The three main causes of strokes are blood clots that form in the brain’s arteries, blood clots that travel to the brain from elsewhere in the body, and bleeding from a ruptured blood vessel in the brain. These three mechanisms account for virtually all strokes, though the underlying conditions that trigger them, particularly high blood pressure, arterial plaque buildup, and irregular heart rhythms, are what ultimately put people at risk.
Blood Clots That Form in the Brain
The most common type of stroke happens when a blood clot develops directly inside an artery that supplies the brain. This is called a thrombotic stroke. It starts with atherosclerosis, a process where fatty deposits called plaque gradually build up along artery walls and narrow the passage for blood flow. Over time, plaque can crack open. When it does, blood platelets rush to the damaged spot and clump together, forming a clot that partially or fully blocks the artery.
Once blood flow is cut off, brain cells begin dying within minutes from lack of oxygen and nutrients. The location and size of the blockage determine how severe the damage is. A clot in a large artery feeding a major brain region causes more widespread injury than one in a smaller branch vessel. Thrombotic strokes often develop in people who have had years of uncontrolled high blood pressure or high cholesterol, both of which accelerate plaque formation inside the arteries.
Blood Clots That Travel From Elsewhere
The second major cause is an embolic stroke, where a clot forms somewhere else in the body and then travels through the bloodstream until it lodges in a brain artery. The most common source is the heart, particularly in people with atrial fibrillation, an irregular heart rhythm that affects millions of adults. When the heart beats erratically, blood can pool in the upper chambers and form clots. If a piece breaks free, it can reach the brain in seconds.
Atrial fibrillation increases the risk of stroke fivefold. It frequently coexists with atherosclerosis, and the two conditions share overlapping risk factors: high blood pressure, diabetes, obesity, and heart failure. Research from the Atherosclerosis Risk in Communities study found that plaque buildup in the carotid arteries (the major vessels in the neck) was associated with 10-year ischemic stroke risk in middle-aged adults who also had atrial fibrillation. So these two causes often compound each other.
Clots can also form on plaque in the carotid arteries themselves. If a fragment breaks loose and travels up into the brain, it blocks a smaller artery downstream. Together, thrombotic and embolic strokes make up about 87% of all strokes.
Bleeding in the Brain
The third cause is a hemorrhagic stroke, which happens when a blood vessel in or around the brain ruptures and bleeds. This accounts for roughly 13% of strokes but tends to be more deadly. The damage comes from two directions at once: brain tissue loses its blood supply, and the escaped blood creates pressure that compresses and irritates surrounding areas, causing additional swelling and injury.
There are two forms. Intracerebral hemorrhage is bleeding from a vessel within the brain itself, most often caused by chronic high blood pressure that weakens artery walls over time until they eventually give way. Subarachnoid hemorrhage is bleeding in the space between the brain and the membranes covering it, typically caused by the rupture of a brain aneurysm, a balloon-like bulge in a weakened blood vessel wall. A tangle of abnormally formed blood vessels called an arteriovenous malformation can also burst and cause this type of bleeding, though it’s less common.
High Blood Pressure Ties All Three Together
If there is one condition that connects every type of stroke, it is high blood pressure. It accelerates the plaque buildup that leads to thrombotic strokes. It puts extra stress on the heart, contributing to the irregular rhythms that cause embolic strokes. And it weakens blood vessel walls, setting the stage for hemorrhagic strokes. The 2024 stroke prevention guidelines from the American Heart Association and American Stroke Association recommend keeping blood pressure below 130/80 for people at elevated cardiovascular risk.
Sodium intake plays a direct role. Reducing daily sodium from around 3,500 mg to 1,500 mg lowers systolic blood pressure by roughly 8 points in adults over 55, and a meta-analysis found that sustained sodium reduction cuts cardiovascular events by 20%. One estimate suggests that even a modest 4% annual reduction in population-wide sodium intake over a decade could prevent between 30,000 and 83,000 stroke deaths.
Transient Ischemic Attacks: A Warning Sign
A transient ischemic attack, or TIA, involves the same mechanism as an ischemic stroke (a temporary blockage) but resolves on its own, usually within minutes. Because there’s no lasting damage, people sometimes dismiss it. That’s a mistake. About 1 in 3 people who have a TIA will eventually have a full stroke, and roughly half of those strokes occur within a year.
A TIA is essentially the same disease process caught before it causes permanent injury. It signals that the conditions for a major stroke, whether from plaque, clots, or both, are already in place.
Recognizing a Stroke as It Happens
Knowing the causes matters for prevention, but recognizing a stroke in progress is what saves brain tissue. The BE-FAST acronym covers the key warning signs:
- Balance: Sudden loss of balance or coordination, trouble walking
- Eyes: Sudden vision loss, blurred vision, or double vision
- Face: One side of the face droops or feels numb
- Arm: Weakness or numbness in one arm (or leg)
- Speech: Slurred or confused speech, difficulty understanding others
- Time: Call emergency services immediately
The older FAST acronym missed strokes that presented mainly with balance or vision problems. Adding “B” and “E” catches a larger proportion of cases, particularly posterior strokes affecting the back of the brain. Every minute without treatment means more brain cells lost, so speed is the single most important factor in outcomes.