A stroke happens when blood flow to part of the brain is cut off, either by a clot blocking an artery or by a blood vessel bursting and bleeding into brain tissue. Within minutes, brain cells in the affected area begin to die. The specific cause depends on which type of stroke occurs, but the vast majority trace back to a handful of well-understood risk factors, with high blood pressure at the top of the list.
Two Main Types of Stroke
Ischemic strokes, caused by blood clots, account for roughly 87% of all strokes. Hemorrhagic strokes, caused by bleeding in or around the brain, are less common but more likely to be fatal. The causes and risk factors overlap significantly, but the underlying mechanisms are different.
How Blood Clots Cause Ischemic Stroke
An ischemic stroke starts one of two ways. In a thrombotic stroke, a clot forms directly inside an artery that supplies the brain. This usually happens because fatty deposits (atherosclerosis) have been narrowing the vessel for years, and a clot finally forms at that narrowed spot and blocks blood flow entirely.
In an embolic stroke, the clot forms somewhere else in the body, breaks loose, and travels through the bloodstream until it lodges in a brain artery too small to pass through. The most common origin is the heart. When the heart beats irregularly, as it does in atrial fibrillation, blood pools in the upper chambers and can form clots. Those clots then get pumped out into circulation and may end up in the brain.
The pooling process in atrial fibrillation is well documented: the heart’s poor contractions cause blood to stagnate in the left atrium, creating a sludge-like environment that activates the clotting system. Patients with atrial fibrillation have a measurably heightened tendency to form clots, sometimes called a prothrombotic state, which persists as long as the irregular rhythm continues.
What Triggers Brain Bleeds
Hemorrhagic strokes fall into two categories. An intracerebral hemorrhage means a blood vessel inside the brain ruptures and bleeds into surrounding tissue. A subarachnoid hemorrhage means bleeding occurs in the space between the brain and the thin tissues covering it, usually from a burst aneurysm (a weak, ballooned-out spot on an artery wall).
High blood pressure is the leading cause of intracerebral hemorrhage. Years of elevated pressure weakens the walls of small arteries deep inside the brain until one gives way. Other causes include blood-thinning medications, a condition called cerebral amyloid angiopathy (where abnormal protein deposits weaken blood vessel walls, particularly in older adults), and abnormal tangles of blood vessels known as arteriovenous malformations. Subarachnoid hemorrhage is most often caused by a ruptured aneurysm, though vascular malformations and inflammation of blood vessel walls can also be responsible.
High Blood Pressure: The Biggest Risk Factor
Hypertension contributes to both types of stroke. For ischemic stroke, it accelerates the buildup of fatty deposits in arteries. For hemorrhagic stroke, it directly damages vessel walls. A large study published in JAMA Network Open found that for every 10-point increase in average systolic blood pressure, the risk of ischemic stroke rose by 20% and the risk of intracerebral hemorrhage rose by 31%. That relationship held across all racial and ethnic groups studied.
Current prevention guidelines from the American Heart Association emphasize keeping systolic blood pressure below 130 mm Hg. Clinical trials have shown that more aggressive blood pressure control, targeting below 120 mm Hg, provides additional protection. For context, “normal” blood pressure is considered below 120/80, and stage 1 hypertension starts at 130/80.
Diabetes, Cholesterol, and Metabolic Risk
Type 2 diabetes nearly doubles the overall risk of stroke and more than doubles the risk of ischemic stroke specifically. High blood sugar damages blood vessel linings over time, promotes inflammation, and accelerates atherosclerosis. These effects compound with high blood pressure, which is common in people with diabetes.
High LDL cholesterol contributes to the fatty plaques that narrow arteries and set the stage for clot formation. While no major clinical trial has tested cholesterol-lowering drugs with stroke as the primary outcome, multiple large trials of statin medications have shown significant stroke reduction as a secondary benefit.
Sedentary Lifestyle and Physical Inactivity
Sitting for extended periods is an independent risk factor for stroke, separate from its effects on weight, blood pressure, and blood sugar. A meta-analysis of seven studies covering nearly 680,000 people found that sedentary behavior increased stroke risk by 16% overall. The dose-response relationship was nonlinear: risk started climbing once daily sedentary time exceeded about 3.7 hours, and at 11 hours per day, the risk was 21% higher than baseline. This doesn’t mean all sitting is dangerous, but it does mean that prolonged, unbroken inactivity carries measurable risk even in people who are otherwise healthy.
Stroke in Younger Adults
While stroke is far more common after age 65, it does happen in younger people, and the causes are often different. Carotid artery dissection, where the inner lining of the main neck artery tears and blood collects within the vessel wall, is the most common cause of stroke in people under 40, accounting for about 20% of strokes in young adults. The tear can happen spontaneously or after trauma to the neck. As blood pools between the artery’s layers, it narrows or completely blocks the vessel, cutting off flow to the brain.
A small hole in the heart called a patent foramen ovale (PFO) is another important cause in younger stroke patients. About 25% of the general population has a PFO, a flap-like opening between the heart’s upper chambers that normally closes after birth but in some people never seals completely. A clot from the venous system, such as from a deep vein in the leg, can pass through this opening into the arterial side and travel to the brain. PFO is thought to be responsible for roughly half of all cryptogenic strokes, the term for strokes where no clear cause is found after a full workup.
When the Cause Can’t Be Found
About one-third of ischemic strokes are classified as cryptogenic, meaning standard testing doesn’t reveal a clear source. In many of these cases, the culprit turns out to be atrial fibrillation that comes and goes without symptoms. When patients with unexplained strokes are monitored with implantable heart monitors over weeks or months, undetected atrial fibrillation shows up in as many as 25% of cases. This is why extended heart monitoring has become a standard part of the investigation after a cryptogenic stroke, particularly in patients who also have a PFO.
Transient Ischemic Attacks as a Warning
A transient ischemic attack (TIA) produces stroke-like symptoms that resolve within minutes to hours, typically because a small clot dissolves on its own. TIAs are often called “mini-strokes,” but that label understates the urgency. The risk of a full stroke after a TIA is approximately 3% within two days, 5% within a week, and 9% within 90 days. The first 48 hours are the most dangerous window, which is why a TIA should be treated as a medical emergency even after symptoms have cleared. The same underlying causes that produce TIAs, including atherosclerosis, atrial fibrillation, and carotid disease, are responsible for the strokes that follow.
Risk Factors You Can and Can’t Control
Some stroke risk factors are fixed. Age is the strongest: risk roughly doubles each decade after 55. Family history of stroke, prior stroke or TIA, and certain genetic conditions also raise risk in ways you can’t change.
The controllable risk factors are where prevention happens:
- High blood pressure is the single largest modifiable risk factor for both ischemic and hemorrhagic stroke.
- Atrial fibrillation increases stroke risk roughly fivefold and is present in many strokes that initially appear unexplained.
- Smoking damages blood vessel walls, raises blood pressure, and makes blood more likely to clot.
- Diabetes roughly doubles stroke risk through sustained vascular damage.
- High cholesterol drives the plaque buildup that narrows arteries and creates sites for clot formation.
- Physical inactivity independently raises stroke risk, with prolonged daily sitting compounding the effect.
- Heavy alcohol use raises blood pressure and can trigger atrial fibrillation.
Most strokes are not random events. They result from years of cumulative damage to blood vessels, driven by conditions that are largely treatable. The same factors tend to cluster together: someone with high blood pressure often also has elevated cholesterol and blood sugar, and each additional risk factor compounds the others.