A stroke happens when blood flow to part of the brain is cut off or when a blood vessel in the brain bursts. About 87% of strokes are caused by blockages (ischemic strokes), while the rest involve bleeding in or around the brain (hemorrhagic strokes). The triggers range from long-term conditions like high blood pressure to sudden events like a blood clot breaking loose from the neck.
How Blocked Blood Vessels Cause Stroke
Ischemic strokes, the most common type, happen in two ways. In a thrombotic stroke, a blood clot forms directly inside an artery in the brain. This usually develops where fatty plaque has already narrowed the vessel wall, similar to a clogged pipe that finally seals shut. In an embolic stroke, a clot forms somewhere else in the body, breaks free, and travels through the bloodstream until it lodges in a brain artery too small to let it pass.
One of the most common sources of these traveling clots is the carotid arteries, the two large blood vessels on either side of your neck that supply the brain. When plaque builds up in these arteries (carotid artery disease), pieces can break off and shoot directly into the brain. The risk rises with the degree of narrowing: research from the ACSRS study found annual stroke rates of 0.8% for people with 50 to 69% narrowing, 1.4% for 70 to 89%, and 2.4% for 90 to 99%.
Irregular Heartbeat and Clot Formation
Atrial fibrillation, a condition where the upper chambers of the heart quiver instead of beating in a steady rhythm, is one of the most dangerous stroke risk factors. It increases stroke risk nearly fivefold. When the heart doesn’t contract properly, blood pools and moves sluggishly in a small pouch of the heart called the left atrial appendage. That stagnant blood can form a clot, which then gets pumped out of the heart and travels to the brain. This is why people with atrial fibrillation are often prescribed blood thinners, even if they feel fine otherwise.
How Bleeding in the Brain Causes Stroke
Hemorrhagic strokes happen when a blood vessel in or around the brain ruptures. The most common type, intracerebral hemorrhage, involves bleeding directly into the brain tissue. The less common type, subarachnoid hemorrhage, occurs when bleeding fills the space between the brain and the thin layers of tissue covering it.
Several things can cause these bleeds:
- High blood pressure is the leading cause. Years of elevated pressure gradually weaken the walls of small arteries in the brain until one gives way.
- Aneurysms are balloon-like bulges in weakened artery walls. They can exist for years without symptoms, then rupture suddenly. Subarachnoid hemorrhages often start this way.
- Arteriovenous malformations (AVMs) are tangles of abnormal blood vessels, usually present from birth, that can burst inside the brain.
- Head trauma from falls, accidents, or impacts can cause bleeding severe enough to produce a stroke.
High Blood Pressure: The Biggest Risk Factor
High blood pressure contributes to both types of stroke. It accelerates plaque buildup in arteries, making blockages more likely, and it weakens vessel walls, making bleeds more likely. Current guidelines set the target for stroke prevention at below 130/80 mmHg, a threshold lowered from the previous 140/90 standard. Many people walking around with readings in the 130s or 140s don’t realize they’re already in the danger zone.
The damage is cumulative. Blood pressure doesn’t need to spike dramatically to cause harm. Years of moderately elevated pressure quietly remodel the walls of small brain arteries, making them stiff, narrow, and fragile. This is why strokes can seem to come out of nowhere in people who “felt fine.”
Acute Triggers That Can Spark a Stroke
Beyond chronic risk factors, certain events can trigger a stroke within minutes or hours. Stimulant drugs are among the most dangerous acute triggers. Cocaine use within 24 hours of a stroke is associated with a sixfold increase in ischemic stroke risk among adults under 50, according to a large population-based study. Stimulants like cocaine and amphetamines cause a sudden surge in blood pressure and can trigger spasms in brain arteries, choking off blood flow. That same pressure spike can also rupture existing aneurysms or weak vessel walls, causing hemorrhagic strokes.
Extreme physical exertion, intense emotional stress, and sudden anger can also act as short-term triggers, particularly in people who already have underlying vascular problems. These events share a common thread: they cause rapid spikes in blood pressure or heart rate that vulnerable blood vessels can’t handle.
Warning Strokes That Predict Bigger Ones
A transient ischemic attack (TIA), sometimes called a “mini-stroke,” produces stroke symptoms that resolve within minutes to hours as the blockage clears on its own. It’s tempting to dismiss a TIA because the symptoms go away, but it’s one of the strongest warning signs that a full stroke is coming. The risk of ischemic stroke within 90 days of a TIA ranges from 3 to 20%, with a residual risk of about 4.9% even after the initial high-risk window passes.
A TIA means the conditions for a stroke already exist in your blood vessels or heart. The clot happened to dissolve before permanent damage occurred, but the underlying cause, whether it’s carotid plaque, atrial fibrillation, or small vessel disease, is still there.
Silent Strokes You Never Feel
Not all strokes announce themselves with obvious symptoms. Silent strokes are small areas of brain damage caused by blocked blood flow that produce no noticeable symptoms at the time. They’re remarkably common: in the Rotterdam Scan Study, brain MRIs revealed that 20% of participants had silent infarcts they never knew about. Silent brain infarcts are five times as prevalent as symptomatic strokes in the general population.
These strokes tend to occur in deeper brain structures and individually may not cause problems you’d notice. But they accumulate over time and are linked to cognitive decline, memory problems, and a higher risk of future symptomatic strokes. They’re typically discovered incidentally when someone gets a brain MRI for another reason.
Rare Genetic Conditions
A small number of strokes, particularly in younger adults, are caused by inherited conditions that damage blood vessels over time. CADASIL is the most common single-gene disorder causing stroke and small vessel disease in the brain. It damages the walls of small arteries, leading to recurrent strokes that often begin in a person’s 30s or 40s. Fabry disease, another genetic condition, affects blood vessels throughout the body and is considered an underdiagnosed cause of stroke in younger people. Both conditions are rare, but worth investigating when someone under 50 has a stroke with no obvious explanation.
Risk Factors That Overlap and Compound
Stroke rarely has a single cause. Most people who have one carry several risk factors that interact and amplify each other. Diabetes damages blood vessel linings and accelerates plaque formation. Smoking constricts arteries and makes blood more prone to clotting. Obesity raises blood pressure and increases the likelihood of atrial fibrillation. High cholesterol feeds the plaque deposits that narrow arteries in the neck and brain.
The practical takeaway is that stroke risk is cumulative. Someone with mildly elevated blood pressure, borderline cholesterol, and a sedentary lifestyle may face a higher combined risk than someone with a single dramatic risk factor. Each condition you manage, whether through medication, diet, or exercise, removes one layer of compounding risk.