migraine with aura is also linked to a small increase in stroke risk, particularly in young women. In about half of strokes in young patients, no definitive cause is identified even after thorough testing.
Inherited Conditions That Cause Stroke
Some people carry genetic conditions that make strokes more likely, often at ages when stroke is otherwise rare. CADASIL is one of the most recognized, a condition where mutations cause the walls of small brain arteries to thicken and deteriorate, leading to recurrent strokes starting in middle age. A related condition, CARASIL, follows a similar pattern but is inherited differently and tends to appear earlier.
Other inherited disorders linked to stroke include sickle cell anemia (where abnormally shaped red blood cells block small vessels), Fabry disease (where a missing enzyme leads to fatty deposits in blood vessel walls), Ehlers-Danlos syndrome and Marfan syndrome (both of which weaken connective tissue in artery walls, making tears and ruptures more likely), and hereditary hemorrhagic telangiectasia, which causes fragile, abnormal blood vessels prone to bleeding. For several of these conditions, no cure exists, and treatment focuses on managing symptoms and controlling other stroke risk factors.
Hemorrhagic Stroke: When Vessels Burst
Hemorrhagic strokes have their own set of causes. Intracerebral hemorrhage, bleeding directly into brain tissue, is most commonly caused by chronic high blood pressure. Years of elevated pressure weaken the walls of small arteries deep in the brain until they eventually rupture. In older adults, a condition called cerebral amyloid angiopathy is another leading cause. It involves the buildup of a specific protein in brain blood vessel walls, making them brittle and prone to bleeding.
Subarachnoid hemorrhage, bleeding into the space surrounding the brain, is usually caused by a ruptured aneurysm, a balloon-like weak spot in an artery wall. Aneurysms can exist for years without symptoms and may rupture suddenly during physical exertion or straining. Arteriovenous malformations, tangles of abnormal blood vessels present from birth, are another cause. Blood-thinning medications, whether prescribed anticoagulants or aspirin, also increase the risk of hemorrhagic stroke, especially in people who already have weakened blood vessels.
Warning Signs: Transient Ischemic Attacks
A transient ischemic attack, or TIA, is caused by the same mechanisms as an ischemic stroke, but the blockage clears on its own and symptoms resolve, usually within minutes to hours. TIAs are often called “mini-strokes,” but the name is misleading because the danger they signal is anything but minor. About 1 in 3 people who experience 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 them a critical warning.
The typical TIA involves the same fatty plaque buildup or traveling clots that cause full strokes. The difference is simply that the clot dissolves or dislodges before permanent brain damage occurs. If you experience sudden weakness, vision changes, difficulty speaking, or dizziness that resolves on its own, the underlying cause hasn’t gone away.
Strokes With No Identified Cause
About one-third of all ischemic strokes and TIAs have no clear cause even after standard testing. These are called cryptogenic strokes, and they represent roughly 400,000 cases annually in western Europe alone. Suspected hidden causes include brief episodes of atrial fibrillation too short to catch on routine monitoring, unstable plaque in arteries that isn’t severe enough to show up on imaging, PFOs, and minor structural heart abnormalities. In younger adults, the proportion is even higher: about half of strokes in people under 50 fall into this category. Advances in prolonged heart rhythm monitoring have helped reclassify some of these cases by catching intermittent AFib that standard tests miss.